KR100368218B1 - A manufacturing process of a high strength steel with high temper soften resistance - Google Patents

Abstract

The present invention relates to a method of producing a tensile strength 110kgf / mm2 high tensile steel; The purpose of the present invention is to provide a method for producing a tensile strength 110kgf / mm2 high tensile strength steel sheet having excellent softening resistance by adding a small amount of carbonitride forming elements to the steel of chemical composition similar to the conventional 90kgf / mm2 grade steel and applying a direct quenching process.

The present invention for achieving the above object, in the weight% C: 0.13-0.18%, Mn: 0.2-1.5%, Si: 0.15-0.35%, P: 0.02% or less, S: 0.008% or less, Ni: 2.0- A slab consisting of 4.0%, Cr: 0.5-2.0%, Mo: 0.3-1.0%, Nb: 0.02-0.06%, Ti: 0.02-0.06%, Sol-Al: 0.02-0.06% and the remaining Fe and other unavoidable impurities Hot-rolled steel with high softening resistance, which is heated at 1150-1300 ℃ and hot rolled, then starts cooling within 30 seconds, cooled to room temperature at a cooling rate of 10-50 ℃ / sec, and then treated at 550-700 ℃. The technical subject matter is a manufacturing method.

Description

A manufacturing process of a high strength steel with high temper soften resistance}

The present invention relates to a method of manufacturing a tensile strength 110kgf / mm2 high tensile strength steel, more specifically, a small amount of carbonitride forming element in a chemical composition similar to the conventional 90kgf / mm2 grade steel and softened by applying the direct hardening process It relates to a method of manufacturing a high tensile strength 110kgf / mm2 high tensile strength steel sheet.

In general, high tensile steels with a tensile strength of 90kgf / mm2 or more are manufactured by hardening and annealing heat treatment to secure martensitic structure. At this time, the antireflection treatment is performed at a high temperature of 550 ° C or higher. It is done.

However, the problem with the soaking treatment at high temperatures is that the strength of the steel is drastically reduced.

As an example, HY100 steel, widely known as a 90kgf / mm2 high tensile steel, is manufactured by hardening annealing and the typical manufacturing method is as follows (MIL-S-16116K (SH)). Namely, by weight%, C: 0.15-0.20%, Mn: 0.2-1.5%, Si: 0.15-0.38%, P: 0.015% or less, S: 0.008% or less, Ni: 2.75-3.5%, Cr: 1.3- The slab composed of 1.7%, Mo: 0.35-0.45%, V: 0.03% or less and the remaining Fe and other unavoidable elements is sufficiently heated at 1150-1300 ° C., and a reduction ratio of 10-30% per each rolling pass is obtained. And hot rolling under a rolling finishing temperature of 900 ° C. or higher, followed by air cooling and reheating to 900 ° C. or higher, followed by annealing at 550-650 ° C.

In recent years, the strength of steel has been demanded, especially in the defense industry, and there is a demand for improving the strength of the conventional steel. For this purpose, high strength can be obtained by performing heat treatment of conventional steel at a lower temperature. However, when the sour heat treatment temperature is lowered, there is a problem of reducing the toughness due to tempered martensite embrittlement (Source-author: G. Krauss, signature: Principles of Treatment of steel). Therefore, as another method for increasing the strength without harming the toughness, a method of increasing the alloying elements, such as Ni, Cr, Mo, Co, etc., which is expensive with the increase of C, is considered. Decreased weldability, of course, there is a problem that causes a large increase in production costs.

Accordingly, the present inventors have conducted in-depth studies and experiments on the method of increasing the strength of the high tensile steel without a large increase in the alloying elements and a great decrease in the toughness, and have come to propose the present invention based on the results.

That is, the present invention provides a method for producing high tensile strength 110kgf / mm2 high tensile strength steel by adding a small amount of carbonitride forming elements to a chemical composition steel similar to the conventional 90kgf / mm2 grade steel and applying a direct quenching process. There is a purpose.

1 is a graph showing the change in hardness according to the considered temperature of the invention and the comparative material

Figure 2 is a transmission electron micrograph showing the fine precipitate of the invention and the comparative material

3 is a graph showing the tensile strength-impact toughness relationship between the invention material and the comparative material

The method for producing high tensile steel of the present invention for achieving the above object, by weight% C: 0.13-0.18%, Mn: 0.2-1.5%, Si: 0.15-0.35%, P: 0.02% or less, S: 0.008% or less , Ni: 2.0-4.0%, Cr: 0.5-2.0%, Mo: 0.3-1.0%, Nb: 0.02-0.06%, Ti: 0.02-0.06%, Sol-Al: 0.02-0.06%, remaining Fe and other unavoidable The steel slab made of impurity was heated at 1150-1300 ℃ and hot rolled, and then started cooling within 30 seconds, cooled to room temperature at a cooling rate of 10-50 ℃ / sec, and then treated at 550-700 ℃. It is configured to include.

Hereinafter, the above-mentioned ingredient range and the reason for limitation of manufacturing conditions are demonstrated.

The C is a major reinforcing element in high tensile steel, and its content increases as hardening and toughness increase, but on the contrary, when weldable and toughness decreases, hardening and hardening are not possible because its content decreases. It is desirable to limit it to 0.18%.

The Mn also has an effect of increasing the hardenability by increasing the hardenability, but when excessively added, it is harmful to weldability, and therefore, the content thereof is preferably limited to 0.2-1.5%.

The Si is a component to be added as a deoxidizer, the yield strength increases but the increase in the ductile-brittle transition temperature to reduce toughness and also harmful to weldability, the content is preferably limited to 0.15-0.35%.

Since P and S are inevitable impurity elements in the steelmaking process, they are harmful to toughness and weldability, so P and S contents are preferably limited to 0.02% or less and 0.008% or less, respectively.

Ni is an essential element of high tensile strength steel which increases the hardenability of the steel and improves the toughness by lowering the ductile-brittle transition temperature. However, the Ni content is preferably limited to 2.0-4.0%.

The Cr and Mo are not only essential elements for improving the hardenability of steel, but also significantly increase the strength by suppressing softening at high temperatures of more than 550 ° C. and suppressing softening at high temperatures when the amount is added. Increases. On the other hand, when Cr and Mo are added for the purpose of suppressing softening at high temperature above 550 ° C, it is known that 5.0% of Cr and 1.0% of Mo should be added (Source-Author: G.Krauss, Signature: Principles of Treatment of steel). However, it is not preferable because the addition amount of Cr and Mo impairs weldability and the manufacturing cost increases significantly. Therefore, in the present invention, mainly added for the purpose of increasing the hardenability, Cr is 0.5-2.0%, Mo is limited to 0.3-1.0%, respectively.

When the addition amount of Ti and Nb is small, the added Ti and Nb precipitate into a considerably large second phase through casting and rolling, and there is almost no content in solid solution in the steel after the rolling is completed. It exhibits a soothing softening behavior similar to that of conventional steels, in which the strength sharply decreases during the souring treatment in the vicinity. On the other hand, as the amount of Ti and Nb is increased, the content of Ti and Nb in solid solution after rolling is increased to increase the softening resistance at around 600 ° C, but when excessively added, the nozzle clogging during continuous casting and There is a problem of reducing the toughness due to the formation of oxides, emulsions, and the like, and since the manufacturing cost increases, the amount of addition is preferably limited to 0.02-0.06%.

Sol-Al has an effect of improving toughness as an essential element for deoxidation, but when the content is excessively increased, the toughness is rather deteriorated due to the increase of aluminum oxide in the steel, so the content is limited to 0.02-0.06%. desirable.

In the present invention, it is preferable to heat the steel slab having the composition as described above to 1150-1300 ℃, the reason is that when the heating temperature is less than 1150 ℃ causes an excessive rolling load due to an increase in deformation resistance during rolling, 1300 ℃ This is because in the above case, unevenness of the tissue due to abnormal growth of the austenite crystal grains is caused and the toughness is subsequently damaged.

The slab heated as described above is hot rolled and then cooled, and the holding time until the start of cooling at this time can sufficiently secure the effect of increasing strength and toughness by control rolling and direct quenching, and the holding time If it is too long, the control rolling effect is reduced by recrystallization of austenite, so it is preferable to limit it to 30 seconds or less.

In the case of cooling the hot rolled steel sheet, it is preferable to set the cooling rate to 10-50 ° C./sec, because the faster the cooling rate, the more advantageous in terms of increasing the strength of martensite by securing martensite structure. If it is too fast to cause severe plate deformation, if the cooling rate is too slow because the soft tissues such as the upper bainite causes a decrease in strength.

After cooling to room temperature as described above, it is treated in a temperature range of 550-700 ℃. Sour temperature is an important factor for strength and toughness.The temperature increases below 550 ℃, but the toughness decreases significantly due to the sour martensite embrittlement. It is impossible to secure the strength of mm 2.

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

EXAMPLE

Steel slabs composed as shown in Table 1 below are sufficiently heated at 1250 ° C., rolled and heat treated under the conditions shown in Table 2 below, and then the mechanical properties of the prepared specimens are measured and the results are shown in Table 3 below. It was.

Steel grade Chemical component (% by weight) C Mn Si P S Ni Cr Mo V Nb Ti Sol.Al Invention steel 0.15 0.60 0.26 0.003 0.004 3.08 1.50 0.78 - 0.04 0.04 0.024 Comparative steel 0.17 0.27 0.25 0.004 0.005 2.92 1.53 0.39 0.026 - - 0.020

Psalm Number Reduction rate in the unrecrystallized temperature range (%) Hardening Method Cooling rate (℃ / sec) Consideration temperature (℃) River bell Invention a 50 Direct water cooling after rolling 35 600 Invention steel b 50 Direct water cooling after rolling 35 650 Invention steel c 0 Direct water cooling after rolling 35 600 Invention steel d 0 Direct water cooling after rolling 35 650 Invention steel Comparative material One 0 Water cooling after reheating 35 600 Invention steel 2 0 Water cooling after reheating 35 650 Invention steel 3 0 Water cooling after reheating 35 450 Comparative steel 4 0 Water cooling after reheating 35 500 Comparative steel 5 0 Water cooling after reheating 35 600 Comparative steel 6 0 Water cooling after reheating 35 650 Comparative steel 7 50 Direct water cooling after rolling 35 600 Comparative steel 8 50 Direct water cooling after rolling 35 650 Comparative steel

Mechanical Property Specimen Number Yield strength (kgf / ㎡) Tensile Strength (kgf / ㎡) Elongation Room Temperature Shock Toughness (Joule) Invention a 112.4 127.5 16.6 89 b 103.9 118.1 17.9 113 c 110.6 125.7 18.8 85 d 103.2 117.4 17.7 107 Comparative material One 90.4 102.7 17.0 115 2 83.9 95.3 19.3 136 3 109.0 124.5 17.9 72 4 106.0 118.0 19.4 94 5 96.5 107.5 22.0 156 6 81.5 92.5 23.5 204 7 97.1 108.4 22.8 161 8 84.2 95.6 23.7 207

As can be seen from the above Tables 1, 2 and 3, only the inventive material (a, b, c, d) produced by the present invention and the comparative material (3, 4) subjected to the soaking treatment at a relatively low temperature were 110 kgf / mm 2. It can be seen that the above tensile strength is satisfied. However, in the case of the comparative materials 3 and 4, the softening brittleness occurred.

In addition, the comparative materials (5, 6) manufactured by reheating and quenching the conventional steel and the soaking treatment at 600 ° C. or higher and the comparative materials manufactured by the same direct quenching method as the invention material and polished at 600 ° C. or higher ( 7,8) shows a strength about 20 kgf / mm 2 lower than the invention. On the other hand, the comparative materials (1, 2) produced by the reheating and quenching method, which is a conventional manufacturing method, also show a strength of 20 kgf / mm 2 or more lower than the inventive material.

As described above, when the soaking treatment was performed at the same soaking temperature, the invention material showed about 20 kgf / mm2 higher strength than the comparative material, and in view of metallurgy, an appropriate amount of Ti and Nb, which are carbonitride forming elements, was added. It is an effect which controlled the manufacturing method suitably so that carbonitride may precipitate finely. That is, high strength is secured in the present invention by controlling the alloying component and the production method by increasing the resistance to softening during high temperature polishing at 550 占 폚 or higher.

The results for showing this more clearly are shown in FIG. 1. According to FIG. 1, when direct quenching is applied to conventional steels, the strength is evenly increased in the range of temperature, which is a well-known control rolling-direct quenching effect (source-author: KA Tayloy et al., Signature: Physical Metallargy of Direct-). Quenched Steels)

On the other hand, in the case of directly quenching the invention steel with a small amount of Ti and Nb added to the conventional steel, the strength reduction is significantly suppressed at 550 ° C. or more, in addition to the direct quenching effect shown in the conventional steel, that is, the soot softening resistance is remarkably improved. have. This is the effect of precipitation strengthening due to the precipitation of fine particles when Ti and Nb newly added to the inventive steel exist in solid solution in the steel by direct quenching and then treated at around 600 ℃.

2 shows a transmission electron micrograph showing the composite carbonitride of Ti and Nb formed from the inventive material (a) prepared by the direct quenching method and the comparative material (1) prepared by the reheat quenching method. In the case of Comparative Material 1 (FIG. 2A), very large precipitated particles were distributed, whereas in the case of Inventive Material (a) (FIG. 2B), very fine precipitated particles were distributed. The difference in size of the precipitated particles is due to the precipitated particles of the comparative material formed during the reheat quenching heat treatment at 900 ° C. and the precipitated particles of the inventive material formed during the soaking treatment at 600 ° C. It is well known thermodynamically that the precipitated particles become fine as the temperature is lowered.

In addition, according to the precipitation strengthening theory, as the precipitated particles are finer, the reinforcing effect is increased, so that the comparative materials (a, b, c, d) manufactured by the direct quenching method of the inventive steel are manufactured by the reheat quenching method ( This is the reason for showing the strength superior to 1, 2).

On the other hand, the strength-toughness relationship between the invention material and the comparative material securing a tensile strength of 110kgf / mm 2 or more is shown in Figure 3, as shown in Figure 3, the invention materials (a, b, c, d) are It can be seen that the toughness is excellent at a similar strength compared to the comparative materials (3, 4). This is due to the low toughness of the sour martensite embrittlement in the case of comparative material treated at 450 ℃ and 500 ℃ from the metallurgical point of view. Therefore, the increase in strength due to decrease of sour temperature is accompanied by the decrease of toughness. It can be seen that there is a problem.

As described above, the present invention has the effect of economically producing 110kgf / mm2 high-tensile strength steel which is significantly suppressed sour softening resistance at a high temperature of more than 550 ℃ by the addition of a small amount of Ti and Nb and the direct quenching process.

Claims (1)

In the manufacturing method of high strength steel, By weight% C: 0.13-0.18%, Mn: 0.2-1.5%, Si: 0.15-0.35%, P: 0.02% or less, S: 0.008% or less, Ni: 2.0-4.0%, Cr: 0.5-2.0%, A steel slab consisting of Mo: 0.3-1.0%, Nb: 0.02-0.06%, Ti: 0.02-0.06%, Sol-Al: 0.02-0.06%, remaining Fe and other unavoidable impurities is heated at a temperature of 1150 to 1300 ° C. After rolling, cooling is started within 30 seconds, cooled to room temperature at a cooling rate of 10-50 ° C./sec, and then treated at 550-700 ° C. for high softening resistance.