JP2008274405A - High-strength steel sheet excellent in sr resistant property and deformation performance, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel sheet having X100 grade or higher showing excellent SR resistance property and deformation performance and a method for manufacturing the same. <P>SOLUTION: The steel containing, by mass%, 0.03-0.10% C, 0.01-0.5% Si, 1.5-2.5% Mn, 0.1-0.5% Mo, 0.08% or less Al, one or more elements of Ti, Nb, and V and the balance being Fe and unavoidable impurities, and having a PCM value of not less than 0.19 and not more than 0.25, and a P value of 0.20 or more, is heated to 1,100-1,300°C, hot-rolled at a rolling finishing temperature of 750°C or more, then treated with accelerated cooling at a cooling speed of 20°C/s or more to reach not less than 350°C and not more than 550°C, thereafter, reheated to 550-700°C at a temperature increase rate of 0.5°C/s or more. The PCM value is represented by the formula (1): C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×B and the P value is represented by the formula (2): [Mo]+[Ti]+[Nb]+[V]. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-strength steel sheet having a strength of X100 grade or more suitable for a structural pipe such as a riser pipe or a conductor casing pipe used in a submarine resource drilling vessel, and a method for manufacturing the same, and in particular, a stress relief annealing (SR) performed after welding. It relates to those having excellent strength and toughness even after treatment.

  In recent years, the demand for high strength steel pipes or steel materials of APIX 100 grade or higher has been increased from the viewpoint of improving operational efficiency due to pressure increase and reducing material costs, and they are used as riser steel pipes for oil or gas drilling.

  Riser steel pipes are welded to forged products (for example, connectors) with a large amount of alloying elements by circumferential welding, so SR treatment (stress relief annealing) is required, and excellent strength and toughness are required after SR treatment. Is done.

  Moreover, since there is concern about buckling as the seabed resource collection area becomes deeper, it is also required to have excellent deformability in the longitudinal direction of the pipe.

  Patent Document 1 relates to a low-cost high-strength riser steel pipe steel sheet having excellent high-temperature SR characteristics at 600 ° C. or higher and HIC resistance, and 0.30 to 1.00% Cr—0.005 to 0.0030% Ti—. It is described that an Nb essential addition system of 0.060% or less can be accelerated and cooled to obtain an X-80 class strength after SR treatment at 650 ° C.

  Patent Document 2 relates to a high-strength welded steel pipe in which the yield strength of the base metal and the weld metal is 551 MPa or more and excellent in toughness before and after the SR treatment of the welded part, and the base material is selected as an optional element for the Nb-Ti essential addition system. Steel pipes having a composition containing Cu, a yield strength of 551 MPa or more, and a weld metal having a specific component composition and a yield strength of 551 MPa or more are described.

Patent Document 3 relates to a high-strength steel plate that is APIX80 or higher and has excellent SR resistance without containing a large amount of alloying elements. One or two of Ti, Nb, and V are added to a 0.1 to 0.5% Mo essential additive system. It is described that steel added with seeds or more is hot-rolled at a finish rolling temperature of 750 ° C. or more, accelerated cooling, and then reheated at 550 to 700 ° C. to disperse and precipitate fine carbides of 10 nm or less.
Japanese Patent Laid-Open No. 11-50188 JP 2001-158939 A JP 2004-269964 A

  However, since the steel sheet described in Patent Document 1 compensates for the strength reduction due to the SR treatment by precipitation of Cr carbide during the SR treatment, a large amount of Cr is necessary, and not only the material cost is high, but also the weldability and toughness. There is concern about the decline.

  Further, in order to compensate for the strength reduction due to tempering, the method of precipitating Cr carbide or the like during the SR treatment easily coarsens the carbide and causes a decrease in toughness.

  The steel pipe described in Patent Document 2 is mainly aimed at improving the characteristics of the seam weld metal. No special consideration is given to the base metal, and a reduction in the base metal strength due to the SR treatment cannot be avoided. It is necessary to increase the strength before SR treatment by accelerated cooling.

  The steel sheet described in Patent Document 3 contains expensive Mo in an amount of 0.1 to 0.5%, has a high production cost, and causes a stable precipitation of finely dispersed carbide.

  Accordingly, the present invention solves the above-mentioned problems of the prior art, and is a high-strength steel plate of APIX100 grade or higher, which has excellent SR resistance and high deformation performance without adding a large amount of alloy elements. It aims at providing the manufacturing method of.

  By the way, since the chemical composition of the steel plate for welded steel pipes and the steel plate for welded structures is severely restricted from the viewpoint of weldability, high-strength steel sheets of X65 grade or higher are manufactured by accelerated cooling after hot rolling.

  Therefore, the microstructure is mainly bainite or a structure containing island martensite (MA) in bainite. However, when SR treatment is applied to a microstructure steel having a high area ratio of second-phase cementite or MA. Further, since the cementite structure or MA in bainite is decomposed by tempering, a decrease in strength is inevitable.

  Thus, it is difficult to ensure strength and toughness after SR treatment without controlling the second phase structure. In order to achieve both high strength and excellent SR resistance, the present inventors have made a steel material by SR treatment. Detailed studies on the microstructural changes of the following were made and the following findings were obtained. Excellent SR resistance refers to small changes in strength and toughness before and after SR treatment.

  a) In order to obtain excellent SR resistance, the microstructure of the steel needs to be a microstructure that allows precipitation of alloy carbides even if the shape changes before and after the SR treatment, and is decomposed by the SR treatment. It is effective to suppress MA to 10% or less and to disperse and precipitate carbon in steel as thermally stable fine carbides. When the YR of the steel sheet before SR treatment is 90% or less, the MA area ratio is controlled to be 2% or more.

  b) A composite carbide composed of one or more of Ti, Nb, and V and Mo and a very fine precipitate of 10 nm or less is thermally stable.

  c) The above-mentioned fine carbide contains Mo as an essential additive, and a steel having one or two or more alloy components of Ti, Nb, and V is directly cooled below the bainite transformation start temperature in the process of cooling by hot cooling after hot rolling. Is obtained by stopping the cooling and immediately performing rapid reheating.

  d) That is, immediately after the cooling of accelerated cooling is stopped, a bainite structure containing untransformed austenite is formed, but the generation of cementite is suppressed by Mo, and carbon exists in a supersaturated state.

  Thereafter, when reheating is performed, supersaturated carbon is combined with Mo, Ti, Nb or V, and is preferentially deposited on the dislocations as fine carbides. Furthermore, untransformed austenite containing supersaturated carbon changes to MA in the cooling process after reheating.

  e) A steel having a metal structure in which a composite carbide composed of one or more of Ti, Nb, and V and Mo and Mo is dispersed and precipitated and has a certain fraction of MA is high before SR treatment. Strength is obtained.

  In addition, heating at about 700 ° C. does not cause the fine carbides to decompose or coarsen, and the high strength is maintained by the decomposition of MA and subsequent precipitation of new alloy carbides. Maintained. By making the structure before SR treatment a mixed structure of hard MA and soft bainite that has been tempered by reheating, the steel has a low YR and has excellent deformation performance.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. In mass%, C: 0.03-0.10%, Si: 0.01-0.5%, Mn: 1.5-2.5%, Mo: 0.1-0.5%, Al: ≦ 0.08%,
Further, Ti: 0.005 to 0.035%, Nb: 0.005 to 0.07%, V: 0.005 to 0.1% of one type or two or more types,
The PCM value according to the following formula (1) satisfies 0.19 or more and 0.25 or less, the P value according to the following formula (2) satisfies 0.20 or more, and consists of the balance Fe and inevitable impurities,
The microstructure of the steel sheet is a bainite structure including island-shaped martensite (MA) having an area ratio of 2% or more and 10% or less at the center of the plate thickness, and is mainly composed of Mo having an equivalent circle diameter of 10 nm or less in the entire cross section in the plate thickness direction. Mo, Nb, V, Ti) 30% or more of composite carbide per 1 μm ² is dispersed, and the total amount of precipitation is 0.03% by mass or more. High strength with excellent SR resistance and deformation performance steel sheet.
PCM value = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1)
However, each element symbol shows content (mass%).
P value = [Mo] + [Ti] + [Nb] + [V] (2)
However, each element symbol indicates atomic% of each contained element.
2. Furthermore, by mass%, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Ca: 0.0005 to 0.0035%, REM: 0.0005 to 0.01 %, B: 1 type or 2 types or less of 0.001% or less, The high-strength steel plate excellent in SR resistance property and deformation performance of Claim 1 characterized by the above-mentioned.
After heating the steel which consists of a component composition of 3.1 or 2 to the temperature of 1100-1300 degreeC, and hot-rolling at the rolling completion temperature of 750 degreeC or more, it is 350 degreeC with the cooling rate of 20 degrees C / s or more. Accelerated cooling to a temperature of 550 ° C. or lower, and then immediately reheating to 550 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or higher. Excellent in SR resistance and deformation performance A method for producing a strength steel plate.

  According to the present invention, after SR treatment, SR 100 having high strength of X100 grade or higher, YR: 90% or less, and does not buckle even when used as a riser pipe used in a deep sea area, and A method for producing a high-strength steel sheet having excellent deformation performance is obtained, which is extremely useful industrially.

The chemical composition, manufacturing method, and metal structure of the high-strength steel sheet excellent in SR resistance and deformation performance according to the present invention will be described. In the following description, all units represented by% are mass%.
[Chemical composition]
C: 0.03-0.10%
C is an element that increases the strength of steel, and in order to obtain a desired structure and obtain desired strength and toughness, it is necessary to contain 0.03% or more. On the other hand, when it contains exceeding 0.10%, weldability will deteriorate, it will become easy to produce a weld crack, and a base material toughness and HAZ toughness will fall. For this reason, C is limited to the range of 0.03-0.10%. In addition, Preferably it is 0.06-0.08%.

Si: 0.01 to 0.5%
Si is an element that acts as a deoxidizer and increases the strength of the steel by solid solution strengthening. However, if it is less than 0.01%, there is no effect, and if it exceeds 0.50%, the HAZ toughness deteriorates significantly. Let For this reason, Si is made 0.01 to 0.50%. In addition, Preferably it is 0.05 to 0.20%.

Mn: 1.5 to 2.5%
Mn is an element that has the effect of improving the hardenability of steel and improving the strength and toughness, and needs to contain 1.5% or more, but inclusion exceeding 2.5% deteriorates weldability. There is a fear. For this reason, Mn is limited to 1.5 to 2.5% of range. In addition, Preferably it is 1.8%-2.0%.

Mo: 0.1 to 0.5%
Mo is an important element in the present invention, and by containing 0.1% or more, forming a fine composite carbide with Ti, Nb, V while suppressing pearlite transformation during cooling after hot rolling, It greatly contributes to strength increase. However, if it exceeds 0.5%, the weld heat-affected zone toughness is deteriorated, so the Mo content is specified to be 0.1 to 0.5%.

Al: ≦ 0.08%
Al acts as a deoxidizing material at the time of steel making, and inclusion exceeding 0.08% causes a decrease in toughness. In addition, Preferably, it is 0.01 to 0.05%.

One or more of Ti, Nb and V Ti: 0.005 to 0.035%
Ti, like Mo, is an important element in the present invention. By adding 0.005% or more, Mo and composite carbide are formed, which greatly contributes to an increase in strength. However, addition exceeding 0.035% leads to deterioration of weld heat affected zone toughness and base metal toughness. Therefore, when Ti is added, the content is specified to be 0.005 to 0.035%.

Nb: 0.005 to 0.07%
Nb improves toughness by refining the structure, but forms a composite carbide with Mo and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.07%, the toughness of the weld heat affected zone deteriorates. Therefore, when Nb is added, the content is specified to be 0.005 to 0.07%. To do.

V: 0.005 to 0.1%
V, like Nb, forms a composite carbide with Mo and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.1%, the toughness of the weld heat affected zone deteriorates. Therefore, when V is added, the content is specified to be 0.005 to 0.1%. To do.

PCM value: 0.19 or more and 0.25 or less PCM value is expressed by the following formula (1) using the mass% of the alloy element, and if the PCM value is less than 0.19, high strength of X100 grade cannot be obtained. It is specified to be 0.19 or more. Further, if the PCM value exceeds 0.25, there is a risk of cold cracking in the weld heat affected zone during welding, so it is specified to be 0.25 or less.
PCM value = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1)
However, each element symbol shows content (mass%).

P value: 0.20 or more The P value is represented by the following formula (2). If the P value is less than 0.20, high strength of X100 grade cannot be obtained after SR heat treatment at less than 700 ° C. This is specified above. The high-strength steel sheet according to the present invention satisfies desired characteristics when both the PCM value and the P value satisfy specified values.
P value = [Mo] + [Ti] + [Nb] + [V] (2)
However, each element symbol indicates the content (atomic%).

The total amount of the above elements in atomic% is the ratio of the sum of the number of atoms of Mo, Ti, Nb, V contained in the steel and the total number of atoms of Fe, Mo, Ti, Nb, V and other alloy elements. However, it can also be obtained by the following equation (3) using the content in terms of mass% of Mo, Ti, Nb, and V. Each element symbol in the following formula (3) is a content (% by mass).
(Mo / 95.9 + Nb / 92.91 + V / 50.94 + Ti / 47.9) / (100 / 55.85) × 100 (3)
The present invention contains one or more of Cu, Ni, Cr, and Ca when further improving the strength and toughness of the steel sheet.

Cu: 0.5% or less Cu is an element effective for improving toughness and increasing strength, but if added in a large amount, weldability deteriorates, so when added, the upper limit is 0.5%.

Ni: 0.5% or less Ni is an element effective for improving toughness and increasing strength, but if added in a large amount, the SR resistance is lowered, so when added, the upper limit is 0.5%.

Cr: 0.5% or less Cr is an element effective for obtaining sufficient strength even at low C like Mn. However, if added in a large amount, the weldability deteriorates. And

Ca: 0.0005 to 0.0035%
Ca is an element effective in improving toughness by controlling the morphology of sulfide inclusions, but if it is less than 0.0005%, the effect is not sufficient, and even if added over 0.0035%, the effect is saturated, Since the toughness is deteriorated due to a decrease in the cleanliness of the steel, the Ca content is specified to be 0.0005 to 0.0035% when added.

REM: 0.0005 to 0.01%
REM is also an element effective for improving toughness by controlling the form of sulfide inclusions in steel, but if it is less than 0.0005%, its effect is not sufficient, and even if added over 0.01%, it is also effective. Saturates, but rather deteriorates the toughness due to a decrease in the cleanliness of the steel. Therefore, when added, the REM content is specified to be 0.0005 to 0.01%.

B: 0.001% or less B is effective for high strength, but if added over 0.001%, the SR characteristics deteriorate, so when added, the content is made 0.001% or less.

The balance other than the components described above consists of Fe and inevitable impurities.
[Microstructure and precipitation form of composite carbide]
The thick steel plate according to the present invention is a bainite structure containing island-shaped martensite (MA) having an area ratio of 2% or more and 10% or less in the center portion of the plate thickness. More than 30 Mo-based (Mo, Nb, V, Ti) composite carbides having an equivalent diameter of 10 nm or less are dispersed per 1 μm ² , and the total precipitation amount of the composite carbide (a composite carbide having an equivalent circle diameter of 10 nm or less) is The microstructure needs to be 0.03% by mass or more. Here, the equivalent circle diameter of the composite carbide is the diameter of the circle when the area of the composite carbide obtained by image processing is converted into the area of the circle.

  If the microstructure before SR treatment is controlled as described above, a tensile strength of 760 MPa or more is achieved before and after SR treatment while suppressing the decrease in tensile strength to 50 MPa or less even after SR treatment at less than 700 ° C. It is possible.

  In order to set the YR of the steel sheet before SR treatment to 90% or less, it is necessary to have a bainite structure including island martensite (MA) having an area ratio of 2% or more at the center of the plate thickness. On the other hand, if the area ratio exceeds 10%, the base material toughness decreases, so it is specified to be 10% or less.

  Insular martensite (MA) may decompose after SR treatment at less than 700 ° C., leading to a decrease in tensile strength. In order to suppress the amount of decrease in the tensile strength after the SR treatment to 50 MPa or less, precipitation strengthening is performed by finely dispersing a thermally stable composite carbide before and after the SR treatment.

That is, 30 or more Mo-based (Mo, Nb, V, Ti) composite carbides having an equivalent circle diameter of 10 nm or less are dispersed per 1 μm ² , and the total of the composite carbides (composite carbides having an equivalent circle diameter of 10 nm or less). The amount of precipitation needs to be 0.03% by mass or more.

  The area ratio of the island-like martensite may be identified by observing at least 10 visual fields at random with a scanning electron microscope (magnification 2000 times) at the center of the plate thickness. Further, the Mo-based composite carbide may be identified by observing at least 10 fields of view at random with a transmission electron microscope (magnification: 30000 times) at the center of the plate thickness.

Next, the manufacturing method of the high strength steel plate of this invention using the steel of the said composition is demonstrated.
[Production conditions]
In the present invention, the cooling is stopped at a temperature lower than the bainite transformation start temperature by accelerated cooling, and the rapid reheating is immediately performed, so that the control of the MA area ratio and the precipitation strengthening can be combined and used most effectively. Become.

Heating temperature: 1100-1300 ° C
If the heating temperature is less than 1100 ° C., the required strength cannot be obtained because the solid solution of the carbide is insufficient, and if it exceeds 1300 ° C., the toughness deteriorates, so the heating temperature is set to 1100 to 1300 ° C.

Hot rolling Hot rolling is performed at a rolling end temperature of 750 ° C. or higher. When the rolling end temperature is low, a soft ferrite phase is generated before accelerated cooling and the strength is lowered.

Immediately after the completion of accelerated cooling rolling, cooling is performed at a cooling rate of 20 ° C./s or more. When the cooling rate is less than 20 ° C./s, a soft ferrite phase and cementite are precipitated, so that sufficient strength cannot be obtained after accelerated cooling. Moreover, since the amount of solid solution C decreases due to precipitation of cementite, strengthening due to precipitation of fine carbides during SR treatment cannot be obtained. Therefore, the cooling rate after the end of rolling is regulated to 20 ° C./s or more.

  The cooling stop temperature is 350 ° C. or higher and 550 ° C. or lower. In the present invention, the fraction of untransformed austenite that contributes to the formation of MA is controlled by stopping cooling just below the bainite transformation start temperature after the end of rolling.

  Not only the precipitation strengthening by fine precipitates is obtained during the subsequent rapid heating, but MA is generated in the cooling process after the heating, and the YR of the steel sheet can be reduced. However, if the cooling stop temperature is less than 350 ° C., the MA area ratio is not 2% or more, and YR cannot be 90% or less.

  On the other hand, if the cooling stop temperature exceeds 550 ° C., the MA area ratio exceeds 10%, the strength increase before SR becomes significant, the strength decreases after SR, and sufficient strength cannot be obtained. Therefore, the accelerated cooling stop temperature is specified to be 350 ° C. or higher and 550 ° C. or lower.

After accelerated cooling, reheating is performed immediately after accelerated recooling to a temperature of 550 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or more. This process is an important production condition in the present invention, and fine composite carbides contributing to precipitation strengthening are precipitated during the reheating.

  In order to obtain fine composite carbide, it is rapidly reheated to a temperature range of 550 to 700 ° C. immediately after accelerated cooling. If the heating rate is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency deteriorates, and coarse cementite precipitates on the grain boundaries, and the fine composite carbide Dispersion precipitation cannot be obtained, sufficient strength cannot be obtained, and toughness also deteriorates.

  If the reheating temperature is less than 550 ° C, the amount of precipitation is not sufficient and sufficient precipitation strengthening cannot be achieved. If the reheating temperature exceeds 700 ° C, the composite carbide becomes coarse and sufficient strength and toughness cannot be obtained. Specified at 550 to 700 ° C.

  There is no need to set the temperature holding time at the reheating temperature. Even if it cools immediately after reheating if the manufacturing method of this invention is used, the high intensity | strength by fine precipitation will be obtained. Even if the cooling rate after reheating is air cooling, coarsening of the fine composite carbide does not occur. Furthermore, untransformed austenite containing supersaturated carbon changes to MA in the cooling process after reheating.

  The manufacturing equipment of the steel plate according to the present invention is not particularly specified. FIG. 1 shows an example of equipment suitable for carrying out the production method of the present invention.

  In the rolling line 1, a hot rolling mill 3, an accelerated cooling device 4, an in-line induction heating device 5, and a hot leveler 6 are arranged from upstream to downstream. By installing the in-line type induction heating device 5 or other heat treatment device on the same line as the hot rolling mill 3 as a rolling facility and the accelerated cooling device 4 as a subsequent cooling facility, the rolling and cooling can be quickly performed. Since a reheating process can be performed, it can heat without reducing the steel plate temperature after rolling cooling too much.

  Steel of chemical composition (steel types A to L) shown in Table 1 was made into a slab by a continuous casting method, and a thick steel plate (Nos. 1 to 18) having a plate thickness of 15 to 25 mm was produced using this.

  The heated slab was rolled by hot rolling and immediately cooled using a water-cooled accelerated cooling facility. Table 2 shows the production conditions of each steel plate (No. 1 to 18).

  As for the tensile properties of the steel sheet produced as described above, a tensile test was performed using a full thickness test piece in the same direction as the rolling direction as a tensile test piece, and the tensile strength was measured. According to the APIX100 standard, the yield strength before SR treatment was 690 MPa or more, the tensile strength was 760 MPa or more, and the yield ratio was 90% or less.

  The weld heat affected zone (HAZ) toughness is measured by a Charpy test using a test piece to which a heat history corresponding to a heat input of 40 kJ / cm is added using a reproducible thermal cycle apparatus, and the Charpy absorbed energy at −10 ° C. is 100 J or more. The thing was made into the scope of the present invention.

  Further, in order to investigate the SR resistance, each steel plate was subjected to SR treatment using a gas atmosphere furnace. The heat treatment was performed at 650 ° C. for 2 hours, and then removed from the furnace and cooled to room temperature by air cooling. The tensile properties and Charpy impact properties of the steel sheets before and after SR treatment were measured. The measurement results are also shown in Table 2.

  In Table 2, No. 1 as an example of the present invention. 1 to 9 all have chemical components and production methods within the scope of the present invention, and have a high strength such as a yield strength of 690 MPa or more and a tensile strength of 760 MPa or more before and after the SR treatment. The toughness was also good. In addition, the yield ratio before SR treatment was 90% or less, indicating high deformation performance.

  On the other hand, Comparative Example No. 10-15, the chemical components are within the scope of the present invention, but the production method is outside the scope of the present invention, so fine composite carbides may not be dispersed and precipitated, and the base material strength or base material toughness deteriorates. did. Moreover, when the area ratio of MA was not appropriate, the yield ratio exceeded 90%, or the strength after SR decreased.

  Comparative Example No. Since chemical components 16 to 18 are outside the scope of the present invention, sufficient base material strength and toughness cannot be obtained, or the weld heat affected zone toughness is inferior.

Schematic which shows an example of the manufacturing line for enforcing the manufacturing method of this invention.

Explanation of symbols

1: rolling line,
2: Steel plate,
3: Hot rolling mill,
4: Accelerated cooling device,
5: Inline type induction heating device,
6: Hot leveler

Claims (3)

In mass%, C: 0.03-0.10%, Si: 0.01-0.5%, Mn: 1.5-2.5%, Mo: 0.1-0.5%, Al: ≦ 0.08%,
Further, Ti: 0.005 to 0.035%, Nb: 0.005 to 0.07%, V: 0.005 to 0.1% of one type or two or more types,
The PCM value according to the following formula (1) satisfies 0.19 or more and 0.25 or less, the P value according to the following formula (2) satisfies 0.20 or more, and consists of the balance Fe and inevitable impurities,
The microstructure of the steel sheet is a bainite structure including island-shaped martensite (MA) having an area ratio of 2% or more and 10% or less at the center of the plate thickness, and is mainly composed of Mo having an equivalent circle diameter of 10 nm or less in the entire cross section in the plate thickness direction. Mo, Nb, V, Ti) 30% or more of composite carbide per 1 μm ² is dispersed, and the total amount of precipitation is 0.03% by mass or more. High strength with excellent SR resistance and deformation performance steel sheet.
PCM value = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1)
However, each element symbol shows content (mass%).
P value = [Mo] + [Ti] + [Nb] + [V] (2)
However, each element symbol indicates atomic% of each contained element.   Furthermore, by mass%, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Ca: 0.0005 to 0.0035%, REM: 0.0005 to 0.01 %, B: 1 type or 2 types or less of 0.001% or less, The high-strength steel plate excellent in SR resistance property and deformation performance of Claim 1 characterized by the above-mentioned.   After heating the steel which consists of a component composition of Claim 1 or Claim 2 to the temperature of 1100-1300 degreeC, and hot-rolling at the rolling completion temperature of 750 degreeC or more, with the cooling rate of 20 degreeC / s or more. Excellent SR resistance and deformation performance, characterized by accelerated cooling to a temperature of 350 ° C. or more and 550 ° C. or less, and then immediately reheating to 550 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or more. A method for producing high strength steel sheets.

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