JP2017155333A - Low yield ratio high strength thick steel sheet excellent in large heat input weld heat-affected zone toughness and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio high tension thick steel sheet having stable base material performance and capable of enhancing weld heat affected zone toughness of large heat input with weldment heat input of over 400 kJ/cm and a manufacturing method therefor.SOLUTION: There is provided a low yield ratio high strength thick steel sheet excellent in large heat input heat-affected zone toughness with yield strength of 650 MPa or more and yield ratio of 85% or less, having a component composition containing, by mass%, C:0.03 to 0.10%, Si:0.01 to 0.08%, Mn:1.4 to 3.0%, P:0.015% or less, S:0.0050% or less, Al:0.005 to 0.1%, Ti:0.004 to 0.03%, N:0.0015 to 0.0065%, having Ceq defined in the following formula (1) of 0.50 to 0.70%, Ti/N satisfying over 2.0 and less than 4.2 and the balance Fe with inevitable impurities, a micro structure having a concentrated area with C of 0.2 to 1.0 mass%, the concentrated area having an average circle equivalent diameter of 1.0 to 5.0 μm and area fraction of 5 to 15%. Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 (1), where C, Mn, Si, Ni, Cr, Mo and V are contents of each element (mass%) and 0 when they are not contained.SELECTED DRAWING: None

Description

  The present invention is excellent in toughness of the weld heat affected zone, yield strength (YP) is 650 MPa or more, and yield ratio (YR) is 85% or less, even in high heat input welding where the heat input of welding exceeds 400 kJ / cm. Thus, the present invention relates to a low-yield-ratio high-strength steel plate having a thickness of 12 mm or more, which is suitable for construction, and a method for producing the same.

  In recent years, with the increase in size and span of building structures, it has been required to increase the thickness and strength of steel used. From the viewpoint of safety of steel structures, it has high allowable stress and yield. There is a need to reduce the ratio.

  When the yield ratio is reduced, even if a stress higher than the yield point is applied, the stress allowed until failure increases, and the uniform elongation increases, so that the steel material is excellent in plastic deformability.

  In particular, in a high-tensile steel sheet having a tensile strength (TS) exceeding 780 MPa, it is common to add a large amount of an alloy for securing the strength, so that the yield ratio tends to increase and the toughness also decreases.

  Conventionally, a multistage heat treatment including reheating and quenching in a ferrite + austenite two-phase region is generally used as a manufacturing process of a low yield ratio high strength thick steel plate. However, the microstructure obtained is mainly composed of a ferrite phase and disperses bainite or martensite as a hard second phase. Therefore, depending on the volume fraction of the ferrite phase, the tensile strength is 780 MPa or more and the yield strength is 650 MPa or more. It is difficult to achieve stably.

  On the other hand, when a steel plate is used for the structure, generally, welded joint is used, and from the viewpoint of safety, the base metal toughness of the steel material used, as well as the weld heat affected zone (sometimes referred to as HAZ). It is required to have excellent toughness.

  In recent years, as described above, with the increase in the size of building structures, the use of thicker steel materials has been demanded, and the application range of large heat input welding has been expanded from the viewpoint of improving the construction efficiency of the structures and reducing the construction costs. doing. For box columns used for high-rise buildings, super-high heat input welding such as submerged arc welding, electroslag welding, or the like in which the heat input exceeds 400 kJ / cm is applied.

  In recent years, improvement in earthquake resistance of building structures has been demanded, and welded joints are also required to have high toughness. For example, the column-beam joint is required to have high toughness such that the Charpy absorbed energy at 0 ° C. exceeds 47 J.

  Generally, when high heat input welding is applied to a steel material, the most serious problem is toughness deterioration in the bond portion of the weld heat affected zone. The bond portion is exposed to a high temperature just below the melting point during high heat input welding, the austenite crystal grains become the most coarse, and the subsequent cooling transforms into a fragile upper bainite structure, which is an island-like martensite that is an embrittled structure. Sites form and toughness decreases. Therefore, a thick steel plate having both base metal mechanical properties such as high strength, low yield ratio, and high toughness and weld heat affected zone toughness has been demanded, and various proposals have been made.

  In Patent Document 1 and Patent Document 2, after quenching the hot-rolled steel sheet, it is again heated to the two-phase region of ferrite + austenite for quenching to achieve high strength and low yield ratio. Have been described.

  In Patent Document 3, after rolling, by direct quenching immediately after quenching, the microstructure after quenching is changed to a bainite phase or a martensite phase, and then again heated to a ferrite + austenite two-phase region and subjected to normalization. It is described that high strength and low yield ratio are achieved.

  In Patent Document 4, after a certain period of time has passed after rolling, ferrite is precipitated, and then a direct quenching method in which quenching is performed to obtain a two-phase structure of ferrite phase + martensite phase, thereby increasing strength and reducing yield ratio. It is described to achieve.

  In Patent Document 5, after the components are adjusted, residual austenite (also referred to as residual γ) is generated by direct quenching after rolling, thereby increasing the strength of the base metal, reducing the yield ratio, and welding. Achieving high toughness is described.

  In Patent Document 6, martensite or island-shaped martensite is contained in a bainite-based structure, and the volume fraction, particle size, and aspect ratio are appropriately controlled, whereby a tensile strength of 590 MPa or more and 80% or less. A base material having a low yield ratio is described, and as its manufacturing method, after component adjustment, it is directly hardened after rolling, and further, the reheating treatment after cooling stop is optimized.

JP 2001-288512 A JP-A-6-248337 JP-A-5-230530 JP-A-7-97626 JP 2001-226740 A JP 2006-291348 A

  However, the techniques described in Patent Document 1, Patent Document 2, and Patent Document 3 are concerned with an increase in manufacturing cost due to a complicated heat treatment process. In the techniques described in Patent Document 4 and Patent Document 5, the volume fraction of the ferrite and martensite phases is likely to change depending on the manufacturing conditions and the position in the steel sheet, and a high strength and a low yield ratio can be stably obtained. Therefore, the operation load for adjusting the manufacturing conditions is large.

  In the technique described in Patent Document 6, there is no specific suggestion regarding a thick steel plate having a strength level such as a tensile strength of 780 MPa or more and a yield strength of 650 MPa or more and a manufacturing method thereof.

  Furthermore, in the techniques described in Patent Documents 1 to 6, it is not assumed that the welding heat affected zone toughness of high heat input welding in which the welding heat input exceeds 400 kJ / cm is stably achieved.

  In view of such circumstances, the present invention provides a stable base material performance and can improve the weld heat affected zone toughness of high heat input welding with a heat input of welding exceeding 400 kJ / cm. And it aims at providing the manufacturing method.

In order to achieve the above-mentioned problems, the present inventors have actively used island martensite from the viewpoint of the strength of the base metal and a low yield ratio, and on the other hand, a large heat input from the viewpoint of weld heat affected zone toughness. In the weld zone, we conducted intensive research on how to suppress the formation of island martensite as much as possible, and obtained the following knowledge.
(1) The inventors of the present invention focused on a C-enriched region that tends to be island martensite. When high heat input welding with a welding heat input exceeding 400 kJ / cm is performed, if the concentration region of C in the microstructure of the base metal is coarse, the concentration of coarse C is increased even when heated to a high temperature by welding. The region tends to remain. As a result, the remaining portion of the C-enriched region easily develops into coarse island martensite during the cooling process of welding, resulting in a decrease in toughness. Therefore, by finely dispersing the C-enriched region, which is likely to be island-like martensite, in the base metal microstructure, the coarse C-enriched region almost disappears when heated to a high temperature by welding. It is thought that coarse island martensite is less likely to be generated during the cooling process, and the high heat input welding heat-affected zone toughness is improved.
(2) As a result of intensive studies by the present inventors, the microstructure of the base material has a concentration region where C is 0.2 to 1.0% by mass, and the concentration region has an average equivalent circle diameter. 1.0 to 5.0 μm and by including 5 to 15% in area fraction, the area fraction of island martensite generated in the high heat input welding heat-affected zone can be reduced and refined. . As a result, it is possible to achieve toughness securing of the high heat input heat affected zone such that the welding heat input exceeds 400 kJ / cm.
(3) A steel having a component composition with a reduced Si and P content is subjected to hot rolling, and then subjected to a cooling treatment in which the cooling rate and the cooling stop temperature are optimized. By carrying out the reheating process with the temperature rate and reheating temperature optimized, it is possible to obtain a microstructure in which the concentrated region of C is finely dispersed in the microstructure, and the yield strength of 650 MPa or more as the characteristics of the base material. And a low yield ratio of 85% or less can be achieved stably.

The present invention has been completed by further studying the above knowledge. The gist of the present invention is as follows.
[1] Component composition is mass%, C: 0.03-0.10%, Si: 0.01-0.08%, Mn: 1.4-3.0%, P: 0.015% Hereinafter, S: 0.0050% or less, Al: 0.005-0.1%, Ti: 0.004-0.03%, N: 0.0015-0.0065%, and the following (1) Ceq defined by the formula is 0.50 to 0.70%, Ti / N is more than 2.0 and less than 4.2, the balance is made of Fe and inevitable impurities, and the microstructure is C Has a concentrated region of 0.2 to 1.0% by mass, and the concentrated region has an average equivalent circle diameter of 1.0 to 5.0 μm and an area fraction of 5 to 15%. A low-yield-ratio high-strength thick steel plate excellent in high heat input heat affected zone toughness with a yield strength of 650 MPa or more and a yield ratio of 85% or less.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
However, C, Mn, Si, Ni, Cr, Mo, and V are the contents (% by mass) of each element, and 0 when not contained.
[2] Further, by mass%, Cu: 0.1 to 1.0%, Ni: 0.1 to 2.0%, Cr: 1.5% or less, Mo: 1.0% or less, Nb: 0.0. 1 type or less, V: 0.2% or less, Ca: 0.005% or less, REM: 0.02% or less, Mg: 0.005% or less, and B: 0.005% or less A low-yield-ratio high-strength thick steel plate excellent in high heat input welding heat-affected zone toughness having a yield strength of 650 MPa or more and a yield ratio of 85% or less as described in [1].
[3] A steel material having the component composition described in [1] or [2] is heated to 1000 to 1250 ° C., and the rolling reduction is 30% or more in a temperature range of 950 ° C. or less at the surface temperature. The hot rolling is performed at a surface temperature of 900 ° C. or lower and the Ar ₃ transformation point or higher, and then Ar ₃ −300 to Ar at an average cooling rate of 5 to 100 ° C./s from the temperature range of the Ar ₃ transformation point or higher. ₃ After cooling to a cooling stop temperature of −150 ° C., after reheating at a temperature increase rate of 0.5 ° C./s or more to a temperature range of cooling stop temperature + 100 ° C. to less than Ac ₁ transformation point, 0.5 A method for producing a low yield ratio high strength thick steel plate excellent in high heat input heat affected zone toughness having a yield strength of 650 MPa or more and a yield ratio of 85% or less, characterized by holding for 3 min and air cooling.
[4] The high heat input heat affected zone toughness having a yield strength of 650 MPa or more and a yield ratio of 85% or less according to [3], further tempering at 400 ° C. or more and less than the Ac ₁ transformation point. A method for producing a high-strength steel sheet with a low yield ratio and excellent strength.

  According to the present invention, a thick steel plate excellent in weld heat affected zone toughness of high heat input welding in which the yield strength of the base material is 650 MPa or more, the yield ratio is 85% or less, and the heat input of welding exceeds 400 kJ / cm. It can be manufactured stably without complicated heat treatment. For this reason, it greatly contributes to the enlargement of the steel structure, the improvement of the earthquake resistance of the steel structure and the improvement of the construction efficiency, and has a remarkable industrial effect.

FIG. 1 is a schematic diagram showing a groove shape of electroslag welding. FIG. 2 is a schematic diagram showing a sampling position of a Charpy impact test piece from the electroslag welded portion shown in FIG.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<Ingredient composition>
Hereinafter, each component will be described. In addition, “%” representing the content of a component means “% by mass”.

C: 0.03-0.10%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material, and needs to contain 0.03% or more. On the other hand, the content exceeding 0.10% significantly deteriorates the toughness of the heat-affected zone particularly affected by high heat input welding. Further, in order to deteriorate the weld crack resistance and the low temperature toughness of the base material, the content is limited to 0.03 to 0.10%. Preferably, it is 0.05 to 0.08%.

Si: 0.01 to 0.08%
Since Si also has an effect as a deoxidizing material and an effect of increasing the strength of the base material, it is set to 0.01% or more. Si is an element that promotes the formation of island martensite. Therefore, generation of island martensite generated by controlling Si to 0.08% or less can be suppressed, and the size of island martensite can be reduced. Preferably it is 0.03 to 0.07%. More preferably, it is 0.03 to 0.05%.

Mn: 1.4 to 3.0%
Mn has the effect of increasing the strength of the steel. In the present invention, in order to reduce the island-like martensite in the microstructure of the high heat input welding heat-affected zone and make it finer to ensure toughness and to ensure the yield strength of the base material to be 650 MPa or more, 1 It needs to contain 4% or more. On the other hand, if the content exceeds 3.0%, the toughness of the base metal and the weld heat-affected zone toughness deteriorate significantly, so the content is limited to the range of 1.4 to 3.0%. Preferably, it is 1.5 to 2.8%.

P: 0.015% or less P is concentrated in island-like martensite in the HAZ structure, and P promotes the formation and coarsening of island-like martensite by suppressing pearlite transformation, thus reducing HAZ toughness. Let Therefore, reduction of P is desirable for improving HAZ toughness. Therefore, it is made 0.015% or less.

S: 0.0050% or less S is an element that degrades the low temperature toughness of the base material, and it is desirable to reduce it as much as possible. If the content exceeds 0.0050%, this tendency becomes remarkable, so the upper limit is set.

Al: 0.005 to 0.1%
Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process for high strength steels. In addition, N in the steel is fixed as AlN and contributes to the improvement of the toughness of the base metal. However, if the content exceeds 0.1%, the toughness of the base material decreases, and the weld metal part is mixed during welding, In order to deteriorate the toughness, the content is limited to 0.1% or less. In addition, such an effect is recognized by containing 0.005% or more. Preferably, it is 0.01 to 0.07%.

Ti: 0.004 to 0.03%
Ti is an important element that has a strong affinity with N and precipitates as TiN during solidification, and contributes to increasing the toughness of the weld heat affected zone by suppressing the coarsening of austenite grains in the high heat input weld heat affected zone. . In order to ensure such an effect, the content of 0.004% or more is necessary. On the other hand, if it exceeds 0.03%, the TiN particles are coarsened, and the expected austenite grain coarsening suppression effect is saturated, so the content is limited to the range of 0.004 to 0.03%. Preferably, it is 0.006 to 0.025%.

N: 0.0015 to 0.0065%
N is an element necessary for securing TiN, and if it is less than 0.0015%, a sufficient amount of TiN cannot be secured. On the other hand, if the content exceeds 0.0065%, the toughness of the base metal and the welded portion is significantly reduced due to an increase in the amount of solute N, so the content is limited to 0.0065% or less. Preferably, it is 0.0030 to 0.0060%.

Ceq: 0.50 to 0.70%
In this invention, content is adjusted within the range of the component composition mentioned above so that the carbon equivalent Ceq defined by Formula (1) may be 0.50 to 0.70%.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
However, C, Mn, Si, Ni, Cr, Mo, V: Content (mass%) of each element, and 0 when not contained.

  When Ceq is less than 0.50%, the hardenability of the high heat input weld heat affected zone is insufficient, and the microstructure of the weld heat affected zone is transformed into a fragile upper bainite structure including island martensite which is an embrittled structure. However, the high toughness of the desired high heat input weld cannot be ensured. On the other hand, when Ceq exceeds 0.70%, the toughness of the base material is remarkably deteriorated and the weld crack resistance is also deteriorated, so the content is limited to the range of 0.50 to 0.70%. Preferably, it is in the range of more than 0.58 to 0.70%.

  Further, in the present invention, the Ti and N contents are adjusted within the above-described component range so that Ti / N (where Ti and N are contents (mass%)) is more than 2.0 and less than 4.2. To do.

  When Ti / N is 2.0 or less, the amount of TiN necessary for improving the toughness via the effect of suppressing the coarsening of the structure of the high heat input welding heat affected zone cannot be secured due to the pinning effect. On the other hand, when Ti / N is 4.2 or more, since the base metal toughness and the weld heat affected zone deteriorate due to the generation of TiC particles and the coarsening of TiN, Ti / N is more than 2.0 and less than 4.2. Limited to range.

  In the present invention, in addition to the basic components described above, one or more of Cu, Ni, Cr, Mo, Nb, V, Ca, REM, Mg, and B can be contained as necessary.

Cu: 0.1-1.0%, Ni: 0.1-2.0%
Cu and Ni are elements that can increase the strength while maintaining high toughness, and are also useful elements for increasing the strength because they have a small effect on the high heat input welding heat-affected zone toughness. Can be selected according to the content. When it contains, it is preferable to contain Cu 0.1% or more. However, if the amount of Cu exceeds 1.0%, hot brittleness is caused and the surface properties of the steel sheet are deteriorated. In addition, Preferably, it is 0.2 to 0.7%.

  When Ni is contained, it is preferable to contain 0.1% or more. However, even if it contains more than 2.0%, the effect is saturated, the effect commensurate with the content can not be expected, and it becomes economically disadvantageous. To do. In addition, Preferably it is 0.2 to 1.7%.

Cr: 1.5% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.2% or less Cr, Mo, Nb, and V are all elements that contribute to improving the strength of steel. And can be appropriately contained depending on the desired strength.

  When Cr is contained, it is preferable to contain 0.05% or more. However, the content exceeding 1.5% degrades the high heat input welding heat-affected zone toughness, so it is desirable to limit the content to 1.5% or less.

  When Mo is contained, it is preferable to contain 0.05% or more. However, the content exceeding 1.0% adversely affects the base material toughness and the high heat input welding heat affected zone toughness, so it is desirable to limit the content to 1.0% or less.

  When Nb is contained, it is preferable to contain 0.005% or more. However, if the content exceeds 0.1%, the base metal toughness and the high heat input welding heat-affected zone toughness deteriorate, so it is desirable to limit the content to 0.1% or less.

  When V is contained, it is preferable to contain 0.01% or more. However, if the content exceeds 0.2%, the high heat input welding heat-affected zone toughness is deteriorated, so it is desirable to limit the content to 0.2% or less.

Ca: 0.005% or less, REM: 0.02% or less, and Mg: 0.005% or less Ca, REM, and Mg are elements that contribute to toughness improvement, and are selected according to desired characteristics. Can be contained.

  Ca is a useful element that improves toughness through refinement of crystal grains. When Ca is contained, 0.001% or more is preferably contained. However, even if the content exceeds 0.005%, the effect is saturated.

  When REM is contained, 0.002% or more is preferably contained. However, even if the content exceeds 0.02%, the effect is saturated, so when it is contained, the upper limit is made 0.02%.

  Mg is a useful element that improves toughness through refinement of crystal grains, and is preferably contained in an amount of 0.001% or more. However, even if the content exceeds 0.005%, the effect is saturated.

B: 0.005% or less B has an effect of increasing the strength of steel through improvement of hardenability. In addition, during high heat input welding, an element useful for improving toughness by suppressing the fragile upper bainite phase in the heat affected zone, promoting the formation of the lower bainite phase, and fixing solute nitrogen as nitrides. It is. On the other hand, the content exceeding 0.005% remarkably increases the hardenability and brings about deterioration of the toughness and ductility of the base material. For this reason, when it contains, it is 0.005% or less. In addition, Preferably, it is 0.0003 to 0.0020%.

  The balance other than the above components is Fe and inevitable impurities.

<Microstructure>
In the present invention, the microstructure has a concentrated region where C is 0.2 to 1.0% by mass, and the concentrated region has an average equivalent circle diameter of 1.0 to 5.0 μm and an area fraction. It is characterized by containing 5 to 15%.

  The C-enriched region tends to be a martensite structure or island-like martensite containing residual γ in both the base material structure and the HAZ structure, and has a high dislocation density. Further, the concentration of C results in a hard phase that is harder than the parent phase described later. Therefore, TS is improved and movable dislocations introduced in a large amount suppress the increase in YP, which is effective for achieving both high strength and a low yield ratio.

  If there is a C-concentrated region in the microstructure of the base metal, the C-concentrated region is exposed to a high temperature during welding, so C diffuses and the C concentration gradually decreases. And finally it falls to the C component value of the base material. However, when the time of exposure to a high temperature is short, C does not diffuse sufficiently and a coarse C-enriched region remains in the HAZ structure.

  Here, as the temperature is lowered from the high-temperature austenite region, ferrite nucleates from a portion having a low C concentration or an old γ grain boundary. And since the austenite is the most stable at the place where the C concentration is high, the ferrite transformation is finally performed. On the other hand, C is expelled to the untransformed austenite side in the course of ferrite transformation (ferrite can contain C only up to about 0.02%, and austenite contains several percent C. As the ferrite transformation progresses, the C concentration in the untransformed austenite becomes higher and higher. Finally, C in the untransformed austenite in which the C concentration is high, that is, the C enriched region is transformed into island martensite. At this time, if the C enriched region is coarse, the island-like martensite formed in the HAZ structure also becomes large. Therefore, if the C-enriched region of the base material is small, the island-like martensite that can be finally formed into the HAZ structure is also small.

  In other words, if the C-enriched region, which tends to be island martensite, is finely dispersed in the microstructure of the base metal, the area fraction of island martensite generated in the high heat input weld heat affected zone is reduced. As a result, the toughness of the high heat input weld heat-affected zone having a welding heat input exceeding 400 kJ / cm can be ensured.

  In the present invention, the microstructure of the base material has a concentration region where C is 0.2 to 1.0% by mass, and the concentration region has an average equivalent circle diameter of 1.0 to 5.0 μm. By including 5-15% in a certain area fraction, the area fraction of the island-like martensite produced | generated in a high heat input welding heat affected zone can be reduced, and it can refine | miniaturize. As a result, it is possible to achieve toughness securing of the high heat input heat affected zone such that the welding heat input exceeds 400 kJ / cm.

  In the concentrated region, if C is less than 0.2% by mass, the above-described effects of increasing strength and reducing the yield ratio cannot be obtained, and if it exceeds 1.0% by mass, the base material ductility and low temperature toughness are obtained. Deteriorates.

  Further, when the average equivalent circle diameter of the C enriched region is less than 1.0 μm, the above-described effects of high strength and low yield ratio cannot be obtained. On the other hand, if it exceeds 5.0 μm, the toughness of the welded portion deteriorates. For this reason, the average equivalent circle diameter is limited to a range of 1.0 to 5.0 μm.

  Further, if the area fraction of the C-enriched region is less than 5%, the above-mentioned effects of increasing the strength and reducing the yield ratio cannot be obtained, while if it exceeds 15%, the ductility of the base material and the low temperature Toughness deteriorates. For this reason, the area fraction is limited to a range of 5 to 15%. In addition, Preferably, it is 6 to 12%.

  The C enriched region is obtained by controlling the cooling stop temperature and the reheating temperature, which will be described later, and by setting the Si content to 0.08% or less and the P content to 0.015% or less. If the Si content exceeds 0.08% or the P content exceeds 0.015%, the size of the C enriched region exceeds 5.0 μm and / or even if other elements and manufacturing conditions are appropriate, and / or The area fraction exceeds 15%, and the target weld zone toughness cannot be obtained. This is because when Si is 0.08% or less, cementite is likely to be generated, and when P is 0.015% or less, pearlite is likely to be generated, and it is difficult to form a coarse C-enriched region. is there.

  The area of the C enriched region can be obtained using EPMA.

  The parent phase excluding the C-concentrated region, which is likely to be island martensite, is essentially a mixed structure of bainite phase and ferrite phase, and the strength decreases when a structure such as pearlite and cementite is mixed. The smaller the area fraction of these tissues, the better. However, when the structure of pearlite, cementite, or the like is 15% or less in area fraction, the influence can be ignored, so it may be contained. From the viewpoint of securing strength, the area fraction of the bainite phase is preferably 60% or more.

<Manufacturing method>
Next, a manufacturing method will be described. It should be noted that the temperature is a temperature at a half-thickness position unless particularly limited. The temperature at the center of the plate thickness is obtained by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer. Further, the temperature condition in the cooling condition after rolling is the temperature at the center of the plate thickness, and the cooling rate is also an average cooling rate calculated based on the temperature at the center of the plate thickness.

Heating the steel material to 1000 ° C. to 1250 ° C. The molten steel having the above-described composition is melted by a conventional method such as a converter, electric furnace, vacuum melting furnace or the like, and the obtained steel material is heated to 1000 ° C. to 1250 ° C. If the heating temperature is less than 1000 ° C., the deformation resistance in hot rolling becomes high, and the amount of reduction per pass cannot be made large. Therefore, the number of rolling passes increases and the rolling efficiency decreases, and the steel material ( The casting defect in the slab may not be able to be crimped. On the other hand, when the reheating temperature exceeds 1250 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. For this reason, the reheating temperature of a steel raw material shall be the range of 1000-1250 degreeC.

Cumulative rolling reduction in the temperature range of 950 ° C. or less at the steel sheet surface temperature is 30% or more. Next, in order to refine the microstructure, the cumulative rolling reduction in the temperature range of 950 ° C. or less at the steel sheet surface temperature is 30% or more. Hot rolling is performed. If the cumulative rolling reduction in the temperature range of 950 ° C. or lower is less than 30%, the microstructure becomes coarse, the desired structure cannot be refined, and the desired high toughness cannot be ensured. For this reason, the cumulative rolling reduction in the temperature range of 950 ° C. or less at the steel sheet surface temperature in hot rolling is limited to 30% or more. In addition, in the case of a very thick steel plate having a plate thickness exceeding 80 mm, it is desirable to secure at least one rolling pass with a reduction rate of 15% or more per pass for the Zaku pressure bonding.

Rolling end temperature: 900 ° C or less at the steel sheet surface temperature Ar ₃ transformation point or more If the rolling end temperature exceeds 900 ° C at the steel sheet surface temperature, the microstructure becomes coarse and the desired base metal toughness cannot be secured, and the hardenability increases. Therefore, a desired microstructure cannot be secured. On the other hand, when the rolling end temperature is less than the Ar ₃ transformation point at the steel sheet surface temperature, a ferrite phase is generated and coarsened during rolling or immediately after rolling, and the toughness of the base material is lowered. For this reason, the rolling end temperature was limited to 900 ° C. or lower and Ar ₃ transformation point or higher at the steel sheet surface temperature.

Cooling from the temperature range above the Ar ₃ transformation point to the temperature range of Ar ₃ -300 to Ar ₃ -150 ° C. at an average cooling rate of 5 to 100 ° C./s After rolling, the resulting thick steel plate has an Ar ₃ transformation point. at an average cooling rate of 5 to 100 ° C. / s from the above temperature _range, cooled to cooling stop temperature of Ar ₃ -350~Ar 3 -150 ℃. If the cooling rate after the end of rolling is less than 5 ° C./s, the microstructure after accelerated cooling becomes a ferrite main structure, and the formation of island martensite is also inhibited, so that it is impossible to secure a yield strength of 650 MPa or more. . On the other hand, when the cooling rate exceeds 100 ° C./s, it becomes difficult to control the temperature at each position in the steel sheet, and material variations are likely to occur in the sheet width direction and the rolling direction. Occurs.

The cooling stop temperature is a particularly important control factor in the production method of the present invention. When the cooling stop temperature is lower than Ar ₃ -300 ° C., the bainite transformation is completed at the time of cooling stop and there is no residual austenite, and there is no generation of a hard phase from the residual austenite at the time of subsequent reheating and air cooling. Since it becomes impossible to make a C enriched region, the yield ratio of 85% or less cannot be satisfied. On the other hand, when the cooling stop temperature after cooling becomes higher than Ar ₃ -150 ° C., the bainite transformation does not proceed at the time of cooling stop, and the diffusion of C to the retained austenite does not proceed. The above yield strength and yield ratio of 85% or less cannot be satisfied.

The above-described cooling rate after rolling is in the average cooling rate range of 5 to 100 ° C./s and the accelerated cooling stop temperature satisfies the range of Ar _{3 to} 300 ° C. to Ar _{3 to} 150 ° C. In addition, a microstructure in which retained austenite is finely dispersed in the bainite main structure is obtained.

Cooling stop temperature + 100 ° C to Ac ₁ Transformation point temperature range is reheated at a rate of temperature rise of 0.5 ° C / s or more, then held for 0.5 to 3 minutes, air cooling After the completion of accelerated cooling, After reheating at a temperature increase rate of 0.5 ° C./s or higher to a temperature range of stop temperature + 100 ° C. to less than Ac ₁ transformation point, hold for 0.5 to 3 minutes and air cool. Since reheating is performed from the state of the microstructure in which the untransformed γ is finely dispersed in the bainite main structure, the concentrated region of C of the obtained base material can be finely dispersed.

  If the rate of temperature rise is less than 0.5 ° C./s, it takes a long time to reach the desired reheating temperature, so that the production efficiency is lowered, and pearlite transformation occurs, so that island-like martensite is not generated and the yield ratio is 85. % Or less cannot be satisfied.

Further, if the cooling stop temperature is less than + 100 ° C., C cannot be sufficiently concentrated to the untransformed γ. On the other hand, when the reheating temperature is equal to or higher than the Ac ₁ transformation point, the desired yield strength of 650 MPa or more cannot be satisfied due to softening of bainite.

  The holding time after reheating is set to 0.5 to 3 min so as not to inhibit the productivity. As means for reheating, atmospheric furnace heating, gas flame, induction heating and the like can be used, and in consideration of economy and controllability, induction heating is preferable.

The above-described reheating and air cooling to the Ac ₁ transformation point or less at a temperature rising rate of 0.5 ° C./s or more causes C to diffuse into the finely dispersed residual austenite to form island martensite. As a result, a high strength and a yield strength of 650 MPa or more and a low yield ratio of 85% or less can be achieved.

The Ar ₃ transformation point can be obtained by the following equation (2).
Ar ₃ = 868-396C + 25Si-68Mn-21Cu-36Ni-25Cr-30Mo (2)
However, C, Si, Mn, Cu, Ni, Cr, and Mo are contents (mass%) of each alloy element, and set to 0 when not contained.

Further, the Ac ₁ transformation point can be obtained by the following equation (3).
Ac ₁ = 751-27C + 18Si-12Mn-23Cu-23Ni + 24Cr + 23Mo-40V-6Ti + 233Nb-169Al-895B (3)
However, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, and B are the contents (mass%) of each alloy element, and are 0 when not contained.

In the present invention, the steel sheet may be cooled to room temperature and then reheated and tempered. In the tempering step, toughness can be improved by tempering to 400 ° C. or higher and lower than the Ac ₁ transformation point.

As the microstructure after the tempering treatment, if the hard phase is sufficiently harder than the parent phase, the effect of achieving both high strength and a low yield ratio can be obtained. In order to obtain such an effect, the tempering temperature must be 400 ° C. or higher. However, when the temperature exceeds the Ac ₁ transformation point, the strength is lowered. Therefore, the tempering treatment is desirably performed at 400 ° C. or higher and below the Ac ₁ transformation point. The holding time is preferably 0 to 20 minutes.

  By using the steel material having the above-described composition and performing hot rolling, cooling and reheating, and air cooling under the above-described conditions, a desired C enriched region can be dispersed in the microstructure of the base material. As a result, it becomes possible to disperse and generate island martensite in the HAZ structure after welding, and the base metal characteristics with a yield strength of 650 MPa or more and a yield ratio of 85% or less, and a welding heat input of 400 kJ / cm. It is possible to produce a low-yield-ratio high-strength steel sheet that combines the high toughness of the heat-affected zone with a large heat input welding that exceeds.

  The steel materials prepared by the converter-ladder refining-continuous casting method were made into thick steel plates having various thicknesses by hot rolling-accelerated cooling-reheating-air cooling and tempering.

Table 1 shows the component composition of the steel material, and Table 2 shows the manufacturing conditions, the area fraction of the concentrated region of C, and the average equivalent circle diameter. The concentration of C was measured with EPMA. The C concentration was calculated based on a calibration curve prepared from a standard sample. The concentrated region was measured by taking an SEM structure photograph in the region measured by EPMA. In Table 1, the Ar ₃ transformation point (° C.) was determined by equation (2), and the Ac ₁ transformation point (° C.) was determined by equation (3).

  A JIS No. 4 tensile test piece was sampled from the plate thickness ½ position of each thick steel plate obtained, and a tensile test was performed in accordance with the provisions of JIS Z2241, and the tensile properties were investigated.

In addition, V-notch test specimens were collected from the plate thickness 1/2 position of each thick steel plate obtained according to JISZ2202, and subjected to Charpy impact test according to JISZ2242, and absorbed at 0 ° C. The energy (vE ₀ ) was obtained and the base metal toughness was evaluated.

Moreover, the groove | channel shown in FIG. 1 was prepared for the test plate for coupling extract | collected from each thick steel plate, and the welded joint was produced by electroslag welding (welding heat input> = 400kJ / cm). Thereafter, as shown in FIG. 2, a JIS No. 4 impact test piece having a notch position as a bond portion was taken from the welded joint portion, and a Charpy impact test was conducted at a test temperature of 0 ° C. The absorption energy (vE ₀ ) was determined. The thickness t of the joint test plate was the same as that of the base material.

The tensile strength (TS) of the base metal is 780 MPa or more, the yield strength (YP) is 650 MPa or more, the yield ratio (YR) is 85% or less, the base material toughness (vE ₀ ) is 100 J or more, and the joint bond portion at 0 ° C. Absorption energy (vE ₀ ) 47J or more was regarded as acceptable.

  The obtained results are shown in Table 3.

In all of the examples, the tensile strength is 780 MPa or more, the yield strength is 650 MPa or more, the yield ratio is 85% or less, the absorbed energy vE ₀ at 0 ° C. is 100 J or more, the strength is high, the yield ratio is low, and the toughness is high. Has material properties. Moreover, even if it is a case where a high heat input welding construction is performed, it is recognized that vE ₀ in a bond part is 47J or more and excellent high heat input heat affected zone toughness is obtained.

  On the other hand, in a comparative example that is out of the scope of the present invention, one or more of the base material strength, the yield ratio, the base material toughness, and the high heat input welding heat affected zone toughness do not satisfy the target value.

Claims (4)

Component composition is mass%, C: 0.03-0.10%, Si: 0.01-0.08%, Mn: 1.4-3.0%, P: 0.015% or less, S : 0.0050% or less, Al: 0.005 to 0.1%, Ti: 0.004 to 0.03%, N: 0.0015 to 0.0065%, defined by the following formula (1) Ceq is 0.50 to 0.70%, Ti / N is more than 2.0 and less than 4.2, the remainder is made of Fe and inevitable impurities, and the microstructure is C is 0.00. Yield characterized in that it has a concentration region of 2 to 1.0% by mass, and the concentration region is 1.0 to 5.0 μm in average circle equivalent diameter and 5 to 15% in area fraction. A low yield ratio high strength thick steel plate excellent in high heat input heat affected zone toughness having a strength of 650 MPa or more and a yield ratio of 85% or less.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
However, C, Mn, Si, Ni, Cr, Mo, and V are the contents (% by mass) of each element, and 0 when not contained.   Further, by mass, Cu: 0.1 to 1.0%, Ni: 0.1 to 2.0%, Cr: 1.5% or less, Mo: 1.0% or less, Nb: 0.1% or less V: 0.2% or less, Ca: 0.005% or less, REM: 0.02% or less, Mg: 0.005% or less, and B: 0.005% or less. The low-yield-ratio high-strength thick steel plate excellent in large heat input welding heat-affected zone toughness having a yield strength of 650 MPa or more and a yield ratio of 85% or less. The steel material having the component composition according to claim 1 or 2 is heated to 1000 to 1250 ° C, the cumulative rolling reduction in the temperature range of 950 ° C or less at the surface temperature is 30% or more, and the rolling end temperature is the surface temperature. The hot rolling is performed at 900 ° C. or lower and the Ar ₃ transformation point or higher, and then at an average cooling rate of 5 to 100 ° C./s from the temperature range of the Ar ₃ transformation point or higher, Ar _{3 to} 300 to Ar _{3 to} 150 ° C. After cooling to the cooling stop temperature, after reheating at a temperature increase rate of 0.5 ° C./s or more to a temperature range of cooling stop temperature + 100 ° C. to less than Ac ₁ transformation point, hold for 0.5 to 3 minutes, A method for producing a low-yield-ratio high-strength thick steel plate excellent in high heat input heat-affected zone toughness having a yield strength of 650 MPa or more and a yield ratio of 85% or less, characterized by air cooling. Furthermore, tempering is performed at 400 ° C. or more and less than the Ac ₁ transformation point, and the yield strength according to claim 3 is 650 MPa or more and the yield ratio is 85% or less. A method for producing a low yield ratio high strength thick steel sheet

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