JPH04141519A - Production of structural steel plate excellent in toughness at low temperature and having high young's modulus - Google Patents

Abstract

PURPOSE:To stably and smoothly produce a structural steel plate excellent in toughness at low temp. and improved in rigidity (Young's modulus) in a specific direction with high productivity by limitatively combining respective technical conditions in a rolling stage in an unrecrystallization region, in an intermediate controlled cooling stage, and in a rolling stage in a two-phase region. CONSTITUTION:As a first means, a slab of a structural steel having a temp. between the Ar3 point and 1250 deg.C is hot-rolled at a temp. between the recrystallization finishing temp. and the Ar3 point at >=20% reduction of area, cooled immediately at >=5 deg.C/sec cooling rate, and subjected to rolling in a two phase region at a temp. lower than the Ar3 point at >=50% reduction of area. Further, as a second means, controlled cooling is exerted down to <=600 deg.C at >=5 deg.C/sec cooling rate after rolling in a two phase region is performed at a temp. lower than the Ar3 point at >=50% reduction of area. Further, in the case where alloying elements are added according to the required properties for the purpose of increasing the strength of a base material or improving joint toughness, one or more kinds among Ni, Cr, Mo, Cu, W, P, Co, V, Nb, Ti, Zr, Ta, Hf, rare earth elements, Y, Ca, Mg, Te, Se, and B can be added but, as to the additive quantities, additive quantities are controlled to <=4.5% in total/because deformation resistance in the two phase region is increased and rolling is made difficult when transformation temp. is lowered excessively.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a structural steel plate that has excellent low-temperature toughness and has dramatically improved Young's modulus in the direction perpendicular to the rolling direction (hereinafter referred to as the C direction). The present invention relates to a manufacturing method.

<Prior Art> Generally, the rigidity of a steel plate is proportional to Young's modulus if the shape is constant.

In conventional steels, the Young's modulus is considered to be constant at approximately 21,000 kgf/■■2, excluding special cases such as single crystals and electromagnetic steel sheets, and was not considered to be a particularly noteworthy material property.

However, in recent years, there has been a demand for improved rigidity in a specific direction during use, and consideration has been given to applying the C direction of C direction high Young's modulus steel plates to this purpose.

According to this method, it is possible to increase the rigidity of the structure without increasing the plate thickness or changing the shape.

On the other hand, there are various proposals regarding manufacturing methods for high Young's modulus steel materials, all of which involve developing rolling texture2 through rolling in the two-phase region or ferrite region to improve the Young's modulus of the steel sheet in a specific direction1. .

For example, Japanese Patent Publication No. 5B-14849 discloses a method for producing high-young steel materials. The high Young's modulus feed material disclosed herein is obtained by subjecting steel with specified chemical composition to phase range rolling, controlling the cooling rate to 300°C after finishing rolling, and then
It has been shown that the Young's modulus in the C direction is increased by approximately 10% by tempering at a temperature of 00° C. or lower.

In addition, in Japanese Patent Publication No. 62-4448, C is 0.03
By winding steel with less than M content % at a temperature of 450°C or higher and 720°C or higher with a specified rolling reduction rate in the warm range of Ar2 or lower and 600°C or higher, the Young's modulus in the direction up to 24,300 kgf/dust 12 A method to increase this is described.

<Problems to be Solved by the Invention> However, all of the above-mentioned proposals have inherent problems in practical use as described below, and improvements are awaited for each of them.

That is, the guaranteed temperature of molybdenum according to the proposal of Japanese Patent Publication No. 58-M849 is 0°C. This does not meet the requirement for guaranteed toughness of -60°C or less, which has been required for important structural steel plate members in recent years from the viewpoint of strengthening safety.

This is because toughness deteriorates due to significant formation of processed ferrite due to rolling in the two-phase region.

In addition, it is essential to perform tempering treatment by heating to a temperature of 700° C. or less after hot rolling, which requires a large amount of thermal energy, which inevitably increases manufacturing costs and reduces productivity.

In addition, Japanese Patent Publication No. 62-4448 discloses C50.03%
It describes the limitation of ingredients as an essential requirement, and it is essentially related to the manufacturing method of extremely mild steel, which cannot meet the required strength of structural steel.

Furthermore, if the temperature is waited for by air cooling until the two-phase region is reached, ferrite grains and austenite grains grow and the target Young's modulus improves, but it is extremely difficult to obtain low-temperature toughness that guarantees -60°C. There is a strong possibility that productivity will decrease due to waiting for the temperature to reach the two-phase region.

Therefore, the present invention provides a method for manufacturing structural steel plates with high productivity and low cost, which have excellent low-temperature toughness that guarantees -60°C and improves Young's modulus by about 10% or more. The goal is to provide the following.

<Means for Solving the Problems> In order to achieve the above-mentioned problems, the present invention has the following objectives: (1) Structural steel pieces having a temperature of Ar 2 points or more and 1250°C or less are rolled at a reduction rate ≧ at Ar 1 point or more below the recrystallization end temperature. 20%
A high Young's modulus structure with excellent low-temperature toughness, characterized by hot rolling, immediately cooling at a cooling rate of 5°C/sec or more, and rolling in a two-phase region with a rolling reduction of 50% or less at less than the Ar2 point. (2) Structural steel pieces with a temperature of Ar 2 points or more and 1250"C or less are rolled at Ar 2 points or more below the recrystallization end temperature and a reduction rate ≧ 20.
% hot rolling, immediately cool at a cooling rate of 5°C/sec or more, and after performing two-phase region rolling with a reduction rate ≧50% below the Ar□ point, At the cooling rate,
The second method is a method for manufacturing a high Young's modulus structural steel plate with excellent low-temperature toughness, which is characterized by controlled cooling to a temperature of 600° C. or lower.

The structural steel to which the present invention is directed is described, for example, in the above-mentioned Japanese Patent Publication No. 5B-14849, and as described below,
In order to obtain the required material properties of ordinary welded structural steel, the types and amounts of additive elements that have been determined based on the relationship between action and effect that has been confirmed through use in five industrial fields are used in the same way. The same effect and effect can be obtained. Therefore, steels containing these materials are the target steels of the present invention.

Each of these component elements, the reason for their addition, and their amounts are shown below.

C is added as an effective component to improve the strength of steel, but if the content exceeds 0.20%, it not only increases the deformation resistance during two-phase filling rolling, but also makes rolling difficult. Since island-like martensite precipitates in the welded area and significantly deteriorates the toughness of the steel, the mucus is limited to 0.20% or less.

Sl is necessary as a deoxidizing element for steel 78, and is also useful as a strength-increasing element, but when added in excess of more than 1.0%, it reduces the workability of steel and deteriorates the toughness of welds. . Moreover, since the deoxidizing effect is insufficient if it is less than 0.01%, the amount added is regulated to 0.01 to 1.0%.

Mn is also necessary as a deoxidizing element, and if it is less than 0.3%, the cleanliness of the steel decreases and the workability is impaired. Additionally, it is necessary to add 0.3% or more of the component to improve the strength of the rope. However, since Mn lowers the transformation temperature, excessive addition will lower the two-phase region rolling temperature too much and cause an increase in deformation resistance, so the upper limit is set at 2.0%.

AI and N are added in order to have an effective function in matching the crystal orientation and recrystallization of the steel through solid solution and precipitation during the rolling process, in addition to the refinement of the steel by A1 nitride. However, if the amount added is small, it has no effect, and if it is excessive, it deteriorates the toughness of the steel, so AI: 0.001 to 0
．． 20%, N: limited to 0.020% or less.

The above are the basic components of the steel targeted by the present invention, but for the purpose of increasing base metal strength or joint toughness,
When adding alloying elements according to the required properties,
If the transformation temperature is lowered too much, the deformation resistance in the two-phase region will increase and rolling will become difficult.
r, No, Cu, JP, Co.

V, Nb, Ti, Zr, Ta, Hf, rare earth element + Y,
Ca+ Mg, Te, Se.

One or more types of B may be added, but the total amount added is 4.5
It is regulated within %.

<Function> In order to achieve the problems of the prior art described above, the present inventors have repeatedly conducted various experimental studies using general structural steel having the following chemical components.

C: 0.10~0.15% Si: 0.15~
0.25%Mn: 0.8~1.6%Al:
0.01~0.05%N: 0.0020~0.00
The 50% results are shown in FIG.

FIG. 1 shows the relationship between the intermediately controlled cooling rate and the low temperature toughness expressed as νTrs when the reduction amount in the non-recrystallized region is 25% and the reduction amount in the two-phase region is 50%.

As shown in the figure, it has been found that when the controlled cooling rate is 5° C./sec or more, the toughness is improved to a level that can guarantee toughness at temperatures below -60° C.

This is because the residence time at high temperatures is shortened, so ferrite grains,
Alternatively, it is thought that toughness is improved because grain growth of austenite grains is suppressed.

The present invention has been made based on the above findings.

<Example> (1) Test steel The steel composition of the present invention may be any combination of elements and addition amounts of the above-mentioned general structural steels, but Table 1
The chemical components used in the examples are shown together with comparative examples.

This is also the chemical composition of typical structural steels with different strength levels in the field of structural steels.

(2) Manufacturing conditions and material results The manufacturing conditions and the obtained materials are shown in Table 2.

The test steels shown in Table 1 are Fragrance 1, 2 is 40 kg class steel, and Fragrance 3.
~6 is 50 kg class steel, and fragrance 7 is 60 kg class steel. Further, the sample steel may be V, Nb, Ni, Ti, or Cu as required.

Alloying elements such as Ni, Cr, and No are added.

The invention examples with No. and AI=A7 were all manufactured with good productivity without any heat treatment after rolling.

The Young's modulus in the C direction is maintained without any decrease in low temperature toughness.
Conventional 21. OOOkgf /■■1 for 2 levels
An improvement of 0% to 16% was obtained, and a structural steel plate that fully satisfied the target was obtained.

Especially A6. Directly rolled materials A and 7 exhibited superior low-temperature toughness compared to heated materials at the same temperature level, likely due to the effect of their thermal history.

On the other hand, Comparative Examples Nos. 81 to B7 had their own problems, and structural steel plates satisfying the above requirements could not be obtained.

That is, in Comparative Example No. 2 and Old, in which the two-phase region reduction rate at Ar2 point or lower was less than 50%, the improvement in Young's modulus did not reach the required range.

Comparative example No. B2 where the heating temperature is as high as 1300° C. Comparative example No. 84 to B7 where the unrecrystallized reduction rate is less than 20%
could not be used for the intended purpose due to poor toughness.

Comparative example No. B3° B5. No. No. B3° B5. No. No. B3° B5. No. No. B3° B5. B
The toughness of No. 7 did not reach the -60°C level.

<Effects of the Invention> The present invention described above has excellent low-temperature toughness and toughness in a specific direction by combining the technical conditions of rolling in the non-recrystallized region, intermediate controlled cooling, and rolling in the two-phase region in a limited manner. This makes it possible to smoothly and stably manufacture structural steel plates with an increase in rigidity (Young's modulus) of approximately 10% or more at extremely high productivity by omitting post-rolling tempering treatment.
The economic effects it will bring to industry, especially in this field, will be extremely large.

[Brief explanation of the drawing]

FIG. 1 shows the relationship between the intermediately controlled cooling rate and the low temperature toughness expressed as vTrs. Patent applicant Nippon Steel Corporation

Claims (2)

[Claims] (1) Structural steel pieces whose temperature is Ar_3 points or more and 1250°C or less are rolled at Ar_3 points or more below the recrystallization completion temperature, and the reduction rate is 2 or more.
0% hot rolling, immediately cooling at a cooling rate of 5°C/second or more, and rolling in a two-phase region with a rolling reduction of 50% or less below the Ar_3 point. A method for manufacturing steel plates for structural use. (2) Structural steel pieces whose temperature is Ar_3 points or more and 1250°C or less are rolled at Ar_3 points or more below the recrystallization completion temperature, and the reduction rate is 2 or more.
0% hot rolling, immediately cooled at a cooling rate of 5°C/sec or more, and after performing two-phase region rolling with a rolling reduction of ≥50% below the Ar_3 point, the cooling rate was increased to a cooling rate of 5°C/sec or more. Te, 6
A method for producing a high Young's modulus structural steel plate with excellent low-temperature toughness, characterized by controlled cooling to a temperature of 00°C or less.