KR900001892B1 - Making process for steel plate of low alloy - Google Patents

Abstract

An untempered low alloy high tension steel plate has the composition of: 0.04-0.20 wt.% C, below 0.50% Si, 0.05-2.00 wt.% Mn, 0.01-0.10 wt.% Al and below 0.03 wt.% Ti, and the carbon equivalent of 0.30- 0.33. The steel plate is characterized by being produced from controlled cooling with the cooling rate of 10-15oC/sec from 780-820oC to 480-520oC after pressing. The process can avoid the reduction of productivity.

Description

Manufacturing method of non-alloy low alloy high tensile steel sheet

1 is a graph showing the relationship between the strength and toughness according to the cooling rate changes.

2 is a microscopic histology of cooling conditions.

3 is a graph showing the change in strength and toughness according to the carbon equivalent.

The present invention relates to a method for producing a non-coarse low alloy high tensile strength steel sheet, in particular, to a method for producing a non-coarse low alloy high tensile strength steel sheet excellent in high strength and low temperature impact resistance of 50kg / mm ² or more tensile strength.

Non-coarse low alloy high strength thick steel sheets are widely used for shipbuilding and cold chilled marine structures. Recently, the use environment of shipbuilding and offshore structures is becoming more severe and lighter. The demand for them is increasing. Conventional manufacturing methods for producing steel for shipbuilding and offshore structures have been made by adding alloying elements such as V, Cu, Ni, and Cr, and performing heat treatment such as normalizing.

However, this method has a problem in that the strength is increased due to the increase in carbon equivalent by the addition of these alloying elements, but the toughness and weldability are deteriorated, and the manufacturing cost is expensive because the heat treatment process is added in the manufacturing process. On the other hand, the addition of alloying elements such as Nb, V, etc., and controlled rolling can achieve grain refinement and improve strength, toughness and weldability, but this method reduces the productivity (T / H) due to the rolling process control. There is an unavoidable problem.

Therefore, the production cost is the development of shipbuilding and marine structural steel having a low-cost way in a tensile strength of 50kg / mm ² or more and cold high toughness (impact absorption energy and transition temperature) that can be used in support of -60 ℃ has been required.

Accordingly, the present invention has been made in view of the above-described circumstances to provide a method for manufacturing a non-coarse low alloy high tensile strength steel sheet having excellent tensile strength of 50kg / mm ² or more, high strength and low temperature impact toughness, and low manufacturing cost.

Hereinafter, the present invention will be described in detail.

In the present invention, in manufacturing a steel sheet after an amorphous low alloy high tensile strength, C: 0.05-0.20 wt /% (hereinafter referred to as%), Si: 0.5% or less, Mn: 0.05-2.0%, Al: 0.01-0.10% , Ti: 0.03% or less, while the carbon equivalent (Ceq) is in the range of 0.30-0.33, after rolling, the steel is controlled at a cooling rate of 10-15 ℃ / sec from 780-820 ℃ to 480-520 ℃ It relates to a method for producing a non-coarse low alloy high tensile strength steel sheet characterized in that the cooling. In the above, when C is less than 0.05%, it is insufficient to obtain a predetermined strength, while at 0.20% or more, the impact toughness and weldability are impaired, so the range is set to 0.05-0.20%. Si is a major deoxidizer of steel and is a solid solution strengthening element, but containing 0.5% or more causes deterioration of toughness and weldability, so the upper limit is 0.5%.

Mn is an element that increases the strength of the steel, such as C, requires 0.05% or more in order to obtain a predetermined strength. However, when it contains 2.0% or more, the carbon equivalent is greatly increased to reduce toughness and weldability, so the Mn content is set to 0.05-2.0% in the present invention.

Al is an element necessary for deoxidation and has a grain refining effect, but the effect is insufficient at 0.01% or less, and if it exceeds 0.10%, the inclusion is not only increased, but also the weldability is limited to 0.01-1.0%.

Ti is an essential additive element in the present invention, combined with N in the steel at high temperature, precipitated in the form of TiN, the growth inhibition and uniformity of austenite grains during slab heating, as well as the controlled cooling process after rolling Precipitates precipitated fine in the role of improving the hardness (Hardness) of the matrix (Matrix) by interaction with the dislocation (Dislocation). However, since 0.03% or more impairs weldability, 0.03% was made into an upper limit.

〉탄소당량이 높을수록 강도는 증가하지만 반대로 인성 및 용접성이 저하하게 된다. 탄소당량이 0.30이하에서는 소정의 강도와 인성이 불충분하며 한편 0.33 이상인 경우 강도는 우수한지만 인성이 저하하므로 탄소당량범위는 강도뿐만 아니라 인성 및 용접성이 동시에 우수한 특성을 갖는 0.30-0.33으로 한정하였다.In general, the carbon equivalent (Ceq) can be expressed as the following equation: <Carbon equivalent (Ceq.) = C +

> The higher the carbon equivalent, the higher the strength, but the lower the toughness and weldability. If the carbon equivalent is 0.30 or less, the predetermined strength and toughness are insufficient, while if the carbon equivalent is 0.33 or more, the strength is good but the toughness is lowered, so the carbon equivalent range is limited to 0.30-0.33, which has not only strength but also toughness and weldability.

On the other hand, the control cooling technology is already known as a technology that can improve the strength without heat treatment by cooling the hot rolled plate in a specific temperature range in the manufacturing process.

However, the material of the steel varies greatly depending on the chemical composition, rolling conditions and cooling conditions of the steel.

In addition, even in the same chemical composition and rolling conditions, the strength and toughness change greatly depending on the cooling conditions, so strict management of each condition is required according to the material properties required. In the present invention, the tensile strength is 50kg / mm ² or more and can be used even in cold papers. In order to obtain low temperature impact toughness, the cooling start temperature was limited to 780-820 ° C., the cooling end temperature was 480-520 ° C., and the cooling rate was 10-15 ° C./sec. This cooling condition is a condition for obtaining a uniform structure of fine ferrite and bainite, and if the cooling condition is out of this range, the predetermined strength and toughness cannot be obtained.

Hereinafter, the present invention will be described in detail through examples.

Example 1

Steel made of the components shown in Table 1 is made of slab 70 mm thick. The slab is heated to 1200 ° C. and a 12 mm thick plate is produced. In order to investigate the effect of cooling conditions on the material properties, the cooling start temperature was converted to 900 ℃ and 800 ℃, and the cooling end temperature was converted to 600 ℃, 500 ℃ and 400 ℃, and the cooling rate was air-cooled (1 ℃ / sec) to water cooling (50 ° C./sec) to six conditions.

Table 1

Carbon equivalent (Ceq) = C +

Mechanical properties, that is, yield strength, tensile strength, impact energy, and ductility-brittle transition temperature of steels prepared as described above are measured and shown in Table 2.

[Table 2]

* V ^E -60 ℃: Shock absorption energy measured at Sarpi impact tester for specimens with V notch (at -60 ℃)

vTrs: Ductile-brittle transition temperature.

As can be seen in Table 2, the invention example is superior to the mechanical properties compared to the comparative example. On the other hand, Figure 1 is a graph showing the strength and toughness according to the cooling rate for the C steel having the composition of the present invention shows that the strength and toughness in the cooling rate range of the present invention 10-15 ℃ / sec.

The reason for this is that as shown in the tissue photograph of FIG. 2, the fine ferrite and bainite are uniformly distributed at the cooling rate of 10 ° C / sec (a), but the strength and toughness are excellent. When the cooling rate is 1 ℃ / sec (b) exhibits a pearlite band (pearlite band) structure, it can be seen that the strength is increased but toughness is greatly reduced. On the other hand, Figure 3 is a graph showing the strength and toughness of the carbon equivalent of C steel having the composition of the present invention and A and B steel deviating from the composition of the present invention shows that the material of C steel is excellent. As described above, the present invention provides an amorphous low alloy high tensile strength steel sheet excellent in high strength and low temperature impact toughness of 50kg / mm ² or more tensile strength.

Claims (1)

In manufacturing non-coarse low alloy high tensile strength steel sheet, C: 0.05-0.2wt /% (hereinafter referred to as%), Si: 0.5% or less, Mn: 0.05-2.0%, Al: 0.01-0.10%, and Ti : Controlled cooling at a rate of 10-15 ℃ / sec from 780-820 ℃ to 480-520 ℃ after general rolling of steel with carbon equivalent (Ceq) in the range of 0.30-0.33 Method for producing a non-coarse low alloy high tensile strength steel sheet, characterized in that.

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