KR101799886B1 - Steel sheet for use at lower temperatures, and method for producing same - Google Patents

Abstract

A low temperature steel sheet having high strength and excellent in low temperature toughness and brittle crack propagation stopping property, and a process for producing the same. (1/4) t, the residual austenite content is 2.2% to 14% by volume, and the plate thickness (1/4) The average aspect ratio of the old austenitic grains is not more than 4.0 and the average grain size of the old austenitic grains in the cross section parallel to the rolling direction is not less than 10 占 퐉 and not more than 60 占 퐉, The {100} planarity degree of 1.3 or more and the {100} planarity parallel to the plate surface is 0.90 or less, and the plate thickness (1/2) of the steel plate is 1 mm or less, temperature steel sheet having a structure in which the {111} planarity parallel to the sheet surface is in the range of 1.2 to 2.5 and has a tensile strength of 700 MPa or more and excellent in low-temperature toughness and brittle crack propagation stopping properties.

Description

TECHNICAL FIELD [0001] The present invention relates to a low-temperature steel sheet and a method for manufacturing the steel sheet,

The present invention relates to a low-temperature steel sheet which is suitable for use in an LNG storage tank or the like, has a high strength and is excellent in low temperature toughness and brittle crack propagation stopping property, and a method for producing the same.

In recent years, the demand for natural gas (LNG) as a clean energy source has been rapidly increasing due to the increasing global demand for energy and deterioration of the global environment accompanying it. Along with this, the construction of LNG storage tanks has been actively promoted at home and abroad, and the demand for low temperature steel sheets used in tank bodies is also increasing.

Since the LNG storage tank is always required to have a high degree of safety, the low temperature steel sheet used in the tank body is required to have excellent toughness at the temperature at which the LNG becomes liquid (about -162 DEG C) Is required. Particularly, in consideration of the seriousness of the tank destruction accident assumed when a crack occurs in the tank main body, the brittle crack propagation stopping property is important.

Further, the LNG storage tank tends to be enlarged for the purpose of effectively utilizing the site, and it is desired that the low-temperature steel sheet used for the tank main body has better strength (yield strength, tensile strength). In addition, the steel sheet for low temperature is required to have further excellent brittle crack propagation stopping characteristics with thickening of the tank body due to enlargement of the tank body.

In general, it is known that brittle crack propagation stopping properties are correlated with toughness (brittle and ductile fracture transition temperature), and improvement of low-temperature toughness of a low-temperature steel sheet is one of the effective means for improving the brittle crack propagation stopping property I think. Therefore, conventionally, a 9% Ni steel sheet excellent in low-temperature toughness has been widely used as a low-temperature steel sheet used for a tank main body of an LNG storage tank.

However, applying 9% Ni steel containing about 9% Ni as a high-priced alloy element to the tank main body of the LNG storage tank causes an increase in the construction cost of the tank. Therefore, from the viewpoint of cost reduction, the low temperature steel sheet used for the tank main body is provided with characteristics (strength, low temperature toughness, brittle crack propagation stop characteristics, etc.) equal to or less than 9% Development of a low-temperature steel sheet is desired.

In general, when the Ni content of the low-temperature steel sheet is reduced, the steel sheet properties including low-temperature toughness are deteriorated, making it difficult to ensure high safety required for the LNG storage tank. To such a problem, a technique for improving the properties of a steel sheet such as low-temperature toughness has been proposed for a low-temperature steel sheet having a reduced Ni content.

For example, in Patent Documents 1 to 9, a steel (slab) having a Ni content of about 5 to 10% is heated and controlled at a relatively low temperature, and then subjected to direct hardening. Subsequently, A technique has been proposed in which the steel is tempered by heating or quenched by heating at a temperature not lower than the Ac ₁ transformation point following the direct quenching and then tempering by heating to a predetermined temperature to make the steel structure finer. In Patent Documents 1 to 9, it is described that the low temperature toughness of a steel in which the Ni content is reduced is improved by making the steel structure finer as described above. In the above-described Patent Documents 1 to 9, the following technologies are proposed.

Patent Document 1 discloses a low temperature steel material in which the Ni content is more than 6% by mass but less than 8%, the area ratio of austenite is not less than 1.7%, and the aspect ratio of the austenite is averaged 3.5 or less, and the mean circle-equivalent particle diameter is 1.0 占 퐉 or less. In Patent Document 1, by specifying the area ratio of the austenite, the aspect ratio, and the average circle equivalent grain size as described above, it is possible to obtain the characteristics (YS, TS, -196 DEG C The Charpy absorbed energy in the steel material is obtained.

Patent Document 2 proposes a technique of making a steel composition satisfying Ni: more than 5% and less than 7.5%, and 3Si + 5Al + 50N? 0.65 in mass% with respect to cryogenic steel. In addition, Patent Document 2 discloses that the above steel composition has low temperature toughness (Charpy absorbed energy at -196 deg. C) at least Ni content higher than 9% Ni steel, It is described that a cryogenic steel excellent in CTOD (Crack Tip Opening Displacement) characteristic of a weld heat affected portion including a welded portion (welded portion) is obtained.

Patent Document 3 discloses that the Ni content is more than 5.0% by mass but less than 8.0%, the amount of retained austenite at the plate thickness (1/4) t position is 3.0% by volume or more, And a technique of setting the average effective crystal grain size to 5.5 탆 or less has been proposed. Patent Document 3 discloses that the brittle crack propagation stopping property and the brittle crack generation inhibiting property of the post-steel sheet are improved by defining the retained austenite amount and the mean effective crystal grain size as described above, A Ni-reduced low-temperature steel sheet having excellent toughness (Charpy absorption energy at -196 DEG C) is obtained.

Patent Document 4 discloses a cryogenic steel sheet in which the Ni content is more than 5.0% by mass but less than 10.0%, the amount of retained austenite at the plate thickness (1/4) t position is 3.0% by volume or more, A technique has been proposed in which the effective grain size is set to 5.5 m or less on average at the plate thickness (1/4) t position. In Patent Document 4, brittle crack generation inhibiting characteristics (Charpy absorbed energy at -196 ° C, limit CTOD value) and arrestability (-196 ° C) are obtained by defining the residual austenite amount and the effective crystal grain size as described above. Lt; 0 > C, and the absorption energy at three-plane slit Charpy absorption at a temperature of -20 [deg.] C).

Patent Document 5 discloses that the Ni content, the retained austenite amount and the effective crystal grain size are the same as those of Patent Document 4 with respect to the steel sheet for cryogenic temperature and the average value GAM (misorientation misorientation) Of 0.85 DEG or more has been proposed. Patent Document 5 discloses that brittle fracture occurrence suppression characteristics (Charpy absorbed energy at -196 deg. C, limit CTOD value) after tensional aging can be obtained by specifying the amount of retained austenite, the effective crystal grain size and GAM as described above A steel sheet having excellent cryogenic temperature is obtained.

Patent Document 6 proposes a technique of setting the Ni content to 5.5 to 8.5% by mass% and the average effective crystal grain size in the region of 0.2 mm or less from the steel material surface to 5.0 탆 or less with respect to the cryogenic steel material. Patent Document 6 discloses that a cryogenic steel excellent in low-temperature resistance to fracture (low-temperature CTOD value at -165 deg. C) after being subjected to deformation is obtained by making the metal structure of the steel surface region finer .

Patent Document 7 discloses a cryogenic steel material in which the Ni content is more than 5.0% by mass but less than 10.0%, the amount of retained austenite at the plate thickness (1/4) t position is 3.0% by volume or more, , A reduction ratio of the amount of retained austenite when receiving 1% of plastic deformation under an environment of -165 占 폚 is 25% or less. In Patent Document 7, the brittle crack generation inhibiting characteristic (Charpy absorbed energy at -196 캜, limit CTOD value) and the arresting characteristic (three-plane slit Charpy absorption at -196 캜) Energy) is obtained.

Patent Document 8 discloses a Ni-added steel sheet in which the Ni content is 5.0% or more and 7.5% or less by mass%, and Ni is present in a proportion of 1/4 of the plate thickness in the depth direction from the plate surface, A technique in which the segregation ratio is 1.3 or less, the amount of austenite after deep cooling is 2% or more, the austenite non-uniformity index after cooling is 5.0 or less, and the mean circle equivalent diameter of austenite after cooling is 1 탆 or less Has been proposed. Patent Document 8 discloses that a Ni addition steel sheet excellent in toughness (CTOD value at -165 deg. C) and excellent in aesthetic performance is obtained even when the Ni segregation ratio and after-cooling austenite are defined as above, Is obtained.

Patent Document 9 discloses that the Ni content of low-temperature Ni-containing steel is 7.0 to 10.5% by mass and the degree of integration of the {110} texture of the surface parallel to the steel sheet surface in the range of 3 mm from the surface of the steel sheet is 1.2 or more and the degree of integration of the {100} and {211} texture in the plane parallel to the steel sheet surface at the center of the thickness of the steel sheet is 1.2 or more and 3.0 or less, respectively. Patent Document 9 discloses that a Ni-containing steel for low temperature which is excellent in low temperature toughness (Charpy absorbed energy at -196 캜) and brittle crack propagation stopping property is obtained by developing a predetermined texture.

International Publication No. 2007/034576 International Publication No. 2007/080646 Japanese Laid-Open Patent Publication No. 2011-241419 Japanese Laid-Open Patent Publication No. 2011-219848 Japanese Laid-Open Patent Publication No. 2011-219849 Japanese Laid-Open Patent Publication No. 2013-14812 Japanese Laid-Open Patent Publication No. 2013-14811 Japanese Patent Publication No. 4975888 Japanese Laid-Open Patent Publication No. 2011-214099

 Watanabe Yuki, et al., "Fracture Toughness of 9% Ni Steel and Safety of LNG Reservoir," Japan Steel Pipe Gibo, Japan Steel Pipe Co., Ltd., No.104, 1984, p. 2-12

However, in the techniques proposed in Patent Documents 1 to 3, 5 and 6, although the low temperature toughness (Charpy absorption energy or fracture generation characteristic) of the steel is examined, the relationship between the low temperature toughness and the brittle crack propagation stopping property It has not been reviewed. Therefore, when the Ni content of the low-temperature steel sheet is reduced to less than 9%, there is a possibility that the brittle crack propagation stopping property becomes insufficient. When such a low-temperature steel sheet is applied to the LNG storage tank, its safety can not be guaranteed .

In the techniques proposed in Patent Documents 4 and 7, the brittle crack propagation stopping characteristics (the arresting characteristics) of the low-temperature steel sheet have been examined and the brittle crack propagation stopping characteristics (the arresting characteristics) are evaluated by the triple slit Charpy test . However, in these techniques, the above test is carried out using the test specimens collected from the center of the thickness of the low-temperature steel sheet, and the brittle crack propagation stopping characteristics in the entire thickness of the steel sheet including the front and back layers of the steel sheet are not clear . Therefore, it is difficult to say that the brittle crack propagation stopping characteristic, which is important for securing the safety of the LNG storage tank, is sufficiently verified.

In the technique proposed in Patent Document 8, the Ni segregation ratio and the after-cooling austenite nonuniformity index are specified for the purpose of improving the brittle crack propagation stopping property (the arresting property) of the low-temperature steel sheet. However, in this technique, in the step of producing a low-temperature steel sheet having a desired Ni segregation ratio and a post-cooling austenite nonuniformity index, a band segregation reduction treatment for holding the steel strip before rolling at a heating temperature of 1250 DEG C or higher for 8 hours or more . Heating and holding the steel strip prior to rolling at such a high temperature is not normally performed, requires a very large amount of energy, and increases manufacturing cost. In addition, there is also a possibility that not only the manufacturing cost increases but also the surface quality of the steel sheet becomes poor.

In the technique proposed in Patent Document 9, improvement of the brittle crack propagation stopping property is achieved by controlling the texture of the low-temperature steel sheet having the Ni content of 7.0 to 10.5% by mass%. However, with this technique, the strength and the low-temperature toughness of the steel sheet decrease as the Ni content decreases. When the Ni content is smaller (for example, less than 7.35%), it becomes difficult to achieve both strength of the steel sheet and low temperature toughness and brittle crack propagation stopping characteristics.

As described above, in the prior art, when the Ni content is reduced to less than 9% by mass, particularly, the low-temperature steel sheet reduced to about 7% or less than 7% by mass% has high strength and excellent low temperature toughness , And it is also difficult to obtain a low temperature steel sheet excellent in brittle crack propagation stopping characteristics.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a low-temperature steel sheet having a reduced Ni content with respect to a conventional 9% Ni steel sheet which has become popular as a material for a tank main body of an LNG storage tank, Temperature steel sheet having a strength equal to or higher than that of an Ni steel sheet, a low-temperature toughness and a further excellent brittle crack propagation stopping property, and a method for stably producing the low-temperature steel sheet.

The present inventors have found that means for securing sufficient strength and low temperature toughness and improving brittle crack propagation stopping property for a low temperature steel sheet having a basic component having a Ni content of 5.5% or more and 8.0% or less by mass% Were examined at different points of view. As a result, in order to make the low temperature toughness and the brittle crack propagation stopping property of the low temperature steel sheet having the basic component equal to or more than that of the 9% Ni steel sheet, the steel sheet structure is made into a tempering martensite structure in which the retained austenite is finely dispersed , It was recognized that it is necessary to control the amount of retained austenite dispersed in the tempering martensite and control the average grain size and the average aspect ratio of the old austenite grains of the steel sheet to make the old austenite grains finer.

Further, it was found that the characteristics of the front and back surfaces of the steel sheet affected the brittle crack propagation stopping characteristics of the steel sheet as a whole, and the {110} surface was developed near the surface of the steel sheet. On the other hand, It was recognized that the brittle crack propagation stopping property of the low-temperature steel sheet was further improved.

By specifying the steel sheet structure as described above, even if the Ni content is reduced to 8.0% by mass or less, it is recognized that sufficient strength, low temperature toughness and brittle crack propagation stopping property can be obtained to secure the safety of the LNG storage tank ≪ / RTI > Further, it was recognized that low-temperature toughness and brittle crack propagation stopping characteristics are further improved by securing a predetermined amount of austenite retained at a low temperature.

The present inventors have also studied a method for stably producing a low-temperature steel sheet having a Ni content of 5.5% or more and 8.0% or less in mass% and having a desired structure as described above. As described above, control rolling is performed in a relatively low temperature zone on a steel having a Ni content of about 5 to 10%, followed by direct quenching, followed by tempering by heating to a predetermined temperature, _And then quenched by heating at a temperature not lower than ₁ transformation point, and then tempering by heating to a predetermined temperature, whereby the steel structure can be made finer. However, as a result of the studies conducted by the inventors of the present invention, it has been found that, in the conventional control rolling, the average grain size and the average aspect ratio of the old austenite grains of the steel sheet are controlled, and the {110} surface is developed near the steel sheet surface, It has been found that it is very difficult to develop the {111} face at the center.

Therefore, as a result of further investigation, it has been found that, in order to sufficiently miniaturize the old austenitic grains and reduce the average aspect ratio and to develop a desired crystal face in the vicinity of the steel sheet surface and in the central portion, It was recognized that it is particularly effective to control the cumulative rolling reduction ratio not only in the temperature range but also in the recrystallization temperature range. In addition to controlling the cumulative rolling reduction ratio in each temperature range between the recrystallization temperature region and the non-recrystallization temperature region, by specifying other rolling conditions, direct quenching, tempering, or additional conditions of secondary quenching, It has been found that a low-temperature steel sheet having a desired structure can be stably produced without forming a step of holding the billet at 1250 DEG C or longer for a long period of time.

The present invention has been completed on the basis of the above-described recognition, and its gist is as follows.

The steel sheet according to any one of claims 1 to 3, characterized in that the steel sheet comprises at least one of C: 0.03 to 0.10%, Si: 0.02 to 0.30%, Mn: 0.65 to 1.20%, P: 0.005% Or more and 0.10% or less, N: 0.0015% or more and 0.0045% or less, Ni: 5.5% or more and 8.0% or less, the balance being Fe and inevitable impurities, and made of tempering martensite in which retained austenite is dispersed (1/4) t, the amount of retained austenite in the volume ratio is 2.2% or more and 14% or less at the plate thickness (1/4) t, The average grain size of the nightlips is not less than 10 占 퐉 and not more than 60 占 퐉 and the average aspect ratio of the old austenitic grains is not more than 4.0 and not more than {110} planes The degree of integration is more than 1.3, and the {100} planarity parallel to the surface is 0.90 Hayigo, according to the plate thickness (1/2) t position of the steel sheet, is a {111} plane integration parallel to the printing plate has a 1.2 to 2.5 or less tissue, the low-temperature steel sheet characterized in that not less than the tensile strength 700㎫.

[2] The low temperature steel sheet according to [1], wherein the retained amount of retained austenite after the sub zero treatment at the plate thickness (1/4) t position is 1.7% or more and 11% or less by volume.

[3] The low temperature steel sheet according to [1] or [2], further comprising, in mass%, Mo: 0.05% or more and 0.50% or less in addition to the above composition.

[4] The low temperature steel sheet according to [1] or [2], further comprising, in mass%, Mo: 0.10% to 0.30%

[5] The low temperature steel sheet according to any one of [1] to [4], further comprising, in mass%, not more than 1.00% of Cr.

[6] The cold-rolled steel sheet according to any one of [1] to [4], further comprising, in mass%, less than 0.20% of Cr in addition to the above composition.

[7] The steel sheet according to any one of [1] to [6], further comprising, in addition to the above composition, at least one selected from the group consisting of Cu in an amount of less than 0.40%, Nb in an amount of 0.05% Or two or more of them.

[8] The low temperature steel sheet according to any one of [1] to [7], further comprising, in addition to the above composition, 0.03% or less of Ti by mass%.

[9] The powder according to any one of [1] to [8], further comprising, in addition to the above composition, at least one selected from the group consisting of Ca in an amount of not more than 0.007%, REM in an amount not more than 0.010% Or two or more of them.

[10] A steel material having a composition as described in any one of [1] and [3] to [9], which is heated to a temperature of 1000 ° C or higher and 1200 ° C or lower and has a cumulative reduction ratio at a temperature range of 950 ° C or lower and 840 ° C or higher The rolling reduction is carried out at a rolling reduction rate of 30% or more and 75% or less at a temperature range of 840 DEG C or lower and a rolling reduction temperature of 820 DEG C or lower at 700 DEG C or higher, (1/2) t, the cooling is carried out at an average cooling rate of not lower than 550 캜 and not lower than 300 캜 at 1 캜 / s or higher and a cooling termination temperature of 300 캜 or lower, And then tempering at a temperature in the range of 550 ° C to 650 ° C.

[11] A steel material having a composition as described in any one of [1] and [3] to [9], which is heated at a temperature of 1000 ° C. or higher and 1200 ° C. or lower, and a cumulative reduction rate at 950 ° C. or lower in a temperature range exceeding 840 ° C. The rolling reduction is performed at a rolling reduction temperature of 850 DEG C or lower and 730 DEG C or higher at a rolling reduction rate of 30% or higher and 75% or lower at a temperature range of 840 DEG C or lower, (1/2) t, the cooling is carried out at an average cooling rate of not lower than 550 캜 and not lower than 300 캜 at 1 캜 / s or higher and a cooling termination temperature of 300 캜 or lower, The average cooling rate at a temperature range of at least 550 캜 and not less than 300 캜 at a plate thickness (1/2) t position of the steel sheet after heating to a temperature range of 650 캜 or more and less than the Ac ₃ transformation point is 3 캜 / s or more, and cooling is carried out at a cooling termination temperature of 200 DEG C or lower And then tempering the steel sheet at a temperature in the range of 500 ° C to 650 ° C.

According to the present invention, even if the Ni content is reduced to 5.5% or more and 8.0% or less by mass%, the low-temperature steel sheet suitable for use in an LNG storage tank or the like has high strength and low temperature toughness equal to or higher than 9% , A low-temperature steel sheet excellent in brittle crack propagation stopping characteristics can be obtained. Further, according to the present invention, a low-temperature steel sheet having a Ni content of 5.5% or more and 8.0% or less and having characteristics suitable for use in an LNG storage tank or the like can be stably produced.

Fig. 1 is a view showing the shape of a test piece used in a double tensile test with a surface cut-away. Fig.

(Mode for carrying out the invention)

Hereinafter, the present invention will be described in detail.

First, the reason for limiting the composition of the low-temperature steel sheet of the present invention will be described. In addition,% representing the following compositional composition means mass% (mass%) unless otherwise specified.

C: not less than 0.03% and not more than 0.10%

C is an important element for imparting desired strength to the steel sheet, and its content needs to be 0.03% or more. On the other hand, if the C content exceeds 0.10%, the low temperature toughness of the steel sheet is lowered. Therefore, the C content is 0.03% or more and 0.10% or less. And preferably 0.04% or more and 0.08% or less.

Si: not less than 0.02% and not more than 0.30%

Si is an element contributing to the improvement of the strength of the steel sheet and is an element having an action as a deoxidizing agent. For the purpose of developing these effects, the Si content is set to 0.02% or more in the present invention. However, if the Si content is excessively high, the susceptibility of the steel sheet to tempering embrittlement increases. Therefore, the Si content is 0.02% or more and 0.30% or less. , Preferably not less than 0.03% and not more than 0.20%.

Mn: not less than 0.65% and not more than 1.20%

Mn is an element contributing to the enhancement of the strength of the steel sheet by enhancing the quenching property of the steel. If the Mn content is less than 0.65%, the quenching property of the steel is lowered, and the strength of the steel sheet as well as the low temperature toughness are lowered. On the other hand, when the Mn content exceeds 1.20%, the effect of improving the strength of the steel sheet is reduced, and conversely the low temperature toughness is lowered and the susceptibility to tempering brittleness is also increased. Therefore, the Mn content should be 0.65% or more and 1.20% or less. Preferably not less than 0.70% and not more than 0.95%.

P: not more than 0.005%, S: not more than 0.003%

P and S are both inevitable impurities and are harmful elements that adversely affect the low temperature toughness and brittle crack propagation stopping properties of the steel sheet. For example, in order to obtain a sound base material and weld joint when the steel sheet is welded to form a welded structure, it is preferable to suppress the content of P and S as much as possible. In the present invention, the P content is set to 0.005% The content should be 0.003% or less.

Al: not less than 0.01% and not more than 0.10%

Al is an element required as a deoxidizer. When the content is less than 0.01%, the effect as a deoxidizing agent is insufficient, while when it is more than 0.10%, the cleanliness of the steel is impaired. Therefore, the Al content is set to 0.01% or more and 0.10% or less. , Preferably not less than 0.02% and not more than 0.05%.

N: 0.0015% or more and 0.0045% or less

N is a precipitate formed in the steel, and when the content exceeds 0.0045%, it becomes a cause of deterioration of the toughness of the base material and the weld heat affected zone when the steel sheet is welded to be a welded structure. However, N is also an element contributing to grain refinement of the base material by forming AlN. This effect is obtained by setting the N content to 0.0015% or more. Therefore, the N content should be 0.0015% or more and 0.0045% or less.

Ni: not less than 5.5% and not more than 8.0%

Ni is an element that is very effective for improving the low-temperature toughness of a steel sheet, but because it is an expensive element, the steel sheet costs increase as its content increases. The present invention is characterized in that the Ni content is reduced to 8.0% or less to provide a low-temperature steel sheet having excellent strength, low-temperature toughness, brittle crack propagation stopping property, and low cost. However, if the Ni content is less than 5.5%, the strength of the steel sheet is lowered. In addition, stable retained austenite at a low temperature is not obtained. As a result, low temperature toughness and brittle crack propagation stopping characteristics of the steel sheet also deteriorate.

For the above reason, the Ni content is set to 5.5% or more and 8.0% or less. , Preferably not less than 6.0% and not more than 7.5%.

In the production of the low-temperature steel sheet according to the present invention, when the production process is not carried out, which will be described later, the Ni content may be set to 5.5% or more and less than 7.0%.

The above are the basic components of the low temperature steel sheet of the present invention. The low temperature steel sheet of the present invention may further contain the following elements in addition to the above basic components.

Mo: 0.05% or more and 0.50% or less

Mo is an element effective for suppressing the susceptibility of the steel sheet to tempering embrittlement and is also an element for obtaining steel sheet strength without damaging the low temperature toughness. In order to obtain such an effect, the Mo content is preferably 0.05% or more. However, if the Mo content exceeds 0.50%, low temperature toughness is lowered. Therefore, when Mo is contained, the content thereof is preferably 0.05% or more and 0.50% or less. More preferably, it is more than 0.10% but not more than 0.30%.

Cr: not more than 1.00%

Cr is an element exhibiting the same effect as Mo. However, when the content exceeds 1.00%, the low temperature toughness of the steel sheet tends to decrease. Therefore, when Cr is contained, the content thereof is preferably 1.00% or less, and more preferably 0.01% or more and less than 0.20%.

At least one selected from the group consisting of Cu: less than 0.40%, Nb: less than 0.05%, V: less than 0.05%

Cu, Nb and V are all effective elements for increasing the strength of the steel sheet, and can be contained in the following ranges as required.

Cu: less than 0.40%

Cu is an effective element for increasing the steel sheet strength by improving the quenching property. However, when the content is 0.40% or more, the low-temperature toughness of the steel sheet is lowered, and the property of the surface of the steel (slab) after casting may deteriorate. Therefore, when Cu is contained, the content thereof is preferably less than 0.40%, more preferably from 0.10% to 0.30%.

Nb: not more than 0.05%, V: not more than 0.05%

Nb and V are all effective elements for increasing the steel sheet strength by precipitation strengthening. However, if the content of these elements is excessively high, the low-temperature toughness of the steel sheet is lowered. Therefore, when Nb is contained, the content thereof is preferably 0.05% or less, more preferably 0.01% or more and 0.03% or less. When V is contained, the content thereof is preferably 0.05% or less, more preferably 0.01% or more and 0.04% or less.

Ti: not more than 0.03%

Ti is an element having a function of increasing the toughness of a welded portion although it does not particularly affect the mechanical properties of the base metal when the steel sheet is welded to form a welded structure. Therefore, if necessary, Ti may be contained in a range of 0.03% or less.

0.007% or less of Ca, 0.010% or less of REM, and 0.070% or less of Mg

Ca, REM and Mg are elements having an effect of improving the low-temperature toughness of the steel sheet by controlling the form of inclusions in the steel. However, if the content of these elements is excessive, the cleanliness of the steel may be impaired. Therefore, when Ca is contained, the content thereof is preferably 0.007% or less, and more preferably 0.001% or more and 0.004% or less. When REM is contained, the content thereof is preferably 0.010% or less, and more preferably 0.002% or more and 0.008% or less. When Mg is contained, the content thereof is preferably 0.070% or less, and more preferably 0.001% or more and 0.004% or less.

Further, in the low temperature steel sheet of the present invention, the other components are Fe and inevitable impurities. Examples of the inevitable impurities include Sn, As, Pb and the like in addition to P and S described above. The content of Sn, As, and Pb is preferably 0.001% or less in total.

Next, reasons for limiting the structure of the low temperature steel sheet of the present invention will be described.

The steel sheet for low temperature of the present invention is a structure composed of tempering martensite in which the retained austenite is dispersed, and the volume percentage (volume percentage of the total austenite) of the retained austenite (residual austenite dispersed in tempering martensite) is 2.2 The average grain size of the old austenite grains is not less than 10 占 퐉 and not more than 60 占 퐉 and the average aspect ratio is not more than 4.0; the {110} planes are accumulated in the vicinity of the surface of the steel sheet; Have an integrated organization. The low-temperature steel sheet of the present invention preferably has a structure in which the retained amount of retained austenite after the subzero treatment is 1.7% or more and 11% or less by volume.

The low temperature steel sheet of the present invention has a structure in which the tempering martensite has a matrix structure and residual austenite is finely dispersed in the tempering martensite as a matrix. By making the retained austenite finely dispersed in the tempered martensite, the tempered martensite is excellent in strength and low temperature toughness, and the strength of the steel sheet can be improved without damaging the low temperature toughness and the like. Since the low-temperature toughness is lowered when soft ferrite or hard and coarse pearlite is contained, the base structure of the low-temperature steel sheet of the present invention is preferably composed of only tempering martensite. Further, since it is difficult to distinguish the tempering lower bainite from the tempering martensite, the tempering martensite in the present invention includes the tempering lower bainite. Tempering Lower bainite is also excellent in strength and low-temperature toughness as in tempering martensite, so that there is no problem in terms of properties.

Amount of retained austenite: 2.2% to 14% by volume

The retained austenite contributes to improvement of low temperature toughness and brittle crack propagation stopping property of the steel sheet. In the low-temperature steel sheet of the present invention, since the content of Ni having the effect of improving low-temperature toughness or the like is reduced from about 9% to 8.0% or less of the conventional steels, by making the structure including a predetermined amount of retained austenite, It is necessary to increase the toughness and brittle crack propagation stopping characteristics. In the low-temperature steel sheet of the present invention having a Ni content of 5.5% or more and 8.0% or less, it is necessary to set the amount of retained austenite to 2.2% or more in terms of volume ratio in order to obtain sufficient low temperature toughness and brittle crack propagation stopping property. However, if the amount of retained austenite in the steel sheet exceeds 14% by volume, excessively high tensile strength and yield strength are lowered, and low temperature toughness is impaired. Therefore, the volume percentage of retained austenite is set to 2.2% or more and 14% or less. , Preferably not less than 2.4% and not more than 12%. The amount of these retained austenite (2.2% or more and 14% or less, preferably 2.4% or more and 12% or less by volume) is the amount of retained austenite before the subzero treatment is performed on the steel sheet for low temperature. The amount of retained austenite (2.2% to 14% by volume, preferably 2.4% to 12%) is the amount of retained austenite at the plate thickness (1/4) t of the steel sheet.

Amount of retained austenite after sub-zero treatment: 1.7% to 11% by volume

The low temperature steel sheet used in the tank main body of the tank for storing LNG is required to have excellent low temperature toughness and brittle crack propagation stopping property even at the temperature (about -162 캜) at which LNG liquefies. Therefore, in the low-temperature steel sheet of the present invention, it is necessary that the retained austenite stably exists at a low temperature service temperature or a test temperature, and it is preferable that the retained austenite amount after stabilizing the subzero treatment is 1.7% Do. However, as described above, if the amount of retained austenite in the steel sheet excessively increases, the tensile strength is extremely increased and the yield strength is lowered, and the low temperature toughness is impaired. Therefore, the amount of retained austenite after the subzero treatment is preferably 1.7% or more and 11% or less by volume. More preferably, it is 2.2% or more and 9.5% or less. Here, the subzero treatment is to treat the material to be treated (low temperature steel sheet) in liquid nitrogen at -196 DEG C for 15 minutes or longer. The amount of retained austenite (1.7% or more and 11% or less by volume, more preferably 2.2% or more and 9.5% or less by volume) is the amount of retained austenite after sub- It is the amount of austenite.

Average particle diameter of old austenitic grains: 10 占 퐉 or more and 60 占 퐉 or less

Average aspect ratio of old austenitic grains: not more than 4.0

In order to improve the low-temperature toughness and brittle crack propagation stopping properties of the steel sheet, it is important to make the old austenitic grains finer and control their shape. In the present invention, in order to impart the desired low temperature toughness and brittle crack propagation stopping property to the steel sheet, the average grain size of the old austenitic grains is set to 60 탆 or less and the average aspect ratio of the old austenitic grains is set to 4.0 or less. Further, the old austenitic grains are preferably finer. The lower limit value of the average grain size of the old austenitic grains substantially obtained in the present invention is about 10 탆. Therefore, in the present invention, the average grain size of the old austenitic grains is set to 10 μm or more and 60 μm or less. And preferably 10 占 퐉 or more and 50 占 퐉 or less. On the other hand, the lower limit value of the average aspect ratio of the austenite grains substantially obtained in the present invention is about 1.3. Therefore, the average aspect ratio of the old austenitic grains is preferably 1.3 or more and 4.0 or less, more preferably 1.5 or more and 3.0 or less.

The average grain size and the average aspect ratio are the average grain size and the average aspect ratio in the cross section parallel to the rolling direction of the sheet thickness (1/4) t of the steel sheet.

As described above, by optimizing the amount of retained austenite in the steel sheet and the grain size and shape of the old austenite grains, a low-temperature steel sheet having high strength and excellent low temperature toughness and brittle crack propagation stopping characteristics can be obtained. However, in the case of a low-temperature steel sheet in which the content of Ni is reduced to 8.0% or less, particularly in the case of a low-temperature steel sheet in which the Ni content is reduced to about 7% or less than 7%, the amount of retained austenite and the grain size and shape of the old austenite grains are optimized The brittle crack propagation stopping property may still be insufficient as the material for the tank main body of the tank for storing LNG. Therefore, in order to further improve the brittle crack propagation stopping property, the present invention is a structure in which predetermined crystal faces are integrated at the surface layer and the plate thickness (1/2) t position of the steel sheet.

The {110} planarity parallel to the plate surface in the surface layer of the steel sheet: 1.3 or more

The {100} plane parallel to the plate surface in the surface layer of the steel sheet: 0.90 or less

The low temperature steel sheet of the present invention has a {110} surface density of 1.3 or more parallel to the sheet surface at a position of 1 mm from the surface layer of the steel sheet, specifically, from the surface of the steel sheet in the sheet thickness direction. Further, the {110} plane integration degree is preferably 1.5 or more and 2.5 or less. Further, the degree of integration of {100} planes parallel to the plate surface is set to 0.90 or less at a position of 1 mm in the plate thickness direction from the surface of the steel sheet. Further, the degree of integration of the {100} plane is preferably 0.60 or more and 0.90 or less. In the case where the {110} plane integration degree is less than 1.3 or the {100} plane integration degree exceeds 0.90, the effect of improving the brittle crack propagation stopping performance can not be expected.

Plate thickness of steel plate (1/2) Degree of integration of {111} plane parallel to plate surface at t position: 1.2 or more and 2.5 or less

At the sheet thickness (1/2) t position of the steel sheet, the degree of integration of {111} planes parallel to the sheet surface is set to 1.2 or more and 2.5 or less. And preferably 1.4 or more and 2.2 or less. When the {111} plane integration degree is less than 1.2, an effect of improving the brittle crack propagation stopping performance can not be expected. On the other hand, if the degree of {111} plane integration exceeds 2.5, absorption energy (low-temperature toughness) may be lowered due to development of separation.

In the present invention, the {hkl} surface integration degree refers to the relative intensity ratio of the diffracted X-ray intensity I from the {hkl} plane of the subject to the diffracted X-ray intensity I ₀ from the {hkl} I / I ₀ .

As described above, even when the Ni content is reduced to 8.0% or less by optimizing the composition and the structure of the steel sheet, it is possible to provide a low temperature tantalum having sufficient strength, low temperature toughness and brittle crack propagation stopping property as the material for the tank main body of the LNG storage tank A steel sheet is obtained. Further, the present invention is a low-temperature steel sheet having a tensile strength of 700 MPa or higher.

Tensile strength: 700MPa or more

Since the low temperature steel sheet of the present invention is mainly used for low temperature tanks such as LNG storage tanks, strength as a structural steel sheet is required. When the strength of the low-temperature steel sheet as the material of the tank main body is not sufficient, the plate thickness of the material steel sheet (low-temperature steel sheet) must be increased in order to secure the strength of the tank. As a result, . Therefore, the tensile strength of the steel sheet is 700 MPa or more. And preferably 710 MPa to 810 MPa.

The thickness of the low-temperature steel sheet of the present invention is not particularly limited, but is preferably 6 mm or more and 50 mm or less.

Next, a manufacturing method of the low temperature steel sheet of the present invention will be described.

In the present invention, the steel material (steel slab) having the above composition is heated, followed by hot rolling (controlled rolling), followed by direct quenching, followed by tempering by heating to a predetermined temperature, An intermediate heat treatment (secondary quenching) in which the steel sheet is cooled to a temperature lower than the Ac ₁ transformation point and lower than the Ac ₃ transformation point is carried out, and then the steel sheet is tempered by heating to a predetermined temperature.

In addition, when the intermediate heat treatment (secondary quenching) is not performed, a steel sheet for low temperature steel having higher strength than that in the case of performing the intermediate heat treatment can be produced. Therefore, in the case of producing a low-temperature steel sheet having a low strength and having a relatively low requirement for low-temperature toughness, the Ni content of the steel material is reduced to less than 7.0% and the intermediate heat treatment (secondary quenching) It is possible to employ a manufacturing method that does not perform the manufacturing process. On the other hand, in the case of producing a low-temperature steel sheet particularly with a low temperature toughness and a brittle crack propagation stopping property, the intermediate heat treatment (secondary quenching) may be employed.

In the present invention, the method of the solvent of the steel is not particularly limited, and a known solvent method such as a converter, an electric furnace or the like can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, from the viewpoint of productivity and quality, it is preferable to make the slab (steel material) by the continuous casting method. In addition, a slab may be formed by a known casting method such as a roughing-crushing rolling method.

The heating conditions, rolling conditions, direct quenching conditions, intermediate heat treatment (secondary quenching) conditions and tempering conditions of the steel material (slab) are as follows.

Heating temperature of steel material: 1000 ℃ or more and 1200 ℃ or less

When the heating temperature of the steel material is less than 1000 캜, the coarse AlN precipitated at the casting stage of the cast steel is not solid solution, and the low temperature toughness of the steel sheet is lowered. Further, the added elements in the steel material are not diffused sufficiently uniformly, and the low temperature toughness of the steel sheet is lowered. In addition, it becomes difficult to satisfy the rolling conditions to be described later, particularly the rolling conditions at the recrystallization temperature range (950 DEG C or lower and 840 DEG C or higher). On the other hand, when the heating temperature of the steel material exceeds 1200 ° C, the austenite grains coarsen and the low-temperature toughness of the steel sheet lowers and is also a waste.

For the above reason, the heating temperature of the steel material is specified to be 1000 ° C or more and 1200 ° C or less. And preferably 1000 ° C or more and 1100 ° C or less.

After the steel material is heated to 1000 ° C or more and 1200 ° C or less, hot rolling is performed under the following conditions. In the present invention, the fineness of the austenite lips and the appropriate flattening are achieved by rolling the steel material, and the martensite structure obtained by direct quenching after rolling is made finer. By tempering the steel sheet having the martensite structure thus micronized, a low-temperature steel sheet having a desired average grain size (10 μm or more and 60 μm or less) and austenite grains having an average aspect ratio (4.0 or less) can be obtained. Further, in the present invention, the desired crystal faces are developed in each of the steel sheet surface layer and the steel sheet center portion by rolling the steel material.

Strict control of the rolling conditions is necessary in order to make the austenite grains finer and adequately flatten, and to develop the desired crystal plane.

Cumulative rolling reduction at a temperature range of 950 DEG C or lower and 840 DEG C or higher: 30% or higher

In order to make the old austenite grains of the low-temperature steel sheet finer (average grain size: 10 탆 or more and 60 탆 or less), it is important to apply a suitable pressure reduction on the low temperature side in the recrystallization temperature range. By making the cumulative reduction ratio at 950 占 폚 or lower and 840 占 폚 at a temperature in the range of 30% or higher, recrystallized grains that are fine and equiaxed (equiaxial) are obtained and combined with rolling at the non- The martensite structure after direct quenching becomes finer. As a result, the tempered martensite structure after tempering is refined to obtain a desired austenite grain having an average grain size (10 탆 or more and 60 탆 or less). However, from the viewpoint of securing a sufficient reduction rate at the non-recrystallized temperature region, it is preferable that the cumulative reduction rate at 950 占 폚 or below and 840 占 폚 is 75% or less.

The above temperature range (950 占 폚 or lower and higher than 840 占 폚) is a temperature range measured at the surface position of the steel sheet.

Cumulative rolling reduction at a temperature range of 840 DEG C or lower: 30% or more and 75% or less

The purpose of the rolling at the non-recrystallized temperature is to finer the austenite grains finerized by rolling at the recrystallization temperature and introducing many dislocations. When quenching is performed directly after forming a fine austenitic grain having a dislocation density as high as described above, martensite transformation is performed from the finely processed austenite to obtain a martensite having a fine grain of the effective grain size. As a result, it can be considered that high strength and high toughness of the steel sheet are achieved. Further, by controlling the cumulative reduction ratio at the non-recrystallized temperature region, {110} planes are accumulated near the surface of the steel sheet, and {111} planes are accumulated at the center of the steel plate.

In order to control the aspect ratio of the old austenitic grains to a predetermined value (4.0 or less) while exhibiting the same effect as described above, the cumulative reduction ratio at the temperature range of 840 DEG C or less, that is, the non- 30% or more and 75% or less. , Preferably not less than 40% and not more than 75%. The temperature range (840 DEG C or lower) is set to the temperature range measured at the surface position of the steel sheet.

Rolling end temperature when intermediate heat treatment is not performed: 820 DEG C or lower 700 DEG C or higher

Rolling end temperature when intermediate heat treatment is performed: 850 DEG C or lower 730 DEG C or higher

Another important condition for the rolling conditions is the rolling finish temperature. When the reheating (intermediate heat treatment) is not performed at 650 ° C or more and less than the Ac ₃ transformation point in the post-process, the rolling finish temperature is set to 820 ° C or lower and 700 ° C or higher. On the other hand, in the case of reheating (intermediate heat treatment) at 650 ° C or higher and lower than the Ac ₃ transformation point in the post-process, the rolling finish temperature is 850 ° C or lower and 730 ° C or higher.

When the intermediate heat treatment is not applied, the strength of the steel sheet is stabilized. However, when the intermediate heat treatment is not applied, residual austenite is produced in the subsequent tempering process. However, when the rolling end temperature exceeds 820 DEG C, sufficient amount of retained austenite is not produced during the tempering process. Toughness and brittle crack propagation stopping characteristics are degraded. When the rolling finish temperature is less than 700 占 폚, the crystal grains elongate and the aspect ratio exceeds 4.0, so that absorption energy (low-temperature toughness) is lowered due to occurrence of separation. Therefore, when the intermediate heat treatment is not applied, the rolling finish temperature is set to 820 캜 or lower and 700 캜 or higher. Preferably 800 ° C or lower and 720 ° C or higher. These rolling finish temperatures are all the temperatures measured on the surface of the steel sheet.

On the other hand, when the intermediate heat treatment is applied, relatively retained austenite tends to be obtained easily, and the low temperature toughness and brittle crack propagation stopping property of the low temperature steel sheet are stabilized, but the steel sheet strength tends to decrease. Therefore, in the case of applying the intermediate heat treatment, it is necessary to set the rolling finishing temperature to a somewhat higher value to obtain a desired steel sheet strength, and to set the rolling finishing temperature to 850 캜 or lower and 730 캜 or higher. Preferably 830 DEG C or lower and 740 DEG C or higher. These rolling finish temperatures are all the temperatures measured on the surface of the steel sheet.

After completion of the rolling, forced cooling such as water cooling is performed under the following conditions to directly perform quenching. The forced cooling is started within 300 seconds after completion of rolling.

550 ° C or less Average cooling rate in a temperature range of 300 ° C or more: 1 ° C / s or more

Cooling end temperature: 300 ℃ or less

When the average cooling rate at a temperature range of 550 DEG C or lower and 300 DEG C or higher after rolling is less than 1 DEG C / s after rolling is completed, the number of high-temperature transformation textures having low toughness including crude cementite is increased, do. Therefore, the average cooling rate at least in the temperature range of 550 DEG C or lower and 300 DEG C or higher is set to 1 DEG C / s or higher. Preferably 3 DEG C / s or more. The upper limit of the average cooling rate is not particularly limited, but it is preferable that the upper limit of the average cooling rate is 100 占 폚 / s or less, which is a practically achievable cooling rate. When the forced cooling is terminated at a temperature exceeding 300 캜, the martensitic transformation is not completed and a uniform martensite structure is not obtained, so that the strength of the steel sheet and the low temperature toughness are lowered. Therefore, forced cooling is performed until the center temperature of the steel sheet becomes 300 DEG C or less. Preferably 250 DEG C or less. By performing the cooling as described above, the steel sheet structure becomes a uniform martensite structure. In addition, there may be a structure including a lower bainite in addition to martensite.

In addition, the above-mentioned temperature (cooling termination temperature) is the temperature at the plate thickness (1/2) t of the steel sheet. Further, the average cooling rate is obtained based on the temperature at the plate thickness (1/2) t of the steel sheet. The temperature at the sheet thickness (1/2) t position of the steel sheet is obtained by calculation such as simulation calculation from the sheet thickness, the surface temperature, and the cooling condition. For example, the temperature at the sheet thickness (1/2) t position is obtained by calculating the temperature distribution in the thickness direction of the steel sheet using the difference method.

The steel sheet is forcedly cooled to a temperature of 300 deg. C or less at the center temperature of the steel sheet, and then tempered. Alternatively, the steel sheet is forcedly cooled to a steel sheet center temperature of 300 ° C or lower, further subjected to an intermediate heat treatment, and then tempered. The intermediate heat treatment and tempering are performed under the following conditions.

Heating temperature for intermediate heat treatment: 650 ° C or more and less than Ac ₃ transformation point

After the rolling is completed, the steel sheet cooled to 300 캜 or less is heated to (γ + α) 2 inferior to the Ac ₃ transformation point and quenched, whereby the structure becomes finer and alloying elements are distributed. For this reason, martensite (which may include lower bainite) and a small amount of retained austenite are formed with tempering martensite (which may include tempering lower bainite) and concentrated alloying elements. When this mixed structure is tempered near the Ac ₁ transformation point, the stable austenite in which the alloying elements are concentrated further precipitates, and impurities which are harmful to the toughness such as C and N in the tempering martensite migrate to austenite. As a result, a mixed structure of tempered martensite (which may include tempering lower bainite) which is fine and extremely high in toughness and retained austenite which is highly stable even at a very low temperature is obtained, and the low temperature toughness Remarkably improves.

In the case of the steel material of the present invention, the Ac ₁ transformation point is about 570 to 615 ° C. Therefore, in order to exhibit the above effect, the heating temperature for the intermediate heat treatment is set to be lower than the Ac ₃ transformation point of 650 ° C or higher. And preferably not lower than 670 캜 (Ac ₃ transformation point-15 캜). The heating temperature is a temperature at a plate thickness (1/2) t of the steel sheet, which is calculated by calculation. The entire steel plate is heated to the above-mentioned temperature range (650 ° C or more and less than the Ac ₃ transformation point), and then forced cooling is carried out under the following conditions.

550 ° C or less Average cooling rate in a temperature range of 300 ° C or more: 3 ° C / s or more

Cooling end temperature: 200 ° C or less

When the entire steel sheet is heated to the above-mentioned temperature range (less than 650 ° C and less than the Ac ₃ transformation point) and the average cooling rate is less than 3 ° C / s at 550 ° C or lower and 300 ° C or more, a part of the austenite is not transformed into martensite And the strength of the steel sheet is lowered. Therefore, the average cooling rate at least in the temperature range of not higher than 550 캜 and not lower than 300 캜 is not lower than 3 캜 / s. Preferably 5 [deg.] C / s or more. The upper limit of the average cooling rate is not particularly limited, but it is preferable that the upper limit of the average cooling rate is 100 占 폚 / s or less, which is a practically achievable cooling rate. When the forced cooling is terminated at a temperature exceeding 200 캜, the transformation of the retained austenite into martensite or bainite progresses, and the amount of retained austenite decreases. Therefore, in the intermediate heat treatment, forced cooling is performed until the steel sheet becomes 200 DEG C or lower. The above-mentioned temperature (cooling termination temperature) is a temperature at a plate thickness (1/2) t position of the steel sheet, which is calculated by calculation. Further, the average cooling rate is obtained based on the temperature at the plate thickness (1/2) t of the steel sheet.

Tempering temperature when no intermediate heat treatment is performed: 550 deg. C or higher and 650 deg. C or lower

Tempering temperature in the case of performing the intermediate heat treatment: 500 deg. C or higher and 650 deg. C or lower

In the present invention, the tempering temperature is determined depending on the presence or absence of the intermediate heat treatment, and the steel sheet after tempering is a steel sheet having a desired retained austenite amount.

When the intermediate heat treatment is not performed, the steel sheet structure before tempering is a uniform martensite structure. In addition to martensite, lower bainite may be contained. Therefore, when the intermediate heat treatment is not performed, it is necessary to generate the retained austenite in the steel sheet by tempering, and therefore, the tempering temperature needs to be 550 캜 or higher. However, if the tempering temperature exceeds 650 ° C, the steel sheet strength is lowered. For the reasons described above, the tempering temperature in the case where the intermediate heat treatment is not carried out is set to 550 deg. C or higher and 650 deg. C or lower. And preferably 570 DEG C or more and 630 DEG C or less.

The above tempering temperature is a temperature at a plate thickness (1/2) t of the steel sheet, and is a temperature obtained by calculation.

On the other hand, in the case of performing the intermediate heat treatment, the steel sheet structure before tempering is a mixed structure of tempering martensite, concentrated martensite of alloying elements and a small amount of retained austenite. Further, the mixed structure may include a tempering lower bainite or a lower bainite. Therefore, in the case of performing the intermediate heat treatment, the main purpose of the tempering is to obtain a proper steel sheet strength and low temperature toughness by tempering the quartz martensite or the lower bainite. Taking this purpose into consideration, the tempering temperature in the case of performing the intermediate heat treatment is set to 500 deg. C or higher at which the effect of tempering can be obtained. However, if the tempering temperature exceeds 650 ° C, the strength of the steel sheet is lowered. And preferably 520 ° C or higher and 620 ° C or lower.

The above tempering temperature is a temperature at a plate thickness (1/2) t of the steel sheet, and is a temperature obtained by calculation.

As a result of tempering under the above conditions, a structure composed of tempered martensite having a structure in which retained austenite at a volume ratio of 2.2% or more and 14% or less (or 1.7% or more and 11% or less after the subzero treatment) is dispersed in the tempering martensite A low-temperature steel sheet is obtained. In addition, the tempering martensite may include a tempering lower bainite.

The steel sheet after tempering may be cooled, but it may be cooled at a cooling rate faster than the cooling rate (for example, forced air cooling, water cooling, or the like).

Example

The slabs (steel materials) having a chemical composition shown in Table 1 having a thickness of 125 to 250 mm were heated and hot-rolled, then forcedly cooled to within 150 seconds after completion of rolling, and then tempered to obtain a steel plate having a thickness of 8 to 50 mm . For some slabs (steel material), an intermediate heat treatment was provided between forced cooling after hot rolling and tempering. Table 2 shows the heating temperature of the slab (steel material), the hot rolling condition, the forced cooling condition after hot rolling, the conditions of the intermediate heat treatment, the conditions of tempering and the plate thickness of the steel sheet.

The Ac ₃ transformation point shown in Table 1 was obtained from a thermal expansion curve obtained by heating a sample for thermal expansion measurement taken from each steel sheet at room temperature to 850 캜 at 5 캜 / min.

The temperature (950 DEG C or lower and 840 DEG C or higher and 840 DEG C or lower) specified in the " cumulative reduction ratio " shown in Table 2 is a value of the temperature measured at the surface position of the steel sheet. The "rolling finish temperature" shown in Table 2 is the value of the temperature measured at the surface position of the steel sheet. Average cooling rate " and " cooling end temperature " of " cooling after hot rolling " described in Table 2 are the average cooling rate and cooling termination temperature (both calculated values) to be. The " heating temperature " in " intermediate heat treatment " in Table 2 is the temperature (calculated value) at the plate thickness (1/2) t position of the steel sheet. The "average cooling rate" and the "cooling end temperature" in the "intermediate heat treatment" in Table 2 are the average cooling rate and the cooling termination temperature (both calculated values) at the plate thickness (1/2) t of the steel sheet. The " tempering temperature " in Table 2 is the temperature (calculated value) at the plate thickness (1/2) t of the steel sheet.

The steel sheet thus obtained was observed for texture and the steel sheet structure was identified to obtain the average grain size and average aspect ratio of the old austenitic grains. In addition, by the X-ray diffraction method, the volume percentage of retained austenite, the {110} planarization degree and the {100} planarity degree parallel to the plate surface of the steel sheet surface, The directivity of the {111} plane parallel to the plane of the plate was obtained.

The volume ratio of the retained austenite in the steel sheet subjected to the subzero treatment was obtained by the X-ray diffraction method.

The steel sheet thus obtained was subjected to a tensile test, a Charpy impact test (test temperature: -196 ° C) and a double tensile test with a surface cut-out (test temperature: -196 ° C) Brittle crack propagation stopping characteristics were evaluated.

Tissue observation, X-ray diffraction and various tests were carried out as follows.

(1) Tissue observation

Identification of steel sheet structure

A test piece for tissue observation was taken from the steel sheet thus obtained and subjected to nital erosion by polishing a cross section (L section) parallel to the rolling direction. At a plate thickness (1/4) t position, (Magnification: 400 times), and the photographs were taken at least five o'clock or more. It was confirmed that, in any of the steel sheets obtained by the above, the steel sheet had a tempered martensite structure free of pro-eutectoid ferrite and having a lath-shaped structure. Further, the tempering martensite may be a structure including a part of the tempering lower bainite. This is because the tempering martensite and the tempering lower bainite both have a lath-like structure and have a structure in which fine carbides or retained austenite are dispersed in the structure, so that it is difficult to distinguish between the tempering martensite and the tempering lower bainite .

The average particle diameter and average aspect ratio of the old austenitic grains

The tempered martensite structure was corroded in accordance with JIS G 0551 (2005) with respect to the cross section parallel to the rolling direction of the plate thickness (1/4) t position of the steel sheet obtained by the above, and an optical microscope (magnification: 400 Times) with respect to the austenite grains, and the circle-equivalent diameter and the aspect ratio of the old austenitic grains were measured, and the average values (average grain size, average aspect ratio) were obtained.

(2) X-ray diffraction

Volume ratio of retained austenite

The test piece for X-ray diffraction was taken from the steel sheet obtained in the above manner in parallel with the surface of the steel sheet, and subjected to grinding and polishing (chemical polishing), and the surface of the test piece after polishing was regarded as the plate thickness (1/4). Thereafter, the diffraction intensity of (200), (211) plane of?, (200), (220) and (311) plane of? Were obtained by X-ray diffractometry using the test piece, and the volume ratio of? did.

Volume ratio of retained austenite after sub-zero treatment

X-ray diffraction test pieces were collected from the steel sheet obtained in the above manner in parallel with the plate surface, and the test pieces were subjected to sub-zero treatment. The subzero treatment was carried out under the condition that the test piece was held in liquid nitrogen at -196 캜 for 15 minutes or longer. After the sub-zero treatment, the test piece was ground and polished (chemical polishing), and the surface of the test piece after polishing was regarded as the plate thickness (1/4) t position of the steel plate. Thereafter, the volume percentage of the retained austenite was determined by the above-mentioned method using the test piece.

Density of crystal plane

A surface parallel to the plate surface was cut out from the steel sheet obtained by the above, and the machined structure was removed so that the surface of the test piece after the mechanical polishing and etching was positioned 1 mm below the surface of the steel sheet to prepare a test piece. Thereafter, the degree of integration of {110} planes parallel to the surface of the steel sheet and the degree of {100} surface parallel to the surface of the steel sheet were measured by the inversion method using a test piece.

Further, a surface parallel to the plate surface was cut out from the steel sheet obtained by the above, and the machined structure was removed so that the surface of the test piece after the mechanical polishing and etching became the plate thickness (1/2) t position of the steel sheet, thereby producing a test piece. Thereafter, the degree of integration of {111} planes parallel to the surface of the steel sheet was measured by the inverse method using a test piece.

(3) Tensile test

A tensile test piece having a parallel portion diameter of 14φ was taken from the plate thickness (1/2) t position of the steel sheet obtained above in a direction (C direction) perpendicular to the rolling direction, and the tensile test piece according to JIS Z 2241 (2011) A room temperature tensile test was conducted. However, for a steel sheet having a plate thickness of less than 25 mm (steel sheet No. 7 in Table 2), a tensile test piece having a flat thickness of a full thickness was taken and subjected to a room temperature tensile test in accordance with JIS Z 2241 (2011) .

(4) Charpy impact test (evaluation of low temperature toughness)

A V-notch Charpy test piece having a longitudinal direction perpendicular to the rolling direction (direction C) was taken from the plate thickness (1/2) t position of the steel sheet obtained as described above, and according to JIS Z 2242 (2005) Charpy impact test at 196 ° C was carried out. For a steel plate having a thickness of less than 10 mm (steel sheet No. 7 in Table 2), a half size Charpy impact test piece was taken and measured according to JIS Z 2242 (2005) Charpy impact test was carried out. In the Charpy impact test, each steel sheet was subjected to three tests to measure the absorbed energy, and the average value thereof was determined.

(5) Double tensile test with surface cut-out (evaluation of brittle crack propagation stopping characteristics)

From the steel sheet thus obtained, a total thickness test piece having a notch on the surface (both surfaces) as shown in Fig. 1 was sampled so that the crack propagation direction was perpendicular to the rolling direction (direction C), and in Non-Patent Document 1 A double tensile test was conducted with a surface cut at a test temperature of -196 캜 and a load stress of 375 MPa. In the double tensile test with surface cutout, the absorbed energy measured by the Charpy impact test of the above (4) is higher than the steel sheet having reached the target value of the present invention described later and a part of the steel sheet 1 to 7, 12, 14, 15, 18, 19, 25 to 43, and 51).

(YE): 600 MPa or more, tensile strength (TS): 700 MPa or more, and Charpy absorbed energy ( _-196 ) at -196 ° C. is 150 J or more when the intermediate heat treatment is performed And a surface stress of 375 MPa at -196 deg. C), and a surface roughness of not less than 100 J (not less than 50 J in the case of a half-size test piece) The brittle crack propagation stopping property in which the brittle cracks stop in the double tensile test satisfies the target characteristics of the present invention. Table 3 shows the above results.

In Table 3, Steel Nos. 1 to 7 and 26 to 43 were produced under the chemical composition and production conditions of the present invention, and the strength, Charpy absorption energy (low temperature toughness) at -196 캜, and brittle crack propagation stopping property We are meeting our goals.

On the other hand, Steel sheets Nos. 8 and 9 have a low Charpy absorption energy (low temperature toughness) at -196 DEG C because the heating temperature of the slab (steel material) is higher than the range of the present invention.

Since the rolling conditions are out of the range of the present invention, the steel plates Nos. 10 to 19 do not obtain the target Charpy absorbed energy (low temperature toughness) at -196 캜 or fail to achieve the target brittle crack propagation stopping property.

Steel plates Nos. 20 and 21 have a tempering temperature higher than the range of the present invention, so that the target strength can not be obtained.

Since the tempering temperature is lower than the range of the present invention, the desired residual austenite amount can not be obtained and the target Charpy absorbed energy (low temperature toughness) at -196 캜 is not obtained for Steel Nos. 22 and 23.

Since the intermediate heat treatment temperature is out of the range of the present invention, the desired residual austenite amount can not be obtained and the Charpy absorbed energy (low temperature toughness) at -196 캜 is low for the steel sheets Nos. 24 and 25.

The steel plates Nos. 44 to 51 do not have the desired characteristics because the chemical components are out of the range of the present invention.

Claims (12)

0.003% or less of S, 0.003% or less of S, 0.01% or more and 0.10% or less of Al, 0.10% or less of C, 0.02% or more and 0.30% or less of Si, , N: not less than 0.0015% and not more than 0.0045%, Ni: not less than 5.5% and not more than 8.0%, the balance being Fe and inevitable impurities,
(1/4) t, the amount of retained austenite is 2.2% or more and 14% or less by volume, and the plate thickness (1/4) t The average aspect ratio of the old austenite grains having a cross section parallel to the rolling direction is 10 탆 or more and 60 탆 or less and the average aspect ratio of the old austenite grains is 4.0 or less, The degree of integration of {100} planes parallel to the surface of the steel sheet is not more than 0.90 and the degree of {110} planarity parallel to the surface of the steel plate is not less than 1.3, And has a parallel {111} plane integrated density of 1.2 to 2.5,
Characterized in that it has a brittle crack propagation stopping property in which a brittle crack is stopped in a double tensile test in which the tensile strength is 700 MPa or more and the surface is cut at a load stress of 375 MPa at a test temperature: . The method according to claim 1,
Wherein the volume percentage of retained austenite after the subzero treatment is 1.7% or more and 11% or less at a plate thickness (1/4) t position. The method according to claim 1,
Further comprising, in mass%, at least one of the following groups A to E in addition to the above composition.
Group A: Mo: 0.05% or more and 0.50% or less
Group B: Cr: not more than 1.00%
Group C: at least one kind selected from the group consisting of Cu: less than 0.40%, Nb: not more than 0.05%, and V: not more than 0.05%
Group D: Ti: not more than 0.03%
Group E: 0.007% or less Ca, 0.010% or less REM, and 0.070% or less Mg 3. The method of claim 2,
Further comprising, in mass%, at least one of the following groups A to E in addition to the above composition.
Group A: Mo: 0.05% or more and 0.50% or less
Group B: Cr: not more than 1.00%
Group C: at least one kind selected from the group consisting of Cu: less than 0.40%, Nb: not more than 0.05%, and V: not more than 0.05%
Group D: Ti: not more than 0.03%
Group E: 0.007% or less Ca, 0.010% or less REM, and 0.070% or less Mg The method of claim 3,
The low temperature steel sheet according to the above group A, wherein the Mo content is more than 0.10% but not more than 0.30% by mass%. 5. The method of claim 4,
The low temperature steel sheet according to the above group A, wherein the Mo content is more than 0.10% but not more than 0.30% by mass%. The method of claim 3,
The steel sheet for low temperature as set forth in the above group B, wherein Cr is contained in an amount of less than 0.20% by mass. 5. The method of claim 4,
The steel sheet for low temperature as set forth in the above group B, wherein Cr is contained in an amount of less than 0.20% by mass. 6. The method of claim 5,
The steel sheet for low temperature as set forth in the above group B, wherein Cr is contained in an amount of less than 0.20% by mass. The method according to claim 6,
The steel sheet for low temperature as set forth in the above group B, wherein Cr is contained in an amount of less than 0.20% by mass. A method for producing a low temperature steel sheet according to any one of claims 1 to 10, wherein the steel material having the above composition is heated to a temperature of 1000 캜 to 1200 캜 and a temperature of 950 캜 to 840 캜 Wherein the cumulative rolling reduction at a temperature range of 840 占 폚 or lower is 30% or more and 75% or less and the rolling finish temperature is 820 占 폚 or lower and 700 占 폚 or higher; , The average cooling rate in the temperature range of at least 550 캜 and not less than 300 캜 is set to 1 캜 / s or more and the cooling end temperature is set to 300 Deg.] C or lower, and then tempering the steel sheet at a temperature in the range of 550 DEG C to 650 DEG C. A method for producing a low temperature steel sheet according to any one of claims 1 to 10, wherein the steel material having the above composition is heated to a temperature of 1000 캜 to 1200 캜 and a temperature of 950 캜 to 840 캜 The cumulative rolling reduction at a temperature range of 840 占 폚 or lower is 30% or more and 75% or less and the rolling finish temperature is 850 占 폚 or lower and 730 占 폚 or higher, , The average cooling rate in the temperature range of at least 550 캜 and not less than 300 캜 is set to 1 캜 / s or more and the cooling end temperature is set to 300 subjected to cooling to a ℃ below, was then heated to a temperature range of less than 650 ℃ Ac ₃ transformation point, the temperature of the plate thickness (1/2) t position of the steel sheet, at least 550 or more than 300 ℃ temperature ℃ The average cooling rate in the station is set at 3 DEG C / s or higher , Subjected to intermediate heat treatment which performs cooling to the cooling termination temperature is below 200 ℃, method for producing Then, the low-temperature steel sheet characterized in that the tempering in the temperature range of less than 500 ℃ 650 ℃.

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