KR900005374B1 - Making process for high tensile strength steel - Google Patents

Abstract

High tensile strength steel is obtained by two-stage quenching process after hot rolling steel slab comprising (in wt.) 0.010.20% C, 0./005-1.0% Si, 0.20-2.0% Mn, up to 0.025% P and S, 0.050.30% Cu, up to 9.0% Ni, up to 3% Cr and Mo, 0.001-0.15% sol. Al, up to 0.05 T and Nb, up to 0.10% V, upto 0.0025% B and the balance Fe. The first stage quenching is carried out within the range of (reheating temp.+20 deg.C) to (reheating temp. + 70 deg.C). The second stage quenching is carried out within the range of (Ac3 + 30 deg.C) to (Ac3 + 100 deg.C) following by tempering treatment.

Description

Manufacturing method of tempered high tensile strength steel by 2 times hardening method

1 is a micrograph of the present invention and the prior art.

2 is a graph showing the tensile test results of the present invention and the prior art material.

3 is a graph showing the tensile test results of the present invention and the prior art material.

4 is a graph showing the impact toughness of the present invention and conventional materials.

5 is a graph showing the flexural strength of the present invention and the prior art material.

6 is a graph showing the present invention and the conventional heat treatment method.

The present invention relates to a method of manufacturing high tensile strength steel having high strength and impact toughness, susceptibility to stress corrosion cracking and low surface corrosion rate by two hardening methods.

Up to 80 kg / mm2 of tensile steel has been used for bridges, pens tock, submarines and deep-sea wells, including offshore structures, but the strength has been increased due to the large size, high performance, thinness and economic pursuit. The use of high 100 kg / mm2 grade tempered (annealed and heat treated) high tensile strength steels has become necessary. In addition, when applying 100㎏ / ㎠ grade steel, it is possible to weld with the same level of construction management as 80㎏ / ㎜ grade steel, and it has excellent strength and low temperature impact toughness, and is sensitive to stress corrosion cracking and low surface corrosion rate. This is required.

Conventional methods for achieving the above objectives include the addition of expensive alloying elements, such as Ni, Mo, Cr, in large quantities, slab high temperature heating, hot rolling after air rolling, and then quenching and heat treatment to provide strength, impact toughness and corrosion resistance. Method to improve the particle size, and add a small amount of elements such as Nb, V, Ti, B, and Al, which have significant hardening effect, and then carry out low-temperature slab heating, controlled rolling, and reheating repeated annealing to refine the grains and precipitates. It has been proposed a method of improving the quality characteristics by heat treatment and a method of directly quenching and soaking heat at high temperature without air cooling after hot rolling.

However, in the method by adding a large amount of expensive alloying elements such as Ni, Mo, Cr, etc., the cost increases and the economical efficiency decreases, and the carbon equivalent and the weld cracking sensitivity are increased, thereby deteriorating the weldability and the welding construction. There was a drawback to the difficulty. In addition, in the above-described method of controlled rolling and reheating repeated quenching, the thickness due to overload of the roll due to the nonuniformity of the quality characteristics according to the sensitization of the production manufacturing conditions for the low temperature rolling and the reheating residual quenching method and the low temperature rolling Increasing deviation, flatness, and product defect rate for plate deformation have not only lowered productivity, but also increased strength and toughness.

In the above method of direct quenching, when the direct quenching is performed, the alloying element is dissolved at higher than its equilibrium solubility at the quenching temperature, so that the dislocation density increases and fines, but the grain refining effect is higher than that of the reheating heat treatment. It is small and the rolling processing structure remains, so that the non-uniform detail structure is obtained, so that the uniform quality characteristics cannot be obtained.

Accordingly, the present invention is to provide a high-tensile steel with excellent mechanical properties by direct quenching without hot air after hot rolling and then reheated and quenched again.

Hereinafter, the present invention will be described in detail.

In the present invention, C: 0.01-0.20%, Si: 0.005-1.0%, Mn: 0.20-2.0%, PS: 0.25% or less, Cu: 0.05-0.30%, Ni: 9% or less, Cr, Mo: 3.0% or less, Sol Al: 0.001-0.15%, Ti, Nb: 0.05% or less, V: 0.10% or less High strength and impact toughness are achieved by the two-annealing method, which is directly annealed in the temperature range 20-70 ℃ higher than the reheating temperature, and then reheated and annealed in the temperature range of 30-100 ℃ higher than the Ac ₃ transformation point. The present invention relates to a method of manufacturing high tensile strength steel having low susceptibility to stress corrosion cracking and low surface corrosion rate.

6 (a) showing a conventional heat treatment method and FIG. 6 (b) showing a heat treatment method according to the present invention).

The said component and a limiting method are demonstrated.

C is at least 0.01% in order to secure sufficient strength as a high tensile strength steel, but an increase of more than necessary deteriorates weldability and impact toughness, and is highly susceptible to stress corrosion cracking due to formation of coarse carbide, so 0.20% or less is preferable. .

Si is an element necessary as a deoxidizer in steelmaking. It should contain 0.005% as a basic, and if it exceeds 1.0%, Si not only forms non-metallic inclusions such as SiO ₄ , but also forms island-like martensite, which leads to weldability, toughness and stress corrosion cracking. 0.005-1.0% is preferable because it impairs sensitivity.

Mn is a deoxidizer in steelmaking. It is effective by adding at least 0.2% to increase the strength. If it exceeds 2.0%, Mn forms non-metallic inclusions such as MnS, lowering the susceptibility to impact toughness and stress corrosion cracking and damaging the surface corrosion. It is preferable to set it as%.

P solidifies and segregates in the steel pressure gauge, so the weldability and toughness of the steel are severely degraded, so the content of P is made 0.025% or less.

S decreases impact toughness and susceptibility to stress corrosion cracking by forming MnS and performing grain boundary segregation, so the upper limit is set to 0.025%.

Cu is an element that strengthens the matrix structure to increase strength, susceptibility to stress corrosion cracking and lowers the rate of surface corrosion. It is effective only when added at 0.05% or more, and when added at 0.30% or more, it damages the hot workability during hot rolling. 0.05-0.30% is preferable because it lowers.

Cr and Mo form fine carbides and nitrides and microstructures to increase strength, impact toughness, stress corrosion cracking susceptibility and surface corrosion rate, but 3% of each adds sufficient effects. As there is no economic feasibility, the upper limit is 3%.

Ni is an element that greatly improves the hardenability, and the grain size is refined and the matrix structure is strengthened so that the low temperature toughness is greatly improved despite the increase in strength, the susceptibility to stress corrosion cracking and the surface corrosion rate are lowered. By increasing the amount of produced, the effect of raising the soothing effect can be obtained even at a low cooling rate when a large amount is added.If the addition of more than 9.0%, the increase in strength and low temperature toughness almost reaches the labeling state, so the upper limit is lower than 9.0%. It is limited to.

Nb, V, Ti, and B improve hardenability and form fine carbides and nitrides to refine the grain size, thereby increasing strength and impact toughness, susceptibility to stress corrosion cracking, and lowering the surface corrosion rate. Since embrittlement is formed and said quality is reduced, Ti and Nb are respectively 0.05% or less, V is 0.10% or less, and B is limited to 0.025% or less.

So1 and Al are required as deoxidizers in steelmaking and are effective when added over 0.001% to refine crystal grains and increase impact toughness and corrosion resistance. 0.001% -0 15%.

Before hot rolling, it is heated (soaking) at 1000-1300 ℃ in the heating furnace. If it is heated above 1300 ℃, coarsening of grains occurs. If it is heated below 1000 ℃, precipitates are not completely dissolved and mechanical properties are degraded.

In case of carrying out direct quenching and reheating quenching by the method according to the present invention, the alloying element is dissolved at higher than the parallel solubility at the quenching temperature (sostenitization temperature) to increase the dislocation density and finer the carbonitride during sourcing. Microdispersion precipitation is possible, and the amount of fine precipitates increases, and after hot rolling, air cooling, which is a conventional method, is performed in the ideal mixed zone of austenite and ferrite, which is a temperature section between Ac ₃ and Ac _1. At the interface between austenite and ferrite, elements such as Nb, V, and B form carbides and nitrides to precipitate in three dimensions, degrading hot workability and facilitating grain embrittlement.

So, by performing direct point quenching, the method according to the invention prevents critical brittleness, which is detrimental to the quality characteristics by quenching the temperature-absorption temperature zone, and when the elements such as Nb, V, B and Ti are evenly dispersed in the mouth, In air cooling, since the cooling time is long after hot rolling, complex precipitates such as Mn, Mo, Cr, Cu, etc. are easily formed in grains and grain boundaries along dislocations and crystal boundaries almost parallel to the rolling direction. While it grows first, it grows preferentially in the mouth and grain boundary at the time of solubility, whereas direct quenching, which is a method according to the present invention, does not have time for long growth of precipitates, thereby forming spherical composite precipitates.

After hot rolling, the direct quenching temperature should be higher than the reheating quenching temperature, which means that the difference between the amount of precipitates employed by the quenching effect at the direct quenching temperature and the amount of precipitates employed by the quenching effect at the reheating quenching temperature should be large. Because it can double the effect. If the temperature difference is within the range of 20 ℃, the effect on the difference of the precipitates employed is less. If the temperature is over 70 ℃, it becomes 170 ℃ above the Ac ₃ transformation point, resulting in grain coarsening phenomenon and impact toughness. Lose. Therefore, the optimum direct quenching temperature is limited to 20-70 ℃ higher than the reheating quenching temperature.

The optimum reheat quenching temperature is directly related to the Ac ₃ transformation point. If the reheating quenching cell is in the range of 30 ° C on the Ac ₃ transformation point, the temperature of the steel sheet is higher than that of the Ac ₃ transformation point due to an error in the steel sheet heat treatment conditions during operation. It is much lower and the ferrite is more likely to precipitate, so work should be done at intervals of 30 ° C or higher.

If the reheating quenching temperature is more than 100 ℃ above the Ac ₃ transformation point, the effect of refining grains becomes less, so limit the optimal reheating quenching temperature to 30-100 ℃ higher than the Ac ₃ transformation point.

Compared with the conventional steel, the inventive steel has an increased dislocation density to finer, fine precipitates of carbonitrides are formed, and the amount of fine precipitates increases.

In addition, the grain boundary embrittlement can be eliminated by evenly dispersing the precipitate evenly in the mouth without excessive precipitation of elements such as Nb, V, and B in the grain boundary, and the growth of long-growing composite precipitates is suppressed and spherical composite precipitates are formed. Excellent toughness, stress corrosion cracking resistance and surface corrosion resistance.

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

Example 1

The steel ingots formed as shown in Table 1 were heated at 1300 ° C. for 1 hour in order to remove component segregation and casting structure therein, using a vacuum induction furnace, and then hot forged into small slabs.

TABLE 1

Steel grades A and B, as shown in Table 1, were hot-rolled under the same conditions as in Table 2, followed by air cooling in the case of the conventional materials, and water cooled in the case of the present invention at 910 ° C.

TABLE 2

Next, the prior art material and the invention material was austenized at 880 ° C. for 50 minutes and quenched in water at 20 ° C., followed by 60 minutes at 600 ° C., followed by invention material (1) and prior art material (3). The microscopic tissue was observed and shown in FIG.

(D), (e), (f) of FIG. 1 showing the present invention material (1) directly quenched, and (a), (b), (c) showing the air-cooled conventional material (3). As described above, the present invention (1) is generally finer in structure than the conventional material (3) (see (d) and (a)).

In addition, in this invention material 1, spherical precipitates are formed in a grain and a grain boundary compared with the prior art material 3 in which a precipitate is scattered long in a grain and a grain boundary. (see (b: intragranular), (c: intergranular) and (e: intragranular), (f: intergranular)).

The precipitate is composed of a composite precipitate mixed with Mn, Mo, Cr, Fe, etc., the small and fine precipitate is composed of a composite precipitate, such as Nb, Ti, B, the amount of fine precipitate is directly cooled in the case of the present invention Larger amounts were observed than one prior art.

Example 2

By measuring the mechanical length, that is, the tensile strength of the invention material (1,2) and the conventional material (3,4) manufactured as in Example 1, the second invention is about the invention material (1) and the conventional material (3) The invention material (b) and the prior art material 4 are shown in Fig. 3, respectively.

As can be seen in FIGS. 2 and 3, the present invention has excellent tensile strength (about 8%) as compared with the conventional material.

Example 3

The inventive material (1) and the conventional material (3) prepared as in Example 1 were subjected to an impact test at room temperature to -80 ° C, and the results are shown in FIG.

As shown in FIG. 4, the present invention (1) showed an average increase of 2 Kg.m over the seed material (3) over the entire temperature range, and the soft brittle transition temperature also moved toward the lower temperature by 9 ℃.

Example 4

In order to measure the stress corrosion cracking susceptibility for the inventive material (1) and the conventional materials (3, 4) prepared as in Example 1, a small tensile test piece (16-25 mmφ) bent at 45 ° C, an equilibrium length The 32 mm) was maintained at 50 ° C. in a 3.5% NaCl solution, and then immersed for 30 days, followed by bending test in the reverse direction to show flexural strength in FIG. 5.

As can be seen in FIG. 5, the present invention material (1) is superior in flexural strength as compared with the prior art (3) containing the same Ni, the present invention material (1) is stress corrosion compared to the conventional material (3) Lower susceptibility to cracking

Example 5

In order to measure the surface corrosion resistance of the inventive materials (1, 2) and the conventional materials (3, 4) prepared as in Example 1, after maintaining 50 ℃ in 5% NaCl solution to immerse for 30 days to reduce the corrosion loss It was measured as a corrosion rate (g / ㎡.h) divided by time is shown in Table 3, respectively.

TABLE 3

As can be seen in Table 3, when the same Ni content, the present invention appeared to have a lower corrosion rate than the prior art.

As described above, the present invention can reduce the increase in production manufacturing cost according to the large amount of Ni, Mo, Cr, etc., and can reduce the productivity decrease and increase of defective products by low temperature rolling and reheating repeated pressure method. By the heat treatment method, it is possible to reduce the uneven quality characteristics due to the non-uniform tissue with little refining effect.

Claims (1)

In the preparation of tempered high tensile steel, C: 0.01-0.20%, Si: 0.005-1.%, Mn: 0.20-2.0%, P, S: 0.025% or less, Cu: 0.05-0.3% or less, Ni: 90% or less, Cr. After Mo: 3% or less, Sol Al: 0.001-0.15% T, Nb: 0.05% or less, V: 0.10 or less, B: 0.0025% or less Tempered high tensile strength steel by two times hardening method characterized by direct heat quenching in the temperature range 20-70 ℃ higher than reheating temperature, then reheat quenching in the temperature range 30-100 ℃ higher than Ac ₃ transformation point Manufacturing method.

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