KR20000041271A - Method for producing super high hardness steel - Google Patents

Abstract

PURPOSE: A method is provided to economically produce a steel having excellent abrasion resistance, super high hardness, smooth surface and inner quality. CONSTITUTION: A molten steel is composed of 0.48 to 0.53wt% of C, 0.15 to 0.30wt% of Si, 0.80 to 1.00wt% of Mn, less than 0.020wt% of P, less than 0.005wt% of S, 0.85 to 1.05wt% of Cr, 0.15 to 0.25wt% of Mo, less than 2ppm of H and the remain of Fe and incidental impurities. The molten steel is continuously cast and secondly cooled by spraying 0.32 to 0.40liter/kg of water to be a casting piece. The casting piece is reheated in a temperature of 1100 to 1280°C in a heating furnace. The casting piece is also depressed in a roughing mill to be a thick steel strip. The steel strip is in a temperature of 550 to 600°C for 48 to 72hours and cooled with air. Then, the strip is heated in a temperature of 820 to 850°C and tempered in a temperature of 140 to 180°C to be a super high hardness steel.

Description

Ultra Hard Steel Manufacturing Method

The present invention relates to a method for manufacturing ultra-high hardness steel used for mechanical parts such as tramway tracks requiring surface abrasion resistance after heat treatment, and more specifically, Brinell hardness (HB600) class having ultra-high hardness, excellent wear resistance and excellent internal and surface quality. It relates to a method of manufacturing ultra-high hardness steel.

In general, when the carbon content is 0.40% C or more, the occurrence of cracks during quenching becomes high, and if there is a problem in the internal quality of segregation, internal cracks, etc., the sensitivity of the cracks is increased.

This is due to the difference in surface and internal tissues due to rapid heating and quenching during heat treatment, and the volume change causes stress. Especially, if cracks remain, it acts as a starting point for stress and the degree of stress is generated. It may be exceeded, in which case a crack will occur.

Generally, in order to secure high hardness, the amount of C added is increased and Cr and Mo are added to improve the quenching property, but the quenching property must be satisfied, and the crack generation should not occur during the heat treatment.

However, in the case of increasing the amount of C added as described above and securing high hardness by adding Cr and Mo, segregation is caused by addition of a large amount of C, which is easy to segregate, and cracks are generated on the surface of the cast steel by addition of Cr and Mo. There will be a factor.

On the other hand, it is well known that the continuous cast material (soft cast material) is a problem in the quality of the center of thickness, it is the key to the manufacturing conditions to secure the internal quality and surface quality at the same time.

At present, the most commonly used method of manufacturing HB 600 grade orbital steel is manufactured from ingots processed at forging ratio 4S or higher after ingot casting in terms of securing internal quality.

However, this process has been able to obtain a high internal quality, but in terms of production, the manufacturing cost is increased by going through many processes, and as a result, there is a problem that the product cost increases.

MEANS TO SOLVE THE PROBLEM The present inventor made the research and experiment in order to solve the said trouble of the prior art, and based on the result, this invention proposes the present invention, The present invention is the continuous casting of ultra-hard steel (CONTINIOUS CASTING) process-thick plate rolling It is an object of the present invention to provide a method of manufacturing ultra-high hardness steel, which can more economically produce steel having excellent surface and internal quality, excellent wear resistance and ultra high hardness, by manufacturing by a process-heat treatment process.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is a method for producing ultra-high hardness steel, in weight%, C: 0.48-0.53%, Si: 0.15-0.30%, Mn: 0.80-1.00%, P: 0.020% or less, S: 0.005% or less, Continuous casting of molten steel composed of Cr: 0.85-1.05%, Mo: 0.15-0.25%, H: 2ppm or less, remaining Fe and other unavoidable impurities, and spraying water at a water spray of 0.32-0.40ℓ / kg during continuous casting After making the cast by secondary cooling, after multi-stage air-cooling, reheat the cast to 1100-1280 ℃ in the heating furnace, and then at least 1 pass in roughing mill (RM) at low speed to 0.7 or more After rolling down the thick plate to produce the thick steel sheet,

After maintaining for 48-72 hours at a temperature of 550-600 ℃, air-cooled, and then quenched at 820-850 ℃, and then treated at 140-180 ℃ to produce a very hard steel.

Hereinafter, the reason for limiting the composition of the composition, the operation operation control and the heat treatment condition will be described.

The carbon (C) is an element that increases hardness and strength by increasing the hardenability during heat treatment of steel, and when the amount is too large, the hardenability becomes too high and causes cracks, and when the amount is too small It is preferable to limit the carbon amount to 0.48-0.53% because the hardenability is not low to secure the hardness.

The silicon (Si) forms a carbide and is dissolved in Fe to increase the elastic limit tensile strength and to improve the surface state of the slab with deoxidation element, and when added in a large amount, the ferrite (FERRITE) structure and the formation of non-metallic inclusion (SILICATE) toughness When the amount is added in small amounts, it is difficult to expect a sufficient deoxidation effect and adversely affect the surface quality. Therefore, the amount of silicon is preferably limited to 0.15-0.30%.

The manganese (Mn) contributes to the hardness improvement during heat treatment as an element for improving the hardenability, and when the excessive amount is added, the weldability is impaired, and when the small amount is added, the hardness is unstable due to the decrease in the hardenability. It is desirable to.

Since the phosphorus (P) forms a hardened structure that is fragile in the segregation door at the center of the steel sheet, the internal impurities are deteriorated as the largest impurity that impairs low-temperature impact toughness, so the content thereof is preferably limited to 0.020%.

The sulfur (S) is a harmful element, such as phosphorus (P) component to form the MnS inclusions in the center of the thickness in the thick plate product, the MnS inclusions are elongated during rolling to cause deterioration of internal quality, so the sulfur ( The content of S) is preferably limited to 0.005% or less.

The Cr increases the hardness and strength as an effective element for hardening, which is detrimental to weldability when a large amount is added, and is disadvantageous for securing hardness when a small amount is added, and therefore, the content thereof is preferably limited to 0.85-1.05%.

The Mo prevents embrittlement generated when the temperature is maintained at a temperature above 500 ° C as well as improving the strength by depositing a dense MoC compound in the grain boundary and in the grain by combining with carbon during the solidification and transformation of steel, and also improves the hardenability. As an element to improve the strength, since a large amount may cause coarse brittleness due to coarsening and excessive precipitation, the amount is preferably limited to 0.15-0.25%.

The H is an element that causes hydrogen to form hydrogen molecules after precipitation in inclusions or pores in an atomic state, and causes cracks due to an increase in internal pressure, and the content thereof is preferably limited to 2 ppm or less.

In the present invention, after continuously casting a steel having a composition range as described above in the appropriate conditions to produce a cast steel, and then rolled the plate in a suitable thick plate rolling conditions, the steel plate is maintained at a constant temperature section, and then air-cooled, This will be described as follows.

That is, in the related art, high internal quality was obtained through ingot, but the present invention controls air cooling of the secondary cooling method and casting during combustion casting in order to secure excellent internal quality in continuous casting.

In other words, the secondary cooling during the combustion casting is water-cooled to the surface of the cast steel, the intermediate cooling is preferably selected in the range of 0.32-0.40L / kg.

When the water spray amount exceeds 0.40ℓ / kg, a large amount of Cr and Mo, which is a hardenable alloy element, is contained in the slab (casting) due to quenching during the playing operation, and there is a fear of cracking, and the water spray amount is 0.32ℓ. In case of less than / kg, there is a possibility that impurities inside the slab may segregate in the center.This is due to the increase of molten steel flow rate at the end of the solidification, and the thickening steel existing between the dendrites is ejected to promote the center segregation as well as the bulging. This is because it causes internal cracks. These factors are not preferable because the final product may be defective due to cracks during stress caused by heat or tissue deformation during heat treatment.

After continuous casting as described above, the air is cooled in multiple stages in order to diffuse hydrogen, which is a harmful gas remaining inside, in the slab state.

In the present invention, any means other than multi-stage stack air cooling may be applied as long as it can diffuse hydrogen, which is a harmful gas remaining inside the slab.

When the slab manufactured as described above is subjected to normal thick plate rolling, hydrogen remaining inside the product diffuses along the grain boundary of the stretched inclusions while the inclusions are elongated in the rolling direction during the thick plate rolling. There is.

Therefore, in the present invention, to accelerate the diffusion of hydrogen, at least one pass should be subjected to low-speed dropping, which is to promote the release of non-diffusing hydrogen to the outside by compressing the porosity remaining in the center. .

In the present invention, at the time of the rolling under reduced pressure, one or more passes must be performed so that the aspect ratio obtained by the following formula (1) is 0.7 or more.

[Where r is the radius of the rolling roll (mm)

h0: entry slab thickness

h1: exit slab thickness]

After rolling the thick plate as described above, cooling should be delayed at a high temperature as much as possible so that hydrogen is sufficiently diffused in the slow cooling furnace and released to the outside.

That is, since hydrogen has a large diffusion coefficient at a temperature of about 550-600 ° C., it is preferable to charge it in a slow cooling furnace immediately after rolling, and to maintain it at a temperature of about 550-600 ° C. for 48-72 hours, followed by air cooling.

Briquette hardness (HB) 600 grade ultra-high hardness having super high hardness, excellent wear resistance, and excellent surface and internal quality by performing hardening treatment at 820-850 ° C. and then hardening at 140-180 ° C. Steel is produced.

The quenching treatment is heated to 820-850 ℃ the temperature to become austenite, and after holding for a certain time water cooled to around 300 ℃ it is preferable to air-cooled afterwards to prevent cracking.

If the quenching temperature is too low during the quenching treatment, sufficient quenchability cannot be secured, and if the quenching temperature is too high, it is not preferable because coarse grains are coarsened, so the quenching temperature is preferably set at 820-850 ° C.

The soaking treatment is performed for imparting toughness. If the soaking temperature is low, it is difficult to secure toughness. If it is too high, it is difficult to secure the target hardness or toughness due to precipitation of carbides at the interface or recovery of dislocations. It is preferable to select 140-180 degreeC.

In the case of cutting the thick plate prepared as described above before the quenching treatment, in order to minimize thermal shock during cutting, the thick plate is preferably preheated to a temperature of 200-300 ° C. and then cut and then treated with an anti-shrink treatment. Do.

In addition, in the present invention, when the thick plate is cut to a predetermined size before the quenching treatment, the plate is maintained at about 550-600 ° C. for 48-72 hours, and then air-cooled to 200-300 ° C. in the air cooling process, and then cut. Then, it is preferable to carry out the soaking treatment.

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

Example 1

After dissolving in the converter to have the composition as shown in Table 1, after the furnace refining process to continuously cast to prepare a cast steel having a cross-sectional thickness of 203mm, these were rolled respectively to finally produce a 44, 45 mmt thick plate. The manufactured plate is subjected to heat treatment by dividing the plate into a holding material that is maintained in a slow cooling furnace and an unmaintained material that is not maintained in a slow cooling furnace. The specimens are collected from the heat-treated plate and subjected to a mechanical test. The results are shown in Table 2 below.

In the case of the holding material to be maintained in a slow cooling furnace in Table 2 it was maintained at 590 ℃ 62 hours.

Evaluation of the mechanical properties and internal quality shown in Table 2 was based on MIL-A-12560H presented in the US Navy standards, for Rockwell hardness (HRc) 55-60, surface No Crack for 44-45mmt track material If satisfied, it was evaluated to have desirable properties.

Steel grade Chemical composition (% by weight, but H is ppm) C Si Mn P S Cu Cr Mo V H Conventional Steel 1 0.511 0.24 0.93 0.008 0.033 0.20 1.05 0.20 0.021 Conventional Steel 2 0.515 0.25 0.88 0.010 0.030 0.23 1.01 0.20 0.020 0.14 Inventive Steel 1 0.496 0.26 0.89 0.014 0.002 - 0.91 0.18 - 0.12 Inventive Steel 2 0.501 0.24 0.90 0.015 0.003 - 0.95 0.20 - 0.16 Invention Steel 3 0.507 0.25 0.89 0.014 0.002 - 0.93 0.18 - 0.15

Example Steel grade Control condition Heat treatment Hardness (HRc) Crack occurrence (surface / inside) Cooling performance (ℓ / kg), whether appropriate Heavy plate rolling process, maintenance in slow cooling furnace Hardening temperature (℃) x time (Hr) Consideration temperature (℃) x time (Hr) Requirements - - - - 55-60 No crack Conventional Materials 1 Conventional Steel 1 0.60, ○ General Rolling, × 840 x 1.2 200 x 2.0 53.1 Yu / Yu Conventional material 2 0.60, × Low Speed Rolling, × 790 x 1.2 150 x 2.0 55.1 No / Yes Conventional Materials 3 Conventional Steel 2 0.50, ○ Low speed rolling, ○ 840 x 1.2 200 x 2.0 53.7 Nothing Conventional Materials 4 0.50, × General Rolling, × 840 x 1.2 400 x 2.0 46.2 No / Yes Invention 1 Inventive Steel 1 0.36, ○ Low speed rolling, ○ 850 x 1.2 150 x 2.0 58 Nothing Invention 2 Inventive Steel 1 0.40, ○ Low speed rolling, ○ 840 x 1.2 160x2.0 56 Nothing Invention 3 Inventive Steel 2 0.38, ○ Low speed rolling, ○ 850 x 1.2 180 x 2.0 55 Nothing Invention 4 Inventive Steel 2 0.36, ○ Low speed rolling, ○ 830 x 1.2 170x2.0 56 Nothing Invention 5 Invention Steel 3 0.32, ○ Low speed rolling, ○ 820x1.2 140x2.0 60 Nothing

As shown in Table 2, the conventional materials (1), (2), and (4) to which the general rolling is applied, the CRACK appeared on the surface or inside, the quenching temperature of 790 ℃ material (conventional material) It showed the level near the pass or the lower limit, and the crack did not meet the requirements, and even when the sour temperature was applied at 200 and 400 ° C (prior materials (3) and (4)), the hardness was lower than the required value. It can be seen that.

On the other hand, in the case of the invention material (1-5) according to the present invention it can be seen that it shows a level satisfying all the requirements.

As described above, the present invention manages the slab quality through the operation control conditions in the appropriate C, Si, Mn, Cr, Mo, and low P, S component design and performance in steel, and the rolling, slow cooling and heat treatment conditions in the thick plate It is possible to economically manufacture steel that can secure super high hardness, excellent wear resistance and good surface and internal quality through control. Therefore, mechanical parts manufacturing field such as tramway track requiring high hardness of about 600 Brinell hardness and excellent wear resistance. There is an effect that can be applied more effectively to.

Claims (3)

In the method for producing ultrahard steel, in weight%, C: 0.48-0.53%, Si: 0.15-0.30%, Mn: 0.80-1.00%, P: 0.020% or less, S: 0.005% or less, Cr: 0.85 -1.05%, Mo: 0.15-0.25%, H: 2ppm or less, continuous casting of molten steel composed of the remaining Fe and other unavoidable impurities, followed by secondary cooling by spraying water with a water spray amount of 0.32-0.40ℓ / kg during continuous casting After casting the cast steel by multi-stage air-cooling, reheat the cast steel to 1100-1280 ℃ in the heating furnace, and then lower the pressure drop to at least one pass in the roughing mill (RM) to 0.7 or more After the heavy plate rolling to produce a thick steel sheet, After maintaining 48-72 hours at a temperature of 550-600 ℃, air-cooled, then quenched at 820-850 ℃, ultra-hardness steel, characterized in that the treatment at 140-180 ℃ The method of claim 1, wherein when the thick steel sheet is cut before the quenching treatment, the steel sheet is preheated to a temperature of 200-300 ° C. and then cut. According to claim 1, If the thick steel plate is cut before the quenching treatment to maintain 48-72 hours at a temperature of 550-600 ℃, and then air-cooled to 200-300 ℃ in the process of air cooling and then cut Manufacturing method of ultra hard steel