JPH08144019A - Bainitic steel material reduced in material dispersion and its production - Google Patents

Abstract

PURPOSE: To produce a steel material free from restrictions in a manufacturing process and reduced in material dispersion. CONSTITUTION: This steel material has a composition containing, by weight, 0.001-<0.030% C, <=0.60% Si, 1.00-3.00% Mn, 0.005-0.20% Nb, 0.0003-0.0050% B, and <=0.100% Al and also has a structure in which bainitic structure comprises >=90%. If necessary, 0.7-2.0% Cu is further incorporated.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a thick steel plate, steel strip, shaped steel or steel bar having a thickness of 30 mm or more, which is used in the fields of construction, marine structures, pipes, shipbuilding, storage tanks, civil engineering, construction machinery and the like. The present invention relates to a steel material such as a steel material, particularly a steel material having little variation in material quality and a manufacturing method thereof.

[0002]

2. Description of the Related Art As described above, thick steel materials represented by thick steel plates are used in various fields and have been improved in properties such as high strength and high toughness. In recent years, it has been required that these characteristics are uniform in the thickness direction and that the variations among steel materials are small.

For example, “Iron and Steel No. 74 (1988) No. 6
On pages 11 to 21 of the "No.", as the height of buildings rises,
It has been reported that, due to the deformation of buildings, it is designed to absorb vibration energy and prevent collapse due to a huge earthquake. Specifically, when an earthquake occurs, the framework of the building is collapsed into a predetermined shape, and the collapse of the building is prevented by plasticizing the framework material. That is, it is premised that the frame of a building behaves as intended by the designer when an earthquake occurs, and the designer must fully understand the load bearing ratio of steel materials such as columns and beams of the building. Is. Therefore, it is indispensable that steel materials such as steel plates and H-section steels used for columns and beams are homogeneous, and variations in strength of steel materials pose a serious problem.

Since high tensile strength and high toughness are required for steel materials used for construction and shipbuilding, this type of steel material is usually manufactured by the controlled rolling control cooling method, so-called TMCP method. Is. However, when a thick steel material is manufactured by this TMCP method, the cooling rate varies in the thickness direction or between the steel materials, and the microstructure changes, resulting in variations in the material thickness direction or between the steel materials. Of. Variations in material include those that appear in the thickness direction especially in thick steel plates, those that appear between webs and flanges due to uneven cooling between the webs and flanges in H-section steel, or those that appear between lots, etc. There is.

Therefore, in Japanese Patent Laid-Open No. 4-224623, Nb
Is added, the cooling rate after rolling is set to 3 ° C / s or more, and the upper limit of the cooling stop temperature is set to 500 ° C, so that the structure in the plate thickness direction is a structure in which ferrite and bainite are mixed, It has been proposed to increase the strength of the central portion and reduce the hardness difference in the plate thickness direction. However, the cooling rate must be strictly controlled to 3 ° C./s or more even in the central part of the plate thickness, and if the cooling rate distribution occurs in the plate thickness direction, the material will be dispersed immediately, so the manufacturing process is strict. Therefore, it was not suitable for production on an industrial scale.

Further, in Japanese Patent Laid-Open No. 62-130215, the strength is secured by Cu precipitation strengthening, while at the
It is possible to improve low temperature toughness by cooling to 300 to 700 ° C. at a cooling rate of 5 ° C./s or more, then maintaining the temperature range of 500 to 650 ° C. for a certain period of time, and then cooling to room temperature.
Proposed. However, this technology is aimed at improving low temperature toughness, and by suppressing the variation in materials in the various forms described above, it is possible to satisfy the homogeneity of materials required for recent structural steels. Difficult to do.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above problems, that is, there are no restrictions in the manufacturing process.
It is an object of the present invention to provide a steel material with less material variation in the thickness direction and between steel materials, and also to propose a method for manufacturing the steel material.

[0008]

[Means for Solving the Problems] The material variation of thick steel materials, typically thick steel sheets, is caused by a large change in the cooling rate in the thickness direction from the steel sheet surface to the central portion or variation in manufacturing conditions in the cooling process. This is because the change in the cooling rate caused by the change causes the microstructure variation. In order to avoid this structure fluctuation, it is important to obtain a uniform structure in a wide cooling rate range.

[0009] Therefore, the inventors of the present invention have repeatedly studied the method for obtaining a uniform structure even if the manufacturing conditions change, and as a result, by redesigning the component composition, the cooling rate changes. However, it has been found that a steel sheet having a uniform structure in the thickness direction and having little material variation can be obtained.

That is, in order to make the structure a bainite single phase without depending on the cooling rate, the Ar ₃ point is lowered.
In addition to adding appropriate amounts of n and Nb, in order to prevent ferrite from precipitating even at a low cooling rate, B that reduces the grain boundary energy of the former austenite grain boundary is added, and further, by limiting the amount of C, carbide precipitation in bainite By suppressing the above, the component composition that completely eliminates the strength change due to the change of the cooling rate and the change of the precipitation form of the carbide was realized. By complying with this component composition, the structure becomes a bainite single phase by a normal manufacturing process without being affected by rolling conditions and cooling conditions, and therefore variations in strength and toughness can be minimized.

This invention is (1) C: 0.001 wt% or more 0.03
Less than 0 wt%, Si: 0.60 wt% or less, Mn: 1.00 to 3.00 wt%,
Nb: 0.005-0.20 wt%, B: 0.0003-0.0050 wt% and
Al: a composition containing 0.100 wt% or less, and 90% or more having a bainite structure, which is a bainite steel material with little material variation (first invention), (2) In the first invention, the steel material further comprises Cu: A bainite steel material having a composition containing 0.7 to 2.0 wt% and less variation in material (second invention), (3) In the first invention or the second invention, the steel material is
Further, in the bainite steel material (third invention) having a composition containing Ti: 0.005 to 0.20 wt% and less variation in material, (4) the first invention, the second invention or the third invention, the steel material further has V: 0.005 to Bainite steel with a composition containing 0.20 wt% and little variation in quality (4th invention), (5) 1st
In the invention, the second invention, the third invention, or the fourth invention, the steel material further contains Ni: 2.0 wt% or less, Cr: 0.5 wt% or less, M
o: 0.5 wt% or less, W: 0.5 wt% or less and Zr: 0.5 wt%
% Bainite steel material having a composition containing one or two or more selected from the
(Invention), (6) In the first invention, the second invention, the third invention, the fourth invention or the fifth invention, the steel material further comprises REM and Ca.
It is a bainite steel material (sixth invention) having a composition containing at least one selected from 0.02 wt% or less and having little material variation.

The thick steel material can be manufactured by the following three methods using steel materials having various compositions according to the components defined in each of the first to sixth inventions. . That is, when hot rolling the (A) steel material, after heating to a temperature of Ac _{3 to} 1350 ° C., the rolling is finished in the austenite unrecrystallized temperature range of 800 ° C. or higher, and then cooled. Manufacturing method (B) When hot-rolling a steel material, after heating to a temperature of Ac _{3 to} 1350 ° C., rolling is completed in the austenite unrecrystallized temperature range of 800 ° C. or higher, and after cooling, 500 ° C. or higher 800 ° C
Manufacturing method characterized by performing a precipitation treatment of reheating and holding in a temperature range of less than (C) during hot rolling of a steel material, after heating to a temperature of Ac _{3 to} 1350 ° C, austenite of 800 ° C or more Rolling was completed in the non-recrystallization temperature range, and then 500 ° C, which is the precipitation temperature range.
After accelerating cooling at a cooling rate of 0.1 to 80 ° C / s to a specified temperature of 800 ° C or more and less than 800 ° C, keep isothermal for 30s or more in the temperature range of 500 ° C or more and less than 800 ° C, or 1 ° C / s or less in the temperature range. The manufacturing method is characterized in that a precipitation treatment of cooling for 30 s or more is performed at a cooling rate, and then cooling is performed.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the reasons for limiting each chemical component of the steel material of the present invention will be explained. C: 0.001 wt% or more and less than 0.030 wt% C is required to be 0.001 wt% or more in order to form a bainite single phase without depending on the cooling rate. On the other hand, when the content is 0.030 wt% or more, carbide precipitates inside the bainite structure or at the lath boundary, and when the cooling rate changes, the precipitation form of the carbide changes, so that it becomes difficult to obtain constant strength in a wide cooling rate range. Here, the difference (hardness change) between the maximum value and the minimum value of the hardness in the thickness direction was investigated for each thick steel plate having a thickness of 80 mm in which the C content was changed in the component system according to the present invention. The composition of components other than C is Si: 0.02 wt%,
Mn: 1.6 wt%, Nb: 0.020 wt%, B: 0.0018 wt% and
Al: 0.03 wt%. As shown in Fig. 1, the results of this investigation show that when the C content is less than 0.001 wt% and 0.030 wt% or more,
It can be seen that the hardness variation exceeds H _V : 20 and the strength variation becomes remarkable. Therefore, the C content is limited to 0.001 wt% or more and less than 0.030 wt%.

The upper limit of C is 0.02% by weight.
In addition to the homogeneous structure, excellent sulfide stress corrosion cracking resistance can be obtained. That is, in oil and natural gas pipe transportation, LPG storage tanks, and the like, crude oil and natural gas often contain hydrogen sulfide, and the hydrogen sulfide atmosphere corrodes the surface of the steel sheet and penetrates into the steel from this corroded surface. Since the atomic hydrogen thus formed locally concentrates in the steel, the susceptibility to cracking becomes high. At the same time, during the transportation of crude oil or natural gas, stress generated in the circumferential direction of the transportation pipe causes stress corrosion cracking in the concentrated region of atomic hydrogen, which leads to the destruction of the steel material. It is important to prevent this unique stress corrosion cracking in the sulfide environment, so-called sulfide stress corrosion cracking. And, by limiting the C content to 0.02 wt% or less, the transformation strain caused by the shear transformation is eliminated, the concentration of atomic hydrogen that enters the steel in the sulfide corrosion environment is prevented, and the strength is further increased. By achieving the increase by Cu precipitation strengthening, it is possible to obtain extremely excellent sulfide stress corrosion cracking resistance even in a high hydrogen sulfide concentration environment while having strength and low temperature toughness equivalent to or better than conventional ones. You can do it.

Si: 0.60 wt% or less If Si exceeds 0.60 wt%, the toughness of the welded portion deteriorates.
Limit to less than 0.60wt%. In addition, it is preferable to add 0.02 wt% or more for deoxidation and securing strength.

Mn: 1.00 to 3.00 wt% Mn is the bainite single phase, especially the volume fraction of the bainite structure.
To achieve 90% or more, 1.0 wt% or more, preferably 1.50
Although the content of more than 3.0% by weight is required, the content of more than 3.00% by weight significantly increases the hardening by welding and deteriorates the toughness of the heat affected zone (HAZ).

Nb: 0.005 to 0.20 wt% Nb has the effect of lowering Ar ₃ in particular and extending the bainite production range to the low cooling rate side, and is necessary for obtaining a stable bainite structure. Furthermore, it contributes to precipitation strengthening and is also effective in improving toughness. To expect these effects, 0.005 wt% or more is necessary. On the other hand, if it exceeds 0.20 wt%, the effect of improving the toughness is saturated and it is economically disadvantageous, so 0.20 wt% is the upper limit.

B: 0.0003 to 0.0050 wt% B needs to be 0.0003 wt% or more in order to form a bainite single phase, but if it exceeds 0.0050 wt%, BN precipitates and the weldability deteriorates. ~ 0.0050wt% limited.

Al: 0.100 wt% or less Al exceeds 0.100 wt% and the weldability is impaired, so the content is set to 0.100 wt% or less. 0.010 wt% for deoxidation
It is preferable to add the above.

According to the present invention, by adjusting the components to the above basic composition, a homogeneous structure, specifically a bainite structure of 90% or more, can be obtained with almost no dependence on the manufacturing conditions, particularly the cooling rate. Is characterized by. This feature is apparent from the experiment whose results are shown in FIG.

That is, regarding the steel adjusted to the composition according to the present invention (invention example) and the conventional steel used in building materials (conventional example), the cooling rate in the manufacturing process is 0.1 to
FIG. 2 shows the results of an examination of the tensile strength of the steel sheets obtained by variously changing it at 50 ° C./s. From the figure, it can be seen that by adjusting the components according to the present invention, a constant strength can be obtained without depending on the cooling rate. In particular, the YS and TS values show less variation over a wide range of cooling rates that cannot be predicted conventionally. This is where the limitation of the amount of C, and the addition of Mn and Nb, and further an appropriate amount of B, contribute as described above. Therefore,
Even if the cooling rate changes in the thickness direction of the thick steel sheet, the strength does not change depending on the cooling rate, and a thick steel sheet with little material variation in the thickness direction can be obtained.

In the invention examples, C: 0.007 wt%, Si: 0.
02wt%, Mn: 1.55wt%, Nb: 0.024wt%, B: 0.0018wt
% And Al: 0.032 wt%, the composition becomes a balance of iron and unavoidable impurities, while the conventional example has a composition of C: 0.
14 wt%, Si: 0.4 wt%, Mn: 1.31 wt%, Al: 0.024 wt
%, Nb: 0.015 wt%, Ti: 0.013 wt%. Then, in the same manufacturing process, the cooling rate was changed, a large number of thick steel plates with a thickness of 15 mm were manufactured, and the tensile strength was measured on the test pieces taken from each thick steel plate.

In the present invention, the strength and toughness level can be freely controlled by adding a predetermined chemical component to the basic component. At this time, since the already obtained homogeneous structure is less affected by the addition of a new component, a thick steel plate having high strength and / or high toughness with little material variation can be easily obtained.

First, in order to improve the strength, Cu: 0.7 to 2.0 wt% and further Ti: as a precipitation strengthening component.
0.005-0.20wt% and / or V: 0.005-0.20wt%
Can be added. In addition, when these precipitation strengthening components are added, further strengthening can be performed by performing a precipitation strengthening treatment described later.

Cu: 0.7 to 2.0 wt% Cu is added for the purpose of precipitation strengthening and solid solution strengthening, but if it exceeds 2.0 wt%, the toughness deteriorates sharply, while
If it is less than 0.7 wt%, the effect of precipitation strengthening is small, so 0.7-
2.0 wt%

Ti: 0.005 to 0.20 wt% In order to lower the Ar ₃ and contribute to the formation of a bainite structure, Ti becomes TiN to improve the toughness of the welded portion and to enhance the precipitation strengthening. Is required, while 0.
If the content exceeds 20 wt%, the toughness deteriorates, so 0.005 to 0.20
wt% range.

V: 0.005 to 0.20 wt% V is added in an amount of 0.005 wt% or more for precipitation strengthening, but even if added over 0.20 wt%, the effect is saturated, so 0.20 wt% is added. The upper limit.

Further, in order to improve the strength, Ni: 2.
0 wt% or less, Cr: 0.5 wt% or less, Mo: 0.5 wt% or less,
One or more selected from W: 0.5 wt% or less and Zr: 0.5 wt% or less can be added. Since these components are effective even in a small amount, the lower limits can be set appropriately.

Ni: 2.0 wt% or less Ni is effective in improving strength and toughness and preventing Cu cracking during rolling when Cu is added, but is expensive and excessively added. Even so, the effect is saturated,
Add within 2.0 wt% or less. If the addition amount is less than 0.05 wt%, the above effect is insufficient, so the addition amount is 0.05 wt%.
% Or more is preferable.

Cr: 0.5 wt% or less Cr has the effect of increasing the strength, but if added over 0.5 wt%, the toughness of the welded portion deteriorates, so Cr is added in the range of 0.5 wt% or less. The lower limit is preferably 0.05 wt%.

Mo: 0.5 wt% or less Mo has the effect of increasing the strength at normal temperature and high temperature, but if it exceeds 0.5 wt%, the weldability deteriorates.
Add in the range of wt% or less. The lower limit is 0.05 wt% because the effect of increasing strength is insufficient with addition of less than 0.05 wt%.
% Is preferable.

W: 0.5 wt% or less W has the effect of increasing the high temperature strength, but it is expensive, and if it exceeds 0.5 wt%, the toughness deteriorates, so 0.5
Add in the range of wt% or less. If the addition amount is less than 0.05 wt%, the strength increasing effect is insufficient.
% Or more is preferable.

Zr: 0.5 wt% or less In addition to the effect of increasing the strength, Zr has the effect of improving the plating crack resistance when galvanizing, for example.
If added in excess of 0.5 wt%, the toughness of the weld will deteriorate, so add it in the range of 0.5 wt% or less. The lower limit is 0.05
It is preferably set to wt%.

In order to improve the toughness of HAZ,
0.02wt of at least one selected from REM and Ca
% Or less can be added. REM turns into oxysulfide and suppresses the grain growth of austenite grains, and HAZ
The toughness is improved, but if added in excess of 0.02wt%, the cleanliness of the steel will be impaired, so the content is made 0.02wt% or less. In addition, 0.
Since the effect of improving the HAZ toughness is insufficient with the addition of less than 001 wt%, the addition amount is preferably 0.001 wt% or more.

Ca is effective in improving the toughness of HAZ, and
It is also effective in improving the material in the plate thickness direction by controlling the morphology of sulfides in steel, but if added over 0.02 wt%, the amount of nonmetallic inclusions increases and causes internal defects, so 0.02 wt% or less To do. If the addition amount is less than 0.0005 wt%, the above effect is insufficient, so the addition amount is preferably 0.0005 wt% or more.

Since the steel sheet of the present invention can obtain a homogeneous structure by adjusting the components to the above-mentioned basic composition, it is not necessary to strictly control the production conditions, and it is usual to produce this type of steel sheet. It may be manufactured according to.

For example, it is recommended that the steel slab whose composition is adjusted to the above-mentioned basic composition is heated to a temperature of Ac _{3 to} 1350 ° C., rolled at a temperature of 800 ° C. or higher, and then cooled.

[0038] That is, the heating temperature becomes insufficient homogenization can not be completely austenite is less than Ac _3, whereas, since the surface oxidation exceeds 1350 ° C. intensifies, the Ac ₃ to 1350 ° C. It is preferable to heat to a temperature range. If the rolling finishing temperature is lower than 800 ° C, the rolling efficiency is lowered, so that it is preferably 800 ° C or higher.

Cooling after rolling does not have to be strictly controlled as in the conventional case, and either air cooling or accelerated cooling can be used, but it is preferably 0.5 to 80 ° C./s. This is because when cooling is performed at a cooling rate exceeding 80 ° C / s, the bainite-laser spacing becomes close and the strength increases depending on the cooling rate, while at less than 0.5 ° C / s ferrite is less likely to form bainite single phase. .

Also in the manufacturing method, the strength and toughness level can be freely controlled by adding various treatment steps as in the case of the above-mentioned additive component. First, in the rolling process after heating to a temperature of Ac _{3 to} 1350 ° C, rolling is performed in the austenite unrecrystallized temperature range of 800 ° C or higher to improve the toughness.

That is, rolling in the austenite unrecrystallized region has the effect of refining the bainite structure by the introduction of work dislocations and improving the toughness. Here, as shown in FIG. 3, which shows the results of examining the relationship between the rolling reduction and the fracture surface transition temperature in the non-recrystallized region, since the rolling reduction ratio is 30% or more, the toughness improving effect becomes remarkable. 30% or more is recommended. In addition, the finishing temperature in the experiment shown in FIG.
The composition of the steel sheet used in the experiment was C: 0.007 wt%, Si:
0.02wt%, Mn: 1.55wt%, Al: 0.32wt%, Nb: 0.024wt
% And B: 0.0018 wt%, the balance becomes iron and unavoidable impurities. On the other hand, although the upper limit of the rolling reduction in the unrecrystallized region is not specified, it may be disadvantageous in operation to reduce the rolling reduction by 95% or more due to the problem of rolling load.

Further, as a precipitation strengthening component, Cu: 0.7-
When 2.0 wt%, and further, Ti: 0.005 to 0.20 wt% and / or V: 0.005 to 0.20 wt% is added, after rolling is finished, a predetermined temperature of 500 ° C or higher and lower than 800 ° C, which is the precipitation temperature range, is reached. After accelerating cooling at a cooling rate of 0.1 to 80 ° C./s, it is kept isothermal for 30 s or more at the predetermined temperature or at a cooling rate of 1 ° C./s or less in the temperature range.
It is effective to improve the strength to carry out a precipitation treatment for cooling for s or more.

That is, when the cooling rate from the end of rolling to the precipitation treatment temperature is less than 0.1 ° C./s, ferrite is formed in the bainite structure, while when it exceeds 80 ° C./s, the bainite-laser spacing becomes dense. The strength increases depending on the cooling rate, and at less than 0.1 ° C / s, ferrite is generated and the bainite single phase is not formed, so the cooling rate is 0.1 to 80
The range is ° C / s.

Then, after this accelerated cooling, 800 ° C. or more and 800 ° C. or more
Cu, Ti (CN) and V (CN) can be obtained by holding isothermally for 30 s or more in a temperature range of less than ℃ or by performing precipitation treatment for cooling for 30 s or more at a cooling rate of 1 ℃ / s or less in the temperature range.
It is possible to increase the strength by precipitating any one or more of the above, and further Nb (CN). Further, this precipitation treatment makes the structure uniform and further reduces the variation in material in the plate thickness direction.

Here, when the temperature of the precipitation treatment is 800 ° C. or higher, the precipitation components remain dissolved and the precipitation is less likely to occur. Therefore, in order to achieve sufficient precipitation, it is necessary to perform the precipitation treatment below 800 ° C. On the other hand, if the temperature is less than 500 ° C, the precipitation reaction does not easily occur, so the temperature range was set to 500 ° C or more and less than 800 ° C. Further, the holding time is set to 30 s or more because sufficient precipitation strengthening cannot be achieved when the holding time is less than 30 s. Then, precipitation strengthening can be obtained by maintaining the cooling rate of 1 ° C./s or less in the temperature range for 30 seconds or more, and sufficient precipitation strengthening cannot be obtained at a cooling rate of more than 1 ° C./s. A cooling rate of 0.1 ° C./s or less is desirable in order to sufficiently strengthen the precipitation.

Further, the above-mentioned precipitation treatment can be carried out after cooling following rolling. That is, after cooling, it may be reheated to a temperature range of 500 ° C. or higher and lower than 800 ° C. and held.

When the C content of the steel slab is limited to 0.02 wt% or less to obtain the above-mentioned excellent resistance to sulfide stress corrosion cracking, the holding time in the temperature range of 500 ° C. or higher and less than 800 ° C. or The cooling time is preferably set to 300 s or more. By this precipitation treatment, the elimination of the surface defects of bainite grains that have inherited the rolling strain at 950 ° C and below and the surface defects generated during shear transformation proceed simultaneously.
The concentration of atomic hydrogen that has penetrated into the steel in a sulfide corrosive environment is prevented, and the sulfide stress corrosion cracking resistance is improved.

[0048]

【Example】

Example 1 Steel slabs adjusted to various compositional compositions shown in Table 1 were treated with 1150
After heating to ℃, finish rolling with a total reduction of 74% Temperature:
Finished at 800 ℃, then accelerated cooling (cooling rate: 7 ℃ /
s) was performed to produce a thick steel plate having a thickness of 80 mm.

Each of the thick steel plates thus obtained was subjected to a tensile test and a Charpy test to investigate its mechanical properties and to evaluate the variation in strength in the thickness direction. The hardness distribution in the plate thickness direction was investigated by measuring the pitch. Furthermore, in order to evaluate the toughness of HAZ, after heating the steel plate to 1350 ° C
After a thermal cycle of cooling to 500 ° C in 300 s (corresponding to the heat history of HAZ when welding with a heat input of 500 kJ / cm), a Charpy test piece was sampled and the Charpy absorbed energy at 0 ° C was measured. It was measured.

As shown in Table 2 of the results of each of these investigations, the thick steel plate according to the present invention has a tensile strength of 400 MPa or more and has a uniform structure, so that there is a variation in hardness in the thickness direction. It is extremely small as compared with the comparative example, and it is understood that the difference between the maximum value and the minimum value of hardness is within 20 in H _v . The volume ratio of the bainite structure was measured by the point calculation method from an optical micrograph taken at 400 times.

[0051]

[Table 1]

[0052]

[Table 2]

Example 2 Steel slabs adjusted to various compositional compositions shown in Table 3 are shown in Table 4
The steel plate having a thickness of 80 mm was manufactured by performing the treatment according to the respective conditions shown in. Each of the thick steel plates thus obtained was subjected to a tensile test and a Charpy test in the same manner as in Example 1 to investigate the mechanical properties and also the variation in strength in the thickness direction.

The results of these investigations are shown in Table 4 below.
It can be seen that the thick steel sheet according to the present invention has a tensile strength of 400 MPa or more and has a uniform structure, so that the variation in hardness in the thickness direction is extremely small as compared with the comparative example. It is also found that the strength increase is realized by adding the precipitation strengthening element and performing the precipitation strengthening treatment, as compared with the invention examples in which the precipitation strengthening element whose characteristics are shown in Table 2 are not added.

[0055]

[Table 3]

[0056]

[Table 4]

Further, the thick steel plate having the composition shown in Table 5 was heated to 1150 ° C. and rolled to 50 ° C. by 50%,
After performing reheat precipitation treatment at 550 ° C. for 40 minutes and then air cooling, the sulfide stress corrosion cracking resistance was evaluated. That is, the test piece shown in Fig. 4 (a) was taken from the central area of the thickness of the thick steel plate, stress was applied to the test piece with the device shown in Fig. 4 (b), and then the NACE solution (5% NaCl + 0. 5% CH ₃ COOH + saturated H ₂
S) for 720 hours. The applied stress is equivalent to 40 to 120% of the 0.5% proof stress of the steel sheet in the tensile test,
0.5% of applied stress that did not fracture after soaking for hours
The sulfide stress corrosion cracking resistance was evaluated by the ratio to the proof stress. The larger the evaluation value, the better the sulfide stress corrosion cracking resistance. This evaluation result is shown in FIG.
As shown in (1), it is understood that the steel sheet in which C is limited to 0.02 wt% or less is also excellent in sulfide stress corrosion cracking resistance.

[0058]

[Table 5]

[0059]

The thick steel sheet of the present invention has a bainite single phase structure at any cooling rate used in the cooling step in industrial scale production. Therefore, it is possible to industrially stably supply a thick steel plate, which is expected to have an increased demand in the future and in which the material variation in the thickness direction is extremely small. The invention is also advantageously adapted to the field of shaped steel.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a C content and a strength variation in a thick steel plate.

FIG. 2 is a diagram showing a relationship between a cooling rate and strength of a thick steel plate.

FIG. 3 is a diagram showing the relationship between the rolling reduction of unrecrystallized region rolling and the fracture surface transition temperature of the obtained steel sheet.

FIG. 4 is a diagram showing a test piece and a test apparatus used for a test for evaluating sulfide stress corrosion cracking resistance.

FIG. 5 is a diagram showing the relationship between the C content and the resistance to sulfide stress corrosion cracking in thick steel plates.

Front page continued (72) Inventor Mitsuhiro Okazu 1-chome, Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture (no address) Inside Mizushima Steel Works, Kawasaki Steel Co., Ltd. (72) Kenji Oi 1-chome, Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture No.) Inside the Mizushima Steel Works, Kawasaki Steel Co., Ltd. (72) Inventor Fumaru Kawabata, Mizushima Kawasaki Dori, 1-chome, Kurashiki City, Okayama Prefecture (No house number) Inside the Mizushima Steel Works, Kawasaki Steel Co., Ltd. (72) Tomoya Koseki, Kurashiki City, Okayama Prefecture Mizushima Kawasaki Dori 1-chome (No house number) Kawasaki Steel Co., Ltd. Mizushima Steel Works (72) Inventor Koji Itakura Kurashiki, Okayama Prefecture Mizushima Kawasaki Dori 1 house (No house number) Kawasaki Steel Co., Ltd. Mizushima Steel Mfg. (72) Invention Yuuki Ota 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (no street number) Kawasaki Steel Works Mizushima Works

Claims (14)

[Claims] 1. C: 0.001 wt% or more and less than 0.030 wt%, Si: 0.60 wt% or less, Mn: 1.00 to 3.00 wt%, Nb: 0.005 to 0.20 wt%, B: 0.0003 to 0.0050 wt% and Al: 0.100. A bainite steel material with less variation in material, characterized by having a composition containing less than wt% and having a bainite structure of 90% or more. 2. The bainite steel material according to claim 1, wherein the steel material has a composition further containing Cu: 0.7 to 2.0 wt%, and there is little material variation. 3. The bainite steel material according to claim 1 or 2, wherein the steel material has a composition further containing Ti: 0.005 to 0.20 wt%, with little material variation. 4. The steel material according to claim 1, 2 or 3,
Furthermore, a bainite steel material having a composition containing V: 0.005 to 0.20 wt% with little material variation. 5. The steel material according to claim 1, 2, 3 or 4, further comprising Ni: 2.0 wt% or less, Cr: 0.5 wt% or less, Mo: 0.5 wt% or less, W: 0.5 wt% or less and Zr: A bainite steel material that has a composition containing one or more selected from 0.5 wt% or less and has little material variation. 6. The method according to claim 1, 2, 3, 4 or 5.
A bainite steel material that has a composition that contains at least one selected from REM and Ca in an amount of 0.02 wt% or less and has little material variation. 7. C: 0.001 wt% or more and less than 0.030 wt%, Si: 0.60 wt% or less, Mn: 1.00 to 3.00 wt%, Nb: 0.005 to 0.20 wt%, B: 0.0003 to 0.0050 wt% and Al: 0.100. When hot rolling a steel material having a composition containing less than wt%, after heating to a temperature of Ac _{3 to} 1350 ℃, finish rolling in the austenite unrecrystallized temperature range of 800 ℃ or more, and then cool. And a method for manufacturing a bainite steel material with less material variation. 8. C: 0.001 wt% or more and less than 0.030 wt%, Si: 0.60 wt% or less, Mn: 1.00 to 3.00 wt%, Nb: 0.005 to 0.20 wt%, B: 0.0003 to 0.0050 wt% and Al: 0.100. At the time of hot rolling of a steel material having a composition containing wt% or less, after heating to a temperature of Ac _{3 to} 1350 ° C., rolling is completed in the austenite unrecrystallized temperature range of 800 ° C. or more, and then, after cooling, 500 A method for producing a bainite steel material with little material variation, which comprises performing a precipitation treatment in which the material is reheated to a temperature range of ℃ or more and less than 800 ℃ and held. 9. C: 0.001 wt% or more and less than 0.030 wt%, Si: 0.60 wt% or less, Mn: 1.00 to 3.00 wt%, Nb: 0.005 to 0.20 wt%, B: 0.0003 to 0.0050 wt% and Al: 0.100. At the time of hot rolling of a steel material having a composition containing wt% or less, after heating to a temperature of Ac _{3 to} 1350 ° C, the rolling is completed in the austenite unrecrystallized temperature range of 800 ° C or more, and then the precipitation treatment temperature range. After accelerated cooling at a cooling rate of 0.1 to 80 ° C / s to a specified temperature of 500 ° C or higher and lower than 800 ° C,
Dispersion of materials that is characterized by holding isothermally for 30 s or more in a temperature range of 500 ℃ or more and less than 800 ℃ or cooling for 30 s or more in the temperature range at a cooling rate of 1 ℃ / s or less, and then cooling Method for producing bainite steel products with low energy consumption. 10. The method for producing a bainite steel material according to claim 7, 8 or 9, wherein the steel material has a composition further containing Cu: 0.7 to 2.0 wt%. 11. The method for producing a bainite steel material according to claim 7, 8, 9 or 10, wherein the steel material has a composition further containing Ti: 0.005 to 0.20 wt%. 12. The method for producing a bainite steel material according to claim 7, 8, 9, 10 or 11, wherein the steel material has a composition further containing V: 0.005 to 0.20 wt%. 13. The steel material according to claim 7, 8, 9, 10, 11 or 12, further comprising: Ni: 2.0 wt% or less, Cr: 0.5 wt% or less, Mo: 0.5 wt% or less, W: 0.5 wt%. % Or less and Zr: 0.5 wt% or less, a method for producing a bainite steel material having a composition containing one or more selected from the following. 14. A method according to claim 7, 8, 9, 10, 11, 12 or 13.
In the method for producing a bainite steel, the steel material further has a composition containing 0.02 wt% or less of at least one selected from REM and Ca.