JPS589814B2 - Manufacturing method for high-toughness, high-tensile tempered steel sheet with reduced rolling anisotropy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-toughness, high-tensile-temperature tempered steel sheet with reduced rolling anisotropy.

In the production of normal high-strength steel sheets, blooming is performed at high temperatures to increase productivity, with a finishing temperature of 1150°C.
After that, for dehydrogenation treatment, it is usually slowly cooled or isothermally maintained in a dehydrogenation furnace.

Therefore, the following drawbacks arise.

That is, since the finishing temperature during blooming rolling is high, recrystallization progresses quickly, resulting in significantly coarsening of austenite grains.

During slow cooling for dehydrogenation treatment after blooming, AlN compounds concentrate and precipitate at coarse austenite grain boundaries (
Figure 1).

If the heating temperature during subsequent plate rolling is high (1150°C or higher), this AtN compound will re-dissolve into solid solution and once disappear, and will precipitate uniformly and finely when the product plate cools, so there is no problem. .

However, if the heating temperature is lower than 1150° C., solid solution does not occur again, and as the base iron is deformed by rolling, it is aligned in the rolling direction as shown in FIG.

Therefore, the ductility in the direction perpendicular to the rolling direction deteriorates, and the impact value also decreases.

A rolling direction test piece L and a rolling direction test piece C were taken and subjected to an impact test. The results are shown in FIG.

This result shows that the impact value decreases significantly in the direction perpendicular to rolling.

The heating temperature in the actual thick plate rolling process is kept at a low temperature at which the AlN compound does not dissolve again in order to prevent scale flaws, prevent the effect of improving boron hardenability from decreasing, and save energy. Anisotropy in impact properties caused by rolling anisotropy poses a problem.

This invention is based on the above-mentioned knowledge that the finish temperature of blooming roll has an important influence on the precipitation behavior of AlN compounds, and the present invention manages the finish humidity and processing rate of blooming roll, and heats the plate during rolling of thick plates. This paper proposes a method for manufacturing high-toughness, high-tensile tempered steel sheets that reduces rolling anisotropy by limiting the temperature.

This invention has C0.05-0.18%. Si0.50%
Below, Mn 0.30 to 1.50%, P 0.02% or less,
S0.015% or less, CuO. 05-0.50%. Ni
O. 30-2.00%. CrO. 10-1.50%. M
oO. 15~0.70%, V0.01~0.10%, S
olAl0.04-0.15%. B.O. 0005~0.
0.030%, N0.002 to 0.015%, and the carbon equivalent (Ceq) when the plate thickness is 50 mm or less is 0.
5 lower (Ceq=C+~Si+-Mn+-Ni+Cr10"
After manufacturing a steel ingot from molten steel that satisfies the condition that the carbon equivalent (Ceq) is 0.60 or less when the plate thickness exceeds 50 mm and is 250 mm or less, and the balance is Fe and unavoidable impurities, the steel The lump is heated to 1,250 to 1,330°C to start blooming rolling, and the slab is rolled to a predetermined shape by performing a reduction of at least 10% with the temperature at the center of the slab being 1,050°C or less. After dehydrogenation annealing by slow cooling at a cooling rate of less than 1 day, or after blooming and cooling to complete transformation, the plate thickness is 50 mm at AC 1 point or less.
After dehydrogenation treatment, either by holding isothermally for 10 hours or more and dehydrogenation annealing, further 115
It is characterized by being reheated to a temperature of 0°C or lower, rolled into a thick plate product of a predetermined shape, cooled, reheated and quenched at a temperature of 3 Ac or higher, and tempered at a temperature of 1 AC or lower. This is a method for producing high-toughness, high-tensile tempered steel sheets with reduced rolling anisotropy.

In this invention, the steel ingot heating temperature during blooming rolling is set to 125
The reason why the temperature is set at 0 to 1330°C is because if the temperature is lower than this range, the effect of dissipating the micro-segregation of the steel ingot by diffusion will be reduced, and if it is higher than this range, the life of the furnace will be shortened.

In addition, in blooming rolling, applying a reduction of at least 10% at a slab center temperature of 1050°C or lower causes recrystallization of austenite grains and precipitation of AlN compounds at the same time after reduction, resulting in concentration of AlN compounds at coarse austenite grain boundaries. This is to prevent precipitation and ensure uniformly dispersed precipitation.

If the AlN compound is uniformly dispersed and precipitated, even if the heating temperature during thick plate rolling is low and the A7N compound does not disappear due to re-solid solution, the lined AlN will be removed during ductile fracture in the rolling direction test piece. A situation where a large number of voids occur continuously along the compound is avoided.

The inventor experimentally investigated the influence of the blooming finishing temperature and found that 105
It has been found that by applying a rolling reduction of 10% or more at 0° C. or lower, the elf energy (upper shelf energy) in the impact test in the direction perpendicular to the rolling direction is improved.

That is, an impact test was conducted in the direction perpendicular to the rolling direction when the finishing temperature of the blooming rolling was varied and a rolling reduction of 15% was applied at the finishing humidity in each pass schedule.

The results are shown in FIG. Each sample was heated to 1280℃ for 1
Heating for hours, blooming and slow cooling (300℃/day)
After that, it was heated to 1080° C. for 1 hour to perform thick plate rolling and allowed to cool, further heated to 930° C. for 1 hour for quenching, and then heated to 635° C. for 1 hour and allowed to cool.

From this result, it can be seen that when the blooming finishing temperature is 1050°C or lower, the impact value in the rolling direction improves from 9 Kg-m to 14 Kg-m, and the rolling anisotropy is significantly reduced. .

The reason for slow cooling after blooming at a cooling rate of 400° C./day or less is to perform dehydrogenation treatment during slow cooling.

Cooling at a cooling rate exceeding 400° C./day is not desirable because dehydrogenation becomes insufficient and hydrogen cracking occurs.

Another method for dehydrogenation annealing is to allow the dehydrogenation annealing to occur after blooming, allowing the austenite to ferrite + bainite + pearlite transformation to complete, and then holding it just below the ACl point (for more than 10 hours for dehydrogenation annealing). This is because the diffusion rate of hydrogen is faster in ferrite at the same temperature, so it is advantageous to perform dehydrogenation treatment in the ferrite structure, and in this case, it is necessary to hold it for 10 hours or more per plate thickness of 50 mm. .

The reason why the thick plate rolling heating temperature is set to 1150° C. or lower is to prevent the occurrence of scale defects and a decrease in the hardenability improving effect of B.

When heated to 1150° C. or higher, scale is generated, and since the scale is difficult to peel off in the alloy steel that is the subject of this invention, it becomes the main cause of indentation defects during rolling.

Furthermore, the hardenability improving effect of B is also reduced, resulting in poor hardenability.

Next, the reason for limiting the chemical composition of the alloy steel that is the subject of this invention will be explained.

C is required in an amount of 0.05% or more to ensure strength, but if it exceeds 0.18%, it impairs weldability, which is not desirable.

Si is necessary for deoxidation and is effective in increasing hardenability and resistance to temper softening, but if it is less than 0.05%, the effect cannot be obtained sufficiently, and if it exceeds 0.50%, it deteriorates toughness. Therefore, the range of 0.05 to 0.50% is desirable.

Mn' is effective for increasing hardenability, and must be contained at 0.30% or more to ensure toughness, but Mn' is 1.50% or more.
% is undesirable because it promotes rolling anisotropy and tempering brittleness.

Since P causes embrittlement due to burning, it is desired that the content be as small as possible, and is limited to 0.02% or less.

S increases rolling anisotropy and deteriorates ductility, so 0.01
It is necessary to limit it to 5% or less.

Cu improves toughness without impairing weldability, so 0.
0.051% or more is required, but if it exceeds 0.50%, the surface quality deteriorates, which is not desirable.

Ni needs to be 0.30% or more to improve low-temperature toughness, but because it is expensive, it is set to 2.0% or less.

Cr needs to be present in an amount of 0.10% or more to improve hardenability, but if it exceeds 1.50%, toughness decreases and weldability deteriorates, which is not desirable.

Mo improves hardenability. , and increases the resistance to softening during tempering, so it is contained at 0.15% or more, but 0.70%
If it exceeds V, the toughness decreases, so it is desirable that V is effective in increasing the resistance to temper softening, and in order to ensure strength, V is 0.0.
1% or less is required, but if it exceeds 0.10%, the toughness deteriorates, which is not desirable.

At is effective in fixing N and exhibiting the effect of improving the hardening of B, and is required in an amount of 0.04% or more as SolAl; however, if it exceeds 0.15%, the toughness decreases and is therefore undesirable.

B is effective in improving hardenability without impairing weldability, but it is ineffective at less than 0.00051%, and o. o
If it exceeds 0.30%, the ductility and toughness will decrease;
．． It was limited to a range of 0.0005% to 0.0030%.

N is required to be 0.002% or more in order to adjust the austenite grain size, but if it exceeds 0.015%, it is not desirable because it impairs weldability.

In addition, it is desirable that the carbon equivalent is above a certain value in order to improve hardenability, but in order to ensure weldability, αq is 0.53% or less under JM plate thickness of 50 mm, and 50 mm
Ceq must be 0.60% or less below 250 mm.

Example 1 CO. 12%. SiO. 27%. MnO. 82%, P0
．． 015%. S0.005%. CuO. 26%. NiO
．． 91%, Cr0.44%. Mo0.42%, vO, 0
5%. Solk10.077%, B0.0025%, N
O. 0074% CeqO. 487% steel ingot 1
It is heated to 280℃ and subjected to blooming rolling, and the finishing temperature is 1000℃.
A pressure reduction of 15% was carried out at 150° C., and the mixture was slowly cooled at a cooling rate of 150° C./day.

Then, it was heated to 1080℃ and rolled into a thick plate of 5.0m.
After finishing the plate into a thick plate with a thickness of m, it was heated to 930°C for 1 hour and water quenched, then heated to 635°C for 1 hour and allowed to cool (steel value, A-1).

For comparison, among the above processing conditions, the blooming finishing temperature was set to 11.
One with a heating temperature of 50°C (steel type No. A-2) and one with a thick plate rolling heating temperature of 1200°C (steel type No. A-3)
made.

The subsequent steps are the same as A-1.

The ACl point of a low alloy steel such as the steel of this invention is approximately 720°C, which is almost equal to the eutectoid impregnation of carbon steel.

A test piece L in the rolling direction and a test piece C in the direction perpendicular to the rolling direction were taken from these samples and subjected to a tensile test and an impact test. Table 1 shows the results.

Example 2 C0.13%. SiO. 25%, Mn0.92%. P0
．． 012%. S0.003%, Cu0.27%. Ni1
．． 32%, CrO. 51%, Mo0.48%. VO. 0
4%. SoAAt0.065%, BO. 0018%,N
A steel ingot containing 0.0066% Ceq and 0.552% Ceq was heated to 1280°C to perform blooming rolling, and the finishing temperature was 100°C.
A pressure reduction of 15% was performed at 0°C, and the mixture was slowly cooled at a cooling rate of 150°C/day.

Then, it is heated to 1080℃ and rolled into a thick plate of 100mm.
After finishing it into a thick plate and water quenching it by heating it to 930℃ for 2 hours,... Heated to 635°C for 2 hours and allowed to cool (
Steel type No. B-1).

For comparison, among the above processing conditions, the blooming finishing temperature was set to 11.
One was made at 50°C (steel type No. B-2), and the other was made at a thick plate rolling heating humidity of 1200°C (steel type &.B3).

The ACl point of a low alloy steel such as the steel of this invention is approximately 720°C, which is almost equal to the eutectoid temperature of carbon steel.

A test piece L in the rolling direction and a test piece C in the direction perpendicular to the rolling direction were taken from these samples and subjected to a tensile test and an impact test. The results are shown in Table 2.

From the test results of Examples 1 and 2 above, it can be seen that the steel according to the present invention has reduced rolling anisotropy compared to the comparative example, and also has high strength and excellent impact properties.

Although B-2 has sufficient strength, vEs in the C direction is extremely low.

B-3 does not exhibit rolling anisotropy in vEs, but due to the high heating temperature during thick plate rolling, it is inferior in strength and vTs, which would allow B to fully exhibit its hardenability improvement effect.

Example 3 The same steel as in Example 1 was used under the same processing conditions, but the cooling method after blooming was changed, and after blooming, it was allowed to cool to room temperature, and then reheated to 680°C, held for 50 hours, and then allowed to cool. (Steel type No.A
-1') and the same steel as in Example 2 under the same processing conditions, but with a different cooling method after blooming and cooling to room temperature after blooming,
A sample (steel type No. B-1') was prepared by reheating it to 680°C, holding it for 50 hours, and then cooling it. From these samples, a test piece L in the rolling direction and a test piece C in the direction perpendicular to the rolling direction were taken. The results of the tensile test and the guard test are shown in Table 3.

This test result shows that even if the dehydrogenation treatment after blooming is changed, the same effects as in Examples 1 and 2 can be obtained.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the state in which AlN compounds are concentrated and precipitated at the austenite grain boundaries, which occur in steel slabs when the finishing temperature during blooming rolling is high.
An explanatory diagram showing the state of AtN compounds when a steel slab with precipitated AtN compounds is rolled into a thick plate. Figure 3 shows the impact test results for a test piece L in the rolling direction and a test piece C in a direction perpendicular to the rolling direction from a thick plate in which AlN compounds are lined up in the rolling direction. The fourth is a chart showing the effect of the finishing temperature of blooming on the impact value in the direction perpendicular to rolling.

Claims (1)

[Claims] 8%. Si 0.50% or less, 1.50%. P 0.02% or less, S 0.015% or less, CuO. 0 5 ~ 0.5 0%,
NiO%, Cr 0.10-1.50%, Mo0.
15-0.70%. V0.01~0.10%, Solk
t0.0 4~0.1 5%, B0.0005~0.0
030%, NO. 002 to 0.015%, and carbon equivalent Ceq (Ceq=C+1/24Si+1/6
Mn+l/40Ni +1/5Cr+1/4Mo+1/
14V) is 0.53 or less if the plate thickness is 50mm or less,
0.6 if the plate thickness exceeds 50mm and is less than 250mm
After manufacturing a steel ingot from molten steel that satisfies the conditions of 0 or less and consists of the balance Fe and unavoidable impurities, the steel ingot is
Start blooming by heating to 330°C, at least 10
% reduction table After rolling the slab into a predetermined shape with the temperature at the center of the slab being 1050°C or less, dehydrogenation annealing is performed by slow cooling at a cooling rate of 400°C/day or less, or
Or, after performing dehydrogenation treatment, either by allowing cooling after blooming rolling to complete the transformation, and then performing dehydrogenation annealing by holding isothermally at a temperature below the ACl point for 10 hours or more per plate thickness of 50mm, and then further The product is characterized by being reheated to a temperature of 1150°C or less, rolled into a thick plate product of a predetermined shape, cooled, then reheated and quenched at a temperature of 3 Ac or higher, and tempered at a temperature of 1 Ac or lower. A method for manufacturing high-toughness, high-tensile tempered steel sheets with reduced rolling anisotropy.