JPH0445224A - Production of steel material reduced in yield ratio - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method of manufacturing a steel material with a low yield ratio.

(Conventional technology) In recent years, in order to improve competitiveness in various fields such as shipbuilding and industrial machinery, various efforts have been made to reduce welding work, pursue the ultimate properties of steel materials such as bending workability, improve weldability, and reduce steel material costs. Demand is increasing. Among these, in order to improve the bending workability of thick steel plates, it is necessary to develop thick steel plates with a low yield ratio of 80% or less.

Furthermore, in the fields of architecture and bridges, a reduction in yield ratio is desired to improve the safety of structures. In addition to simply lowering the yield ratio, introducing a yield shelf significantly increases the amount of energy absorbed by the structural member during an earthquake.

For this reason, the introduction of a yield shelf is also required.

Recently, there has been active development of steel plates with a strength of 50) cg f/- or more using accelerated cooling technology after steel plate rolling, with the aim of improving base material low-temperature toughness and weldability, mainly for use in shipbuilding, line pipes, etc. , the production of steel materials with good bending workability and low yield ratios has not been studied.

In the conventional controlled rolling-controlled cooling process, in order to improve low-temperature toughness, hot rolling is performed to make the grains as fine as possible, and accelerated cooling is performed from the austenite phase region.

However, according to this method, there is a problem that the yield point increases due to grain refinement and hardening of the ferrite and the conversion of some barite to bainite, resulting in an increase in the yield ratio and a decrease in bending workability.

Some of the inventors of the present invention have published Japanese Patent Application Laid-Open No. 59-211528, Japanese Patent Application No. 62-52856, and Japanese Patent Application No. 62-5542.
In No. 8, we proposed a method for producing low yield ratio non-tempered steel. As a result of studying a method of lowering the yield point using a controlled rolling-controlled cooling process, we developed a method for producing steel with a low yield point and low yield ratio while obtaining good low-temperature toughness with fine-grained ferrite. It depends.

That is, after heating to 900 to 1200°C, Ar
After applying a cumulative reduction of 30% or more at a temperature of s or more to refine the grains, air cooling to Ar3 or less to properly precipitate soft pro-eutectoid ferrite, followed by forced cooling, the soft pro-eutectoid ferrite and remaining austenite are separated. By appropriate mixing of the resulting ferrite-pearlite bainite,
It was discovered that only the yield point decreased without a decrease in tensile strength or low-temperature toughness.

(Problem to be solved by the invention) However, after that, the demand for a lower yield ratio became even stronger.
The content proposed earlier was insufficient to meet the strict requirements.

In addition to simply lowering the yield ratio, introducing a yield shelf significantly increases the amount of energy absorbed by the structural member during an earthquake. For this reason, there is a need to introduce a yield shelf, but there is no knowledge of how to achieve this.

(Means for solving the problem) For this reason, in order to continue to lower the yield ratio, we conducted numerous experiments and detailed studies, and found that in order to lower the yield ratio, the microstructure of steel should be changed to ferrite and secondary. Create a two-phase mixed structure of carbide phases. In order to further reduce the yield ratio,
It is important to lower the yield point and increase the tensile strength.

In order to lower the yield point, the area ratio of ferrite should be increased and the grains should not be made too fine, and in order to increase the tensile strength, the second phase carbide (bainite or martensite) that has become hard due to rapid cooling should be tempered. They discovered that it is important not to soften the material more than necessary.

Furthermore, regarding the yield shelf, it was found that by applying a light reduction between the two-phase region temperature and water cooling, active dislocations are introduced into the steel, which results in the presence of a yield shelf during the tensile test.

Based on this knowledge, the present invention has made it possible to manufacture steel materials with a low yield ratio, and its gist is that in weight %, c: o, oa ~ 0.30%, S 1:
0.05-0.60%, Mn: 0.50-2.5%,
Ag: Based on steel containing 0.005 to 0.1%,
In addition, Cu: 2.0% or less, Ni: 4.0% or less, Cr: 1.0% or less, Mo: 0.50% or less, Nb:
0.10% or less, V: 0.10% or less, T1: 0.
IN or two or more selected from the strength improving element group consisting of 15% or less, B: 0.005% or less, or Ca: 0
．． 01% or less, or one or more selected from the strength-improving element group of Cu=H and Ca, with the remainder being Fe and unavoidable impurities.
Heated to 1250℃, hot rolled, then rapidly cooled to 250℃ or less, then reheated to Ac +20℃ to Ac1 + 80℃, applied a light reduction of 1% to 30%, successively water-cooled, and then cooled to 200 to 600℃. The present invention relates to a method for manufacturing a steel material with a low yield ratio, which is characterized by tempering in a temperature range.

(Function) In the present invention, the heating temperature is raised and then the Ac
- The ferrite area ratio is significantly increased by heating to a lower temperature between the -Ac3 transformation points and water cooling from there.

Furthermore, by lowering the tempering temperature, the second phase portion is not softened more than necessary, which has a synergistic effect, making it possible to manufacture steel materials with a low yield ratio.

Next, the reason for limiting the ingredients in the present invention will be described.

C is required at 0.03% or more to ensure strength, but
If the content is too large, it will impair the toughness and weldability of the steel, so the content should be limited to 0.30%.

St is added as necessary in an amount of 0.05% or more for deoxidation, but if it increases, weldability will be impaired, so the content should be 0.6% or less.

Ml is useful as an element that increases strength at a low cost, and 0.5% or more is necessary to ensure strength, but if it increases, weldability will be impaired, so the content should be 2.5% or less.

An) is required to be 0.005% or more for deoxidation, but if it is too large, inclusions in the steel will increase too much and the properties of the steel will deteriorate, so the upper limit is set at 0.1%.

In the present invention, steel containing the above-mentioned elements as basic components is heated, rolled, and heat treated as specified in the present invention to ensure a steel material with a low yield ratio. However, depending on the required characteristics of the steel, the following strength-improving elements One or more types of inclusion spheroidizing elements may be added.

V, Tj, and Nb are elements that increase strength through precipitation strengthening, but in large amounts they impair toughness, so V
: 0.10%, Ti:o, 15%, Nb:0.10
The upper limit was %.

Ni, Cu, Cr, and Mo are elements that increase strength through solid solution strengthening, but in large amounts they impair toughness, so Ni: 4.0%, Cu: 2.0%, Cr: 1.0
%, Mo: 0.50% was the upper limit.

B is an element that increases strength by improving hardenability, but if the amount increases, it deteriorates toughness, so 0.005
The upper limit was %.

Ca is added because it is useful for spheroidizing inclusions, but if too much Ca forms inclusions in the steel and deteriorates the properties of the steel, the upper limit of the content is 0.01%.

Next, heating, rolling, and cooling conditions, which are important requirements of the present invention, will be described.

The lower limit of the heating temperature is 1050° C. so as not to make the austenite grains finer than necessary during heating.

On the other hand, if the temperature is too high, there will be no effect on the quality of the material, and on the contrary, it will be disadvantageous in terms of energy saving, so the upper limit is set at 1250°C.

As for rolling, normal hot rolling is performed.

Next, the reason why the cooling stop temperature for accelerated cooling after rolling was set to 250°C or less is because if cooling is stopped in a high temperature range exceeding 250°C and then tempering heat treatment is performed, the strength will decrease slightly and the low-temperature toughness will decrease at the same time. .

In the accelerated cooling, the water density is preferably 0.3rd/d·win or more so that the steel plate is uniformly cooled.

Next, set the reheating temperature to Ac + 20℃ or higher Ac1 + 80℃
The reason for setting the temperature below is that heating to this temperature range significantly improves the ferrite area ratio. That is, directly above Ac1, the transformation does not proceed sufficiently and the hardening of the second phase carbide portion is insufficient, whereas A c 1+
When the temperature is 20° C. or higher, the transformation will proceed sufficiently and the second phase portion will also be sufficiently hardened.

The ferrite area ratio decreases as the heating temperature becomes higher than this A c 1+ 20°C. And A C1+
If the temperature falls below 80° C., the ferrite area ratio to obtain the low yield ratio targeted by the present invention cannot be obtained, so this is set as the upper limit.

In this way, the reheating temperature is changed from Ac +20℃ to Ac 1
The reason why it is limited to the lower temperature side than the middle of the temperature range of +80℃ and A c - A c s is that due to heating on the side closer to ACl, the ferrite part becomes larger in the area ratio of ferrite to austenite during heating. By freezing this state by rapid cooling as specified below, we aim to increase the ferrite area ratio and achieve a low yield ratio.

Ac +20°C to Ac t +80°C Water cooling after reheating will sufficiently harden the part where C, which has become austenitized during reheating, becomes a quenched structure, increasing the tensile strength and obtaining a low yield ratio. It's for a reason. The water cooling conditions may be water cooling by immersion or roller quenching, which makes it easy to obtain a rapidly quenched and quenched structure.

Prior to this cooling, a light pressure reduction of 1% to 30% is applied. Reheating temperature A C1+ 20°C ~ A Cl
By applying a light reduction in the two-phase region at +80° C., active dislocations are introduced into the steel, which can lead to the presence of a yield shelf during tensile testing. If the reduction rate in the two-phase region is less than 1%, there is no yield shelf, so the lower limit is set to 1%, and if it exceeds 30%, the yield ratio increases, which is contrary to the purpose of the present invention, so the upper limit was set to 30%. .

Furthermore, regarding the tempering temperature, for the two-phase mixed structure of ferrite and second phase carbide, if the second phase part, which has been sufficiently hardened by previous water cooling, is tempered at too high a temperature, it will become too soft, resulting in a decrease in tensile strength and eventually yielding. In order to increase the ratio,
The upper limit is 600°C.

However, if the tempering temperature is too low to be less than 200°C, the tempering effect will be almost gone and the toughness will decrease, so the lower limit of the tempering temperature is set at 200°C.

The treatment of direct quenching and tempering has slightly lower toughness than the treatment of hot rolling, reheating, and quenching and tempering, but higher strength can be obtained.

(Example) Table 1 shows the chemical composition of the test materials, and Table 2 shows the heating, rolling, and cooling conditions and the mechanical properties of the obtained steel sheets.

Steel A, G, H, I, J, K, L, M, N
, O.

P is 50kg f/+oJ class, steel B, C, D, E,
F.Q.

L60) cgf/- class for R, S, T, U, ■ is 80 kg
The composition system aims for f/m4 class strength, and as shown in Table 2, steel plate N (LAI, AI0.Bl, CI, DI,
El.

Fl, Gl, Hl, II, Jl, Kl, L’l.

Ml, Nl, 01. Pi, Ql, R1, Sl.

TI, Ul, and Vl are examples of the present invention, and each is 50
kg f /at l110 kg f /d, 80)
It has sufficient strength and good low-temperature toughness as a cg f /d class steel, and has a low yield ratio (yield ratio of 70%), which is the aim of the present invention.
(below) and has a yield shelf.

On the other hand, steel plates No. and A2 have a high yield ratio because the heating temperature is too low. In A3, the cooling stop temperature is too high, so the low-temperature toughness is reduced. The reheating temperature of A4 is too low, and the reheating temperature of A5 is too high, resulting in a high yield ratio. A6 has a high yield ratio because the tempering temperature is too high.

Since A7 was not tempered, its low-temperature toughness was reduced. A8 does not have a yield shelf because there is no light reduction after reheating. A9 has a high yield ratio of 35% after reheating. B2 has a high reheating temperature, and B3 has a high yield ratio because the tempering temperature is too high.

(Effects of the Invention) The present invention provides a yield shelf with high strength of 50 kgf/- or more and good bending workability without using particularly expensive alloying elements or without reheating after rolling. This invention enables the production of steel materials with a low yield ratio at low cost, and is a highly effective invention industrially.

Claims (1)

[Claims] 1. In weight%, C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0. Steel consisting of 0.005~0.1% balance Fe and unavoidable impurities is heated to 1050
Heating to ~1250℃, hot rolling, then rapidly cooling to 250℃ or less, then reheating to Ac_1+20℃~Ac_1+80℃, applying a light reduction of 1% to 30%, followed by water cooling, and then 200 to 600℃. A method for producing a steel material with a low yield ratio, characterized by tempering in a temperature range. 2. In weight%, Cu: 2.0% or less, Ni: 4.0% or less, Cr: 1.0% or less, Mo: 0.50% or less, Nb: 0.10% or less, V: 0 2. The method for producing a steel material with a low yield ratio according to claim 1, which contains one or more of the strength improving element group consisting of .10% or less, Ti: 0.15% or less, and B: 0.005% or less. 3. The method for producing a steel material with a low yield ratio according to claim 1, which contains Ca: 0.01% or less in weight%. 4. In weight%, Cu: 2.0% or less, Ni: 4.0% or less, Cr: 1.0% or less, Mo: 0.50% or less, Nb: 0.10% or less, V: 0 .10% or less, Ti: 0.15% or less, B: 0.005% or less, and Ca: 0.01% or less. A method of manufacturing steel materials with a low ratio.