JP2015189984A - Low yield ratio high strength and high toughness steel plate, method for producing low yield ratio high strength and high toughness steel plate, and steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio high strength and high toughness steel plate showing high toughness in such a manner that Charpy impact absorption energy at -20°C is 200 J or higher, and having a low yield ratio of 85%, and in which the low yield ratio and high toughness can be made consistent even in the case of API 5L×80 or higher.SOLUTION: Provided is a low yield ratio high strength and high toughness steel plate having a steel structure made of bainite and island martensite, and having a steel structure where the area ratio of the island martensite is 2 to 15%, the number of the island martensite having a major axis of 3.5 μm or higher is 0.010 pieces/μmor lower by number density, in which yield ratio is 85% or lower and Charpy impact absorption energy at -20°C is 200 J or higher.

Description

  The present invention relates to a low yield ratio high strength high toughness steel sheet that can be preferably used as a material for a line pipe with little material deterioration after a coating treatment at 300 ° C. or less, a method for producing the low yield ratio high strength high toughness steel sheet, and low yield The present invention relates to a steel pipe manufactured using a high strength and high toughness steel plate.

  In recent years, line pipes used for transportation of natural gas and crude oil are required to have high strength in order to improve transportation efficiency by increasing pressure and to improve local welding efficiency by reducing wall thickness. In particular, when a ductile crack occurs in a natural gas transportation pipeline, the ductile crack progresses and large-scale damage is assumed to occur. In order to suppress the crack propagation speed of the ductile cracks, the steel sheet as the material of the line pipe is required to have a high absorption energy in the Charpy impact test (hereinafter sometimes referred to as Charpy impact absorption energy). That is, high toughness is required.

  In addition to high strength and high toughness, the steel plate used as the material for the line pipe is also required to have a low yield ratio (hereinafter sometimes referred to as YR) from the standpoint of ground fluctuation. .

  In general, as a method of reducing the yield ratio, a method of adjusting the steel structure of a steel plate is known. Specifically, it is known that it is effective to have a structure in which hard phases such as bainite and martensite are appropriately dispersed in a soft phase such as ferrite.

  As a production method for obtaining a structure in which a hard phase is moderately dispersed in the soft phase as described above, Patent Document 1 discloses a heat treatment method in which quenching is performed between two phases of ferrite and austenite between quenching and tempering. It is disclosed.

As a technique for achieving a low yield ratio without performing the complex heat treatment disclosed in Patent Document 1, Patent Document 2 discloses that rolling of a steel sheet is finished at an Ar ₃ temperature or higher, and the subsequent accelerated cooling rate and cooling stop temperature. A method of controlling is disclosed. According to the method of Patent Document 2, it is said that a low yield ratio can be achieved with a two-phase structure of acicular ferrite and martensite.

  In Patent Documents 3 and 4, a hard phase such as island martensite (hereinafter, sometimes referred to as MA) is dispersed in ferrite or bainite, so that low YR and high It is disclosed that the Charpy impact absorption energy is compatible.

JP-A-55-97425 Japanese Patent Laid-Open No. 1-176027 JP 2010-219758 A

  However, the technique described in Patent Document 1 has a problem that the heat treatment process increases, so that the productivity is lowered and the manufacturing cost is increased.

Moreover, in the technique described in Patent Document 2, as shown in the examples, in order to obtain a steel sheet having a tensile strength of 490 N / mm ² (50 kg / mm ² ) or more, the carbon content of the steel sheet is increased, Or it is necessary to set it as the component composition which increased the addition amount of the other alloy element. For this reason, the technique described in Patent Document 2 not only causes an increase in the manufacturing cost of the material, but also may cause a problem of deterioration of the weld heat affected zone toughness.

  Furthermore, in the technique described in Patent Document 3, the steel structure is a three-phase structure composed of bainite, MA, and pseudopolygonal ferrite, and the area ratio of bainite and MA and the equivalent circle diameter of MA are adjusted. As a result, low yield ratio and high Charpy impact absorption energy are achieved. However, since Patent Document 3 targets API 5L X65 and X70, there is a concern about a decrease in Charpy impact absorption energy at X80 or higher.

Therefore, the present invention has a strength equivalent to X80 with a tensile strength of 625 N / mm ² or more, a Charpy impact absorption energy at −20 ° C. of 200 J or more and high toughness, and a yield ratio is as low as 85%. An object of the present invention is to provide a low yield ratio high strength high toughness steel sheet that can achieve both the above low yield ratio and high toughness even when API 5L X80 or higher.

  The inventors quantified the allowable MA size and the number density of MAs to achieve the target Charpy impact absorption energy. Note that MA can be easily identified by etching a steel sheet with a 3% nital solution (nitral alcohol solution) and then observing the steel sheet by electrolytic etching.

  As a result of observing the cross section of the test piece after the Charpy impact test as shown in FIG. 1 using a scanning electron microscope (SEM), it was confirmed that voids were generated starting from coarse MA. In FIG. 1, 1 indicates MA and 2 indicates a void.

  No. in Table 2 described later. Using steel No. 3, the frequency at which voids were generated starting from MA was measured by the Charpy impact test, and the relationship between the frequency and the MA major axis was determined. Then, the relationship as shown in FIG. 2 was obtained. From the relationship shown in FIG. 2, it was found that the frequency of occurrence of voids from MA increases rapidly when the major axis of MA is 3.5 μm or more.

Further, as shown in FIG. 3, the relationship between the number density of MA having a major axis of 3.5 μm or more and the Charpy impact absorption energy at −20 ° C. was obtained. From these results, in order to obtain a high Charpy impact absorption energy exceeding 200 J at −20 ° C., the number density of MA having a major axis of 3.5 μm or more needs to be 0.010 / μm ² or less. I found out. In addition, the result shown in FIG. It was obtained by using steels of 3, 4, 6, 8, and 17.

Furthermore, in order to obtain a very high Charpy impact absorption energy exceeding 250 J at −20 ° C., the number density of MA having a major axis of 3.5 μm or more needs to be 0.005 / μm ² or less. I found out.

  Next, as a result of intensive studies on the proper component composition and the method for producing the steel sheet, the following findings (a) to (c) were obtained.

  (A) Austenite is stabilized by adding an appropriate amount of Mn, which is an austenite stabilizing element. This stabilization makes it possible to produce hard MA without adding a large amount of expensive alloy elements such as Cu and Ni.

  (B) By making the cumulative reduction ratio during hot rolling 10% or more in the austenite recrystallized region at 900 ° C. or higher and 50% or more in the austenite non-recrystallized region below 900 ° C., fine MA has a steel structure It can be uniformly dispersed in. As a result, it is possible to improve the Charpy impact absorption energy while maintaining a low yield ratio.

  (C) In accelerated cooling after hot rolling, cooling is stopped during the bainite transformation, that is, in a temperature region where untransformed austenite exists, and then reheating is performed from the bainite transformation finish temperature (hereinafter referred to as Bf point) or higher. Thus, the steel structure of the steel sheet can be a two-phase structure in which MA is uniformly generated in bainite. This two-phase structure makes it possible to reduce the yield ratio.

  The present invention has been made by further studying any of the above findings. Specifically, the present invention provides the following.

(1) An island martensite having a steel structure composed of bainite and island martensite, wherein the island martensite has an area ratio of 2 to 15% and a major axis of 3.5 μm or more. Is a low yield ratio, high strength and high, characterized in that the number density is 0.010 / μm ² or less, the yield ratio is 85% or less, and the Charpy impact absorption energy at −20 ° C. is 200 J or more. Tough steel plate.

  (2) By mass%, C: 0.03-0.08%, Si: 0.05-0.5%, Mn: 1.2-2.5%, P: 0.015% or less, S: 0.002% or less, Mo: 0.05 to 0.5%, Al: 0.01 to 0.08%, N: 0.01% or less, and O: 0.0050% or less, with the balance being Fe And the low yield ratio high strength high toughness steel sheet according to (1), which is an inevitable impurity.

  (3) Further, by mass%, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, V : 0.1% or less, Ca: 0.005% or less, and B: One or two or more types selected from 0.005% or less are contained, The low yield ratio height according to (2) High strength and toughness steel plate.

(4) A method for producing a low yield ratio high strength high toughness steel sheet according to any one of (1) to (3), wherein the slab is heated to a temperature of 1000 to 1300 ° C., and The slab after the slab heating process is hot under the condition that the cumulative reduction ratio at 900 ° C. or higher and 980 ° C. or lower is 10% or higher, the cumulative reduction ratio below 900 ° C. is 50% or higher, and the rolling end temperature is Ar ₃ temperature or higher. A hot rolling step of rolling into a hot-rolled sheet, and the hot-rolled sheet is accelerated and cooled to an arbitrary temperature selected from 500 to 650 ° C. at a cooling rate of 5 ° C./s or more, and then 0.5 ° C. / and a reheating step of reheating to an arbitrary temperature selected from 550 ° C. to 750 ° C. at a temperature rising rate of s or more. A method for producing a low yield ratio, high strength, high toughness steel sheet.

  (5) A steel pipe manufactured using the low yield ratio high strength high toughness steel sheet according to any one of (1) to (3).

  The low yield ratio, high strength, high toughness steel sheet of the present invention has a yield ratio of 85% or less and a Charpy impact absorption energy of 200 J or more at −20 ° C. without containing a large amount of alloy elements. And even if it is API 5L X80 or more, the said low yield ratio and high toughness can be made compatible.

  Moreover, according to the present invention, a steel sheet having a low yield ratio and high toughness can be produced with high productivity and low cost. For this reason, the steel plate which can be preferably used mainly for the material of a line pipe can be stably manufactured in large quantities at a low cost, and the productivity and economical efficiency in laying the pipeline can be remarkably improved.

It is a figure which shows the result of having observed the cross section of the test piece after a Charpy impact test using SEM (scanning electron microscope). It is a figure which shows the relationship between the frequency which a void generate | occur | produces from MA by the Charpy impact test, and MA major axis. It is a figure which shows the relationship between the number density of MA whose major axis is 3.5 micrometers or more, and the Charpy impact absorption energy in -20 degreeC.

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

  The low yield ratio high strength high toughness steel sheet of the present invention (hereinafter sometimes referred to as the steel sheet of the present invention) has a steel structure adjusted, so the yield ratio is 85% or less, and at −20 ° C. Charpy impact absorption energy is 200J or more. First, the steel structure will be described.

  The steel structure of the steel sheet of the present invention is substantially a two-phase structure composed of bainite and island martensite (MA). The steel structure is a structure in which MA, which is a hard phase, is uniformly dispersed in bainite. Here, “uniformly dispersed” means a state of being dispersed in bainite, not in the form of center segregation or band. By adopting such a steel structure, both low yield ratio and high toughness are achieved. In particular, it is one of the features of the present invention that a low yield ratio and high toughness can be achieved even when API 5L X80 or higher. More specifically, the steel structure of the steel sheet of the present invention is as follows.

  In the steel structure of the steel sheet of the present invention, the area ratio of MA is 2 to 15%. If the area ratio of MA is less than 2%, it is difficult to make the yield ratio 85% or less. Moreover, when the area ratio of MA exceeds 15%, the toughness of the steel sheet may be deteriorated.

  Moreover, as above-mentioned, the steel structure of the steel plate of this invention is a 2 phase structure substantially, and all or most of components other than MA are bainite. Therefore, in the steel structure, the area ratio of bainite is 85 to 98%.

  In addition, “substantially” in the “substantially two-phase structure” means that a phase other than bainite and MA may be included as long as the effects of the present invention are not impaired. Specifically, ferrite, pearlite, etc. may be included up to 5% in total area ratio.

In the steel structure of the steel sheet of the present invention, MA having a major axis of 3.5 μm or more is 0.010 pieces / μm ² or less in number density. If MA having a major axis of 3.5 μm or more exceeds 0.010 / μm ² in number density, the toughness of the steel sheet may deteriorate. Preferably, MA having a major axis of 3.5 μm or more is 0.005 / μm ² or less in number density.

  The area ratio of the phase such as MA is calculated by calculating the area ratio occupied by MA or the like from image processing of a microstructure photograph of at least four visual fields obtained by SEM (scanning electron microscope observation), for example. Can be obtained. In addition, the number density of MA having a major axis of 3.5 μm or more is obtained by subjecting a microstructure photograph obtained by SEM observation to image processing, measuring the major axis of each MA from the image obtained by this image processing, and measuring the major axis of 3. It can be obtained by obtaining the number of 5 μm MAs and dividing them by the measurement area.

  Although the component composition of the steel plate of the present invention is not particularly limited, in mass%, C: 0.030 to 0.08%, Si: 0.05 to 0.5%, Mn: 1.2 to 2.5%, P: 0.015% or less, S: 0.002% or less, Mo: 0.05 to 0.5%, Al: 0.01 to 0.08%, N: 0.01% or less, and O: 0.0. It is preferable that it contains 0050% or less, and the balance is Fe and inevitable impurities.

  Moreover, the steel plate of this invention is the mass% as needed, Cu: 0.5%, Ni: 0.5% or less, Cr: 0.5% or less, Nb: 0.1% or less, Ti: It is preferable to contain one or more selected from 0.1% or less, V: 0.1% or less, Ca: 0.005% or less, and B: 0.005% or less.

  Hereafter, the said component is demonstrated. In the description of the component, “%” means “mass%”.

C: 0.030 to 0.08%
C is an element that contributes to precipitation strengthening as a carbide and generates MA. If the C content is less than 0.030%, MA may be insufficiently produced and the yield ratio may be increased. Moreover, when content of C exceeds 0.08%, the deterioration of the toughness of a steel plate and a weld heat affected zone (HAZ) toughness may be caused. Therefore, the steel sheet of the present invention preferably has a C content of 0.030 to 0.08%. More preferably, it is 0.05 to 0.08%.

Si: 0.05-0.5%
Si is added for deoxidation. If the Si content is less than 0.05%, the deoxidation effect may not be sufficient. Further, if the Si content exceeds 0.5%, the toughness and weldability may be deteriorated. Therefore, in the present invention, the Si content is preferably 0.05 to 0.5%. More preferably, it is 0.05 to 0.3%.

Mn: 1.2 to 2.5%
Mn is an element that improves the strength, toughness, and hardenability of the steel sheet. Mn is an element that promotes the production of MA. If the content of Mn is less than 1.2%, the effect is not sufficient, the production of MA becomes insufficient, and the yield ratio may be increased. On the other hand, if the Mn content exceeds 2.5%, the toughness and weldability of the steel sheet may deteriorate. Therefore, the Mn content is preferably 1.2 to 2.5%. More preferably, it is 1.5 to 2.0%.

P: 0.015% or less In the present invention, if the content of P in the steel sheet is large, central segregation is remarkable, and the toughness and weldability of the steel sheet may deteriorate. For this reason, in this invention, it is preferable to make content of P 0.015% or less. More preferably, it is 0.01% or less. In the present invention, P may be included as an inevitable impurity element, but the steel sheet of the present invention may not include P.

S: 0.002% or less S generally becomes MnS inclusions in steel and deteriorates the toughness of the steel sheet. For this reason, the content of S is preferably as small as possible, and the steel sheet of the present invention may not contain S. However, if the S content is 0.002% or less, the effect of the present invention is not impaired. For this reason, in this invention, it is preferable that the upper limit of S content shall be 0.002%. More preferably, it is 0.0015% or less.

Mo: 0.05-0.5%
Mo is an element that improves hardenability and promotes the formation of MA. If the Mo content is less than 0.05%, the effect is not sufficient, the production of MA is insufficient, and the yield ratio may be high. If the Mo content exceeds 0.5%, the weld heat affected zone toughness may be deteriorated. Therefore, in the present invention, the Mo content is preferably 0.05 to 0.5%. More preferably, it is 0.10 to 0.3%.

Al: 0.01 to 0.08%
Al is added as a deoxidizer. If the Al content is less than 0.01%, the deoxidation effect may be insufficient. If the Al content exceeds 0.08%, the cleanliness of the steel may decrease and the toughness may deteriorate. Therefore, the Al content is preferably 0.08% or less. More preferably, it is 0.01 to 0.07%.

N: 0.010% or less N may deteriorate the toughness of the weld heat affected zone. Therefore, in the present invention, it is preferable that the upper limit of the N content is 0.010%. A more preferable N content is 0.007% or less. In the present invention, N may be included as an inevitable impurity element, but the steel sheet of the present invention may not include N.

O: 0.0050% or less O may cause generation of coarse inclusions and may deteriorate the toughness of the steel sheet. Therefore, in the present invention, the upper limit of the O content is preferably 0.0050%. A more preferable range of the O content is 0.0045% or less. In the present invention, O may be included as an inevitable impurity element, but the steel sheet of the present invention may not include O.

  The above is a preferable basic component contained in the steel plate of the present invention. Further, for the purpose of further improving the strength and toughness of the steel sheet and improving the HIC resistance, the steel sheet of the present invention is one or more of Cu, Ni, Cr, Nb, Ti, V, Ca, B. It may contain.

Cu: 0.5% or less Cu contributes to improving the hardenability of steel. In order to improve hardenability, the Cu content is preferably 0.05% or more. However, if the Cu content exceeds 0.5%, the toughness of the steel sheet may deteriorate. For this reason, when it contains Cu, it is preferable to make the content into 0.5% or less.

Ni: 0.5% or less Ni is an element effective for improving toughness and increasing strength. In order to obtain the effect of improving toughness and strength, the Ni content is preferably 0.05% or more. However, if the Ni content exceeds 0.5%, the effect of containing Ni is saturated, which is disadvantageous in terms of cost. For this reason, when it contains Ni, it is preferable to make the content into 0.5% or less.

Cr: 0.5% or less If Cr is contained, sufficient strength can be imparted to the steel sheet even when the C content is low. In order to obtain this effect, the Cr content is preferably 0.05% or more. However, when the Cr content exceeds 0.5%, the weldability deteriorates. For this reason, when it contains Cr, the content is preferably 0.5% or less.

Nb: 0.1% or less Nb refines the structure and improves the toughness of the steel sheet. Nb forms carbides and contributes to an increase in the strength of the steel sheet. In order to obtain these effects, the Nb content is preferably 0.005% or more. However, if the Nb content exceeds 0.1%, the weld heat affected zone toughness deteriorates. For this reason, when it contains Nb, it is preferable to make the content into 0.1% or less.

Ti: 0.1% or less Due to the pinning effect of TiN, Ti suppresses austenite coarsening during slab heating and improves the toughness of the steel sheet. Ti also reduces solute N and suppresses the yield ratio increase due to strain aging. In order to obtain these effects, the Ti content is preferably 0.005% or more. However, if the Ti content exceeds 0.1%, the weld heat affected zone toughness deteriorates. For this reason, when it contains Ti, it is preferable to make the content into 0.1% or less.

V: 0.1% or less V refines the structure and improves the toughness of the steel sheet. V forms carbides and contributes to the improvement of the strength of the steel sheet. In order to obtain these effects, the V content is preferably 0.0005% or more. However, if the V content exceeds 0.1%, the toughness of the weld heat affected zone deteriorates. For this reason, when it contains V, it is preferable to make the content into 0.1% or less.

Ca: 0.005% or less Ca is an element effective in improving toughness by controlling the form of sulfide inclusions. In order to obtain this effect, the Ca content is preferably 0.0005% or more. However, even if the Ca content exceeds 0.005%, the above effect is saturated without increasing, but rather the toughness may be deteriorated due to a decrease in the cleanliness of the steel. For this reason, when it contains Ca, it is preferable to make the content 0.005% or less.

B: 0.005% or less B is an element effective for increasing the strength and improving the toughness of the weld heat affected zone. In order to obtain these effects, the B content is preferably 0.0005% or more. However, if the B content exceeds 0.005%, the weldability deteriorates. For this reason, when it contains B, it is preferable to make the content 0.005% or less.

  The balance other than the above is Fe and inevitable impurities. However, the steel plate of the present invention may contain other elements other than the above as long as the effects of the present invention are not impaired. The inevitable impurities include not only impurities contained in raw materials and impurities mixed in during the manufacturing process, but also other elements that are intentionally added. For example, for the purpose of improving the properties of the welded portion, it is also acceptable in the present invention to contain 0.005% or less of Mg or 0.01% or less of REM.

  The steel pipe of this invention can be manufactured using the said steel plate of this invention. Since the steel sheet of the present invention has a low yield ratio and high toughness, the steel pipe of the present invention is preferable for line pipes. The steel pipe of the present invention can be produced by a conventionally known method.

Then, the manufacturing method of the steel plate of this invention is demonstrated. Although the manufacturing method of the steel plate of this invention is not specifically limited, It is preferable to have the following processes.
(Slab heating step) The slab is heated to a temperature of 1000 to 1300 ° C.
(Hot rolling step) The slab after the slab heating step has a cumulative reduction ratio of 900% to 980 ° C of 10% or more, a cumulative reduction ratio of less than 900 ° C of 50% or more, and a rolling end temperature of Ar ₃ temperature. It hot-rolls on the above conditions to make a hot-rolled sheet.
(Cooling / reheating step) The hot-rolled sheet is accelerated and cooled to an arbitrary temperature selected from 500 to 650 ° C. at a cooling rate of 5 ° C./s or more, and then at a temperature rising rate of 0.5 ° C./s or more. Reheat to any temperature selected from 550 ° C to 750 ° C.

  Hereinafter, each step will be described. The temperature in the description of each step is the temperature at the center in the thickness direction of the steel plate. The center temperature in the plate thickness direction is measured directly by inserting a thermocouple in the center of the slab or steel plate, or by the difference method using parameters such as plate thickness and thermal conductivity from the surface temperature of the slab or steel plate. It can be grasped by calculating by heat transfer calculation. 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 divided by the time required to reheat the temperature difference necessary for reheating up to the reheating temperature after cooling.

  In the slab heating step, for example, a slab having the above component composition is heated to a temperature of 1000 to 1300 ° C. If the heating temperature is less than 1000 ° C., the solid solution of the carbide is insufficient and the required strength may not be obtained. Moreover, when heating temperature exceeds 1300 degreeC, the toughness of a steel plate may deteriorate. Therefore, in the present invention, the heating temperature is preferably set to 1000 to 1300 ° C.

  In the hot rolling step, the cumulative rolling reduction at 900 ° C. or higher and 980 ° C. or lower is set to 10% or higher. By making the cumulative reduction amount in the temperature range of 900 ° C. or more and 980 ° C. or less 10% or more, the austenite grains can be uniformly refined, and then the MA production sites generated in the prior austenite grain boundaries can be uniformly dispersed. it can. The toughness can be improved by making the MA finer and uniformly dispersed. Further, by making the cumulative reduction amount in the temperature range of 900 ° C. or more and 980 ° C. or less 10% or more, the number of MA production sites is further increased, and the coarsening of MA can be suppressed. Therefore, it is preferable to set the cumulative reduction amount at 900 ° C. or more to 10% or more. More preferably, the cumulative rolling reduction at 900 ° C. or higher and 980 ° C. or lower is set to 15% or higher.

  In the hot rolling process, the cumulative rolling reduction at less than 900 ° C. is set to 50% or more. By setting the cumulative rolling reduction in the temperature range below 900 ° C. to 50% or more, the austenite grains can be refined, the number of MA production sites generated at the prior austenite grain boundaries can be increased, and the coarsening of the MA can be suppressed. . The refinement of MA can improve toughness. For this reason, the cumulative rolling reduction below 900 ° C. is preferably 50% or more. More preferably, the cumulative rolling reduction at less than 900 ° C. is 55% or more.

In the hot rolling step, the rolling end temperature is set to Ar ₃ temperature or higher. When the rolling end temperature is lower than Ar ₃ temperature, the subsequent ferrite transformation rate decreases, plastic strain due to rolling remains in the ferrite phase, and the ferrite strength increases, and the hardness difference between the ferrite phase and the bainite phase decreases. . As a result, the desired yield ratio cannot be achieved. Therefore, it is preferable that the rolling end temperature is Ar ₃ temperature or higher. More preferably, the rolling end temperature is (Ar ₃ temperature + 50 ° C.) or higher.

Incidentally, Ar ₃ temperature is calculated from the following equation.
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
In addition, an element symbol shows content (mass%) of each element.

  In the cooling / reheating process, first, the hot-rolled sheet after the hot rolling process is accelerated and cooled. The cooling rate during accelerated cooling is preferably 5 ° C./s or more. When the cooling rate is less than 5 ° C./s, pearlite is generated during cooling, and sufficient strength and low yield ratio may not be obtained.

In the accelerated cooling, the cooling start temperature is preferably Ar ₃ temperature or higher because it is easy to suppress toughness deterioration due to ferrite formation.

  In the accelerated cooling, the cooling stop temperature is an arbitrary temperature selected from 500 to 650 ° C. The cooling stop temperature 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. When the cooling stop temperature exceeds 650 ° C., C is consumed in the pearlite precipitated during cooling, and MA is not generated. Therefore, in the present invention, the cooling stop temperature for accelerated cooling is preferably set to an arbitrary temperature selected from 500 to 650 ° C.

  In the present invention, it is preferable to perform reheating immediately after accelerated cooling. “Immediately” is preferably within 120 seconds after cooling is stopped, from the viewpoint of reducing manufacturing efficiency and fuel cost required for heat treatment. This is because it is preferable to perform reheating from a state in which untransformed austenite exists.

  In order to reliably concentrate C that undergoes ferrite transformation to untransformed austenite, it is desirable to raise the temperature by 50 ° C. or more from the reheating start temperature during reheating.

  In the reheating step, it is preferable to set the heating rate to 0.5 ° C./s or more and the reheating temperature (the reached temperature when reheating) to an arbitrary temperature selected from 550 to 750 ° C. This process is also an important production condition in the present invention. Due to the ferrite transformation from the untransformed austenite at the time of reheating and the accompanying discharge of C to the untransformed austenite, the untransformed austenite enriched with C during the air cooling after the reheating transforms to MA. In order to obtain such MA, it is necessary to reheat from the temperature above the Bf point to an arbitrary temperature selected from 550 to 750 ° C. after accelerated cooling.

  When the rate of temperature increase during the reheating is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency is deteriorated. On the other hand, if the rate of temperature rise is less than 0.5 ° C./s, pearlite transformation occurs, so MA cannot be obtained, and a sufficiently low yield ratio cannot be obtained.

  If the reheating temperature during the reheating is less than 550 ° C., ferrite transformation does not occur sufficiently and C is not sufficiently discharged into untransformed austenite, so that MA is not generated and a low yield ratio cannot be achieved. If the reheating temperature exceeds 750 ° C., sufficient strength cannot be obtained due to softening of bainite. Therefore, it is preferable to set the reheating temperature to an arbitrary temperature selected from 550 to 750 ° C.

  Note that the cooling rate after reheating is basically preferably air cooling.

  The present invention makes a steel pipe using the steel plate manufactured by the above-mentioned method. Examples of the method for forming a steel pipe include a method for forming a steel pipe into a shape by cold forming such as a UOE process or a press bend (also called a bending press).

  In the UOE process, after performing groove processing on the width direction end of the thick steel plate used as a raw material, the end bending of the width direction end of the steel plate is performed using a press machine, and then the steel plate is processed using a press machine. By forming it into a U shape and an O shape, the steel plate is formed into a cylindrical shape so that the widthwise ends of the steel plate face each other. Next, the opposing widthwise ends of the steel plates are brought together and welded. This welding is called seam welding. In this seam welding, a cylindrical steel plate is constrained, the widthwise ends of opposing steel plates are butted against each other in a tack welding process, and welding is performed on the inner and outer surfaces of the butt portion of the steel plate by the submerged arc welding method. A method having a two-stage process, that is, a main welding process for performing the above-described process is preferable. After seam welding, pipe expansion is performed to remove residual welding stress and improve roundness of the steel pipe. In the pipe expansion process, the pipe expansion ratio (ratio of the outer diameter change amount before and after the pipe expansion to the outer diameter of the pipe before the pipe expansion) is usually performed in the range of 0.3% to 1.5%. From the viewpoint of the balance between the roundness improvement effect and the capacity required for the tube expansion device, the tube expansion rate is preferably in the range of 0.5% to 1.2%.

  In the case of a press bend, a steel pipe having a substantially circular cross-sectional shape is manufactured by sequentially forming a steel plate by repeating three-point bending. Thereafter, seam welding is performed in the same manner as the above-described UOE process. Also in the case of press bend, tube expansion may be performed after seam welding.

  Steel plates having a thickness of 20 mm to 28 mm were manufactured under the conditions shown in Table 2 using steels having the composition shown in Table 1 (steel types A to J). In Table 2, 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 is calculated directly by inserting a thermocouple in the center of the slab or steel plate, or by heat transfer calculation such as the differential method using parameters such as plate thickness and thermal conductivity from the surface temperature of the slab or steel plate. Calculated by

  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 divided by the time required to reheat the temperature difference necessary for reheating up to the reheating temperature after cooling.

  A sample for steel structure observation was collected from the central part of the plate width of the steel plate produced as described above, and the plate thickness section parallel to the rolling longitudinal direction was mirror-polished, and then MA appeared using a two-step etching method. . Thereafter, using SEM, steel structure photographs were randomly taken for 5 fields of view at a magnification of 2000 times, and the area ratio and major axis of MA in the photographs were measured with an image analyzer. Most of the materials except MA were bainite.

  Next, JIS Z2202 (1980 revised edition) from the full thickness tensile test piece based on API-5L so that the direction perpendicular to the rolling longitudinal direction of each steel sheet is the longitudinal direction of the test piece, and the plate thickness center part. V-notch Charpy impact test pieces were collected and subjected to a tensile test and a Charpy impact test (JIS Z2242) to evaluate strength and toughness.

  Next, a full-thickness tensile test piece based on API-5L was taken from each steel plate so that the direction parallel to the rolling longitudinal direction was the specimen longitudinal direction, and a tensile test was performed. Yield ratio was calculated from% yield strength and tensile strength.

  Table 2 shows the results of image analysis of the metal structure of the base metal and the results of the strength and toughness investigation.

  In Table 2, No. 1 as an example of the present invention. Each of Nos. 1 to 6 showed a tensile strength of 625 MPa or more, a yield ratio of 85% or less, and Charpy impact absorption energy at −20 ° C. of 200 J or more.

Further, in the present invention example, the structure of the steel sheet is bainite and MA, the volume fraction of island martensite is 2 to 15%, and MA having a major axis of 3.5 μm or more is 0.010 / μm in number density. ² or less.

  On the other hand, No. which is a comparative example. In Nos. 7 to 13, either the yield ratio or the Charpy impact absorption energy was insufficient.

1 Island martensite (MA)
2 void

Claims (5)

The steel structure consists of bainite and island martensite,
In the steel structure, the area ratio of the island-shaped martensite is 2 to 15%,
Insular martensite having a major axis of 3.5 μm or more is 0.010 pieces / μm ² or less in number density,
The yield ratio is 85% or less,
A low yield ratio high strength high toughness steel sheet having a Charpy impact absorption energy at −20 ° C. of 200 J or more.   In mass%, C: 0.030 to 0.08%, Si: 0.05 to 0.5%, Mn: 1.2 to 2.5%, P: 0.015% or less, S: 0.002 %: Mo: 0.05-0.5%, Al: 0.01-0.08%, N: 0.01% or less and O: 0.0050% or less, the balance being Fe and inevitable The low yield ratio high strength high toughness steel sheet according to claim 1, which is an impurity.   Further, in terms of mass%, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, V: 0.0. The low yield ratio, high strength, high toughness according to claim 2, comprising one or more selected from 1% or less, Ca: 0.005% or less, and B: 0.005% or less. steel sheet. A method for producing a low yield ratio high strength high toughness steel sheet according to any one of claims 1 to 3,
A slab heating step of heating the slab to a temperature of 1000 to 1300 ° C;
The slab after the slab heating step is heated under the condition that the cumulative rolling reduction at 900 ° C. or higher and 980 ° C. or lower is 10% or higher, the cumulative rolling reduction at less than 900 ° C. is 50% or higher, and the rolling end temperature is Ar ₃ temperature or higher. A hot rolling step of hot rolling into a hot rolled sheet;
The hot-rolled sheet is accelerated and cooled to an arbitrary temperature selected from 500 to 650 ° C. at a cooling rate of 5 ° C./s or higher, and then from 550 ° C. to 750 ° C. at a temperature rising rate of 0.5 ° C./s or higher. And a reheating step of reheating to a selected arbitrary temperature. A method for producing a low yield ratio high strength high toughness steel sheet.   A steel pipe manufactured using the low yield ratio high strength high toughness steel sheet according to any one of claims 1 to 3.