KR101008820B1 - Heat-treatment Hardening Steel-sheet having Excellent Low-temperature impact toughnss, and method for producing the same - Google Patents

Abstract

The present invention relates to a heat-treated hardened steel sheet excellent in low temperature impact toughness and a method of manufacturing the same. Specifically, in weight%, carbon (C): 0.19-0.40%, manganese (Mn): 0.5-2.5%, chromium (Cr): 0.1-0.5%, boron (B): 0.0015-0.0040%, silicon ( Si): 0.5% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.05% or less, aluminum (Al): 0.03% or less, molybdenum (Mo): 0.05 to 1.0% further contained The remainder is then heated and maintained in a steel consisting essentially of Fe and unavoidable impurities above the austenite recrystallization temperature, and then cooled to an average cooling rate of 50 to 150 ° C./s to below the Ms point temperature through hot forming. It is manufactured by quenching. According to the present invention, it is possible to obtain a heat-treated hardened steel sheet having a high tensile strength of 1200 to 1500 MPa and excellent low temperature impact toughness having a transition temperature range of -80 ° C or less.

Description

Heat-treatment Hardening Steel-sheet having Excellent Low-temperature impact toughnss, and method for producing the same

The present invention relates to a heat-treated hardened steel sheet having excellent low temperature impact toughness and a method for manufacturing the same. More particularly, the present invention relates to a high temperature and low temperature impact toughness, and is applicable to various mechanical structural parts including vehicle structural parts. The present invention relates to a heat treated hardened steel sheet and a method of manufacturing the same.

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 facing the automotive industry. Increasingly, international regulations on the amount of CO ₂ emissions, which are environmental pollutants, are increasingly increasing and inevitably require the development of fuel-efficient vehicles. 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.

Korean Patent Laid-Open Publication No. 2006-0018860 discloses that the average cooling rate is suppressed within a predetermined range at a temperature range below the Ms point (temperature at which martensite starts to form from austenite) during cooling after molding, for example, during mold cooling. Disclosed is a technique for producing a hot forming member having a stable strength and toughness by hot forming. This conventional invention has a mass% of C: 0.15 to 0.45%, Mn: 0.5 to 3.0%, Cr: 0.1 to 0.5%, Ti: 0.01 to 1.0%, B: 0.0002 to 0.004%, Si: 0.5% or less, P: A steel sheet having a steel composition consisting essentially of Fe and an unavoidable impurity, containing one or two or more alloy elements including 0.05% or less and Nb: 1% or less, by heating to Ac ₃ or more, It is a technique of hot-molding to the final product shape after maintaining time.

In Japanese Patent Laid-Open No. 8-269615, C: 0.18 to 0.30%, Si: 0.01 to 1.0%, Mn: 0.2 to 1.5%, P: 0.03% or less, S: 0.02% or less, Al: 0.08% or less, Cr: 0.1 ~ 0.5%, B: 0.0006 ~ 0.0040%, N: 0.01% or less, optionally contain at least one of Cu: 0.5% or less, Ni: 0.3% or less, Ti: 0.01 ~ 0.05% A steel sheet made of Fe is disclosed. After the steel sheet is cold worked, the steel sheet is heated to a high temperature to be molded in a press (quenching in a mold) to increase its strength.

However, the conventional hot forming method (hot press method) has a problem in that the high strength steel having a martensite structure of Hv 450 or more having low low temperature impact toughness and low toughness against low temperature impact.

Accordingly, the present invention has been made to solve the above problems, and an object of the present invention is to provide a heat-treated hardened steel sheet having a high tensile strength of 1200 ~ 1500 MPa and excellent low temperature impact toughness and a method of manufacturing the same.

Heat-treated hardened steel sheet excellent in low-temperature impact toughness according to the present invention, carbon (C): 0.19 to 0.40%, manganese (Mn): 0.5 to 2.5%, chromium (Cr): 0.1 to 0.5%, boron (B ): 0.0015 to 0.0040%, silicon (Si): 0.5% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.05% or less, aluminum (Al): 0.03% or less, molybdenum (Mo) It further contains 0.05 to 1.0%, and the balance is characterized by consisting of a composition consisting essentially of Fe and inevitable impurities.

The composition may further contain one or two or more of nickel (Ni): 2.0 wt% or less, niobium (Nb): 0.10 wt% or less, and copper (Cu): 1.0 wt% or less.

Further, in the composition, nitrogen (N): 0.008 to 0.012 wt%, and titanium (Ti): 0.02 to 0.05 wt% may be added.

Heat-treated hardened steel sheet excellent in low-temperature impact toughness according to the present invention, by weight% carbon (C): 0.19 ~ 0.40%, manganese (Mn): 0.5 ~ 2.5%, chromium (Cr): 0.1 ~ 0.5%, boron ( B): 0.0015 to 0.0040%, silicon (Si): 0.5% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.05% or less, aluminum (Al): 0.03% or less, and molybdenum (Mo) ) 0.05 to 1.0%, and the remainder of the steel consisting of the composition consisting essentially of Fe and unavoidable impurities after heating and holding above the austenite recrystallization temperature, the average cooling rate to below the Ms point temperature through hot forming It is manufactured by performing a hardening process cooled at 50-150 degreeC / s.

According to the present invention, it is possible to obtain a heat-treated hardened steel sheet having a high tensile strength of 1200 to 1500 MPa and excellent low temperature impact toughness having a transition temperature range of -80 ° C or less.

Hereinafter, a preferred embodiment of a heat-treated hardened steel sheet excellent in low temperature impact toughness according to the present invention and a manufacturing method thereof.

The heat-treated hardened steel sheet of the present invention has a weight (wt)% of carbon (C): 0.19 to 0.40%, manganese (Mn): 0.5 to 2.5%, chromium (Cr): 0.1 to 0.5%, boron (B): 0.0015 to Molybdenum (Mo) 0.05 to 1.0% in a composition of 0.0040%, silicon (Si): 0.5% or less, phosphorus (P): 0.05% or less, sulfur (S): 0.05% or less, aluminum (Al): 0.03% or less It further contains, and remainder consists of a composition which consists essentially of Fe and an unavoidable impurity.

The steel sheet having such a composition was heated and maintained at the austenite recrystallization temperature or more (about 950 ° C.) in a heating furnace, and then cooled at an average cooling rate of 50 to 150 ° C./s to a temperature below the Ms point temperature (about 150 ° C.) through hot forming. It is manufactured by performing a quenching treatment.

In the present invention, in order to improve the hardenability, nickel (Ni): 2 wt% or less, niobium (Nb): 0.10 wt% or less, and copper (Cu): 1.0 wt% or less may further include one kind or two or more kinds. It may contain.

In addition, nitrogen (N): 0.008 to 0.012 wt%, and titanium (Ti): 0.02 to 0.05 wt% may be added to the composition, and the titanium (Ti) is due to the precipitation of nitrogen (N) due to Al-N. TiN is formed so as not to reduce hardenability, thereby increasing austenite grain size to improve hardenability.

Hereinafter, the steel composition and manufacturing conditions of the heat-treated hardened steel sheet excellent in low temperature impact toughness according to the present invention will be described in more detail.

Carbon (C): 0.19-0.40 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. By stabilizing austenite and lowering the Ac ₃ point, the lowering of the quenching treatment temperature is promoted. If the carbon content is less than 0.15 wt%, the effect is not sufficient, and if the carbon content exceeds 0.45 wt%, toughness deterioration of the quenched portion becomes remarkable. Preferably it is 0.19-0.40 wt%.

Manganese (Mn): 0.5 ~ 2.5 wt%

Manganese is an effective element that suppresses the formation of pearlite phase and promotes austenite formation and carbon enrichment inside, contributing to the formation of residual austenite, enhancing the hardenability of steel sheet and securing strength after quenching. However, when the manganese content is less than 0.5 wt%, a very fast cooling rate is required and industrially it is impossible to prevent the production of pearlite, and when it exceeds 2.5 wt%, manganese band structure is formed and segregation increases rapidly, which impairs the workability and weldability of the steel. Manganese content is regulated from 0.5 to 2.5 wt%.

Chromium (Cr): 0.1 ~ 0.5 wt%

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

Titanium (Ti): 0.02 ~ 0.05 wt%

Titanium is an effective element to increase the hardenability of steel sheet and to secure the strength stably, and to prevent Al-N precipitation, which forms TiN in steel and reduces hardenability. It is preferable to contain Ti: 0.02-0.05 wt% with respect to the composition which has N: 0.008-0.12 wt%.

Boron (B): 0.0015 ~ 0.0040 wt%

Boron is an element that increases the hardenability of the 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 exceeds 0.0040 wt%, the effect is saturated and the cost increases.

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

These elements have the effect of increasing the hardenability of the steel sheet and stabilizing the strength after the hardening. However, if their content exceeds the upper limit, the effect is saturated and the cost increases.

Nickel (Ni): 2 wt% or less, Niobium (Nb): 0.10 wt% or less, Copper (Cu): 1.0 wt% or less

These elements also have the effect of increasing the hardenability of the steel sheet and stabilizing the strength after the hardening. However, if their content exceeds the upper limit, the effect is saturated and the cost increases.

The molybdenum may further contain Molybdenum (Mo): 0.05 ~ 1.0%, the upper limit is preferably 0.2 wt%. Above 0.2 wt%, it is saturated with little change of transition temperature, and above 1.0 wt% is not economical due to the increase in cost.

The steel sheet used in the present invention is heated to an austenite temperature range at the time of heating prior to hot forming to cause austenite transformation, so that the mechanical properties at room temperature before heating are not important, but are manufactured to a strength that can be wound by a coil. The metal structure before heating is not particularly limited, but as described above, suppressing the formation of Al-N in steel can increase the size of the austenite grains as much as possible to increase hardenability and toughness. As the steel sheet, any of hot rolled steel sheet, cold rolled steel sheet and plated steel sheet may be used, but a steel sheet having a thickness that satisfies the cooling rate during hot press working is used. Examples of the plated steel sheet include aluminum-based plated steel sheets and zinc-based plated steel sheets. The plated steel sheet may also be used as an electroplated steel sheet, a hot dip steel sheet and an alloyed hot dip steel sheet.

In order to perform quenching after forming into a molding member, that is, a hot press member, in the mold cooling during hot forming (hot press molding), the steel sheet is first heated to an austenite temperature range (Ac ₃ or higher, preferably 950 ° C.), It is maintained at that temperature for 1 minute or more under normal conditions. Although the upper limit of the holding time is not particularly set, it is preferable to set the upper limit of the holding time to about 10 minutes in consideration of actual production efficiency.

The cooling rate during hot press (in mold) or after hot press (after demolding) is a parameter that plays a very important role in obtaining stable strength and toughness in the hot press member. In this embodiment, the cooling is performed at an average cooling rate of 50 to 150 ° C./s to about 150 ° C. which is below the Ms point temperature.

According to the embodiment of the present invention, it is possible to obtain a heat-treated hardened steel sheet having a high tensile strength of 1200 ~ 1500 MPa and excellent low temperature impact toughness having a transition temperature range of -80 ℃ or less.

Hereinafter, the above-described high strength steel sheet and a method of manufacturing the same will be described by comparing with a comparative example through an invention example.

Table 1 shows the component ratio of the steel plate of the invention example and a comparative example of this invention.

division
Chemical composition (wt%) Remarks
C Mn Cr B Si P S Al Mo Psalm 1 0.21 1.2 0.2 0.002 0.25 0.008 0.002 0.02 - Comparative example Psalm 2 0.21 1.2 0.2 0.002 0.25 0.008 0.002 0.02 0.05 Inventive Example Psalm 3 0.21 1.2 0.2 0.002 0.25 0.008 0.002 0.02 0.10 Inventive Example Psalm 4 0.21 1.2 0.2 0.002 0.25 0.008 0.002 0.02 0.20 Inventive Example

Table 2 is a quenching treatment for cooling the steel sheet (thickness 1.0mm) by the composition of the invention examples and comparative examples of Table 1 after heating at 950 ° C. for about 5 minutes and cooling to 150 ° C. at a cooling rate of 100 ° C./s through press work. Is a table showing the (ductile-brittle) transition temperature range as a result of the low temperature impact test. The low temperature impact test used a well-known general test method.

division Temperature (℃) Remarks
-60 -70 -80 -90 -100 -105 -110 Psalm 1 ○ × × × × × × Comparative example Psalm 2 ○ ○ ○ × × × × Inventive Example Psalm 3 ○ ○ ○ ○ × × × Inventive Example Psalm 4 ○ ○ ○ ○ ○ ○ × Inventive Example

As shown in Table 2, specimen 2 containing 0.05 wt% of Mo has a transition temperature of -80 ° C, and specimen 3 containing 0.10 wt% of Mo has a transition temperature of -90 ° C and contains 0.2 wt% of Mo. 4 indicates that the transition temperature is -105 ℃. There was almost no change in the transition temperature for Mo 0.2 wt% or more.

Claims (4)

% By weight Carbon (C): 0.19 to 0.40%, Manganese (Mn): 0.5 to 2.5%, Chromium (Cr): 0.1 to 0.5%, Boron (B): 0.0015 to 0.0040%, Silicon (Si): Over 0 ~ 0.5%, phosphorus (P): more than 0 ~ 0.05%, sulfur (S): more than 0 ~ 0.05%, aluminum (Al): more than 0 ~ 0.03% In addition to the composition, molybdenum (Mo) 0.05 ~ 0.2% A heat-treated hardened steel sheet having excellent low-temperature impact toughness, wherein the balance is composed of Fe and inevitable impurities. The method according to claim 1, Nickel (Ni): greater than 0 to 2 wt%, niobium (Nb): greater than 0 to 0.10 wt%, and copper (Cu): greater than 0 to 1.0 wt%, further comprising one or two or more Heat-hardened steel sheet having excellent low temperature impact toughness. The method according to claim 1 or 2, A heat-treated hardened steel sheet having excellent low temperature impact toughness, characterized by further containing nitrogen (N): 0.008 to 0.012 wt%, and titanium (Ti): 0.02 to 0.05 wt%. By weight%, Carbon (C): 0.19-0.40%, Manganese (Mn): 0.5-2.5%, Chromium (Cr): 0.1-0.5%, Boron (B): 0.0015-0.0040%, Silicon (Si): 0 Molybdenum (Mo) 0.05 to 0.2% in a composition of greater than 0.5%, phosphorus (P): greater than 0 and 0.05%, sulfur (S): greater than 0 and 0.05%, aluminum (Al): greater than 0 and 0.03%. Further comprising, the balance being a steel having a composition consisting of Fe and inevitable impurities, The low temperature impact toughness characterized in that the quenching treatment is produced by maintaining the austenite recrystallization temperature to 950 ° C and then performing a quenching treatment to cool the Ms point temperature to 150 ° C at an average cooling rate of 50 to 150 ° C / s through hot forming Excellent heat treatment hardening steel sheet production method.