JPS6283420A - Manufacture of non-heattreated high tensile steel excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To manufacture stably a non-heattreated high tensile steel excellent in toughness at low temp. by subjecting a steel containing specific percentage of C, Si, Mn and Al to rolling, cooling, light draft, and cooling successively under specific conditions. CONSTITUTION:Rolling is applied to the steel consisting of, by weight, 0.005-0.20% C, 0.05-0.5% Si, 0.5-2.5% Mn, 0.005-0.08% Al and the balance Fe with inevitable impurities in the temp. range from (Ar3 transformation point + 150 deg.C) to Ar3 transformation point at >=30% draft. After rolling, the steel is cooled to >=500 deg.C and is subjected to light draft at 700-500 deg.C at 1-10% draft, followed by cooling down to 300 deg.C at 1-5 deg.C/sec cooling rate. In this way, high tensile steel excellent in toughness at low temp. after welding can be manufactured stably without recourse to temper treatment.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing non-temperature high tensile strength steel with excellent low-temperature toughness, and in particular to a method for manufacturing thick steel plates by warm rolling to reduce the deterioration of toughness. Regarding the manufacturing method for increasing strength, it is used in the field of high-strength thick steel plates that require welding and low-temperature toughness.

[Conventional technology]

Conventionally, high tensile strength steel plates that require low-temperature toughness due to welding, such as steel plates for shipbuilding and offshore structures, steel plates for tanks and pressure vessels, steel plates for pipelines, and steel plates for industrial machinery, have been processed by normalizing and baking. Although it is manufactured using a heating and tempering process, it has the disadvantage of increasing manufacturing costs due to the rising cost of heat treatment.

On the other hand, as a manufacturing method that does not perform heat treatment, so-called non-heat treatment, to achieve high tensile strength and high toughness, for example, JP-A No. 5
Publication No. 7-134514 or JP-A-58-6122
There is a method of performing accelerated cooling after controlled rolling, as disclosed in Japanese Patent No. 4.

However, it is possible to reduce the deterioration of toughness, increase strength (higher TS), and reduce alloy components by using the accelerated cooling method by reducing the cooling stop temperature to 500°C or less. If the temperature is below 500°C, the cooling rate increases rapidly, making it difficult to control the target cooling stop temperature, causing uneven cooling in the width and length directions of the steel plate, resulting in distortion and material properties. It was difficult to produce uniform steel plates.

As a method for achieving high strength by reducing cooling unevenness, Japanese Patent Application Laid-open No. 58-144419 or Japanese Patent Application Laid-Open No. 59-1
23713, after controlled rolling or controlled rolling.
There is a method of performing warm rolling after accelerated cooling.

However, high strength can be achieved only by the warm rolling method in which rolling is performed by 15% or less in the temperature range of 600 to 300°C;
The disadvantage is that the toughness deteriorates.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the above-mentioned problems of the prior art, and to be able to stably produce high-strength steel with less distortion and material variation within the steel plate and excellent low-temperature toughness after welding, without using heat treatment. We are here to provide you with a method.

[Means and effects for solving the problems] The above objects of the present invention are achieved by the following two inventions.

The gist of the first invention is as follows.

That is, in terms of weight ratio, C: 0.005-020%, Si: 0.05-05%,
Mn: 0.5-2.5%, Al: 0005-008
% and the remainder is Fe and unavoidable impurities, the steel is rolled at a reduction rate of 30% or more in the temperature range of (Ar3 transformation point + 150 ° C.) to Ar3 transformation point, and after the rolling, the steel is heated to 500 ° C. or more. A step of cooling and applying light reduction at a rolling reduction rate of 1 to 10% in a temperature range of 700 to 500°C, and a step of cooling to 300°C at a cooling rate of 1 to 5°C/s after the light reduction. This is a method for producing non-thermal high tensile strength steel with excellent low temperature toughness, which is characterized by:

The gist of the second invention is as follows.

That is, the same C, Si, Mn, A as in the first invention
In addition to basic components such as I, Nb: 0.01-0.10%, v: 001-010%
Cu: 0.5% or less, Cr: 0.5% or less, Mo
: 0.5% or less, Ni: 0.5% or less, Ti: 0
．． 005-003%, B: 0.0003-0.0020%, Ca:
0.0 0 1 to 0.0 1 0%, rare earth metal: 0.
A steel with excellent low-temperature toughness is rolled and cooled by the same method as the first invention. This is a method for producing tempered high tensile strength steel.

As a result of various studies on the method of reducing toughness deterioration and increasing strength using a warm rolling method, the present inventors found that the cooling rate after warm rolling to a temperature range of 300°C or higher was set to 1-b. We have newly discovered that strength and toughness can be achieved at the same time.

The experiment that formed the basis of the present invention will be explained.

C: 0.08%, Si: 0.25%, Mn: 1.5%,
Al70.020%, Nb:0.04%, V:0.05
(Ar3 transformation point +150℃)
Applying a pressure reduction of 60% in the temperature range from to Ar3 transformation point,
After finishing rolling at 790, it was rolled at 65 at a cooling rate of 10°C/S.
Accelerated cooling to 0℃, further reducing the rolling reduction rate to 0℃ at 630℃
The tensile strength (TS) and yield strength (ys ) and fracture surface transition temperature (vTrs) were investigated, and the results are shown in FIG.

In the case of accelerated cooling shown by the solid line in FIG. 1, compared to the case of air cooling shown by the dotted line, it is clear that YS and TS are higher (vTrs has shifted to the lower temperature side. In other words, after warm rolling, It can be seen that high strength and toughness can be achieved at the same time by increasing the cooling rate.

The reason for the improvement in toughness by increasing the cooling rate after warm rolling is that it suppresses the grain growth of warm-rolled ferrite, and also suppresses the grain growth of ferrite generated from untransformed γ. This is thought to be due to the refining effect of γ and the refining and dispersion effect of second phase structures such as bainite and martensite generated from untransformed γ.

Next, the reason for limiting the component composition of the present invention will be explained.

C: If C is less than 0.005%, the strength of the steel plate will decrease, and the weld heat affected zone (hereinafter referred to as HAZ) will become significantly softened.
On the other hand, if it exceeds 0.020%, the toughness of the base metal deteriorates, and the hardening of the welded part and cracking resistance deteriorate significantly, so the C content must be within the range of 0.005 to 0.020%.

Sl: Si is an element that is inevitably included for deoxidation during steel refining, but if it is less than 0.05%, the toughness of the base material will deteriorate, while if it exceeds 0.05%, the cleanliness of the steel will deteriorate and the toughness will decrease. Therefore, S
l must be within the range of 005 to 05%.

Mn: If Mn is less than 0.5%, the strength and toughness of the steel plate will decrease,
Furthermore, the softening of the HAZ increases, and if it exceeds 25%, the toughness of the HAZ deteriorates, so Mn needs to be within the range of 05 to 25%.

Al: In order to deoxidize the steel, it is necessary to add at least 0005% of Ajl as a solid solution.On the other hand, if it exceeds 008%, not only the toughness of the HAZ but also the toughness of the weld metal will deteriorate significantly. must be within the range of 0005 to 008%.

The above are the basic components of the high-strength steel in the present invention, but if necessary, limited amounts of Nb, v, Cu, Cr, Mo,
At least one selected from Ni 2 , Ti 2 , B, Ca 2 , and rare earth metals can be added and contained, and specific effects can be added by appropriately containing each of them as described below. The reasons for limiting these selective addition elements are as follows.

Nb: Nb is effective in making ferrite grains finer, but at 0.01%
Nb is limited to a range of 0.01 to 0.10% because if it is less than 0.01%, it has no effect, and if it exceeds 0.010%, it will diffuse into the weld metal during welding and reduce the toughness of the weld metal.

V: ■ is added to improve the strength and toughness of the base material of the steel plate and ensure the strength of the joint, but if it is less than 0.01%, it has no effect, while if it exceeds 0.10%, it will reduce the toughness of the base material and HAZ. Since it causes significant deterioration, (2) is limited to a range of 001 to 010%.

Cu: Cu not only has almost the same effect as Ni, but also improves corrosion resistance, but if it exceeds 0.5%, cracks are likely to occur during hot rolling and the surface quality of the steel sheet deteriorates. It is necessary to keep it below 0.5%.

Cr: Cr is added to ensure the strength of the base metal of the steel plate and the strength of the joint, but if it exceeds 0.5%, not only the toughness of the base metal but also the toughness of the weld will deteriorate, so it must be kept below 0.5%. .

MO: Mo improves strength and toughness by regulating the γ grains during rolling and producing fine bainite, but it is not necessary to add more than 0.05% to achieve the purpose of this invention. Mo content is limited to 0.05% or less since any more than this will increase manufacturing costs.

N1: N1 improves the strength and toughness of the base material without adversely affecting the hardenability and toughness of HAZ, but if it is added in excess of 0.5%, it will increase the manufacturing cost and will also meet the objectives of the present invention. Since it is not necessary to achieve the effect, set it to 0.5% or less.

T1: Ti is added for the purpose of improving the toughness by refining the γ grains and increasing the strength of the T1 carbonitride. However, 0005%
If it is less than 0.003%, there is no effect, and if it exceeds 0.003%, the toughness deteriorates, so Tl was limited to a range of 0.005 to 0.03%.

z B improves strength and toughness because it generates fine bainite, but if Q, OO is less than 03%, this effect is absent,
On the other hand, if it exceeds 0.0020%, the hardness of the HAZ will increase significantly.
Since B is raised twice, B is limited to a range of 0.0003 to 0.0020%.

Ca: If Ca is less than 0,001%, it is insufficient to control the morphology of MnS and is not effective in improving the toughness of the steel plate in the direction perpendicular to the rolling direction.On the other hand, if it exceeds 0,010%, the cleanliness of the steel deteriorates and may cause internal defects. Therefore, Ca was set within the range of 0001% to 0010%.

Rare earth metal (hereinafter referred to as REM); REM of less than 0001% is insufficient for controlling the morphology of MnS and is not effective in improving the toughness of the steel plate in the direction perpendicular to rolling, while on the other hand, if it exceeds 0010%, the cleanliness of the steel is poor. Since it is also disadvantageous in terms of arc welding, REM is 0001 to 00.
It needs to be within 10%.

Next, the reasons for limiting the manufacturing conditions in the present invention will be explained.

In the method of the present invention, the slab heating temperature is not particularly limited, but the slab heating temperature is preferably in the range of 900 to 1250°C.

The reason why the rolling reduction in the temperature range from (Ar3 transformation point + 150°C) to Ar3 transformation point (non-recrystallized γ region) was set to 30% or more is because if the rolling reduction is less than 30%, the γ grains will elongate and the deformation zone will occur. This is because the introduction of a small amount of crystal grains results in insufficient grain refinement. Also, the temperature range is from (Ar3 transformation point +150℃) to A
The reason for reaching the r3 transformation point is the reduction rate of 3 in the uncrystallized γ region.
This is because even if rolling is carried out at 0% or more, if the rolling temperature exceeds (Ar3 transformation point + 150°C), elongation of the γ grains and introduction of deformation bands will not be sufficient and the crystal grains will not become fine. In addition, if the rolling temperature is lower than the Ar transformation point, rolling occurs in the (γ + α) two-phase region and the ferrite is work hardened, so even if the method of the present invention is carried out under other conditions, the toughness will deteriorate significantly.
r transformation point or higher.

Next, in the present invention, the cooling conditions after non-recrystallized γ region rolling are not particularly limited, but are preferably in the range of air cooling to 30° C./S. Even if the cooling after rolling in the non-recrystallized γ region is air cooling, as long as the amount of rolling in the non-recrystallized γ region is 30% or more, the ferrite grains, which is the main objective of the present invention, can be sufficiently refined. When accelerated cooling is performed at 30°C/S or less after unrecrystallized γ region rolling,
Because the ferrite grains before warm rolling, which will be described later, become finer,
This is because the final ferrite grain size becomes fine and the toughness is improved compared to air-cooled materials.

In addition, the reason why the cooling stop temperature after rolling in the non-recrystallized γ region is set to 500°C or higher is that when accelerated cooling is performed, the cooling stop temperature is 500°C or higher.
If the temperature is below .degree. C., significant distortion occurs in the plate wall, and even if warm rolling is performed in the next step, the distortion in the plate wall will not be resolved.Therefore, the lower limit of the cooling stop temperature was set at 500.degree.

Furthermore, if the reduction ratio in warm rolling is less than 1%, the increase in tensile strength will be insufficient, and if it exceeds 10%, the toughness will deteriorate significantly, so it was limited to a range of 1 to 10%.

If the cooling rate after warm rolling is less than 1°C/S, the effect of refining the ferrite grains, which is the main objective of the present invention, cannot be achieved, and if it exceeds 5°C/S, the second phase structure will be finely dispersed. However, since lumpy bainite and martensite are generated and the toughness is significantly deteriorated, the cooling rate after warm rolling is 1~b.Also, if the accelerated cooling stop temperature after warm rolling is less than 300℃, the fine ferrite grains No effect was observed at 300°C.
If the temperature is less than 1 to b degrees, the cooling rate becomes too fast and distortion occurs in the plate thickness, so the lower limit of the cooling stop temperature after warm rolling needs to be limited to 300 degrees Celsius.

〔Example〕

Two kinds of test steels whose compositions are shown in Table 1 were manufactured into 16 mm thick steel plates under the heating-rolling-cooling conditions shown in Table 2. The mechanical properties of these thick steel plates were investigated, and the results are also shown in Table 2.

In Table 1 and Table 2, Comparative Example No. 1 is not subjected to accelerated cooling and warm rolling, and Comparative Example No. 2 is accelerated cooling but not warm rolling, so the tensile strength is TS is low.

Comparative example No. 3 was warm rolled but not accelerated cooled after warm rolling, so N05 had a small rolling reduction of less than 30% in the unrecrystallized γ region, so No. 6 (γ+α
) Since rolling in the two-phase region is carried out, the warm rolling ratio of No. 10 exceeds 10%, so the fracture surface transition temperature vTrs is significantly deteriorated in both cases. Also, comparative example No. 1
In No. 1, since cooling after warm rolling, which is the most important step in the present invention, was performed by air cooling, the toughness was also significantly deteriorated.

In contrast, invention example No. 4. Since Nos. 7 to 9 and Nos. 12 all satisfy the structural conditions of the present invention, it can be seen that they have high strength and sufficient low temperature toughness.

〔Effect of the invention〕

As is clear from the above examples, the present invention is capable of producing high-strength steel with excellent low-temperature toughness after welding by limiting the chemical composition of non-tempered high-strength steel and limiting the rolling-cooling manufacturing conditions. It was possible to stably produce the product without any quality treatment.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the rolling reduction at 630° C. and the tensile strength, yield strength, and fracture surface transition temperature when air cooling or accelerated cooling at 4° C./S is performed after rolling.

Claims (2)

[Claims] (1) C: 0.005-0.20%, Si: 0.05-0.5 in weight ratio
%, Mn: 0.5-2.5%, Al: 0.005-0.
08% and the balance is Fe and unavoidable impurities, rolling the steel at a reduction rate of 30% or more in the temperature range of (Ar_3 transformation point + 150 ° C.) to Ar_3 transformation point;
After the rolling, cooling to 500°C or higher and applying light rolling at a rolling reduction rate of 1 to 10% in a temperature range of 700 to 500°C, and cooling to 300°C at a cooling rate of 1 to 5°C/s after the light rolling. A method for producing non-thermal high tensile strength steel with excellent low temperature toughness, comprising the steps of: (2) C: 0.005-0.20%, Si: 0.05-0.5 in weight ratio
%, Mn: 0.5-2.5%, Al: 0.005-0.
08%, further Nb: 0.01-0.10%, V: 0.01-0.10
%Cu: 0.5% or less, Cr: 0.5% or less, Mo: 0
．． 5% or less, Ni: 0.5% or less, Ti: 0.005~
0.03%, B: 0.0003 to 0.0020%, Ca: 0.001 to 0.010%, and rare earth metals: 0.001 to 0.010%. Steel in which the remainder is Fe and unavoidable impurities is (Ar_3
30 in the temperature range of transformation point +150℃) to Ar_3 transformation point
% or more rolling, and 500% after the rolling.
℃ or higher and applying light reduction at a rolling reduction rate of 1 to 10% in a temperature range of 700 to 500℃, and 300% after the light reduction.
A method for producing a non-temperature high tensile strength steel having excellent low-temperature toughness, comprising the step of cooling to a temperature of 1 to 5 °C/s at a cooling rate of 1 to 5 °C/s.