JP2000345239A - Production of steel excellent in toughness of welding heat-affected zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing steel small in the variation of the material in the thickness direction and also excellent in the toughness of the welding heat-affected zone. SOLUTION: As to this method for producing steel, a steel stock, which contains, by weight, 0.01 to 0.03% C, 0.05 to 0.10% Nb, 0.0003 to 0.0050% B and 0.005 to 0.050% Ti, in which the ratio of the Ti content and Al content, i.e., Ti/Al satisfies >=5.0, moreover contains one or two kinds of Ca and rear earth metals, and in which oxide inclusions having an inclusion compsn. composed of, by weight, 20 to 90% Ti oxide, <=70% Al2O3, one or two kinds of Ca oxide and REM oxide by 5 to 50% in total and <=15% MnO are dispersed, is heated to the temp. range of 950 to 1,250 deg.C and is thereafter subjected to hot rolling in which the cumulative draft in the temp. region of <=950 deg.C is controlled to >=20%, and the rolling finishing temp. is controlled to >=800 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel material suitable for use in civil engineering, construction, bridges, marine structures, pipes, ships, storage tanks, construction machines, and the like, and has a small variation in characteristics particularly in the thickness direction. And a high-strength and high-toughness steel material having excellent toughness in a heat-affected zone. Note that the steel material in the present invention includes a thick steel plate, a steel strip, a shaped steel, and a steel bar.

[0002]

2. Description of the Related Art In recent years, with the enlargement of structures, high-strength, thick-walled steel materials have been frequently used. Thick steel materials represented by thick steel plates have been improved in properties such as higher strength and higher toughness.In recent years, however, there has been little variation in characteristics in the thickness direction and variation between steel materials. Is also required to be small.

[0003] For example, "Iron and Steel 74th Year (1988) No. 6
No. 11, p.11-21, with the rise of buildings, to prevent the collapse of buildings in the event of a huge earthquake, the design to absorb vibration energy due to the deformation of buildings has come to be taken Have been reported. Specifically, when an earthquake occurs, the framework of the building is collapsed in a predetermined shape, and the collapse of the building is prevented by plasticizing the framework.

In order to make such a design possible, a designer completely grasps the strength ratio of steel materials used for columns, beams, and the like, and when an earthquake occurs, the framework of the building changes the behavior intended by the designer. Is assumed. Therefore, it is indispensable that steel materials such as a steel plate and an H-shaped steel used for columns and beams are homogeneous, and therefore, there is a great problem that there is a variation in strength within steel materials or between steel materials.

Recently, steel materials used for construction are required to have high tensile strength and high toughness.
It is customary to manufacture according to a controlled cooling method, a so-called TMCP method. However, when a thick steel material is manufactured by the TMCP method, the cooling rate during controlled cooling differs in the thickness direction or between the steel materials, and the structure of the steel material changes in the thickness direction or between the steel materials. Material variations occur between steel materials. As for the variation of the material,
In particular, in addition to those appearing in the thickness direction of a thick steel plate, those appearing between the web and the flange due to uneven cooling between the web and the flange in the H-section steel, those appearing between the lots, and the like.

To solve such a problem, see, for example,
Japanese Patent Application No. 179020 proposes to reduce a difference in hardness in a cross section in the thickness direction by controlling a chemical composition, a reduction amount, a cooling rate, and a cooling end temperature. However, in the case of thick steel plates, particularly in the case of extremely thick steel plates exceeding 50 mm, a cooling rate distribution in the thickness direction inevitably occurs. It is difficult to suppress the hardness difference in the above.

In Japanese Patent Application Laid-Open No. 61-67717, the strength difference in the sheet thickness direction is greatly reduced by using an extremely low carbon content. However, as shown in FIG. In particular, even for extremely thick steel plates exceeding 50 mm, the variation in strength due to the inevitable change in cooling rate has not yet been eliminated. Japanese Patent Application Laid-Open No. 58-77528 describes that a stable hardness distribution can be obtained by adding Nb and B in combination, but a cooling rate of 15 to 40 ° C. is used to make the structure bainite. / S range. Since it is difficult to control the cooling rate strictly even in the center of the sheet thickness, it is difficult to obtain a uniform structure in the sheet thickness direction and the strength varies, or martensite islands are formed and the ductility and toughness deteriorate. There was a disadvantage.

Japanese Patent Application Laid-Open No. 54-132421 discloses a line pipe for performing hot rolling in which the carbon content is extremely low and the finishing temperature is 800 ° C. or lower in order to improve the weldability. A method for producing a high toughness high tensile bainite steel has been proposed. However, in the technique described in Japanese Patent Application Laid-Open No. 54-132421, there is a problem that productivity is low because hot rolling is completed in a low temperature range, and when stripping is performed on a thick steel plate or the like. However, there is a problem that distortion occurs due to the cutting.

[0009]

On the other hand, the inventors of the present invention disclosed in Japanese Patent Application Laid-Open No. 8-144019 that by using an extremely low C content, the material variation was small and the welding heat affected zone (HAZ) was reduced. A method for producing a steel material having excellent impact characteristics at 0 ° C. was proposed. However, recently, HA
A steel material with further improved Z toughness has been required, and an increase in the impact absorption energy of HAZ at −40 ° C. has been desired.

[0010] The present invention advantageously solves the above-mentioned problems of the prior art, has less material variation in the thickness direction, and
An object of the present invention is to provide a method for producing a steel material having high impact absorption energy at 40 ° C. and excellent in toughness of a heat affected zone.

[0011]

Means for Solving the Problems Material variation of a thick steel material is caused by a large change in the cooling rate in each part in the thickness direction from the steel surface to the center, or a change in the cooling rate in each part of the steel material due to a variation in manufacturing conditions. Due to organizational changes caused by the To avoid this tissue variation, it is important to be able to obtain a homogeneous tissue at a wide range of cooling rates.

Therefore, the present inventors have returned to the origin and made intensive studies on a technique for obtaining a homogeneous structure at a wide range of cooling rates. As a result, they came to the point that it was necessary to redesign the steel composition. We have:
By setting the C content to an extremely low C content of 0.03 wt% or less and further adding an appropriate amount of Nb and B, the structure can be made an extremely low C bainite structure without depending on the cooling rate. It has been found that by using the main structure, a steel material having a desired strength can be obtained.

First, the experimental results on which the present invention is based will be described. By weight%, C: 0.015%, Si: 0.30%,
Mn: 1.56%, Nb: 0.029%, B: 0.0013%, Ti: 0.009
%, Ca: 0.0023%, is heated to 1150 ° C and hot-rolled into a 15mm thick steel sheet. The cooling rate after hot-rolling varies between 0.1 and 50 ° C / s. I let it. For these steel sheets, a tensile test was performed to determine the tensile properties. The result is shown in FIG. 1A in relation to the yield strength YS, the tensile strength TS, and the cooling rate after hot rolling.

From FIG. 1 (a), it is found that the C content is reduced to 0.03% by weight or less, and that Nb and B are further added so that the C content is 0.1 to 50%.
It can be seen that the tensile strength TS and the yield strength YS show almost constant values regardless of the cooling rate over a wide cooling rate range of ° C / s. On the other hand, in a conventional steel material for building having a high C content and containing no B, as shown in FIG. 1B, the yield strength YS and the tensile strength TS greatly change depending on the cooling rate. ing. In addition, conventional steel materials for building
By weight%, C: 0.14%, Si: 0.4%, Mn: 1.31%, Al:
The composition contained 0.024%, Nb: 0.015%, and Ti: 0.013%. After hot rolling, the cooling rate was similarly varied.

As described above, by reducing the C content to 0.03% by weight or less and adding an appropriate amount of Nb and further B, it is possible to remarkably reduce the cooling rate dependence of the steel material strength, It was found that a steel material with little material variation at the same time or a steel material with little strength variation between lots can be obtained. In addition, the present inventors further improve the toughness of the heat affected zone by dispersing the oxide-based inclusions uniformly and finely and preventing the coarsening of the crystal grains in the heat affected zone (pinning effect). I thought I could do it. The present inventors have further studied a method of uniformly and finely dispersing oxide-based inclusions in a steel material without nozzle clogging, and as a result, the oxide-based inclusions were mainly made of Ti oxide,
It has been found that the composition of the oxide-based inclusions needs to be in the optimum range.

Next, the results of studies conducted by the present inventors on the optimum composition range of the oxide-based inclusions will be described. First, in order to achieve a fine and uniform dispersion of oxide-based inclusions, it is necessary to improve the wettability between the deoxidized product inclusions and the molten steel. To achieve this, the Al ₂ O ₃ In order to reduce the concentration to 70% by weight or less, and to prevent coarsening of crystal grains in the heat affected zone, the concentration of Ti oxide in the oxide-based inclusions should be at least 20% by weight or more. MnO concentration in oxide inclusions should be 15% by weight or less. CaO or REM oxide concentration in oxide inclusions should be 50% by weight.
% By weight or less.

Further, in order to prevent nozzle clogging, it is necessary to lower the melting point of the deoxidized product. For this purpose, Ca treatment or REM treatment requires CaO in the inclusions.
Alternatively, the REM oxide concentration should be at least 5% by weight, the Al ₂ O ₃ concentration should be 70% by weight or less, and the Ti oxide concentration should be 90% by weight.
It has been found that the following is important.

Based on these findings, the present inventors have found that the optimum composition range of the oxide-based inclusions is, as shown in FIG. 2, 20 to 90% by weight of Ti oxide, CaO, and REM oxide. Either one or two kinds in total: 5 to 50% by weight, Al ₂ O ₃ :
It was determined to be 70% by weight or less. In addition, MnO is 15% by weight or less. By controlling the composition of the oxide-based inclusions to fall within the range shown in FIG. 2, the ability to suppress the coarsening of crystal grains of the inclusions (pinning effect) without causing nozzle clogging and generation of harmful inclusion clusters Can be used effectively.

As described above, the present inventors set the C content to 0.1.
In addition to the extremely low content of not more than 03% by weight, the inclusion of appropriate amounts of Nb and B, and the inclusion of oxide-based inclusions with an appropriate inclusion composition mainly composed of Ti oxides, It has been found that it is possible to obtain a steel material with further improved affected zone toughness. In addition, the present inventors have excellent weld heat affected zone toughness, further consist of bainite-based structure, less material variation in the thickness direction, and in order to stably obtain a steel material having the desired strength and toughness, In addition to the above-mentioned restrictions on the chemical components and inclusion compositions, it has been found that it is necessary to limit the hot rolling conditions or further the cooling conditions after the hot rolling.

The present invention has been made based on the above findings. That is, the present invention provides a method for preparing C:
0.001 to 0.03%, Si: 1.0% or less, Mn: 0.8 to 3.0%,
Nb: 0.005 to 0.10%, B: 0.0003 to 0.0050%, Ti: 0.00
5 to 0.050%, and the ratio of Ti content to Al content, Ti
/ Al satisfies 5.0 or more, and Ca: 0.0010 to 0.0100
%, REM: 0.0010% to 0.0100%, containing one or two kinds, the balance being Fe and inevitable impurities, and as oxide-based inclusions, by weight%, Ti oxide: 20 to
_{90%, Al 2 O 3:} 70% or less, Ca oxide, the sum of one or two of REM oxides: 5 to 50%, MnO: oxide having a composition of inclusions of 15% or less After heating the steel material in which inclusions are dispersed to a heating temperature of 950 to 1250 ° C, the cumulative rolling reduction in the temperature range of 950 ° C or less is 20% or more.
A method for producing a steel material having excellent toughness in a weld heat-affected zone, characterized by performing hot rolling at a rolling end temperature of 800 ° C. or higher. , Cu: 0.05-2.0%, Ni: 0.05-1.5%, Cr: 0.
05-1.0%, Mo: 0.02-0.7%, V: 0.005-0.1%,
One or more of these may be contained.

In the present invention, C: 0.001 to 100% by weight.
0.03%, Si: 1.0% or less, Mn: 0.8 to 3.0%, Nb: 0.005
~ 0.10%, B: 0.0003 ~ 0.0050%, Ti: 0.005 ~ 0.050
% And the ratio of Ti content to Al content, Ti / Al is 5.0
Satisfies the above, Ca: 0.0010-0.0100%, REM
: Containing one or two of 0.0010% to 0.0100%, having a composition consisting of balance Fe and inevitable impurities,
As oxide-based inclusions, Ti oxide: 20 to 90% by weight
%, Al ₂ O ₃ : 70% or less, Ca oxide or REM oxide, one or two of them in total: 5 to 50%, MnO: 15% or less After heating the steel material in which the material is dispersed to a heating temperature of 950 to 1250 ° C, the cumulative reduction in the temperature range of 950 ° C or less is 20% or more and the rolling end temperature is 800 ° C or more. A method for producing a steel material having excellent toughness in the heat affected zone of a welding, characterized by rolling, followed by accelerated cooling at a cooling rate of 2 to 50 ° C./s and a cooling stop temperature of 600 ° C. or less. In the present invention, Cu: 0.05 to 2.0% by weight in addition to the above composition,
Ni: 0.05-1.5%, Cr: 0.05-1.0%, Mo: 0.02-0.7
%, V: 0.005 to 0.1%.

[0022]

First, the reasons for limiting the chemical composition of a steel material will be described. In the following, “%” in the composition means “% by weight”. C: 0.001 to 0.03% C is an element that increases the strength of steel, and a content of 0.001% or more is necessary to secure desired strength.
Further, the upper limit of the C content is set to 0.03% in order to eliminate the formation of the pearlite phase in an equilibrium state without impairing the weldability of the steel. Incidentally, the content is preferably in the range of 0.002 to 0.02%.

Si: 1.0% or less Si is an effective element for increasing the strength of steel by solid solution strengthening irrespective of cooling conditions. However, if it exceeds 1.0%, the base metal toughness will be remarkably increased. Deteriorate. For this reason, Si was limited to 1.0% or less. Incidentally, preferably 0.60
% Or less. Mn: 0.8 to 3.0% Mn is an element that greatly affects the continuous cooling transformation behavior of steel in the extremely low C region. To obtain a bainite structure in the extremely low C region, the content of Mn is required to be 0.8% or more. But 3.0
%, The base material toughness is deteriorated. For this reason, Mn was limited to the range of 0.8 to 3.0. Incidentally, the content is preferably 1.0 to 2.5%.

Nb: 0.005 to 0.10% Nb, like Mn, is an element that greatly affects the continuous cooling transformation behavior of steel in the extremely low C region. To obtain a bainite structure in the extremely low C region, Nb is 0.005% or more. Is required.
However, the content exceeding 0.10% deteriorates the base material toughness. For this reason, Nb was limited to the range of 0.005 to 0.10%.
In addition, Preferably, it is 0.01 to 0.08%.

B: 0.0003% to 0.0050% B, like Mn and Nb, is an element that greatly affects the continuous cooling transformation behavior of steel in the extremely low C region, and 0.0003% to obtain a bainite structure in the extremely low C region. % Or more is required. However, if the content exceeds 0.0050%, the effect is saturated,
The effect corresponding to the content cannot be expected. Therefore, B is 0.
It was limited to the range of 0003 to 0.0050%. In consideration of weld toughness, the range of 0.0010 to 0.0030% is preferable.

Ti: 0.005 to 0.050% Ti is one of the important elements in the present invention. The most important element of the present invention is to perform Ti deoxidation and to effectively use the oxide generated by Ti deoxidation. The Ti oxide dispersed in the steel has an effect of suppressing austenite grain growth by a crystal grain pinning effect. Further, Ti remaining in the steel after deoxidation generates TiN in the subsequent cooling process. T
iN contributes to the suppression of coarsening of austenite grains in the HAZ portion and improves HAZ toughness. Further, by fixing N in the steel, it becomes possible to secure B important for obtaining extremely low C bainite steel without changing it to BN. In order to obtain these effects, it is necessary to contain 0.005% or more of Ti, but if it exceeds 0.050%, the increase of solid solution Ti or the precipitation of Ti carbide deteriorates the base material and HAZ toughness. Let it. For this reason, Ti was limited to the range of 0.005 to 0.050%.

Ti / Al: 5.0 or more In the present invention, Ti / Al is set to 5.0 or more in order to prevent deoxidation of Ti and generation of Al ₂ O ₃ cluster. From the Ti-Al-O equilibrium, if Ti / Al is less than 5.0, Al ₂ O ₃ clusters are formed,
Oxide-based inclusions cannot be uniformly and finely dispersed. In addition,
Preferably, Ti / Al is at least 6.0.

Ca: 0.0010-0.010%, REM: 0.0010-0.
One or two of Ca and REM of 010% contribute to lowering the melting point of inclusions and improve wettability.
It is an essential element for realizing fine and uniform dispersion of the deoxidation product. For this purpose, the content of each of them is required to be 0.0010% or more. On the other hand, the content of each exceeding 0.010% decreases the cleanliness of the steel and impairs the base metal toughness. For this reason, Ca and REM were each limited to the range of 0.0010 to 0.010%.

Cu: 0.05-2.0%, Ni: 0.05-1.5%, C
r: 0.05 to 1.0%, Mo: 0.02 to 0.7%, V: 0.005 to 0.1
%, One or more of Cu, Ni, Cr, Mo, and V may contain one or more of them as necessary to increase the strength of the steel. Cu, Ni, C
r, Mo, and V are all effective elements for improving hardenability. They promote bainite transformation in the extremely low C region, increase transformation strain generated during bainite transformation, and increase dislocation density. Has the effect of increasing the strength of Such an effect is at least 0.05% for Cu, Ni, and Cr, and 0.1% for Mo.
It is recognized at a content of not less than 02% and not less than 0.005% in V. On the other hand, if the content exceeds 2.0% of Cu, 1.5% of Ni and 0.1% of V, the effect saturates and the effect corresponding to the content cannot be expected, resulting in an economic disadvantage. Therefore, Cu is 0.05-2.0
%, Ni is preferably in the range of 0.05 to 1.5%, and V is preferably in the range of 0.005 to 0.1%. In addition, Cr and Mo are each 1.0%,
If the content exceeds 0.7%, the weldability and toughness deteriorate.
Therefore, it is preferable that Cr is limited to the range of 0.05 to 0.50% and Mo is limited to the range of 0.02 to 0.20%.

The balance other than the above components is Fe and inevitable impurities. As an inevitable impurity, Al: 0.005
%, P: 0.03% or less, S: 0.004% or less, N: 0.00
Less than 6% is acceptable. Although Al acts as a deoxidizing agent, in the present invention, it can be used as a preliminary deoxidizing agent to adjust the O concentration before deoxidizing Ti. However, when added in a large amount, the concentration of Al ₂ O _{3 in the} inclusions increases, causing large cluster inclusions and clogging of the nozzle. Therefore, the content of Al is preferably set to 0.005% or less.

The molten steel having the above-described composition is melted by deoxidizing Ti. It goes without saying that preliminary deoxidation with Al may be performed. The smelting method is not particularly limited.
Any commonly known melting method such as an electric furnace and a vacuum melting furnace can be suitably used. When the deoxidation method is Ti deoxidation, the deoxidized product becomes an inclusion mainly composed of Ti oxide. The composition of the deoxidation product (oxide-based inclusion) is preferably adjusted by the amount of alloying element added and the procedure of preliminary deoxidation.

The molten steel is then passed through a continuous casting method, an ingot casting method, or the like.
Any of the commonly known casting methods can be suitably used, such as slabs and the like.
Of steel for rolling. In the present invention, the steel material
Oxide-based inclusions are finely dispersed. Finely dispersed oxidation
Material inclusions are mainly composed of Ti oxides, and
Compound: 20-90%, Al_Two0 _Three: 70% or less, Ca oxide, REM acid
Of one or two of the compounds: 5-50%, MnO:
It has an inclusion composition of 15% or less.

Ti oxide: 20 to 90% Ti oxide has a crystal grain pinning effect of suppressing coarsening of austenite grains in the heat affected zone. For this reason,
In the present invention, the oxide-based inclusion has a composition mainly composed of Ti oxide. For this purpose, the concentration of the Ti oxide in the oxide-based inclusion is 20% or more. If it is less than 20%, a crystal grain pinning effect cannot be expected. On the other hand, if the concentration of Ti oxide in the oxide-based inclusions exceeds 90%, the melting point of the oxide-based inclusions becomes high, and the inclusion of the inclusions on the immersion nozzle wall is likely to occur, causing nozzle clogging. Easier to do. For this reason,
The concentration of Ti oxide in oxide-based inclusions is limited to 20 to 90%. In addition, Preferably, it is 50-85%. Further, TiO ₂ , Ti ₂ O _{3 and the} like are preferable as the Ti oxide in the present invention.

Al ₂ O ₃ : 70% or less Al ₂ O ₃ easily forms large cluster inclusions and hinders uniform and fine dispersion of oxide-based inclusions. Therefore, in the present invention, it is preferable to reduce the Al ₂ O ₃ concentration in the oxide-based inclusions as much as possible. Al ₂ O ₃ concentration in oxide-based inclusions
When it exceeds 70%, the wettability of inclusions with molten steel is reduced,
Furthermore, nozzle clogging becomes remarkable. For these reasons, the Al ₂ O ₃ concentration in the oxide-based inclusions is set to 70% or less.

Total of one or two of Ca oxide and REM oxide: 5 to 50% In the present invention, in order to lower the melting point of oxide-based inclusions,
One or two of Ca oxide (CaO 2) and REM oxide are contained in the oxide-based inclusions in a total amount of 5% or more. Also, since Ca and REM easily combine with S to form a sulfide, if the concentration of Ca oxide (CaO) or REM oxide in the oxide-based inclusions exceeds 50 wt%, the inclusions will be reduced. CaS around
, REM sulfide is formed. For this reason, the inclusions are coarsened, and the ability to pin the crystal grains of the oxide-based inclusions is reduced. From these facts, oxide inclusions
One or two of Ca oxide and REM oxide are limited to a range of 5 to 50% in total.

MnO: 15% or less MnO has an effect of reducing the crystal grain pinning ability of Ti oxide. For this reason, MnO 2 in oxide-based inclusions is limited to 15% or less. In the steel material of the present invention, the content of oxide inclusions is preferably set to 0.005 to 0.025% by weight. The size of the oxide-based inclusions contained is 3 μm
It is preferable to set the following. If it exceeds 3 μm, the ability to suppress austenite grain coarsening will decrease.

In the present invention, the amount of inclusions is determined by a cleanliness test using an optical microscope or by quantification of an extraction residue, and the composition of the inclusions is determined by a scanning electron microscope (SE).
M), and measurement is performed by a procedure called quantitative analysis using EDX. The steel material for rolling is reheated to a temperature of 950 to 1250 ° C., or hot rolled without being reheated to be a steel material. Next, the reasons for limiting the hot rolling conditions will be described.

Heating temperature: 950 to 1250 ° C. If the heating temperature is lower than 950 ° C., the added Nb does not completely form a solid solution, so that the effect of Nb on the continuous cooling transformation of steel cannot be sufficiently obtained. On the other hand, when the heating temperature exceeds 1250 ° C., the austenite grains become extremely coarse, the structure after rolling becomes coarse, and the toughness decreases. Therefore, the heating temperature is
Limited to the range of 950-1250 ° C. Preferably, 950
~ 1200 ° C. If Nb is completely dissolved, hot rolling may be performed without reheating.

Cumulative rolling reduction in a temperature range of 950 ° C. or less: 20%
The increase in the rolling reduction in the temperature range of 950 ° C. or lower, that is, the non-recrystallization temperature range, makes the packet size of bainite transformed from austenite grains fine, and improves the toughness of the bainite structure. Note that a packet refers to an organizational unit in which laths are accumulated. In addition, an increase in the amount of reduction in the non-recrystallization temperature range increases the austenite grains and increases the dislocation density accumulated in the austenite grains.
Therefore, at the time of transformation, part of the dislocation is inherited by the bainite structure after transformation, and the strength is further increased. Such an effect becomes remarkable according to the cumulative rolling reduction when the cumulative rolling reduction in the temperature range of 950 ° C. or less is 20% or more. For this reason, the cumulative rolling reduction in the temperature range of 950 ° C or less was limited to 20% or more.

Hot rolling end temperature: 800 ° C. or higher As the hot rolling end temperature becomes lower, the rate at which strains (dislocations) introduced into austenite grains by rolling are accumulated in the grains increases. As a result, the inherited amount of dislocations to the bainite structure after transformation is significantly increased, so that the strength is increased. However, even if the hot-rolling end temperature is less than 800 ° C., the tendency of the increase in strength is saturated, and the rolling efficiency decreases due to the increase in deformation resistance. Therefore, in the present invention,
The rolling end temperature was limited to 800 ° C. or higher.

In the present invention, after hot rolling, air cooling or further accelerated cooling may be performed. Wall thickness 10-150 mm
The cooling rate of the steel material during air cooling is approximately 1.0 ° C./s to 0.05 ° C.
° C / s. The steel material of the present invention has a bainite structure with no material variation within such a cooling rate range.
Therefore, in the present invention, there is no problem even if the cooling after the hot rolling is air cooling. By performing accelerated cooling after hot rolling, the generated bainite structure is refined, and the toughness can be further improved.

The accelerated cooling conditions are as follows: cooling rate: 2 to 50 ° C./s
The cooling stop temperature is preferably set to 600 ° C. or lower. If the cooling rate is less than 2 ° C./s, further refinement of the bainite structure cannot be obtained, and the effect of accelerated cooling is small. Also,
Cooling rates exceeding 50 ° C./s are difficult to achieve industrially. Therefore, the cooling rate of the accelerated cooling is 2 to 50 ° C./s.
It is preferable to set it in the range.

When the cooling stop temperature exceeds 600 ° C.,
The effect of accelerated cooling is small and the effect of improving toughness is small. For this reason, the cooling stop temperature is preferably set to 600 ° C. or lower.

[0044]

EXAMPLES Steel having the composition shown in Table 1 was melted in a vacuum melting furnace. The composition of the oxide-based inclusion is mainly Ti / Al
And the amounts of Ca and REM added were adjusted.
In addition, molten steel was injected into the mold from a ladle using a nozzle to form a steel ingot. Regarding the state of adhesion of inclusions in the nozzle during casting, the inside of the nozzle was visually observed after casting, and the presence / absence of inclusions was also investigated.

As a comparative example, to make the composition of the oxide-based inclusions out of the range of the present invention and increase the amount of Ti oxide,
In order not to deoxidize Al, increase Ti / Al, and increase CaO and REM oxides, increase the amount of Ca or REM added and increase the amount of Al ₂ O
_In order to increase the number ₃ , the ratio of Ti / Al was decreased, and to increase the amount of MnO 2, preliminary deoxidation with Mn was performed and the addition amount of Al, Ti, and Ca was decreased.

These ingots were slab-rolled into 100 mm thick slabs. Then, hot rolling was performed under the conditions shown in Table 2, and after hot rolling, the steel was cooled under the conditions shown in Table 2 to obtain a steel sheet having a thickness of 30 mm. The obtained steel plate was subjected to a tensile test and a Charpy impact test of the base material. Further, in order to evaluate the strength variation in the thickness direction, the cross-sectional hardness of the obtained steel sheet was measured at a pitch of 2 mm from the surface, and the hardness distribution in the thickness direction was investigated.
Further, in order to evaluate the toughness of the HAZ portion (the toughness of the reproduced HAZ portion), a test piece taken from a steel sheet was heated to 1350 ° C.
After a heat cycle of cooling from 800 ° C to 500 ° C in 300 sec (corresponding to the heat history of the welded joint HAZ welded with a heat input of 500 kJ / cm), a Charpy test specimen was collected.
The Charpy absorbed energy (vE _-40 ) at -40 ° C was measured. In addition, the test material was sampled from the as-rolled material, and the composition of oxide-based inclusions in the steel material was investigated. As described above, the composition was analyzed by a quantitative analysis method using EDX attached to the SEM. These results are shown in Table 3.

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

[Table 3]

The example of the present invention has high strength and high toughness, and the reproduced HAZ portion of the example of the present invention shows a high absorption energy of 200 J or more in a Charpy impact test at -40 ° C. Further, in the example of the present invention to which accelerated cooling is applied, the toughness of the base material is further improved as compared with the air-cooled material. On the other hand, the comparative examples out of the range of the present invention show the base material characteristics or the reproduction H
The toughness of the AZ part is reduced. Steel materials No. 15 to No. 19 whose components and inclusions deviate from the range of the present invention have vE- _{40 of the} reproduced HAZ part as low as less than 100 J, and steel materials No. 15 to No. 17,
In No. 19, the concentration of Ti oxide, the concentration of Al ₂ O ₃ or the concentration of Ti / Al were out of the range of the present invention, and nozzle clogging occurred. In addition, any one of C, Nb, and B is out of the range of the present invention.
In o. 20, No. 21, and No. 22, the hardness distribution in the thickness direction, strength, base metal toughness, and HAZ toughness are not all satisfactory steel materials.

Further, a steel material whose rolling conditions are out of the range of the present invention.
In No. 2 and No. 7, either the base metal strength or the base metal toughness was deteriorated. Compared with steel No. 4 (Example of the present invention), the base material strength toughness of steel No. 23 in which the rolling end temperature was lowered by 80 ° C.
o. Approximately the same as 4 and the base material characteristics are saturated. Even if the rolling end temperature is lower than 800 ° C., only the rolling efficiency is reduced and there is no advantage in characteristics.

[0052]

According to the present invention, it is possible to inexpensively produce a steel material having a small material variation in the thickness direction, a high impact absorption energy at −40 ° C., and an excellent toughness of the weld heat affected zone. It works. INDUSTRIAL APPLICABILITY The present invention is advantageously applicable not only to the field of thick steel plates but also to the fields of section steel and steel bars.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between steel material tensile properties and a cooling rate after hot rolling.

FIG. 2 is a ternary phase diagram showing a preferable range of an oxide-based inclusion composition.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Fumimaru Kawabata 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. 1-chome (without address) Mizushima Works, Kawasaki Steel Co., Ltd. F-term (reference) 4K032 AA01 AA02 AA04 AA08 AA11 AA14 AA15 AA16 AA17 AA19 AA22 AA23 AA24 AA31 AA35 AA36 AA40 BA01 BA02 CA03 CD03 CD02

Claims (4)

[Claims] C. 0.001 to 0.03%, Si: 1.0% or less, Mn: 0.8 to 3.0%, Nb: 0.005 to 0.10%, B: 0.0003 to 0.0050%, Ti: 0.005 to 0.050% by weight%. And the ratio of Ti content to Al content, Ti / Al satisfies 5.0 or more, Ca: 0.0010 to 0.0100%, REM: 0.
Contains one or two of 0010% to 0.0100%, with the balance being
It has a composition consisting of Fe and unavoidable impurities, and as oxide-based inclusions, by weight%, Ti oxide: 20-90%, Al
₂ O ₃ : 70% or less, Ca oxide or REM oxide, one or two kinds in total: 5 to 50%, and MnO: 15% or less. After heating the steel material to a temperature range of 950 to 1250 ° C, the cumulative reduction in the temperature range of 950 ° C or less is 20% or more, and the rolling end temperature is
A method for producing a steel material having excellent toughness in a heat-affected zone of a weld, characterized by performing hot rolling at 800 ° C. or higher. 2. The composition according to claim 1, further comprising:
u: 0.05-2.0%, Ni: 0.05-1.5%, Cr: 0.05-1.0
%, Mo: 0.02 to 0.7%, V: 0.005 to 0.1%, the steel material having excellent toughness in the weld heat affected zone according to claim 1, characterized in that it contains one or more of the following. Method. 3. In% by weight, C: 0.001 to 0.03%, Si: 1.0% or less, Mn: 0.8 to 3.0%, Nb: 0.005 to 0.10%, B: 0.0003 to 0.0050%, Ti: 0.005 to 0.050% And the ratio of Ti content to Al content, Ti / Al satisfies 5.0 or more, Ca: 0.0010 to 0.0100%, REM: 0.
Contains one or two of 0010% to 0.0100%, with the balance being
It has a composition consisting of Fe and unavoidable impurities, and as oxide-based inclusions, by weight%, Ti oxide: 20-90%, Al
₂ O ₃ : 70% or less, Ca oxide or REM oxide, one or two kinds in total: 5 to 50%, and MnO: 15% or less. After heating the steel material to a temperature range of 950 to 1250 ° C, the cumulative reduction in the temperature range of 950 ° C or less is 20% or more, and the rolling end temperature is
Hot rolling to 800 ° C or higher, and then cooling rate: 2
A method for producing a steel material having excellent toughness in a heat affected zone of a weld, characterized in that accelerated cooling is performed at a cooling stop temperature of 600 ° C. or less at a temperature of 50 ° C./s or less. 4. The composition according to claim 1, further comprising:
u: 0.05-2.0%, Ni: 0.05-1.5%, Cr: 0.05-1.0
%, Mo: 0.02 to 0.7%, V: 0.005 to 0.1%, the steel material having excellent toughness in the weld heat affected zone according to claim 3, characterized by containing one or more of the following. Method.