JP5413537B2 - High strength steel plate and high strength steel pipe excellent in deformation performance and low temperature toughness, and methods for producing them - Google Patents

Description

  The present invention is suitably used as a line pipe for transportation of natural gas, crude oil and the like, and in particular, a high-strength steel plate and a high-strength steel pipe excellent in deformation performance and low-temperature toughness with a large deformation tolerance against ground fluctuation and the like, and production thereof Regarding the method.

  In recent years, line pipes have become increasingly important as a long-distance transportation method for natural gas and crude oil. Line pipes are laid in various environments. For example, ground pipes are frozen in summer and winter in frozen land belts, are subject to external pressure due to ocean currents on the seabed, and are subject to ground deformation due to earthquakes. Under such an environment, the line pipe may bend and displace due to ground fluctuation or the like, and therefore, a steel pipe excellent in deformation performance that hardly causes buckling or the like even when the line pipe is deformed is desired.

  Conventionally, as steel pipes excellent in deformation performance, steel pipes that have been improved by paying attention to the work hardening index (n value) as disclosed in Patent Document 1, and those disclosed in Patent Document 2 Steel pipes have been proposed in which the yield ratio, which is the ratio of the yield strength to the tensile strength, is focused on and improved.

JP 11-279700 A JP 2005-15823 A

  Conventionally proposed techniques are techniques for improving steel sheet and steel pipes used for line pipes and the like by focusing on work hardening index and yield ratio in order to improve deformation performance.

  However, in particular, line pipes used in cold regions such as frozen land zones are required to have excellent low-temperature toughness, but for technology to obtain steel plates and steel pipes that are excellent in deformation performance and low-temperature toughness. Sufficient consideration has not been made.

  The present invention has been devised in view of the above-described problems, and is capable of suppressing the amount of reduction in thickness during deformation, a high-strength steel plate and a high-strength steel pipe excellent in deformation performance and low-temperature toughness, and these It is an object to provide a manufacturing method.

  The present inventors have intensively studied to solve the above problems. As a result, it has been found that the deformation performance of steel plates and steel pipes used in pipelines and the like can be improved by paying attention to the Rankford value.

  Conventionally, studies have not been made on steel sheets and steel pipes used for line pipes, etc., focusing on the amount of reduction in wall thickness when deformed due to ground fluctuation or the like. As an index value for evaluating the amount of reduction in thickness at the time of deformation, the Rankford value is known in the field of automobile steel sheets and the like. For steel plates and steel pipes used in pipelines, no technology has been proposed that aims to improve deformation performance by focusing on the Rankford value.

  The present inventors have intensively studied to obtain a high-strength steel plate and a high-strength steel pipe excellent in deformation performance and low-temperature toughness. As a result, in order to obtain a high-strength steel sheet and a high-strength steel pipe excellent in deformation performance and low-temperature toughness, the size of the effective crystal grain size should be optimized while optimizing the amount of texture having a predetermined crystal orientation. Was found to be particularly effective. The present inventors have further studied, and in order to optimize the amount of texture having a predetermined crystal orientation, it is particularly effective to control various production conditions including the reduction ratio during hot rolling. In particular, it has been found that the rolling reduction per rolling pass in the temperature range above the recrystallization temperature is very important.

  This invention was made | formed as a result of examination based on said knowledge, The summary is as follows.

  (1) By mass%, C: 0.03 to 0.08%, Si: 0.01 to 0.50%, Mn: 1.50 to 2.50%, P: 0.015% or less, S: 0.0050% or less, Al: 0.001 to 0.080%, N: 0.0010 to 0.0060%, Ti: 0.005 to 0.030%, Nb: 0.010 to 0.050% And the balance consists of Fe and inevitable impurities, Ceq represented by the following formula (A) is 0.35 to 0.50%, and Pcm represented by the following formula (B) is 0.15 to 0.25%, composed of a composite structure of ferrite and either one or two of bainite and martensite, the effective crystal grain size at the center of thickness being 20 μm or less, and a plate at the center of thickness X-ray random intensity ratio of {111} plane parallel to the plane is 0.5 to 5.0, {5 4} plane X-ray random intensity ratio is 1.0 to 3.0, {100} plane X-ray random intensity ratio is 3.0 or less, and each of {112} plane and {223} plane X-ray random intensity ratio Is a high-strength steel sheet excellent in deformation performance and low-temperature toughness, characterized by having a thickness of 25 mm or more and a tensile strength of 565 MPa or more.

    Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (A)

Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15
+ V / 10 + 5B (B)

  Here, C, Mn, Ni, Cu, Cr, Mo, V, Si, and B are contents in mass% of each element.

  (2) The deformation performance and low temperature toughness of (1) above, wherein the Rankford value rD in the 45 ° direction and the Rankford value rC in the sheet width direction are 1.0 or more with respect to the rolling direction of the steel sheet, respectively. Excellent high strength steel plate.

(3) Further, in mass%, V: 0.010 to 0.100%, Ni: 1.0% or less, Cu: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% Hereinafter, one or more elements selected from B: 0.0001 to 0.0020%, Ca: 0.0040% or less, Mg: 0.0017 % or less, REM: 0.005% or less A high-strength steel sheet excellent in deformation performance and low-temperature toughness as described in (1) above.

  (4) A high-strength steel pipe excellent in deformation performance and low-temperature toughness, comprising the steel plate of any one of (1) to (3).

(5) By mass%, C: 0.03 to 0.08%, Si: 0.01 to 0.50%, Mn: 1.50 to 2.50%, P: 0.015% or less, S: 0.0050% or less, Al: 0.001 to 0.080%, N: 0.0010 to 0.0060%, Ti: 0.005 to 0.030%, Nb: 0.010 to 0.050%, The balance consists of Fe and inevitable impurities, Ceq represented by the following formula (A) is 0.35 to 0.50%, and Pcm represented by the following formula (B) is 0.15. The steel slab of ˜0.25% is heated at a heating temperature of 1000 to 1150 ° C., and then the reduction rate per pass in the temperature range above the recrystallization temperature, the heating temperature is 1000 ° C. or more and less than 1050 ° C. When the heating temperature is 1050 ° C. or more and 1150 ° C. or less, the heating temperature is 10%. And 15%, further rolling a cumulative reduction ratio as 35% or more, then rolling a cumulative reduction ratio as 70% to 80% in a temperature range of less than the Ar ₃ transformation point recrystallization temperature, followed, Ar ₃ transformation A high-strength steel sheet excellent in deformation performance and low-temperature toughness characterized by being water-cooled with a temperature range from −50 ° C. to less than the Ar ₃ transformation point as a cooling start temperature and a temperature range of 200 to 500 ° C. as a cooling end temperature. Production method.

    Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (A)

Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15
+ V / 10 + 5B (B)

  Here, C, Mn, Ni, Cu, Cr, Mo, V, Si, and B are contents in mass% of each element.

(6) The steel slab is further in mass%, V: 0.010 to 0.100%, Ni: 1.0% or less, Cu: 1.0% or less, Cr: 1.0% or less, Mo : 1.0% or less, B: 0.0001 to 0.0020%, Ca: 0.0040% or less, Mg: 0.0017 % or less, REM: 0.005%, one selected from the following Or the manufacturing method of the high strength steel plate excellent in the deformation | transformation performance and low-temperature toughness of said (6) characterized by containing 2 or more types of elements.

  (7) A method for producing a high-strength steel pipe excellent in deformation performance and low-temperature toughness, characterized in that the steel plate obtained by the production method of (5) or (6) is formed into a tubular shape and the butt portion is welded.

  According to the present invention, it is possible to obtain a high-strength steel plate and a high-strength steel pipe excellent in deformation performance and low-temperature toughness that can suppress a reduction in thickness during deformation.

It is a structure | tissue photograph in the thickness center part of the steel plate of this invention.

  First, the reason for limiting the numerical range of the composition of the steel plate and steel pipe according to the present invention will be described. In the following, “%” represents “mass%”.

  C is an element necessary for ensuring the strength of steel. If the C content is less than 0.03%, the strength of the final product is insufficient. When the C content is more than 0.08%, the low temperature toughness of the base material and HAZ is remarkably lowered. Therefore, the C content is 0.03 to 0.08%.

  Si is an element that acts as a deoxidizing agent and contributes to improvement in strength. If the Si content is less than 0.01%, the strength of the final product may be insufficient. If the Si amount is more than 0.50%, the HAZ toughness is remarkably lowered. Therefore, the Si amount is set to 0.01 to 0.50%.

  Mn is an element that contributes to improving the strength of steel. If the Mn content is less than 1.50%, the strength of the final product may be insufficient. When the amount of Mn exceeds 2.50%, the low temperature toughness of the base material and the HAZ is remarkably lowered. Therefore, the Mn content is 1.50 to 2.50%. Preferably, it is 1.50 to 2.00%.

  Al is an element that serves as a deoxidizing element and contributes to the refinement of the metal structure. If the Al content is less than 0.001%, this effect cannot be sufficiently obtained. If the Al content exceeds 0.080%, Al-based non-metallic inclusions increase in the steel and the cleanliness of the steel deteriorates. Therefore, the amount of Al is limited to 0.080% or less. A preferable range is 0.001 to 0.050% or less.

  Ti is an element that precipitates as TiN in the steel to suppress the coarsening of the austenite grains of the HAZ during reheating of the slab and to refine the metal structure and improve the low temperature toughness of the base material and the HAZ. However, if the Ti content is less than 0.005%, this effect cannot be sufficiently obtained. On the other hand, if the Ti content exceeds 0.030%, the low temperature toughness deteriorates due to the coarsening of TiN and precipitation hardening by TiC. Therefore, the Ti content is 0.005 to 0.030%.

  Nb has the effect of suppressing the recrystallization of austenite during hot rolling to refine the structure and improving the low temperature toughness of the base material and HAZ, but this effect is achieved when the Nb content is less than 0.010%. Is not enough. On the other hand, if the Nb content exceeds 0.050%, the HAZ toughness and on-site weldability are adversely affected. Therefore, the amount of Nb is made 0.010 to 0.050%.

  P is an impurity inevitably contained in the steel, and the low temperature toughness of the base metal and HAZ deteriorates by causing grain boundary segregation and center segregation, but if the amount of P is 0.015% or less This is an acceptable range for low temperature toughness. Therefore, the P content is limited to 0.015% or less.

  S is an impurity inevitably contained in the steel, and the ductility and toughness are reduced by producing a sulfide that is stretched by hot rolling in the steel, but the amount of S is 0.0050% or less. If it exists, it becomes an allowable range for ductility and toughness. Therefore, the amount of S is limited to 0.0050% or less.

  N is an element that improves the low temperature toughness of the base material and HAZ by precipitating as TiN in the steel, thereby suppressing the coarsening of the austenite grains of the HAZ during reheating of the slab. If the N content is less than 0.0010%, this effect cannot be sufficiently obtained. If the N content exceeds 0.0060%, the toughness decreases due to an increase in the solid solution N content. Therefore, the N content is 0.0010 to 0.0060%.

  Moreover, in this invention, the carbon equivalent Ceq represented by the following numerical formula (A) computed from content in the mass% of C, Mn, Ni, Cu, Cr, Mo, V is 0.35-0. .50%. The carbon equivalent Ceq is a value that serves as an index of hardenability.

  If the Ceq value is less than 0.35%, the target tensile strength of 565 MPa or more cannot be obtained. In addition, when the Ceq value is more than 0.50%, MA (Martensite-Austenite Constituent: a mixture of martensite and austenite) that deteriorates toughness becomes remarkable, and the toughness deteriorates. In addition, in the following numerical formula (A), the element not contained in the steel is calculated as 0.

    Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (A)

  Moreover, in this invention, Pcm represented by the following numerical formula (B) calculated from the content in the mass% of C, Si, Mn, Cu, Cr, Ni, Mo, V, and B is 0.15. -0.25%. Pcm is a value that serves as an index of weldability.

  When the Pcm is more than 0.25%, the low temperature toughness of the base material and the HAZ deteriorates. When the Pcm is less than 0.15%, deterioration of the low temperature toughness of the base material and the HAZ can be suppressed, but the target tensile strength cannot be obtained. In addition, in the following numerical formula (B), the element not contained in the steel is calculated as 0.

Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15
+ V / 10 + 5B (B)

  The above is the reason for limiting the basic elements of the steel plate and steel pipe according to the present invention. In addition to this basic element, the balance of the steel sheet and steel pipe according to the present invention is composed of Fe and inevitable impurities.

  Moreover, the steel plate and the steel pipe according to the present invention include one or more elements selected from V, Ni, Cu, Cr, Mo, B, Ca, Mg, and REM, as necessary. It may be further contained in a numerical range as described in (1). Even if these elements are contained in the following ranges, the X-ray random intensity ratio and the Rankford value in the steel plate and steel pipe are within the ranges specified in the present invention.

  V has almost the same effect as Nb, but the effect is weaker than Nb. Moreover, it has the effect which suppresses softening of a welding part. If the amount of V is less than 0.010%, the effect of improving the low temperature toughness of the base metal and HAZ and suppressing the softening of the welded portion will be insufficient. If the amount of V exceeds 0.100%, it adversely affects the toughness of HAZ and on-site weldability. Therefore, the V amount is set to 0.010 to 0.100%.

  Ni, Cu, Cr, and Mo are elements that increase the hardenability and contribute to increasing the strength of steel. However, when the content is too large, the toughness of the HAZ and the field weldability are lowered in addition to the reduction in economic efficiency. Therefore, Ni, Cu, Cr, and Mo each have a content of 1.0% or less.

  B is an element that enhances hardenability and contributes to increasing the strength of steel. If the amount of B is less than 0.0001%, this effect cannot be sufficiently obtained. If the amount of B exceeds 0.0020%, the toughness of HAZ and on-site weldability will deteriorate. Therefore, the B amount is set to 0.0001 to 0.0020%.

  Ca and REM are elements that control the form of sulfide and contribute to the improvement of low temperature toughness. If the Ca content exceeds 0.0040% and the REM content exceeds 0.005%, a large amount of CaO-CaS and REM-CaS precipitate to form large clusters and large inclusions, which impairs the cleanliness of the steel. Also, there is a risk of adversely affecting on-site weldability. Therefore, the Ca content is 0.0040% or less, and the REM content is 0.005% or less.

Mg is an element that is dispersed and precipitated as a fine oxide and contributes to the improvement of low temperature toughness by suppressing the coarsening of the HAZ grain size. If the Mg content exceeds 0.0017 %, the toughness deteriorates due to the coarsening of the oxide. Therefore, the Mg content is 0.0017 % or less.

  Next, the reasons for limiting the metal structure, texture, thickness, tensile strength, and Rankford value (r value) of the steel sheet and steel pipe according to the present invention will be described.

  The metal structure needs to be composed of a composite structure of soft ferrite and either one or two of hard bainite and martensite in order to improve work hardening characteristics.

  Further, the metal structure needs to have an effective crystal grain size of 20 μm or less at the thickness center portion. This is because the low temperature toughness deteriorates when the effective crystal grain size exceeds 20 μm. The effective crystal grain size is a crystal grain size at a circle-equivalent diameter measured by an EBSP (Electron Backscatter Diffraction Pattern) method at a portion surrounded by a boundary of a structure having an orientation difference of 15 ° or less. Means.

  The texture needs to satisfy the conditions as described below for the X-ray random intensity ratio in order to obtain a preferable Rankford value for the steel plate and the steel pipe. In the present invention, attention is paid to the Rankford value rD in the 45 ° direction with respect to the rolling direction of the steel sheet and the Rankford value rC in the sheet width direction. By increasing the Rankford value, the deformation performance of the steel plate and the steel pipe can be enhanced.

  The crystal orientations to be described below all relate to a plane parallel to the plate surface. The X-ray random intensity ratio is a numerical value representing the degree of integration of crystal planes having respective orientations, and indicates the ratio of X diffraction intensities of crystal planes having respective orientations to a random standard sample having no texture.

  Since the texture having the {111} plane crystal orientation can increase rC and rD as it develops, it is preferably developed as much as possible. From the viewpoint of obtaining preferable rC and rD, the X-ray random intensity ratio of the {111} plane needs to be 0.5 or more. If the X-ray random intensity ratio of the {111} plane exceeds 5.0, the target value may not be obtained for the X-ray random intensity ratio of other crystal orientations. The intensity ratio is 5.0 or less.

  Since the texture having the {554} plane crystal orientation can increase rC as it develops, it is preferably developed as much as possible. From the viewpoint of obtaining preferable rC, the X-ray random intensity ratio of the {554} plane needs to be 1.0 or more. Further, if the X-ray random intensity ratio of the {554} plane is more than 3.0, a target value may not be obtained for the X-ray random intensity ratio of other crystal orientations. The line random intensity ratio is 3.0 or less.

  A texture having a {100} plane crystal orientation causes rC and rD to decrease as it grows, so it is preferable that development is suppressed as much as possible. From the viewpoint of obtaining preferable rC, the X-ray random intensity ratio of the {100} plane needs to be 3.0 or less.

  The texture having the {112} plane and {223} plane crystallographic orientation is preferably developed as much as possible because rD can be increased as it grows. From the viewpoint of obtaining a preferable rD, the X-ray random intensity ratio of each of the {112} plane and the {223} plane needs to be 0.5 or more. If the X-ray random intensity ratio of each of the {112} plane and the {223} plane exceeds 4.0, the target value may not be obtained for the X-ray random intensity ratio of other crystal orientations. } The X-ray random intensity ratio between the {223} plane and the {223} plane is 4.0 or less.

  For the X-ray random intensity ratio of the present invention, a measurement value measured by X-ray diffraction at the center of thickness is used. This is because the texture that can increase the rC and rD of the {111} plane, etc. is easy to develop in the thick surface layer part and difficult to develop in the central thickness part, so the X-ray random intensity ratio in the thick central part By making the evaluation object, it is possible to exhibit a deformation performance of a certain level or more in the entire thickness direction.

  The steel sheet has a thickness of 25 mm or more and a tensile strength of 565 MPa or more (API standard of X70 or more from the viewpoint of preventing breakage due to internal pressure when used as a line pipe while ensuring the strength required as a final product. Grade).

  In the present invention, the deformation performance improves as the Rankford value rD in the 45 ° direction and the Rankford value rC in the sheet width direction increase with respect to the rolling direction of the steel sheet. In order to reduce the risk of buckling or the like due to a decrease in thickness when the steel plate or steel pipe is deformed, rD and rC are preferably 1.0 or more, more preferably 1.1 or more.

  Next, the manufacturing method of the steel plate which concerns on this invention is demonstrated.

  First, after melting molten steel having the above-described composition by a known melting method using a converter or the like, a steel piece is obtained from molten steel obtained by a known casting method such as continuous casting.

  Subsequently, the obtained steel slab is heated to a temperature of 1000 to 1150 ° C. If the heating temperature is less than 1000 ° C., sufficient recrystallization of austenite cannot be achieved, and sufficiently high low temperature toughness cannot be obtained. When the heating temperature is higher than 1150 ° C., the austenite grains are coarsened, the effective crystal grain size is increased, and the low temperature toughness is lowered.

  Subsequently, in the temperature range above the recrystallization temperature, the reduction rate per pass, that is, the value of the cumulative reduction rate / pass number is 5 to 10% when the heating temperature is 1000 ° C. or more and less than 1050 ° C., and the heating When the temperature is 1050 ° C. or higher and 1150 ° C. or lower, the rolling is performed at 10 to 15%, and the cumulative rolling reduction is 35% or more. If the cumulative rolling reduction is less than 35%, the austenite grain size cannot be sufficiently reduced by recrystallization, the effective crystal grain size increases, and the low temperature toughness decreases.

  The rolling reduction per pass is particularly important in obtaining the target texture of the crystal orientation. Conventionally, the rolling reduction per pass has not been increased due to equipment restrictions and the like. However, in order to obtain the target structure in the steel plate and steel pipe of the present invention, the rolling reduction per pass needs to be in the above range. If the rolling reduction per pass is out of the above range, the target texture distribution cannot be obtained.

  The reduction rate of individual paths may be outside the above range due to reasons such as the pass schedule, but it is preferable that the reduction rate is in the above range in passes that are more than half of the number of passes. It is more preferable that the above range be in the pass.

Subsequently, rolling is performed at a cumulative rolling reduction of 70% or more in a temperature range not lower than the Ar ₃ transformation point and lower than the recrystallization temperature. If the cumulative rolling reduction is less than 70%, the development of the texture of the {554} plane is suppressed, the target X-ray random intensity ratio cannot be obtained, and the rC value decreases.

Subsequently, water cooling is performed with an Ar ₃ transformation point of −50 ° C. or more and less than the Ar ₃ transformation point as a cooling start temperature and a temperature range of 200 to 500 ° C. as a cooling end temperature. When the cooling start temperature is less than Ar ₃ transformation point −50 ° C., the formation of ferrite is promoted and the target strength cannot be obtained. When the cooling start temperature is equal to or higher than the Ar ₃ transformation point, the development of the texture of each of the {112} plane and the {223} plane is suppressed, and the target X-ray random intensity ratio cannot be obtained. Decreases.

  When the cooling end temperature is less than 200 ° C., it causes a decrease in productivity and a hydrogen defect. If the cooling end temperature exceeds 500 ° C., the target strength cannot be obtained. The cooling rate is not particularly limited, but is about 1 to 10 ° C./s.

The Ar ₃ transformation point is obtained from the following mathematical formula (C). C, Si, etc. in the following mathematical formula (C) mean the content of each element in mass% in steel.

Ar ₃ = 868-396 × C + 24.6 × Si-69.1 × Mn-36.1
× Ni-20.7 × Cu-24.8 × Cr + 29.6 × Mo (C)

  The steel plate thus manufactured is further formed into a tubular shape, and a steel pipe is obtained by joining the butted portions. A known UOE method, a bending roll method, or the like is used as a tube forming method for forming the steel sheet into a tubular shape, and arc welding, laser welding, or the like is used as a welding method for the butt portion.

  As mentioned above, although the example of embodiment of this invention was demonstrated in detail, said embodiment is only what showed the example of actualization in implementing this invention, and the technical scope of this invention is by these. It should not be interpreted in a limited way.

  Hereinafter, the effects of the present invention will be described with reference to examples.

  Molten steel having the composition of each steel type A to F shown in Table 1 below was melted in a converter and made into a steel piece by continuous casting. The obtained steel slab was hot-rolled and cooled under the conditions shown in Table 2 below. No. 1-5 and 8-15 steel plates, and No. 1 in which the steel plates were formed into a tubular shape and the butted portions were joined. 6-7 steel pipes were obtained. No. The diameter of the 6-7 steel pipe is 48 inches (1219.2 mm).

  About the obtained steel plate and steel pipe, the tensile strength etc. which are demonstrated below were measured. The results are shown in Table 3.

  The tensile strength was measured by cutting out a JIS No. 5 plate-like test piece whose longitudinal direction was parallel to the rolling direction from the obtained steel plate and performing a tensile test based on the method described in JIS Z2241 using this test piece. Further, the tensile strength was also obtained for the grades in the API standard. For the steel pipe, the tensile strength was measured with a full thickness test piece in the longitudinal direction of the steel pipe based on the API standard.

  The metal structure was observed with an optical microscope. Effective crystal grain size is measured by the EBSP method, the boundary of the structure having an orientation difference of 15 ° or more is regarded as a grain boundary, the area inside one crystal is obtained, and the area converted to the equivalent circle diameter is effective. The crystal grain size was evaluated.

  The X-ray random strength ratio was obtained by cutting a test piece of 10 mm in the rolling direction × 10 mm in the plate width direction from the obtained steel plate, polishing the test piece to the vicinity of the thickness center by mechanical polishing, and polishing it to a mirror surface by buffing, The strain was removed by electrolytic polishing or the like, and at the same time, the thickness center layer was adjusted to be a measurement surface, and evaluation was performed by measuring the diffraction intensity of each crystal orientation by X-ray diffraction.

  The Rankford value is obtained by cutting out a JIS No. 5 plate-shaped test piece from the obtained steel plate, a test piece whose longitudinal direction is parallel to the rolling direction, a test piece parallel to the 45 ° direction with respect to the rolling direction, and the plate width direction. Parallel test specimens were manufactured, and tensile tests based on the method described in JISZ2241 were performed using these test specimens. When the specimen was given a uniaxial tensile strain of 3%, the width strain of each test specimen and Each rankford value rC, rD, rL was evaluated from the thickness strain ratio. rL is the Rankford value in the rolling direction.

  In the Charpy impact test, a V-notch Charpy test piece is produced from the obtained steel plate from a 1/4 position in the thickness direction, and the Charpy absorbed energy when the test temperature is −40 ° C. according to the method described in JISZ2242. Evaluation was made by measuring.

  In each example, an example in which the tensile strength was 565 MPa or more was accepted, and an example in which the Charpy absorbed energy was 200 J or more was accepted for low-temperature toughness. In each table, the underline indicates that it is outside the scope of the invention.

  Production No. Nos. 1 to 7 are invention examples. Nos. 1 to 5 are steel plates, no. 6 to 7 are examples of steel pipes. In all of these, the composition, metal structure, effective crystal grain size, X-ray random strength ratio, wall thickness, and tensile strength satisfy the conditions of the present invention, the rD value is 1.0 or more, and the rC value is 1. A high-strength steel sheet excellent in low-temperature toughness with a Charpy absorbed energy of 200 J or more is obtained.

  In addition, since the composition, thickness, and manufacturing method of these all satisfy the conditions of the present invention, the metal structure, effective crystal grain size, X-ray random strength ratio, and tensile strength satisfy the conditions of the present invention. What has been obtained.

  In FIG. 1, an example of the structure | tissue photograph in the thickness center part of the steel plate of the example of an invention is shown. The photograph in FIG. Two. In the structure photograph, a white portion having no fine structure inside is ferrite, and a portion other than ferrite, which is gray as a whole and has a fine structure inside, is bainite or martensite.

  Production No. 8 to 15 are comparative examples. Production No. No. 8 is an example in which the effective crystal grain size is large because the heating temperature is high, and the low temperature toughness is deteriorated.

  Production No. No. 9 has a low rolling reduction per pass in a temperature range equal to or higher than the recrystallization temperature, so the target texture distribution was not obtained, and the rD and rC values, which are indicators of deformation characteristics, deteriorated. It is an example.

  Production No. No. 10 is an example in which the cumulative reduction rate in the temperature range above the recrystallization temperature is low, so that the austenite grain size cannot be sufficiently reduced by recrystallization, the effective crystal grain size is increased, and the low temperature toughness is deteriorated. is there.

Production No. No. 11 is an example in which the development of the texture of the {554} plane is suppressed and the rC value is deteriorated because the cumulative rolling reduction in the temperature range from the Ar ₃ transformation point to the recrystallization temperature is low.

  Production No. No. 12 is an example in which, since the cooling start temperature is high, the development of the texture of each of the {112} plane and the {223} plane is suppressed, and the rD value is deteriorated.

  Production No. No. 13 is an example in which the strength is lowered because the cooling end temperature is high.

  Production No. No. 14 is an example in which the strength, the rD value, and the rC value are deteriorated because both Ceq and Pcm are low and the rolling reduction per pass in the temperature range equal to or higher than the recrystallization temperature is low.

  Production No. No. 15 is an example in which both Ceq and Pcm are high, the effective crystal grain size is large because the heating temperature is high, and the toughness is lowered by increasing the strength.

Claims (7)

% By mass
C: 0.03-0.08%,
Si: 0.01 to 0.50%,
Mn: 1.50 to 2.50%,
P: 0.015% or less,
S: 0.0050% or less,
Al: 0.001 to 0.080%,
N: 0.0010 to 0.0060%
Ti: 0.005 to 0.030%,
Nb: 0.010 to 0.050%
And the balance consists of Fe and inevitable impurities,
Ceq represented by the following formula (A) is 0.35 to 0.50%,
Pcm represented by the following formula (B) is 0.15 to 0.25%,
Composed of a composite structure of ferrite and one or two of bainite or martensite,
The effective crystal grain size at the center of the wall thickness is 20 μm or less,
In the thickness center portion, the X-ray random intensity ratio of the {111} plane parallel to the plate surface is 0.5 to 5.0, the X-ray random intensity ratio of the {554} plane is 1.0 to 3.0, {100 } The X-ray random intensity ratio of the plane is 3.0 or less, and the X-ray random intensity ratio of each of the {112} plane and the {223} plane is 0.5 to 4.0,
The wall thickness is 25 mm or more,
A high-strength steel sheet excellent in deformation performance and low-temperature toughness, characterized by having a tensile strength of 565 MPa or more.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (A)
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15
+ V / 10 + 5B (B)
Here, C, Mn, Ni, Cu, Cr, Mo, V, Si, and B are contents in mass% of each element.   The excellent deformation performance and low temperature toughness according to claim 1, wherein the Rankford value rD in the 45 ° direction and the Rankford value rC in the sheet width direction are 1.0 or more, respectively, with respect to the rolling direction of the steel sheet. High strength steel plate. Furthermore, in mass%,
V: 0.010-0.100%
Ni: 1.0% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less B: 0.0001 to 0.0020%
Ca: 0.0040% or less,
Mg: 0.0017 % or less,
REM: 1 type or 2 or more types of elements chosen from 0.005% or less are contained, The high strength steel plate excellent in the deformation performance and low-temperature toughness of Claim 1 characterized by the above-mentioned.   A high-strength steel pipe excellent in deformation performance and low-temperature toughness, comprising the steel plate according to any one of claims 1 to 3. % By mass
C: 0.03-0.08%,
Si: 0.01 to 0.50%,
Mn: 1.50 to 2.50%,
P: 0.015% or less,
S: 0.0050% or less,
Al: 0.001 to 0.080%,
N: 0.0010 to 0.0060%
Ti: 0.005 to 0.030%,
Nb: 0.010 to 0.050%
And the balance consists of Fe and inevitable impurities,
Ceq represented by the following formula (A) is 0.35 to 0.50%,
A steel slab having a Pcm of 0.15 to 0.25% represented by the following formula (B) is heated at a heating temperature of 1000 to 1150 ° C,
Subsequently, in the temperature range above the recrystallization temperature, the reduction rate per pass is 5 to 10% when the heating temperature is 1000 ° C. or higher and lower than 1050 ° C., and the heating temperature is 1050 ° C. or higher and 1150 ° C. or lower. Is 10 to 15%, and further rolled with a cumulative rolling reduction of 35% or more,
Next, rolling is performed at a cumulative reduction ratio of 70 to 80% in a temperature range of not less than the Ar ₃ transformation point and less than the recrystallization temperature,
Subsequently, the deformation performance and the low temperature toughness are characterized in that the temperature range from Ar ₃ transformation point −50 ° C. to less than Ar ₃ transformation point is the cooling start temperature, and the water temperature is 200 to 500 ° C. as the cooling end temperature. A method for producing excellent high-strength steel sheets.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (A)
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15
+ V / 10 + 5B (B)
Here, C, Mn, Ni, Cu, Cr, Mo, V, Si, and B are contents in mass% of each element. The billet is further in mass%,
V: 0.010-0.100%
Ni: 1.0% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less B: 0.0001 to 0.0020%
Ca: 0.0040% or less,
Mg: 0.0017 % or less,
REM: The manufacturing method of the high strength steel plate excellent in the deformation | transformation performance and low-temperature toughness of Claim 5 characterized by including 1 type, or 2 or more types of elements chosen from 0.005% or less.   A method for producing a high-strength steel pipe excellent in deformation performance and low-temperature toughness, characterized in that a steel plate obtained by the production method according to claim 5 or 6 is formed into a tubular shape and a butt portion is welded.