JP2583659B2 - Manufacturing method of high Young's modulus structural steel sheet with excellent low temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial application field> The present invention is a TS40kgf / TS which excels in low-temperature toughness and dramatically improves the Young's modulus in a direction perpendicular to the rolling direction (hereinafter referred to as C direction). The present invention relates to a method for producing a structural steel sheet having a thickness of ² mm or more.

<Conventional Technology> Generally, the rigidity of a steel sheet is proportional to the 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 / mm ² except for special examples such as single crystals and magnetic steel sheets, and was not regarded as a material property to be particularly noted.

However, in recent years, improvement in rigidity in a specific direction in use has been demanded, and application of the C direction of the C direction high Young's modulus steel sheet to this has been studied.

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 a method for producing a steel material having a high Young's modulus, and all of them propose that a rolling texture is developed by rolling in a two-phase region or a ferrite region to improve the Young's modulus in a specific direction of the steel sheet.

For example, Japanese Patent Publication No. 58-14849 discloses a method for producing a high Young's modulus steel material. The high Young's modulus steel material disclosed herein is obtained by subjecting a steel having a defined chemical composition to two-phase rolling, controlling the cooling rate to 300 ° C. after rolling, and then tempering at a temperature of 700 ° C. or less. About 10% of Young's modulus in the direction
It has been shown that it can be increased to some degree.

Japanese Patent Publication No. 62-4448 discloses that a steel having a C content of less than 0.03% by weight is regulated at a temperature range of not less than Ar _{3 but not} less than 600 ° C. and not less than 450 ° C. and not more than 720 ° C. Describes a method for increasing the Young's modulus in the C direction up to a maximum of 24,300 kgf / mm ² .

<Problems to be Solved by the Invention> However, each of the above proposals has the following problems in practical use, and improvements are waiting for each.

That is, in the proposal of Japanese Patent Publication No. 58-14849, a rolling method (α-γ dual-phase reduced rolling under pressure) is applied as a production method for improving the Young's modulus, which significantly promotes the formation of texture. However, it is difficult to secure the base material toughness. Therefore, the guaranteed temperature is 0 ° C. From the viewpoint of ensuring safety more strictly, it does not satisfy the low temperature toughness of -60 ° C or less, which is recently required for important members of structures. In addition, in recent years, in response to the reduction in operating fuel efficiency that shipbuilding manufacturers are promoting, in response to the weight reduction of hulls, Young's modulus is low as a steel sheet that can obtain the required strength and rigidity even if the thickness is reduced. It does not satisfy the request.

Further, the proposal in Japanese Patent Publication No. 62-4448 discloses that C ≦ 0.03
% Of content restrictions have been a prerequisite relates to a manufacturing method intended for TS30kgf / mm ² or less of the steel sheet, the present invention does not meet the structural steel TS40kgf / mm ² or more intensity in question.

Therefore, the present invention provides an excellent structural steel sheet having excellent low-temperature toughness that guarantees −60 ° C., and a Young's modulus required as a structure is improved by about 15% or more with respect to conventional steel. It is an object of the present invention to provide a low-cost manufacturing method.

<Means for Solving the Problems> The present invention to achieve the above object, reduction in (1) temperature recrystallization finish temperature below Ar ₃ point or more structural steel s pieces of 1250 ° C. or less than the Ar ₃ point Perform hot rolling at a rate of ≧ 20%, immediately cool at a cooling rate of 5 ° C./sec or more,
A structure with excellent low-temperature toughness and high Young's modulus characterized by performing a two-phase decompression rolling with a reduction rate ≧ 50% at less than ₃ Ar points and then performing a tempering treatment for 5 minutes or more and 30 minutes or less in a temperature range of 700 ° C. or less. (1) hot rolling of a structural steel piece having a temperature of not less than ₃ points Ar and not more than 1250 ° C. with a rolling reduction of ≧ 20% at a temperature not lower than the recrystallization end point and not less than ₃ points Ar And immediately cool at a cooling rate of 5 ° C./sec or more,
After performing two-phase decompression rolling with a reduction ratio of ≧ 50% at less than ₃ Ar points, control cooling to a temperature of 600 ° C. or less at a cooling rate of 5 ° C./sec or more, and then 5 minutes in a temperature range of 700 ° C. or less As described above, a method for producing a structural steel sheet having excellent low-temperature toughness and high Young's modulus characterized by performing a tempering treatment for 30 minutes or less is used as the second means.

The structural steel to which the present invention is directed is described, for example, in the above-mentioned Japanese Patent Publication No. 58-14849, and as described below, a conventional welded structural steel obtains a required material. Equivalent functions and effects can be obtained by using the types and amounts of the additional elements determined based on the relation between the functions and effects confirmed in the application in the field. Therefore, the present invention is intended to include steels including these.

The constituent elements, the reasons for their addition, and their amounts are shown below.

C is added as an effective component for improving the strength of steel. However, an excessive content of more than 0.20% not only increases the deformation resistance during two-phase decompression rolling and makes rolling difficult, but also increases the weldability. The content is regulated to 0.20% or less because island martensite precipitates in the steel and significantly deteriorates the toughness of the steel.

Si is necessary as a deoxidizing element of molten steel and is useful as a strength increasing element. However, if added in excess of 1.0%, the workability of the steel is reduced, and the toughness of the weld is deteriorated. Further, if it is less than 0.01%, the deoxidizing effect is insufficient, so the amount added is regulated to 0.01 to 1.0%.

Mn is also necessary as a deoxidizing component element, and if it is less than 0.3%, the cleanliness of the steel is reduced and workability is impaired. Also, it is necessary to add 0.3% or more as a component for improving the strength of the steel material.
However, since Mn lowers the transformation temperature, the excessive addition lowers the rolling temperature in the two-phase region too much and increases the deformation resistance, so the upper limit is 2.0%.

Al and N are added to refine the steel by Al nitrides, and also to provide a solid solution and precipitation during the rolling process, so that the crystal orientation of the steel can be adjusted and the steel can be effectively worked. However, when the addition amount is small, there is no effect, and when it is excessive, the toughness of the steel is deteriorated, so that Al: 0.001 to 0.20%, N: 0.020%
% Or less.

The above are the basic components of the steel targeted by the present invention. For the purpose of increasing base metal strength or improving joint toughness,
When adding alloying elements according to the required properties,
If the transformation temperature is too low, deformation resistance in the two-phase region increases and rolling becomes difficult.
o, Cu, W, Co, V, Nb, Ti, Zr, Ta, Hf, Rare earth element, Y, Ca, Mg, Te, S
One or more e and B may be added, but the total amount is 4.5%
Regulations within.

<Function> In order to achieve the object of the related art, the present inventors repeated various experimental studies using a general structural steel having the following chemical components.

C: 0.07 to 0.15% Si: 0.15 to 0.25% Mn: 0.8 to 1.6% Al: 0.01 to 0.05% N: 0.0020 to 0.0050% As a result, FIGS. 1 to 3 were obtained.

FIG. 1 shows the relationship between the controlled cooling rate in the middle and the low-temperature toughness represented by vTrs when the rolling reduction in the unrecrystallized region is 25% and the rolling reduction 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./second or more, the toughness is improved to a level at which toughness can be guaranteed at −60 ° C. or less.

This is presumably because the residence time at a high temperature is shortened, so that the grain growth of ferrite grains or austenite grains is suppressed, so that the toughness is improved.

FIG. 2 shows that the steel manufactured under the rolling conditions shown in FIG. 1 is subjected to a tempering treatment at various temperatures at a heating rate of 0.5 ° C./sec for a holding time of 10 minutes, and the tempering temperature and the Young's modulus in the C direction are obtained. improved margin shows the relationship between the (relative 21,000kgf / mm ² level conventional steel).

The present inventors have found from this figure that there is an appropriate tempering temperature range in which the Young's modulus is improved.

This is because during the rolling in the α + γ range or the α range, crystal grains having a main orientation of {211} <111> are formed in a direction perpendicular to the rolling direction, and the crystal grains not having the main orientation are the recrystallization temperature of ferrite. In the above tempering process, {211} <111>
This is considered to be the result of substitution of the crystal grains having the main orientation of.

Further, it has been found that when the tempering treatment is performed at 700 ° C. or higher, the improvement of the Young's modulus can no longer be expected.

This is because crystal grains having a main orientation of {211} <111> disappear during the process in which a specific and small number of crystal grains grow rapidly and consume other crystal grains, and the entire crystal orientation becomes random. It seems to be.

FIG. 3 shows the tempering temperature, the holding time, and the C-direction when the steel shown in FIG. 1 and manufactured under rolling conditions was tempered at a tempering temperature of 650 ° C. and a heating rate of 0.5 ° C./sec. shows the relationship between the improvement margin of Young's modulus (relative 21,000kgf / mm ² level conventional steel).

From this figure, the inventors found that the retention time at the tempering temperature was
It has been found that setting the time to 5 minutes or more and 30 minutes or less forms a texture having a {211} <111> direction in a direction perpendicular to the above-described rolling direction and increases the Young's modulus.

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

<Examples> (1) Test steel The steel component of the present invention may be in any combination as long as it is an element and the amount of addition of the above-mentioned general structural steel. These are shown in Table 1.

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

(2) Manufacturing conditions and material results Table 2 shows the manufacturing conditions and the obtained materials.

The test steels shown in Table 1 are steel grades 1 and 2 of 40 kg class steel and steel grade 3
６6 is a 50 kg class steel, and steel type 7 is a 60 kg class steel. Further, the test steel contains alloying elements such as V, Nb, Ni, Ti, Cu, Ni, Cr, and Mo as necessary.

In all of the inventive examples Nos. A1 to A7, structural steel sheets having excellent low-temperature toughness and Young's modulus characteristics were obtained.

That is, the low-temperature toughness is excellent at −61 to −110 ° C., and the Young's modulus in the C direction is clear in comparison with the Young's modulus of B9 to B15 of the comparative example in which tempering was not performed using the same steel type as A1 to A7. As mentioned above, the improvement cost is improved by 5.0-8.0%, and the conventional 21,000kgf / mm ²
Structural steel sheets exhibiting an improvement of 15% or more with respect to the level and sufficiently achieving the object of the present invention were obtained.

In addition, the A6 and A7 direct-rolled materials exhibited superior low-temperature toughness as compared with the heated materials at the same temperature level due to the effect considered to be the effect of the heat history.

On the other hand, Nos. B1 to B22 of Comparative Examples each had a problem, and a structural steel sheet satisfying the above demand could not be obtained.

That is, Comparative Example N in which the two-phase decompression rate of Ar ₃ points or less was less than 50%.
For o.B1, B5 and B16, the improvement in Young's modulus did not reach the required range.

Nos. B2 and B17 of Comparative Examples having heating temperatures as high as 1300 ° C. and Comparative Examples Nos. B4 and B19 to B22 having recrystallization reduction rates of less than 20% were all poor in toughness and could not be used for the intended use.

The toughness of the sample not cooled in the middle and the comparative example No. B18 in which the cooling rate of the middle cooling was less than 5 ° C./sec did not reach the −60 ° C. level.

B3 and B7 with a retention time of less than 30 minutes at the tempering temperature, B8 with a retention time of 30 minutes or more, and B6 with a high tempering temperature of 800 ° C did not achieve the required Young's modulus improvement.

<Effects of the Invention> The present invention described above provides low-temperature toughness by restrictively combining the technical conditions of the rolling in the unrecrystallized region, the controlled cooling and the rolling process in the two-phase region, and the subsequent tempering process. It enables smooth and stable production of structural steel sheets with excellent rigidity (Young's modulus) of about 15% or more in a specific direction under extremely high productivity. The economic effects on the industrial world are extremely large.

[Brief description of the drawings]

FIG. 1 shows the relationship between the controlled cooling rate on the way and the low temperature toughness expressed by vTrs. FIG. 2 shows the relationship between the tempering temperature and the margin for improving the Young's modulus in the C direction. FIG. 3 shows the relationship between the holding time at the tempering temperature and the improvement in the Young's modulus in the C direction.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hiroshi Takezawa 1 Nishinosu, Oita, Oita City, Oita Prefecture Inside Nippon Steel Corporation Oita Works (72) Inventor Toshiaki Hashi 1 Nishinosu, Oita City, Oita City, Oita Prefecture New Japan (56) References JP-A 4-141519 (JP, A) JP-A 4-141518 (JP, A) JP-A 62-47430 (JP, A) Japanese Patent Publication Sho 60- 4887 (JP, B2)

Claims (2)

(57) [Claims] 1. A temperature performs recrystallization finish temperature below Ar rolling reduction ≧ 20% of the heat rolling at _three points or more structural steel s pieces of 1250 ° C. or less than ₃ points Ar, immediately 5 ° C. / sec or more Cooling at a cooling rate, after two-phase decompression rolling at a reduction rate of ≧ 50% at less than the Ar ₃ point, and tempering at a temperature range of 700 ° C. or less for 5 minutes or more and 30 minutes or less. A method for manufacturing structural steel sheets with excellent Young's modulus. 2. Structural steel slabs having a temperature not lower than the Ar ₃ point and not higher than 1250 ° C. are subjected to hot rolling with a reduction ratio ≧ 20% at an Ar ₃ point or lower below the recrystallization end temperature and immediately at 5 ° C./sec or higher. After cooling at a cooling rate and performing two-phase decompression rolling with a rolling reduction of ≧ 50% at less than ₃ points of Ar, controlled cooling to a temperature of 600 ° C or less at a cooling rate of 5 ° C / sec or more, then 700 ° C or less A tempering treatment for 5 minutes or more and 30 minutes or less in the above temperature range.