KR101062122B1 - Heat-hardening type hot press steel plate and manufacturing method thereof - Google Patents

Abstract

The present invention is to shorten the heat treatment process by using a heat treatment process for applying heat to the steel sheet material in the press mold in the process of manufacturing the hot press steel sheet.

According to the present invention, after heating the steel sheet to 700 ° or more, which is an Ar3 or higher temperature in a heating furnace, the steel sheet is supplied into the press mold and press-molded at a temperature of 600 to 800 ° C in the press mold, and then the cooling end temperature is in the range of 100 to 500 ° C. In the state that the mold is heated so that the heat treatment at the desired temperature is maintained until the phase transformation of the microstructure.

According to this, the present invention has a useful effect of shortening the manufacturing time of the hot press steel sheet and reducing the manufacturing cost.

In addition, the present invention can obtain a heat-hardening type hot press steel sheet having excellent fatigue characteristics when adopting a steel type added with boron, which is a hardenability securing element.

Description

Heat treatment hardened hot press steel sheet and its manufacturing method {Quenched hot-rolled press steel sheet and method for producing the same}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment hardening hot press steel sheet and a method for manufacturing the same, and more particularly, to a heat treatment hardening hot press steel sheet and a method for manufacturing the same, which are capable of eliminating a post heat treatment process by heating a steel sheet material to a press die. .

The steel industry is a representative infrastructure of the country, and is closely related to the industrial competitiveness of the country as well as the electronics, shipbuilding and automobile industries.

Among them, the automobile industry is an industry in which technologies in various fields such as machinery, materials, and electronics are integrated, and numerous companies are involved in the entire industry.

Therefore, the impact of the automobile industry on the domestic manufacturing industry has a greater ripple effect than any other industry, and the economy of the automobile industry is directly related to the domestic manufacturing industry.

However, there are many difficulties due to the strengthening of global environmental regulations and intensifying competition due to oversupply.

Among these, environmental regulation is the biggest problem faced by the automotive industry. Increasingly, international regulations on carbon dioxide emissions, which are environmental pollutants, are increasing. Inevitably, high fuel economy vehicles are required.

Therefore, the automobile industry is pushing for structural changes and material development for light weight, and applying hot press steel sheets (hot forming steel sheets) that have higher strength than general high strength press steel sheets.

The hot press steel plate developed for a vehicle is mostly used for a frame or an impact member of a vehicle, and the frame or impact member is for protecting a driver and a passenger in a vehicle crash, and thus impact toughness is required.

In addition, the hot press steel sheet is manufactured by pressing at a high temperature of about 800 ℃ through a hot press method, and then cooled to room temperature to obtain a martensitic structure having a tensile strength of 1,500 MPa, and then after tempering It adopts the method of going through several heat treatment processes such as improving the elongation through the heat treatment process and performing a cooling process to cool later.

However, the above-described process has a problem in that the production cost of the post-heat treatment process and the cooling is not only a cost increase factor, but also goes through several heat treatment processes, which takes a long time to produce and thus lowers productivity.

The present invention has been proposed in view of the above-mentioned problems, and its object is to heat the steel plate base material in the press during the manufacturing process of the hot press steel sheet using the hot press method, thereby to perform the post heat treatment process and the cooling process in the press. It is to provide a heat treatment hardened hot press steel sheet and a method for manufacturing the same that can be processed in the.

The present invention for achieving the above object is the step of supplying a steel sheet material heated in the heating furnace to the press mold,

Press molding the steel sheet material;

Maintaining a mold temperature at 100 to 500 ° C. such that the microstructure of the press-formed steel sheet is phase transformed;

Extracting the steel sheet in which the phase transformation is completed in the press die;

And scaling to remove the surface scale of the steel sheet extracted from the mold.

The steel sheet material is a weight%, contains carbon (C): 0.15 ~ 0.30% silicon (Si): 0.03 ~ 0.30%, manganese (Mn): 1.0 ~ 2.0%, boron (B): 0.0005 ~ 0.0040%, S: 0.003% or less, P: 0.012% or less, calcium (Ca): 0.0010-0.0040%, titanium (Ti): 0.005-0.1%, niobium (Nb): 0.0005-0.5%, sum of chromium and molybdenum (Cr + Mo): 0.1-2.0%, and the remainder has a composition of iron (Fe).

The heat treatment hardening hot press steel sheet which is another characteristic element of this invention is a weight%, Carbon (C): 0.15-0.30% Silicon (Si): 0.03-0.30%, Manganese (Mn): 1.0-2.0%, Boron ( B): 0.0005 to 0.0040%, S: 0.003% or less, P: 0.012% or less, calcium (Ca): 0.0010 to 0.0040%, titanium (Ti): 0.005 to 0.1%, niobium (Nb): 0.0005 to 0.5%, the sum of chromium and molybdenum (Cr + Mo): 0.1 to 2.0%, the rest of the steel sheet having a composition of iron (Fe) is heated in a heating furnace,

After the steel sheet heated in the heating furnace is press-molded in a press die, the tissue is phase-transformed while being heated to a temperature of 100 to 500 ° C. in the press die.

The present invention is to reduce the heat treatment process by using a one-pass heat treatment process in which the post-heat treatment step is omitted in the process of manufacturing a hot press steel sheet, according to the present invention is a hot press steel sheet It has a useful effect to shorten the manufacturing time and reduce the manufacturing cost.

In addition, the present invention can obtain a heat-hardening type hot press steel sheet having excellent fatigue characteristics when adopting a steel grade to which boron, which is a hardenability securing element, is adopted.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The steel slab is reheated to a temperature of 1200 ° C. or higher and maintained for 1-2 hours. After hot finishing rolling at an Ar 3 point or more and Ar 3 + 50 or less, the steel slab is wound at 500-700 ° C., and then subjected to a cold treatment at room temperature for 1 hour.

The hot rolled steel sheet is pickled and cold rolled at a reduction ratio of about 50% to produce a cold rolled steel sheet. The cold-rolled steel sheet may be used as it is, or surface treatment may be performed by hot dip galvanizing or electro galvanizing, hot dip galvanizing, polymer oxidation coating, or the like to prevent oxidation.

Steel sheets manufactured in the above manners may be hot press formed in the present invention.

Heat treatment hardening hot press steel sheet according to the present invention and a manufacturing method thereof, will be described with reference to Figures 1 and 2, after pressing one of the above steel sheets in a heating furnace to a temperature of more than Ar3 or more than 700 degrees (press mold) 12,14) After supplying to the inside (S1) and press molding at 600 ~ 800 ℃ temperature in the press mold (12,14) (S2), the heat treatment is performed at the desired temperature in the cooling end temperature range 100 ~ 500 ℃ The mold is maintained in the heated state until 10 seconds to the end of phase transformation (time varies depending on the steel type) (S3).

The press mold is composed of upper and lower press molds 12 and 14, and a heat source that heats the press molds 12 and 14 to supply heat until the phase change of the steel sheet is finished is a heating element inside the body of the upper and lower press molds. 20 can be incorporated.

For example, the heating element 20 may adopt a channel through which a fluid, which is a medium for transferring heat from an external heat source, is circulated.

After press forming is completed, the steel sheet has a cooling process in which the microstructure is extracted from the mold after the phase transformation is completed (S4), and then selectively cooled during the cooling process of oil cooling, water cooling, and air cooling.

Thereafter, there is a scaling process of removing the scale of the surface of the steel sheet through a known pickling process or a shot blast process (S5).

Through the above method, the present invention can produce a steel sheet capable of securing a tensile strength of 1,000 MPa and an elongation of 10% or more under a condition of a material thickness of 5 mm in a press product state.

At this time, the composition components of the steel sheet material, for example, in weight%, carbon (C): 0.15 to 0.30%, silicon (Si): 0.03 to 0.30%, manganese (Mn): 1.0 to 2.0%, boron (B) : 0.0005 to 0.0040%, S: 0.003% or less, P: 0.012% or less, calcium (Ca): 0.0010 to 0.0040%, titanium (Ti): 0.005 to 0.1%, niobium (Nb): 0.0005 to 0.5% The sum of chromium and molybdenum (Cr + Mo) is 0.1 to 2.0%, and the remainder may be one having a composition of iron (Fe).

In this case, the addition of boron ensures quenchability and obtains sufficient tensile strength and elongation.

The chemical composition and the production conditions of the present invention will be described in detail.

Carbon [C]: 0.15-0.30 wt%

Carbon is an indispensable element for imparting high strength to the steel sheet, and is a major element for increasing the hardenability of the steel sheet and determining the strength after the hardening. If the carbon content is less than 0.15 wt%, the effect is not sufficient. If the carbon content exceeds 0.30 wt%, the toughness of the quenched portion becomes remarkable.
Manganese [Mn]: 1.0-2.0 wt%

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Manganese contributes to the formation of austenite and promotes austenite formation and carbon enrichment therein, contributing to the formation of residual austenite, and is an effective element for enhancing the hardenability of steel sheet and securing strength after quenching. In order to secure tensile strength of 1400MPa, the content is regulated within the range of 1.0 ~ 2.0 wt%.
Boron [B]: 0.0005-0.0040 wt%

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Boron is an element that increases the hardenability of steel sheet by delaying the transformation of austenite into ferrite during continuous cooling transformation, and further increases the stable securing effect of strength after quenching. If the boron content is less than 0.0005 wt%, the effect is insignificant, and if the boron content exceeds 0.0040 wt%, the effect is saturated and the material is waste compared to the effect.
Silicon [Si]: 0.03 to 0.3 wt%

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It is usually added as a deoxidizer in the steelmaking process to remove oxygen in the steel, and also has a function of improving hardenability. However, at least 0.03 wt% is retained during the steelmaking process, and the amount of addition is increased, so that the Mn / Si ratio is low, which may cause brittleness such as welding cracking. Therefore, it is desirable to limit the maximum value to 0.3 wt%.
Phosphorus [P]: 0.012 wt% or less

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In the steelmaking process, the content is about 0.020 wt% after the normal dephosphorization process. However, since the hot workability of the steel is degraded at a high temperature, a very small amount of control is required to improve the high temperature workability. Due to the recent development of steelmaking technology, it is possible to control less than 0.012 wt%, so set 0.012 wt% as the maximum value of phosphorus content.
Sulfur [S]: 0.003 wt% or less

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As an element inevitably contained in the steelmaking process, the content is about 0.015 wt% after a general desulfurization process, but like phosphorous, the hot workability of steel is degraded at a high temperature, so a very small amount of control is required to improve high temperature workability. Since control of less than 0.003 wt% is possible due to the recent development of steelmaking technology, set 0.003 wt% as the maximum value of sulfur content.
Calcium [Ca]: 0.001 ~ 0.004 wt%

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After the normal desulfurization process in steelmaking, Ca inclusions are spheroidized to control the inclusion inclusions. Such inclusions reduce impact and toughness when present in a linear manner, maximizing toughness by managing in a range of 0.001 to 0.004 wt%. In addition, the Ca content in the steelmaking process can not technically exceed 0.0040 wt%.

Titanium [Ti]: 0.005 ~ 0.1 wt%

It is usually added to meet nitrogen (N) to form a compound at high temperature to control nitrogen. Nitrogen in the steel contained in the steelmaking process is an inevitable element. Nitrogen is added in order to suppress the quenchability by forming boron and compounds. Titanium should be added at least 0.005 wt%, the effect is expected, and when added more than 0.1 wt% loses its commercial significance. Therefore, the amount of titanium added is limited to 0.005 to 0.1 wt%.
Niobium [Nb]: 0.0005 ~ 0.5 wt%

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The addition of niobium enables the metal particles to be controlled to be small during hot rolling and has an effect of increasing the stabilization of strength after quenching by utilizing its hardenability. The addition of niobium also contributes to the improvement of the impact value of the material through the effect of inhibiting grain growth of the welded portion to secure welding strength and making grains fine.

Chromium (Cr), molybdenum (Mo): 0.1 ≦ Cr + Mo ≦ 2.0 wt%

Chromium and molybdenum are usually added to secure hardenability, and have an effect of increasing strength after quenching by utilizing hardenability. In order to secure quenchability, the chromium and molybdenum should be included at least 0.1 wt%. However, if more than 2 wt% of chromium and molybdenum are added together, its function is saturated.

As the steel sheet used in the present invention, any of a hot rolled steel sheet, a cold rolled steel sheet, and a plated steel sheet may be used. The plated steel sheet may also be an electroplated steel sheet, a hot dip steel sheet, and an alloyed hot dip galvanized steel sheet. It can also be used as a cold rolled steel sheet coated with a high molecular weight oxidation paint. The processing method of the product is applicable by the usual commercial hot press method. In order to facilitate product molding, a method of performing hot pressing through preforming may also be applied.

Figure 3 is a graph showing the change in the phase transformation material when the steel sheet material having the composition described above different mold heat treatment, the number 2, 3 shown in the arrow is an embodiment corresponding to the present invention, the remaining 1, 4, 5, 6 is compared An example is shown.

The heat treatment conditions and material change of these Examples and Comparative Examples are shown in Table 1 below.

number Steel plate material temperature (℃) Mold Heat Treatment Temperature (℃) When holding
Liver (sec) Material properties
division Tensile Strength (MPa) Yield strength (MPa) Elongation (%) Fatigue Limit (MPa) Microstructure
One 800 50 - 1,550 1080 8 450 Martensite Comparative Example 1 2 800 280 50 1,080 980 12 820 Temper Martensite Example 1 3 800 420 40 1,030 920 14 780 Bainite Example 2 4 800 50 - 980 750 13 610 Bainite Comparative Example 2 5 800 50 - 1,220 1100 9 550 Temper Martensite Comparative Example 3 6 800 50 - 1,050 520 12 430 Ferrite + Martensite Comparative Example 4

According to the above Table 1, Examples 1 and 2 of the present invention corresponding to Nos. 2 and 3 are heat-treated steel sheets until a phase transformation occurs in press molding of a press die, and comparisons corresponding to Nos. 1 and 6 Examples 1 and 4 are not heat-treated until the phase transformation at the time of press molding, Comparative Examples 2 and 3 are subjected to a separate post-heat treatment process.

Therefore, as in Examples 1 and 2 of the present invention, it is possible to control the microstructure through heat treatment during press molding without undergoing a separate post-heat treatment process, to secure sufficient tensile strength and elongation, and to have excellent fatigue characteristics. It can be seen that a steel sheet can be obtained.

And the microstructure of the steel sheet manufactured by the present invention can be controlled by adjusting the temperature of the mold according to the required characteristics of the parts, it is possible to secure the low-temperature structure, such as martensite, bainite.

In addition, the ferrite phase can be obtained at room temperature when heated first after heating at the temperature of A1 ~ A3 of 800 ℃ or lower, and if applied, a two-phase structure of ferrite + martensite (or bainite) can be obtained. .

1 is a flow chart sequentially showing a method of manufacturing a heat treatment hardening hot press steel sheet according to the present invention.

Figure 2 is a schematic view showing the configuration of a press die of the present invention.

3 is a graph illustrating a state in which a steel sheet having a composition of the present invention is changed according to press heat treatment conditions.

Explanation of symbols on the main parts of the drawings

12,14: upper and lower press mold

20: heating element

Claims (3)

By weight%, contains carbon (C): 0.15 to 0.30% Silicon (Si): 0.03 to 0.30%, manganese (Mn): 1.0 to 2.0%, boron (B): 0.0005 to 0.0040%, sulfur (S) : 0.003% or less, phosphorus (P): 0.012% or less, calcium (Ca): 0.0010 ~ 0.0040%, titanium (Ti): 0.005 ~ 0.1%, niobium (Nb): 0.0005 ~ 0.5%, sum of chromium and molybdenum ( Cr + Mo): 0.1 to 2.0%, and the remainder of the steel sheet material having a composition of iron (Fe) is heated in a heating furnace above the Ar3 temperature, and then supplied to the press mold, Press molding the steel sheet material at 600 to 800 ° C. in the press mold; Maintaining the press mold temperature at 100 to 500 ° C. such that phase transformation of the microstructure of the press-formed steel sheet is performed in the press mold; Extracting the steel sheet in which the phase transformation is completed in the press die; And scaling to remove the surface scale of the steel sheet extracted from the mold. delete By weight%, contains carbon (C): 0.15 to 0.30% Silicon (Si): 0.03 to 0.30%, manganese (Mn): 1.0 to 2.0%, boron (B): 0.0005 to 0.0040%, sulfur (S) : 0.003% or less, phosphorus (P): 0.012% or less, calcium (Ca): 0.0010 ~ 0.0040%, titanium (Ti): 0.005 ~ 0.1%, niobium (Nb): 0.0005 ~ 0.5%, sum of chromium and molybdenum ( Cr + Mo): 0.1 ~ 2.0%, the rest of the steel sheet having a composition of iron (Fe) is heated above the Ar3 temperature in the heating furnace, After the steel sheet heated in the heating furnace is press-molded at 600 to 800 ° C. in a press die, the phase is transformed while the substrate is heated to a temperature of 100 to 500 ° C. in the press die, Heat-treated hardened hot press steel sheet having a tensile strength of 1030 ~ 1080MPa, elongation 12 ~ 14% and fatigue limit of 780 ~ 820MPa.

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