JPH0211652B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method (direct rolling) for producing a homogeneous, non-directional, strong steel plate by hot rolling without reheating after casting. In other words, steel used for line pipes, various low-temperature storage tanks/pressure vessels, shipbuilding/official structures, and other various steel structures used in the room temperature range or lower temperature range is immediately hot rolled after casting. Therefore, after finishing the material to a predetermined size, accelerated cooling is performed during the cooling process to create a fine Widmann-Schüttesten-like bainitic transformation structure caused by Ti oxide. The objective is to impart strength and low-temperature toughness equivalent to or higher than steel plates manufactured by quenching, tempering, normalizing, and accelerated cooling immediately after rolling, and is mainly used in direct rolling of thick steel plates. It provides a powerful means to meet the desire to obtain the desired results. (Prior Art) In recent years, there has been a strong trend towards simplifying or omitting processes in the manufacturing technology of hot rolled steel sheets. This is reflected in the elimination of reheating steps for intermediate products due to the introduction of continuous casting and controlled cooling processes. Amid these technological trends,
In the field of hot strip manufacturing technology, direct rolling has been put into practical use, in which continuously cast slabs are immediately rolled without being reheated. However, such direct rolling has not yet been put into practical use as a manufacturing technology for thick steel plates. It is well known that direct rolling of thick steel plates can be carried out metallurgically in the fields of mild steel and general ordinary steel, where the low-temperature toughness of the steel plate is not so strictly required. However, for low-temperature steels and high-strength steels that require low-temperature toughness, direct rolling results in inferior low-temperature toughness compared to conventional manufacturing methods. For this reason, as a means to compensate for this, as disclosed in JP-A-57-131320, rough rolling is started when the slab surface temperature reaches 1100 to 750°C, and then finishing is carried out to below Ar ₃ . There is a method in which rolling is performed and the steel sheet temperature after rolling is cooled at a predetermined cooling rate between 650°C and 400°C. However, such a method in which high-intensity controlled rolling precedes controlled cooling requires that rolling be carried out in a low temperature range to ensure sufficient rolling reduction, and that time-temperature control during rolling must be carefully controlled. As a result, the productivity of the rolling process is significantly reduced. In addition, the types of steel plates that can be manufactured using this process are limited to those that are relatively thin, such as line pipe materials. The reason for this is that this process uses refinement of the steel structure to ensure the low-temperature toughness of the steel sheet, but this requires the refinement of austenite grains and ferrite grains through rolling. It is. (Problems to be Solved by the Invention) The present invention utilizes a method of refining grains through conventional rolling when producing high-strength steel sheets with excellent low-temperature toughness by direct rolling. Instead, the aim is to refine the transformed structure of the steel sheet by using Ti oxide precipitates that are finely dispersed and precipitated within the slab during the steel melting and casting process. In other words, in the method of the present invention, the transformation of steel does not occur from the γ grain boundaries, but instead occurs in the form of intragranular transformation from Ti oxide-based inclusions that are finely dispersed and precipitated in the steel material, independent of the γ grain boundaries. Therefore, the refinement of the steel structure is achieved by a mechanism that is completely independent of the grain refinement caused by rolling. By utilizing such a transformation mechanism, there is no need for complicated and complicated rolling process control in the rolling process, and the productivity of the rolling process is dramatically increased, and the thickness of the steel plate that can be produced is not only thin but also It is an object of the present invention to enable a direct rolling process that can be applied to steel plates as thick as slabs. (Means for solving the problems) In order to achieve the above object, the present invention employs the following means. That is, the structure of the steel in the solidified state has a weight percent concentration of C: 0.001 to 0.300%, Si: 0.8% or less,
Mn: 0.4-2.0%, Ti: 0.003-0.050%, O:
0.0010 to 0.0100%, and 0.007% or less of Al, which is unavoidably mixed in as an impurity element during the melting process of steel,
Contains the balance iron and impurity elements, and has a particle size of
After solidification, a cast slab containing oxide-based inclusions of 3 μm or less and containing a composite crystal phase of one or two of TiO and Ti ₂ O ₃ in a range of 0.004% to 0.100% by weight. After rolling to the final thickness of the steel plate while the substantial part of the slab is still at a temperature of 900℃ or higher during cooling, the slab is rolled from 900℃ to 500℃.
Cooling between ℃ in a time of 20 seconds or more and 100 seconds or less,
A method for manufacturing a steel plate having a fine bainitic structure consisting of fine Widmann-Schüttetten-like ferrite plates, and a composition of the steel in a solidified state in terms of weight percent concentration of C: 0.001 to 0.300%, Si:
0.8% or less, Mn: 0.4~2.0%, Ti: 0.003~0.050
%, O: 0.0010-0.0100% as the basic component, Cu:
1.5% or less, Ni: 10% or less, Cr: 1% or less,
Mo: 1% or less, Nb: 0.2% or less, V: 0.5% or less, B: 0.0025% or less, REM: 0.05% or less,
Contains one or more of Ca: 0.008% or less, Al, which is unavoidably mixed in as an impurity element during the melting process of steel, is 0.007% or less, and the balance contains iron and impurity elements, and the particle size is 3 μm. below
0.004% by weight of oxide inclusions containing a composite crystal phase of one or both of TiO and Ti ₂ O ₃
% or more and 0.100% or less is rolled to the final thickness of a steel plate while the substantial part of the slab is at a temperature of 900°C or more during cooling after solidification, and then Between 900℃ and 500℃
This is a method for manufacturing a steel plate with a fine bainite structure consisting of fine Widmann-Schüttesten-like ferrite plates, which is cooled for a period of 20 seconds or more and 100 seconds or less. Below, first, the reason for limiting the composition of the steel material related to the present invention will be described. C, Si, and Mn are used to increase the strength of steel materials.
An appropriate amount is required as it promotes hardening of the HAZ tissue.
It must not be too high. From this point of view, in steel materials to which the method of the present invention is intended, C should be set at 0.001 to 0.300%, and Si should be set at 0.001 to 0.300%.
0.8% or less, and Mn in the range of 0.4 to 2.0%. Al, Ti, and O play a fundamental role in the formation of the fine bainite structure (hereinafter referred to as the "fine bainite structure") consisting of fine Widmann-Schüttesten-like ferrite plates that characterize the structure of the steel produced by the method of the present invention. Involved. When Al is higher than 0.007%, oxides containing a composite crystal phase of one or two of TiO and Ti ₂ O ₃ (hereinafter referred to as "Ti- Oxide-based inclusions (called "oxide-based inclusions") are not formed during the steel melting process. Therefore, it is necessary to not add Al, and to prevent it from being mixed in from the ferroalloy or refractory, and the amount of Al that is unavoidably mixed is set to 0.007% or less. If Ti is added to steel with Al content within the above range, Ti-Oxide inclusions will be formed, but if the amounts of Ti and O are too large, they will become coarse and the frequency of occurrence of fine bainite structures will be too low for practical use. loses the above effect. On the other hand, if the amounts of Ti and O are too small, Ti-Oxide inclusions will not be formed. Therefore, Ti is 0.003~0.050%, O
0.0010 to 0.0100%, and Ti−
The total amount of oxide inclusions was set to 0.004% or more and 0.100% or less. As a result, oxide-based inclusions having a grain size of 3 μm or less and a composite of one or two of TiO and Ti ₂ O ₃ are formed in the slab solidified in the normal casting process. As is well known, these oxides produce fine ferrite=plates from within the grains, independent of the austenite grain boundaries, during the cooling process from austenite in the steel, contributing to toughening of the steel. Cu is effective in improving the corrosion resistance and strength of steel materials, but too much Cu can cause hot cracking in the weld metal.
It was set to 1.5% or less. Ni increases the strength and low-temperature toughness of steel materials at the same time, so it is preferable to add it for such purposes.
Steels containing more than 10% Ni are difficult to obtain the economic effects of the present invention. Therefore, it was set to 10% or less. Cr, Mo, and B improve the hardenability of steel, and are effective in stabilizing the fine bainite structure in the process according to the present invention. However, if the amount is too high, hot cracking will occur during the transformation process from the γ phase. Therefore
Cr and Mo were each 1% or less, and B was 0.0025% or less. In the method of the present invention, Nb and V precipitate as fine carbonitrides in the cooling process after rolling and increase the strength of the steel, but if they are in too large a quantity, they impair the low-temperature toughness of the steel material. Therefore, Nb was set to 0.2% or less, and V was set to 0.5% or less. Ca and REM have the function of fixing S in steel and reducing MnS, which is harmful to the ductility and notch toughness of steel materials, so they are added for such uses. However, if there is too much Ca, it will reduce the cleanliness of the steel and cause internal defects in the steel plate, so the upper limit should be set for Ca.
0.008%, and 0.05% for REM. Note that P, S, and N have no primary significance with respect to the technical requirements of the method of the present invention, but they are important for the toughness of the welded joint (heat affected zone: HAZ and weld metal). Since it is unfavorable for P and S, it is desirable that the content be as low as possible, and it is desirable that the content be 0.025% or less for P and S, and 0.0050% or less for N. Next, the reasons for limiting the rolling method and post-rolling cooling conditions of the present invention will be described. In the method of the present invention, rolling of a slab that satisfies the above-mentioned requirements is completed while a substantial portion of the slab remains at a temperature of 900°C or higher during the cooling process after casting. It must be rolled like this. The reason for this is that rolling at a temperature lower than this causes the γ grains to become finer and the rolled structure to remain in the γ phase, which is harmful to the formation of a fine bainite structure. Further, if the cooling rate after rolling is too high, the steel structure becomes martensite, and if it is too low, the steel structure becomes ferrite=pearlite, and in either case, the fine bainitic structure, which is a characteristic of the method of the present invention, cannot be obtained. Therefore, it is necessary to cool the temperature between 900°C and 500°C in a period of 20 seconds or more and 100 seconds or less. (Function) According to the method of the present invention, there is no need to reheat the as-cast slab with extremely coarse austenite grains, and there is no need to try to refine the crystal grains by hot rolling. By simply cooling from ℃ to 500℃ for 20 seconds or more and 100 seconds or less, it becomes possible to produce a homogeneous, non-directional steel plate with excellent low-temperature toughness. The reason why a steel plate with excellent low-temperature toughness can be obtained regardless of the refinement of the γ grains or α grains by reheating or rolling the slab is as follows. In other words, conventional methods, such as reheating or rolling slabs to refine the γ grains, and this method and γ/
In a method that aims at grain refinement by rolling recrystallization of α grains in combination with rolling in the α coexistence region,
This aims to refine the γ grains before the start of γ/α transformation, allowing γ/α transformation to occur from γ grain boundaries or deformation bands that are thought to play a role similar to γ grain boundaries, resulting in a fine α grain structure. In contrast to trying to obtain a rolled steel plate consisting of
The method of the present invention uses γ/
Cause alpha metamorphosis. For this reason, it is not only unnecessary to re-fine the γ grains, but the γ grains must be rather coarse. In other words, even in steel that satisfies the requirements for the composition of the present invention, when the γ grains become fine, transformation from the γ grain boundaries occurs preferentially, so that fine and large amounts of γ grains exist within the γ grains. There is
α transformation from Ti oxide no longer occurs. For this reason, rolling in the present invention must be completed while a substantial portion of the steel is at a high temperature of 900° C. or higher so as not to cause the γ grains to become finer. In this way, high-temperature steel made of coarse γ grains is
When cooled between ℃ and 500℃ for 20 seconds or more and 100 seconds or less, plate-shaped ferrite is generated from within the γ grains with Ti oxide as the core. FIG. 1 shows the structure of a steel plate produced by the method of the present invention. Example 1 Table 1 shows the case where a slab that satisfies the requirements for the composition described in claim 1 of the present application is rolled and cooled under the conditions according to the method of the present invention in the cooling process after casting, and
The test steel and process conditions for the comparative rolling method are shown. Steel plates A and C are produced by the method of the present invention, and although they are rolled at an extremely high temperature range compared to the rolling temperature range of ordinary thick plates, they become fine bainitic structures due to appropriate cooling after rolling, and have excellent properties. It shows notch toughness. Although the rolling conditions of steel plates B and D satisfy the requirements of the present invention, the cooling rate after rolling is lower than that of steel plate B.
In steel plate D, it is too large, and in both cases, it is too small to form a fine bainite structure. On the other hand, in steel plate E, the cooling conditions after rolling satisfy the conditions of the present invention, but the rolling temperature range was too low and the γ grains became finer, so transformation from the grain boundaries took priority and the fine bainite structure was formed. Not obtained.

[Table] Example 2 Table 2 is related to the statement in claim 2 of the present application, and the steel plate F is an ultra-low N steel with Nb addition,
Steel plate G is an extremely low N Cu-Ni-Mo-Nb-Ca additive steel, and steel plate H is an extremely low N Cr-V-B-REM additive steel. Inter-rolling process conditions. Itpo steel plate I
, the process conditions meet the requirements of the present invention, but Al
is out of the range required by the present invention, and does not contain Ti oxide, so the steel structure becomes a coarse upper bainitic structure, and the shear peace notch toughness is poor.

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[Table] (Effects of the invention) Since the method of the present invention aims to refine the steel structure through the mechanism described above, it has the following superior effects compared to conventional methods and conventional steel. . (1) It becomes possible to produce steel sheets with excellent low-temperature toughness by hot rolling as-cast slabs without reheating them. (2) Hot rolling is not intended to refine the gamma grains, but merely to obtain the final dimensions, shape, or surface properties of the steel sheet, or to improve internal properties such as roughness and residual porosities in the slab. The purpose is to crimp this in case of insufficient
It is sufficient to carry out the rolling at a high temperature where the hot deformation resistance of the steel is small, so there is no need to apply an excessive load to the rolling mill, and therefore the productivity of rolling can be significantly increased. (3) The intragranular transformation of plate-shaped ferrite, which occurs with Ti oxide as the core, occurs stably over a wide range of cooling rates, so it is difficult to use thick steel plates where the cooling rate in the surface layer and the interior of the steel plate are significantly different. Also, there is little difference in the structure when viewed in the thickness direction, and a homogeneous material can be obtained. Therefore, a steel plate having a thickness close to that of a cast piece, excellent low-temperature toughness, and good homogeneity can be obtained.

[Brief explanation of drawings]

FIG. 1 is a microscopic photograph showing the structure of a steel plate produced by the method of the present invention.

Claims (1)

[Claims] 1. The composition of the steel in the solidified state is C: 0.001 to 0.300%, Si: 0.8% or less, Mn: 0.4 to 2.0%, Ti: 0.003 to 0.050%, O: 0.0010 to 0.0100. %, Al, which is unavoidably mixed in as an impurity element during the steel melting process, is 0.007% or less, and the balance contains iron and impurity elements, and the particle size is 3 μm or less, including TiO,
0.004% or more by weight of oxide inclusions containing a composite crystal phase of one or two types of Ti ₂ O ₃
When a slab containing 0.100% or less is cooled after solidification, a substantial portion of the slab becomes 900%
From a fine Widmann-shaped ferrite plate, which is rolled to the final thickness of the steel plate while still at a temperature above ℃, and then cooled from 900℃ to 500℃ for a period of 20 seconds to 100 seconds. A method for producing a steel sheet with a fine bainite structure. 2 The composition of the steel in the solidified state has the following basic components in weight percent concentration: C: 0.001 to 0.300%, Si: 0.8% or less, Mn: 0.4 to 2.0%, Ti: 0.003 to 0.050%, O: 0.0010 to 0.0100%. , Cu: 1.5% or less, Ni: 10% or less, Cr: 1% or less, Mo: 1% or less, Nb: 0.2% or less, V: 0.5% or less, B: 0.0025% or less, REM: 0.05% or less, Ca : Contains 0.008% or less of one or more types of Al, which is unavoidably mixed in as an impurity element during the melting process of steel.
0.007% or less, the remainder contains iron and impurity elements, and the particle size is 3 μm or less and contains oxide-based inclusions containing a composite crystal phase of one or two of TiO and Ti ₂ O ₃ in weight percent. After rolling a slab containing in the range of 0.004% to 0.100% to the final thickness of a steel plate while the substantial part of the slab is at a temperature of 900℃ or higher during cooling after solidification. , a method for producing a steel sheet with a fine bainitic structure consisting of fine Widmann-Schutetzten-like ferrite plates, which is cooled between 900°C and 500°C for a period of 20 seconds or more and 100 seconds or less.