JP2005232513A - High-strength steel sheet and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet having ≥750 MPa tensile strength and having excellent low-temperature toughness and also excellent low-temperature toughness of weld zone even in a very low temperature environment of -46°C and also to provide its manufacturing method. <P>SOLUTION: The high strength steel sheet is composed of steel having a composition which consists of 0.01 to 0.10% C, ≤0.30% Si, 1.20 to 2.50% Mn, ≤0.010% P, ≤0.002% S, 0.2 to 1.5% Ni, 0.1 to 0.8% Mo, 0.005 to 0.06% Nb, 0.004 to 0.025% Ti, ≤0.05% sol.Al, ≤0.0050% N, ≤0.003% O(oxygen) and the balance Fe with impurities or further contains prescribed amounts of one or more elements among Cr, Cu, V, B, Ca, Mg, Zr and REM and in which the value of A represented by equation A=12P+45S+67(N+O) (where the symbol of an element means the content of the element contained in the steel) is made to ≤0.5%. The steel sheet has ≥0.18° half-width of X-ray diffraction intensity from the (110) plane and ≥750 MPa tensile strength or further has a prescribed metallic structure. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-strength steel sheet excellent in cryogenic toughness and having a tensile strength of 750 MPa or more, and suitable for use in line pipes and various pressure vessels for transporting natural gas and crude oil, and a method for producing the same.

  In pipelines for transporting natural gas and crude oil over long distances, there is an increasing need to increase the strength of the pipe material itself and limit the increase in wall thickness, with the aim of reducing laying costs and transportation costs. At present, in the American Petroleum Institute (API), X80 grade (tensile strength 620 MPa or more) steel is standardized and put into practical use, and has higher strength X100 (tensile strength 750 MPa or more) and more than X100 (for example, The application of high-strength grade steel having a tensile strength of 900 MPa or more is also being studied.

  On the other hand, in a line pipe, it is important to combine both strength characteristics and base metal low temperature toughness among the required characteristics to be provided as a structural material. In particular, when laying in an extremely cold region, the low temperature toughness of the base material is extremely important. If this low temperature toughness is not sufficient, the pipeline may be destroyed. However, in general, increasing the strength characteristics deteriorates the toughness, and conversely, increasing the toughness results in a lack of strength. It is necessary to have.

  In order to meet the demand for such high strength and excellent low-temperature toughness, for example, Patent Document 1 and Patent Document 2 include a high-strength steel excellent in low-temperature toughness of X100 super grade with a high Mn content and a high Strength line pipes and their manufacturing methods have been proposed.

  Patent Document 3 discloses a high-strength steel material having a tensile strength excellent in strain aging resistance of 600 MPa or more and a desired low-temperature toughness and a manufacturing method thereof, and Patent Document 4 further describes an X100 grade. With the recognition that the above-mentioned high-strength line pipes need to be evaluated by a destructive test using a test piece with a full thickness of steel pipe in particular, there is a problem based on the evaluation of the fracture characteristics in actual pipes by the DWTT test specified by the API. A high-strength steel excellent in stable fracture resistance characteristics is disclosed.

  However, in the techniques disclosed in Patent Documents 1 to 3, the low temperature toughness is evaluated only by the Charpy impact test. Also, the DWTT test described in Patent Document 4 is only conducted at −30 ° C., and the toughness in a cryogenic environment such as −40 ° C. to −50 ° C. expected in a very cold region is examined. It has not been.

Japanese Patent Application Laid-Open No. 8-199292

JP 2000-199036 A JP 2002-220634 A JP 2003-3233 A

  The present invention has been made in view of the above circumstances, and its purpose is to have a tensile strength of 750 MPa or more, excellent low temperature toughness even in a cryogenic environment of −46 ° C. (−50 F), and An object of the present invention is to provide a high-strength steel sheet having a weld joint with an absorbed energy of 80 J or more under the same environment and excellent in low-temperature toughness at the weld and a method for producing the same.

  Specifically, it aims at providing the high strength steel plate provided with all the following performances, and its manufacturing method.

Base material strength: Tensile strength (TS) ≧ 750 MPa
Base material toughness: Shock absorption energy ( _v E _{-46 ° C} ) ≥200J
Base material toughness: Ductile surface fracture ratio (SA- _{46 ° C} ) ≧ 75% by DWTT test
Welded joint toughness: shock absorption energy ( _v E _{-46 ° C} ) ≧ 80J

  As a result of repeated experiments to achieve the above-mentioned problems, the present inventor has found the following.

  In general, in order to achieve both high strength and high toughness, it is well known that the structure of a steel material is bainite, martensite or a mixed structure thereof. The microstructure of its own structure is not sufficient. However, by miniaturizing the developed dislocation substructures formed during hot working and passing them on to the bainite and martensite structures, the ability to suppress fracture generation at low temperatures and stop fracture propagation (these Here, it has been found that the performance of "breakage suppression characteristics" and "destructive propagation stop characteristics" are particularly improved.

  Furthermore, it is extremely effective to limit the total amount of P, S, N and O (oxygen) in the base material for the refinement of the dislocation substructure, and appropriate hot rolling and post-rolling treatment (cooling conditions) In order to achieve both high strength and low temperature toughness, it is possible to obtain bainite, martensite, or a mixed structure thereof having a finely developed dislocation substructure by a combination of It was found that this is made possible by controlling the half-value width of the X-ray diffraction intensity from the (110) plane in a steel having a composition. Moreover, it was confirmed that the restriction on the total amount of P, S, N and O is also effective for miniaturization of the weld heat affected zone of the high strength steel.

  The “dislocation substructure” as used herein refers to a structure formed by dislocations introduced into the structure by hot working or the like, and the “finely developed dislocation substructure” refers to a fine subgrain or cellular structure. , Refers to a structure in which the angle between adjacent subgrains is 0.5 degrees or more. That is, a large amount of dislocations are introduced into austenite by hot working, but the introduced large amounts of dislocations form fine subgrains, etc., by appropriate base material components and processing after hot working and rolling, Subgrains and the like remain in the structure after transformation due to cooling, and contribute to improving the toughness of the steel material.

  Thus, a finely developed dislocation substructure that contributes to the improvement of toughness of high-strength steel cannot be obtained only by manufacturing a normal material by a normal method. In order to obtain this structure, it is necessary to satisfy the following conditions (a), (b) and (c) at the same time.

  (A) The total ratio of the martensite phase and the bainite phase in the metal structure of the surface layer portion and the thickness center portion is 95% by volume or more and 80% by volume or more, respectively.

  (B) The total amount of P, S, N, and O in the base material is limited. That is, the A value in the following formula (1) is limited to 0.5% or less. Note that each element symbol in the formula (1) means the content (% by mass) of each element contained in the steel.

A = 12P + 45S + 67 (N + O) (1)
(C) Furthermore, the half width of the X-ray diffraction intensity from the (110) plane is set to 0.18 degrees or more, desirably 0.22 degrees or more, and more desirably 0.25 degrees or more.

  By satisfying these conditions, the base material can have both high strength of 750 MPa or more and excellent Charpy impact characteristics and DWTT characteristics at −46 ° C.

  The present invention has been made on the basis of the above findings, and the gist of the present invention is the following (1) and (2) high-strength steel plates, (3) high-strength welded steel pipes, and (4) high-strength steels. In the manufacturing method.

(1) By mass%, C: 0.01 to 0.10%, Si: 0.30% or less, Mn: 1.20 to 2.50%, P: 0.010% or less, S: 0.002 %: Ni: 0.2-1.5%, Mo: 0.1-0.8%, Nb: 0.005-0.06%, Ti: 0.004-0.025%, sol. Al: not more than 0.05%, N: not more than 0.0050% and O (oxygen): not more than 0.003%, or further comprising components of the following first group or / and second group,
Components of the first group ··· Cr: 1.0% or less, Cu: 0.1 to 1.5%, V: 0.1% or less and B: one or more of 0.0030% or less Second group Components: ··· Ca: 0.01% or less, Mg: 0.01% or less, Zr: 0.01% or less and REM: 0.05% or less, the balance being Fe and impurities, It is made of a steel having an A value of 0.5% or less represented by the formula (1), the half width of the X-ray diffraction intensity from the (110) plane is 0.18 degrees or more, and the tensile strength is 750 MPa or more. Is a high strength steel plate.

A = 12P + 45S + 67 (N + O) (1)
Here, all the element symbols in formula (1) are for each element contained in the steel.
It means content (mass%).

  (2) The high-strength steel sheet according to (1), wherein a total ratio of the martensite phase and the bainite phase in the metal structure of the surface layer portion and the central thickness portion is 95% by volume or more and 80% by volume or more, respectively. .

  (3) A high-strength welded steel pipe obtained by processing the high-strength steel plate according to (1) or (2).

(4) By mass%, C: 0.01 to 0.10%, Si: 0.30% or less, Mn: 1.20 to 2.50%, P: 0.010% or less, S: 0.002 %: Ni: 0.2-1.5%, Mo: 0.1-0.8%, Nb: 0.005-0.06%, Ti: 0.004-0.025%, sol. Al: 0.05% or less, N: 0.0050% or less, and O (oxygen): 0.003% or less, or in addition to the above-described components, the following first group or / and second group Containing the ingredients of
Components of the first group: mass%, Cr: 1.0% or less, Cu: 0.1 to 1.5%, V: 0. 1% or less and B: one or more of 0.0030% or less, second group components, Ca: 0.01% or less, Mg: 0.01% or less, Zr: 0.01% or less, and REM: One or more of 0.05% or less of the balance is Fe and impurities, and a steel having an A value of 0.5% or less represented by the following formula (1) is heated to 950 to 1200 ° C., and then hot rolled. The rolling is finished at a finishing temperature of 900 to 600 ° C., and accelerated cooling is performed at a cooling rate of 4 ° C./second or more from a temperature range not lower than 600 ° C. to a temperature of 550 ° C. or less. A method for producing high-strength steel, characterized in that the cooling is terminated so that the temperature is not higher than ° C.

A = 12P + 45S + 67 (N + O) (1)
Here, all the element symbols in formula (1) are for each element contained in the steel.
It means content (mass%).

  The above-mentioned “half-value width of X-ray diffraction intensity” is the distribution width at the half of the intensity value where the diffraction intensity is the highest in the X-ray diffraction intensity data (“diffraction intensity-angle” data). It is an angle. The “half width” will be described in detail later.

  “REM” is a rare earth element and refers to scandium (Sc), yttrium (Y), and lanthanoid (15 elements). The content of REM is the total content of the contained rare earth elements.

The high-strength steel plate of the present invention and the welded steel pipe obtained by processing this steel plate have high strength and are excellent in toughness at cryogenic temperatures including the welded portion. Since the unstable fracture resistance characteristics based on the evaluation by the DWTT test are excellent, for example, the effect of dramatically improving the safety when the steel pipe of the present invention is used as a line pipe in a cryogenic environment can be obtained. .
This steel plate can be reliably and stably manufactured by the method of the present invention.

  Hereinafter, the reason why the high-tensile steel plate of the present invention, the manufacturing method thereof, and the welded steel pipe are specified as described above will be described in detail. In the following, “%” of the alloy element means “mass%”.

  First, the chemical composition of steel will be described.

C: 0.01 to 0.10%
C is contained for the purpose of securing the strength. However, if the content is less than 0.01%, the hardenability is insufficient and it is difficult to secure a tensile strength of 750 MPa or more, and the toughness is not sufficient. On the other hand, if the content exceeds 0.10%, the toughness of the steel (base material) and the welded portion, particularly the weld heat affected zone (HAZ) is lowered. Moreover, the weldability at the time of welding construction also falls. For this reason, C content was made into 0.01 to 0.10%. A preferable range is 0.02 to 0.08%, and a more preferable range is 0.03 to 0.08%.

Si: 0.30% or less Si is usually added as a deoxidizing agent, but when its content exceeds 0.30%, the toughness of the steel and its welded portion decreases. For this reason, Si content was 0.30 or less. A preferable upper limit is 0.20%, and a more preferable upper limit is 0.15%. The lower limit is not particularly defined, but the Si content is preferably 0.01% or more in order to obtain a sufficient deoxidation effect.

Mn: 1.20 to 2.50%
Mn is contained in order to improve hardenability and increase strength, but if the content is less than 1.20%, it is difficult to ensure a tensile strength of 750 MPa or more. On the other hand, if the content exceeds 2.50%, the toughness of the steel and the welded portion thereof decreases. For this reason, Mn content was made into 1.20-2.50%. A preferable range is 1.2 to 2.2%, and a more preferable range is 1.2 to 1.7%.

P: 0.010% or less P is an impurity element, and not only lowers the low temperature toughness of the steel and its welded portion, particularly the weld heat affected zone, but also lowers the weldability. Therefore, the lower the P content, the better. However, inevitable mixing is inevitable, and excessive reduction leads to an increase in cost. Therefore, the upper limit is set to 0.010% as a limit that does not cause actual harm. A preferable upper limit is 0.008%, and a more preferable upper limit is 0.005%. In addition, P content needs to satisfy | fill (1) Formula mentioned later.

S: 0.002% or less S is an impurity element similar to P described above, and not only lowers the low temperature toughness of the steel and its welded portion, particularly the weld heat affected zone, but also lowers the weldability. In the present invention, reduction of the content is an essential element. That is, in order to ensure sufficient cryogenic toughness for a high strength steel sheet having a tensile strength of 750 MPa or more, it is preferable to reduce the S content as much as possible, but inevitable mixing is inevitable, and excessive reduction increases costs. Therefore, the upper limit is set to 0.002% or less as a limit that does not cause actual harm. A preferable upper limit is 0.0008%, and a more preferable upper limit is 0.0005%. In addition, S content needs to satisfy | fill (1) Formula mentioned later.

Ni: 0.2 to 1.5%
Ni has the effect of improving weldability as well as improving the low temperature toughness and brittle crack propagation stopping performance of steel. These effects can be obtained even at the impurity level, but become significant when the content is 0.2% or more. However, even if the content exceeds 1.5%, not only the cost for improving the effect becomes small for the cost increase, but also excessive retained austenite is generated by quenching and tempering treatment, and the yield strength decreases. May end up. For this reason, the Ni content in the case of positively adding and containing is preferably 0.2 to 1.5%.

Mo: 0.1 to 0.8%
Mo improves hardenability and improves the strength and toughness of steel by solid solution strengthening. Further, at the time of compound addition with Nb, it has the effect of promoting refinement of the structure and at the same time dispersing moderate retained austenite in the steel and improving the toughness of the base metal and the welded portion. These effects become significant when the content is 0.1% or more. However, if the content exceeds 0.8%, the strength is excessively increased and the toughness of the base material and its welded portion is impaired. For this reason, Mo content was made into 0.1 to 0.8%.

Nb: 0.005 to 0.06%
Nb is an element that refines the structure of the steel and greatly improves the toughness of the high-strength steel, but the above effect cannot be obtained with a content of less than 0.005%. On the other hand, if the content exceeds 0.06%, weldability is impaired. For this reason, Nb content was made into 0.005 to 0.06%. A preferable range is 0.005 to 0.03%, and a more preferable range is 0.005 to 0.02%.

Ti: 0.004 to 0.025%
Ti is an element that refines the structure of steel and its heat-affected zone and improves the low-temperature toughness of the base metal and its heat-affected zone, but the above effect can be obtained with a content of less than 0.004%. Absent. On the other hand, if the content exceeds 0.025%, not only the low temperature toughness of the steel and its welded portion, particularly the weld heat affected zone, is impaired, but also the weldability is lowered. For this reason, Ti content was made into 0.004-0.025%. A preferable range is 0.004 to 0.015%, and a more preferable range is 0.004 to 0.010%.

sol. Al: 0.05% or less Al is an element usually added as a deoxidizer, and has an effect of fixing N contained as an impurity in steel as AlN. If the Al content exceeds 0.05%, not only will the weld properties deteriorate, but weldability will also decrease, and the MA ratio (existence ratio of island martensite structure) will increase, and toughness will increase. Deteriorates. For this reason, the content of Al is sol. Al content was 0.05% or less. A preferable upper limit is 0.035%, and a more preferable upper limit is 0.025%. The lower limit is not particularly required, but sol. The Al content is preferably 0.0005% or more.

N: 0.0050% or less N is an impurity element that is a harmful element that lowers the toughness of steel. If the content exceeds 0.0050%, the desired low-temperature toughness cannot be secured. For this reason, N content was made into 0.0050% or less. The preferable upper limit is 0.0025%, and the more preferable upper limit is 0.0020%, but the lower the N content, the better. In addition, N content needs to satisfy | fill (1) Formula mentioned later.

O (oxygen): 0.003% or less O is an impurity element similar to N described above, and is an extremely harmful element that lowers the toughness of steel. If the content exceeds 0.003%, the desired content is Low temperature toughness cannot be secured. Therefore, the O content is set to 0.003% or less. The preferable upper limit is 0.0018%, and the more preferable upper limit is 0.0012%, but the lower the O content, the better. The O content must satisfy the following formula (1).

A value: 0.5% or less Each element of P, S, N, and O is suppressed to a content within the above-described range, and the following formula (1) (element symbols in the formula are each included in steel) The A value expressed by the element content (%) is required to be 0.5% or less.

A = 12P + 45S + 67 (N + O) (1)
That is, in a steel material having an A value exceeding 0.5%, a fine dislocation substructure is not sufficiently introduced into the steel structure even if appropriate hot rolling and post-rolling treatment (accelerated cooling described later) are performed. Also, the welded portion is not sufficiently miniaturized, and the intended high strength and high toughness cannot be obtained. For this reason, A value was made into 0.5% or less. A preferable upper limit of the A value is 0.4%.

  The high-strength steel sheet of the present invention is sufficient for the chemical composition of the steel as long as it consists of the above-described components, the balance Fe, and impurities, but if necessary, Cr, Cu, V, B, Ca, Any one or more of Mg, Zr and REM may be positively added and contained. In this case, high strength can be obtained without impairing the low temperature toughness and weldability of the steel and its welded portion, particularly the weld heat affected zone, and a thicker steel plate or steel pipe can be obtained.

Cu: 0.1 to 1.5%
Cu has the effect of improving hardenability and toughening steel (base material) without significantly impairing the weldability, and the effect becomes significant when the content is 0.1% or more. However, if the content exceeds 1.5%, not only the toughness of the base material and its welded portion is impaired, but also the hot ductility may be greatly reduced. For this reason, when added, the Cu content is preferably 0.1 to 1.5%.

Cr: 1.0% or less V: 0.1% or less B: 0.0030% or less Any of these elements has an effect of improving hardenability and strengthening the steel. However, if Cr, V, and B are contained in amounts exceeding 1.0%, 0.1%, and 0.0030%, respectively, the strength rises excessively, and the toughness of the steel and its welds is impaired. Sometimes. For this reason, the contents of Cr, V, and B are set to 1.0% or less, 0.1% or less, and 0.0030% or less, respectively. The above effects can be obtained at any impurity level for any element. Accordingly, the lower limit is not particularly defined, but a significant effect is seen with a content of 0.01% or more for Cr, 0.005% or more for V, and 0.0003% or more for B. In this case, the Cr, V, and B contents are preferably 0.01 to 1.0%, 0.005 to 0.1%, and 0.0003 to 0.0030%, respectively.

Ca: 0.01% or less Mg: 0.01% or less Zr: 0.01% or less REM (rare earth element): 0.05% or less These elements all control the form of inclusions in steel. In addition to improving the toughness and corrosion resistance of steel and its welds, it has the effect of stabilizing elements (N, C, O) that are harmful to low temperature toughness. However, if Ca, Mg and Zr are all contained in excess of 0.01%, and REM is contained in excess of 0.05%, the cleanliness of the steel decreases, and the steel and its weld The toughness of the part decreases. Therefore, the Ca, Mg, and Zr contents are all 0.01% or less, and the REM content is 0.05% or less.

  The above effects can be obtained at any impurity level for any element. Accordingly, the lower limit is not particularly defined, but since a significant effect is observed at a content of 0.0005% or more, the contents of Ca, Mg, and Zr in the case of positively adding and containing are all 0. 0005 to 0.01%, and the content of REM is preferably 0.0005 to 0.05%.

In consideration of weldability and steel cleanliness, it is preferable that the carbon equivalents C _eq and V _s shown below are within the ranges of 0.40 to 0.58 and 0.28 to 0.42%, respectively.

C _eq (%) = C + (Mn / 6) + {(Cu + Ni) / 15}
+ {(Cr + Mo + V) / 5}
V _s (%) = C + (Mn / 5) + 5P− (Ni / 10)
-(Mo / 15) + (Cu / 10)
Here, each element symbol means the content (%) of the element.

The reason why C _eq is preferably 0.40 to 0.58% is wide not involving deterioration of toughness by using a mixed structure of bainite and martensite not only in the base material but also in the weld heat affected zone. This is because it becomes possible to obtain steel having a desired structure under manufacturing conditions. When the carbon equivalent is less than the lower limit, it becomes difficult to maintain the tensile strength of the base material at 750 MPa or more due to insufficient hardenability. Moreover, when a carbon equivalent exceeds an upper limit, the toughness in a welding heat affected zone and the toughness in the steel plate surface deteriorate from the excessive raise of hardenability.

The reason why V _s is 0.28 to 0.42% is to reduce the center segregation of the continuous cast slab. When the V _s value exceeds 0.42%, the center segregation is strongly generated, and in the case of a high strength steel having a tensile strength of 750 MPa or more, the toughness of the center portion is deteriorated. On the other hand, if it is less than 0.28%, center segregation does not occur, but a tensile strength of 750 MPa or more cannot be ensured.

  The high-strength steel plate described in (1) is a steel plate made of steel having the chemical composition described above and having a half width of X-ray diffraction intensity from the (110) plane of 0.18 degrees or more. The half width of the X-ray diffraction intensity is set to 0.18 degrees or more for the following reason.

  FIG. 1 is a diagram schematically showing X-ray diffraction intensity data on the (110) plane of steel, and is a diagram for explaining the half-value width of the X-ray diffraction intensity. FIG. 1A shows a case where there is one peak, and FIG. 1B shows a case where the peak is divided into two. As shown in FIG. 1, the half width is an angle representing the width of the distribution at the half intensity value of the highest diffraction intensity (peak value) at this diffraction intensity peak. is there. As shown in FIG. 1B, when the peak is divided into two, it takes a value half that of the higher peak intensity value.

  The half width of the X-ray diffraction intensity is one of the evaluation parameters of the lattice defect density in the metal structure, etc., but the high strength steel sheet of the present invention uses this half width to achieve high toughness in a low temperature range. The lattice defect density, which is an index for doing so, was limited. That is, if the condition that the half-value width of the X-ray diffraction intensity in the (110) plane of the bainite phase or martensite phase satisfies 0.18 degrees or more, the lattice defect density related to the formation of fine subgrains increases. Excellent toughness is exhibited at low temperatures. The half width of the X-ray diffraction intensity is preferably 0.22 degrees or more, and more preferably 0.25 degrees or more.

  The reason why the crystal plane for X-ray diffraction is the (110) plane is that this crystal plane is most commonly used in X-ray diffraction. In the high-strength steel sheet of the present invention, in order to obtain good low-temperature toughness, it is specified that “the half-value width of the X-ray diffraction intensity on the (110) plane is 0.18 degrees or more”, but the tensile strength is 750 MPa class. In the case of a steel sheet having the above strength, the half width is preferably 0.20 to 0.30 degrees from the viewpoint of balance of strength and the like.

  When the Kα1 and Kα2 peaks appear independently in the diffraction pattern, the half width takes the value at the half intensity value of the Kα1 peak, and is the half intensity value of the Kα1 and Kα2 peak. By the way, when the values appear partially overlapping, measure with the total width. The half width is measured on a plane parallel to the rolling surface at a portion 1 mm inside the steel sheet surface in the thickness direction.

  The high-strength steel sheet described in (1) is a steel sheet that defines the chemical composition (including the condition that the A value is 0.5 or less) and the half-value width of the X-ray diffraction intensity as described above. Setting the half width to 0.18 degrees or more is possible by appropriately performing hot rolling and post-rolling treatment, as will be described later, whereby bainite having a finely developed dislocation substructure, Martensite or a mixed structure thereof is obtained. The steel sheet that quantitatively defines the metal structure is the steel sheet described in (2) below.

  In the high-strength steel sheet according to (2), in the high-strength steel sheet according to (1), the total proportion of the martensite phase and the bainite phase in the metal structure of the surface layer portion is 95% by volume or more. There is a steel plate in which the total proportion of the martensite phase and the bainite phase in the metal structure at the center of the wall thickness is 80% by volume or more.

  In this steel plate, the above conditions (a) to (c) are clearly satisfied, and bainite, martensite or a mixed structure thereof having a sufficiently fine and developed dislocation substructure is obtained. Therefore, since the fracture occurrence suppression characteristics and the fracture propagation stop characteristics at low temperatures are remarkably excellent, the unstable fracture resistance characteristics are further improved, and both the high strength of 750 MPa or more, the Charpy impact characteristics and DWTT characteristics at -46 ° C. are achieved. Can do.

  In a steel sheet having a half width of less than 0.18 degree, the dislocation substructure is not sufficiently developed and is not fine, so that even if the metal structure satisfies the above conditions, sufficient Charpy impact characteristics and DWTT characteristics are obtained. Can't get.

  Conversely, when the chemical composition of the steel is out of the range defined by the present invention, or when the steel structure is not the structure defined by the present invention, the half width of the (110) plane peak is 0.18 degrees or more. Even so, a fine dislocation substructure is not obtained, and a steel sheet having both high strength and high toughness cannot be obtained.

  Next, the manufacturing method (the method (4)) of the high-strength steel of the present invention described above will be described.

  As long as the chemical composition of the steel satisfies the conditions stipulated in the present invention, the high-strength steel sheet of the present invention can be tempered by reheating and quenching after normal hot rolling, or directly quenched after normal hot rolling. It can also be produced by a method of tempering, and also a method of accelerated cooling after normal hot rolling. However, for reliable and stable production, it is desirable to produce by an accelerated cooling treatment after hot rolling under the following conditions.

Heating temperature: 950-1200 ° C
If the heating temperature is less than 950 ° C., a tensile strength of 750 MPa or more may not be ensured. On the other hand, if the heating temperature exceeds 1200 ° C., the occurrence of brittle fracture and the stabilization of elements (N, C, O) harmful to low temperature toughness after subsequent hot rolling become insufficient, and the desired low temperature toughness is ensured. It may not be possible. For this reason, the heating temperature is desirably 950 to 1200 ° C.

Hot rolling finishing temperature: 900-600 ° C
When the finishing temperature of hot rolling is less than 600 ° C., a tensile strength of 750 MPa or more may not be ensured. Moreover, when the finishing temperature of hot rolling exceeds 900 ° C., the structure is not sufficiently refined by rolling and subsequent accelerated cooling, and the occurrence of brittle fracture and stabilization of elements (N, C) harmful to low temperature toughness May become insufficient and the desired low-temperature toughness may not be ensured. For this reason, the finishing temperature of hot rolling shall be 900-600 degreeC.

In order to set the X-ray half width of the (110) plane to 0.18 degrees or more, after sufficiently reducing in the low temperature austenite region, lower bainite is nucleated from within the austenite grains, and the lower bainite It is necessary to suppress growth. For this reason, high-density dislocation is necessary, and for that purpose, it is desirable to perform rolling at 50% or more in the austenite non-recrystallization temperature region (975 ° C. or lower and _{Ar 3} transformation point or higher). On the other hand, if the reduction ratio of the austenite in the non-recrystallization temperature region exceeds 90%, the anisotropy of the mechanical properties becomes remarkable. Therefore, the reduction rate in the non-recrystallization temperature region is desirably 90% or less.

Water cooling start temperature: 600 ° C. or more When the water cooling start temperature during accelerated cooling is less than 600 ° C., a tensile strength of 750 MPa or more may not be ensured. For this reason, the water cooling start temperature at the time of accelerated cooling is preferably 600 ° C. or higher.

Cooling rate: 4 ° C./second or more When the cooling rate during accelerated cooling is less than 4 ° C./second, coarse bainite is mixed in the structure, and good low temperature toughness cannot be ensured. In addition, the developed dislocation substructure introduced by rolling may not remain in the transformed structure and may not be brought to the final product. For this reason, the cooling rate during accelerated cooling is preferably 4 ° C./second or more. The upper limit of the cooling rate is not particularly specified.

Water cooling stop temperature: 550 ° C. or less When the water cooling stop temperature during accelerated cooling exceeds 550 ° C., a tensile strength of 750 MPa or more cannot be secured, and not only the structure is not sufficiently refined, but also introduced by rolling. The developed dislocation substructure is not brought into the final product after transformation, and the desired unstable fracture resistance characteristics may not be ensured. For this reason, the water cooling stop temperature during accelerated cooling is preferably 550 ° C. or lower.

In addition, from the viewpoint of improving toughness due to the tempering effect and preventing hydrogen defects, it is desirable to stop at 500 to 300 ° C. and then gradually cool without cooling to room temperature. The water cooling stop temperature is limited to a temperature range of 500 to 300 ° C., and water cooling stop at a temperature higher than that is directly connected to insufficient quenching. When it is desired to further enhance the tempering effect, tempering is performed below the A _c1 transformation point.

Recuperation temperature range: 70 ° C. or less Cooling is completed so that the recuperation temperature range becomes 70 ° C. or less. Here, the “recuperation temperature range” is the difference between the temperature reached when the cooling is stopped and the temperature of the steel plate when the surface temperature rises due to the heat inside the steel plate after cooling stops and becomes stable. Means. Specifically, it is the difference between the steel plate temperature measured immediately after leaving the water cooling device and the steel plate temperature measured after 20 to 50 seconds (depending on the plate thickness).
If the recuperation temperature range from the accelerated cooling stop to the end of cooling exceeds 70 ° C, the dislocation substructure introduced and developed in the rolling process is not brought into the final product after transformation, and the fracture propagation stoppage characteristic Deteriorates. In order to reduce the recuperation temperature range, it is desirable to reduce the temperature difference between the steel plate surface layer and the center portion during cooling and to end the phase transformation of at least the surface layer portion at the end of cooling.

  According to the method described above, the high-strength steel sheet according to (1) or (2) having both high strength and excellent cryogenic toughness can be produced reliably and stably.

  Furthermore, the high-strength welded steel pipe of the present invention (the welded steel pipe of (3) above) will be described.

  The high strength welded steel pipe of the present invention is a welded steel pipe obtained by processing the above-described high strength steel sheet of the present invention. As long as this welded steel pipe is a base material for the high-strength steel sheet of the present invention, it may be obtained by processing by any well-known welding pipe making method.

  For example, a high-strength steel sheet is processed into a U-shape, then processed into an O-shape, a steel pipe is manufactured by a method of welding and expanding by a normal method (UO press method), quenching, and tempering as necessary. Applicable. The UO press, welding, and pipe expansion can be performed by using an apparatus provided in a normal pipe manufacturing factory. Welding may be performed by a submerged arc welding method using a commercially available welding material.

  Steel having the chemical composition shown in Table 1 was melted in a laboratory vacuum to form a slab having a thickness of 100 to 160 mm, subjected to hot rolling under various conditions, and then cooled under various conditions to obtain a thickness of 14 It was set as a -35 mm thick steel plate. Table 2 shows hot rolling conditions and cooling conditions.

  About the obtained steel plate, the structure of steel, the half width of X-ray diffraction intensity, the tensile characteristics, and the toughness were investigated by the following methods. The toughness of the welded joint was also investigated.

  In the investigation of the steel structure, the cross section of the sample taken from the portion corresponding to 1/4 of the plate thickness was polished, and the surface etched with 2% nital etchant was summed with martensite and bainite by optical microscope observation. The area ratio (total ratio) was measured. Ten fields of view were measured for one sample, and the average of the ten measurements was taken as the total ratio of the steel sheet.

  The half width of the X-ray diffraction intensity was measured by taking a 25 mm square test piece including a surface parallel to the rolling surface of the portion 1 mm inside the steel sheet surface in the thickness direction, and electropolishing the surface. Of these, a range of 20 mm in diameter was used as a measurement surface. For the measurement, RU-200 manufactured by Rigaku Corporation was used, and a cobalt radiation source was used. The output was 30 KV and 100 mA.

  Regarding the tensile properties, a 14A tensile test piece defined in JIS Z 2201 is taken from the center of the plate thickness in parallel with the rolling direction and subjected to a tensile test. Yield strength YS (MPa), tensile strength TS ( MPa).

As for toughness, No. 4 Charpy impact test piece defined in JIS Z 2202 was sampled perpendicularly to the rolling direction from the center of the plate thickness and subjected to Charpy impact test, and impact absorption energy _v E _{-46 ° C} (J) and The ductile-brittle fracture surface transition temperature _v T _s (° C.) was determined. Further, a DWTT test piece defined in API 5L was taken perpendicular to the rolling direction and subjected to a DWTT test. The ductile fracture surface ratio SA _{−46 ° C.} (area%) at _{−46 ° C.} and 75% ductile-brittle fracture surface. The transition temperature SATT _75% (° C.) was measured.

In addition, a welded joint was prepared by submerged arc welding (heat input: 3.2 to 7.6 kJ / mm) for each pass on the front and back surfaces, and the Charpy test piece was welded to the FL portion (welded metal and base metal). The Charpy impact test was conducted by sampling from the part 1 mm inside the surface perpendicular to the weld line so as to be located at the boundary line), and the impact absorption energy _v E _{−46 ° C.} (J) of the part was measured. .

The steel structure (total ratio of martensite and bainite) and the half width of the X-ray diffraction intensity are also shown in Table 2 above. Further, in Table 3, the tensile properties of the base material (YS, TS), and toughness of the base material _{(v E -46 ℃, v T} s and SA _{-46 ℃,} SATT _75%) and the welding FL zone toughness _(v E- _{46 ° C} ).

As is apparent from Table 3, in the present invention example, the base material has a tensile strength (TS) of 750 MPa or more, a Charpy absorbed energy ( _v E _{−46 ° C.} ) of 200 J or more at _{−46 ° C.,} and 75% or more. The DWTT ductile fracture surface ratio (SA _{−46 ° C.} ) was exhibited, and the joint Charpy test also showed high Charpy absorbed energy ( _v E _{−46 ° C.} ) significantly exceeding 80 J. That is, the steel sheet has achieved all of the target performances shown in the column of “Problems to be Solved by the Invention”, has high strength and excellent cryogenic toughness, and has excellent unstable fracture resistance characteristics. Understand.

  On the other hand, in the comparative example in which the chemical composition of the steel sheet deviates from the range specified in the present invention, or the full width at half maximum or the metal structure deviates from the definition of the present invention even if the specified chemical composition is satisfied, the DWTT ductile fracture surface ratio Was significantly lower than 75% of the target, and the high strength of 750 MPa, the Charpy impact property and DWTT property at -46 ° C., and the welded FL part Charpy property could not be satisfied at the same time.

  The high-strength steel sheet of the present invention and the welded pipe obtained by processing this steel sheet are high-strength, and also have excellent toughness at cryogenic temperatures, including welds, and excellent unstable fracture resistance characteristics. ing. Therefore, even in a cryogenic environment, it can be safely used for a line pipe for transporting natural gas or crude oil, various pressure vessels, and the like.

It is a figure which shows typically the data (diffraction pattern) of the X-ray diffraction intensity of steel, (A) is a case where a peak is one, and (B) is a case where a peak is divided into two.

Claims (6)

In mass%, C: 0.01 to 0.10%, Si: 0.30% or less, Mn: 1.20 to 2.50%, P: 0.010% or less, S: 0.002% or less, Ni: 0.2-1.5%, Mo: 0.1-0.8%, Nb: 0.005-0.06%, Ti: 0.004-0.025%, sol. Al: 0.05% or less, N: 0.0050% or less, and O (oxygen): 0.003% or less, with the balance being Fe and impurities, and the A value represented by the following formula (1) being 0 A high-strength steel plate made of steel of 5% or less and having a half-value width of X-ray diffraction intensity from the (110) plane of 0.18 degrees or more and a tensile strength of 750 MPa or more.
A = 12P + 45S + 67 (N + O) (1)
Here, all the element symbols in formula (1) are for each element contained in the steel.
It means content (mass%). In addition to the components according to claim 1, further, by mass, Cr: 1.0% or less, Cu: 0.1 to 1.5%, V: 0.1% or less and B: 0.0030% or less X-ray diffraction intensity from the (110) plane, comprising at least one of the above, with the balance being Fe and impurities, A value of 0.5% or less represented by the following formula (1) A high-strength steel sheet having a half width of 0.18 degrees or more and a tensile strength of 750 MPa or more.
A = 12P + 45S + 67 (N + O) (1)
Here, all the element symbols in formula (1) are for each element contained in the steel.
It means content (mass%). In addition to the components according to claim 1 or 2, further, by mass, Ca: 0.01% or less, Mg: 0.01% or less, Zr: 0.01% or less, and REM: 0.05% or less It contains one or more of them, the balance is Fe and impurities, the A value represented by the following formula (1) is 0.5% or less, and the X-ray diffraction intensity from the (110) plane A high strength steel plate having a half width of 0.18 degrees or more and a tensile strength of 750 MPa or more.
A = 12P + 45S + 67 (N + O) (1)
Here, all the element symbols in formula (1) are for each element contained in the steel.
It means content (mass%).   The high strength according to any one of claims 1 to 3, wherein a total proportion of the martensite phase and the bainite phase in the metal structure of the surface layer portion and the thickness center portion is 95% by volume or more and 80% by volume or more, respectively. steel sheet.   A high-strength welded steel pipe obtained by processing the high-strength steel plate according to claim 1. In mass%, C: 0.01 to 0.10%, Si: 0.30% or less, Mn: 1.20 to 2.50%, P: 0.010% or less, S: 0.002% or less, Ni: 0.2-1.5%, Mo: 0.1-0.8%, Nb: 0.005-0.06%, Ti: 0.004-0.025%, sol. Al: 0.05% or less, N: 0.0050% or less, and O (oxygen): 0.003% or less, or in addition to the above-described components, the following first group or / and second group Containing the ingredients of
Components of the first group: mass%, Cr: 1.0% or less, Cu: 0.1 to 1.5%, V: 0. 1% or less and B: one or more of 0.0030% or less, second group components, Ca: 0.01% or less, Mg: 0.01% or less, Zr: 0.01% or less, and REM: One or more of 0.05% or less of the balance is Fe and impurities, and a steel having an A value of 0.5% or less represented by the following formula (1) is heated to 950 to 1200 ° C., and then hot rolled. The rolling is finished at a finishing temperature of 900 to 600 ° C., and accelerated cooling is performed at a cooling rate of 4 ° C./second or more from a temperature range not lower than 600 ° C. to a temperature of 550 ° C. or less. A method for producing high-strength steel, characterized in that the cooling is terminated so that the temperature is not higher than ° C.
A = 12P + 45S + 67 (N + O) (1)
Here, all the element symbols in formula (1) are for each element contained in the steel.
It means content (mass%).

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