KR20100035829A - Method for manufacturing of quenched steel sheet, press mold used in the method - Google Patents

Abstract

PURPOSE: A manufacturing method of a thermal process curing steel sheet and a press mold which is applied in the same method are provided to improve workability by applying different cooling speed according to the processing amount. CONSTITUTION: A steel material is mold pressured by using a top and bottom mold. The cooling speed is differently set according to the processing amount(A). A plurality of cooling lines is formed on the top and bottom mold(b). The mixed-refrigerant of the oil and water is provided to a plurality of cooling lines(C). The tensile strength of the small processing amount part is 1400~1500MPa. The tensile strength of the large processing amount part is 1200~1400MPa.

Description

Method for manufacturing heat treated hardened steel sheet and press mold applied thereto {Method for manufacturing of Quenched steel sheet, press mold used in the method}

The present invention relates to a method for manufacturing a heat-treated hardened steel sheet and a press mold applied thereto, and more particularly, to a method for manufacturing a heat-treated hardened steel sheet and a mold applied thereto, which can be applied to a component having a large amount of processing.

Recently, the automotive industry is increasing the application of ultra-high strength steel sheet to meet the demand for safety and light weight. However, in the case of ultra-high strength steel sheet, the formability of the steel sheet is deteriorated, and thus the application to the automotive parts requiring processing into a complicated shape is limited.

Therefore, there is an increasing demand for a heat-curing hardened steel sheet which is capable of securing high strength by heating the steel sheet and pressing it in a high temperature state and then rapidly cooling the steel sheet.

Heat-treated hardened steel sheet contains more than 0.0005wt% of boron (B) and press-forms materials based on carbon (C) and manganese (Mn) at a high temperature of 900 ℃ or higher and rapidly cools them in a mold to increase the strength of parts. .

Since the heat-treated hardened steel sheet has a very high strength of 1,400 MPa or more, it is very excellent in terms of specific strength divided by the yield strength by density, which can greatly contribute to the weight reduction of automobiles, and it is difficult to form because there is almost no springback after processing. It is applied to ultra high strength demanding parts.

However, the conventional heat treatment hardened steel sheet has the following problems.

Heat-treated hardened steel sheet is processed into parts after it is extracted by heating in a molding apparatus called a press mold. In the case of a complex part such as a center piler of a vehicle, the temperature of a large amount of processing is processed during cooling. It cools rapidly compared to the small amount.

In this case, there is a problem that a wave is generated or bent in a part due to a difference in elongation due to a temperature difference between a portion having a large amount of processing and a portion having a small amount of processing after pressing. Therefore, it is difficult to freely implement the desired part shape.

The present invention is to solve the conventional problems as described above, the object of the present invention is to control the cooling rate of the large amount of processing during the press molding of the heat-treated hardened steel sheet to be able to freely implement a complex shaped parts without defects It is to provide a manufacturing method and a press die applied thereto.

According to a feature of the present invention for achieving the above object, the present invention contains boron (B) in the range of 0.025 ~ 0.0040wt%, carbon (C), manganese (Mn), chromium (Cr), silicon The steel based on (Si) is heated to a high temperature above the Ac3 transformation point and press-molded, and press-molded by using upper and lower molds having a plurality of cooling lines supplied with a water refrigerant or a mixed refrigerant of water and oil, By varying the cooling rate for the part with small amount of processing and the part with large amount of processing, the part with small amount of processing should have tensile strength of 1400 ~ 1500MPa and the part with large amount of processing.

In the press molding part, the cooling amount is supplied to the water coolant at a cooling rate of 200-300 ° C./s up to 150 ° C. or lower by the cooling line supplied with water refrigerant. Cooling at a cooling rate of 50 ~ 150 ℃ / s to 150 ℃ below the Ms temperature by a cooling line supplied with.

In the press mold to cool the material at the same time by using the upper and lower molds, the upper and lower molds are provided with a plurality of cooling lines, the cooling line is 100% water refrigerant or a mixed refrigerant of water and oil Is optionally supplied.

The cooling line bent at the upper and lower molds and provided at the portion of the steel sheet with a large amount of processing amount is supplied with a mixed refrigerant mixed with 50 to 80% of water and 20 to 50% of the oil. Water 100% refrigerant is supplied.

In the present invention, while forming a press, different cooling rates are applied to a portion having a small amount of processing and a portion having a large amount of processing to improve workability.

In addition, boron (B) is contained in the range of 0.025 to 0.0040 wt%, and the cooling rate is lowered by adjusting the alloy component system of carbon (C), manganese (Mn), silicon (Si), niobium (Nb), and chromium (Cr). Even high tensile strength is ensured.

Therefore, since the heat treatment hardened steel sheet having excellent workability and high strength is molded without defects, there is an effect of improving product productivity and reliability of the product.

Hereinafter, with reference to the accompanying drawings, a preferred embodiment of a method for manufacturing a heat-treated hardened steel sheet according to the present invention and a press die applied thereto will be described in detail.

Heat-curing steel sheet of the present invention is carbon (C) 0.19 ~ 0.40wt%, manganese (Mn) 0.5 ~ 2.5wt%, chromium (Cr) 0.1 ~ 0.5wt%, silicon (Si) 0.5wt% or less, phosphorus (P) 0.05 wt% or less, sulfur (S) 0.05 wt% or less, aluminum (Al) 0.03 wt% or less, and one of nickel (Ni), niobium (Nb), and copper (Cu) in steel having an alloy composition of the remaining iron (Fe) Species or more are contained in the basic composition, and boron (B), which is a hardenable element, is added in a range of 0.025 to 0.0040 wt%.

More specifically, boron (B) is contained in the range of 0.025 to 0.0040 wt%, and the steel based on carbon (C), manganese (Mn), chromium (Cr), and silicon (Si) is about 950 ° C, which is higher than Ac3 transformation point. After heating to a high temperature of fully austenitized, the steel sheet is press-molded into a product shape at a time in a press die, and different cooling rates are applied to a portion having a small amount of processing and a portion having a large amount of processing to improve workability.

Boron increases the hardenability of the steel sheet by delaying the transformation of austenite into ferrite during continuous cooling transformation, and secures the strength after quenching.

When boron content is lowered to less than 0.0025wt%, hardenability decreases, but it decreases further at cooling rate 30 ~ 150 ℃ / s. And if it exceeds 0.0040 wt%, the effect is saturated.

This is confirmed in the graphs of Figures 1 and 2, the boron content of 0.0040wt% or more has no effect of improving strength, and 0.0025wt% or less of the hardenability according to the cooling rate.

On the basis of this fact, for parts with a small amount of processing during press molding, cooling is carried out at a cooling rate of 200 to 300 ° C / s up to 150 ° C, which is below the Ms temperature. Cool at a cooling rate of / s. This is to prevent quenching due to the difference in elongation rate by preventing rapid cooling of a large amount of processing.

In the case of a small amount of processing, if the cooling rate exceeds 300 ° C / s, there is a risk of breakage due to subcooling, if the cooling rate is less than 200 ° C / s it is not possible to secure the desired tensile strength. In addition, in the case of a large amount of processing, if the cooling rate exceeds 150 ℃ / s, the fracture occurs due to the difference in elongation due to subcooling, if the cooling rate is less than 50 ℃ / s can not secure the desired tensile strength.

When press molding, 100% of water is basically used as the refrigerant in the cooling line for cooling the product, and oil is mixed to control the cooling rate. The oil to be mixed is an quenching oil that uses a water soluble mixture with water.

According to the experimental result of FIG. 2, when water and oil are mixed with the refrigerant of the cooling line during press molding, the cooling rate is slower than when water is used. However, in a steel sheet containing 0.025 wt% or more of boron (eg, 25 ppm), even if water and oil are mixed as a refrigerant in a cooling line, hardenability is not significantly lower than that of water.

As a result, water is supplied to the cooling line of the part with a small amount of processing so that the steel plate is cooled at a cooling rate of 200 to 300 ° C./s. If necessary, mix water and oil.

In addition, the press mold according to the present invention employs upper and lower molds, and a refrigerant is supplied to the cooling line side respectively provided in the upper and lower molds to cool the steel sheet.

Referring to the center pillar part shown in FIG. 3 as an example, 100% water is applied to the portion (a) of which the processing amount is small, and portions (b) and (c) where the processing amount and the large portion are mixed Apply 100% water or a mixture of water and oil.

That is, as shown in Figure 4, the cooling line is bent in the upper and lower molds, the cooling line provided in the portion of the processing amount of the steel sheet is supplied with a refrigerant (B) mixed with water 50 ~ 80%, oil: 20 ~ 50% In the cooling line provided at a portion where the processing amount is flat and low, 100% of water is supplied to the refrigerant A.

Hereinafter, the function and content of the alloying elements of the present invention will be described.

Carbon (C): 0.19 ~ 0.40wt%

Carbon is an indispensable element for imparting high strength to the steel sheet. And it is an important element to increase the hardenability of the steel sheet and determine the strength of the steel sheet after quenching. Such carbon stabilizes austenite and lowers the Ac 3 point to promote lowering of the quenching treatment temperature. If the content of carbon (C) is less than 0.19 wt%, the effect is not sufficient. If carbon (C) exceeds 0.40 wt%, toughness deterioration of the quenched portion becomes remarkable.

Manganese (Mn): 0.5-2.5 wt%

Manganese (Mn) inhibits perlite production and promotes austenite formation and carbon enrichment therein, contributing to the formation of residual austenite. And it has a function which raises the hardenability of a steel plate and ensures the strength of a steel plate stably after hardening.

Manganese requires industrially very fast cooling rate when added below 0.5wt%, preventing the production of pearlite.In addition, when manganese is added above 2.5wt%, manganese band structure is formed and segregation increases rapidly, which impairs workability and weldability of steel. do.

Chromium (Cr): 0.1-0.5 wt%

Chromium is an effective element to increase the hardenability of the steel sheet and to secure the strength after the hardening. However, if the content of chromium is less than 0.1wt%, the effect is not sufficient, and if the content is more than 0.5wt%, the effect is not increased.

Silicon (Si): 0.5 wt% or less, Phosphorus (P): 0.05 wt% or less, Sulfur (S): 0.05 wt% or less, Aluminum (Al): 0.03 wt% or less

These elements have the effect of increasing the hardenability of the steel sheet and stabilizing the strength after quenching. However, when these contents exceed each upper limit, the effect will be saturated.

Nickel (Ni): 2 wt% or less, Niobium (Nb): 0.10 wt%, Copper (Cu): 1 wt% or less

These elements have the effect of increasing the hardenability of the steel sheet and stabilizing the strength after the hardening. However, when these contents exceed each upper limit, the effect will be saturated.

The present invention includes the components of the alloy steel, the remainder is iron (Fe) and inevitable elements, and the elements contained in accordance with the situation of raw materials, materials, manufacturing facilities, etc. of the inevitable impurities such as nitrogen (N), oxygen (O) Fine incorporation is also allowed.

The steel slab having the composition as described above is obtained by ingot or continuous casting process after obtaining molten steel through the steelmaking process, and is subjected to a process of hot-rolled, cold-rolled, press-molded and heat-treated reinforced steel sheet. .

Press molding heats it to 950 degreeC or more which is more than Ar3, and presses it, and manufactures a part. Cooling takes place simultaneously with the press working.

Cooling is performed at a cooling rate of 200 ~ 300 ℃ / s up to 150 ℃ which is below Ms temperature for the part with small amount of processing, and at 50 ~ 150 ℃ / s to 150 ℃ below Ms temperature for the part with high processing amount. This prevents quenching of parts with high throughput.

Accordingly, a portion having a small amount of processing has a tensile strength of 1400 to 1500 MPa and a portion having a large amount of processing has a tensile strength of 1200 to 1400 MPa.

Hereinafter, the method of manufacturing the heat treatment hardened steel sheet described above will be described in detail with reference to Examples.

Table 1 shows the tensile test results for each cooling rate according to the boron content by dividing the present and conventional boron content into Comparative Examples and Inventive Examples.

division Boron content (wt%) Cooling rate (℃ / sec) Tensile Strength (MPa) Elongation (%) Inventive Example 0.0030 300 1550 6 100 1450 7 60 1350 9 Comparative Example 1 0.0015 300 1530 6 100 1320 7 60 1050 10 Comparative Example 2 0 300 1480 6 100 950 9 60 620 20

Looking at Table 1, it can be seen that after hot press molding according to the content of boron, it is divided into areas where tensile strength is secured by cooling rate and areas that are not.

That is, in the case of the present invention containing more than 0.0025wt% of boron, even if the cooling rate for each part is uniform, even tensile strength is secured, whereas in the case of Comparative Examples 1 and 2 in which the boron content is less than 0.0025wt%, the cooling rate is different. Uniform tensile strength tends not to be secured. In addition, when no addition of boron, it can be seen that the tensile strength is sharply reduced as in Comparative Example 2.

Within the scope of the basic technical idea of the present invention, many other modifications are possible to those skilled in the art, and the scope of the present invention should be interpreted based on the appended claims. will be.

1 is a graph showing the change in strength of the steel sheet according to the boron content and cooling rate.

Figure 2 is a graph showing the change in strength of the steel sheet according to the cooling rate when the refrigerant content of the cooling line in the boron content and press molding.

Figure 3 is a perspective view showing a center filler component for an example of the heat-curing steel sheet of the present invention.

4 is a configuration diagram showing a cooling line of a press die for hot press molding of the center pillar component of FIG.

Claims (4)

Contain boron (B) in the range of 0.025 ~ 0.0040wt%, and heat-press and press-form steels based on carbon (C), manganese (Mn), chromium (Cr), and silicon (Si) to a temperature higher than Ac3 transformation point. , Press molding using upper and lower molds with a plurality of cooling lines supplied with a water refrigerant or a mixed refrigerant of water and oil, thereby reducing the amount of processing by varying the cooling rate for a portion with a small amount of processing and a large amount of processing. Silver 1400 ~ 1500MPa, a large amount of processing is a method for producing a heat-treated hardened steel sheet, characterized in that it has a tensile strength of 1200 ~ 1400MPa. The method according to claim 1, In the case of the press molding, the portion with a small amount of processing is cooled by a cooling line supplied with a water refrigerant at a cooling rate of 200 to 300 ° C / s to 150 ° C below the Ms temperature, Method for producing a heat-treated hardened steel sheet characterized in that for the portion with a large amount of processing is cooled at a cooling rate of 50 ~ 150 ℃ / s to 150 ℃ below the Ms temperature by a cooling line supplied with a mixed refrigerant of water and oil. The upper and lower molds are employed in the press mold to cool the material at the same time as the hot working, The upper and lower molds are provided with a plurality of cooling lines, wherein the cooling line is 100% refrigerant, or a mixed refrigerant of water and oil is selectively supplied to the press mold. The method according to claim 3, The cooling line bent at the upper and lower molds and provided at the portion of the steel sheet with a large amount of processing amount is supplied with a mixed refrigerant mixed with 50 to 80% of water and 20 to 50% of the oil. Press mold, characterized in that the supply of water 100% refrigerant.

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