JPH08269542A - Production of high tensile strength steel plate of 950n/mm2 class or above, excellent in weldability - Google Patents

Abstract

PURPOSE: To produce a high tensile strength steel plate excellent in weldability and used for various structures. CONSTITUTION: A slab, having a steel composition essentially containing, by weight ratio, 0.04-0.1% C, 0.5-2% Cu, 1-4.5% Ni, 0.2-1% Cr, 0.1-1% Mo, and 0.01-0.1% V, is heated at 900-1000 deg.C and rolled. At the time of its finish rolling, reduction of >=60% finish draft is done at 800-880 deg.C finish biting temp., followed by water cooling without delay. The resulting steel plate is heated to a temp. not lower than the Ac3 point, hardened, and successively tempered at a temp. not higher than the Ac1 point. By this method, the >=950N/mm class high tensile strength steel plate with excellent weldability can be obtained.

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 having excellent weldability and a tensile strength of 950 N / mm ² or more.

[0002]

2. Description of the Related Art With the recent increase in demand for huge constructions such as long bridges, the characteristics required for steel materials used for these structures are becoming severe. In particular, good weldability, that is, reduction of preheating temperature during welding, is indispensable from the viewpoint of ensuring safety in construction and cost reduction. Since it greatly affects the construction cost, further strengthening of steel materials is an important condition along with good weldability. Actually, the span is several thousand.
There is also a plan to construct an m-class suspension bridge, and there is an inevitable demand for high-strength steel plates with good weldability of 950 N / mm ² or higher for these structures.

As a method for producing a high-weldability steel having good weldability used for such purpose, there is, for example, the production method described in Japanese Patent Laid-Open No. 5-1323. However, the conventional matching method does not sufficiently consider the control during rolling, that is, the heating temperature and the direct quenching temperature, and as a result, the strength over the entire thickness of the steel sheet satisfies the strength level of 950 N / mm ² class or higher. Not.

[0004] invention itself Hei 5-1323 JP is an excellent invention for obtaining a good weldability tensile strength 780N / mm ² class steel sheet of 950 N / mm ² or more Grade As the manufacturing method, the heating temperature during rolling and the direct quenching temperature are not appropriate, so the strength is low, and it is not suitable for application to huge structures such as super-large span suspension bridges as described in the preceding paragraph.

[0005]

SUMMARY OF THE INVENTION The present invention was devised in view of such circumstances, and basically, a steel slab having a chemical composition system having excellent weldability is used at 900 ° C. or higher and 1000 ° C. or higher.
By heating in a low temperature range of less than ℃, controlled rolling, and then directly quenching from a high temperature of 800 ℃ or more after the completion of rolling, the surface layer of the steel sheet is basically a fine grained and expanded lower portion transformed from austenite crystal grains. It is a bainite-based structure, and the central part of the steel sheet is a lower bainite transformed from granular austenite crystal grains, and as a result, it is intended to obtain a high-strength steel sheet having a tensile strength of 950 N / mm ² or more over the entire thickness of the steel sheet. .

[0006]

The gist of the present invention is as follows. (1) By weight ratio, C: 0.04 to 0.1%, Si:
0.02-0.5%, Mn: 0.4-1.5%, Cu:
0.5 to 2%, Ni: 1 to 4.5%, Cr: 0.2 to 1
%, Mo: 0.1 to 1%, V: 0.01 to 0.1%, A
1: 0.01 to 0.1%, B: 0.0004% or less,
N: 0.006% or less, a slab having a steel composition consisting of the balance Fe and unavoidable impurities, 900 ° C to 1000
After heating at less than ℃, after finishing rolling of hot rolling at a final biting temperature of 800 to 880 ℃, a reduction of finishing reduction of 60% or more, and then water cooling from a temperature of 800 ℃ or more,
Subsequently, a method for producing a high-strength steel having good weldability of 950 N / mm ² grade or more, characterized by tempering at a temperature of Ac ₁ point or less.

(2) The steel composition of (1) is characterized by further containing 0.01 to 0.1% of Nb (1)
A method for manufacturing the steel sheet described. (3) Ti is added to the steel composition of (1) or (2) in an amount of 0.1.
The method for producing a steel sheet according to (1), characterized in that the steel sheet contains 0.1 to 0.1%. (4) In addition to the steel composition of (1), (2) or (3), C
a is contained in an amount of 0.001 to 0.01%.

[0008]

The main feature of the steel sheet of the present invention is basically a steel slab having a chemical composition system having excellent weldability, that is, a steel composition having a chemical composition system having low weld hardenability due to low hardenability.
After control rolling in a recrystallization temperature range in which the biting temperature of finish rolling is 800 to 880 ° C., after heating in a temperature range that is 900 ° C. or more and less than 1000 ° C., that is, a temperature range that can avoid the dissolution of AlN that suppresses austenite grain coarsening. By direct quenching, the surface layer part is a fine grained lower bainite-based structure with high toughness, and since the center part of the plate thickness is a relatively coarse austenite grain due to recrystallization, it is easy to quench and form lower bainite. This is a manufacturing method.

Next, the reasons for limitation of the present invention will be described. C
In order to secure the minimum hardenability, 0.04% or more is necessary, but if added in excess of 0.1%, the weldability will be significantly reduced. Therefore, the addition range of C is 0.04 to 0.1%. Si is important as a deoxidizing material, and is at least 0.02%.
However, the addition of more than 0.5% deteriorates the weldability, roughens the weld joint bond structure, and causes the island martensite structure to appear to significantly reduce the toughness of this portion. Therefore, the addition range of Si is 0.02-
0.5%.

Mn is an essential element from the viewpoint of the effect of fixing S to prevent grain boundary cracking during hot working and the hardenability, and requires at least 0.4%, but 1.5% is required. If added over, it deteriorates weldability and Cu to grain boundaries during hot working.
Promote segregation. Therefore, the addition range of Mn is 0.4 to
1.5%.

Cu is an important position in the present invention in which the hardenability is suppressed. That is, it is an essential element for securing strength by precipitation strengthening and requires a minimum of 0.5%, but addition of more than 2% reduces weldability and causes segregation cracks at grain boundaries during hot working. . Therefore C
The addition range of u is 0.5 to 2%.

Ni is an important element indispensable for ensuring hardenability and matrix toughness, and at least 1% is necessary.
Addition of more than 4.5% significantly reduces weldability and hot workability. Therefore, the addition range of Ni is 1 to 4.
5%. Cr is an important element for ensuring the hardenability and requires a minimum of 0.2%, but if it is added in excess of 1%, the weldability is significantly reduced. Therefore, the Cr addition range is 0.2 to 1%.

Mo requires a minimum addition of 0.1% from the viewpoint of strengthening effect by precipitation strengthening and solid solution strengthening and securing hardenability, but addition of more than 1% remarkably improves weldability and hot workability. Lower. Therefore, the Mo addition range is 0.1 to 1%. V requires at least 0.01% from the viewpoint of hardenability and the strengthening effect by precipitation strengthening of carbonitrides such as wing-shaped V carbonitride, but addition of more than 0.1% of the base metal and welded joint It significantly reduces toughness and also reduces weldability. Therefore, the addition range of V is 0.01 to
It is 0.1%.

When Nb is added, it precipitates as a carbonitride and is effective for precipitation strengthening and grain refinement, so 0.01% is required at a minimum, but it exceeds 0.1%. And reduces the toughness of the base metal and welded joint. Therefore, the addition range of Nb is 0.01 to 0.1%. Ti
When added, it precipitates as a carbonitride and is effective for precipitation strengthening and grain refinement.
01% is required, but if it exceeds 0.1%, the toughness of the base material and the weld joint is lowered. Therefore, the addition range of Nb is 0.01 to 0.1%.

Al is not only important as a deoxidizing agent, but also has an effect of fixing N to form AlN and suppressing remarkable coarsening of austenite grains, so that at least 0.01% is required. An excessive addition of more than 0.1% not only impairs weldability but also causes precipitation of coarse AlN and significantly impairs toughness, so the upper limit is 0.1%. Therefore, the range of Al is 0.01 to 0.1%.

B is an element that segregates at the austenite grain boundaries and suppresses the formation of ferrite to ensure hardenability, but in the present invention that aims at weldability, it is an element that causes weld hardenability and is included. Is required to be limited, and it is necessary to suppress it to 0.0004% or less.

Since N has an adverse effect on overall toughness due to the formation of carbonitrides, its content must be limited to 0.006% or less. Ca is extremely effective in spheroidizing non-metallic inclusions such as MnS and is effective in reducing anisotropy, and 0.001% or more is necessary for this effect, but 0.01% The above additions rather reduce toughness. Therefore, the range of addition of Ca is 0.001 to 0.01%.

Next, the steel plate manufacturing conditions which are the main features of the present invention will be described. The slab heating temperature needs to be 900 ° C. or higher in order to secure the rolling temperature in the recrystallization region,
When heated at a temperature of not less than 0 ° C., AlN that prevents the austenite grains from coarsening is dissolved, resulting in a decrease in the toughness of the surface layer portion. Therefore, the rolling slab heating temperature range is 900 ° C.
The temperature is not less than 1000 ° C. Furthermore, the heating temperature is set to AlN
If the temperature is lower than the temperature obtained from the solubility product of AlN, the dissolution of AlN can be completely prevented, so that the toughness of the surface layer is more preferable.

In the finish rolling during the controlled rolling, the biting temperature is 8
If the recrystallization zone controlled rolling with a rolling reduction of 60% or more is performed in the temperature range of 00 to 880 ° C. and water cooling is performed immediately after the rolling is completed, the surface layer portion is fine-grained and a high toughness value is obtained, but the biting temperature is 8
In finish rolling at 80 ° C or higher, the crystal grains in the surface layer portion become coarse again after rolling, so there is no effect of ensuring toughness. At low temperatures of 800 ° C or lower, unrecrystallized rolling is performed and hardenability is reduced, resulting in significant strength and toughness. Get lower. Further, the cumulative rolling reduction (= (thickness before finishing-thickness after finishing) / thickness before finishing) at the time of rolling in the controlled recrystallization region requires 60% or more for fine graining of the surface layer portion, and below that. The effect is negligible.

Further, the direct quenching temperature after the recrystallization zone controlled rolling needs to be 800 ° C. or higher in order to avoid precipitation of Cr, Mo, and V, which are important for hardenability, before quenching.
After rolling, the matrix must be tempered at a temperature below the Ac ₁ point for the purpose of securing the toughness of the matrix and strengthening the precipitation by forming precipitates of Mo and V.

[0021]

【Example】

[Example 1] After smelting steel having a composition range shown in Table 1, 970
After heating at ℃ and rolling to a plate thickness of 150 mm by hot rolling, finishing rolling at 820 ° C. was started to finally make a steel plate having a thickness of 60 mm, and then immediately water cooling was performed. After cooling with water, tempering was performed at a temperature of 630 ° C. to obtain a test steel plate. Yield strength (0.2% yield strength) of each steel plate surface 7mm below and steel plate center part,
The results of tensile strength, brittle fracture transition temperature (vTrs), and crack stop temperature in the y-type weld crack test are shown in the same table.

The weldability referred to here is determined by the level of crack stop temperature, and the lower this is, the better. When the preheating temperature was less than 50 ° C., the cracking of the weld metal could occur, so the cracking of the steel itself could not be evaluated. The tensile test was carried out according to JIS Z 2204 using JIS Z 2201.4 test pieces. The impact test was carried out according to JIS Z 2242 using a JIS Z 2202.4 test piece. The longitudinal directions of the tensile / impact test pieces were both in the direction perpendicular to the rolling (C direction). Furthermore, the y-type weld cracking test complies with JIS Z 3158, and the temperature is 30 ° C and the humidity is 6
The welding was carried out in an atmosphere of 0%, and an extremely low hydrogen type welding rod L-80EL manufactured by Nippon Steel Welding Co., Ltd. was used as the welding rod.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

Of the steels shown in Table 1, No. 1 to No. 7 are examples of the present invention, and No. 8 to No. 29 are comparative examples outside the scope of the present invention. Comparative steel No. 8 has a lower strength and toughness at the center of the steel plate because C is below the lower limit of the present invention, and No. 9 has a lower weldability because it exceeds the upper limit. Comparative steel No. 10 is Si
Is higher than the upper limit of the present invention, the toughness is low. Comparative Steel No.
No. 11 has a lower strength and toughness at the center of the steel plate because Mn is below the lower limit of the present invention, and No. 12 has a lower weldability because it exceeds the upper limit. Comparative Steel No. 13 has a lower strength and toughness at the center of the steel plate because Cu is below the lower limit of the present invention, and No. 14 has a lower weldability because it exceeds the upper limit. Comparative example steel No. 15 has a lower strength and toughness at the center of the steel plate because Ni is below the lower limit of the present invention, and No. 16 has a lower weldability because it exceeds the upper limit.

Comparative Steel No. 17 has a lower strength and toughness at the center of the steel plate because Cr is below the lower limit of the present invention, and No. 18 has a lower weldability because it exceeds the upper limit. Comparative Steel No. 19 is
Since Mo is below the lower limit of the present invention, the strength and toughness at the center of the steel sheet are low, and No. 20 is above the upper limit, the weldability is low. Comparative Steel No. 21 has a lower strength and toughness at the center of the steel plate because V is less than the lower limit of the present invention, and No. 22 has higher toughness and weldability because it exceeds the upper limit. Comparative Steel No. 23 has a low toughness because Nb exceeds the upper limit of the present invention. Comparative Steel No. 24
Has a low toughness because Ti exceeds the upper limit of the present invention.

In Comparative Example Steel No. 25, Al is below the lower limit of the present invention, so the toughness of the steel sheet surface layer portion is low, and in No. 26, above the upper limit, the toughness and weldability are low. Comparative Example No. 27 is
Since B exceeds the upper limit of the present invention, the weldability is low. Comparative example N
O.28 has generally low toughness because N exceeds the upper limit of the present invention. Comparative Example No. 29 has a low toughness because Ca exceeds the upper limit of the present invention. On the other hand, the present invention example No. 1
No. 7 shows a property in which the balance between strength and toughness is satisfactory over the entire steel sheet.

[Example 2] A steel having a composition within the scope of the present invention shown in Table 2 was melted, and then 60 mm under the manufacturing conditions shown in Table 3.
The results of yield strength (0.2% proof stress), tensile strength, and brittle fracture transition temperature (vTrs) are shown in the same table. The tensile test was carried out according to JIS Z 2204 using JIS Z 2201.4 test pieces. The impact test was carried out according to JIS Z 2242 using a JIS Z2202.4 test piece. The longitudinal directions of the tensile / impact test pieces were both in the direction perpendicular to the rolling (C direction).

[0031]

[Table 5]

[0032]

[Table 6]

Among the sample steels shown in Table 3, A and B are examples of the present invention, and C to H are comparative examples having different heat treatments. In Comparative Example C, the heating temperature is below the lower limit of the present invention, so the strength / toughness of the steel plate center portion is low, and since D is above the upper limit, the toughness of the surface layer portion is low. In Comparative Example E, the biting temperature of finish rolling is lower than the lower limit of the present invention, so the strength and toughness at the center of the steel sheet are low, and F is higher than the upper limit, so the surface layer toughness is low. Comparative Example G
, The direct quenching temperature is lower than 800 ° C., so that the strength and toughness at the center of the steel sheet are low. In Comparative Example H, the rolling reduction in finish rolling is below the lower limit of the present invention, and the rolling reduction is insufficient, so the effect of fine grain in the surface layer of the steel sheet is small and the toughness is low. On the other hand, the invention examples A to
B shows the characteristics that strength and toughness are all satisfactory.

[0034]

EFFECTS OF THE INVENTION According to the present invention, it is possible to supply a high-strength steel sheet having a tensile strength of 950 N / mm ² or more, which has a high balance of strength and toughness with high weldability, and it can be said that the effect on the industrial world is extremely great.

Claims (4)

[Claims] 1. By weight ratio, C: 0.04 to 0.1%, Si: 0.02 to 0.5%, Mn: 0.4 to 1.5%, Cu: 0.5 to 2% , Ni: 1 to 4.5%, Cr: 0.2 to 1%, Mo: 0.1 to 1%, V: 0.01 to 0.1%, Al: 0.01 to 0.1%, B: 0.0004% or less, N: 0.006% or less, a slab having a steel composition consisting of the balance Fe and unavoidable impurities, and 900 ℃ or more 100
After heating at less than 0 ° C., finish rolling of hot rolling is performed at a finish biting temperature of 800 to 880 ° C. at a finishing reduction rate of 60% or more, and then water cooling is performed at a temperature of 800 ° C. or more, and then Ac ₁ A method for producing a high-strength steel sheet with good weldability of 950 N / mm ² grade or more, which is characterized by tempering at a temperature below the point. 2. The steel composition according to claim 1 further containing 0.01 to 0.1% of Nb, and good weldability and high tension of 950 N / mm ² grade or higher according to claim 1. Steel plate manufacturing method. 3. The steel composition according to claim 1 or 2, further comprising T
The method for producing a high-strength steel sheet with good weldability of 950 N / mm ² grade or higher according to claim 1, wherein i is contained in an amount of 0.01 to 0.1%. 4. A high-strength steel sheet with good weldability of 950 N / mm ² grade or higher according to claim 1, further containing 0.001 to 0.01% of Ca in the steel composition of claim 1, 2 or 3. Production method.