JPH1121625A - Production of thick steel plate excellent in strength and toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a thick steel plate excellent in strength and toughness without causing residual stress and strain by applying hot rolling to a steel of specific composition at specific heating temp. and further applying cooling while specifying cooling temp. and a magnetic field of atmosphere, respectively. SOLUTION: A steel, having a composition consisting of, by weight, 0.05-0.18% C, 0.05-0.6% Si, 0.60-2.00% Mn, 0.005-0.050% Al, <=0.020% P, <=0.015% S, 0.0050-0.0150% N, either or both of 0.04-0.15% V and 0.01-0.15% Ti, and the balance Fe with inevitable impurities, is prepared. This steel is heated and soaked to and at a temp. not lower than the Ar3 transformation point and hot-rolled. At this time, rolling is finished at a temp. not lower than the Ar3 transformation point represented by an equation defined from the contents of respective components. Subsequently, cooling is applied through the temp. region from the completion of rolling down to (Ar3 -100) deg.C in a magnetic field of >=2T, followed by air cooling or accelerated cooling down to room temp. By this method, the thick steel plate, useful for pillar material and beam material for building construction, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the manufacture of steel plates, and more particularly, to steel strips, shaped steels, and the like used in the fields of construction, marine structures, pipes, shipbuilding, storage tanks, civil engineering, construction machinery, and the like. The present invention is also applicable to a method for manufacturing a non-heat treated steel material such as a bar.

[0002]

2. Description of the Related Art As a method for securing strength, toughness, and weldability of a thick steel plate, a thermal mechanical control (TMCP) method is used.
l Process) is known. For example,
JP-A-3-223419 discloses that hot rolling (Ar ₃ points + 150
After applying the pressure of 30% or more at ° C.) or more recrystallization region, (Ar ₃
A method of miniaturizing the structure by introducing a deformation zone by applying a reduction of 50% or more at _three points of + 150 ° C. to Ar is disclosed. Further, in JP-A-64-73019, (Ae ₃ -200
By subjecting the 2-phase region rolling at _{℃) ~ (Ae 3 -250 ℃} ) below a temperature range, cause ferrite, method of a fine ferrite structure is disclosed during processing.

However, in these prior arts, in a very thick steel plate, when light pressure is applied, the deformation zone is not sufficiently introduced, so that the ferrite nucleus is reduced. There were problems such as the effects of anisotropy, residual stress, and strain being large. further,
Long-term temperature control during rolling to apply rolling in the non-recrystallized region significantly reduces rolling efficiency, and there is also a problem that the deformation behavior of the steel sheet cannot be sufficiently controlled due to the occurrence of residual stress and strain. .

[0004] Also, Iron and Steel No. 77 (1991) No. 1 P.1
The properties of high-strength as-rolled steel material to which V and N are added are shown in No. 71.
No toughness was obtained.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to propose a method for manufacturing a thick steel plate which can ensure high strength and high toughness without generating residual stress and strain, and is excellent in strength and toughness.

[0006]

Means for Solving the Problems The inventors have conducted various experiments and studies in order to achieve the above object, and have found that
For steels containing Ti, V, N contents, TiN, VN or
It was found that the transformation of ferrite using TiN and VN composite precipitates as nuclei was promoted in a strong magnetic field, and it was clarified that the formation of a fine ferrite-pearlite structure could improve the strength and toughness. Further, in steels containing REM, Ca, and O, it was found that the promotion of ferrite transformation centered on oxide-based inclusions of REM and Ca occurred in a strong magnetic field. Similarly, it was revealed that the formation of a fine ferrite-pearlite structure can improve strength and toughness. Furthermore, it has been found that in a steel containing B and N, promotion of ferrite transformation with BN as a nucleus occurs in a strong magnetic field.

[0007] The present invention has been made based on these findings, and the gist configuration thereof is as follows. [First invention] C: 0.05 to 0.18 wt%, Si: 0.05 to 0.6 wt%, Mn: 0.60 to
2.00 wt%, Al: 0.005 to 0.050 wt%, P: 0.020 wt% or less, S: 0.015 wt% or less, N: 0.0050 to 0.0150 wt%, V: 0.04 to 0.15 wt% and Ti: 0.01 to 0.15 wt% wt%
After heating and equalizing the steel consisting of Fe and unavoidable impurities to the Ar ₃ transformation point or higher, the remainder is subjected to hot rolling. Rolling is completed at a temperature equal to or higher than the Ar ₃ transformation point defined by the content of the component, and from the end of the rolling, the temperature up to the Ar ₃ transformation point −100 ° C. is cooled in a magnetic field of 2 T or more, and then air-cooled to room temperature or A method for manufacturing a thick steel plate having excellent strength and toughness, characterized by accelerated cooling. Ar ₃ (° C.) = 910−230C + 25Si−74Mn−56Ni−16Cr−9Mo−5Cu−1620Nb (1) [Second invention] C: 0.05 to 0.18 wt%, Si: 0.05 to 0.6 wt%, Mn: 0.60 ~
2.00 wt%, Al: 0.005 to 0.050 wt%, P: 0.020 wt% or less, S: 0.015 wt% or less, and N: 0.0050 to 0.0150 wt%, and V: 0.04 to 0.15 wt% and Ti: 0.01 to 0.01 wt%. 0.15wt
% Of one or two kinds, and further contains Cu:
0.05-2.0 wt%, Ni: 0.05-0.60 wt%, Nb: 0.003--0.
020 wt%, Cr: 0.05-0.50 wt%, Mo: 0.02-0.10 wt%,
A group consisting of one or more of the following, B: 0.0002 to 0.0020 wt
%, REM: 0.0010 to 0.0200 wt%, Ca: 0.0010
One or two kinds of O-0.0100wt% and O: 0.0040-0.0150wt%
And at least one of the group consisting of Fe and a steel consisting of unavoidable impurities is heated and soaked at a temperature equal to or higher than the Ar ₃ transformation point, and then hot-rolled. The rolling is completed at a temperature equal to or higher than the Ar ₃ transformation point defined by the content of each component, and from the end of the rolling to the temperature of the Ar ₃ transformation point −100 ° C.
A method of manufacturing a steel plate having excellent strength and toughness, characterized by cooling in a magnetic field of 2 T or more, and then cooling by air or accelerated cooling to room temperature. Ar ₃ (° C.) = 910−230C + 25Si−74Mn−56Ni−16Cr−9Mo−5Cu−1620Nb (1) [Third invention] C: 0.05 to 0.18 wt%, Si: 0.05 to 0.6 wt%, Mn: 0.60 ~
2.00 wt%, Al: 0.005 to 0.050 wt%, P: 0.020 wt% or less, S: 0.015 wt% or less, O: 0.0040 to 0.0150 wt%, and Ti: 0.01 to 0.15 wt% REM: 0.0010 to 0.0200
wt%, Ca: 0.0010-0.0100wt%, containing one or more kinds, and the balance consisting of Fe and unavoidable impurities is heated and soaked to the Ar ₃ transformation point or higher, and then hot-rolled. At that time, rolling is completed at a temperature not lower than the Ar ₃ transformation point defined by the content of each component in Equation (1), and from the end of rolling, the Ar ₃ transformation point −
A method for manufacturing thick steel sheets with excellent strength and toughness, characterized by cooling to a temperature of 100 ° C in a magnetic field of 2 T or more, and then cooling to room temperature by air or accelerated cooling. Ar ₃ (° C.) = 910−230C + 25Si−74Mn−56Ni−16Cr−9Mo−5Cu−1620Nb (1) [4th invention] C: 0.05 to 0.18 wt%, Si: 0.05 to 0.6 wt%, Mn: 0.60 ~
2.00 wt%, Al: 0.005 to 0.050 wt%, P: 0.020 wt% or less, S: 0.015 wt% or less, O: 0.0040 to 0.0150 wt%, and Ti: 0.01 to 0.15 wt% REM: 0.0010 to 0.0200
wt%, Ca: one or more of 0.0010 to 0.0100 wt%, and Cu: 0.05 to 2.0 wt%, Ni:
0.05 to 0.60 wt%, Nb: 0.003 to 0.020 wt%, Cr: 0.05 to
A group consisting of one or more of 0.50 wt%, Mo: 0.02 to 0.10 wt%, B: 0.0002 to 0.0020 wt%, V: 0.04 to 0.15 wt
% Or more, and at least one group of N: 0.0050 to 0.0150 wt%, and the balance was heated and soaked at a temperature above the Ar ₃ transformation point of steel consisting of Fe and unavoidable impurities. Thereafter, hot rolling is performed, and at that time, the rolling is completed at a temperature equal to or higher than the Ar ₃ transformation point defined by the content of each component in the formula (1), and the Ar ₃ transformation point −100 ° C. 2 T up to temperature
A method for producing a thick steel sheet having excellent strength and toughness, wherein the steel sheet is cooled in the above-described magnetic field, and then cooled to room temperature by air or accelerated cooling. Ar ₃ (° C) = 910-230C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb ... (1)

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for the limitation of the present invention will first be described in terms of chemical composition. C requires 0.05% by weight or more to secure strength, but if it exceeds 0.18% by weight, the base material toughness and weldability are reduced. Therefore, C is limited to 0.05 to 0.18% by weight.

Although Si is effective in deoxidizing steel and increasing strength,
If the content is less than 0.05 wt%, the effect is not obtained, and if added over 0.60 wt%, the HAZ toughness is significantly deteriorated.
Limited to 60 wt%.

Mn is a component effective for increasing the strength, and the lower limit is set to 0.60 wt% from the viewpoint of securing the strength. However, the amount of Mn is 2.
When the content exceeds 00 wt%, the rolled air-cooled structure changes from ferrite + pearlite to ferrite + bainite, and the base material toughness is reduced. Therefore, the upper limit is set to 2.00 wt%.

[0011] Al is required to be at least 0.005 wt% for deoxidation. However, the addition of more than 0.050 wt% saturates the deoxidizing effect, so the upper limit was made 0.050 wt%. Since P deteriorates toughness, it is limited to 0.020 wt% or less. S is mainly M in steel
MnS exists as nS, and MnS elongates by rolling, causing anisotropy in ductility and toughness. Therefore, the content was limited to 0.015 wt% or less. However, since MnS can also be expected to have an effect as a ferrite transformation nucleus, the content is preferably 0.004 to 0.010 wt%.

V precipitates as a VN in austenite during rolling and cooling, and ferrite precipitates using it as a nucleus, thereby making crystal grains finer and greatly contributing to improvement in toughness.
Also, after ferrite transformation, VN precipitates in the ferrite,
Since the base metal strength can be increased without performing strong water cooling at the time of cooling, it is also effective for uniformity of properties within the sheet thickness and for reduction of residual stress and distortion. To exert these effects effectively, it is necessary to add 0.04% by weight or more.However, if it exceeds 0.15% by weight, the toughness and weldability of the base material are greatly impaired.
Limited to 04-0.15 wt%.

Ti is present mainly as TiN, and has an action of suppressing the grain growth of γ during heating of the material and an action of accelerating the precipitation of ferrite-forming nuclei or VN into γ by remaining and precipitating in γ. Therefore, it is an effective component for refining crystal grains. Further, Ti has a ferrite forming ability by being present as an oxide. In order to exert these effects, it is necessary to add 0.01 wt% or more, and if it exceeds 0.15 wt%, the cleanliness and toughness of the steel will be reduced.
Limited to 0.15 wt%.

N forms TiN, VN or BN, suppresses the grain growth of γ during the heating of the material and precipitates ferrite using these as nuclei, thereby making the crystal grains finer and greatly contributing to the improvement of toughness. In addition, VN precipitates in the ferrite after ferrite transformation, and the strength of the base material can be increased without performing strong water cooling during cooling, so it is effective for uniform production of properties within the sheet thickness and reduction of residual stress and strain. is there. In order to exert these effects effectively, it is necessary to add 0.005 wt% or more. However, if it exceeds 0.015 wt%, it greatly impairs the base metal toughness and weldability, so it was limited to 0.005 to 0.015 wt%. .

In the second invention and the fourth invention, Cu, Ni,
One or more of Nb, Cr and Mo can be added. Each of these components is an effective component for improving hardenability, and the effect of grain refinement with TiN and VN is optimized and
VN precipitation strengthening also works effectively. That is, by lowering the Ar ₃ transformation point, ferrite grains are more finely formed and precipitation strengthening of VN is increased.However, if the Ar ₃ transformation point is lowered too much, bainite transformation is mainly performed, and finer ferrite formation is reduced. Since the strength becomes insufficient, the strength increases due to the strengthening of the structure, but the toughness of the base material greatly decreases. In order to improve strength and toughness through lowering of the Ar ₃ transformation point, Cu, Ni, Nb,
Cr and Mo are 0.05wt%, 0.05wt%, 0.003wt%, 0.
More than 05wt%, 0.02wt% is required. In order to compensate for the decrease in hot workability due to Cu, Ni needs to be added in an approximately equivalent amount to Cu. However, if Ni exceeds 0.6 wt%, the production cost becomes too high. The upper limit is 0.60wt%
And However, Cu is a precipitation strengthening component, and may be added up to 2.0 wt% for strength adjustment. However, if it exceeds 2.0 wt%, the weld heat affected zone and the cracking resistance during rolling decrease, so the upper limit is set. 2.0 wt%. Nb, Cr and Mo are each
If it exceeds 0.020 wt%, 0.50 wt%, or 0.10 wt%, the weldability and toughness are impaired.

In the second and fourth inventions, B can be added. B, like Ti and V, becomes a nucleus for ferrite as BN, but segregates at the grain boundary to suppress the formation of coarse grain boundary ferrite, and has the effect of making the ferrite grains after rolling fine. The effect is exhibited by the addition of B in an amount of 0.0002 wt% or more, but the addition of more than 0.0020 wt% lowers the toughness, so the content was limited to 0.0002 to 0.0020 wt%.

In the second, third or fourth invention, one or two of REM and Ca can be added.
REM and Ca are finely dispersed as oxides that are stable even at high temperatures, and have the effect of suppressing the growth of γ grains during material heating and of reducing the ferrite grain size after rolling. Also,
It is also effective in improving HAZ toughness, and when these effects are expected, 0.0010 wt% or more of each is required. However, addition of more than 0.0200 wt% decreases the cleanliness and base metal toughness of steel. Therefore, REM, Ca was set in the range of 0.0010 to 0.0200 wt%.

In the second invention, a predetermined amount of O can be contained, and in the third invention and the fourth invention, O is essential. O is indispensable for securing the production amount of the oxides of REM, Ca and Ti, and O: 0.0040 to 0.0150
The reason for limiting to the range of% is that if the amount is less than 0.0040 wt%, the amount of oxides generated is not sufficient, and if the amount exceeds 0.0150 wt%, the cleanliness and base metal toughness of the steel are determined.

For the above reasons, it is important in the limited range of the chemical composition of the present invention to secure the precipitates and inclusions serving as nuclei for ferrite formation.
N is an essential component when nitride-based inclusions and precipitates such as are used, and O is an essential component when oxide-based inclusions and precipitates such as REM, Ca and Ti are used. Becomes It is also possible to use these effects at the same time.

Next, the reasons for limiting the manufacturing method will be described.
Since the material heating temperature to a uniform austenite structure materials of tissue requires more Ar ₃ transformation point, Ar ₃ to uniform structure after ferrite transformation is less than the transformation point is not obtained, Ar ₃ transformation point or more and Limited. Regarding the rolling conditions, in the case of the present invention, when rolling is performed at a high temperature, intragranular ferrite transformation with precipitates as nuclei is promoted in a magnetic field described below, and when performing rolling at a low temperature, transformation from the precipitate is performed. In addition, ferrite transformation from the transformation zone due to processing is promoted. That is, in any case, it is an important point to cause transformation in a magnetic field. Therefore, it is a feature of the present invention that a magnetic field is applied at Ar ₃ −100 ° C. where the transformation is almost completed from the rolling end temperature at the Ar ₃ point or higher. At this time, a magnetic field of 2 T or more is necessary for promoting the transformation, and a magnetic field of less than 2 T has no effect.

[0021]

EXAMPLE An ingot having the composition shown in Table 1 was used to reduce the thickness from 50 mm to 15 mm.
From the microstructure observation at the center of the thickness of each of the obtained steel sheets, the ferrite grain size was evaluated in terms of a circle-equivalent diameter, and the effect of applying a magnetic field was examined. Table 2 shows the results. From these results, the ferrite grain size was reduced in the manufacturing method according to the present invention, and by applying a high magnetic field during transformation, it was possible to manufacture a steel sheet having excellent strength and toughness.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

According to the present invention, it is possible to manufacture a high-strength steel plate having excellent strength and toughness which has been difficult to manufacture as a pillar or beam for a building structure. To play.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/58 C22C 38/58 (72) Inventor Fumimaru Kawabata 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Pref. Mizushima Works, Ltd. (72) Inventor Kenichi Amano 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Pref. Kawasaki Steel Works, Ltd. Mizushima Steel Works, Ltd. Address Kawasaki Steel Corp.

Claims (4)

[Claims] 1. C: 0.05 to 0.18 wt%, Si: 0.05 to 0.6 wt%, Mn: 0.60 to 2.00 wt%, Al: 0.005 to 0.050 wt%, P: 0.020 wt% or less, S: 0.015 wt% or less and N: comprises 0.0050～0.0150Wt%, and V: 0.04~0.15wt% and Ti: containing 0.01～0.15Wt% of one or, the balance being Fe and unavoidable impurities steel Ar ₃ After heating and soaking above the transformation point, hot rolling is performed.At that time, rolling is completed at a temperature not less than the Ar ₃ transformation point defined by the content of each component in equation (1), and rolling is completed A method for producing a thick steel plate having excellent strength and toughness, comprising cooling from a temperature of up to an Ar ₃ transformation point of −100 ° C. in a magnetic field of 2 T or more, followed by air cooling or accelerated cooling to room temperature. Ar ₃ (° C) = 910-230C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb ... (1) 2. C: 0.05 to 0.18 wt%, Si: 0.05 to 0.6 wt%, Mn: 0.60 to 2.00 wt%, Al: 0.005 to 0.050 wt%, P: 0.020 wt% or less, S: 0.015 wt% or less And N: 0.0050 to 0.0150% by weight, and V: 0.04 to 0.15% by weight and Ti: 0.01 to 0.15% by weight, and Cu: 0.05 to 2.0% by weight as a selective component. , Ni: 0.05 to 0.60 wt%, Nb: 0.003 to 0.020 wt%, Cr: 0.05 to 0.50 wt%, Mo: 0.02 to 0.10 wt%, a group consisting of one or more of: B: 0.0002 to 0.0020 wt %, REM: 0.0010 to 0.0200 wt%, Ca: 0.0010 to 0.0100 wt%, and at least one of the group consisting of O: 0.0040 to 0.0150 wt%, with the balance being Fe and After heating and soaking the steel consisting of unavoidable impurities to the Ar ₃ transformation point or higher, hot rolling is performed, and at that time, the Ar ₃ transformation point or more defined by the content of each component in Equation (1) is used. Finish rolling at the temperature and from the end of rolling Ar ₃ transformation point up to 100 ° C
A method of manufacturing a steel plate having excellent strength and toughness, characterized by cooling in a magnetic field of 2 T or more, and then cooling by air or accelerated cooling to room temperature. Ar ₃ (° C) = 910-230C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb ... (1) 3. C: 0.05 to 0.18 wt%, Si: 0.05 to 0.6 wt%, Mn: 0.60 to 2.00 wt%, Al: 0.005 to 0.050 wt%, P: 0.020 wt% or less, S: 0.015 wt% or less And O: 0.0040 to 0.0150 wt%, Ti: 0.01 to 0.15 wt% REM: 0.0010 to 0.0200 wt%, Ca: 0.0010 to 0.0100 wt%, and the balance is Fe and After heating and soaking the steel consisting of unavoidable impurities above the Ar ₃ transformation point,
Hot rolling is performed, and at that time, the rolling is completed at a temperature equal to or higher than the Ar ₃ transformation point defined by the content of each component in Equation (1), and from the end of the rolling to the temperature of the Ar ₃ transformation point −100 ° C. Is cooled in a magnetic field of 2 T or more, and then cooled to room temperature by air or accelerated cooling. Ar ₃ (° C) = 910-230C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb ... (1) 4. C: 0.05 to 0.18 wt%, Si: 0.05 to 0.6 wt%, Mn: 0.60 to 2.00 wt%, Al: 0.005 to 0.050 wt%, P: 0.020 wt% or less, S: 0.015 wt% or less And O: 0.0040 to 0.0150 wt%, Ti: 0.01 to 0.15 wt% REM: 0.0010 to 0.0200 wt%, Ca: 0.0010 to 0.0100 wt%, and one or more of the following: Cu: 0.05 to 2.0 wt%, Ni: 0.05 to 0.60 wt%, Nb: 0.003 to 0.020 wt%, Cr: 0.05 to 0.50 wt%, Mo: 0.02 to 0.10 wt% B: 0.0002 to 0.0020 wt%, V: 0.04 to 0.15 wt%, and at least one of the group consisting of N: 0.0050 to 0.0150 wt%, the balance being Fe and inevitable Heat the steel consisting of impurities above the Ar ₃ transformation point
After soaking, hot rolling is performed.At that time, rolling is completed at a temperature equal to or higher than the Ar ₃ transformation point defined by the content of each component in Equation (1), and from the end of rolling, the Ar ₃ transformation point − A method for manufacturing thick steel sheets with excellent strength and toughness, characterized by cooling to a temperature of 100 ° C in a magnetic field of 2 T or more, and then cooling to room temperature by air or accelerated cooling. Ar ₃ (° C) = 910-230C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb ... (1)