JPH02209421A - Production of high tensile steel with low yield ratio - Google Patents

Abstract

PURPOSE:To produce a high tensile steel having tensile characteristics as well as low yield ratio by subjecting a steel with a specific composition to heating and rolling at a specific temp., applying quenching to the above without delay to form martensitic structure, and carrying out heating and cooling again under respectively specified conditions. CONSTITUTION:A slab having a composition consisting of, by weight ratio, 0.03-0.20% C, 0.01-1.0% Si, 0.30-2.0% Mn, <=0.010% N, further either or both of 0.005-0.070% Al and 0.01-0.04% Ti, and the balance essentially Fe is heated up to about 1000-1250 deg.C and hot-rolled into a steel plate of prescribed thickness. Subsequently, after rolling is finished at a temp. of the Ar3 point or above, the steel plate is quenched without delay or is cooled after the completion of rolling, heated again up to a temp. of the Ar3 point or above, and then quenched, by which the structure of the steel plate is formed into a martensitic structure. The steel plate whose structure is transformed to martensitic structure is heated again up to a temp. range between Ac3 and (Ac3+50 deg.C) and then cooled at <=100 deg.C/min cooling rate. By this method, the high tensile steel with low yield ratio in which tensile strength and yield ratio are regulated to >=about 60kgf/mm<2> and about 70-85%, respectively, can be obtained.

Description

Detailed Description of the Invention <Industrial Application Field> The present invention has a tensile strength of 60 kgf/mj or more and a yield ratio (yield point or 0.2% yield strength/tensile strength) of 70.
The present invention relates to a method for producing high tensile strength steel with a low yield ratio of ~85%.

<Prior Art> As steel structures become larger in size, there is an increasing demand for materials with higher strength.

Conventionally, high-tensile steel with a tensile strength of 60 kgf/- or more has been manufactured by quenching and tempering, but the characteristics of steel manufactured by this method are high strength and toughness, but a significantly high yield ratio. It is what happens. This high yield ratio is one of the reasons why the use of high-strength steel in structures such as buildings and bridges is restricted from the viewpoint of safety against destruction. Therefore, there has been an increasing demand for steel materials that have a low yield ratio that provides ample time between yielding and fracture.

Under these circumstances, methods for producing high tensile strength steel with low yield ratio have been proposed, for example, in JP-A-53-23817 and JP-A-55974.
This method has been proposed in Publication No. 25 and the like. Although there are some differences in the processing methods, the characteristics of these inventions are that they all involve heating to a two-phase temperature range between the Ac transformation point and the Ac3 transformation point to create a mixed structure of ferrite and martensite or bainite. The purpose of this is to lower the yield ratio. However, with this method, the tensile strength also decreases as the yield point decreases, and in order to obtain the same high strength as that obtained with the conventional quenching and tempering method, it is necessary to increase the amount of C or alloying elements added compared to conventional steel. This has the disadvantage of sacrificing weldability.

<Problems to be Solved by the Invention> As described above, no manufacturing method has been proposed that has material properties equivalent to high-strength steel manufactured by conventional quenching and tempering methods and reduces only the yield ratio.

An object of the present invention is to propose a method for manufacturing high-strength steel having a chemical composition equivalent to that of conventional steel and having a low yield ratio without reducing the level of tensile strength.

<Means for Solving the Problems> As a result of intensive studies by the present inventors on a method for manufacturing steel materials with a low yield ratio and less decrease in tensile strength, the inventors determined that the composition was appropriate according to the strength level, and A step of forming a martensitic structure into a martensitic structure before heat treatment in the next step by cooling or heating and cooling after forming the material into a predetermined size in a rolling process;
It was discovered that by combining the steps of heating again to a temperature range of AC3 to (^c++50°C) and cooling at an appropriate cooling rate, a high tensile strength steel with a low yield ratio and little decrease in tensile strength could be obtained for the first time, and the present invention was made. was configured.

That is, in the present invention, C: 0.03 to 0.03 in terms of weight ratio.
20%, Si: 0.01-1.0%, Mn:
0.30-2.0%.

N: 0.010% or less, and AI: 0.0
05-0.070% and Ti: 0.01-0.04
% or both, and if necessary further Ni: 0.1 to 1.5%, Cu: 0.05 to 0.5%,
Cr: 0.20-0.70%, Mo: 0.2
0 to 0.70%, V: 0.01 to 0.10%, and B: 0.0002 to o, one or two of ooso%
After heating a steel wire containing more than 100% Fe, the remainder being essentially Fe, rolling is finished at a temperature of Ar3 or higher and immediately quenched, or after rolling is finished, the steel is cooled and then heated to Ac, + desired temperature and then quenched. Production of low yield ratio high tensile strength steel characterized by heating to a temperature range of AC2 to (Acs + 50"C) and then cooling at a cooling rate of not less than air cooling and not more than 100°C/min. It's a method.

Function> The reason for limiting the composition of the steel used in the present invention will be described below.

C: 0.03% or more is necessary to ensure strength, but if it is too large, toughness and weldability will deteriorate, so the upper limit should be set at 0.20.
%.

Si: 0.01% or more is required as a deoxidizing agent and reinforcing element, but the upper limit is set to 1.0% in consideration of deterioration in ductility and toughness.

Mn=0.30% or more is required to ensure strength, but 2.
If it exceeds 0%, weldability deteriorates, so the upper limit is set at 2.0%.

N:, /V, forms nitrides with Ti and contributes to grain refinement, but if too large, toughness decreases, so the upper limit should be set at 0.
010%.

AZ+Ti: From grain refinement and N fixation, Al
: 0.005% or more, Ti: 0.01% or more, or both are necessary, but excessive addition deteriorates toughness, so the upper limits are set to 0.070% and 0.04, respectively.
%.

In addition to the above-mentioned components, one or more of the following components can be added depending on the desired strength.Cu: 0.05% or more is added as a reinforcing element, but if added in excess, it may form in the ferrite during tempering. 0.0 to increase precipitation and yield point.
5% or less.

Nj: Added in an amount of 0.1% or more to improve hardenability and toughness, but the upper limit is set to 1.5% from the economic point of view.

Cr, Mo, V: 0 each as strength improving elements
．． 20%, 0.20%, and 0.01% or more are added, but excessive addition increases the yield ratio due to carbide precipitation strengthening, so the upper limit is 0.70%, 0.70%, and 0.10, respectively.
%.

B: 0.0002% or more is added to improve hardenability, but the effect is saturated if it exceeds 0.0050%, so the upper limit was set to o, ooso%.

A steel with a composition system that combines each element within the above range considering the strength and toughness level is made into a rolling slab in a normal process, and then heated to a temperature of preferably 1000 to 1250°C and hot-rolled. A steel plate with a specified thickness is formed by rolling.

Next, the structure before heat treatment in the next step is made into a martensitic structure, but the method to make it into a martensitic structure is to finish rolling at a total temperature of 5 points or more, and immediately quench it, or cool it after rolling and then return it to the Acs point or higher. There is a method of changing the structure of the steel sheet to a martensitic structure by heating it to a temperature of

After forming the martensitic structure, the steel plate was made into a 80mm steel plate.

After reheating to a temperature of ~(＾c!+50℃), air cooling or more 10
Cool at a cooling rate of 0"C/min or less.

Next, the experimental results that formed the basis of the present invention are shown in Figures 1 and 2.
As shown in the figure.

A steel having a tensile strength of 80 kgf/- class shown in Table 1 was heated to 1200°C, then hot-rolled to a thickness of 25 mm, allowed to cool to room temperature, and then heated to C5 point or higher and quenched. , Fig. 1 shows 650-9 with martensitic structure.
Figure 2 shows the relationship between reheating temperature and tensile properties when reheated to 50℃ and cooled at a cooling rate of 10℃/min. This figure shows the relationship between cooling rate and tensile properties when cooling at various cooling rates.

From this, reheat at ^cy ~ (Acs + 50℃) and 1
It can be seen that by combining a cooling rate of 00°C/min or less, there is no decrease in tensile strength and the yield ratio is 85 to 70°C.

<Example> A steel plate having a tensile strength of 60 to 80 kgf/- class shown in Table 2 was melted into a slab for rolling, and then steel plates were manufactured under various rolling conditions and heat treatment conditions, and a reliability test was conducted. . The results are shown in Table 3. The tensile strength of both of them is 60kgf/ij after normal quenching and tempering treatment.

The components have been adjusted to a level of 70 kgf/d and 80 kgf/-. As is clear from Table 2,
The conventional method has a high yield ratio for ropes at each tensile strength level (A
I, Bl, CI, H3, H4 steel), or the yield ratio is lower but the tensile strength is also lower (B3.B4
．． Hl, H2, If steel). In the method of the present invention, the tensile strength level is the same as that of ordinary quenched and tempered materials, and only the yield ratio is reduced (A2°B2.C2, D, E, F, G, H5,
12, JK Steel).

The conventional heat treatments were performed as follows.

A1: Normal reheating quenching and tempering B1: Direct quenching + Ac, point heating quenching and tempering BICR + accelerated cooling + two-phase region heating quenching and tempering B4: Direct quenching + two-phase region heating and tempering C1: Direct quenching and tempering Hl: Direct quenching + two-phase region heating and tempering Tempering B2: Direct quenching + 2-phase region tempering H1 Direct quenching Tempering H1 Direct quenching + Ac, point heating quenching Tempering 11: Direct quenching + 2-phase region heating quenching and tempering <Effects of the invention> According to the present invention, the high A high-strength steel having the same chemical composition and tensile strength as tensile steel and a low yield ratio in tensile properties can be obtained, and the application of high-tensile steel to structures such as buildings and bridges will be expanded.

[Brief explanation of the drawing]

Figure 1 is a graph showing the changes in yield point, tensile strength, and yield ratio due to reheating temperature, and Figure 2 is a graph showing the effect of cooling rate after reheating on yield point, tensile strength, and yield ratio. be.

Claims (1)

[Claims] 1. Weight ratio: C: 0.03 to 0.20%, Si: 0
．． 01-1.0%, Mn: 0.30-2.0%, N: 0
．． 010% or less, further Al: 0.005 to 0.070
% and Ti: 0.01 to 0.04% or both, and the remainder substantially consists of Fe, then Ar
Either finish rolling at a temperature of 3 points or higher and immediately quench it, or after cooling after finishing rolling, heat to a temperature of 3 points or higher and quench to form a martensitic structure, then re-roll the Ac
A method for producing a high tensile strength steel with a low yield ratio, which comprises heating to a temperature range of _3 to (Ac_3+50°C) and then cooling at a cooling rate of not less than air cooling and not more than 100°C/min. 2. Weight ratio: C: 0.03-0.20%, Si: 0
．． 01-1.0%, Mn: 0.30-2.0%, N: 0
．． 010% or less, further Al: 0.005 to 0.070
% and Ti: 0.01 to 0.04% or both, and further Ni: 0.1 to 1.5%, Cu: 0
．． 05-0.5%, Cr: 0.20-0.70%, Mo
:0.20~0.70%, V:0.01~0.10% and B:0.0002~0.0050%, one or two of them
After heating steel containing more than 100% Fe, the remainder being essentially Fe, rolling is finished at a temperature of Ar_3 or higher and immediately quenched, or after rolling is finished, the steel is heated to a temperature of Ac_3 or higher and then rapidly cooled. A method for producing a high tensile strength steel with a low yield ratio, which comprises forming a martensitic structure, heating it again to a temperature range of Ac_3 to (Ac_3+50°C), and then cooling at a cooling rate of not less than air cooling and not more than 100°C/min.