KR100328051B1 - A Method of manufacturing high strength steel sheet - Google Patents

Abstract

PURPOSE: A method for manufacturing high tensile strength steel sheet with a tensile strength of 140 kgf/mm¬2 by micronizing the effective grain size of martensite is provided. CONSTITUTION: The method for manufacturing high tensile strength steel sheet includes step of heating a slab comprising C 0.13 to 0.20 wt.%, Mn 0.5 to 1.5 wt.%, Si 0.15 to 0.35 wt.%, 0.02 wt.% or less of P, 0.008 wt.% or less of S, Cr 0.3 to 1.0 wt.%, B 0.001 to 0.004 wt.%, Ti 0.01 to 0.04 wt.%, Nb 0.01 to 0.04 wt.%, Sol-Al 0.02 to 0.06 wt.%, a balance of Fe and incidental impurities to the temperature range of 1150 to 1300°C; hot rolling the slab in the nonrecrystallization temperature region at a cumulative reduction ratio of higher than 40%; starting water-cooling within 30 seconds after hot rolling to ambient temperature at a cooling rate of 5 to 50°C/sec; reheating the cooled steel sheet to the temperature range of 850 to 910°C; cooling the reheated steel sheet to ambient temperature at a cooling rate of 10 to 50°C/sec; and tempering the cooled steel sheet in the temperature range of 150 to 350°C.

Description

A method of manufacturing high strength steel sheet

The present invention relates to a method for producing a wear-resistant steel sheet used in industries such as mining industry, timber industry, cement industry and transport industry, and more specifically, low-temperature reheating after direct rolling and controlled rolling on a chemical composition similar to conventional steel It relates to a method of manufacturing a tensile strength 140kgf / mm2 high tensile strength steel sheet having excellent impact toughness by applying the hardening process.

Abrasion resistant steel sheets are classified according to their hardness (or strength), which is closely related to abrasion resistance. Commercial abrasion resistant steel sheets include BHN (Brinel Hardness Number) 320, 360, 400, and 500 grades, respectively. , 120, 140 and 190 kgf / mm 2. In recent years, as the application fields and usage environments of wear resistant steels are diversified, in addition to the strength for ensuring wear resistance, there is an increasing demand for wear resistant steels having improved toughness.

Tensile strength 140kgf / mm2 (BHN400 grade) The conventional method for producing high strength wear-resistant steel is as follows. By weight% C: 0.13-0.30%, Mn: 0.4-1.5%, Si: 0.15-0.35%, P: 0.02% or less, S: 0.008% or less, Cr: 0.25-0.65%, Mo: 0-0.25%, The slab consisting of B: 0.001-0.003%, Ti: 0.-0.05% and the rest of Fe and other unavoidable elements is sufficiently heated at 1150-1300 ° C., with a reduction ratio of 10-30% for each rolling pass. After hot rolling, air-cooled and then reheated to 900 ° C. or more to quench and then produce it at 150-300 ° C. By the way, the high tensile strength steel sheet provided by the said conventional method has the problem that toughness is low considering the use environment of the wear-resistant steel in recent years. This problem can be increased by the method of adding expensive alloying elements such as Ni, Cr, Mo, Co, etc. without reducing the strength of the conventional steel sheet. However, such an alloying element addition method has a problem of lowering weldability due to an increase in carbon equivalent and of increasing production cost.

Therefore, the present inventors have conducted in-depth research and experiment on the method of improving the toughness without increasing the alloying elements in high-strength steel. As a result of controlling the cooling and reheating quenching conditions after controlled rolling and controlled rolling, It was confirmed that the solution was possible by the miniaturization effect, and came to propose the present invention.

That is, the present invention provides a method of manufacturing a 140kgf / mm2 high tensile strength steel sheet having excellent toughness and superior toughness by controlling cooling and post-rolling cooling conditions and reheating and quenching conditions without adding alloying elements to conventional high tensile steel. The purpose is.

1 is a graph showing the tensile strength-impact toughness relationship between the inventive material and the comparative material;

2 is an optical micrograph showing the microstructure of the invention and the comparative material.

The method for producing a high tensile strength steel sheet of the present invention for achieving the above object, by weight: C: 0.13-0.20%, Mn: 0.5-1.5%, Si: 0.15-0.35%, P: 0.02% or less, S: 0.008% The slab consisting of Cr: 0.3-1.0%, B: 0.001-0.004%, Ti: 0.01-0.04%, Nb: 0.01-0.04%, Sol-Al: 0.02-0.06% and the remaining Fe and other unavoidable impurities is 1150 After heating at -1300 ℃ to control rolling under the condition that the cumulative reduction ratio is over 40% in the unrecrystallized temperature region, cooling starts within 30 seconds and then cooled to room temperature at a cooling rate of 5-50 ℃ / sec. Reheating to a temperature of 850-910 ° C and water cooled to room temperature at a cooling rate of 10-50 ° C / sec, followed by a soaking treatment at 150-350 ° C.

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

The C is a major reinforcing element of steel, the content of which is increased by increasing the hardenability and solid-solution strengthening effect, but the weldability and toughness are deteriorated. It is desirable to limit it to 0.2%.

The Mn increases the hardenability and the solid-solution strengthening effect to increase the strength, but when added excessively, it is preferable to limit the content to 0.5-1.5%.

The Si is a component added as a deoxidizer to improve the internal quality, but as the content thereof increases, the yield strength increases, but it is desirable to limit the yield to 0.15-0.35% because it increases the ductility-brittle transition temperature of the steel, thereby deteriorating toughness and harmful to weldability. Do.

As P and S are inevitable impurity elements in the steelmaking process, lowering the content leads to an increase in manufacturing cost, but it is desirable to limit P to 0.02% and S to 0.008% or less, respectively, because they are harmful to toughness.

The Cr is an essential element for improving the hardenability of steel, and as the amount thereof is increased, the hardenability increases but it impairs weldability. Therefore, the Cr is preferably limited to 0.3-1.0%.

B is known as an element which greatly improves the hardenability even when a trace amount is added. In the case of the present invention in which the content of Cr is low and expensive Ni and Mo are not added as compared with general high tensile strength steel, the addition of B is essential for pictorial hardening. At least 0.001% must be added to secure the hardenability, and if the content is greatly increased, the amount of addition is limited to 0.001-0.004% because the compound such as BN can be formed to damage the toughness and reduce the hardenability. It is desirable to.

The Ti is added to maximize the effect of B on the hardenability, when solid solution N is present in the steel, it is combined with B to form a BN compound, and as a result, the effect of B on the hardenability disappears. Therefore, it is added for the purpose of removing solid solution N in steel by forming TiN by adding Ti whose affinity with N is stronger than B. As the amount of Ti is increased, the effect of B can be maximized. However, if the content is excessively increased, nozzle clogging may occur during casting, and inclusions such as TiC ₂ S ₂ may be formed to damage toughness. It is desirable to limit it to 0.04%.

The Nb has an effect of increasing the rolling productivity in the case of applying the control rolling by increasing the austenite fine recrystallization temperature, and is an essential element when performing the control rolling, and also has a significant effect on the austenite grain refining during the reheating and heat treatment. However, the weldability worsens as the amount is increased, so the content is preferably limited to 0.01-0.04%.

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

It is necessary to heat the slabs made as described above to 1150-1300 ° C, because when the heating temperature is below 1150 ° C, the deformation resistance during rolling causes an excessive rolling load, and above 1300 ° C, This is because the abnormal growth causes tissue non-uniformity, and as a result, the toughness is not only impaired, but also an increase in the amount of solid solution N in the steel can lead to a decrease in strength due to insufficient hardenability.

In order to sequentially perform hot rolling, direct quenching, reheating quenching, and annealing of the heated slab as described above, it is necessary to refine the austenite grains during the reheating quenching treatment by controlling the respective conditions, and the reason is as follows. . This is because it is necessary to refine the effective grains of martensite in order to simultaneously improve the strength and toughness of the martensitic steel of a given alloy component or to increase the toughness without sacrificing strength. To this end, it is necessary to refine the austenite grains during reheating and annealing heat treatment. (Source-Author: KA Taylor et al. 2, Signature: Physical Metallurgy of Direct-Quenched Steels).

First, the hot slab heated as described above is hot-rolled, where it is necessary to hot-roll the condition that the cumulative reduction rate in the unrecrystallized temperature range is 40% or more, and the reason is as follows. The finer the structure before reheating annealing and the more dislocations, the higher the driving force and number of nucleation sites of the martensite-austenite transformation during reheating annealing. You lose. If the cumulative reduction ratio is less than 40% in the unrecrystallized temperature range, not only the processing potential structure induced by austenite can be secured, but the micronizing effect of the structure is not so great that the austenite grains cannot be refined during the reheat-heat treatment. .

After the hot rolling as described above, the water is cooled. At this time, the holding time until the start of the water cooling is sufficiently secured to control rolling (unrecrystallized rolling) effect, and if the holding time is too long, it is determined by recrystallization of austenite. Since the control rolling effect is halved, it is preferable to limit it to 30 seconds or less. As mentioned above, 5-50 degreeC / sec is preferable for the cooling rate which starts cooling. The reason for this is that when the cooling rate is 5 ° C / sec or less, generation of fine martensite is not sufficient, coarse bainite and ferrite are generated a lot, and dislocation density decreases. Therefore, the increase of the cooling rate is advantageous in terms of miniaturization of the metamorphic tissue and the increase of dislocation density, but a high plate speed of 50 ° C / sec or more causes severe plate deformation.

After cooling to room temperature as described above, it is necessary to reheat in the austenitic single-phase region (above the Ac ₃ temperature), wherein the temperature range needs to be limited to 850-910 ℃. The reason is that lower heating temperature is advantageous in terms of austenite grain refining, but below 850 ℃, austenitization is insufficient, and the strength can be reduced.At 910 ℃ or higher, the austenitic grain coarsening is promoted, so that martensite is effective. This is because grains increase, resulting in a decrease in toughness. The steel sheet heated as described above is water-cooled, wherein the cooling rate is preferably 10-50 ° C / sec. The reason is that the faster the cooling rate is advantageous in terms of securing strength by securing martensite structure, but faster than 50 ℃ / sec causes severe plate deformation. In addition, when the cooling rate is slower than 5 ° C / sec, soft tissues such as upper bainite or ferrite are generated to cause a decrease in strength.

After cooling to room temperature as described above, it is treated in a temperature range of 150-350 ℃. Consideration temperature is an important factor for strength and toughness. If the temperature is above 450 ℃, toughness increases, but strength decreases rapidly due to cementite formation, recovery and recrystallization. As a result, it is impossible to secure strength of 140kgf / mm2. On the other hand, a sudden decrease in strength can be avoided in the temperature range of 350-450 ° C., but the upper limit is preferably limited to 350 ° C. because the soot martensite embrittlement occurs and both strength and toughness decrease. If the temperature is too low or the temperature is too low, the lower limit is 150 ° C because it is known to have high susceptibility to breakage due to stress crossion cracking, which is known as delayed fracture during use. desirable.

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

EXAMPLE

The slabs formed as shown in Table 1 are sufficiently heated at 1150-1300 ° C., and the mechanical properties of the specimens prepared by rolling and heat treatment under the conditions of Table 2 are shown in Table 3 below. In addition, the tensile strength-impact toughness relationship for each specimen was measured and the results are shown in FIG. 1. And the optical microscope microstructure was observed about the invention material (a, b), the comparative material (1), and the comparative material (5), and the result is shown in FIG.

Steel grade Chemical component (% by weight) C Mn Si P S Cr Ti Nb B Sol.Al Fe Invention steel 0.16 1.0 0.31 0.002 0.003 0.61 0.031 0.032 0.0025 0.025 Remainder

Psalm Number Pressure drop in unrecrystallized temperature range (%) Cooling method after rolling Reheating quenching temperature (℃) Consideration temperature (℃) Way Speed (℃ / sec) Invention a 57 Water cooling 32 910 250 b 57 Water cooling 32 850 250 Comparative material One 0 Air cooling 1.9 910 250 2 0 Water cooling 33 910 250 3 57 Air cooling 1.9 910 250 4 57 Water cooling 32 970 250 5 0 Air cooling 1.9 850 250 6 0 Water cooling 33 850 250 7 57 Water cooling 32 910 350

Mechanical Properties Yield strength (kgf / ㎡) Tensile strength (kgf / ㎡) Elongation (%) BHN Hardness Value Impact toughness 20 ℃ -40 ℃ Invention a 98.5 143.3 14.9 406 63.6 40.3 b 98.8 145.8 15.4 409 60.8 42.1 Comparative material One 96.0 141.5 15.4 402 45.7 30.7 2 95.0 141.1 14.7 399 48.3 31.8 3 97.3 141.9 14.8 403 51.6 32.2 4 99.3 140.8 12.7 397 54.8 33.8 5 94.5 135.0 15.1 359 64.3 43.2 6 91.3 130.8 13.7 382 62.9 44.8 7 96.8 135.2 15.3 378 45.6 21.2

For reference, in order to use the BHN400 wear-resistant steel sheet, the BNH hardness value must be 400 or more, which corresponds to the tensile strength of 140kgf / mm2 according to ASTM-E-140 standard.

First, as shown in Table 3, the invention material (a, b) prepared by the present invention was prepared by a conventional method (ie hot-rolled and air-cooled, then reheated and annealed at 910 ℃ and soaked at 250 ℃ Korean) The strength is higher than that of the comparative material (1), which satisfies the strength and hardness requirements of BHN 400 wear-resistant steel sheet, and the impact toughness at -40 ℃ is compared with the conventional method. It can be seen that 30% more than ash (1).

On the contrary, the comparative material which was hot-rolled and directly annealed by a conventional method and then reheat-annealed at 910 ° C. and the comparative material that was controlled-rolled and air-cooled at 910 ° C. after being controlled-rolled and air-cooled at 910 ° C. 3) is similar to the comparative material (1) in terms of strength, and satisfies the requirements of the BHN grade 400 anti-wear steel sheet, but the impact toughness at -40 ° C also shows the level of the comparative material (1). In addition, the comparative material (4) prepared by the same method as the invention material (a, b) and only the reheating quenching temperature was raised to 970 ℃ has no significant improvement in both strength and toughness, and in the same way as the invention material (a) After the rolling and reheat quenching heat treatment, the comparative material 7 treated at 350 ° C. was lower than the comparative material 1 in both strength and toughness. In addition, the comparative material (5) manufactured by the conventional method and lowered only the reheating quenching temperature to 850 ° C., and the comparative material (6) which was hot rolled and directly quenched by the conventional method and lowered only the reheating quenching temperature to 850 ° C. was an invention material ( Impact toughness similar to that of a and b), but the strength was significantly lower.

On the other hand, as shown in Figure 1, according to the present invention to control the microstructure by performing control rolling and direct quench before reheating and reheating the method of controlling the microstructure once again by reheating at a slightly lower temperature than conventional methods Inventive materials (a, b) produced by the present invention shows a superior strength-toughness relationship than the comparative material (1) manufactured by the conventional method. In addition, the inventive material (a, b) is a comparative material manufactured by a method deviating from one or more conditions among the control rolling, direct quenching, reheating and quenching temperature of 850-910 ° C, and the soaking temperature of 150-350 ° C. Compared with (2-7), the strength-toughness relationship is excellent.

This is analyzed from the metallurgical point of view.

The reheating and annealing strength of martensitic steels are strengthened by solid solution of alloy element, dislocation strengthening by dislocations formed during martensite transformation, precipitation precipitation by various kinds of carbides formed during annealing treatment, and finally lath and packet ( It is determined by Hall-Petch strengthening by martensite effective grain refinement (source-author: A. Kelly and RB Nicholson, signature: Strengthening Mechanism in Crystal). The increasing reinforcing mechanism is known to be strengthened by martensite effective grain refining, and the remaining reinforcing mechanisms are known to decrease toughness with increasing strength. On the other hand, according to a theory published in Transaction ASM (vol.59.26 page, author: RA grange) in 1966 and proven by theories after that, in order to refine martensite effective grains, it is necessary to refine the austenite grains before metamorphosis. The finer nit grains improve both the strength and toughness of martensite after transformation.

First, if the reheat quenching treatment condition and the sour treatment condition were compared with the same invention material (a) and the comparative material (1-3), the same reheat quenching was performed, so that the potential enhancing effect was the same, and the soaking treatment was also performed at the same temperature. Although the solid solution strengthening effect and the precipitation strengthening effect are also the same, the inventive material (a) is superior in both strength and toughness to the comparative material (1-3), for the following reasons. In the case of the inventive material (a), the control rolling and direct quenching processes were applied to refine the tissue before reheating and to contain many dislocations, thereby increasing the driving force and nucleation position number of the austenite transformation during reheating. It is a refined grain of austenite. This can be confirmed by looking at the microstructure shown in FIG. 2. That is, as shown in Figure 2 (a, b), the invention material (a) produced by the present invention has a very fine austenite grains of about 11㎛ after reheating firing (Fig. 2a). On the other hand, it can be seen that the austenite grains of the comparative material 1 prepared by the conventional method are larger (16 μm) (FIG. 2B).

In addition, the comparative material 4, which was rolled and cooled in the same manner as the inventive material (a, b) and reheat-annealed at 970 ° C., showed similar strength-toughness as that of the comparative material 1, and controlled rolling and direct quenching. Appeared to disappear. This is due to the increase in the size of the austenite grains with increasing reheating temperature, which is the coarsening of particles by Oswald Ripening, which is well known metallurgically.

In addition, the comparative material 5 which has been hot rolled and cooled in the air by a conventional method and reheated and treated at 850 ° C., and the comparative material 6 which has been hot-rolled and water cooled by a conventional method and then reheated and treated at 850 ° C. Toughness similar to that of the inventive materials (a, b) was shown, but the strength was remarkably low (FIG. 1). This is due to the fact that the reheating and quenching temperatures are too low to make austenitic grossly. This can be confirmed by looking at the microstructure shown in Figure 2c. That is, the comparative material 5 can be seen that very small reheated austenite is formed after reheating and sintering, while the coarse rolled structure remains considerably, resulting in a very uneven structure. It is known that when such a structure arises, hardenability will be inadequate and strength will fall.

On the other hand, the inventive material (b), which was reheated at 850 ° C., lower than the normal reheating temperature, like the comparative materials (5, 6), had lower strength and toughness than the inventive material (a) which was open-heated at 910 ° C. It is further shown to increase (Fig. 1). This is due to the effect of lowering the austenite transformation temperature (Ac ₃ ) by performing a reheating heat treatment from a structure containing many dislocations by performing control rolling and direct quenching. This can be confirmed by looking at the microstructure shown in FIG. 2D. That is, it can be seen that the inventive material (b) is completely austenitized to show a very uniform austenite structure and the grain size becomes fine to about 8 μm.

In addition, it was found that both the strength and toughness of the comparative material 7 produced by the same rolling, cooling, and reheating quenching as the invention material, but with different final treatment, were not as good as those of the invention material (FIG. 1). In general, martensite tends to decrease in strength while increasing toughness, but increases in toughness, but simultaneously decreases in strength and toughness around 350-450 ° C, the temperature at which cementite is formed. This is known metallurgical sour martensite embrittlement phenomenon and this is the cause of low strength and toughness of the comparative material (7).

As described above, in the case of the present invention materials (a, b), the microstructure before reheating and heat treatment was refined by the control rolling and direct quenching processes, and the driving force of the austenite transformation during reheating heat treatment was increased by securing a large number of dislocation structures. The reheating austenite grains were refined by the increase of and austenite transformation temperature (Ac ₃ ), resulting in more than 30% increase in toughness while slightly increasing the strength compared to the comparative material (1) manufactured by the conventional method. Could know.

As described above, according to the present invention, since the toughness can be improved by 30% or more without adding an alloying component to the conventional steel, the toughness can be improved by 30% or more. It is effective to produce economically.

Claims (1)

In the manufacturing method of high tensile steel sheet, By weight% C: 0.13-0.20%, Mn: 0.5-1.5%, Si: 0.15-0.35%, P: 0.02% or less, S: 0.008% or less, Cr: 0.3-1.0%, B: 0.001-0.004%, The slab of Ti: 0.01-0.04%, Nb: 0.01-0.04%, Sol-Al: 0.02-0.06%, and the remaining Fe and other unavoidable impurities was heated at 1150-1300 ° C. to obtain a cumulative reduction in the uncrystallized temperature range. After control rolling under the condition of% or more, cooling starts within 30 seconds, cooled to room temperature at a cooling rate of 5-50 ° C./sec, and then reheated to a temperature of 850-910 ° C. to 10-50 ° C./sec. Water-cooled to room temperature at a cooling rate, and then a method of producing a high tensile strength steel sheet characterized in that the processing at 150-350 ℃.

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