JP4862266B2 - Manufacturing method of thick-walled low yield ratio high-tensile steel sheet - Google Patents

Description

  The present invention relates to a method for producing a low yield ratio thick-walled high-tensile steel plate having a tensile strength of 550 MPa or more and excellent in earthquake resistance, which is used in fields such as bridges, civil engineering, and architecture.

  Steel materials used in the bridge, civil engineering, and construction fields are required to have a low yield ratio as well as high toughness from the viewpoint of earthquake resistance. However, generally, the yield ratio tends to increase as the strength of the steel material increases. For example, a high-strength steel of 590 MPa class or higher has required special heat treatment or the like to ensure a low yield ratio. This is a process aimed at obtaining a high strength and low yield ratio by forming a two-phase structure of a soft phase and a hard phase such as ferrite and martensite or ferrite and bainite.

  For example, as described in Non-Patent Document 1, Q-Q'-T process in which quenching (Q) is performed after rolling, and then the two-phase region is heated and quenched (Q ') and tempered (T). The 590 MPa class strength and the low yield ratio of 80% or less are compatible. However, the manufacturing process shown in this document has many heat treatment steps and is complicated, and it has been desired to omit the heat treatment step from the viewpoint of construction period and heat treatment cost.

  Therefore, as described in Patent Documents 1 and 2, after Q is omitted, direct quenching (DQ) is performed after rolling, and then Q ′ and T are performed, or after air cooling after rolling. A process for performing Q′-T has been developed, and has become a general production method of low-yield ratio high-tensile steel sheets. However, at present, in order to stably obtain high strength, a low yield ratio, and high toughness, the Q 'heat treatment is indispensable.

  As heat treatment methods that do not require Q ', Patent Documents 3 and 4 propose processes in which air cooling is performed after rolling and cooling is performed from a two-phase region. However, in this technique, a two-phase structure of ferrite-bainite (or martensite) is obtained by quenching after partial ferrite transformation, and high strength and a low yield ratio are achieved. It is not a process that can stably obtain high strength and low yield ratio because it is important to reduce the manufacturability due to the necessity to perform and the time required for air cooling after rolling, and the time management and temperature management of air cooling are important. . In such a process of direct quenching or accelerated cooling after rolling or tempering after cooling, it is said that high strength and a low yield ratio can be obtained as described above, but it is more stable than the Q′-T process. Therefore, it cannot be said that it is a process that can achieve high strength and low yield ratio.

Further, in the cooling in these manufacturing processes, the characteristic difference in the plate thickness direction of the thick material is not particularly taken into consideration, and the 1/4 t position of the plate thickness (from the surface of the steel plate to 1/4 of the plate thickness (t)). ) And 1 / 2t position (1/2 position of the sheet thickness (t) from the surface of the steel sheet, that is, the center position of the sheet thickness), and when the low yield ratio is satisfied at the 1 / 4t position, There is a high possibility that the strength at the 1 / 2t position is insufficient, and when the strength at 1 / 2t is secured, the yield ratio (YR) at the 1 / 4t position is likely to exceed 80%. It is very difficult to stably satisfy both characteristics at the 1 / 2t position. This is usually because when the thick steel plate is cooled from the front and back by water cooling or mist cooling, the cooling rate is faster as it is closer to the plate thickness surface, and slow at the plate thickness center (1 / 2t) position. This is because the transformation organization changes. This tendency is particularly strong in the process in which the Q ′ heat treatment is not performed, and the characteristic difference between the 1 / 4t position and the 1 / 2t position tends to increase.
JP-A-1-156421 Japanese Patent Laid-Open No. 3-207814 JP 63-223123 A JP-A 64-55335 Nippon Steel Pipe Technical Report No.122 (1988) 5-10 pages

  The present invention has been made in view of such circumstances. In a thick high-tensile steel sheet having a tensile strength exceeding 40 mm and having a tensile strength of 550 MPa or more, the characteristic difference in the sheet thickness direction is small without requiring a Q ′ process. It aims at providing the manufacturing method of the low yield ratio high-tensile steel plate which can achieve a low yield ratio and high strength in both the / 4t position and the 1 / 2t position.

In order to solve the above-mentioned problem, the present invention heats a slab to 1000 to 1300 ° C., hot-rolls, accelerates cooling to a temperature of 500 ° C. or less by water cooling, and then tempers at a temperature of Ac ₁ transformation point or less. And containing, by mass%, C: 0.01 to 0.18%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.5%, the balance being Fe and inevitable impurities Difference in the cooling rate at a temperature range of at least 700 to 500 ° C. at the position of 1/4 t in the thickness direction and the position of 1/2 t in the thickness direction during accelerated cooling. The cooling from the surface of the steel sheet is controlled to be stronger in the later stage of cooling than in the initial stage of cooling so that the temperature is within 15%, and during the tempering, the difference between the highest ultimate temperature of the surface layer and the highest ultimate temperature at the 1/2 t position Thickness characterized by being 30 ° C or higher A method for producing a high-strength steel sheet having a low meat yield ratio is provided.

Further, in the present invention, the slab is heated to 1000 to 1300 ° C., hot-rolled, accelerated and cooled by water cooling to a temperature of 500 ° C. or lower, and then tempered at a temperature of Ac ₁ transformation point or lower. And C: 0.01 to 0.18%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.5%, Cu: 0.03 to 1%, Ni : 0.03 to 2%, Cr: 0.03 to 0.5%, Mo: 0.03 to 0.5%, V: 0.02 to 0.15%, Ti: 0.005 to 0.1 %, Nb: 0.005-0.05%, B: 0.0003-0.002%, Ca: 0.0005-0.005%, Mg: 0.0005-0.005%, REM: 0.00. Thick high tension with a thickness of 40 mm or more, containing one or more of 0005 to 0.02%, the balance being Fe and inevitable impurities When manufacturing a steel sheet, the surface of the steel sheet is adjusted so that the difference in cooling rate at a temperature range of at least 700 to 500 ° C. between the position of 1/4 t in the thickness direction and the position of 1/2 t in the thickness direction during accelerated cooling is within 15%. The cooling from the cooling is controlled to be stronger in the later cooling stage than in the early cooling stage , and at the time of the tempering, the difference between the highest temperature at the surface layer and the highest temperature at the 1 / 2t position is 30 ° C. or more. A method for producing a thick-walled, low-yield ratio, high-tensile steel sheet is provided.

  In the present invention, the most important point is that a specific temperature range that affects the characteristics by controlling the cooling in order to reduce the difference in characteristics between the 1/4 t position and 1/2 t position of the thick steel plate exceeding 40 mm. This is a point for controlling the cooling rate at the 1 / 4t position and the 1 / 2t position. In addition, by adjusting the rolling / cooling conditions and the like, high strength and low yield ratio can be achieved at both plate thickness positions, and high toughness can be achieved. Further, by performing appropriate component adjustment, a desired strength and yield ratio can be obtained, and further, toughness can be increased.

  In addition, another important point in the present invention is that, at the time of tempering, by making a difference of 30 ° C. or more between the highest temperature of the surface layer and the highest temperature of the 1/2 t position, and heating the surface layer side to a higher temperature, That is, the hardness on the surface layer side can be reduced and the uniformity in the thickness direction can be increased.

According to the present invention, in producing a thick steel sheet by performing hot rolling, accelerated cooling and tempering under predetermined conditions, the position in the thickness direction 1 / 4t and the thickness direction 1 / 2t at the time of accelerated cooling under the rolling and cooling conditions. The Q ′ process is used because the cooling from the surface of the steel sheet is controlled to be stronger in the late cooling stage than in the early cooling stage so that the difference in the cooling rate in the temperature range of at least 700 to 500 ° C. is within 15%. A thick high-tensile strength steel sheet with a tensile strength exceeding 550 MPa and a thickness of over 40 mm that achieves a low yield ratio and high strength in both the 1 / 4t position and the 1 / 2t position with a small characteristic difference in the sheet thickness direction. Obtainable.

Hereinafter, the present invention will be described more specifically.
FIG. 1 shows the effect of the cooling rate on the tensile properties when the steel component composition and rolling / cooling start / stop conditions are constant. Here, the component composition is 0.08C-0.25Si-1.58Mn-0.28Cu-0.30Ni-0.29Cr-0.04V-0.015Ti-0.0014B (numbers are mass%), and manufactured. The conditions were 1100 ° C. heating—800 ° C. rolling finish—accelerated cooling 300 ° C. stop—500 ° C. tempering. As shown in this figure, when the cooling rate is 2 ° C./sec or less, the strength does not satisfy the target (the target in this test is tensile strength TS ≧ 550 MPa), and when the cooling rate is 5 ° C./sec or more, The yield ratio YR exceeds 80%, and there is an optimum cooling rate range (2 to 5 ° C./sec in the case of this condition) in order to achieve both strength and yield ratio. This optimum cooling rate range is the range where the optimum structure of ferrite + bainite can be obtained, and the strength and yield ratio change depending on the components, rolling conditions, tempering conditions, etc. in addition to the cooling rate. Even so, the target performance can be satisfied by adjusting the components.

  However, in the case of a thick steel plate having a thickness of more than 40 mm, the 1/4 t position and the 1/2 t position under normal cooling conditions (for example, cooling by spraying cooling water or mist continuously from the front and back surfaces of the steel plate). It is difficult to make the characteristics of the 1 / 4t position and the 1 / 2t position compatible with each other.

  Therefore, in the present invention, in manufacturing a thick high-tensile steel sheet having a thickness of 40 mm or more, a manufacturing process is adopted in which a slab is heated, hot-rolled, acceleratedly cooled by water cooling, and then tempered. In the range, the cooling from the steel sheet surface is controlled so that the difference in the cooling rate between the 1 / 4t position and the 1 / 2t position during accelerated cooling is within 15%. As a result, the characteristic difference between the 1 / 4t position and the 1 / 2t position was reduced, and both a high strength and a low yield ratio were achieved at both the 1 / 4t position and the 1 / 2t position.

  As described above, the cooling rate control capable of reducing the cooling rate difference between the 1 / 4t position and the 1 / 2t position is a cooling initial stage by a method of water cooling by water cooling or mist cooling from the front and back surfaces of the steel sheet. In other words, the cooling conditions in the later stage of cooling are changed so that the initial stage is weakly cooled and the latter stage is strongly cooled.

  Normally, when a thick steel plate is cooled from the front and back surfaces of the steel plate under a constant cooling condition, it is cooled while having a temperature gradient in the thickness direction, and the cooling rate at the 1/4 t position and 1/2 t position is 1/4 t. The position is faster, and the 1/4 t position is cooled to a lower temperature more quickly. It is the cooling rate in the temperature range of 700 to 500 ° C. in this production method of this steel type that greatly affects the properties of high-strength steel materials such as 550 MPa class steel and 590 MPa class steel. The time at which the 1 / 2t position passes through the temperature zone of 700 to 500 ° C. is earlier at the 1 / 4t position, and the 1 / 2t position passes through this temperature zone with a delay.

  Therefore, by changing the cooling conditions in the initial cooling stage and the late cooling stage and setting the initial weak cooling and the strong cooling in the latter stage, the cooling rate difference between the 1/4 t position and the 1/2 t position at 700 to 500 ° C. is reduced. It becomes possible to do. The strength of the cooling can be adjusted by adjusting the cooling water density or the like, or by adjusting the time interval between water cooling and non-water cooling (air cooling). In addition, the cooling rate should just be a cooling rate in the temperature range of at least 700-500 degreeC, and if the completion | finish temperature of hot rolling exceeds 800 degreeC, it is good also as a cooling rate in the temperature range of 800-500 degreeC.

As described above, in the present invention, the slab is heated to 1000 to 1300 ° C. and hot-rolled to a temperature of 500 ° C. or lower in order to satisfy high strength and low yield ratio and achieve high toughness. A manufacturing process is adopted in which accelerated cooling is performed by water cooling, followed by tempering at a temperature not higher than the Ac ₁ transformation point.
Hereinafter, these reasons for limitation will be described.

Rolling heating temperature: 1000-1300 ° C
The rolling heating temperature is set to 1000 ° C. or more from the viewpoint of securing the strength, but if the heating temperature becomes too high, the toughness deteriorates, so the upper limit is set to 1300 ° C. Preferably it is 1050-1200 degreeC.

Cooling stop temperature: 500 ° C. or less When the cooling stop temperature is higher than 500 ° C., the cooling rate at the 1 / 2t position is slowed and the strength at 1 / 2t is lowered. Therefore, it shall be 500 degrees C or less. Preferably it is 400 degrees C or less.

Tempering temperature: Ac ₁ transformation point or less Tempering is performed as usual below the Ac ₁ transformation point, and the temperature is selected within this range according to the required strength level. When the tempering temperature becomes high, the strength tends to decrease and the YR tends to increase. Therefore, in order to obtain a high strength and a low yield ratio, 500 ° C. or lower is preferable.

  Furthermore, in order to improve the uniformity of the hardness in the thickness direction, it is preferable that the difference between the highest temperature at the surface layer and the highest temperature at the 1/2 t position is 30 ° C. or more during the tempering. Although the characteristics of 1 / 4t and 1 / 2t can be satisfied by the above process, the vicinity of the surface layer tends to be hard due to the high cooling rate, and the hardness distribution in the plate thickness direction is as shown in FIG. It becomes. Therefore, further, in tempering, by making a difference of 30 ° C. or more between the highest temperature of the surface layer and the highest temperature of the 1/2 t position, the surface layer side is heated to a higher temperature, and the hardness of the surface layer side is reduced, The uniformity of hardness in the thickness direction can be improved. As a method of providing such a temperature distribution during tempering, a heating method in which the surface temperature is increased by using an induction heating device, or by inserting into a high-temperature heat treatment furnace, the temperature rises from the surface, and the plate Extraction from the furnace can be mentioned when the thickness center temperature does not reach the furnace temperature.

  Although the rolling temperature and the rolling finishing temperature are not particularly defined, the rolling finishing temperature is preferably set to 1000 ° C. to 750 ° C. in order to secure an appropriate crystal grain size and ensure toughness. In addition, the cooling rate and cooling method for accelerated cooling are not particularly specified, but the cooling rate is set within the range in which the upper limit is restricted by the plate thickness and cooling method, depending on the component composition and rolling conditions. It is preferable to select. In consideration of weldability and the like, it is desirable to set the cooling rate to the fastest possible value and adjust the components and rolling conditions to ensure the target strength. As for the cooling method, water cooling from the front and back surfaces of the steel sheet or cooling by mist can be considered, but from the viewpoint of ensuring distortion after cooling of the steel sheet and uniformity within the sheet, the surface of the steel sheet is in a nucleate boiling state with a high water density. Cooling only with cooling is desirable, and the cooling capacity can be controlled by the amount of water, but a method of controlling cooling by repeating strong water cooling and non-water cooling (air cooling) is desirable.

If the above production conditions are satisfied, the component composition of the steel sheet may be normal, but in order to obtain high strength, low yield ratio, and high toughness, C: 0.01-0. It is preferable that it contains 18%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.5%, with the balance being substantially made of Fe and inevitable impurities. Furthermore, from the viewpoint of improving various properties, Cu: 0.03 to 1%, Ni: 0.03 to 2%, Cr: 0.03 to 0.5%, Mo: 0.03 to 0.5% , V: 0.02-0.15%, Ti: 0.005-0.1%, Nb: 0.005-0.05%, B: 0.0003-0.002%, Ca: 0.0005 ˜0.005%, Mg: 0.0005 to 0.005%, REM: 0.0005 to 0.02% are preferably contained in one kind or two or more kinds.
Hereinafter, the reason for limiting the content of each additive element will be described.

C: 0.01 to 0.18%
C is an effective element for stably securing the normal temperature strength and high temperature strength of steel, but when the C content increases, the toughness and weldability deteriorate. For this reason, C content was made into 0.01 to 0.18%.

Si: 0.01 to 0.5%
Si is an effective element as a deoxidizing element, and therefore, it is necessary to add at least 0.01% or more. Further, Si is an element effective for solid solution strengthening, but if it exceeds 0.5%, the ductility is lowered and the room temperature strength is increased more than necessary. For this reason, Si content was made into 0.01 to 0.5%.

Mn: 0.3 to 2.5%
Mn is an element effective in securing strength, and for that purpose, addition of 0.3% or more is necessary. On the other hand, if it exceeds 2.5%, the weldability deteriorates and the room temperature strength is increased more than necessary. For this reason, Mn content was made into 0.3 to 2.5%.

Cu: 0.03 to 1%
Since Cu is an element effective for solid solution strengthening, it can be added as necessary. However, if it is less than 0.03%, such an effect is small, and if it exceeds 1%, there is a problem of surface flaws of the steel sheet in addition to an increase in cost. It is preferable.

Ni: 0.03 to 2%
Ni is an element effective in improving strength through improvement of low temperature toughness and hardenability, and therefore can be added as necessary. However, if it is less than 0.03%, such an effect is small, and Ni is an expensive element, and if it exceeds 2%, the cost increases, so when it is added, it should be in the range of 0.03 to 2%. Is preferred.

Cr: 0.03-0.5%
Since Cr is an element effective for increasing the strength by solid solution strengthening, it can be added as necessary. However, if it is less than 0.03%, such an effect is small, and if added over 0.5%, the weldability is deteriorated, so when added, it should be in the range of 0.03-0.5%. Is preferred.

Mo: 0.03-0.5%
Mo is an effective element that increases the strength of the steel by improving hardenability, precipitation strengthening, and the like, so that it can be added as necessary. However, if it is less than 0.03%, such an effect is small, and if added over 0.5%, the cost is increased, weldability is deteriorated, and YR tends to be higher. When it is, it is preferable that it is 0.03 to 0.5% of range.

V: 0.02-0.15%
V is effective in increasing the strength by hardenability and precipitation strengthening, and therefore can be added as necessary. However, if it is less than 0.02%, such an effect is small, and if it exceeds 0.15%, weldability deteriorates, so when it is added, it should be in the range of 0.02 to 0.15%. Is preferred.

Ti: 0.005 to 0.1%
Ti is effective for refining crystal grains, improves toughness, and has an effect of fixing N as TiN, so it can be added as necessary. And such an effect becomes still larger by adding together with Mo and Nb (and / or V). However, if it is less than 0.005%, such an effect is small, and if it exceeds 0.1%, the weldability deteriorates. Therefore, when it is added, the content is preferably in the range of 0.005 to 0.1%.

Nb: 0.005 to 0.05%
Nb is effective for refining crystal grains and improves toughness, and it is effective for increasing the strength by improving hardenability and precipitation strengthening, so Nb can be added as necessary. However, if the amount is less than 0.005%, such an effect is small, and if added over 0.05%, the weldability is deteriorated and the YR is increased. A range is preferable.

B: 0.0003 to 0.002%
B is an element effective for improving hardenability, and can be added as necessary in order to improve the strength. However, if it is less than 0.0003%, the effect is small, and if added over 0.002%, the weldability is deteriorated. Therefore, when added, the content is preferably in the range of 0.0003 to 0.002%. .

Ca: 0.0005 to 0.005%
Mg: 0.0005 to 0.005%
REM: 0.0005 to 0.02%
Ca, Mg, and REM can be added as necessary because they are effective in controlling the form of inclusions and improving the toughness through fixing of impurity elements such as S. And when adding these, from a viewpoint which can exhibit such a characteristic effectively, Ca: 0.0005-0.005%, Mg: 0.0005-0.005%, REM: 0.00. A range of 0005 to 0.02% is preferred.

  The balance is substantially Fe, and impurity elements inevitably mixed in and other trace additive elements are allowed. For example, P and S are impurity elements that cause problems such as a reduction in ductility, workability, and weldability, and it is desirable to reduce them as much as possible. However, a significant reduction leads to an increase in cost. Therefore, the range that does not cause significant material deterioration is usually 0.03% or less. In addition, Al is a deoxidizing element, and sol.Al is precipitated in the steel as AlN and is effective in refining crystal grains. However, if added excessively, inclusions increase and ductility deteriorates. 0.07% or less. Further, N precipitates as AlN and is effective for refining crystal grains. However, when added in a large amount, the toughness of the welded portion deteriorates, so the content is usually 0.0010 to 0.020%.

Examples of the present invention will be described below.
Table 1 shows the component compositions of the test steels of the examples. Steel 1 is a component system that does not add other than Si and Mn as alloy elements, and Steels 2 to 7 have Cu, Ni, Cr, Mo, V, Nb, Ti, B, Ca, Si, Mn as alloy elements. It is a component system to which one or more of Mg and REM are added.

  For these test steels, steel sheets having various thicknesses were manufactured under the manufacturing conditions shown in Table 2. The mechanical properties at that time are shown together in Table 2. Table 3 shows the relationship between the cooling speeds at the 1/4 t position and 1/2 t position when the cooling condition is changed for each thickness. Symbols A, C, E, and G in Table 3 are normal cooling patterns, and symbols B, D, F, and H are controlled so that the difference in cooling rate between the 1 / 4t position and the 1 / 2t position is small. It is the process of the invention. As shown in Table 3, in the cooling patterns A, C, E, and G, the cooling rate ratio of 1 / 4t and 1 / 2t is 1.15 or more (the difference is 15% or more), whereas In the cooling patterns B, D, F, and H, the cooling rate ratio is 1.15 or less (the difference is 15% or less). The symbols of the cooling patterns in Table 2 correspond to A to H in Table 3, and Table 2 shows mechanical properties when the cooling patterns of A to H are applied to steels 1 to 7. In addition, the steel plate code | symbol of Table 2 attaches | subjects code | symbol AF of the cooling pattern of Table 3 to the end of the steel number except 1G-1L.

  Here, 550 MPa grade steel (YS ≧ 385 MPa, TS ≧ 550 MPa) and 590 MPa grade steel (YS ≧ 440 MPa, TS ≧ 590 MPa) are targeted, and in Table 2, target performance (lower strength limit) of 550 MPa grade steel or 590 MPa grade steel The performance is evaluated with a value of 550 MPa class steel and a strength upper limit value of 590 MPa class steel).

  As shown in Table 2, steel plates 1A and 1C are cases where steel 1 is cooled under normal cooling conditions, and the performance of 1 / 2t satisfies the target, but YR is high at 1 / 4t. Therefore, the performances of 1 / 4t and 1 / 2t cannot be compatible. On the other hand, the steel plates 1B and 1D perform cooling control to reduce the cooling rate difference between the 1 / 4t position and the 1 / 2t position, and satisfy the target performance at both 1 / 4t and 1 / 2t.

  The steel plates 1G to 1I have a cooling pattern of D or B, and the cooling rate is controlled. However, the strength of the steel plate 1G is low because the heating temperature is low, and the strength of 1 / 2t does not satisfy the target. In the steel plate 1H, the heating temperature is high and the tensile properties satisfy the target, but the toughness is low. In particular, the toughness at the 1 / 2t position does not satisfy the target. Is high and the strength is low, and the strength at the 1 / 2t position does not satisfy the target.

  Steel plates 5C and 5E are cases where steel 5 is cooled under normal cooling conditions, and the performance of 1 / 2t satisfies the target, but YS and YR are high at 1 / 4t. / 4t and 1 / 2t are not compatible. On the other hand, the steel plates 5D and 5F perform cooling control to reduce the cooling rate difference between the 1 / 4t position and the 1 / 2t position, and satisfy the target performance at both 1 / 4t and 1 / 2t.

  For steel plates 2D, 3D, 4D, 6D, and 7D, cooling control for reducing the cooling rate difference between the 1 / 4t position and the 1 / 2t position indicated by D in Steels 2, 3, 4, 6, and 7, respectively. The target performance is satisfied at both 1 / 4t and 1 / 2t.

  The steel plate 1J is a case where it is cooled under normal cooling conditions and is subjected to normal tempering. Since both 1 / 4t and 1 / 2t have a high cooling rate with respect to the components, the strength and YR are high. In the hardness distribution shown in FIG. 2, the hardness of the surface layer and ¼t shows a tendency to be higher than ½t. On the other hand, the steel plate 1K has been subjected to cooling rate control and normal tempering, and all the properties in Table 2 are satisfied. However, in the hardness distribution in the thickness direction, a hardened region is recognized in the vicinity of the surface layer. The steel plate 1L is tempered under the condition that the temperature difference between the surface layer and 1/2 t is 40 ° C. in addition to the control of the cooling rate. The hardness distribution is also uniform.

  According to the present invention, there is provided a low-yield-ratio high-tensile steel plate having a thickness of 40 mm or more that has a small characteristic difference in the thickness direction and can achieve a low yield ratio and high strength at both the 1 / 4t position and the 1 / 2t position. Since it is obtained, it is effective as a steel material for bridges, civil engineering, and construction, and its industrial utility value is high.

The figure which shows the yield ratio YR, tensile strength TS, and yield strength YS of a steel plate at the time of changing a cooling rate. The figure which shows the hardness distribution of the steel plate thickness direction by cooling conditions.

Claims (2)

The slab was heated to 1000 to 1300 ° C. and hot-rolled, and then accelerated and cooled by water cooling to a temperature of 500 ° C. or lower. Thereafter, tempering was performed at a temperature of the Ac ₁ transformation point or lower. Thick high-strength steel sheet having a thickness of 40 mm or more, containing 01-0.18%, Si: 0.01-0.5%, Mn: 0.3-2.5%, the balance being Fe and inevitable impurities In the manufacturing process, the difference in cooling rate between the temperature range of at least 700 to 500 ° C. between the position of 1/4 t in the plate thickness direction and the position of 1/2 t in the plate thickness direction during accelerated cooling is 15% or less from the surface of the steel plate. The cooling is controlled to be stronger in the later stage of cooling than in the early stage of cooling, and the difference between the highest temperature at the surface layer and the highest temperature at the 1 / 2t position is 30 ° C. or more during the tempering. A method for producing a high-strength steel sheet with a low meat yield ratio. The slab was heated to 1000 to 1300 ° C. and hot-rolled, and then accelerated and cooled by water cooling to a temperature of 500 ° C. or lower. Thereafter, tempering was performed at a temperature of the Ac ₁ transformation point or lower. 01 to 0.18%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.5%, Cu: 0.03 to 1%, Ni: 0.03 to 2 %, Cr: 0.03-0.5%, Mo: 0.03-0.5%, V: 0.02-0.15%, Ti: 0.005-0.1%, Nb: 0.0. 005 to 0.05%, B: 0.0003 to 0.002%, Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005%, REM: 0.0005 to 0.02% For producing a thick high-tensile steel sheet having a thickness of 40 mm or more, comprising one or more of the above, the balance being Fe and inevitable impurities The cooling from the surface of the steel sheet is performed so that the difference in the cooling rate in the temperature range of at least 700 to 500 ° C. between the position of 1/4 t in the thickness direction and the position of 1/2 t in the thickness direction during accelerated cooling is within 15%. Thick-walled low yield, characterized in that the cooling later is controlled to be stronger cooling than the initial cooling, and the difference between the highest temperature at the surface layer and the highest temperature at the 1 / 2t position is 30 ° C. or higher during the tempering. A method for producing a specific high-tensile steel sheet.

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