JP3259604B2 - Manufacturing method of ultra-thick high strength steel with excellent weldability and low acoustic anisotropy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a construction, a penstock,
The present invention relates to a method for manufacturing a very thick steel plate having a tensile strength of 980 N / mm ² or more and excellent in weldability and low in acoustic anisotropy, among ultra-thick steel plates having a thickness of 70 mm or more used in heavy electric machines and the like.

[0002]

^2. Description of the Related Art Conventionally, 980 N / mm ² class high-strength steel has been subjected to finish rolling by suspending rolling after completion of rough rolling in a high temperature range and waiting for the temperature of the steel sheet to decrease. However, in the case of an extremely thick steel plate having a thickness of 70 mm or more, the waiting time for the temperature drop is long, and the toughness is deteriorated due to the coarsening of austenite grains.

[0003] In addition, there is a problem in that cooling by standing cooling has a small temperature distribution in the sheet thickness direction, and it is difficult to reduce the center of the sheet thickness in finish rolling, so that sufficient grain refinement cannot be achieved. As a means for solving this, a method of performing finish rolling after forcibly cooling the steel sheet surface is known. Usually, in this technique, since the rolling is performed after the center of the thickness of the sheet is reduced to a temperature lower than the recrystallization temperature, the structure is elongated during the finish rolling, and the texture is further developed.

Thus, the ratio of the shear wave velocity (V ₁ ) of the vibration in the rolling direction to the shear wave velocity (Vc) of the vibration in the direction perpendicular to the rolling,
The so-called acoustic anisotropy is increased, and there is a problem that the defect position cannot be detected accurately during ultrasonic flaw detection of the welded portion.

[0005]

The present invention has a thickness of 70 mm.
Reduction or, in the production of tensile strength 980 N / mm ² or higher grade of high strength steel, by achieving low-carbon component composition of the steel, as well as improved weldability, also in the plate thickness center at 950 ° C. or higher It is an object of the present invention to provide a method for producing a very thick steel sheet having a small acoustic anisotropy by adding a steel sheet.

[0006]

The present inventors have made various studies to solve the above-mentioned problems. As a result, even in steel materials in which C is lowered to improve weldability, 980 N / N is obtained by utilizing precipitation strengthening by the combined addition of Nb and V.
It has been found that a strength of mm ² class or higher can be obtained.

[0007] Further, by forcibly cooling the steel sheet surface temperature after heating or after the completion of rough rolling so that the surface temperature of the steel sheet is 950 ° C or less and the temperature at the center of the sheet thickness becomes 1000 ° C or more, coarsening of austenite grains can be prevented. Further, it has been found that the temperature distribution in the thickness direction becomes large, so that the vicinity of the surface layer is hardened, so that the central portion of the thickness is also reduced during finish rolling.

Further, the finish rolling end temperature at the center is set to 950.
It has been found that the acoustic anisotropy can be reduced by setting the temperature to not less than ° C. Furthermore, the steel sheet thus obtained was found to be excellent in both strength and toughness, and led to the following invention.

(1) The invention according to claim 1 is characterized by comprising the following steps (the composition of the components is wt%): production of a high-thickness high-strength steel with excellent weldability and low acoustic anisotropy. Is the way. (A) C: 0.03-0.05%, Si: 0.1-0.
6%, Mn: 0.5 to 2.0%, P: 0.03% or less,
S: 0.01% or less, Ni: 2.5 to 4.5%, Al:
0.01-0.1%, Mo: 0.2-1.5%, Cr:
0.2-2.0%, Cu: 0.1-0.5%, Nb:
0.01-0.05%, V: 0.01-0.05%, N
preparing a steel slab containing b + V ≧ 0.06%, the balance being iron and unavoidable impurities; (b) heating the steel slab to 1100 ° C. or higher; and (c) heating the steel slab. (D) a step of subjecting the steel slab or the coarsely-rolled steel slab to a surface temperature of 950 ° C.
(E) forcibly cooling at a cooling rate of 1 ° C./s or more so that the temperature at the center is 1000 ° C. or more, and
A step of immediately rolling the forcibly cooled steel slab and terminating the rolling when the central temperature is 950 ° C. or higher.

(2) The invention according to claim 2 is that the slab is quenched immediately after the completion of the rolling, and then the slab is heated to 500 to 650 ° C.
2. The method according to claim 1, wherein the tempering is carried out in a temperature range of:

[0011]

The present invention will be described below in more detail. Generally, in the direct quenching type high-tensile steel, in order to obtain high strength and high toughness, refinement of the structure of the steel sheet after finish rolling is an extremely important factor. However, in general, in hot rolling, after the rough rolling is completed, the finish rolling is started after the temperature of the steel sheet is lowered by cooling, so that the waiting time for the temperature drop is long and the austenite grains are coarsened.

Particularly, in a very thick steel plate having a thickness of 70 mm or more, the cooling rate by cooling is extremely slow, and the waiting time for the temperature drop is very long, so that the austenite grains are significantly coarsened. When finish rolling is performed on the austenite structure that has been coarsened in this way, the ferrite structure may be coarsened and mixed grains may occur after the finish rolling, resulting in loss of strength and toughness.

Therefore, the present inventors have made various investigations on the prevention of coarsening of austenite grains. As a result, after the heating or after the end of the rolling, the steel sheet surface temperature becomes 9
By forcibly cooling at a cooling rate of 1 ° C./s or more so that the temperature at the center is not more than 50 ° C. and the temperature at the center is not less than 1000 ° C., the waiting time for temperature decrease can be remarkably reduced, and the austenite grains can be prevented from becoming coarse. Instead, it has been found that the use of the difference in deformation resistance caused by the temperature distribution in the plate thickness direction also applies a reduction of about 20% per pass to the central portion.

Further, when rolling is completed at 950 ° C. or higher, no reduction is applied in the austenite unrecrystallized region, so that austenite grains can be prevented from elongating and the acoustic anisotropy is small. . The present invention has been completed based on the above findings. First, the reasons for limiting the chemical components in the present invention will be described.

C: C is an element that forms carbide with Nb and V and contributes to precipitation strengthening. However, if it is less than 0.03%, it is difficult to secure the strength, and if it exceeds 0.05%, the weldability deteriorates. Therefore, the addition amount is limited to 0.03% to 0.05%.

Si: Si is an element necessary for deoxidation,
If the amount of addition is less than 0.1%, the effect is small.
Addition of more than 6% deteriorates weldability. Therefore,
The addition amount is limited to 0.1 to 0.6%.

Mn: Mn is an element necessary for securing strength and toughness. However, if its addition amount is less than 0.5%, no effect can be obtained. However, if added over 2.0%, the hardenability increases and the weldability and toughness decrease. Therefore,
Mn addition amount is limited to 0.5% to 2.0%.

P: Since P is an element that causes grain boundary destruction,
The lower the content, the better. Therefore, the P content is preferably 0.01% or less, but is limited to 0.03% or less because no significant adverse effect is exerted up to 0.03%.

S: An increase in S causes an increase in MnS and a deterioration in toughness. However, the content is limited to 0.01% or less because it does not have a significant adverse effect up to 0.01%.

Ni: Ni is an extremely important element for ensuring strength and toughness without deteriorating weldability. To obtain such an effect, at least 2.5% is required. However, adding more than 4.5% significantly increases the cost, so the amount added is limited to 2.5-4.5%.

Al: Al is an element necessary for deoxidation,
If the amount is less than 0.01%, the effect is small, and if it exceeds 0.1%, the toughness is deteriorated. Therefore, the addition amount is limited to 0.01 to 0.1%.

Mo: Mo is an element necessary for improving the strength. To obtain such an effect, 0.2% or more is required. However, if added in excess of 1.5%, weldability and toughness deteriorate. Therefore, the added amount is 0.2
Limited to ~ 1.5%.

Cr: Cr is an element necessary for imparting strength, but has no effect if its addition amount is less than 0.2%.
%, The weldability is remarkably deteriorated. Therefore, the addition amount is limited to 0.2 to 2.0%.

Cu: Cu is an element effective for increasing the strength without lowering the weldability. To obtain such an effect, at least 0.1% is necessary. However, if added in excess of 0.5%, cracking occurs during hot rolling or weldability is reduced. Therefore, the addition amount is limited to 0.1% to 0.5%.

Nb: Nb is an extremely effective element for increasing the strength by precipitation strengthening. In order to obtain a strength of 980 N / mm ² or higher, a minimum of 0.01% is required. But,
If added over 0.05%, the non-recrystallization temperature rises,
Even if rolling is completed at 950 ° C. or more, austenite grains are elongated and acoustic anisotropy is increased. Therefore, the addition amount is limited to 0.01% to 0.05%.

V: V is an element effective for precipitation strengthening like Nb, and at least 0.01% is required to obtain the effect. However, if it exceeds 0.05%, the weldability deteriorates. Therefore, the amount of addition is 0.01-0.05.
%. Further, it is necessary that Nb + V satisfies 0.06 or more to obtain the effect of the present invention.

Next, the reasons for limiting the cooling and rolling conditions in the present invention will be described. The reason for heating the slab to 1100 ° C. or more is to make Nb necessary for securing the strength form a solid solution in the steel. Next, the heated steel sheet is directly subjected to rough rolling, that is, without rough rolling, and thereafter, the steel sheet surface temperature becomes 950.
Forcibly cooling at a cooling rate of 1 ° C./s or more so that the temperature at the center is 1000 ° C. or more at a temperature of 100 ° C. or less.

At this time, if the surface temperature of the steel sheet is higher than 950 ° C., the difference in deformation resistance in the thickness direction becomes small, so that the central part cannot be reduced. Since the acoustic anisotropy is performed in the crystal region, the acoustic anisotropy increases. On the other hand, if the cooling rate is less than 1 ° C./s, the difference in deformation resistance in the thickness direction becomes small, so that the central part cannot be reduced.

The forced cooling in the present invention is a cooling at a cooling rate higher than that of the natural cooling, and the refrigerant and the cooling method can be arbitrarily selected. For example, there is a method of water-cooling the upper and lower surfaces of the steel plate being conveyed. Steel plate surface temperature 950
In order to set the temperature in the central portion to 1000 ° C. or higher, the cooling conditions and the temperature distribution of the steel sheet may be determined in advance by, for example, heat transfer analysis.

After the completion of the forced cooling, finish rolling is performed in a state where the temperature distribution in the thickness direction remains, that is, in a state where the surface layer is hardened, and finish rolling is performed immediately after the forced cooling. At this time, if the finish rolling end temperature is lower than 950 ° C., the austenite grains elongate and the acoustic anisotropy increases, so the finish rolling end temperature must be 950 ° C. or higher.

After the finish rolling, the steel sheet may be left to cool as it is, or may be quenched and tempered. In the present invention, in consideration of the balance between strength and toughness, it is desirable to perform tempering in the range of 500 to 700 ° C.

[0032]

EXAMPLE An example of the method for producing a very thick high-tensile steel having excellent weldability and no acoustic anisotropy according to the present invention will be described. However, the present invention is not limited to only this example.
The steel slab of the low alloy steel having the component composition shown in Table 1 was subjected to hot rolling under the conditions shown in Table 2, immediately quenched, and then tempered at 630 ° C.

In Table 2, the forced cooling was performed in an online direct cooling device. The slab thickness before hot rolling was 210 mm. A test piece was taken from the center of the thickness of the obtained steel sheet, and the tensile properties, toughness, and acoustic anisotropy were evaluated. Table 3 shows the results. The measurement of acoustic anisotropy was performed according to the method specified in JIS Z 3060.

That is, a transverse wave is incident on the rolled surface of the steel sheet using an ultrasonic transverse wave probe, and the transverse sound speed (V ₁ ) of the vibration in the L direction and the transverse wave sound speed (Vc) of the vibration in the C direction are obtained. Calculate the shear wave sound speed ratio from the formula. Shear wave sound speed ratio = V ₁ / Vc When the shear wave sound speed ratio exceeds 1.02, it is determined that there is acoustic anisotropy.

As is clear from Table 3, steel numbers A1 to A
No. 8 of the present invention has high strength and high toughness and low acoustic anisotropy, and all characteristics are achieved in a well-balanced manner. On the other hand, steel No. B1, which is a comparative example, had insufficient strength due to insufficient C content, and steel No. B2 had low toughness due to excessive addition of C.

Steel No. B3 had a remarkable deterioration in toughness due to an insufficient amount of Ni, and steel No. B4 deteriorated toughness due to excessive addition of Al. In addition, steel numbers B5 and B6
Are inferior in Nb and V, respectively, and thus have decreased strength.

In the steel No. B7, since the cooling was allowed to cool, the austenite grains became coarse and the strength and toughness were reduced. Further, in steel No. B8, cooling was allowed to cool, and the finish rolling temperature was lower than 950 ° C., so that strength and toughness were reduced, and acoustic anisotropy was increased.

In steel numbers B9, B12, and B14, since the surface temperature of the steel sheet at the end of cooling exceeded 950 ° C., the difference in deformation resistance in the thickness direction became small, and the central portion of the steel sheet could not be sufficiently reduced in strength. The toughness decreased.

The steel No. B10 has a center temperature at the end of the cooling condition of less than 1000 ° C. and a finish rolling end temperature of 9 ° C.
Since the temperature was as low as 20 ° C., rolling was performed in an austenite non-recrystallized region, so that the acoustic anisotropy increased. further,
B11 and B13 have a finish rolling end temperature as low as 900 ° C.
Since the austenite unrecrystallized region rolling was performed, the acoustic anisotropy increased.

Regarding the weldability, the maximum hardness test,
The evaluation was performed by a crack test. In the invention steels, Hv (10 kg) = approximately 320 in the maximum hardness test, and 50 ° C. for a 100 mm material and 25 ° C. for a 75 mm material in a diagonal y-crack test were 25 ° C., indicating good weldability.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

According to the method for producing a high-thickness high-strength steel sheet of the present invention, there is an excellent effect that a high-thickness high-strength steel sheet having excellent strength and toughness and low acoustic anisotropy can be produced. Even if such an extremely thick high-tensile steel sheet is used as a structural member involving welding, a defect in a welded portion is completely detected by ultrasonic inspection, so that the reliability is high.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. The following step (component composition is wt%)
A method for producing an ultra-thick high-tensile steel having excellent weldability and low acoustic anisotropy, comprising: (A) C: 0.03-0.05%, Si: 0.1-0.
6%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Ni: 2.5 to 4.5%, Al: 0.01 to 0.1 %, Mo: 0.2-1.5%, Cr: 0.2-2.0%, Cu: 0.1-0.5%, Nb: 0.01-0.05%, V: 0. 01-0.05
%, Nb + V ≧ 0.06%, the balance comprising iron and unavoidable impurities; (b) heating the steel slab to 1100 ° C. or higher; and (c) heating the steel slab. A step of coarsely rolling or not rough rolling the steel slab, and (d) the steel slab or the coarsely rolled steel slab,
The surface temperature is 950 ° C. or less and the temperature at the center is 10
A step of forcibly cooling at a cooling rate of 1 ° C./s or more so that the temperature becomes 00 ° C. or more, and (e) a step of immediately rolling the forcibly cooled steel slab and terminating the rolling at a center temperature of 950 ° C. or more. . 2. The steel sheet according to claim 1, wherein the slab is immediately quenched after the rolling, and then tempered in a temperature range of 500 to 650 ° C. For producing ultra-thick high-strength steel without steel.