JP2011074443A - Steel plate superior in strain-aging resistance with low yield ratio, high strength and high uniform elongation, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel plate which has such superior strain-aging resistance as to be an API 5L X70 grade or lower and has a low yield ratio, high strength and high uniform elongation, and to provide a manufacturing method therefor. <P>SOLUTION: The steel plate superior in strain-aging resistance with the low yield ratio, high strength and high uniform elongation has a component composition including, by mass%, 0.03-0.12% C, 0.01-1.0% Si, 1.2-3.0% Mn, at most 0.015% P, at most 0.005% S, at most 0.08% Al, 0.005-0.05% Nb, 0.10-1.0% Cr, 0.005-0.025% Ti, at most 0.010% N, at most 0.005% O, and the balance Fe with unavoidable impurities. The metal structure is formed of a three-phase structure that includes bainite, polygonal ferrite and island-shaped martensite, in which the island-shaped martensite shall have an area fraction of 3-20% and a circle equivalent diameter of at most 3.0 μm, the polygonal ferrite shall have the area fraction of 10-50% and the circle equivalent diameter of at most 20 μm, and the balance shall be bainite. The steel plate also has the uniform elongation of at most 7% and the yield ratio of at most 85%, before and after the strain-aging treatment at a temperature of at most 250°C for at most 60 min. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a low yield ratio, high strength and high uniform stretch steel sheet suitable for use mainly in the field of line pipes and a method for producing the same, and in particular, a low yield ratio, high strength and high strength excellent in strain aging resistance. The present invention relates to a uniformly stretched steel sheet and a manufacturing method thereof.

  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, a steel product for a line pipe applied to an earthquake zone or the like that may be subjected to large deformation may require a high uniform elongation performance in addition to a low yield ratio. 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.

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

  Furthermore, Patent Document 3 discloses Ti / N and Ca—O—S balance as a technique for achieving a low yield ratio and excellent weld heat affected zone toughness without greatly increasing the amount of alloying elements added to steel. A method is disclosed in which a three-phase structure of ferrite, bainite, and island martensite is formed while controlling the above.

  On the other hand, Patent Document 4 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.

  On the other hand, 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. For this reason, there is a problem that strain aging occurs due to processing strain 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 plate. On the other hand, for example, in Patent Documents 5 and 6, fine precipitation of composite carbide containing Ti and Mo, or fine precipitation 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 using a material and a method for producing the same are disclosed.

JP-A-55-97425 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.

Further, if the technique described in Patent Document 2, as the example, in order to ^{490N / mm 2 (50kg / mm} 2) or more steel in tensile strength, increasing the carbon content of the steel, Or since it is necessary to set it as the component composition which increased the additional amount of other alloy elements, not only the cost of a raw material will be raised, but the deterioration of toughness of a welding heat affected zone becomes a problem.

  Furthermore, in the technique described in Patent Document 3, the uniform elongation performance required when used in a pipeline or the like has not necessarily been clarified such as the influence of the microstructure.

  In the technique described in Patent Document 4, 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 deterioration of the weld heat affected zone toughness becomes a problem.

  In the technique described in Patent Document 5 or 6, 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, the conventional technology produces low yield ratio, high strength, high toughness steel sheets with high uniform elongation with excellent weld heat affected zone toughness without reducing productivity or raising material costs. It was difficult to do.

  Therefore, the present invention solves such problems of the prior art, and can be manufactured at high manufacturing efficiency and low cost, and has a low yield ratio, high strength, and high uniform elongation excellent in strain aging resistance below API 5L X70 grade. It aims at providing a steel plate and its manufacturing method.

  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.

  (A) In the accelerated cooling process, during the bainite transformation, cooling is stopped in a temperature region where untransformed austenite exists, and then reheating is performed from a temperature higher than the bainite transformation finish temperature (hereinafter referred to as the Bf point). The metal structure is a two-phase structure in which hard island martensite (hereinafter referred to as MA) is uniformly formed in the bainite phase, and a low yield ratio can be achieved.

  MA can be easily identified by, for example, etching with a 3% nital solution (nital: nitrate alcohol solution), followed by electrolytic etching and observing. When the microstructure of the steel sheet is observed with a scanning electron microscope (SEM), MA is observed as a white floating part.

  (B) Untransformed austenite is stabilized by adding appropriate amounts of Mn, Si, Cr, etc., so that it is possible to produce hard MA without adding a large amount of expensive alloy elements such as Cu, Ni, Mo, etc. It is.

  (C) By applying a cumulative reduction of 50% or more at 900 ° C. or less in the austenite non-recrystallization temperature range, MA can be uniformly finely dispersed, and the uniform elongation can be improved while maintaining a low yield ratio. Is possible.

(D) By starting accelerated cooling in a temperature range of (Ar ₃ −100 ° C.) to Ar ₃ temperature, 10 to 50% polygonal ferrite is generated in area fraction, and then the above (a) By performing the reheating treatment, it is possible to increase the amount of C diffusion and promote the formation of hard MA, and to improve the uniform elongation after prolonged strain aging such as 60 minutes at 250 ° C. Maintaining a low yield ratio is superior to conventional steel.

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

  In the first invention, the component composition is mass%, C: 0.03 to 0.12%, Si: 0.01 to 1.0%, Mn: 1.2 to 3.0%, P: 0 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005 to 0.05%, Cr: 0.10 to 1.0%, Ti: 0.005 to 0 0.025%, N: 0.010% or less, O: 0.005% or less, the balance being Fe and inevitable impurities, the metal structure is a three-phase structure of bainite, polygonal ferrite, and island martensite The island-shaped martensite has an area fraction of 3 to 20% and an equivalent circle diameter of 3.0 μm or less, an area fraction of the polygonal ferrite of 10 to 50% and an equivalent circle diameter of 20 μm or less, and the remainder Bainite, having a uniform elongation of 7% or more, a yield ratio of 85% or less, and 25 Low yield ratio with excellent strain aging characteristics, characterized by a uniform elongation of 7% or more and a yield ratio of 85% or less even after a strain aging treatment at a temperature of 0 ° C. or less for 60 minutes or less. It is a high-strength, high-uniform elongation steel plate.

  In the second invention, Cu is 0.5% or less, Ni is 1% or less, Mo is 0.5% or less, V is 0.1% or less, Ca is 0.0005 to 0. 5% by mass. 00%, B: One or more selected from 0.005% or less, low yield ratio, high strength, and high uniformity excellent in strain aging resistance according to the first invention It is a stretched steel sheet.

3rd invention heats the steel which has the component composition in any one of 1st or 2nd invention to the temperature of 1000-1300 degreeC, and the cumulative reduction rate in 900 degrees C or less becomes 50% or more. after hot-rolled at Ar ₃ temperature or more rolling end temperature _as, (Ar 3 -100 ° C.) or higher Ar ₃ starts accelerated cooling at a temperature below the temperature range, 500 ° C. at 5 ° C. / s or more cooling rate Accelerated cooling to ˜680 ° C., and then immediately reheating to 550 to 750 ° C. at a heating rate of 2.0 ° C./s or more. Low yield ratio and high strength and high strength It is a manufacturing method of a uniformly stretched steel sheet.

  According to the present invention, a low yield ratio high strength high uniform elongation steel sheet with strain aging resistance is produced at low cost without degrading the weld heat affected zone toughness or adding a large amount of alloying elements. be able to. 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 component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.03 to 0.12%
C contributes to precipitation strengthening as a carbide and is an important element for MA formation. However, addition of less than 0.03% is insufficient for formation of MA, and sufficient strength cannot be ensured. Since addition exceeding 0.12% deteriorates the base metal toughness and the weld heat affected zone (HAZ) toughness, the C content is set in the range of 0.03 to 0.12%. Preferably it is 0.05 to 0.08% of range.

Si: 0.01 to 1.0%
Si is added for deoxidation, but if it is added less than 0.01%, the deoxidation effect is not sufficient, and if added over 1.0%, the toughness and weldability are deteriorated, so the amount of Si is 0.1. The range is 01 to 1.0%. Preferably it is 0.01 to 0.3% of range.

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

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 added as a deoxidizing agent, but if added over 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is 0.08% or less. To do. 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, the addition of less than 0.005% is ineffective, and if added over 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%.

Cr: 0.10 to 1.0%
Cr is added to improve hardenability and promote the formation of MA in the same way as Mn. However, if it is less than 0.10%, its effect is not sufficient, and if it exceeds 1.0%, toughness and weldability deteriorate. The Cr content is in the range of 0.10 to 1.0%. From the viewpoint of further improving the weld heat affected zone toughness, the Cr content is preferably 0.10% or more and less than 0.5%.

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 by adding 0.005% or more. However, addition exceeding 0.025% leads to deterioration of the weld heat-affected zone toughness, 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 0.007% or less.

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 causes inclusions that adversely affect toughness, the amount of O is set to 0.005% or less.

  The above are the basic components of the present invention. For the purpose of further improving the strength and toughness of the steel sheet and improving the hardenability and promoting the formation of MA, the following Cu, Ni, Mo, V, Ca, B 1 type (s) or 2 or more types may be contained.

Cu: 0.5% or less Cu may not be added, but it may be added because it contributes to improving the hardenability of the steel. However, if 0.5% or more is added, toughness deterioration occurs. Therefore, when Cu is added, the amount of Cu is preferably 0.5% or less.

Ni: 1% or less Ni does not need to be added, but contributes to improving the hardenability of the steel, and in particular, since it does not cause toughness deterioration even if added in a large amount, it is effective for toughening, It may be added. However, since Ni is an expensive element, when adding Ni, the amount of Ni is preferably 1% or less.

Mo: 0.5% or less Mo does not need to be added, but is an element that improves hardenability, and is an element that contributes to strength increase by strengthening MA generation and bainite phase. Also good. However, if added over 0.5%, the weld heat-affected zone toughness deteriorates, so when added, the Mo amount is preferably 0.5% or less, and the weld heat-affected zone toughness is reduced. From the viewpoint of further improvement, the Mo amount is more preferably 0.3% or less.

V: 0.1% or less V may not be added, but V may be added because it is an element that improves hardenability and contributes to an increase in strength. In order to obtain the effect, it is preferable to add 0.005% or more. However, if added over 0.1%, the toughness of the weld heat affected zone deteriorates. It is preferable to make it 1% or less.

Ca: 0.0005 to 0.003%
Ca may be added 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 added 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. However, if added over 0.005%, the weldability is deteriorated, so when added, the amount of B is 0.005% or less. It is preferable to do.

  In addition, by optimizing Ti / N, which is the ratio of Ti amount and N amount, austenite coarsening of the weld heat affected zone can be suppressed by TiN particles, and good weld heat affected zone toughness can be obtained. Therefore, Ti / N is preferably in the range of 2 to 8, and more preferably in the range of 2 to 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.

  Next, the metal structure of the present invention will be described.

2. Regarding the metal structure In the present invention, in addition to bainite, island-shaped martensite (MA) having an area fraction of 3 to 20% and an equivalent circle diameter of 3.0 μm or less and an area fraction of 10 to 50% and an equivalent circle diameter of 20 μm or less are used. A metal structure uniformly containing polygonal ferrite is used.

  A three-phase structure in which MA and polygonal ferrite are uniformly formed in bainite, that is, a composite structure containing hard MA in soft tempered bainite and polygonal ferrite, thereby reducing the yield ratio of the steel sheet. High uniform elongation is achieved.

  In such a multiphase structure of soft tempered bainite and polygonal ferrite and hard MA, since the soft phase bears deformation, a highly uniform elongation of 7% or more can be achieved.

  The ratio of MA in the structure is an area fraction of MA (calculated from the ratio of the area of MA in an arbitrary cross section of the steel sheet in the rolling direction and the sheet width direction), and is 3 to 20%. If the area fraction of MA is less than 3%, it may be insufficient to achieve a low yield ratio, and if it exceeds 20%, the base material toughness may be deteriorated.

  Further, from the viewpoint of low yield ratio, high uniform elongation, and base metal toughness, the area fraction of MA is desirably 5 to 12%. Note that the area fraction of MA can be calculated from the area ratio occupied by MA by performing image processing on at least four microscopic microstructure photographs obtained by SEM (scanning electron microscope) observation, for example.

  Further, from the viewpoint of ensuring the toughness of the base material and improving uniform elongation, the equivalent circle diameter of MA is set to 3.0 μm or less. Note that the equivalent circle diameter of MA can be obtained as an average value of the diameters obtained by subjecting the microstructure obtained by SEM observation to image processing, obtaining the diameter of a circle having the same area as each MA, and obtaining the diameter of each MA. .

  On the other hand, the ratio of polygonal ferrite in the structure is the area fraction of polygonal ferrite (calculated from the ratio of the area of polygonal ferrite in an arbitrary cross section of the steel sheet in the rolling direction and the sheet width direction), and is 10 to 50%. And If the area fraction of polygonal ferrite is less than 10%, it may be insufficient to achieve a low yield ratio, and if it exceeds 50%, the strength of the base material may be deteriorated.

  Further, from the viewpoint of low yield ratio, high uniform elongation, and base metal strength, the area fraction of polygonal ferrite is desirably 10 to 30%. The area fraction of polygonal ferrite can be obtained, for example, by calculating from the area ratio occupied by polygonal ferrite by image processing a microstructure photograph of at least four visual fields obtained by SEM observation. .

  Further, from the viewpoint of securing the toughness of the base material and improving the uniform elongation, the equivalent circle diameter of polygonal ferrite is set to 20 μm or less. The average particle diameter of polygonal ferrite is obtained by image processing of the microstructure obtained by SEM observation, obtaining the diameter of a circle having the same area as each polygonal ferrite for each polygonal ferrite, and calculating the average of the diameters. It can be obtained as a value.

  In the present invention, in order to produce MA without adding a large amount of expensive alloy elements such as Cu, Ni, and Mo, Mn, Si, and Cr are added to stabilize untransformed austenite, and then reheat, It is important to suppress pearlite transformation and cementite formation during air cooling.

  The mechanism of MA generation in the present invention is as follows. Detailed manufacturing conditions will be described later.

After heating the slab, rolling is finished in the austenite region, and then accelerated cooling is started from a temperature range of (Ar ₃ -100 ° C.) or higher and below the Ar ₃ transformation temperature. Thereby, polygonal ferrite is first generated.

  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 polygonal ferrite, bainite, and untransformed austenite, and transformation from untransformed austenite to bainite occurs by reheating from a temperature higher than the Bf point. In the bainitic ferrite in the bainite produced in step C, the amount of C solid solution is small, so C is discharged into the surrounding untransformed austenite. This phenomenon becomes more prominent when polygonal ferrite has been generated first.

  Therefore, as the bainite transformation proceeds during reheating, the amount of C in the untransformed austenite increases. At this time, if Mn, Si, Cr, or the like, which is an austenite stabilizing element, is contained in a certain amount or more, untransformed austenite enriched with C remains even at the end of reheating, and is cooled to MA by cooling after reheating. It transforms and finally 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.

  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, by using a steel added with a certain amount of Mn, Si, and Cr, the accelerated cooling is stopped in the middle of the bainite transformation, and then reheating is performed continuously immediately thereafter. Can be generated.

  In the steel according to the present invention, the metal structure is a structure that uniformly contains a certain amount of MA in the polygonal ferrite and bainite phases, but other structures and precipitates to the extent that the effects of the present invention are not impaired. The thing containing this is also included in the scope of the present invention.

  Specifically, when one kind or two or more kinds of pearlite or cementite are mixed, the strength is lowered. 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 3% or less, the metal structure other than polygonal ferrite, bainite and MA, that is, pearlite. Or one or more of cementite may be contained.

  The metal structure described above can be obtained by manufacturing the steel having the above-described composition by the method described below.

3. Manufacturing conditions Steel having the above-described composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and a steel material such as a slab is formed by a conventional method such as a continuous casting method or an ingot-bundling method. It is preferable to do. The melting method and the casting method are not limited to the methods described above. Thereafter, the shape is rolled into a desired shape, and after rolling, cooling and heating are performed.

  In the present invention, the heating temperature, rolling end temperature, cooling end temperature, reheating temperature, and other temperatures are the average temperature of the steel sheet. The average temperature is obtained by calculation based on the surface temperature of the slab or steel plate, taking into account parameters such as plate thickness and thermal conductivity. Moreover, a cooling rate is an average cooling rate which divided the temperature difference required for cooling to the completion | finish temperature of cooling (500-680 degreeC) after completion | finish of hot rolling by the time required to perform the cooling.

  The temperature increase rate is an average temperature increase rate obtained by dividing the temperature difference required for reheating up to the reheating temperature (550 to 750 ° C.) by the time required for reheating after cooling. Hereinafter, each manufacturing condition will be described in detail.

As the Ar ₃ temperature, a value calculated from the following equation is used.
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo

Heating temperature: 1000-1300 ° C
If the heating temperature is less than 1000 ° C, the required strength cannot be obtained because the solid solution of the carbide is insufficient, and if the heating temperature exceeds 1300 ° C, the toughness of the base metal deteriorates, so the heating temperature is in the range of 1000 to 1300 ° C.

Rolling end temperature: Ar ₃ temperature or higher If the rolling end temperature is less than Ar ₃ temperature, the subsequent ferrite transformation rate decreases, so that the concentration of C into untransformed austenite at the time of reheating becomes insufficient and MA is not generated. . Therefore, the rolling end temperature is set to Ar ₃ temperature or higher.

Cumulative rolling reduction of 900 ° C. or less: 50% or more This condition is one of important production conditions in the present invention. The temperature range of 900 ° C. or lower corresponds to the austenite non-recrystallization temperature range. Since the austenite grains can be refined by setting the cumulative rolling reduction in this temperature range to 50% or more, the number of MA production sites generated at the prior austenite grain boundaries increases, and the MA coarsening is suppressed. Contribute.

  When the cumulative rolling reduction at 900 ° C. or less is less than 50%, the equivalent circle diameter of the produced MA exceeds 3.0 μm, so that the uniform elongation may be reduced or the base metal toughness may be reduced. Therefore, the cumulative rolling reduction at 900 ° C. or less is set to 50% or more.

Cooling rate: 5 ° C / s or more, cooling stop temperature: 500-680 ° C
After the rolling, cooling is performed at a cooling rate of 5 ° C./s or more from the temperature range of (Ar ₃ −100 ° C.) to Ar ₃ transformation temperature. When the cooling start temperature is lower than (Ar ₃ -100 ° C.), the area fraction of polygonal ferrite exceeds 50%, and the strength of the base material deteriorates. If the cooling start temperature is higher than the Ar ₃ temperature, the area fraction of polygonal ferrite becomes less than 10%, and the effect of promoting the formation of MA in the bainite phase is lowered. Therefore, the cooling start temperature is set to (Ar ₃ −100 ° C.) or higher and Ar ₃ temperature or lower.

  The cooling rate is 5 ° C./s or more. When the cooling rate is less than 5 ° C./s, pearlite is generated during cooling, so that sufficient strength and low yield ratio cannot be obtained. Therefore, the cooling rate after completion of rolling is set to 5 ° C./s or more.

  In the present invention, it is possible to complete the bainite transformation during reheating without maintaining the temperature during subsequent reheating by supercooling to the bainite transformation region by accelerated cooling.

  Cooling stop temperature shall be 500-680 degreeC. This process is an important production condition in the present invention. In the present invention, C-concentrated untransformed austenite present after reheating is transformed into MA upon subsequent air cooling.

  That is, it is necessary to stop the cooling in a temperature range where untransformed austenite during the bainite transformation exists. If the cooling stop temperature is less than 500 ° C., the bainite transformation is completed, so MA is not generated during air cooling, and a low yield ratio cannot be achieved. If it exceeds 680 ° C, C is consumed in the pearlite that precipitates during cooling and MA is not generated, so the accelerated cooling stop temperature is set to 500 to 680 ° C. From the viewpoint of securing a suitable MA area fraction for giving better strength and toughness, it is preferably 550 to 660 ° C.

  Any cooling equipment can be used for this accelerated cooling.

Temperature increase rate after accelerated cooling: 2.0 ° C./s or more, reheating temperature: 550 to 750 ° C.
Immediately after the accelerated cooling is stopped, reheating is performed to a temperature of 550 to 750 ° C. at a temperature rising rate of 2.0 ° C./s or more.

  Here, reheating immediately after stopping accelerated cooling means reheating at a temperature rising rate of 2.0 ° C./s or more within 120 seconds after stopping accelerated cooling.

  This process is also an important production condition in the present invention. The untransformed austenite is transformed into bainite during reheating after the accelerated cooling, and C is discharged to the remaining untransformed austenite. Sometimes transformed into MA.

  In order to obtain MA, it is necessary to reheat from a temperature higher than the Bf point to a temperature range of 550 to 750 ° C. after accelerated cooling.

  If the heating rate is less than 2.0 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency is deteriorated and MA may be coarsened. Can't get. Although this mechanism is not necessarily clear, by increasing the heating rate of reheating to 2 ° C./s or more, the coarsening of the C-enriched region is suppressed and the coarsening of MA generated in the cooling process after reheating is increased. Is considered to be suppressed.

  When the reheating temperature is less than 550 ° C., the bainite transformation does not occur sufficiently, and the discharge of C into the untransformed austenite becomes insufficient, MA is not generated, and a low yield ratio cannot be achieved. When the reheating temperature exceeds 750 ° C., sufficient strength cannot be obtained due to the softening of bainite, so the reheating temperature range is set to a range of 550 to 750 ° C.

  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.

  In order to reliably concentrate C during bainite transformation to untransformed austenite during reheating, it is desirable to raise the temperature by 50 ° C. or more from the reheating start temperature. There is no need to set the temperature holding time at the reheating temperature.

  If the production method of the present invention is used, sufficient MA can be obtained even after cooling immediately after reheating, so that a low yield ratio and a high uniform elongation can be achieved. However, in order to further promote the diffusion of C and secure the MA volume fraction, the temperature can be maintained within 30 minutes during reheating.

  If the temperature is maintained for more than 30 minutes, recovery may occur in the bainite phase and the strength may decrease. The cooling rate after reheating is preferably basically air cooling.

  As equipment for performing reheating after accelerated cooling, a heating device can be installed downstream of the cooling equipment for performing accelerated cooling. As the heating device, it is preferable to use a gas combustion furnace or induction heating device capable of rapid heating of the steel sheet.

  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 the uniform elongation can be improved to 7% or more as compared with the prior art while maintaining a low yield ratio of 85% or less. Furthermore, in the present invention, since the coarsening of the MA is suppressed by increasing the heating rate of reheating after accelerated cooling, the equivalent circle diameter of the MA can be refined to 3.0 μm or less.

  As a result, even if a conventional steel is subjected to a thermal history that deteriorates characteristics due to strain aging, the steel of the present invention has little MA decomposition, and a predetermined metal comprising a three-phase structure of bainite, polygonal ferrite, and MA. It becomes possible to maintain the organization. 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 heat history in a general steel pipe coating process at 250 ° C. for 60 minutes. It is possible to suppress an increase in yield ratio and a decrease in uniform elongation, and even in the case of a conventional steel, even when subjected to a thermal history that deteriorates characteristics due to strain aging, the present invention steel has a uniform elongation: 7% or more. Ratio: 85% or less can be secured.

  Steels (steel types A to J) having the component compositions shown in Table 1 were made into slabs by a continuous casting method, and thick steel plates (Nos. 1 to 16) having thicknesses of 20 and 33 mm were produced.

  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 16). The heating temperature, rolling end temperature, cooling stop (end) temperature, reheating temperature, and other temperatures were the average temperature of the steel sheet. The average temperature was calculated from the surface temperature of the slab or steel plate using parameters such as plate thickness and thermal conductivity.

  The cooling rate is an average cooling rate obtained by dividing the temperature difference required for cooling to the cooling stop (end) temperature (460 to 630 ° C.) by the time required for the cooling after the hot rolling is completed. . The reheating rate (temperature increase rate) is the average temperature increase rate divided by the time required to reheat the temperature difference required for reheating up to the reheating temperature (540 to 680 ° C.) after cooling. is there.

  The mechanical properties of the steel sheet produced as described above were measured. Table 3 shows the measurement results. Tensile strength was evaluated by taking two full thickness tensile test pieces in the vertical direction of rolling, conducting a tensile test, and evaluating the average value.

  The tensile strength of 517 MPa or more (API 5L X60 or more) was determined as the strength required for the present invention. Yield ratio and uniform elongation were evaluated by the average value of two tensile test specimens taken in the rolling direction. Yield ratio of 85% or less and uniform elongation of 7% or more were defined as the deformation performance required 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. was determined to be 200 J or more.

  For the weld heat affected zone (HAZ) toughness, three specimens with a thermal history corresponding to a heat input of 40 kJ / cm were collected by a reproducible thermal cycle apparatus and subjected to a Charpy test. And the absorbed energy in -20 degreeC was measured and the average value was calculated | required. Those having Charpy absorbed energy at −20 ° C. of 100 J or more were considered good.

  In addition, after hold | maintaining the manufactured steel plate at 250 degreeC for 60 minute (s) and carrying out a strain aging treatment, the tensile test of the base material, the Charpy test, and the Charpy test of the welding heat affected zone (HAZ) were similarly implemented and evaluated. The evaluation criteria after the strain aging treatment were determined based on the same criteria as the evaluation criteria before the strain aging treatment described above.

  In Table 3, all of Nos. 1 to 7 which are invention examples are within the scope of the present invention in the component composition and production method, and before and after strain aging treatment at 250 ° C. for 60 minutes, In terms of strength, the yield ratio was 85% or less, the low yield ratio was 7% or more and the uniform elongation was high, and the toughness of the base metal and the weld heat affected zone was good.

  Moreover, the structure of the steel sheet was a structure in which MA was generated in the phase of polygonal ferrite and bainite, and the area fraction of polygonal ferrite was in the range of 10 to 50%, and the area fraction of MA was in the range of 3 to 20%. . The area fraction of polygonal ferrite and MA was determined by image processing from the microstructure observed with a scanning electron microscope (SEM).

  On the other hand, Nos. 8 to 13, which are comparative examples, have a component composition within the scope of the present invention, but the manufacturing method is outside the scope of the present invention, so the area fraction of polygonal ferrite in the steel sheet structure or circle Equivalent diameter, MA area fraction or equivalent circle diameter is outside the scope of the present invention, and yield ratio, uniform elongation is insufficient or sufficient strength either before or after strain aging treatment at 250 ° C. for 60 minutes The toughness was not obtained.

  Since No.14-16 has a component composition outside the range of the present invention, No.14 and No.15 were both out of the range of the yield ratio and uniform elongation. In No. 16, the HAZ toughness was out of the scope of the invention.

Claims (3)

Ingredient composition is mass%, C: 0.03-0.12%, Si: 0.01-1.0%,
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%, Cr: 0. 10 to 1.0%, Ti: 0.005 to 0.025%, N: 0.010% or less, O: 0.005% or less, and the balance is Fe and inevitable impurities, and the metal structure is bainite. , Polygonal ferrite and island martensite, and the island martensite has an area fraction of 3 to 20% and an equivalent circle diameter of 3.0 μm or less, and the polygonal ferrite has an area fraction of 10-50%, equivalent circle diameter of 20 μm or less, balance is bainite, uniform elongation is 7% or more, yield ratio is 85% or less, and strain aging treatment is performed at a temperature of 250 ° C. or less for 60 minutes or less. Even after application, the uniform elongation is 7% or more and the yield ratio is 8 A low-yield-ratio, high-strength, high-uniform-strength steel sheet excellent in strain aging characteristics, characterized by being 5% or less.   Further, in terms of mass%, Cu: 0.5% or less, Ni: 1% or less, Mo: 0.5% or less, V: 0.1% or less, Ca: 0.0005 to 0.003%, B: 0 The low yield ratio, high strength, high uniform stretch steel plate excellent in strain aging resistance according to claim 1, comprising one or more selected from 0.005% or less. The steel having the component composition according to claim 1 or 2 is heated to a temperature of 1000 to 1300 ° C, and Ar ₃ temperature or higher so that the cumulative rolling reduction at 900 ° C or lower is 50% or higher. After hot rolling at the rolling finish temperature of, accelerated cooling is started in a temperature range from (Ar ₃ -100 ° C.) to Ar ₃ temperature, and accelerated cooling to 500 ° C. to 680 ° C. at a cooling rate of 5 ° C./s or more. And then immediately reheating to 550 to 750 ° C. at a temperature rising rate of 2.0 ° C./s or more, and producing a low yield ratio high strength high uniform stretch steel plate excellent in strain aging characteristics Method.