KR910003877B1 - Making process for high-tension steel - Google Patents

Abstract

A high tension temper steel consists of 0.1-0.15 wt.% C, up to 0.5 wt.% Si, 1.0-2.0 wt.% Mn, up to 0.02 wt.% P, up to 0.02 wt.% S, 0.015-0.025 wt.% Ti, 0.02-0.04 wt.% sol. Al, 5-20 ppm B, 50-100 ppm N, one or two components selected from up to 0.5 wt.% Ni, up to 0.15 wt.% Mo and up to 0.1 wt.% V and the balance Fe, and has 0.34-0.38 carbon equivalent. The steel is hot-rolled, air- cooled, quenched after reheated to over AC3 point, and tempered at below AC1 point. The steel has excellent impact toughness and 60 kg/mm2 class tensile strength.

Description

Manufacturing method of 60kg / mm2 grade high tensile strength steel with excellent impact toughness

1 is a graph showing the shock absorption energy according to the test temperature of the inventive steel and the comparative steel.

2 is a tissue photograph showing the austenite grain size of the inventive steel.

The present invention relates to a method for producing a tensile strength 60kg / mm grade ² high tensile strength steel excellent in impact toughness.

In general, boron (B) increases the hardenability of the steel even with a small amount of addition, so that the amount of alloying element can be reduced by the increase of the hardenability to the tempered high tensile strength steel produced by hardening and annealing, and the manufacturing cost is reduced so that the tempered high tensile strength steel It is common to add boron. However, boron is easily generated as BN due to its high affinity with nitrogen (N), in which case it is difficult to secure a common boron effective in improving the quenching property.

In addition, the more boron is added, the higher the hardenability, but the appropriate amount of boron exists to increase the hardenability, and 10-15 ppm is known as an appropriate amount.

Therefore, when trying to improve the hardenability of the steel by the addition of boron is the key to how to secure the appropriate amount of boron.

Currently, it is a method that has been put to practical use in order to secure an appropriate amount of boron. Firstly, the amount of solid solution nitrogen added to steel is as low as possible. The method is to double the operation level of 0.03%, and the third method is to add elements such as titanium (Ti) and zirconium (Zr) that are more likely to be formed of nitride than boron. The first method of the above method has a disadvantage that the vacuum degassing treatment for a long time, the second method has a disadvantage that the slab quenching treatment, etc., the third method is not particularly necessary compared to the previous two methods, but in this case The addition of nitrogen should be regulated below normal operating levels.

As such, the reason why the amount of nitrogen should be lowered even when adding elements such as titanium is generally that when a strong nitride forming element is added to fix nitrogen, it is necessary to add a little more than the stoichiometric ratio between the element and nitrogen. Will definitely appear. However, when titanium is added in accordance with the amount of nitrogen of about 50-90 ppm, which is an operation level in a general steelmaking plant, the solubility of TiN is very small, and thus the TiN is coarsened to significantly reduce toughness.

The inventors of the present invention reduce the cost of reducing nitrogen and the addition of alloying elements in boron-added steels for the purpose of reducing the manufacturing cost of general structural tempered high tensile strength steels that do not require much weldability such as crack-free steels and high heat input welding steels. Numerous experiments have been carried out to achieve the reduction of nitrogen, and the amount of nitrogen is in the range of 50-100 ppm, which is the normal operating level, and the amount of titanium is 60 kg, which is excellent in toughness due to the proper heat treatment of steel with a little more or less than stoichiometric ratio. It is to provide a manufacturing method of / mm ² grade tempered high tensile strength steel.

The present invention is a method for producing a tensile strength 60kg / mm grade ² high tensile strength steel excellent in impact toughness, by weight, C: 0.1-0.15%, Si: 0.5% or less, Mn: 1.0-2.0%, P, S : 0.02% or less, Ti: 0.015-0.025%, Sol.Al: 0.02-0.04%, B: 5-20ppm, N: 50-100ppm, Ni: 0.5 or less, Mo: 0.15% or less, V: 0.1% or less One or two kinds are added, and the steel, which is composed of residual Fe and has a carbon equivalent (Ceq) of 0.34-0.38, is hot-rolled and air-cooled in a conventional manner, and then reheated to Ac ₃ or more, and then quenched, and is not more than ₁ Ac The present invention relates to a method for manufacturing 60kg / mm ² grade tempered high tensile strength steel for unidirectional structural having excellent impact toughness.

The reason for limiting the component ratio and type of each additional element in the present invention is as follows. The carbon (C) is the element having the most effect on the increase in strength, but as it is added, the toughness and weldability are lowered, so that it is limited to 0.1% or more to secure the minimum strength required, and 0.15% or less to satisfy the specification.

The silicon (Si) is a deoxidation component of steel, which is 0.5% or less, which is a general operation level of the steelmaking process.

The manganese (Mn) is an element effective for increasing hardenability, improving low temperature toughness, etc., and is less effective at 1.0% or less, and the strength may be excessively increased at 2.0% or higher, and also increases the carbon equivalent to damage weldability by 1.0-2.0%. Limit to the range of.

Phosphorus (P) and sulfur (S) are both impurity elements contained in steel inevitably, and the smaller the amount added, the better, but the general level is limited to 0.02% or less.

The nickel (Ni) is an element that is effective in improving the hardenability and low temperature toughness of the steel, but is extremely expensive and is limited to 0.5% or less.

The molybdenum (Mo) is the strongest synergy with the boron element is added to 0.15% or more there is a risk of excess strength and expensive because it is limited to 0.15% or less.

The vanadium (V) is limited to 0.1% or less because it has a bad effect on the toughness and weldability when the element or a large amount of the effect of improving the strength is excessively added.

The titanium (Ti) is limited to the range of 0.015-0.025% close to the stoichiometric ratio with nitrogen as an element to ensure the effect of improving the hardenability of boron through the formation of BN.

Solid Al is an essential element for deoxidation in steel and is limited to the general operating level of 0.02-0.04%.

The nitrogen (N) is limited to 50-100 ppm, which is a typical operating level in the steelmaking process.

The boron (B) is limited to 5-20 ppm, which is known as an optimal amount as an element that greatly improves the hardenability even with a small amount of addition.

Hereinafter, it demonstrates in detail by an Example.

EXAMPLE

The steels having the chemical composition shown in Table 1 were heated to a slab (SLAB) heating temperature of 1200 ° C., and subjected to air-cooling by normal rolling having a reduction ratio of 20-30% per PASS.

At this time, the rolling end temperature was 900-950 ℃.

The hot rolled steel sheet was re-heated to 900-950 ° C., water cooled, and again heated to 600,630 ° C. and air-cooled, and their mechanical properties are shown in Table 2 below.

TABLE 1

* Ceq (%) = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14

TABLE 2

As shown in Table 2, the present invention (1-12) has a tensile strength of 60-72kg / mm2, which satisfies the specification range, the impact transition temperature is lower than that of the comparative steel, and also known low nitrogen- It can be seen that the impact toughness is also better than that of titanium-containing steel.

Inventive materials 9, 5 and Comparative Material 13 of Table 2 is shown in Figure 1 the shock absorption energy according to the experimental temperature.

In addition, Figure 2 shows the austenite grains of the invention material 1 in Table 2, the size of the grain is ASTM # 9.2 in the case of general rough steel is much finer than the vicinity of ASTM # 8.0, and thus the fine grains Impact toughness is improved.

As described above, the present invention can produce 60kg / mm grade ² high strength steel with excellent impact toughness economically by appropriately heat-treating steel with nitrogen content at a normal operating level and titanium added more or less than stoichiometric ratio. It can be effective.

Claims (1)

In the method for producing a tensile strength 60kg / mm grade ² high tensile strength steel with excellent impact toughness, in weight%, C: 0.1-0.15%, Si: 0.5% or less, Mn: 1.0-2.0%, P, S: 0.02% Or less: Ti: 0.015-0.025%, Sol.Al: 0.02-0.04%, B: 5-20ppm, N: 50-100ppm, Ni: 0.5 or less, Mo: 0.15% or less, V: 0.1% or less After adding two kinds, the steel which is composed of residual Fe and has a carbon equivalent of 0.34-0.38 is hot rolled and air-cooled by a conventional method, and then reheated before Ac ₃ or more, and then quenched and considered below Ac ₁ point. 60kg / mm grade ² high tensile strength steel with excellent impact toughness.

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