JP2627697B2 - Manufacturing method for tough steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a tough steel plate or steel material.

[0002]

2. Description of the Related Art The material of steel plates used as structural members such as offshore structures and bridges is determined by chemical components and heat treatment. Recently, it has become possible to produce thick steel plates having good strength and toughness by a controlled rolling method mainly performed at low temperature and an accelerated cooling method in which cooling is performed after rolling. Such techniques include JP-B-49-7291, JP-B-57-21007, and JP-B-59-14535. In general controlled rolling, austenite is refined by rolling recrystallization in a high temperature range, and austenite is stretched sufficiently in an unrecrystallized state in a low temperature range, and a fine ferrite is obtained in a subsequent transformation process. ing. In order to achieve toughness by this method, it is necessary to increase the rolling reduction in both the recrystallization temperature range and the non-recrystallization temperature range of austenite. However, the reduction ratio that can be taken is limited by the ratio of the slab thickness to the product thickness. Therefore, it has been difficult to obtain a certain degree of mechanical properties by this method. These problems are basically the same even in the case of the accelerated cooling method in which cooling is performed after controlled rolling.

[0003]

SUMMARY OF THE INVENTION The present invention is a method for producing a tough steel material which can advantageously eliminate the above-mentioned drawbacks of the conventional method. The gist of the invention is as follows. (1) C: 0.02 to 0.50%, Si: 0.01 to
2.0%, Mn: 0.30-3.5%, with the balance being the center of the steel material that acts during rolling in the austenite non-recrystallization temperature range when hot rolling a steel material consisting of Fe and inevitable impurities. A method for producing a tough steel material, characterized in that there is at least one or more rolling passes at which the maximum value of the hydrostatic stress of the part becomes 10 kg / mm ² or more. (2) C: 0.02 to 0.50%, Si: 0.01 to
2.0%, Mn: 0.30 to 3.5%, and Nb:
0.001 to 0.10%, Al: 0.002 to 0.10
%, Ti ≦ 0.10%, Cu ≦ 3.0%, Ni ≦ 10.
0%, Cr ≦ 10.0%, Mo ≦ 3.5%, Co ≦ 1
0.0%, W ≦ 2.0%, V ≦ 0.10%, B ≦ 0.0
025%, Rem ≦ 0.10%, Ca ≦ 0.0030%
Hydrostatic stress at the center of the steel material during hot rolling of austenite in the non-recrystallization temperature range when hot rolling a steel material containing one or two or more of the following and Fe and unavoidable impurities A method for producing a tough steel material, characterized in that at least one rolling pass has a maximum value of 10 kg / mm ² or more. (3) C: 0.02 to 0.50%, Si: 0.01 to
2.0%, Mn: 0.30-3.5%, with the balance being Fe and inevitable impurities, after casting, as it is or after reheating, the steel sheet or steel material is cooled before or after the start of rolling. In a manufacturing method in which a deformation resistance difference is given in the thickness direction of steel material and rolling is continued , the cooling
Cooling start temperature, cooling time, cooling rate and subsequent rolling
A method for producing a tough steel material, characterized in that there is at least one or more rolling passes in which the rolling shape ratio is set within a range satisfying the following inequality. t ≧ (T-1100) /c+3.4/c (194.5 / m−267.8) and (T−A _r3 ) / c ≧ t ｛T> 1100 + 3.4 (267.8-194.5) / M) where the value on the right side of the inequality is the presence or absence of cooling (the cooling rate c
Value) irrespective of the value of ΔT ≦ 1100 + 3.
4 (267.8-194.5 / m), cooling was performed
The same effect as when the deformation resistance difference is given
It is sufficient that T ≧ _Ar3 is satisfied. Here, T: cooling start temperature (° C.), m: rolling shape ratio,
t: Cooling time (S), c: Cooling rate (° C./S). m = 2√R (h ₀ −h ₁ ) / (h ₀ + h ₁ ) where R: roll radius (mm), h ₀ , h ₁ : entry side
And delivery side plate thickness (mm). (4) C: 0.02 to 0.50%, Si: 0.01 to
2.0%, Mn: 0.30 to 3.5%, and Nb:
0.001 to 0.10%, Al: 0.002 to 0.10
%, Ti ≦ 0.10%, Cu ≦ 3.0%, Ni ≦ 10.
0%, Cr ≦ 10.0%, Mo ≦ 3.5%, Co ≦ 1
0.0%, W ≦ 2.0%, V ≦ 0.10%, B ≦ 0.0
025%, Rem ≦ 0.10%, Ca ≦ 0.0030%
The steel containing at least one of the following, and the balance consisting of Fe and unavoidable impurities, is cast or directly reheated after casting, and before or after the start of rolling, the steel sheet or the steel material is cooled to reduce the thickness direction of the steel material. To give a difference in deformation resistance,
In the manufacturing method in which rolling is continued , the cooling
A method for producing a tough steel material, characterized in that there are at least one or more rolling passes in which the starting temperature, the cooling time, the cooling rate and the rolling shape ratio of the subsequent rolling satisfy the following inequalities. t ≧ (T-1100) /c+3.4/c (194.5 / m−267.8) and (T−A _r3 ) / c ≧ t ｛T> 1100 + 3.4 (267.8-194.5) / M) where the value on the right side of the inequality is the presence or absence of cooling (the cooling rate c
Value) irrespective of the value of ΔT ≦ 1100 + 3.
4 (267.8-194.5 / m), cooling was performed
The same effect as when the deformation resistance difference is given
It is sufficient that T ≧ _Ar3 is satisfied. Here, T: cooling start temperature (° C.), m: rolling shape ratio,
t: Cooling time (S), c: Cooling rate (° C./S). m = 2√R (h ₀ −h ₁ ) / (h ₀ + h ₁ ) where R: roll radius (mm), h ₀ , h ₁ : entry side
And the delivery side plate thickness (mm).

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the components of the steel of the present invention will be described. C is an indispensable element for strengthening steel materials.If it is less than 0.02%, it is difficult to obtain the required high strength, and if it exceeds 0.50%, the toughness of the weld is impaired, so it is limited to 0.02% or more and 0.50% or less. did. Since Si is an element effective for promoting deoxidation and increasing the strength, it is added in an amount of 0.01% or more. However, if added too much, the weldability is deteriorated, so that the content is limited to 2.0% or less. Mn is effective as an element for improving low-temperature toughness, so it is added in an amount of 0.3% or more. However, if added in an amount of 3.5% or more, there is a risk of promoting welding cracking, so Mn is limited to 3.5% or less. Nb is an element that suppresses austenite rolling recrystallization in a trace amount and is effective for strengthening unrecrystallized rolling.
Excessive addition degrades the weld joint toughness, so keep it below 0.1%. Since Al is effective as a deoxidizing agent, it may be added in an amount of 0.002% or more. However, since an excessive amount of Al generates inclusions harmful to the material, the upper limit is set to 0.1%.

[0005] Since a small amount of Ti is effective in refining crystal grains, it may be added in such an amount that does not deteriorate weld toughness. Therefore, the upper limit of the added amount is 0.10%. Cu, N
i, Cr, Mo, Co, and W are all known as elements that improve hardenability and can increase the strength of steel when added to the steel of the present invention, but excessive addition impairs weldability. Therefore, Cu is 3.0% or less, Ni is 10% or less, Cr is 1
The content was limited to 0% or less, Mo to 3.5% or less, Co to 10% or less, and W to 2% or less. V is effective for increasing the strength due to the precipitation effect,
Excessive addition impairs toughness, so the upper limit is 0.10
%. B is known as an element that improves hardenability and can increase the strength of steel when added to the steel of the present invention, but excessive addition increases the precipitates of B and impairs toughness. , And the upper limit was made 0.0025%. Rem and
Ca is effective for detoxifying S, but excessive addition impairs toughness, so the upper limits were set to 0.10% and 0.0030%, respectively.

Next, the technical concept underlying the present invention will be described. Conventionally, as a processing method for improving the toughness of thick steel plate, the crystal grains are refined by recrystallization by rolling in the recrystallization temperature range of austenite, and subsequently the crystal grains are sufficiently stretched in rolling in the non-recrystallization temperature range, and It has been considered effective to transform in this state. That is, it can be said that the conventional rolling method has aimed at maximizing the effect of the rolling by controlling both the rolling temperature and the rolling reduction. In these methods, the rolling conditions that can be controlled are limited, and it has been difficult to expect a certain degree or more of rolling effects. However, the present inventors have discovered a new fact that makes it possible to overcome the above-mentioned limitations, and have invented a method for producing a tough steel material.

[0007] The present inventors have found conditions in which the stress acting during rolling, in addition to both the rolling temperature and the rolling reduction, affect the microstructure during rolling. The changes in the microstructure during hot rolling are austenite recovery and recrystallization, which are rate-limited by the austenite self-diffusion.
Normally, the effect of stress (pressure) on self-diffusion can be neglected, but the effect cannot be ignored if the acting stress can be increased as in the case of rolling under predetermined rolling conditions.
The effect of stress (pressure) on the self-diffusion coefficient is shown by the following equation. D (p) = D (0) exp (-p ・ Δv / kT) where D (p): self-diffusion coefficient of austenite under stress (pressure) p, D (0): under atmospheric pressure Austenite self-diffusion coefficient, Δv: activation volume, k: coefficient, T: temperature. The stress (pressure) in this case is an isotropic compressive stress and is called a hydrostatic stress component. Further, such a compressive stress during rolling is effectively achieved by cooling the steel material under predetermined conditions and imparting a difference in the deformation resistance in the thickness direction of the steel material, and then rolling at a rolling shape ratio of a predetermined value or more. Can be increased.

Next, the reasons for limiting the manufacturing conditions will be described in detail. As mentioned above, compressive stress during rolling suppresses recovery and recrystallization through reduction of the austenite self-diffusion coefficient,
As a result, the controlled rolling effect is increased. As a result of investigating the value of the self-diffusion coefficient D (p) of austenite under various conditions, as shown in FIG. 1, D (p) decreases as p increases, and in particular, 10 kgf / mm at compressive stress. It was found that when a stress exceeding ² was applied, D (p) was greatly reduced. That is, when a stress exceeding 10 kgf / mm ² is applied by compressive stress, recovery and recrystallization during rolling are suppressed, and even when rolling is performed at the same rolling reduction, the effect of strengthening rolling in the non-recrystallization temperature region is obtained. It turned out to be obtained. Claims 1 and 2, wherein a hydrostatic stress acting on the center of the steel material during rolling of the steel material (compressive stress is defined as positive).
Is the maximum value of was restricted with 10 kg f / mm ² or more, by this reason. Of course, the greater the hydrostatic stress, the greater the effect. The hydrostatic stress of 10 kgf / mm ² or more is the hydrostatic stress of the central portion of the steel material acting during rolling.
The maximum value is the temperature distribution of steel as shown in the description of the example.
Calculation by the finite element method considering the distribution of deformation and deformation resistance
Rolling estimated to be 10 kgf / mm ² or more due to
Achieved by adjusting the conditions (rolling temperature, rolling reduction, etc.)
Although it is effective to operate in at least one or more rolling passes, it is more effective to operate in many rolling passes.

In claims 3 and 4 , cooling is performed before or during rolling, and the surface of the steel material is cooled.
This is a condition under which cooling, a deformation resistance difference is generated in the plate thickness direction, and a higher hydrostatic stress is applied by combining appropriate rolling conditions to enable an increase in the controlled rolling effect. The steel to be rolled may be directly rolled after casting, or may be rolled by reheating after the temperature is lowered after casting. In addition, the time at which the temperature distribution and the deformation resistance difference are imparted to the inside of the steel material by cooling is the same effect as long as the predetermined cooling conditions and rolling conditions are obtained before or during rolling, as long as the predetermined cooling conditions and rolling conditions are obtained. can get. Here, predetermined cooling conditions, rolling conditions and
Means cooling start temperature, cooling time and rolling shape ratio
The cooling start temperature, cooling time and rolling shape ratio are as follows
It is necessary to satisfy the inequality: t ≧ (T-1100) /c+3.4/c (194.5 / m−267.8) and (T−A _r3 ) / c ≧ t ｛T> 1100 + 3. 4 (267.8-194.5 / m) where the value on the right side of the inequality is the presence or absence of cooling (the cooling rate c
Value) irrespective of the value of ΔT ≦ 1100 + 3.
4 (267.8-194.5 / m), cooling was performed
The same effect as when the deformation resistance difference is given
It is sufficient that T ≧ _Ar3 is satisfied. Here, T: cooling start temperature (° C.), m: rolling shape ratio,
t: Cooling time (S), c: Cooling rate (° C./S). _{m = 2√R (h 0 -h 1} ) / (h 0 + h 1) However, R: roll radius _{(mm), h 0, h} 1: entrance side
And the delivery side plate thickness (mm). The reasons for limiting the production method so that the cooling start temperature, cooling time, cooling rate and rolling shape ratio satisfy the following inequalities are as follows.

The second inequality in the third and fourth terms is
This is a condition for preventing the average temperature of the steel material after cooling from dropping below the transformation point and preventing the toughness from deteriorating due to the formation of processed ferrite in the subsequent processing. In addition, the third and fourth terms
The first inequality of the term inequality is the various cooling and rolling
For the conditions, reference (2) in the description of the embodiment of the present invention
Hydrostatics in steel using calculations such as the finite element method shown in
Pressure stress, the maximum value of hydrostatic stress during rolling is 10 kg
f / mm ² condition was created as a regression equation.
is there. As a result, the hydrostatic stress at the center of the plate thickness is reduced by cooling.
Is found to be well regulated by the falling temperature
Was. This is because the temperature of the steel surface drops due to cooling,
A corresponding temperature difference occurs between the surface and the center,
Deformation resistance difference caused by degree difference (the surface part is not easily deformed)
The deformation in the plate length direction and the plate width direction at the center of the plate thickness was constrained by
As a result, the higher hydrostatic pressure
Presumed. In other words, the temperature of the surface decreases due to water cooling.
Therefore, the hydrostatic stress at the center of the plate can be increased,
The amount is the temperature difference between the surface and the center (approximately
Depression amount)
It is. As long as this condition is satisfied, the maximum value of the hydrostatic stress acting on the central portion of the steel material during rolling exceeds 10 kgf / mm ² and effectively acts on increasing the controlled rolling drop. Where the right side of the inequality (T-1100) / c
+3.4 /c(194.5 /m-267.8) is cooling
Negative value regardless of presence / absence (value of cooling rate c)
｛T ≦ 1100 + 3.4 (267.8-194.5 /
In the case of m), the deformation resistance difference is given even without cooling.
It shall have the same effect as if, satisfy the T ≧ A _r3
It should be good. Here, T: cooling start temperature (° C.), m: rolling shape ratio m = 2√R (h ₀ −h ₁ ) / (h ₀ + h ₁ ), where R: roll radius (mm), h ₀ , h ₁ : Entering side
And the delivery side plate thickness (mm) .

[0011]

Next, the present invention will be described in detail based on embodiments. First, when the methods of the present invention and the comparative methods shown in Tables 2, 3 and 4 are applied to the steel types shown in Table 1, Tables 2, 3 and
The mechanical properties shown in Fig. 4 are obtained, and the present invention clearly brings about toughening of the steel material, and the present invention is effective.

[0012]

[Table 1]

[0013]

[Table 2]

[0014]

[Table 3]

[0015]

[Table 4]

(How to Determine Hydrostatic Stress Component) The hydrostatic stress acting on the center of the steel material during rolling was determined by calculation using a rigid-plastic finite element method program. There are many literatures on the calculation of stress in a rolling roll tool by the rigid-plastic finite element method, but the calculation in the present invention followed the method of literatures (1) and (2). (1) Mori, Shima, Kosakada: Opportunities Society Transactions, 45-396 (1979), p.
955. (2) Yamada, Hamazuzu, Mori, Kawanami: Preprints of the Spring Meeting of the Japan Society for Technology of Plasticity, (1986), p235, Tokyo. FIG. 2 shows an example of element division in a rolling roll tool used for calculation. FIG. 3 is an enlarged view of a portion A in FIG. The stress and strain acting during rolling were calculated by the rigid plastic finite element method by giving the temperature and deformation resistance at the time of rolling bite for each element. For the deformation resistance, the equations of the references (3) and (4) were used. (3) Shida: plasticity and working, 9 (1968), p127 (4) Shida: plasticity and working, 10 (1969), p610 FIGS. 4 and 5 are calculation examples. FIG. 4 shows a temperature distribution in the thickness direction at the time of rolling engagement. FIG. 5 shows a change in a hydrostatic stress component in a rolling roll bite acting on a central portion of a sheet thickness when rolling is performed under the rolling conditions shown in the drawing. FIG. 5 also shows the maximum value of the hydrostatic stress component.

[0017]

As described above, the fact that the present invention has been carried out to obtain extremely excellent high-strength and high-toughness steel materials has a great industrial effect.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of hydrostatic stress on the self-diffusion coefficient of γ-Fe.

FIG. 2 is a diagram showing an example of element division in a rolling roll tool used for calculation.

FIG. 3 is an enlarged view of a portion A in FIG. 2;

FIG. 4 is a temperature distribution diagram in the thickness direction at the time of rolling engagement.

FIG. 5 is a diagram showing a change in hydrostatic stress in a roll bite.

Claims (4)

(57) [Claims] [Claim 1] C: 0.02 to 0.50%, Si: 0.01 to 2.0%, Mn: 0.3
When hot-rolling a steel material containing 0 to 3.5%, the balance being Fe and unavoidable impurities, the maximum value of the hydrostatic stress of the steel material center acting during rolling in the austenite non-recrystallization temperature region is 10 kg /. A method for producing a tough steel material, wherein at least one or more rolling passes of mm ² or more are provided. 2.C: 0.02 to 0.50%, Si: 0.01 to 2.0%, Mn: 0.3
0 to 3.5%, Nb: 0.001 to 0.10%, Al: 0.002 to 0.10
%, Ti ≤ 0.10%, Cu ≤ 3.0%, Ni ≤ 10.0%, Cr ≤ 10.0%, Mo ≤
3.5%, Co ≦ 10.0%, W ≦ 2.0%, V ≦ 0.10%, B ≦ 0.0025%, Re
m ≤ 0.10%, Ca ≤ 0.0030% containing one or more types,
When hot rolling a steel material whose balance is Fe and unavoidable impurities, the maximum value of the hydrostatic stress at the center of the steel material acting during rolling in the austenite non-recrystallization temperature range is 10 kg / mm ²
A method for producing a tough steel material, wherein at least one or more rolling passes are provided. 3. C: 0.02 to 0.50%, Si: 0.
01-2.0%, Mn: 0.30-3.5%, balance is F
e. After casting the steel consisting of unavoidable impurities and as it is or after reheating, before the start of rolling or after the start of rolling, the steel plate or the steel is cooled to impart a deformation resistance difference in the thickness direction of the steel, and then rolled continuously. In the law,
Cooling start temperature, cooling time, cooling rate and continued cooling
A method for producing a tough steel material, characterized in that there are at least one rolling pass in which the rolling shape ratio of rolling is set within a range satisfying the following inequality. t ≧ (T-1100) /c+3.4/c (194.5 / m−267.8) and (T−A _r3 ) / c ≧ t ｛T> 1100 + 3.4 (267.8-194.5) / M) where the value on the right side of the inequality is the presence or absence of cooling (the cooling rate c
Value) irrespective of the value of ΔT ≦ 1100 + 3.
4 (267.8-194.5 / m), cooling was performed
The same effect as when the deformation resistance difference is given
It is sufficient that T ≧ _Ar3 is satisfied. Here, T: cooling start temperature (° C.), m: rolling shape ratio,
t: Cooling time (S), c: Cooling rate (° C./S). m = 2√R (h ₀ −h ₁ ) / (h ₀ + h ₁ ) where R: roll radius (mm), h ₀ , h ₁ : entry side
And delivery side plate thickness (mm). 4. C: 0.02 to 0.50%, Si: 0.
01 to 2.0%, Mn: 0.30 to 3.5%, and
Nb: 0.001 to 0.10%, Al: 0.002
0.10%, Ti ≦ 0.10%, Cu ≦ 3.0%, Ni
≦ 10.0%, Cr ≦ 10.0%, Mo ≦ 3.5%, C
o ≦ 10.0%, W ≦ 2.0%, V ≦ 0.10%, B ≦
0.0025%, Rem ≦ 0.10%, Ca ≦ 0.00
After casting a steel containing 30% or more of one or more kinds and the balance consisting of Fe and unavoidable impurities, as it is or after reheating, before or after the start of rolling, the steel sheet or the steel material is cooled and the steel plate is cooled. In a manufacturing method in which a deformation resistance difference is given in the thickness direction and rolling is continued , the cooling
Cooling start temperature, cooling time, cooling rate and subsequent rolling
A method for producing a tough steel material, characterized in that there is at least one or more rolling passes in which the rolling shape ratio is set within a range satisfying the following inequality. t ≧ (T-1100) /c+3.4/c (194.5 / m−267.8) and (T−A _r3 ) / c ≧ t ｛T> 1100 + 3.4 (267.8-194.5) / M) where the value on the right side of the inequality is the presence or absence of cooling (the cooling rate c
Value) irrespective of the value of ΔT ≦ 1100 + 3.
4 (267.8-194.5 / m), cooling was performed
The same effect as when the deformation resistance difference is given
It is sufficient that T ≧ _Ar3 is satisfied. Here, T: cooling start temperature (° C.), m: rolling shape ratio,
t: Cooling time (S), c: Cooling rate (° C./S). m = 2√R (h ₀ −h ₁ ) / (h ₀ + h ₁ ) where R: roll radius (mm), h ₀ , h ₁ : entry side
And delivery side plate thickness (mm).