KR100328042B1 - A Method of Manufacturing High strength steel - Google Patents

Abstract

PURPOSE: A method of manufacturing high strength steel sheets with a tensile strength of 150 kgf/mm¬2 for use in the manufacture of armor plate for armored motorcars by controlled rolling and direct quenching process is provided. CONSTITUTION: The passivation method of pickled stainless cold rolled steel sheets includes the steps of reheating a slab comprising C 0.14 to 0.18 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, Ni 2.5 to 3.5 wt.%, Cr 0.5 to 1.0 wt.%, Mo 0.15 to 0.45 wt.%, Nb 0.01 to 0.04 wt.%, Sol-Al 0.02 to 0.06 wt.% at 1150 to 1300°C; controlled-rolling the slab at an accumulation reduction ratio of greater than 50 % in nonrecrystallization region; water cooling the hot rolled steel sheet at a cooling rate of 10 to 50°C/sec to ambient temperature; tempering the cooled steel sheet at 150 to 350°C.

Description

A method of manufacturing high strength steel

The present invention relates to a method for manufacturing high tensile strength steel sheet used as armor material for land combat vehicles, and more specifically, to low temperature impact by applying a control rolling and direct quenching process to a chemical composition steel similar to conventional 125kgf / mm2 grade steel. It relates to a method of manufacturing a high tensile strength 150kgf / mm2 high tensile strength steel sheet.

In recent years, as the maneuverability improvement of land combat vehicles is recognized as important, vehicle weight reduction is inevitably required by increasing the strength of armor materials. In addition, high strength and high toughness of the material are required to improve the protection of the glove material.

Until now, the steel plate used for the combat vehicle armor material is a high tensile steel plate with a tensile strength of 125kgf / ㎜ class, the typical manufacturing method is as follows. By weight%, C: 0.15-0.20%, Mn: 0.1-0.4%, Si: 0.15-0.38%, P: 0.015% or less, S: 0.008% or less, Ni: 2.75-3.5%, Cr: 1.3-1.7% Slab composed of Mo: 0.35-0.45%, V: 0.03% or less and the rest of Fe and other unavoidable elements are sufficiently heated at 1150-1300 ° C., with a reduction ratio of 10-30% and 900 for each rolling pass. After hot rolling under the condition of rolling finish temperature above ℃, it is cooled by air and then reheated to 900 ℃ or higher, and then quenched and then treated at 350-600 ℃ to obtain high tensile steel with tensile strength up to 125kgf / mm2. MIL-A-12560G (MR), 31 August 1988).

However, the high tensile strength steel sheet provided by the conventional method has a problem that the strength characteristics are low in view of the characteristics of the material required for the combat vehicle. This problem can be increased without increasing the toughness of the conventional steel sheet by a method of increasing the alloying elements such as Ni, Cr, Mo, Co, etc., which is expensive with increasing C. However, this method is accompanied by a problem of lowering the weldability according to the increase in the carbon equivalent, as well as an increase in the production cost.

Therefore, the present inventors have conducted in-depth research and experiment on the method of simultaneously improving the strength and toughness without increasing the alloying elements in high tensile steel, and as a result of controlling the rolling and direct quenching conditions, the effective grain size and potential of the martensite were refined. It confirmed that the solution was possible by the reinforcement effect, and came to propose this invention.

That is, the present invention is to provide a method of manufacturing a 150kgf / mm2 high tensile strength steel sheet excellent in low-temperature impact toughness by applying a control rolling and direct quenching process of a steel almost similar to the conventional high-tensile steel.

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

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

Method for producing a high tensile strength steel sheet of the present invention for achieving the above object, by weight% C: 0.14-0.18%, Mn: 0.5-1.5%, Si: 0.15-0.35%, P: 0.02% or less, S: 0.008% The slab of Ni: 2.5-3.5%, Cr: 0.5-1.0%, Mo: 0.15-0.45%, Nb: 0.01-0.04%, Sol-Al: 0.02-0.06% and the remaining Fe and other unavoidable impurities is 1150 After heating at a temperature of -1300 ℃, controlled rolling under the condition that the cumulative reduction rate of 50% or more in the unrecrystallized temperature region, starting cooling within 30 seconds and cooled to room temperature at a cooling rate of 10-50 ℃ / sec It is then configured to include a soaking treatment at 150-350 ℃.

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 is increased to increase the hardenability and strength. On the contrary, if the content is decreased, the hardening steel is hard to secure the strength due to the decrease of hardenability and strength, so it is limited to 0.14-0.18%. It is desirable to.

The Mn also improves the hardenability to increase the strength, but when added excessively, the weldability is deteriorated, so it is preferable to limit the Mn to 0.5-1.5%.

The Si is a component to be added as a deoxidizer, but if its content is increased, the yield strength is increased, but it is preferable to limit it to 0.15-0.35% because it increases the ductile-brittle transition temperature, thereby deteriorating toughness and harmful to weldability.

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

Ni is an essential element of high tensile strength steel to improve the low temperature toughness by lowering the ductile-brittle transition temperature of the steel, but is preferably limited to 2.5-3.5% due to its high cost. When the added amount of Ni is 2.5% or more, the required toughness can be obtained, and when it exceeds 3.5%, it is uneconomical.

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. (Source-author: G. Krauss, signature: Principles of Heat Treatment of Steel), while Cr and Mo are known to impair weldability as their content increases. Therefore, in the present invention, which is characterized by treating at a temperature of 350 ° C. or less, Cr and Mo are added only to improve the quenchability. In this case, Cr is 0.5-1.0% and Mo is 0.15-0.45%. It is preferable to restrict to each.

Nb suppresses austenite grain growth during hot rolling and increases the recrystallization temperature to increase rolling productivity during controlled rolling, which is an essential element when performing controlled rolling, but the weldability worsens as its content increases. It is desirable to limit the content to 0.01-0.04%.

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 an increase in aluminum oxide in the steel, so it is preferable to limit the content to 0.02-0.06%. Do.

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 abnormal growth causes tissue non-uniformity, which in turn hurts toughness.

The slab heated as described above is hot rolled, and it is necessary to hot roll the slab under the condition that the cumulative reduction ratio in the unrecrystallized temperature range is 50% or more. The reason is that when the cumulative reduction ratio is less than 50% in the unrecrystallized temperature range, the improvement of low-temperature toughness is not only small due to insufficient refinement of the effective martensite grains transformed from austenite, and the processing potential induced from austenite. This is because organizations cannot be secured.

After hot rolling as described above, the water is subsequently cooled. At this time, the holding time until the start of water cooling is sufficiently secured to increase the strength and toughness by control rolling and direct quenching. Since the control rolling effect is reduced by recrystallization of knight, it is preferable to limit it to 30 seconds or less.

As mentioned above, 10-50 degreeC / sec is preferable for starting and cooling cooling. The reason is that the faster the cooling rate is advantageous in terms of securing the martensite structure and increasing the strength of martensite by uncrystallized rolling, but the higher the cooling rate above 50 ℃ / sec causes severe plate deformation. In addition, if the cooling rate is too slow, the strength is lowered due to the generation of soft tissues such as upper bainite, and the recrystallization rolling effect is reduced, thereby reducing the strength.

After cooling to room temperature as described above, it is treated in a temperature range of 150-350 ℃. The sour temperature is an important factor for strength and toughness. If the temperature is above 450 ℃, the toughness increases but the strength decreases rapidly due to the formation of cementite and the disappearance of the potential generated by controlled rolling. In the temperature range of 350-450 ° C, soot martensite embrittlement occurs and the toughness decreases rapidly. Therefore, the upper limit is preferably 350 ° C. If the soaking temperature is too low, the strength is excellent, but toughness is difficult to secure, so the lower limit is preferably 150 ° C.

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 yield strength-impact toughness relationship for each specimen was measured and the results are shown in FIG. 1. And the optical microscope microstructure of the comparative material 5 was observed in invention material (a) and the comparative material 2, and the result is shown in FIG.

Steel grade Chemical component (% by weight) C Mn Si P S Ni Cr Mo V Nb Sol.Al Invention steel 0.161 1.02 0.23 0.002 0.004 2.96 0.98 0.30 - 0.017 0.026 Conventional Steel 0.169 0.27 0.25 0.004 0.005 2.92 1.53 0.39 0.026 - 0.020

Psalm Number Pressure drop in unrecrystallized temperature range (%) Hardening Method Cooling rate (℃ / sec) Consideration temperature (℃) River bell Invention a 65 Direct Annealing After Rolling 35 200 Invention steel b 65 Direct Annealing After Rolling 35 300 Invention steel Comparative material One 65 Direct Annealing After Rolling 35 400 Invention steel 2 0 Direct Annealing After Rolling 35 200 Invention steel 3 0 Direct Annealing After Rolling 35 300 Invention steel 4 0 ^* Reheat quenching 35 200 Invention steel 5 0 ^* Reheat quenching 35 200 Comparative steel 6 0 ^* Reheat quenching 35 300 Comparative steel 7 0 ^* Reheat quenching 35 400 Comparative steel * Reheating quenching completes the normal rolling, and after reheating, quenching at a temperature of about 910 ℃

Mechanical Properties Yield strength (kgf / ㎡) Tensile strength (kgf / ㎡) Elongation Impact toughness 20 ℃ -40 ℃ Invention a 127.7 161.9 14,8 47.0 44.8 b 127.5 154.1 12.9 45.8 43.9 Comparative material One 125.5 145.6 12.5 42.9 36.8 2 100.9 140.0 14.9 50.4 46.5 3 106.5 140.6 14.2 52.6 44.5 4 111.0 136.8 15.9 62.0 49.9 5 108.4 135.6 15.1 66.0 59.0 6 106.8 131.4 14.8 66.2 33.2 7 101.2 128.6 15.7 64.1 29.4

For reference, the impact toughness at -40 ℃ should be more than 40joule to be used as armor material of land combat vehicle (MIL-A-12560G (MR), 31 August 1988).

First, as shown in Table 3, it can be seen that only the inventive material (a-b) manufactured by the present invention can guarantee the toughness at -40 ° C and satisfy the tensile strength of 150 kgf / mm 2 or more.

On the other hand, the comparative material (2-3) which manufactured the invention steel by the conventional rolling-direct quenching method, the comparative material (4) which produced the invention steel by the conventional rolling-reheating quenching method, and the conventional steel were rolled conventionally -The comparative materials (5-7) manufactured by the conventional method of reheating hardening did not satisfy the tensile strength of 150 kgf / mm 2 or more, and only the comparative materials (2-5) satisfied the impact toughness at -40 ° C.

On the other hand, as shown in Figure 1, the invention material (a, b) produced by the control rolling and direct quenching method according to the present invention is significantly superior in strength compared to the comparative material (4-7) produced by the reheat quenching method -The relationship was showing humanity.

From a metallurgical point of view, the first reason is that austenite grains that are severely deformed by recrystallization rolling encourage nucleation of martensite lath, resulting in the effective grain refinement of martensite (Source- Author: KA Taylor et al. 2, Signature: Physical metallurgy of Direct-Quenched Steels). The second reason is that the potentials formed by unrecrystallization rolling remain in martensite by direct quenching, thus allowing for additional strength increase.

Evidence for this can be confirmed by looking at the microstructure shown in FIG. 2. That is, as shown in Figure 2, the invention material (a) produced by the present invention shows that not only the austenite is severely deformed but also the deformed tissue remains after the martensite transformation (Fig. 2a). On the other hand, it can be seen that the case of the comparative material (5) manufactured by the reheat quenching method (Fig. 2c).

In addition, as shown in FIG. 1, the comparative materials (2, 3) in which the inventive steel was directly annealed after ordinary hot rolling without unrecrystallized rolling showed similar toughness as the inventive material, but the yield strength was 15-20 kgf / mm2. About low This is mainly due to the effect of direct quenching in the case of the comparative material (2-3) but insufficient effective grain refining and dislocation strengthening effect of martensite caused by simultaneous control rolling and direct quenching. As a proof of this, when looking at the microstructure of the comparative material 2 shown in Fig. 2 (Fig. 2b), it can be seen that the austenite was not largely deformed as well as very large as compared with the invention material.

And, as shown in Figure 1, it was found that in the case of the comparative material (1) manufactured by the same rolling and quenching process as the invention material, but only the final consideration treatment, both strength and toughness are not as good as the invention material. In general, martensite tends to decrease in strength while increasing toughness, but increases in toughness, but decreases in strength and toughness at around 350-450 ° C, the temperature at which cementite is formed. Martensite embrittlement is the cause of low strength and toughness of the comparative material (1).

In view of the above, in the case of the present invention (a, b), a significant increase in strength was achieved due to the processing potential remaining in martensite by a controlled rolling and direct quenching process, and an effective grain of martensite by controlled rolling and direct quenching. It can be seen that the increase in strength is achieved by suppressing the increase in strength as well as the decrease in strength due to the miniaturization of 20 kgf / mm 2 or more while exhibiting toughness similar to that of conventional steel.

As described above, the present invention can economically produce 150kgf / mm2 high tensile strength steel which can improve the strength by more than 20kgf / mm2 while showing toughness equivalent to that of conventional steel by fine-adjusting the component system of conventional steel and direct quenching after control rolling. It works.

Claims (1)

In the manufacturing method of high tensile steel sheet, By weight% C: 0.14-0.18%, Mn: 0.5-1.5%, Si: 0.15-0.35%, P: 0.02% or less, S: 0.008% or less, Ni: 2.5-3.5%, Cr: 0.5-1.0%, After slab consisting of Mo: 0.15-0.45%, Nb: 0.01-0.04%, Sol-Al: 0.02-0.06%, and the remaining Fe and other unavoidable impurities, the slab was heated at a temperature of 1150-1300 ° C., and then the cumulative pressure was reduced in the unrecrystallized temperature range. After control rolling under the condition that the rate is 50% or more, cooling starts within 30 seconds, cooled to room temperature at a cooling rate of 10-50 ° C./sec, and then treated at 150-350 ° C. for high tension steel sheet. Manufacturing method.

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