JP2000129351A - Production of thick high tensile strength steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an on-line type method for producing thick high tensile strength steel plate excellent in the toughness of the center part in the plate thickness. SOLUTION: A rolling stage is composed of two stages, as to the 1st rolling stage, the slab heating temp. is controlled to >=1200 deg.C, and rolling in a temp. region of at least >1150 deg.C is made possible, and, as to the 2nd rolling stage, the slab heating temp. is controlled to <1075 deg.C, and rolling of at least one pass in the recrystallization temp. region is executed. The former rolling solves the defect of the size equal to or below that detectable by UST in the center part of the plate thickness (micro cavity not rolled-in yet), and, in the latter rolling, crystal grain refining is executed. After that, heat treatment is executed in on-line. Moreover, the chemical components are formed into the ones of suppressing the coagulating and coarsening of precipitated carbides, and, the improvement of the toughness in the center part of the plate thickness is executed also from the point of fracture generating characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet used for welding structures such as bridges, tanks, penstocks, warehouses, buildings, and the like. The present invention relates to a thick high-strength steel sheet manufactured by an online process such as quenching. High-strength steel sheets subjected to normalizing heat treatment, annealing heat treatment, and quenching heat treatment offline after rolling are out of the scope of the present invention.

[0002]

2. Description of the Related Art In high-tensile steels used for welded structures, there has been a strong demand for thicker steel than ever before.
It is based on an off-line heat treatment step such as 1-110568 and JP-A-2-170917, and many studies have been made.

However, the off-line heat treatment type high-strength high-strength steel sheet generally has poor weldability due to its C content of 0.1 to 0.2% and a large amount of alloy addition. At present, high production costs are associated with the off-line heat treatment process.

In order to improve the weldability, it is effective to reduce the amount of C. However, it is necessary to increase the amount of expensive alloying elements and to add B, thereby increasing the cost and reducing the toughness due to the hardening of the heat-affected zone of the welded joint. And so on. However, since the improvement in weldability results in a reduction in the total cost of the welded structure,
Without increasing production costs or impairing the soundness of the welded joints,
The potential demand for thickening technology is strong.

[0005] An on-line high-strength steel sheet is one of the techniques to meet this demand. However, since off-line heat treatment is not performed, austenite crystals before transformation are generally coarse grains, and the central part of the sheet thickness ( It is said that it is difficult to secure toughness of (ｔt). In particular, when manufacturing thick steel plates from continuous cast billets, even with large rolling mills owned by steel plate manufacturing plants operating in Japan and overseas, the plate thickness is 75 m.
m, the compressive stress acting on the central part of the sheet thickness becomes
/ 4t portion, significantly reduced, and the thickness of the continuous cast steel slab is generally 300 mm or less, and the amount of reduction cannot be increased. You.

JP-A-57-127504, JP-A-5-25-2
No. 61403 and No. 5-261404 are manufacturing techniques for efficiently deforming the center porosity by using a deformed steel slab in the rough rolling step and devising a rolling method. JP-A-56-84104, JP-A-61-238404, JP-A-2-55605 and JP-A-5-228502 disclose a difference in deformation resistance due to a high shape ratio high-pressure rolling, low-speed rolling, and a temperature difference between the surface and the center of a material to be rolled. In this method, toughness is improved while reducing center porosity detected as an ultrasonic defect. JP-A-60-56017, JP-A-3-4
4417 is rolling using deformation resistance difference due to temperature difference,
The improvement of continuous casting method, and the combination of high shape ratio heavy reduction rolling, aiming at improvement of toughness while reducing center porosity, but there is no specific description of toughness stability, The former applied plate thickness is only 80 mm.

JP-A-63-2433220 and JP-A-63-3
07216, JP-A-5-228506, in addition to the application of the above-mentioned temperature difference rolling, utilize the surface layer cooling-reheating process of the material to be rolled to control the structure of the central portion of the sheet thickness, and particularly to enhance the effect of controlled rolling. However, there is no mention of stability of toughness. Japanese Unexamined Patent Publication No. Sho 64-5, which improved the toughness by adding the center porosity to the same technology.
The applied plate thickness of 7901 is only 50 mm.

Japanese Patent Application Laid-Open Nos. 63-50421 to 50428 disclose that Ar ₃ to Ar ₃ +150 are used during rolling of a low-temperature steel slab.
℃ to re-heat the material to achieve a uniform temperature distribution in the sheet thickness direction of the material to be rolled. The purpose is to reduce the difference in properties.

Japanese Patent Application Laid-Open No. Sho 59-211529 has a plate thickness of 70 mm.
High-strength steel with a thickness of more than 50 mm has been refined by low-temperature heating and high-shape-ratio rolling under reduced pressure to improve the toughness at the center of the thickness. However, its stability is not mentioned.

JP-A-60-258410, JP-A-3-1
3524, 4-165015, 8-295959,
8-302427 is a low-temperature heating and controlled rolling,
Alternatively, a high-strength high-strength steel is manufactured by further combining high-pressure rolling, but no consideration is given to the stability of the toughness at the center of the sheet thickness.

JP-A-8-325668 and JP-A-9-1314
3, No. 10-17929 is a technology relating to a 600 N / mm2 class ² steel which is manufactured by combining microstructure optimization to secure the toughness of a central portion of the sheet thickness, and furthermore, to produce a high shape ratio strong rolling reduction.
No consideration is given to center porosity.

[0012]

As described above, the improvement of the toughness at the center of the thickness of a thick-walled high-tensile steel sheet is achieved by controlling the structure of the steel sheet with a focus on reducing the center porosity and ensuring the effect of controlled rolling. Several proposals have been made from a viewpoint.

However, in the case of an on-line thick high-strength steel sheet by continuous casting, the improvement of both the toughness and the stability of the central part of the thickness, which are particularly required, is discussed.
Techniques for solving these at the same time have not yet been proposed.

That is, in the production of an extremely thick steel plate, in order to stably obtain a good thickness central portion toughness, a central defect of a continuously cast steel slab is pressed and the austenite structure before transformation is refined. There must be.

However, rolling in a high temperature range is necessary for defect compression at the center of the steel slab, and the austenite structure inevitably becomes a mixed grain structure containing coarse crystal grains. In order to refine the microstructure, there is no rolling mill that can give an effective rolling force in the recrystallization low temperature range, and, on the other hand, the microstructure can be obtained by low temperature heating,
Due to insufficient defect compression at the center of the billet, no rolling method has been found industrially to solve the above problems. Therefore, in the present invention, the toughness of the central portion of the plate thickness is stable,
Further, an object of the present invention is to provide a method for producing an on-line thick high-strength steel sheet by good continuous casting.

[0016]

Means for Solving the Problems The present inventors have conducted intensive studies on the effects of various factors on the toughness of the central part of the thickness of a thick-walled high-strength steel manufactured using a continuous cast billet. And reduction of fine center porosity (uncompressed zigzag defects) of about 0.1 mm or less, which is below the detection limit of UST (ultrasonic testing) existing in the center of the sheet thickness, the effect of which was not known at all. It was found that when crystal grain refinement was compatible, stable and good toughness could be secured. As a specific solution, the rolling process was divided into two processes (first rolling process, first rolling process, 2 rolling process) is effective.

[0017] Further, the chemical components are also examined, and a composition that suppresses agglomeration and coarsening of precipitated carbides, promotes fine dispersion, and improves toughness and stability from the viewpoint of improving fracture initiation characteristics is found. In addition, the requirements for ensuring strength, weldability, and weld joint toughness were clarified.

That is, the present invention provides: (1) a first rolling step in which a slab is subjected to rolling to eliminate defects at the center of the sheet thickness, and a metal structure is formed on the slab rolled in the first rolling step. A method for producing a high-strength high-strength steel sheet, comprising: a second rolling step of performing rolling to reduce the size; and a heat treatment step of performing a heat treatment online after the rolling.

(2) The first rolling step includes a step of increasing the slab heating temperature to 1200 ° C. or higher, and a step of rolling to a sheet thickness at least 15 mm or more larger than the final product sheet thickness.
In the second rolling step, the slab heating temperature is 900 ° C. or higher and 107
A step of lowering the temperature to less than 5 ° C., and a step of setting the sheet thickness after finish rolling to a final product sheet thickness. Between the first rolling step and the second rolling step, once after the first rolling step, The method for producing a thick high-tensile steel sheet according to the above (1), further comprising a step of cooling to a temperature below the transformation point.

(3) The high-strength steel sheet according to (1) or (2), further comprising, before the first rolling step, a soaking step for reducing center segregation. Production method. (4) The method for producing a thick high-tensile steel sheet according to the above (3), wherein the soaking step includes a soaking step at a temperature of 1250 ° C. or more for 3 hours or more.

(5) In the first rolling step and / or the second rolling step, one rolling pass satisfying ld / hm ≧ 0.7 is set.
The method for producing a thick high-tensile steel sheet according to any one of the above (1) to (4), comprising a step of performing the step more than once.

Where ld = √ (R * (hi-h
o)), hm = (hi + 2 * ho) / 3, where R is the rolling roll radius, hi is the thickness of the rolled side, and ho is the thickness of the rolled out side. (6) The chemical composition of the steel sheet is 0.06 ≦ C ≦ 0.
10%, 1.0 ≦ Mn ≦ 2.0%, 0.02 ≦ Al ≦
0.1%, 0.001 ≦ N ≦ 0.006%
cm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
The Pcm value given by / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is less than 0.21%, and the high wall thickness excellent in weldability and toughness according to any one of (1) to (5) above. Manufacturing method of high tension steel sheet.

(7) Cu ≦ 1%, Ni ≦ 1%, Cr ≦ 1
%, Mo ≦ 0.5%, 0.005% ≦ Nb ≦ 0.05
%, 0.001% ≦ V ≦ 0.1%, and Ceq = C + Mn / 6 + Si / 24 + Ni /
Ceq expressed by 40 + Cr / 5 + Mo / 4 + V / 14
(6) wherein the value is 0.37% or more.
The method for producing a thick high-strength steel sheet excellent in weldability and toughness described in 1 above.

(8) Ti ≦ 0.005%, B ≦
The method for producing a thick high-strength steel sheet excellent in weldability, weld joint toughness, and toughness according to the above (6) or (7), characterized in that the thickness is regulated to 0.0003%.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a manufacturing method in which the rolling process is divided into two steps to optimize the steps for compensating for the drawbacks of rolling independently and to combine them. The first rolling process aims at eliminating the center defect,
In the second rolling step, the slab heating temperature and the rolling conditions were optimized for the purpose of uniform micronization of the structure, and a stable toughness was achieved at the center of the sheet thickness by a synergistic effect of these.

Further, before the first rolling step, a soaking treatment for eliminating center segregation is performed, and by dividing the rolling into two steps, the operation and effect are not limited to the above-mentioned disadvantages.
The elimination of the cast structure and the refinement of the structure are achieved, and the toughness can be further improved.

The configuration of the present invention will be described below. 1. Soaking process: In order to reduce the center segregation of the continuous cast material, the slab is soaked at a temperature of 1250 ° C. or more for 3 hours or more. This is performed when the toughness improving effect of the first and second rolling steps is further ensured.

2. First rolling process: This rolling aims to eliminate the defects at the center and improve the toughness. The defects targeted in the present invention are defects below the UST detection limit, which have not been studied at all (FIG. 1 shows a method for detecting such minute defects in the present invention. A specimen having a total thickness * arbitrary width * about 10 mm is sampled, a bending test is performed with the plate thickness center at the apex, a minute defect is opened, and evaluation is performed.) Therefore, the slab heating temperature (rolling temperature) and the rolling force are defined. In addition, in order to secure a reduction amount effective for reducing the grain size in the second rolling step, the finished plate thickness is specified.

(1) The slab heating temperature is set to 1200 ° C. or higher so that rolling is completed in a temperature range exceeding at least 1150 ° C. in order to press the center defect. (2) In rolling, a compressive stress acts on the center of the sheet thickness.
Rolling at a rolling shape ratio (ld / hm) ≧ 0.7 is performed at least for one pass. The rolling force is the rolling shape ratio (ld / h
m), and as a result of studying the relationship between the rolling shape ratio and the rolling force acting on the center of the plate thickness, in the case of a rolling pass of ld / hm ≧ 0.7, the minimum value of the hydrostatic pressure distribution is defined as the compressive stress. Further, when ld / hm ≧ 1.2, the maximum value of the hydrostatic pressure acting on the center of the sheet thickness exceeds the deformation resistance of the material to be rolled. Is preferably set as high as possible. Here, ld = √ (R * (hi-ho)), hm =
(Hi + 2 * ho) / 3, R: rolling roll radius, hi:
Rolled-in side thickness, ho: Rolled-out side thickness.

The effects of these rules will be described below. FIG. 3 shows the relationship between the heating temperature of the slab and the frequency of defects at the center of the thickness. When the slab heating temperature is set to 1200 ° C. or more, preferably 1250 ° C., and the high-deformation rolling is performed at a high shape ratio, it is found that the frequency of defects at the center of the sheet thickness is reduced.
As described above, as a result of reducing the defects at the central portion of the plate thickness, it is possible to stably obtain good toughness.

FIG. 2 shows the effect of the defect frequency (pieces / m) at the center on the toughness. It is apparent from FIG. 2 that the toughness is stable and good when the frequency of the center defect is reduced. In the case of a 550 N / mm ² class thick steel sheet, it is desirable that the fracture surface transition temperature be stable at the center of the sheet thickness and be about −40 ° C. or less, and be achieved when the frequency of center defect is less than 100 pieces / m. Is done.

(3) In the second rolling step, the thickness of the finished plate is made 15 mm or more larger than the final finished plate thickness (product plate thickness) so as to secure a necessary amount of reduction in order to refine the structure. 3. Cooling step: To further improve the effect of the second rolling step, after the first rolling step, the slab is cooled to a transformation temperature or lower to eliminate a solidified structure by continuous casting.

4. Second rolling step: In this rolling, the slab heating temperature and the rolling conditions are specified in order to make the austenite crystal grain size before transformation after rolling uniform and fine. (1) The slab heating temperature is premised on low-temperature heating. In order to enable recrystallization zone rolling, the slab is heated to 900 ° C. or more. On the other hand, in order to make the austenite grain size before rolling fine, 1075 ° C.
Less than

(2) Rolling is performed in a redrawing temperature range (950 ° C. for Nb-added steel in which the recrystallization temperature range is reduced) in order to refine the structure in the center of the thickness in a rolling allowance of 15 mm or more up to the product thickness. Above, preferably at 1000 ° C.), high-pressure rolling is performed in at least one pass with a high shape ratio (ld / hm ≧ 0.7). The rolling pass having a high rolling shape ratio also has an effect of reducing defects in the central portion of the sheet thickness remaining after the first rolling step. In the main rolling, it is necessary that both the average crystal grain size and the maximum crystal grain size be equal to or smaller than a certain size.

FIG. 4 shows the effect of the austenite grain size before transformation on the toughness at the center of the sheet thickness. In order to obtain a toughness of −40 ° C. or less at the center of the sheet thickness, the average crystal grain size must be 70 μm or less. As for the stability of toughness, it is necessary to set the maximum grain size to 100 μm or less according to FIG. The stability of the toughness is determined as a direct target value of 200 J or more in terms of the MOTE value of the Charpy absorbed energy at −5 ° C. (minimum value of the number of the three tests stochastically). In addition, the toughness evaluation method by Mote was evaluated by the Japan Welding Association as a toughness evaluation test for high-strength steel sheets.
It has been proposed (WES1109-1995) as a means for eliminating the variation in the results for the TOD test.

On the other hand, the effects of the slab heating temperature and the rolling shape ratio in the main rolling on the refinement of the structure are shown in FIGS. From these figures, it is confirmed that when the slab heating temperature is less than 1075 ° C. and the rolling shape ratio is 0.7 or more, a required uniform fine structure in the austenite structure before transformation is obtained.

The rolling method according to the present invention exerts an effect on general components as a high-strength steel sheet for a welded structure.
Particularly preferred component compositions are those shown below, which suppress the agglomeration and coarsening of precipitated carbides, promote the fine dispersion, and secure the improvement of toughness and stability from the viewpoint of improving the fracture occurrence characteristics.

C: added to ensure base material strength. C
If the amount is less than 0.06%, a large amount of hardenability improving elements such as Cu, Ni, Cr, and Mo must be added, resulting in high cost and deterioration of weldability. Since the dilution of C becomes small and it becomes difficult to secure the joint strength, 0.06% or more is added. Excessive addition impairs weldability, promotes carbide coarsening at the center of the sheet thickness, and lowers toughness.

Si: 0.0 since it works effectively for strength and deoxidation
Although 1% or more may be added, addition exceeding 0.4% should be avoided because it deteriorates the weld cracking susceptibility and the weld joint toughness.

Mn: 1.0% or more is added to secure base metal strength and welding strength. Excessive addition deteriorates weld cracking susceptibility, and at the same time promotes central segregation during continuous casting,
In order to harden the microstructure and impair the toughness at the center of the sheet thickness and its stability, the upper limit is 2.0% or less, preferably 1.8%.
% Or less, more preferably 1.6% or less.

Al: usually 0.005% for deoxidizing steel
Although it is contained as described above, it is added in an amount of 0.02%, preferably 0.03% or more in order to refine the microstructure. Excessive addition causes surface scratches and the like, so the upper limit is made 0.1%, preferably 0.06%.

N: Reacts with Al, Nb, etc. to form precipitates, thereby refining the microstructure. At the time of tempering when Nb and V are added, the strength is increased by precipitation hardening, so that the content is made 0.001%, preferably 0.002% or more. Excessive addition impairs the toughness of the base metal and the welded joint, so the upper limit is made 0.006%, preferably 0.005%.

Ti, B: Ti is a fine grained microstructure,
In the case of B-added steel, it is added positively in order to secure B effective for hardenability. However, in the present invention, B is treated as an impurity to prevent hardening of the weld heat-affected zone, and the amount of B is regulated. Therefore, it is not necessary to add Ti actively, and when Nb and V are added. Has a remarkable effect of fixing N and changes the carbon nitride precipitation behavior of Nb and V, which is not preferable.
Restrict to less than 5%. In addition, B is regulated to less than 0.0003%.

P, S: Both are impurity elements, and 0.015% or less to obtain a sound base metal and a welded joint.
Preferably, the content is regulated to 0.01% or less. The above is the concept of the basic component composition in the component design. Ceq and Pcm are defined in accordance with the required properties (plate thickness, strength, preheating step, functional properties, etc.). The addition of elements is not a problem.

Cu, Ni, Cr, Mo, Nb, V: When obtaining 600 N / mm ² class high strength steel as the steel of the present invention or when weather resistance is required, one or more kinds are added. In this case, the upper limit of Cu, Ni, and Cr is preferably set to 1%, and the upper limit of Mo is set to 0.5% in order to secure weldability and prevent unnecessary hardenability. Nb is an extremely effective material that maintains the strength in the thickness direction of a thick-walled high-strength steel sheet at a constant thickness (hardenability adjusting function), and at the same time, secures the strength, and at the same time, finely disperses carbides in the microstructure to ensure toughness. Element. In order to obtain these effects, at least 0.005% or more is added, but if it exceeds 0.03%, the hardenability adjusting function decreases,
Further, if it exceeds 0.05%, a tendency of deterioration of the weld joint toughness is also observed, so the upper limit is 0.05%, preferably 0.0%.
3%.

The function of adjusting hardenability means that, for example, in the case of a directly hardened type high-thickness steel sheet, a remarkable improvement in hardenability is observed in the central part of the steel sheet which is slowly cooled with respect to the surface layer which is strongly cooled. This is an effect. In the case of the Nb-added system, the effect of the precipitation effect is taken into consideration, and the amount of alloy addition necessary for securing the strength at the center of the plate thickness can be reduced. V has the same effect as Nb, so in the case of 600 N / mm ² class high-strength steel, 0.0
It is desirable to add 1% or more. If the addition amount exceeds 0.1%, the weld cracking sensitivity and the base material toughness deteriorate, so the upper limit is made 0.1%. The effect similar to Nb is 0.0
Since the content tends to be saturated at 8% or more, the upper limit of the preferable addition amount is set to 0.08%.

Ceq: When a thick steel plate having a tensile strength of 550 N / mm ² or more and a thickness of more than 75 mm is used, Ceq
Value (Ceq = C + Mn / 6 + Si / 24 + Ni / 40 +
The component is adjusted so that (Cr / 5 + Mo / 4 + V / 14) becomes 0.37 or more.

Pcm: Pcm value (C + Si / 30 + Mn / 20 + Cu / 2) in order to omit the preheating step or reduce the preheating temperature in welding work in a normal environment.
0 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 +
5B) is regulated to less than 0.21.

The strength and thickness of the high-strength steel sheet of the present invention is not particularly limited.
50N / mm It is very effective for improving the quality of thick steel plate exceeding 75mm of ^2nd class.

[0050]

EXAMPLES Table 1 shows the chemical components of steel used for investigating the effects of the present invention. Each steel has a composition suitable for obtaining the effects of the production method according to the present invention. The test steel was made into a steel ingot after smelting, hot-rolled to a predetermined thickness under various conditions shown in Tables 2 and 3, and then directly quenched as an example of an online die manufacturing method. Tempered.

Tables 2 and 3 show the thickness of the steel sheet, the frequency of defects at the center of the thickness, the average crystal grain size, the maximum crystal grain size, and the mechanical properties (strength and toughness). A JIS No. 4 test piece sampled from a direction perpendicular to the main rolling direction at a sampling position: plate thickness center was used as a test piece having mechanical properties. For toughness, 6 to 18 Charpy impact tests were conducted at a test temperature of -5 ° C, and the stability was evaluated. To determine the fracture transition temperature, three tests were performed at each temperature at different test temperatures. Was. The austenite grain size before the transformation was 0.5 * 0.5 mm ² at the same site as the mechanical property evaluation position.
It was determined from the microscopic observation of the transformed structure for the degree surface.
The defect frequency at the center of the sheet thickness was evaluated from the Z-direction bending test of the center of the sheet thickness shown in FIG.

In Tables 2 and 3, Example No. 1-19 are A
It is a present invention example and a comparative example using steel. No. 7-1
1, 13 to 16 and 19 are examples of the present invention. Since both the first rolling step and the second rolling step are performed under predetermined conditions, the defect frequency at the center of the sheet thickness is less than 100 / m. And the average crystal grain size before transformation is 70 μm or less,
A maximum value of 100 μm or less was realized. As a result, the fracture surface transition temperature was lower than −40 ° C., and the MOT value of the absorbed energy at −5 ° C. was 200 J or more, showing a stable and good toughness.

On the other hand, in Comparative Example No. 1 to 6 are based on a conventional method (one-step rolling). Comparative Example No. 1-3 are the second of the present invention
Of the austenite crystal structure before transformation in the rolling process of No .: Although the low-temperature heating and the strong rolling were satisfied, the requirements of the first rolling process were not satisfied. M / m, the mote value of the absorbed energy at -5 ° C is low, and the toughness is unstable and not good. Comparative Example No. Since Nos. 4 and 5 do not satisfy the requirements of the first and second rolling steps, the austenite crystal structure before transformation together with the defect frequency is not appropriate and the toughness is poor. Comparative Example No. In Nos. 6 and 12, the first rolling process that satisfies the requirements of the present invention achieved a significant improvement in the frequency of defects at the center of the sheet thickness, but the heating temperature in the second rolling process exceeded 1075 ° C. The average grain size and the maximum grain size of the austenite crystal structure did not reach the target, and the toughness was poor.

Comparative Example No. Nos. 17 and 18 are Nos. 6,12
Similarly to the above, the heating temperature in the second rolling step exceeds 1075 ° C. and is outside the scope of the present invention. No. Since the second rolling step was performed at a higher rolling reduction in comparison with Nos. 6 and 12, the average crystal grain size was less than 70 μm, but coarse grains contained in the mixed grain structure were refined by recrystallization. This is difficult, the maximum grain size does not satisfy the target value of the present invention, and the toughness is poor.

Example No. 20 to 31 are examples of the present invention and comparative examples using A to D steel. No. 24-26,29
-31 are examples of the present invention and show good and stable toughness.
In addition, in all the examples having a Ceq value exceeding 0.37 and a component composition containing Nb and V, the tensile strength exceeded 550 N / mm ² , 30 has a Ceq of less than 0.37 and does not contain alloying elements such as Nb,
Tensile strength is below 500 N / mm ² . No. No. 31 contains Nb and V. Although the tensile strength is higher than 30, the Ceq value is less than 0.37, and the tensile strength is 5
Lower than 50 N / mm ² .

No. Nos. 20 and 22 are comparative examples in which the reduction of the defect frequency at the center of the sheet thickness and the uniform refinement of the structure were attempted only by one-step rolling by high-pressure rolling.
As in the case of 4, 5, it is difficult to achieve both of them with a thick material.

[0057]

[Table 1]

[0058]

[Table 2]

[0059]

[Table 3]

[0060]

The present invention relates to a method for producing a thick high-strength steel having excellent mechanical properties at the center of a sheet thickness. Rolling (first rolling process) suitable for eliminating zack defects) and rolling (second rolling process) suitable for grain refinement
After that, heat treatment is performed on-line, and the chemical composition is optimized, so that even a thick material with a thickness of 75 mm or more, high strength steel with excellent weldability and weld joint performance can be used. In the center of the thickness, it is possible to stably maintain good toughness (200 J or more in terms of the MOTE value of Charpy absorbed energy at -5 ° C), which is extremely significant industrially.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a defect frequency evaluation method (Z-direction bending test) of an uncompressed zuck defect at a center portion of a sheet thickness.

FIG. 2 is a graph showing the influence of the frequency of defects and the maximum grain size of the austenite structure before transformation on the stability of toughness.

FIG. 3 is a diagram showing the influence of the heating temperature in the first rolling step and the rolling shape ratio in rolling on the frequency of defects at the center of the sheet thickness.

FIG. 4 is a view showing the influence of the average crystal grain size of the austenite structure before transformation on the fracture surface transition temperature at the center of the sheet thickness.

FIG. 5 is a view showing the effect of the heating temperature in the second rolling step on the austenite grain size before transformation.

FIG. 6 is a diagram showing the effect of the heating temperature and the rolling shape ratio in the second rolling step on the austenite crystal grain size before transformation.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takayuki Furume 1-2-2 Marunouchi, Chiyoda-ku, Tokyo F-term in Nihon Kokan Co., Ltd. 4K032 AA01 AA02 AA04 AA14 AA16 AA19 AA21 AA22 AA23 AA31 AA35 AA36 BA01 CA03 CB02 CF02 CF03

Claims (8)

[Claims] 1. A first rolling step for rolling a slab to eliminate defects at the center of its thickness, and a rolling step for reducing the metal structure of the slab rolled in the first rolling step. 2. A method for manufacturing a high-strength high-strength steel sheet, comprising: a rolling step of No. 2; 2. The first rolling step includes a slab heating temperature of 120.
0 ° C. or more, and a step of rolling to a sheet thickness at least 15 mm or more larger than the final product sheet thickness.
A step of lowering the temperature to less than 5 ° C. and a step of setting the sheet thickness after finish rolling to a final product sheet thickness. Between the first rolling step and the second rolling step, once after the first rolling step, The method for producing a thick-walled high-tensile steel sheet according to claim 1, further comprising a step of cooling to a temperature below the transformation point. 3. The method according to claim 1, further comprising a soaking step for reducing center segregation before the first rolling step. 4. The method according to claim 3, wherein the soaking step includes a soaking step at a temperature of 1250 ° C. or more for 3 hours or more. 5. The method according to claim 1, wherein the first rolling step and / or the second rolling step includes a step of performing at least one rolling pass satisfying ld / hm ≧ 0.7. 4. The method for producing a thick high-tensile steel sheet according to any one of 4. Where l
d = √ (R * (hi-ho)), hm = (hi + 2 * h
o) / 3, where R: radius of rolling roll, hi: thickness of rolled side, and ho: thickness of rolled out. 6. The chemical composition of the steel sheet is 0.06 ≦ C by weight%.
≦ 0.10%, 1.0 ≦ Mn ≦ 2.0%, 0.02 ≦ A
1 ≦ 0.1%, 0.001 ≦ N ≦ 0.006%, Pcm = C + Si / 30 + Mn / 20 + Cu / 20
+ Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5
The method for producing a thick high-strength steel sheet excellent in weldability and toughness according to any one of claims 1 to 5, wherein the Pcm value given by B is less than 0.21%. 7. Cu ≦ 1%, Ni ≦ 1%, Cr ≦ 1%, M
o ≦ 0.5%, 0.005% ≦ Nb ≦ 0.05%, 0.
001% ≦ V ≦ 0.1%, and Ceq = C + Mn / 6 + Si / 24 + Ni / 40 +
The method according to claim 6, wherein the Ceq value represented by Cr / 5 + Mo / 4 + V / 14 is 0.37% or more. 8. The method according to claim 1, wherein Ti ≦ 0.005% and B ≦ 0.00.
The method for producing a high-strength high-strength steel sheet having excellent weldability, weld joint toughness, and toughness according to claim 6 or 7, wherein the thickness is regulated to 03%.