KR900004848B1 - Making process for high-tensile steel - Google Patents

Abstract

Method for producing high tension non-tempered steel by normalizing treatment is characterised by holding a rolled steel at Ac3-Ac3+50 deg.C after fining grain size of steel by control hotrolling. The steel is composed of (in wt.%) up to 0.3% C, 0.5-2.0% Mn, up to 0.5% P, up to 0.5% S, up to 0.2% Nb, up to 0.2% V, 0.01-0.2% Ti, 100-300 ppm N2, up to 1.0% at least one element selected from the group consisting of Ni, Mo, Cr and Cu, and the balance Fe.

Description

Manufacturing Method of Non-Quality High Tensile Steel by Normalizing Heat Treatment

1 is a microscopic picture of the change in rolling method.

2 is a precipitate distribution picture according to the amount of nitrogen.

The present invention relates to a method of manufacturing high tensile strength steel by normalizing heat treatment, and in particular, maximizes grain refinement and precipitation of carbonitrides, and thus has high strength, excellent toughness, and low structural and mechanical properties in the hot working process. It relates to a method of manufacturing high tensile steel. In the conventional method for producing high strength steel by normalizing heat treatment, since many alloy elements are used to obtain high strength steel, the carbon equivalent (Ceq) is high as about 0.50%, so that the weldability and crack sensitivity are generated. This is not only bad, but also has a disadvantage in that the manufacturing cost is expensive, and is also a method of manufacturing a steel sheet that does not utilize the reinforcement by refining grains that can simultaneously improve strength, toughness, weldability, and the like.

In recent years, normalizing heat treatment has been developed with the advance of quenching and tempering method, which can obtain excellent quality by rapid cooling, or control rolling technology that can improve quality moisture through grain refinement without heat treatment. High tensile steel production by the method continues to decrease.

However, the steel sheet manufactured by quenching tempering or the steel sheet manufactured by controlled rolling cannot be used for hot processing of steel sheet to manufacture parts of pressure vessels. Steel sheet by is used.

Because the mechanical properties of the steel sheet depend on the metal structure of the steel sheet, the martensite structure obtained by quenching and tempering or the microstructure obtained by controlled rolling are extinguished or coarsened by hot working at 600-800 ° C. Compared to the deterioration of the mechanical properties of the original steel sheet, the steel sheet manufactured by normalizing has already undergone the normalizing heat treatment of 800-900 ℃ after hot rolling. This is because change and mechanical property deterioration hardly occur.

A normalizing heat treatment is usually in the hot rolling organization that is, the steel sheet manufactured by hot rolling with finish rolling temperature of 950 ℃ degree was heated at about 1200 ℃ in the currently being used to prepare the hard decision processing of the steel sheet of the pressure vessel than the Ac ₃ By reheating and maintaining at a high temperature of about 50 ° C., air-cooling was able to remove chemical composition and structure irregularities of the steel sheet and obtain only a slight grain refining effect.

Therefore, the method of reinforcing the normalized heat-treated steel mainly depends on the method of increasing the amount of alloy, so that a high carbon equivalent becomes a problem. In addition, the normalizing heat treatment mainly removes segregation in the steel sheet and removes nonuniformity of the structure, and the effect of grain refinement is not large.

Accordingly, it is an object of the present invention to provide a method for maximizing the miniaturization effect of grains in normalizing heat treatment and lowering the carbon equivalent to 0.45% or less and producing 50 kg / mm ² grade high strength steel.

The method according to the present invention is a method to further refine the grains of the steel sheet after the normalized heat treatment by introducing the control rolling in hot rolling before normalizing heat treatment, the amount of precipitates in the controlled rolling fixed by adding a large amount of nitrogen (N ₂ ) in the steel In order to increase the grain refining effect and increase the amount of precipitation of carbonitride in the air cooling process and the normalizing heat treatment process after the control rolling, it is to manufacture the non-tough high tensile strength steel by maximizing the precipitation strengthening effect.

In other words, the grain size is refined and the precipitation strengthening of carbonitride is increased than the conventional normalized heat-treated steel which is subjected to normalized heat treatment after general rolling to improve the strength and toughness of the thick steel sheet.

According to the present invention, Nb and V are each added in an amount of 0.2% or less to general low carbon steels containing C: 0.3% or less, Mn: 0.5-2.0%, and P and S: 0.5% or less. At least one element of the group consisting of Mo, Cr and Cu: 1.0% or less, Ti: 0.01-0.2% and N ₂ : 100-300ppm added low alloy high tensile strength steel by hot control rolling to refine the grains, and In the method for producing non-tough high tensile strength steel having high strength and toughness by increasing the amount of precipitation of carbonitrides and making finer grains by performing normalizing heat treatment to heat the steel plate from Ac ₃ temperature to Ac ₃ + 50 ° C. It is about.

Hereinafter, the reasons for limiting chemical components and manufacturing conditions in the present invention will be described.

The alloying elements Nb (niobium) and V (vanadium) are each added at 0.2% or less, and at least one element selected from the group consisting of Ni, Mo, Cr and Cu is added in a range of 1.0% or less and 50 kg / It can be used to manufacture high strength steel of mm ² or higher.

The reason for adding and adding Nb and V is as follows.

Nb-based carbonitrides precipitated at normalization at Ac ₃ + 50 ° C suppressed the growth of austenite grains during the normalization heat treatment, while V-based carbonitrides were re-used by normalizing heat treatment and then re-cooled during the cooling process. This is because it precipitates to improve strength. At this time, since a large amount of nitrogen is present, precipitates are generated than before normalizing, and the precipitation strengthening effect is maximized. However, when the added amount exceeds 0.2%, the effect is saturated, so 0.2% is the upper limit.

Ti (Titanium) is precipitated at high temperature in TiN form to suppress the growth of early austenite crystals from the stab heating process to enhance the effect of controlled rolling and to act as a nucleus of precipitates precipitated during hot rolling and normalizing heat treatment processes. It promotes precipitation phenomenon.

The range of Ti addition was made into 0.01-0.2% range in consideration of the effect. Nitrogen is preferably in the range of 100-300ppm because the amount of nitride precipitates less than 100ppm, so the effect of precipitation strengthening is small, and when it is 300ppm or more, free nitrogen, not precipitate form, exists to reduce toughness. Because it is. Hot-rolled rolling has the effect of controlled rolling when the cumulative reduction ratio of 950 ° C. or lower, which is usually applied, is 50 or higher and the finish rolling temperature is 850% or lower. The higher the cumulative reduction ratio and the lower the finishing rolling temperature, the greater the grain refining effect, but the lower the productivity, so that it can be adjusted according to the required quality level. Normalizing temperature is the austenite matrix is employed high as the intermediate temperature range of V-carbonitride as described above as the temperature range of Ac ₃ + 50 ℃ from the Ac ₃ temperature is employed and Nb-based carbonitride is present in a precipitated state Ac ₃ temperature was the lower limit because it must be a tissue, and Ac ₃ + 50 ° C. was the upper limit to prevent coarsening due to grain growth.

The present invention will be described through the following examples.

Example 1

A steel plate made of a chemical component as shown in Table 1 was made into a slab having a thickness of 70 ppm, heated to 1200 ° C., and then subjected to general rolling and control rolling to prepare a 12 mm thick steel plate. At this time, the amount of nitrogen was changed from 60ppm to 340ppm.

The thick plates manufactured by the normal rolling and the control rolling were subjected to normal heat treatment for 2 hours.

Normalizing temperature was changed to three conditions of 800 degreeC, 880 degreeC, and 950 degreeC. Mechanical properties and precipitate distributions were investigated for all the prepared plates, and the mechanical properties thereof are shown in Tables 2 and 3, and the precipitate distributions are shown in FIGS. 1 and 2.

TABLE 1

TABLE 2

TABLE 3

As shown in Table 2 and Table 3, the invention steels B and C in the amount of nitrogen in the range of 100-300 ppm were excellent in strength and impact toughness when controlled rolling (inventive materials a and b), but the invention steels B and When C was rolled in general (Comparative Materials 2 and 3), the strength was similar, but the impact toughness was very bad.

This can be fully seen from the microstructure of FIG.

FIG. 1 shows the microscopic picture of the invention steel B when normalized after normal rolling and when normalized after control rolling.

As shown in FIG. 1, it can be seen that in the case of controlled rolling, the grain size is about 10 µm, and in the case of general rolling, the grain size is about 25 µm and the grain size of the normalized steel that has been controlled is fine.

On the other hand, as shown in Table 2, the comparative steels A and D of the nitrogen content of 59ppm and 340ppm, respectively, can be seen that the strength and impact toughness does not improve even if controlled rolling.

FIG. 2 is an electron micrograph showing the distribution of precipitates after controlled rolling and normalizing heat treatment of steel A and nitrogen B having an appropriate amount of nitrogen.

2, as shown in the micrograph, it can be seen that the invention steel B having a large amount of nitrogen has a higher amount of precipitates and evenly distributed than the comparative steel A having a small amount of nitrogen.

Table 3 shows the mechanical properties change according to the normalized heat treatment temperature after the controlled rolling of the invention steels B and C having a component range consistent with the present invention.

As shown in Table 3, the effect of normalizing temperature on the mechanical properties did not appear as much as the effect of chemical composition or controlled rolling, but the strength and strength when outside the temperature range of Ac ₃ + 50 ° C from the appropriate range of Ac ₃ temperature. It turns out that toughness falls.

As shown in Table 2 and Table 3, according to the present invention was added to the nitrogen of 100-300ppm and subjected to normalized heat treatment at a suitable temperature after the control rolling to produce an amorphous high tensile strength steel excellent in strength and impact toughness.

Example 2

In order to find out whether the inventive steels B and C of the sample steels of Table 1 shown in Example 1 are suitable for hard plate processing with respect to the invention materials a and b controlled and rolled in the same manner as Table 2 of Example 1 The change in mechanical properties after hot working was measured and the results are shown in Table 4 below.

The hot plate processing was carried out under the same conditions as in the actual case by performing a high stress relief annealing deformation with an experimental rolling mill.

TABLE 4

As shown in Table 4, the invention materials a and b subjected to normalized heat treatment after control rolling of the inventive steels B and C according to the present invention, the strength was increased by hot hard plate processing and the impact toughness change was within 5%. As a range, the degree of deterioration is extremely small, and thus it is suitable for hard plate processing.

Claims (1)

In general, low carbon steel containing C: 0.3 or less, Mn: 0.5-2.0%, P and S: 0.5% or less, Nb and V, respectively, is 0.2% or less, and Ni, Mo, Cr and at least one kind of element from the group consisting of Cu: 1.0% or less, Ti: 0.01-0.2% and N _2: after the low alloy high strength steel with 100-300ppm was added refinement to hot rolling to control the crystal grains, the steel sheet Ac ₃ A method for producing an amorphous high tensile strength steel by normalizing heat treatment, characterized in that the heating is maintained in a temperature range of Ac ₃ + 50 ° C. from the temperature.

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