JP2012158784A - High strength steel excellent in toughness at welding heat-affected zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high strength steel having a tensile strength of 590 MPa grade or more, which suppresses usage of Ni, Mo and Mn as much as possible, and is excellent in high heat input HAZ toughness.SOLUTION: High strength steel excellent in toughness at a welding heat-affected zone has a composition including, by mass%, 0.025-0.050% of C, 0.3% or less of Si, 1.2-2.0% of Mn, 0.05% or less of P, 0.01% or less of S, 1.5-3.5% of Cr, 0.05% or less of Al, 0.005-0.050% of Ti, 0.5-2.0% of Ni and 0.0015-0.0060% of N, with the remainder being iron and unavoidable impurities, while satisfying the following expression (1). 2.3≤(Mn+0.4Cr)≤2.7 (1). In the expression, Mn and Cr represent the content (mass%) of the respective elements.

Description

  The present invention relates to a high-strength steel excellent in the toughness of the heat-affected zone having a large heat input, and in particular, a tensile having an excellent toughness in the heat-affected zone when subjected to a high heat input exceeding 500 kJ / cm. The present invention relates to a high strength steel having a strength of 590 MPa or higher.

  In submerged arc welding, electroslag welding, and the like applied to assembly welding of box columns of building structures, large heat input welding exceeding 500 kJ / cm may be performed in order to improve the efficiency of construction. Generally, when the amount of heat input to welding increases, the structure of the weld heat affected zone (hereinafter referred to as HAZ) becomes coarse and the toughness decreases. Various methods for improving the toughness of the HAZ have been proposed so far.

  In Patent Document 1, the generation of island martensite (hereinafter referred to as MA) is suppressed by extremely low C, and the hardenability improving elements Mn, Ni, and Cr are appropriately contained, so that the γ grain boundary A technique for suppressing the HAZ toughness deterioration of 590 MPa grade steel by suppressing the formation of ferrite in the steel and miniaturizing the block size of the transformed structure in the grains is disclosed.

  In Patent Document 2, as a method for improving the HAZ toughness of 780 MPa class steel, Mn, Ni, and Cu are positively added, and the amount of addition of Mo, Nb, and V is limited to mainly contain bainitic ferrite. There is disclosed a technique for preventing the coarsening of austenite crystal grains in the HAZ by making the structure and optimizing the amounts of Ti and N to stabilize TiN at a high temperature.

JP 2007-126725 A JP 2006-118007 A

  The technique disclosed in Patent Document 1 requires the addition of about 1% Ni in order to improve the HAZ toughness of 590 MPa class steel. Ni is an expensive element, and the alloy cost is high, and it is cost competitive as a 590 MPa class steel. Further, since Mn is added in a large amount of about 2%, the solidification segregation of Mn at the center of the slab thickness is strong, and the toughness at the center of the thickness (1 / 2t) tends to be impaired.

  In the technique disclosed in Patent Document 2, about 1 to 2% of Ni is required to improve the HAZ toughness of the 780 MPa class steel, and further 0.2 to 0.5% of Mo is required. Since Ni and Mo are expensive elements, the alloy cost is high and the cost competitiveness is lacking. Further, since Mn is added in a large amount of about 2%, the solidification segregation of Mn is strong at the slab thickness 1 / 2t as in the technique of Patent Document 1, and the toughness of the 1 / 2t part of the steel plate is high. It is easily damaged.

  Therefore, the present invention improves the high heat input HAZ toughness of 590 MPa class steel or more, and suppresses the use amount of expensive Ni and Mo as much as possible to reduce the alloy cost and solidify and segregate to impair the toughness at the center of the plate thickness. An object of the present invention is to provide a production method capable of obtaining high HAZ toughness while suppressing an easy addition amount of Mn as much as possible.

  In order to solve the above-mentioned problems, the present inventors diligently studied to obtain a high-strength steel excellent in HAZ toughness when subjected to high heat input welding. As a result, the following knowledge was obtained.

  In order to stably secure the high toughness of the heat-affected zone, it is possible to reduce the transformation temperature by adding elements that increase the hardenability such as Mn, Cr, Ni, Si, etc. It is necessary to make the whole into a uniform bainite structure.

  On top of that, by adding both austenite-generating elements (Mn, Ni, etc.) and ferrite-forming elements (Cr, Si, etc.) and making the ferrite-forming element amount appropriate for the austenite-forming element amount, the HAZ toughness can be reduced. Can be good.

  Mn is a strong austenite stabilizing element and promotes bainite transformation by lowering the transformation temperature. However, even if Mn is added alone, MA precipitates between the bainite laths and the HAZ toughness is lowered.

  Ni is also a strong austenite stabilizing element after Mn, and lowers the transformation temperature and promotes bainite transformation, but its effect is smaller than that of Mn. For this reason, even if Ni is added alone, the effect is not sufficient unless a large amount is added. When a large amount of Ni is added alone, HAZ has a uniform bainite structure, and unlike Mn addition, there is almost no formation of MA harmful to toughness between bainite laths, so that excellent HAZ toughness is obtained. However, since Ni is expensive and requires a large amount of addition, the alloy cost of the steel material becomes extremely high.

  Therefore, it is practical to mainly use Mn as an austenite-generating element, and additionally use Ni in addition to it.

  When a ferrite stabilizing element is added in addition to these austenite forming elements, excessive austenite stabilization due to the addition of Mn is alleviated, MA between bainite laths is reduced, and a region with excellent HAZ toughness appears.

  In particular, Cr is effective in suppressing excessive austenite stabilization by Mn. Si is also a strong ferrite-forming element. However, since Si strongly suppresses the formation of cementite, if excessively added, the production of MA increases.

  Therefore, it is effective to mainly use Cr as a ferrite-forming element and use an appropriate amount of Si as a supplement.

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

  In the first invention, the component composition is mass%, C: 0.025 to 0.050%, Si: 0.3% or less, Mn: 1.2 to 2.0%, P: 0.05% Hereinafter, S: 0.01% or less, Cr: 1.5 to 3.5%, Al: 0.05% or less, Ti: 0.005 to 0.050%, Ni: 0.5 to 2.0% N: 0.0015 to 0.0060%, further satisfying the following formula (1), consisting of the remaining iron and unavoidable impurities, is a high strength steel excellent in toughness of the weld heat affected zone is there.

2.3 ≦ (Mn + 0.4Cr) ≦ 2.7 (1)
In the formula, Mn and Cr indicate the content (% by mass) of each element.

  A second invention is a welding characterized in that a steel having the component composition described in the first invention is heated to 1000 ° C. or more and 1250 ° C. or less and hot-rolled, and then cooled at a cooling rate of air cooling or more. This is a method for producing a high-strength steel having excellent heat-affected zone toughness.

  The third invention is a direct quenching step in which the steel having the component composition described in the first invention is heated to 1000 ° C. or more and 1250 ° C. or less, hot-rolled, and then water-cooled, or 350 ° C. or less after hot-rolling. A reheating quenching step in which the steel is cooled to 870 ° C. or higher and then cooled with water, and the steel after the quenching is heated to 450 ° C. or higher and 700 ° C. or lower, and then cooled at a rate of air cooling or higher. It is a manufacturing method of the high strength steel excellent in the toughness of the welding heat affected zone characterized by having.

  According to the present invention, even when high heat input welding exceeding 500 kJ / cm or the like is performed, excellent HAZ toughness can be secured, and a highly safe building structure or the like can be manufactured with high efficiency.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. About component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.025 to 0.050%
C is an element necessary for securing the strength of the base material and suppressing the coarsening of the γ grains to ensure the HAZ toughness, and in order to exert its effect, addition of 0.025% or more is necessary. is there. On the other hand, if added over 0.050%, MA increases and the HAZ toughness deteriorates, so the C content is in the range of 0.025 to 0.050%.

Si: 0.3% or less (excluding 0)
Si is an element necessary for deoxidation at the time of steelmaking, but if the purpose of deoxidation is achieved, the addition amount is at least good. Si is a strong ferrite-forming element and has the effect of suppressing the excessive stabilization of austenite by Mn and suppressing the formation of MA. However, since Si strongly suppresses the formation of cementite, it is excessively added. Then, since MA increases and the HAZ toughness deteriorates, the Si amount is set to 0.30% or less.

Mn: 1.2 to 2.0%
Mn is a strong austenite stabilizing element and is an element useful for lowering the transformation point and ensuring the strength of the base material. It is also an element that promotes bainite transformation. In order to ensure the strength of the base material, addition of 1.2% or more is necessary, but if added over 2.0%, the hardness of HAZ becomes too hard and HAZ toughness deteriorates, so the amount of Mn Is in the range of 1.2 to 2.0%.

P: 0.05% or less (excluding 0)
P is preferably as small as possible because it impairs the toughness of the steel material. However, in consideration of the dephosphorization cost in the steel making process, the P content is 0.05% or less.

S: 0.01% or less (excluding 0)
The smaller the S, the more preferable it is because it impairs the toughness of the steel. However, in consideration of the desulfurization cost in the steel making process, the S amount is set to 0.01% or less.

Cr: 1.5-3.5%
Cr is an element useful for improving the hardenability and ensuring the strength and toughness of the base metal, and also is a ferrite stabilizing element, preventing excessive stabilization of austenite due to the addition of Mn, and generating MA. It is an element useful for suppression. In order to exert these effects, addition of 1.5% or more is necessary. However, if added over 3.5%, the hardness of HAZ increases and the HAZ toughness deteriorates. The range is 5 to 3.5%.

Al: 0.05% or less (excluding 0)
Al is an element necessary for deoxidation at the time of steelmaking, but if the purpose of deoxidation is achieved, the addition amount is at least good, but if added over 0.05%, coarse inclusions such as alumina increase. The base material toughness deteriorates. In addition, MA increases and HAZ toughness deteriorates, so the Al content is 0.05% or less.

Ti: 0.005 to 0.050%
Ti is an element that combines with N to form TiN. TiN suppresses the growth of HAZ γ grains and effectively contributes to the improvement of HAZ toughness. In order to exert this effect, 0.005% or more (preferably 0.010% or more) should be added. However, if added over 0.050%, TiN becomes coarse, and the base material toughness, HAZ Since both toughness deteriorates, the amount of Ti is made 0.005 to 0.050%.

Ni: 0.5 to 2.0%
Ni is also a strong austenite stabilizing element, is an element useful for ensuring the strength of the base material by lowering the transformation point, and is an element promoting bainite transformation. Furthermore, since the toughness of the matrix and HAZ is increased by increasing the toughness of the matrix, 0.5% or more is added, but since Ni is an expensive element, the addition amount is not more than 2.0% from the viewpoint of alloy cost And Therefore, the Ni content is in the range of 0.5 to 2.0%.

N: 0.0015 to 0.0060%
N is an element that combines with Ti to form TiN, and this TiN suppresses the growth of HAZ γ grains and contributes effectively to the improvement of HAZ toughness. In order to exert this effect, the N amount is 0.0015% or more (preferably 0.0025% or more), but if it exceeds 0.0060%, TiN becomes coarse and both the base material toughness and the HAZ toughness deteriorate. Therefore, the N amount is set to a range of 0.0015 to 0.0060%.

2.3 ≦ (Mn + 0.4Cr) ≦ 2.7
In the formula, Mn and Cr indicate the content (% by mass) of each element.

The addition amount of Cr should be determined according to the addition amount of Mn. However, when Mn + 0.4Cr ≧ 2.3% Cr is added, excessive stabilization of austenite by Mn is prevented and generation of MA is suppressed. Moreover, since HAZ has a fine bainite structure having an appropriate hardness, the HAZ toughness becomes excellent. However, if Cr becomes excessive and Mn + 0.4Cr> 2.7, the hardness of HAZ becomes too hard and the HAZ toughness decreases, so Mn + 0.4Cr is set in the range of 2.3 to 2.7.
The balance other than the above components in the high-strength steel of the present invention is Fe and inevitable impurities.

2. Manufacturing conditions Steel having the above-described composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and a steel material such as a slab by a conventional method such as a continuous casting method or an ingot-bundling method. It is preferable that Note that the melting method and the casting method are not limited to the methods described above.

  In the present invention, the temperature such as the heating temperature, the rolling end temperature, the cooling end temperature, and the reheating temperature is the average temperature of the steel sheet. The average temperature is obtained by calculation from the surface temperature of the slab or the steel plate in consideration of parameters such as the plate thickness and heat conduction.

  Hereinafter, each manufacturing condition will be described.

  The steel of the present invention is subjected to ordinary hot rolling, and has a high strength of 590 MPa or more after rolling because it has many alloy components and high hardenability even when air-cooled. The tensile strength can be increased to 780 MPa or more by cooling at a speed or by quenching and tempering.

The heating temperature of the hot rolling is 1000 ° C. or higher and 1250 ° C. or lower.
If it is less than 1000 degreeC, since melt | dissolution in austenite, such as C and N, does not advance sufficiently, the heating temperature of hot rolling shall be 1000 degreeC or more. If the heating temperature exceeds 1250 ° C., the austenite crystal grains become coarse and the toughness of the base material decreases, so the heating temperature for hot rolling is set to 1000 ° C. or more and 1250 ° C. or less.

  After hot rolling, the steel sheet is accelerated and cooled to lower the transformation temperature of the steel by air cooling, and the strength of the base material can be increased from that of the air cooling material. Accelerated cooling may be started at any time after completion of hot rolling, but it is desirable to accelerate cooling from a temperature higher than 600 ° C., which is a temperature at which transformation starts during air cooling, in order to increase the strength of the base material. Further, it is desirable to stop the accelerated cooling at 350 ° C. or higher. This is because martensitic transformation occurs when accelerated cooling to less than 350 ° C. and the toughness of the base material decreases. If the cooling rate of accelerated cooling exceeds air cooling, the purpose of increasing the strength of the base material is achieved, but more desirably, the tensile strength can be increased to 780 MPa or higher by setting it to 5 ° C./s or higher.

  Further, after hot rolling, direct quenching in which the steel sheet is immediately water-cooled, or after hot rolling, the steel sheet is cooled to 350 ° C. or lower, and then reheated to 870 ° C. or higher to perform water-cooling reheating quenching.

  The reason why it is once cooled to 350 ° C. or lower during reheating and quenching is that the bainite transformation is completed at a temperature exceeding 350 ° C. (the steel of the present invention has a low C content and a high Mn, Cr, Ni, etc. component composition). There is almost no bainite transformation and almost no martensitic transformation). The reason for reheating to 870 ° C. or higher is to completely transform the austenite into C, which is a solid carbide.

  The quenched steel sheet is tempered because it has high surface hardness and low toughness near the surface. The tempering treatment is performed at 450 ° C. or higher and 700 ° C. or lower. This is because if the tempering temperature is less than 450 ° C., there is no effect of improving toughness, and if it exceeds 700 ° C., the strength is greatly lowered, and the purpose of obtaining high strength by quenching and tempering cannot be achieved.

  As described above, a steel material having a tensile strength of 780 MPa or more can be obtained by adding an accelerated cooling or quenching and tempering step to the steel having the composition of the present invention. As described above, the steel material subjected to accelerated cooling or quenching and tempering treatment exhibits the same or better performance as the steel material in hot rolling as well as the base material toughness and the HAZ toughness except that the tensile strength becomes 780 MPa or more.

  Hereinafter, the present invention will be described more specifically with reference to examples. Steel materials having the composition shown in Table 1 were melted to form slabs, heated to 1200 ° C., and hot-rolled to a finishing temperature of 950 ° C. and a plate thickness of 15 mm. Cooling after hot rolling was air cooling. Using the steel plate thus obtained, the base material strength and toughness were measured and the HAZ toughness was evaluated as follows.

Measurement of base material strength and toughness A round bar test piece (ASTM-F type) is taken from the rolling direction of each steel plate and subjected to a tensile test according to JISZ 2241, yield strength (YS), tensile strength (TS). The elongation (EL) was measured. And the thing whose tensile strength is 590 Mpa or more was evaluated as high strength. In addition, as an evaluation of the toughness of the base material, three Charpy V-notch specimens were sampled perpendicular to the rolling direction and subjected to a Charpy impact test in accordance with JISZ 2242, and the absorbed energy (vE0) at a test temperature of 0 ° C. was measured. did. It was evaluated that the average of the three absorbed energy was 70 J or more and the minimum value of the three was 50 J or more was excellent in toughness. These results are shown in Table 2.

Evaluation of HAZ toughness The heat history corresponding to the heat-affected zone in the vicinity of the bond when electroslag welding is performed by combining a skin plate material (50 mm thick) and a diaphragm material (50 mm thick) and welding heat input is 550 kJ / cm. A high-frequency induction heating apparatus that simulates and heats a 12 mm thick × 12 mm wide square bar-shaped test piece sample taken from the rolling direction and holds it at 1400 ° C. for 1 second and a cooling time of 800 to 500 ° C. for 510 seconds. Was subjected to heat treatment.

Then, three V-notch test pieces of JIS Z 2202 were sampled from the test pieces subjected to heat treatment simulating the thermal history, and Charpy impact test was conducted in accordance with the procedure of JISZ 2242, and the absorbed energy (vE at a test temperature of 0 ° C. ₀ ) was measured. An average of three absorbed energies of 70 J or more and a minimum of three of 50 J or more was evaluated as having excellent HAZ toughness. These results are shown in Table 3.

  Steel types A4, A5, A7, A18 to A20, and A25 to A27 shown in Table 1 are invention examples in which the base material components all satisfy the scope of the invention.

  On the other hand, steel types A1 to A3, A6, A8 to A17, A21 to A24, and A28 to A35 are comparative examples in which one or more of Si, Cr, and Mn + 0.4Cr are out of the scope of the invention.

  Table 2 shows the tensile test and Charpy test results of the base material part. B4, B5, B7, B18-20, B25-27 have a tensile strength (TS) of 590 MPa or more, and the absorbed energy at 0 ° C. satisfies 70 J or more, and the strength and toughness of the base material are excellent. I understand that. Comparative examples are also No. Except for B16, there was no problem in the strength and toughness of the base metal part.

  Table 3 shows the Charpy test results of the HAZ part, and shows the absorbed energy and brittle fracture surface rate at 0 ° C. Inventive example No. B4, B5, B7, B18-20, and B25-27 all have an average value of each of three absorbed energy at 0 ° C. of 70 J or more, and the minimum value of each three is 50 J or more, and the toughness of the HAZ part It turns out that it is excellent. On the other hand, none of the comparative examples has sufficient toughness in the HAZ part.

Steel materials A19 and A25 (invention example) shown in Table 1 were melted to form a slab, heated to 1200 ° C, hot-rolled to a finishing temperature of 950 ° C and a plate thickness of 15 mm, hot-rolled Subsequent cooling was by air cooling to room temperature. And after reheating this steel plate to 910 degreeC x 5 minutes, water quenching was performed immediately. Furthermore, this quenching material was tempered at 500 ° C. for 5 minutes, and then cooled to room temperature by air cooling (reheating quenching-tempering treatment is shown in Table 4 as heat treatment: RQ-T).
In addition, the steel material having the composition of steel type A25 was hot-rolled in the same manner as described above, and then subjected to direct quenching (DQ) by water cooling from 900 ° C., followed by tempering at 500 ° C. for 5 minutes (direct quenching-tempering treatment). In Table 4, heat treatment: labeled DQ-T).
Using the steel plate thus obtained, the base material strength and toughness were measured and the HAZ part toughness was evaluated in the same manner as in Example 1.
Table 4 shows the mechanical properties of the base material, and Table 5 shows the test results of the HAZ toughness. In either case, the steel material No. C1 to C3 exhibit high strength satisfying YS ≧ 700 MPa, TS ≧ 780 MPa, and toughness is sufficient. Furthermore, HAZ toughness is also excellent.

Steel materials A19 and A25 (invention example) shown in Table 1 were melted to form a slab, heated to 1200 ° C, hot-rolled to a finishing temperature of 870 ° C and a plate thickness of 15 mm, hot-rolled The subsequent cooling was from 850 ° C. to 500 ° C. at a cooling rate of 30 ° C./s by an accelerated cooling device. And it cooled to room temperature by air cooling after that. Using the steel plate thus obtained, the base material strength and toughness were measured and the HAZ part toughness was evaluated in the same manner as in Example 1.
Table 6 shows the mechanical properties of the base material, and Table 7 shows the test results of the HAZ toughness. In either case, the steel material No. D1 and D2 exhibit high strength satisfying YS ≧ 700 MPa, TS ≧ 780 MPa, and toughness is sufficient. Furthermore, HAZ toughness is also excellent.

Claims (3)

The component composition is mass%, C: 0.025 to 0.050%, Si: 0.3% or less, Mn: 1.2 to 2.0%, P: 0.05% or less, S: 0.00. 01% or less, Cr: 1.5 to 3.5%, Al: 0.05% or less, Ti: 0.005 to 0.050%, Ni: 0.5 to 2.0%, N: 0.0015 A high-strength steel excellent in the toughness of the weld heat affected zone, characterized by containing ~ 0.0060%, further satisfying the following formula (1), and consisting of the balance iron and inevitable impurities.
2.3 ≦ (Mn + 0.4Cr) ≦ 2.7 (1)
In the formula, Mn and Cr indicate the content (% by mass) of each element.   The steel having the component composition according to claim 1 is heated to 1000 ° C. or higher and 1250 ° C. or lower and hot-rolled, and then cooled at a cooling rate of air cooling or higher, and excellent in toughness of the heat affected zone of welding. A method for producing high strength steel.   The steel having the component composition according to claim 1 is heated to 1000 ° C. or more and 1250 ° C. or less, hot-rolled, then directly cooled in water, or after hot rolling and cooled to 350 ° C. or less, and then 870 ° C. A reheating and quenching step in which the steel is reheated and water-cooled, and a tempering step in which the steel after quenching is heated to 450 ° C. or higher and 700 ° C. or lower and then cooled at a rate higher than air cooling. A method for producing high-strength steel with excellent toughness of weld heat affected zone.