JP5842577B2 - High toughness, low yield ratio, high strength steel with excellent strain aging resistance - Google Patents

Description

  The present invention relates to a high-toughness, low-yield-ratio high-strength steel sheet that is suitable mainly for use in the field of line pipes, and more particularly to a high-toughness, low-yield-ratio high-strength steel sheet excellent in strain aging resistance.

  In recent years, steel materials for welded structures are required to have a low yield ratio and high uniform elongation from the viewpoint of earthquake resistance in addition to high strength and high toughness. For example, low yield ratio and high uniform elongation performance may be required for steel materials for line pipes applied to seismic zones and the like that may be subject to large deformation. Welded steel pipes such as UOE steel pipes and ERW steel pipes used for line pipes are formed by cold forming the steel sheet into a tubular shape and welding the butt, and then the outer surface of the steel pipe is usually coated from the standpoint of corrosion protection. Further, there is a problem that strain aging occurs due to processing distortion during pipe making and heating during coating treatment, yield stress increases, and the yield ratio in the steel pipe becomes larger than the yield ratio in the steel sheet. Moreover, since the use temperature of these steel materials for line pipes is low, it is also required to have high toughness.

  In general, by making the metal structure of steel a structure in which hard phases such as bainite and martensite are moderately dispersed in ferrite, which is a soft phase, it is possible to achieve low yield ratio and high uniform elongation of steel. It is known that

  As a production method for obtaining a structure in which a hard phase is appropriately dispersed in the soft phase as described above, Patent Document 1 discloses that a phase between ferrite and austenite is provided between quenching (Q) and tempering (T). A heat treatment method for quenching (Q ′) is disclosed.

Patent Document 2 discloses a method for delaying the start of accelerated cooling until the temperature of the steel material becomes equal to or lower than the Ar ₃ transformation point at which ferrite is generated after rolling is completed at the Ar ₃ transformation point or higher as a method that does not increase the manufacturing process. Is disclosed.

As a technique for achieving a low yield ratio without performing a complicated heat treatment as disclosed in Patent Document 1 and Patent Document 2, Patent Document 3 discloses that rolling of a steel material is finished at an Ar ₃ transformation point or higher, A method has been disclosed in which a two-phase structure of acicular ferrite and martensite is achieved to achieve a low yield ratio by controlling the subsequent accelerated cooling rate and cooling stop temperature.

  Furthermore, Patent Document 4 discloses Ti / N and Ca—O—S balance as techniques for achieving a low yield ratio and excellent weld heat affected zone toughness without greatly increasing the amount of alloying elements added to the steel. A method is disclosed in which a three-phase structure of ferrite, bainite, and island martensite is formed while controlling the above.

  Patent Document 5 discloses a technique for achieving a low yield ratio and a high uniform elongation performance by adding an alloy element such as Cu, Ni, and Mo.

  For strain aging, for example, Patent Documents 6 and 7 include fine precipitates of composite carbide containing Ti and Mo, or fine precipitates of composite carbide containing any two or more of Ti, Nb, and V. A low yield ratio high strength high toughness steel pipe excellent in strain aging characteristics utilizing precipitates and a method for producing the same are disclosed.

JP-A-55-97425 JP 55-41927 A Japanese Patent Laid-Open No. 1-176027 Japanese Patent No. 40669905 JP 2008-248328 A JP 2005-60839 A Japanese Patent Laid-Open No. 2005-60840

  However, in the heat treatment method described in Patent Document 1, a low yield ratio can be achieved by appropriately selecting the quenching temperature in the two-phase region, but since the number of heat treatment steps increases, the productivity decreases, the manufacturing There is a problem that the cost increases.

  Moreover, in the technique described in Patent Document 2, since it is necessary to cool the temperature range from the end of rolling to the start of accelerated cooling at a cooling rate that is about to cool, there is a problem that productivity is extremely reduced.

Furthermore, in the technique described in Patent Document 3, in order to increase the carbon content of the steel material in order to obtain a steel material having a tensile strength of 490 N / mm ² (50 kg / mm ² ) or more, as shown in the examples. In addition, since it is necessary to have a component composition in which the added amount of other alloy elements is increased, not only the material cost is increased, but also the deterioration of the weld heat affected zone toughness becomes a problem.

  Further, in the technique described in Patent Document 4, the uniform elongation performance required when used in a pipeline or the like has not always been clarified such as the influence of the microstructure.

  In the technique described in Patent Document 5, since it is necessary to obtain a component composition in which the additive amount of the alloy element is increased, not only the material cost is increased, but also the degradation of the weld heat affected zone toughness becomes a problem.

  In the technique described in Patent Document 6 or 7, although the strain aging resistance is improved, the compatibility with the uniform elongation performance required when used in a pipeline or the like is not yet solved.

  Thus, with the conventional technology, it has been difficult to produce a high-toughness, low-yield-ratio, high-strength steel sheet having excellent strain aging characteristics without reducing productivity and raising material costs. It was.

  Accordingly, the present invention aims to solve such problems of the prior art and provide a high-toughness, low-yield-ratio, high-strength steel sheet excellent in strain aging characteristics that can be produced at high production efficiency and low cost. To do.

  In order to solve the above-mentioned problems, the present inventors diligently studied a manufacturing process of a steel sheet, particularly a manufacturing process of controlled rolling and accelerated cooling after controlled rolling, and subsequent reheating, and as a result, obtained the following knowledge.

By making the metal structure of the steel sheet in the accelerated cooling process into a two-phase structure in which hard island martensite (hereinafter referred to as MA) is uniformly formed in the bainite phase, a low yield ratio can be achieved.
Furthermore, as a result of investigating the form of MA by changing the rate of temperature rise, reheat temperature and holding time by cooling-reheat treatment, it is important that the area fraction of the γ phase contained in MA is 10% or less. I found out. Similarly, it was found that the structure morphology, carbon concentration, and carbide content in MA can be controlled by heat treatment conditions. Based on the knowledge about this MA form control, the strain aging characteristic was examined.

  Since strain aging is affected by dislocation fixation and carbide formation due to carbon in the material, the effect of the form of MA on strain aging resistance was investigated. As a result, it was confirmed that as the C concentration in MA increases, MA exists as a stable phase, and as the amount of MA increases, the C concentration in MA tends to decrease.

  As a result of examining the C concentration and fraction of MA, a difference was observed in the anti-strain aging characteristics depending on the product of these values. When this value is large, since a large amount of excess carbon is contained in MA, the yield stress after strain aging increases and the toughness decreases due to carbide formation, and the strain aging resistance deteriorates. On the other hand, when this value is small, MA becomes unstable, yield stress increases after strain aging, and a large amount of solute carbon and precipitates in the matrix phase cause a decrease in toughness after strain aging.

  As a result of intensive studies on the C concentration in MA and the fraction of MA, the upper and lower limits of these products were set to 3.0 to 4.5. It has become possible to optimize the balance between the MA fraction and the C concentration in the MA to achieve a form that can simultaneously satisfy the low yield ratio and strain aging resistance. By this method, by controlling the carbon form in the steel, even if it is subjected to a thermal history such as yield ratio deterioration due to aging in the case of conventional steel, a stable MA form, the desired structure form even after aging And it is possible to maintain the properties.

  The present invention has been made by further studying the above findings, and the gist thereof is as follows.

  In the first invention, the component composition is mass%, C: 0.04 to 0.07%, Si: 0.01 to 1.0%, Mn: 1.2 to 3.0%, P: 0 .015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.025%, N: 0.010% or less , O: 0.005% or less, composed of the balance Fe and inevitable impurities, the metal structure is composed of a two-phase structure of bainite and island martensite, and the island martensite (hereinafter referred to as MA) The area fraction is 3 to 15%, the equivalent circle diameter is 5.0 μm or less, the area fraction of the γ phase contained in MA is 10% or less, the carbon concentration (mass%) in MA and the fraction of MA High toughness, low yield ratio, high strength with excellent strain aging characteristics, characterized by a product of rate (area%) of 3.0 to 4.5 Steel plate.

  The second invention further includes, in mass%, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Ca: 0.0005 to 0.003%, B: One or more selected from 0.005% or less, and excellent strain aging resistance according to the first aspect of the invention High toughness, low yield ratio, high strength steel sheet.

  According to the present invention, low yield ratio high strength steel sheets and steel pipes with excellent strain aging resistance, high toughness, and high uniform elongation characteristics are deteriorated in weld heat-affected zone toughness or a large amount of alloying elements are added. It can manufacture at low cost, without doing. For this reason, the steel plate mainly used for a line pipe can be stably manufactured in a large amount at a low cost, and the productivity and economy can be remarkably improved, which is extremely useful industrially.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. About a component composition The reason which prescribed | regulated the component composition of this steel material is demonstrated. In addition, all component% means the mass%.

C: 0.04 to 0.07%
C contributes to precipitation strengthening as a carbide and is an important element for the formation of MA. However, the content of less than 0.04% is insufficient for the formation of MA, and sufficient strength cannot be ensured. Since the content exceeding 0.07% deteriorates the weld heat affected zone (HAZ) toughness, the C content is set in the range of 0.04 to 0.07%.

Si: 0.01 to 1.0%
Si is contained for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 1.0%, the toughness and weldability are deteriorated, so the amount of Si is 0.00. The range is 01 to 1.0%. Preferably it is 0.01 to 0.2% of range.

Mn: 1.2-3.0%
Mn is contained in order to improve strength and toughness, further improve hardenability and promote MA formation. However, if the content is less than 1.2%, the effect is not sufficient. In addition, since the weldability deteriorates, the amount of Mn is set in the range of 1.2 to 3.0%. In order to stably produce MA regardless of changes in components and production conditions, the content is preferably 1.5% or more. More preferably, it is 1.5 to 1.8% of range.

P: 0.015% or less, S: 0.005% or less In the present invention, P and S are unavoidable impurities and define the upper limit of the amount thereof. When the P content is large, central segregation is remarkable and the base material toughness deteriorates, so the P content is 0.015% or less. If the content of S is large, the amount of MnS produced increases remarkably and the toughness of the base material deteriorates, so the amount of S is made 0.005% or less.

Al: 0.08% or less Al is contained for deoxidation, but if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is 0.08% or less. . Preferably, it is 0.01 to 0.08% of range.

Nb: 0.005 to 0.05%
Nb is an element that improves toughness by refining the structure and contributes to an increase in strength by improving the hardenability of solid solution Nb. However, if the content is less than 0.005%, there is no effect. If the content exceeds 0.05%, the toughness of the weld heat-affected zone deteriorates, so the Nb content is in the range of 0.005 to 0.05%. Preferably it is 0.01 to 0.03% of range.

Ti: 0.005 to 0.025%
Ti is an important element that suppresses austenite coarsening during slab heating and improves the base material toughness due to the pinning effect of TiN. The effect is manifested when the content is 0.005% or more.
However, if the content exceeds 0.025%, the weld heat-affected zone toughness deteriorates, so the Ti content is in the range of 0.005 to 0.025%. From the viewpoint of weld heat affected zone toughness, the range is preferably 0.005% or more and less than 0.02%.

N: 0.010% or less N is treated as an inevitable impurity, but if the N content exceeds 0.010%, the weld heat affected zone toughness deteriorates, so the N content is 0.010% or less. Preferably it is 0.005%.

O: 0.005% or less In the present invention, O is an unavoidable impurity and defines the upper limit of the amount thereof. Since O is coarse and suppresses the formation of inclusions that adversely affect toughness, the O content is 0.005% or less. Preferably it is 0.003% or less.

  The above is the basic component of the present invention, which consists of the balance Fe and unavoidable impurities, but for the purpose of further improving the strength and toughness of the steel sheet and improving the hardenability and promoting the production of MA, the following. It can contain one or more of Cu, Ni, Cr, Mo, V, Ca, B shown.

Cu: 0.5% or less Cu contributes to the improvement of hardenability of steel, but if it exceeds 0.5%, toughness deterioration occurs. Therefore, when Cu is contained, the amount of Cu is 0.5. % Or less is preferable. More preferably, it is 0.3% or less.

Ni: 1% or less Ni contributes to improving the hardenability of the steel, and in particular, even if it is contained in a large amount, it does not cause toughness deterioration, so it can be contained because it is effective for toughening. However, since Ni is an expensive element, when Ni is contained, the amount of Ni is preferably 1% or less. More preferably, it is 0.3% or less.

Cr: 0.5% or less Cr may be contained because it is an effective element for obtaining sufficient strength even at low C as in the case of Mn. In order to acquire the effect, it is preferable to contain 0.1% or more, but if it contains excessively, weldability will deteriorate, so when it contains Cr, the amount of Cr should be 0.5% or less. preferable. More preferably, it is 0.3% or less.

Mo: 0.5% or less Mo is an element that improves hardenability, and can be contained because it is an element that contributes to an increase in strength by strengthening the MA formation and the bainite phase. However, if the content exceeds 0.5%, the weld heat-affected zone toughness is deteriorated. Therefore, when Mo is contained, the amount of Mo is preferably 0.5% or less, and more preferably. 0.3% or less.

V: 0.1% or less V may not be added, but may be contained because it is an element that improves hardenability and contributes to an increase in strength. In order to acquire the effect, it is preferable to contain 0.005% or more, but if it contains more than 0.1%, the toughness of the weld heat-affected zone deteriorates. The content is preferably 0.1% or less. More preferably, it is 0.08% or less.

Ca: 0.0005 to 0.003%
Ca may be contained because it improves the toughness by controlling the form of sulfide inclusions. The effect appears at 0.0005% or more, and when it exceeds 0.003%, the effect is saturated, and conversely, the cleanliness is lowered and the toughness is deteriorated. It is preferable to set it in the range of 0.003%.

B: 0.005% or less B may be contained because it is an element that contributes to strength increase and weld heat affected zone toughness improvement. In order to obtain the effect, it is preferable to add 0.0005% or more, but if added over 0.005%, the weldability is deteriorated. Therefore, when Ca is contained, the B content is 0.005%. The following is preferable. More preferably, it is 0.0025% or less.

  In addition, by optimizing Ti / N which is the ratio of Ti amount and N amount, it is possible to suppress austenite coarsening of the weld heat affected zone by TiN particles, and to obtain good weld heat affected zone toughness. Therefore, Ti / N is preferably in the range of 2-8, and more preferably in the range of 2-5.

  The remainder other than the said component in the steel plate of this invention is Fe and an unavoidable impurity. However, the content of elements other than those described above is not rejected as long as the effects of the present invention are not impaired. For example, from the viewpoint of improving toughness, Mg: 0.02% or less and / or REM (rare earth metal): 0.02% or less can be included.

2. About metal structure In this invention, it is set as the metal structure which uniformly contains the island-like martensite (MA) of 3-15% of area fractions, and an equivalent circle diameter of 5.0 micrometers or less in addition to bainite. Further, the form of MA has a structure in which the γ fraction in MA is 10 area% or less, and the product of the MA fraction and the carbon concentration in MA is 3.0 to 4.5.

  A two-phase structure in which MA is uniformly formed in bainite, that is, a composite structure containing hard MA in soft tempered bainite achieves low yield ratio and high uniform elongation of steel sheet. doing. In such a multiphase structure of soft tempered bainite and hard MA, since the soft phase bears deformation, high uniform elongation can be achieved. Moreover, by controlling the MA structure form, the carbon form in the steel and MA stability during strain aging are controlled, and the influence of the structure due to strain aging is reduced, thereby improving the strain aging resistance.

MA area fraction: 3-15%
From the viewpoints of low yield ratio, high uniform elongation, and base metal toughness, the ratio of MA in the structure is the area fraction of MA (the area of MA in any cross section of the steel sheet in the rolling direction and the sheet width direction). (Calculated from the ratio) to 3-15%. If the area fraction of MA is less than 3%, it may be insufficient to achieve a low yield ratio, and if it exceeds 15%, the base material toughness may be deteriorated. Therefore, the area fraction of MA is 3 The range is ˜15%. More preferably, it is 5 to 12% of range.

MA equivalent circle diameter: 5 μm or less The equivalent circle diameter of MA is 5 μm or less from the viewpoint of securing toughness of the base material and improving uniform elongation. When the average diameter of MA exceeds 5 μm, toughness is deteriorated, and the equivalent circle diameter of MA is set to 5 μm or less.

Area fraction of the γ phase in the MA phase: 10% or less The γ phase in the MA contributes to a low yield ratio and uniform elongation, but if the ratio exceeds 10%, the γ phase during strain aging resistance Is age-hardened and the strain aging characteristics deteriorate, so the upper limit is made 10%.

Product of MA fraction [area%] and C concentration [mass%] in MA: 3.0 to 4.5
The product of the fraction of MA phase and the C concentration in the MA phase is important from the viewpoint of low yield ratio and toughness after strain aging. When this value is large, since a large amount of excess carbon is contained in MA, the yield stress after strain aging increases and the toughness decreases due to carbide formation, and the strain aging resistance deteriorates. Even when this value is small, MA becomes unstable, yield stress increases after strain aging, and a large amount of solid solution carbon and precipitates in the matrix phase, resulting in a decrease in toughness after strain aging. As a result of intensive studies on the C concentration in MA and the fraction of MA, the product of the MA fraction [area%] and the C concentration [mass%] in MA is in the range of 3.0 to 4.5.

Evaluation Method of MA Form The area fraction of MA can be calculated from the area ratio occupied by MA by conducting tissue observation and image processing the microstructural photograph. The equivalent circle diameter of the MA is obtained as an average value of the diameters of circles having the same area as that of each MA by obtaining an image of the tissue and obtaining the diameter of each MA. Specifically, it is necessary to perform SEM (scanning electron microscope) observation with at least four fields of view at about 1000 to 5000 times. As a sample preparation method at this time, it is only necessary to accurately recognize the target MA, and chemical etching, ion etching, colloidal silica polishing, electrolytic polishing, or the like may be performed.

  Besides direct observation by SEM, mapping by EBSD (backward electron diffraction pattern) is also possible. The fraction of the γ phase in MA can be evaluated using the above-described EBSD after the MA phase is specified. At this time, since the γ phase exists finely, it is necessary to perform mapping with a step size of 0.1 μm or less. For each sample, at least 5 or more MAs are selected, the fraction of γ amount in each MA is measured, and the average value is determined.

  The carbon concentration in MA can be evaluated by specifying the MA phase and using a WDS (wavelength dispersive X-ray spectroscopic analysis) or EDS (energy dispersive X-ray spectroscopic analysis) detector attached to the electron microscope. . At this time, the C concentration in the MA region may be determined by paying attention to the contamination under analysis and comparing the composition with a standard or the like. In particular, the WDS method has higher quantitative accuracy, but if it is concentrated by 0.1% or more, the analysis can also be performed by the EDS method.

  In the present invention, in order to generate MA without adding a large amount of expensive alloy elements such as Cu, Ni, and Mo, Mn and Si are added to stabilize untransformed austenite, and pearlite transformation and cementite are performed by heat treatment. It is important to suppress production.

About the control method of MA form Below, the example implemented by the technique by accelerated cooling and reheating in order to form a fixed amount of MA phase is described, The heat processing for implement | achieving the metal structure for implementing this invention Regarding conditions, a certain amount of C may be concentrated in the untransformed γ grains during the transformation of bainite, and a certain amount of MA phase having a particle size of 5.0 μm or less may be formed as a final structure. Not what you want.

After heating the slab, rolling is finished in the austenite region, and then accelerated cooling is started at the Ar ₃ transformation point or higher.

  In the manufacturing process in which accelerated cooling is completed during bainite transformation, that is, in a temperature range where untransformed austenite exists, and then reheated from a temperature higher than the bainite transformation finish temperature (Bf point), and then cooled, the change in microstructure is It is as follows.

  The microstructures at the end of accelerated cooling are bainite and untransformed austenite. Thereafter, when reheating is performed from a temperature higher than the Bf point, transformation from untransformed austenite to bainite occurs. In the bainitic ferrite in bainite that is formed at a relatively high temperature, the amount of C solid solution is Since there is little, C is discharged to surrounding untransformed austenite.

  Therefore, as the bainite transformation proceeds during reheating, the amount of C in the untransformed austenite increases. At this time, if Mn, Si or the like, which is an austenite stabilizing element, is contained in a certain amount or more, untransformed austenite in which C is concentrated remains even at the end of reheating, and is transformed into MA by cooling after reheating. Finally, it becomes a structure in which MA is formed in the bainite phase.

  In the present invention, after accelerated cooling, it is important to perform reheating from a temperature range in which untransformed austenite exists, and when the reheating start temperature falls below the Bf point, bainite transformation is completed and untransformed austenite does not exist. The reheating start needs to be a temperature higher than the Bf point. Further, the amount of MA formed and the amount of γ fraction C concentrated in MA can be adjusted by this cooling-reheating heat treatment.

  In the above heat treatment, it is necessary to set the cooling stop temperature at which the transformation proceeds to 450 ° C. to 550 ° C. which is equal to or higher than the Bf point, and the reheating temperature needs to delay the progress of bainite and promote carbon diffusion. It is desirable to be from 650 ° C to 650 ° C. Cooling immediately after reaching the reheating temperature, or holding for a maximum of 600 seconds between 550 ° C. and 650 ° C., the C amount and MA fraction of the MA phase can be easily adjusted within the scope of the present invention Is possible. However, if the treatment is performed at a high temperature of 650 ° C. or more for a long time of 600 seconds or more, the amount of carbide is increased and stable formation of MA is not realized. Therefore, the holding temperature is preferably 650 ° C. or less.

  In addition, the cooling after reheating is not particularly specified because it does not affect the transformation of MA, but basically it is preferably air cooling. In the present invention, steel with Mn and Si added in a certain amount is used, and accelerated cooling is stopped during bainite transformation, and then reheating is performed immediately thereafter, thereby producing hard MA without reducing production efficiency. Can be made.

  In the steel according to the present invention, the metal structure is a structure that uniformly contains a certain amount of MA in the bainite phase. However, the structure and precipitates other than bainite and MA are added to the extent that the effects of the present invention are not impaired. What is contained is also included in the scope of the present invention. Specifically, when one or more of ferrite, pearlite, cementite, and the like are mixed, the strength decreases. However, since the influence is negligible when the fraction of the structure other than bainite and MA is low, if the area fraction of the whole structure is 10% or less, the metal structure other than bainite and MA, that is, ferrite, pearlite, cementite, etc. 1 type, or 2 or more types may be contained.

  Although the shape control of the MA structure can be performed using the heat treatment described above, the structure can be realized even by adjusting the stop temperature of the accelerated cooling and the subsequent cooling temperature.

3. About the manufacturing conditions of a steel pipe The low yield ratio high-strength steel pipe excellent in buckling resistance performance according to the present invention is obtained by making a base steel plate having the above-described tensile strength characteristics into a cylindrical shape by a U press and an O press according to a conventional method. It is possible to manufacture by performing seam welding.

  The seam welding is performed by submerged arc welding on the inner surface and the outer surface one layer after the tack welding, and the flux used for the submerged arc welding is not particularly limited, and may be a molten type or a fired type. Moreover, preheating before welding or heat treatment after welding is performed as necessary.

  After seam welding, pipe expansion is performed at a pipe expansion rate of 0.4% or more and 2.0% or less according to the required roundness. When the tube expansion ratio is less than 0.4%, it is difficult to achieve the usually required roundness particularly in the case of a thick wall having a thickness of 20 mm or more. On the other hand, if it exceeds 2.0%, the strain concentration at the bond portion at the boundary between the weld metal and the weld heat-affected zone is excessively increased, and there is a concern of pipe expansion cracking. Moreover, there is a concern that the joint characteristics may deteriorate due to excessive strain introduction. From the viewpoint of improving the roundness and securing of joint strength and toughness, it is preferably 0.5 to 1.5%.

4). Strain aging treatment The temperature and time of the strain aging treatment are 250 ° C. or less and 30 minutes or less.
The effects of the present invention (uniform elongation before and after strain aging treatment and no deterioration in yield ratio) can be obtained by increasing the rate of temperature increase during CR strengthening (900 ° C or lower cumulative rolling reduction of 50% or higher) and HOP reheating. By limiting to 2 ° C./s or more and controlling the shape of MA, MA is stably present even when subjected to aging more than before, and it is possible to maintain the two-phase structure form of bainite and MA, There is little deterioration in yield ratio, uniform elongation, and toughness due to YS increase after strain aging.

  As described above, in the present invention, first, by applying a cumulative reduction of 50% or more at 900 ° C. or less in the austenite non-recrystallization temperature region, the MA generation sites are increased through the refinement of austenite grains, and the MA is increased. Uniform and fine dispersion can be achieved, and while maintaining a low yield ratio of 85% or less, uniform elongation can be improved by 7% or more, and Charpy absorbed energy at -20 ° C. can be improved by 220J or more.

  Furthermore, in the present invention, MA and carbide form control by reheating after accelerated cooling, even if subjected to a thermal history that deteriorates characteristics due to strain aging in the case of conventional steel, it consists of a two-phase structure of bainite and MA. It becomes possible to maintain a predetermined metal structure.

  As a result, in the present invention, the yield stress (YS) rises due to strain aging and is accompanied by a high temperature and a long thermal history in a general steel pipe coating process at 250 ° C. for 30 minutes. Yield ratio increase, uniform elongation, and decrease in base metal toughness can be suppressed, and even if the conventional steel receives a thermal history that deteriorates characteristics due to strain aging, the steel according to the present invention has a yield ratio of 85%. Hereinafter, uniform elongation: 7% or more, Charpy absorbed energy at −20 ° C .: 200 J or more can be ensured.

  In addition, the obtained steel sheet is made into a cylindrical shape by U press and O press according to a conventional method, and then seam welding is performed. Depending on the required roundness, the expanded pipe is 0.4% or more and 2.0% or less. If the pipe is expanded at a rate to obtain a pipe and the treatment is within 250 ° C. × 30 minutes, which is a coating treatment, a high-strength steel pipe with a low yield ratio can be realized without deteriorating the strain aging resistance.

  Steels (steel types A to I) having the composition shown in Table 1 were melted in a 100 kg steel ingot, and then a thick steel plate (No. 1 to 14) having a thickness of 25 mm was manufactured by hot rolling.

  After the heated slab was rolled by hot rolling, it was immediately cooled using a water-cooled accelerated cooling facility and reheated using an induction heating furnace or a gas combustion furnace. The induction furnace was installed on the same line as the accelerated cooling equipment.

  Table 2 shows the production conditions of each steel plate (No. 1 to 14). The heating temperature, rolling end temperature, cooling stop (end) temperature, reheating temperature, and other temperatures were the center temperature of the steel sheet. The center temperature was measured directly by inserting a thermocouple into the center of the slab or steel plate.

  The cooling rate is an average cooling rate obtained by dividing the temperature difference required for cooling to the cooling stop (end) temperature after the hot rolling is finished by the time required for the cooling. The reheating rate (temperature increase rate) is an average temperature increase rate obtained by dividing the temperature difference necessary for reheating up to the reheating temperature after cooling by the time required for reheating. The holding time is the time from when the central portion of the steel plate reaches the reheating temperature until the start of air cooling.

  Yield ratio, uniform elongation, and tensile strength were evaluated by taking two full-thickness round bar tensile test pieces in the rolling direction, conducting a tensile test, and evaluating the average values. The tensile strength of 570 MPa or more, the yield ratio of 85% or less, and the uniform elongation of 7% or more were defined as the deformation performance necessary for the present invention.

  For base metal toughness, three full-size Charpy V-notch test pieces in the vertical direction of rolling were sampled, Charpy test was performed, the absorbed energy at −20 ° C. was measured, and the average value was obtained. The absorption energy at −20 ° C. is 220 J or more.

  In addition, 2% distortion was introduce | transduced to the manufactured steel plate by tension | tensile_strength, and it hold | maintained for 30 minutes at 250 degreeC, and carried out the strain aging treatment. Then, using this material, the tensile test of the base material and the Charpy test of the Charpy test were similarly performed and evaluated. Note that the evaluation criteria after the strain aging treatment are those with a yield ratio of 88% or less, uniform elongation of 7% or more, and absorbed energy at −20 ° C. of 200 J or more, similar to the evaluation criteria before the strain aging treatment. Was passed.

  In Table 3, No. which is an invention example. As for 1-6, as for a component composition and MA form, all are in the range of this invention, before strain aging, high strength of tensile strength 570MPa or more, yield ratio 85% or less, low yield ratio of uniform elongation 7% or more The toughness of the base metal was as good as 200 J or more at −20 ° C. with a low yield ratio of 85% or less even after strain aging treatment at 250 ° C. for 30 minutes. Moreover, the structure of the steel sheet was a structure in which MA was generated in the bainite phase, and the area fraction of MA was in the range of 3 to 20%. In addition, the area fraction of MA was calculated | required by the image process from the 2000 times organization photograph observed with the scanning electron microscope (SEM). For each MA, the γ fraction in MA was measured for 10 MAs by the EBSD method, and the C concentration in the MA was determined by the WDS method.

  No. Nos. 7 to 14 are comparative examples. 7-11, the chemical composition is within the scope of the present invention, but the form of MA in the steel sheet structure is outside the scope of the present invention, and before or after the strain aging treatment at 250 ° C. for 30 minutes. In either state, the yield ratio and uniform elongation were insufficient, or good toughness was not obtained. No. Nos. 12 to 14 are outside the scope of the present invention, so the yield ratio and uniform elongation before strain aging are outside the scope of the invention. No. 14, the yield ratio after strain aging was inferior because the amount of γ phase formed in MA was large.

Claims (3)

Component composition, by mass%, C: 0.04~ 0.067%, Si: 0.01~1.0%, Mn: 1.2~3.0%, P: 0.015% or less, S : 0.005% or less, Al: 0.08% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.025%, N: 0.010% or less, O: 0.005 %, And the balance is Fe and inevitable impurities, the metal structure is a structure containing island martensite in bainite, and the area fraction of the island martensite (hereinafter referred to as MA) is 3 to 15%. The equivalent circle diameter is 5.0 μm or less, the area fraction of the γ phase contained in MA is 10% or less, and the product of the carbon concentration (mass%) in MA and the fraction of MA (area%) A high-toughness, low-yield-ratio, high-strength steel sheet excellent in strain aging resistance, characterized in that the value is 3.0 to 4.5. Furthermore, by mass%, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less , V : 0.1% or less, Ca: 0.0005 to 0.003%, B: 0 The high-toughness, low-yield-ratio, high-strength steel sheet excellent in strain aging resistance according to claim 1, comprising one or more selected from 0.005% or less. A method for producing a high toughness, low yield ratio, high strength steel sheet according to claim 1 or 2,
After the slab is heated to 1090 to 1280 ° C., it is rolled under the condition that the cumulative rolling reduction at 900 ° C. or less is 55 to 75%, the rolling end temperature is the austenite region, and the start temperature is Ar. ₃ More than the transformation point, the cooling rate is 12 to 45 ° C./s, the cooling stop temperature is 450 to 550 ° C., accelerated cooling is performed in the temperature range where untransformed austenite exists, and the reheating temperature is 550 to 650 ° C. A method for producing a high-toughness, low-yield-ratio, high-strength steel sheet, characterized by performing reheating, holding at a reheating temperature as necessary, and then cooling.