KR940008057B1 - Making method of high tension steel - Google Patents

Abstract

The low alloy steel includes (in wt.%) 0.12-0.15 C, up to 0.5 % Si, 1.2-1.5 % Mn, up to 0.02 % P, up to 0.01 % S, 0.01-0.03 % Nb, balance Fe and inevitable impurities. The steel is reheated at 1150-1250 deg.C; hot-rolled at temp. range of Ar1-Ar3; and rapid-cooled to 500-550 deg.C at 5-10 deg.C/sec. cooling rate. The steel is manufactured without heat treating process and has above 60 kg/cm2 tensile strength.

Description

Manufacturing method of high strength steel

1 is a tissue control process diagram schematically showing a tissue control process in manufacturing high tensile steel according to a conventional method.

2 is a tissue control process diagram schematically showing a tissue control process in manufacturing high tensile steel according to the present invention.

3 is a microstructure photograph of a hot rolled steel sheet hot rolled under the conditions of finish rolling temperature above Ar ₃ temperature or below Ar ₁ temperature according to the present invention.

4 is a micromanipulation photograph of the invention material prepared in accordance with the present invention and the comparative material produced under conditions outside the scope of the present invention.

The present invention relates to a method for manufacturing high tensile steel, and more particularly, to a method for manufacturing high tensile strength steel having a resistance ratio used in building structures and the like.

High-strength-resistant high-strength steel is generally used in building structures to increase the stability against earthquakes in high-rise buildings. When an external force exceeding the yield strength of steel is applied by an earthquake, the larger the plastic deformation capacity after yielding, the safer the structure is.The yield ratio (the yield strength divided by the tensile strength) is used as a measure for evaluating the plastic deformation capacity of these steels. The lower the yield ratio, the greater the plastic deformation capacity.

Generally, high tensile steels with tensile strength over 60kg / mm2 are usually produced by quenching and tempering after rolling, but this method is suitable for producing high-strength-resistant high-strength steels because the steel structure is single-phase and the yield ratio is high. Not.

In order to reduce the yield ratio of the steel, the steel structure should be made into a composite structure as long as the soft phase and the hard phase are mixed, not the single phase tissue. In this regard, the operation control methods proposed so far are shown in FIG. In FIG. 1 (a), a method for manufacturing an abnormal composite tissue steel by conventional rolling-reheating quenching-lamellarizing-tempering is presented. Due to the complexity, the manufacturing time is required for a long time and the manufacturing cost is high, so it is not applied to actual production.

In FIG. 1 (b), a method of partially cooling the soft ferrite by quenching and direct quenching (direct lamellarzing) and tempering is shown. In FIG. 1 (c), In order to omit the reheating and quenching process, a conventional method of direct quenching, reverse quenching, and heat treatment after rolling is proposed, and in (d) of FIG. 1, in order to omit reheat quenching and abnormal reheating. A method of direct quenching and consideration after controlled rolling is presented.

The methods shown in (b), (c) and (d) of FIG. 1 are different from the methods shown in (a) of FIG. 1, and the tissue control method is applied after the accelerated cooling process is applied to the thick plate process. There is an advantage to reduce the number of processes. However, the methods shown in (b), (c) and (d) of FIG. 1 can also reduce the number of heat treatment steps, but it can be said to be an uneconomical method since heat treatment cannot be omitted completely.

An object of the present invention is to provide an improved method for manufacturing a 60kg / mm2 high tensile steel for construction having a resistive ratio without proper heat treatment by appropriately controlling the composition of steel, hot rolling and accelerated cooling conditions.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the method of manufacturing high tensile steel, the present invention is, in weight%, C: 0.12-0.15%, Si: 0.5% or less, Mn: 1.2-1.5%, P: 0.02% or less, S: 0.01% or less, Nb: Low-alloyed steel, consisting of 0.01-0.03%, balance Fe and other inevitable impurities, is reheated to 1150-1250 ° C and hot rolled to a finish rolling temperature of less than Ar ₃ or below Ar ₁ and then to 500-550 ° C. The present invention relates to a method for manufacturing a high-strength steel for wear-resistant construction by accelerated cooling to a water cooling stop temperature.

Hereinafter, the reason for the resin limitation will be described.

If the content of C is 0.12% or less, the fraction of the second phase tissue is lowered, so that it is difficult to satisfy the required strength of the standard. If the content is 0.15% or more, the second phase tissue is easily transformed into pearlite. Since the reinforcing effect of the second phase tissue is reduced, the C content is preferably limited to 0.12-0.15%.

The Si is a component added as a deoxidizer during steelmaking, but has a solid solution effect, but when the content is 0.5% or more, the weldability is lowered and an oxide film is formed on the surface of the steel sheet so that the Si content is limited to 0.5% or less. desirable.

When the Mn content is 1.2% or less, the hardenability of the steel is lowered, so that the undeformed austenite is not stabilized at low temperature during accelerated cooling, so it is difficult to transform the second phase structure into bainite structure, and the content is 1.5% or more. Since the weldability is lowered, the content of Mn is preferably limited to 1.2-1.5%.

The lower the content of P is, the better, but is inevitable in the steelmaking process, so the content is preferably limited to 0.02% or less so as to have a deleterious effect on the properties of the steel.

S is present as a non-metallic inclusion of MnS and is elongated by low temperature rolling to promote anisotropy of the steel, so the content thereof is preferably limited to 0.01% or less.

The Nb is a component that inhibits the refining of the austenitic coarse austenite during hot rolling and promotes the hybridization of the austenite grains after rolling, and should be added at least 0.01% in order to obtain such an addition effect. When the addition of 0.03% or more deteriorates the high heat input weldability, it is preferable to limit the content of Nb to 0.01-0.03%.

Hereinafter, the structure control process of this invention is demonstrated.

In the present invention, as shown in FIG. 2, the low-alloy steel formed as described above is heated to a temperature of 1150-1250 ° C. and hot-rolled to a finish rolling temperature condition of Ar ₃ or less and Ar ₁ or more. Accelerated cooling treatment to a water cooling stop temperature of ℃, to coarsen the initial austenite grains up to 1150-1250 ℃ as described above.

The reason why the steel heated to this temperature is hot rolled under the finish rolling temperature condition below the Ar ₃ temperature or higher than the Ar ₁ temperature is to adjust the mixed structure of the partially fine ferrite and the coarse amorphous austenite as shown in FIG. It is for. In FIG. 3, F denotes a cornerstone ferrite structure and A denotes unaffected austenite.

In the hot rolling of the present invention, the reduction ratio is appropriately determined according to the initial slab thickness and the final thick steel sheet thickness, and the number of rolling passes is appropriately determined according to the initial slab thickness, the final thick steel sheet thickness and the rolling mill capacity.

In the present invention, in order to satisfy the standard required yield strength (44-47kg / ㎜ according to the thickness) of 60kg / ㎜ high-tension steel grade, hot-rolled in the abnormal area, and then accelerated cooling treatment, this accelerated cooling treatment is unmodified austenite It is cooled to a water cooling stop temperature of 500-550 ℃ at a cooling rate of more than air cooling, preferably 5-10 ℃ / sec so that the transformation to the upper bainite.

If the water cooling stop temperature is less than 500 ℃ water cooling phenomenon is difficult to control the target water cooling stop temperature to enter the transition zone from the nuclear boiling to the nuclear boiling, the structure of fine martensite-Austenite Constituent (Martensite-Austenite Constituent) tissue is formed In the test, the yielding point (Contineous Yielding) is lost, the yield strength is sharply reduced, so the yield strength of the specification is often not satisfied.In the case of the water cooling stop temperature of 550 ℃ or higher, most of the transformational austenite is Since the tensile strength often does not satisfy the specification requirements (58-70 kg / mm 2) due to transformation into light, it is preferable to set the water cooling stop temperature to 500-550 ° C.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The high-alloy steel was prepared by hot rolling and accelerated cooling treatment under the conditions as shown in Table 2, and the high-alloy steels prepared as shown in Table 1 were then measured for their mechanical properties, and the measurement results are shown in Table 2 below.

TABLE 1

TABLE 2

As shown in Table 2, the comparative material (1) containing less carbon (C) than the scope of the present invention and the comparative material (2) containing less Mn than the range of the present invention is subjected to hot rolling and accelerated cooling conditions. Even with the strict control, it is difficult to meet the standard required oral strength (yield strength: 47kg / m2, tensile strength: 58-70kg / m2) of high yield strength steel of 60kg / ㎡.

In addition, it can be seen that the comparative material (3) which does not contain Nb has a tensile strength of only 50 kg / m2 class, and the second phase structure of the steel is also used for the comparative material (4) whose water cooling stop temperature is too high at 600 ° C. It can be seen that the tensile strength is lowered to 56.4kg / ㎡ by transforming into pearlite.

In addition, the comparative material (5) having a water cooling stop temperature of 470 ° C. is so low that fine phase martensite is formed in the second phase structure, so that a continuous yield phenomenon occurs, the yield strength drops sharply to 39.9 kg / m 2, and the rolling finish temperature is It can be seen that the comparative materials (6) and (7) above the Ar ₃ temperature do not work harden the ferrite, resulting in low tensile strength and yield strength.

In contrast, the inventive material (ac) produced under the composition of the steel according to the present invention, hot rolling and accelerated cooling conditions has a yield strength of 47.5-48.2kg / ㎡, tensile strength of 61.5-64.5kg / ㎡ and 74.5-78.4% It has a resistivity ratio of, and it can be seen that it satisfies the standard requirement of high tensile strength steel of 60kg / ㎡.

As shown in FIG. 4 showing the microstructure photographs of the inventive material (a) and the comparative material (4), in the case of the inventive material (a) [FIG. 4 (a)], the soft ferrite structure (F) and the hard While the upper bainite structure (UB) of the phases is mixed, the strength is excellent, whereas in the case of the comparative material 4 (Fig. 4 (b)), most of the second phase structure is made of pearlite (P). It can be seen that away.

As described above, the present invention has the effect that it is possible to economically manufacture 60kg / ㎡ class high tensile strength steel having a resistance ratio by appropriately controlling the components of the steel, hot rolling and accelerated cooling conditions without undergoing a heat treatment process.

Claims (1)

In the method of manufacturing high tensile steel, in weight%, C: 012-0.15%, Si: 0.5% or less, Mn: 1.2-1.5%, P: 0.02% or less, S: 0.01% or less, Nb: 0.01-0.03% Re-heat the low alloy steel, which is composed of the balance Fe and other unavoidable impurities, to 1150-1250 ° C and hot roll to a finish rolling temperature of less than or equal to the Ar ₃ temperature and more than the Ar ₁ temperature, and then to the water cooling stop temperature of 500-550 ° C. A method for producing a high-strength steel for wear-resistant construction, characterized in that the accelerated cooling treatment at a cooling rate of 5-10 ℃ / sec.