KR20160078807A

KR20160078807A - Hot press forming parts having superior ductility and impact toughness and method for manufacturing the same

Info

Publication number: KR20160078807A
Application number: KR1020140189031A
Authority: KR
Inventors: 이재훈; 조열래; 오진근
Original assignee: 주식회사 포스코
Priority date: 2014-12-24
Filing date: 2014-12-24
Publication date: 2016-07-05

Abstract

The hot press member having excellent elongation and impact toughness according to one embodiment of the present disclosure is composed of 0.04 to 0.12% of C, 0.35 to 1.3% of Si, 0.6 to 1.5% of Mn, 0.01 to 0.08% of Al, 0.1% or less of Cr, 0.1% or less of Cu, 0.1% or less of V, 0.1% or less of Cr, 0.03 to 0.1% of Ti, 0.015 to 0.1% At least one ferrite selected from the group consisting of Zr: 0.1% or less, Nb: 0.1% or less and Ni: 0.1% or less, the balance Fe and other unavoidable impurities, wherein at least 75% ferrite and not more than 25% Or 20% or less of bainite. By having the composition of the member and the microstructure, the impact toughness of the member can be improved, and an excellent elongation and uniform strength can be secured.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot press member having excellent elongation and impact toughness and a method of manufacturing the hot press member.

The present disclosure relates to a hot press member having excellent elongation and impact toughness characteristics and uniform strength, and a manufacturing method thereof.

In recent years, the application of high strength steel sheets has been rapidly increasing in order to meet safety and lighter weight demands of automobile materials. However, the formation of low workability of high strength steel sheet is limited to the application of automobile parts requiring complicated shapes.

Some automobile parts require a collision absorbing member having a tensile strength of 550 MPa or more and an elongation of 15% or more in addition to ultrahigh strength base metal.

In order to reduce serious damage to passengers, a collision-absorbing member of an automobile part is a member that absorbs collision while deforming components in the event of an automobile collision. Recently, a dual steel plate made by laser welding an ultrahigh strength stiffener and a collision- It is used for parts contributing to the safety of the vehicle.

Hot-press forming (HPF) technology is a method of manufacturing high strength members for automobiles of complex shapes. As a technique of heating a steel sheet at a temperature of 880 캜 to 950 캜 and press molding it in a mold, molding and quenching heat treatment are performed at the same time.

Conventional hot press members can secure properties with a tensile strength of 1300 MPa and an elongation of 5% or more after hot forming of a material containing 0.0005 wt% or more of boron (B) and carbon (C) and manganese (Mn) The microstructure consists of martensite or bainite-martensite.

Further, the member based on manganese (Mn) may exhibit Mn segregation formed in the matrix after the heat treatment. Mn is an element effective for solid solution strengthening and hardening enhancement, but when added in a large amount, segregation such as center segregation or micro segregation becomes severe, and impact toughness may be lowered.

In order to solve such a problem, a technique having a uniform strength is required while securing an excellent elongation and impact toughness of a hot press member.

The following Patent Document 1 relates to a hot press member including Mn segregation formed in a matrix after heat treatment.

International Patent Publication No. WO 2008-132303

According to one embodiment of the present disclosure, there is provided a hot press member having excellent elongation and impact toughness characteristics and uniform strength, and a manufacturing method thereof.

The hot press member having excellent elongation and impact toughness according to one embodiment of the present disclosure is composed of 0.04 to 0.12% of C, 0.35 to 1.3% of Si, 0.6 to 1.5% of Mn, 0.01 to 0.08% of Al, 0.1% or less of Cr, 0.1% or less of Cu, 0.1% or less of V, 0.1% or less of Cr, 0.03 to 0.1% of Ti, 0.015 to 0.1% At least one ferrite selected from the group consisting of Zr: 0.1% or less, Nb: 0.1% or less and Ni: 0.1% or less, the balance Fe and other unavoidable impurities, wherein at least 75% ferrite and not more than 25% Or 20% or less of bainite.

A method for producing a hot press member having excellent elongation and impact toughness according to an embodiment of the present disclosure is characterized by comprising, by weight%, 0.04 to 0.12% of C, 0.35 to 1.3% of Si, 0.6 to 1.5% of Mn, 0.1% or less of Cr, 0.1% or less of Cu, 0.01% or less of Cu, 0.03 to 0.1% of Ti, 0.015 to 0.1% of Mo, Preparing a steel sheet containing at least one selected from the group consisting of V: 0.1% or less, Zr: 0.1% or less, Nb: 0.1% or less and Ni: 0.1% or less, the balance Fe and other unavoidable impurities; Heating the steel sheet at a temperature equal to or greater than Ac3; And molding and cooling the heated steel sheet by press molding.

According to one embodiment of the present disclosure, it is possible to provide a hot press member and a method of manufacturing the same that can improve impact toughness and ensure an excellent elongation and uniform strength.

1 is an optical microscope photograph showing the microstructure of a hot press member according to one embodiment of the present disclosure;
2 is an electron probe micro-analysis (EPMA) photograph showing Mn segregation formed in the matrix of Comparative Example 2. Fig.

Preferred embodiments of the present disclosure will now be described with reference to the accompanying drawings.

However, the embodiments of the present disclosure are provided to more fully describe the present disclosure to those skilled in the art.

The embodiments of the present disclosure can be modified into various other forms, and the scope of the present disclosure is not limited to the embodiments described below.

In addition, to include an element throughout the specification does not exclude other elements unless specifically stated otherwise, but may include other elements.

Hereinafter, the steel sheet for a hot press member having excellent elongation and impact toughness according to the present disclosure will be described in detail.

The hot press member having excellent elongation and impact toughness according to one embodiment of the present disclosure is characterized by containing 0.04 to 0.12% of C, 0.35 to 1.3% of Si, 0.6 to 1.5% of Mn, 0.01 to 0.08% of Al, 0.01 to 0.08% of Al, 0.1% or less of Cr, 0.1% or less of Cu, or 0.1% or less of V and 0.03 to 0.1% of Mo, 0.015 to 0.1% of Mo, At least one ferrite selected from the group consisting of Zr: not more than 0.1%, Nb: not more than 0.1%, and Ni: not more than 0.1%, the balance Fe and other unavoidable impurities, and contains not less than 75% ferrite and not more than 25% And 20% or less of bainite.

The hot press member according to another embodiment of the present disclosure contains 0.05 to 0.09% of C, 0.35 to 1.0% of Si, 0.6 to 1.3% of Mn, 0.02 to 0.05% of Al, 0.05 to 0.08% of Ti, 0.1% or less of Cr, 0.1% or less of Cu, 0.1% or less of V, 0.1% or less of Zr, 0.10% or less of Cr, 0.01% or less of Mo, , At least one selected from the group consisting of Nb: 0.1% or less and Ni: 0.1% or less, the balance Fe and other unavoidable impurities.

The hot press member can be manufactured by hot forming a hot press steel sheet.

The steel sheet may be one selected from the group consisting of a hot-rolled steel sheet, a cold-rolled steel sheet and a coated steel sheet.

The steel sheet may have a microstructure including ferrite and pearlite.

The plated steel sheet may have a plated layer formed on the surface of the hot-rolled steel sheet or the cold-rolled steel sheet.

The plating layer may be an aluminum alloy plating layer.

Hereinafter, the composition of the hot press member will be described.

C: 0.04 to 0.12 wt%

C is an indispensable element for imparting strength to the member.

The content of C may be 0.04 wt% to 0.12 wt%, preferably 0.05 wt% to 0.09 wt%.

If the content of C is less than 0.04 wt%, the hot press member can attain a tensile strength lower than 550 MPa under any cooling conditions in the step of molding and cooling the cold-rolled steel sheet by the die press molding.

If the content of C exceeds 0.12% by weight, the phase fraction of martensite after cooling becomes too high, and the elongation of the member may be lowered.

Si : 0.35 to 1.3 wt%

The Si is an element having an effect of improving the strength by solid solution strengthening and is an element promoting the grain boundary diffusion of the solid carbon in the ferrite crystal grain. Further, the Si can reduce a change in the elongation of the member sensitive to the cooling rate after the heat treatment.

The Si content may be 0.35 wt% to 1.3 wt%, and preferably 0.35 wt% to 1.0 wt%.

The change of the elongation of the member sensitive to the cooling rate after the heat treatment can be reduced by the Si, and the elongation of the ferrite phase can be diffused into the grain boundary to further increase the elongation at the uniform strength.

When the content of Si is 0.35% by weight or more, it is possible to obtain an effect of improving the strength of the member and reducing the change of elongation.

If the content of Si exceeds 1.3% by weight, a plating defect such as unplated in the plating process after the rolling process can be caused.

Mn : 0.6 to 1.5 wt%

The Mn is an element useful for improving the strength and hardenability of the member.

The content of Mn may be 0.6 wt% to 1.5 wt%, and preferably 0.6 wt% to 1.3 wt%.

When the content of Mn is 0.6% by weight or more, it is possible to obtain an effect of improving the strength of the member and improving the hardenability.

If the content of Mn exceeds 1.5% by weight, it is possible to reduce the impact toughness of the member by forming Mn segregation in the substrate after cooling.

Therefore, by reducing the content of Mn, it is possible to reduce the Mn segregation in the matrix of the member after the heat treatment to improve impact toughness.

Al : 0.01 to 0.08 wt%

The Al is a typical element used as a deoxidizing agent that combines with oxygen in steel to deoxidize it, and is an element effective to improve the hardenability of martensite by distributing the solid carbon in ferrite with austenite.

The content of Al may be 0.01 wt% to 0.08 wt%, preferably 0.02 wt% to 0.05 wt%.

If the content of Al is less than 0.01% by weight, the effect of deoxidation and the effect of distributing the solid carbon in the ferrite become insufficient.

If the content of Al exceeds 0.08% by weight, the surface quality of the slab can be reduced and the manufacturing cost may increase.

Ti : 0.03 to 0.1 wt%

The Ti is an element which forms a TiN type nitride at a high temperature and forms a TiC type carbide at a low temperature.

After hot rolling or cold rolling and annealing annealing, the Ti forms a complex type of carbide with elements such as Mo, Nb, V and Zr. The carbides can inhibit the growth of austenite grains during heating in a furnace for hot forming and can lower the sensitivity to changes in cooling rate.

The content of Ti may be 0.03 wt% to 0.1 wt%, preferably 0.05 wt% to 0.08 wt%.

When the content of Ti is 0.03 wt% or more, the Ti complex carbide may be formed so as to suppress the growth of austenite grains and the sensitivity to changes in the cooling rate.

If the content of Ti exceeds 0.1 wt%, the impact characteristics of the member can be reduced by forming nitride precipitates such as excessive TiN, and the manufacturing cost can be increased.

Mo : 0.015 to 0.1 wt%

The Mo is an element which is formed on the grain boundary with precipitates such as MO (C, N) and (Mo, Ti) C during hot rolling to inhibit grain boundary growth of ferrite or austenite.

The Mo content may be 0.015 wt% to 0.1 wt%, and preferably 0.15 wt% to 0.07 wt%.

When the content of Mo is 0.015% by weight or more, the grain boundary growth of ferrite or austenite can be suppressed and impact toughness of the member can be improved.

If the content of Mo exceeds 0.1% by weight, the ductility of the member can be reduced and the manufacturing cost can be increased.

According to one embodiment of the present disclosure, precipitates such as MO (C, N) and (Mo, Ti) C are formed during hot rolling to suppress the growth of grain boundaries of ferrite or austenite, It is possible to improve the impact toughness of the member.

P: not more than 0.03% by weight

The P is an element capable of strengthening the member and is an element that can be incorporated as an impurity in the production of steel.

The content of P may be 0.03 wt% or less, preferably 0.02 wt% or less.

If the content of P exceeds 0.03% by weight, impact toughness characteristics of the member may be reduced.

S: not more than 0.01% by weight

The S is an impurity inevitably contained in the steel, and is an element that reduces impact toughness and weldability after hot press forming.

The content of S may be 0.01 wt% or less, preferably 0.005 wt% or less.

If the content of S exceeds 0.01% by weight, impact toughness characteristics of the member such as P element may be reduced.

N: not more than 0.01% by weight

The N is a solid solution strengthening element capable of increasing the strength of a steel sheet, and is an element generally incorporated from the atmosphere.

The content of N may be 0.01 wt% or less.

If the content of N exceeds 0.01% by weight, precipitates such as AlN and TiN are excessively formed, and high temperature ductility can be reduced.

Cr : 0.1% by weight or less

The Cr is an element that improves the incombustibility of the steel.

The content of Cr may be 0.1 wt% or less.

If the content of Cr exceeds 0.1% by weight, center segregation may be produced in the structure of the member to reduce impact toughness of the member.

Cu : 0.1% by weight or less

The Cu is an element for increasing the entrapping ability for suppressing ferrite or bainite transformation and is an element that affects the precipitation behavior of MnS and affects the impact toughness of the member.

The content of Cu may be 0.1 wt% or less.

If the content of Cu exceeds 0.1 wt%, a bainite structure susceptible to a change in cooling rate after austenitization in a furnace for hot forming can be largely formed.

V: 0.1 wt% or less

V is an element that inhibits grain boundary growth of ferrite or austenite by forming a carbide of a composite type with Ti after hot rolling or cold rolling and annealing, and the impact toughness of the member Can be improved.

The content of V may be 0.1 wt% or less.

If the content of V exceeds 0.1% by weight, the ductility of the member can be reduced and the manufacturing cost can be increased.

Zr : 0.1% by weight or less

The Zr is an element that inhibits grain boundary growth of ferrite or austenite by forming carbides of a complex form with Ti, such as Mo, Nb and V, after hot rolling and cold rolling and annealing, Impact toughness of the member can be improved due to inhibition of grain boundary growth.

The content of Zr may be 0.1 wt% or less.

If the content of Zr exceeds 0.1 wt%, the ductility of the member can be reduced and the manufacturing cost can be increased.

Nb : 0.1% by weight or less

Nb is an element that inhibits grain boundary growth of ferrite or austenite by forming carbides of a complex form with Ti such as Mo, V, and Zr after hot rolling, cold rolling and annealing, Impact toughness of the member can be improved due to inhibition of grain boundary growth.

The content of Nb may be 0.1 wt% or less.

If the content of Nb exceeds 0.1 wt%, the ductility of the member can be reduced and the manufacturing cost can be increased.

Ni : 0.1% by weight or less

The Ni is an element that improves the impact characteristics of the member.

The content of Ni may be 0.1 wt% or less.

If the content of Ni exceeds 0.1% by weight, the cost may be greatly increased.

The hot press member of one embodiment of the present disclosure may contain 0 wt% or more of at least one member selected from the group consisting of Cr, Cu, V, Zr, Nb and Ni.

The member may include at least one member selected from the group consisting of 0.1% or less of Cr, 0.1% or less of Cu, 0.1% or less of V, 0.1% or less of Zr, 0.1% or less of Nb and 0.1% have.

The Cr, Cu, V, Zr, Nb and Ni are single or complex precipitates such as NbC, VC and TiMC (M is at least one selected from the group consisting of Cr, Cu, V, Zr, Nb and Ni) To increase the strength and toughness of the member by making the austenite grains finer.

The Cr, Cu, V, Zr, Nb and Ni precipitate fine precipitates within the austenite grains in the presence of the sites of the single or multiple precipitates in the austenite grains, .

If the content of Cr, Cu, V, Zr, Nb, and Ni exceeds the upper limit value, a large amount of oxides are produced during the steelmaking process to cause problems in the process and casting in continuous casting, , Toughness, surface quality, and the like.

The remainder of the disclosure is Fe. However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded.

These impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art of manufacturing.

The hot press member having excellent elongation and impact toughness including the above composition has a microstructure composed of at least 75% of ferrite, at most 25% of martensite or at most 20% of bainite.

The microstructure of the hot press member may contain ferrite in an area fraction of 75% or more.

If the area fraction of the ferrite is less than 75%, it may be difficult to ensure elongation and impact toughness.

The microstructure of the hot press member may include up to 25% martensite or up to 20% bainite in an area fraction.

If the area fraction of the martensite of the martensite exceeds 25% or the area fraction of the bainite exceeds 20%, the strength of the member may increase, but it may be difficult to secure the tensile strength and impact toughness.

The mean grain size of the ferrite grains may be less than 10 [mu] m.

If the mean grain size of the crystal grains of the ferrite is less than 10 占 퐉, the impact toughness can be improved due to grain refinement.

The member having the microstructure may have a tensile strength of 550 MPa or more at room temperature, an elongation of 15% or more, and an impact toughness of 70 J / cm ² or more.

The tensile strength is preferably in the range of 550 to 700 MPa.

Hereinafter, a manufacturing method of the hot press member according to the present disclosure will be described.

A method for producing a hot press member having excellent elongation and impact toughness according to an embodiment of the present disclosure is characterized by comprising, by weight%, 0.04 to 0.12% of C, 0.35 to 1.3% of Si, 0.6 to 1.5% of Mn, 0.1% or less of Cr, 0.1% or less of Cu, 0.01% or less of Cu, 0.03 to 0.1% of Ti, 0.015 to 0.1% of Mo, Preparing a steel sheet containing at least one selected from the group consisting of V: 0.1% or less, Zr: 0.1% or less, Nb: 0.1% or less and Ni: 0.1% or less, the balance Fe and other unavoidable impurities; Heating the steel sheet at a temperature equal to or greater than Ac3; And a step of press-working and cooling the heated steel sheet.

The steel sheet may have a microstructure including ferrite and pearlite.

The coated steel sheet may be a hot rolled steel sheet or a coated steel sheet having a plated layer formed on the surface of the cold rolled steel sheet.

The plating layer may be an aluminum alloy plating layer.

Wherein the step of preparing the steel sheet comprises the steps of: C: 0.04 to 0.12%, Si: 0.35 to 1.3%, Mn: 0.6 to 1.5%, Al: 0.01 to 0.08%, Ti: 0.03 to 0.1% 0.1% or less of Cr, 0.1% or less of Cu, 0.1% or less of V, 0.1% or less of Zr, 0.1% or less of Zr, 0.1% or less of P, : 0.1% or less and Ni: 0.1% or less, the balance Fe, and other unavoidable impurities; Reheating the slab at a temperature of at least 1100 占 폚; Hot-rolling the reheated slab to a finish rolling temperature of Ar3 to Ar3 + 50 占 폚 to produce a hot-rolled steel sheet; Winding the hot-rolled steel sheet at a temperature of 400 ° C to 700 ° C; And subjecting the hot-rolled steel sheet to cold rolling and annealing at a reduction ratio of 50% to 60% to produce a cold-rolled steel sheet.

The slab may be produced through ingot casting or a continuous casting process after obtaining molten steel through a steelmaking process.

The reheating step may be performed at a temperature of 1100 DEG C or higher to reuse the slab produced at the time of casting before the hot rolling.

The cold-rolled steel sheet may include fine precipitates.

After the cold rolling, an annealing step may be further performed.

And a step of forming a plated coating on the surface of the cold-rolled steel sheet after the cold rolling, thereby producing a coated steel sheet.

The cold-rolled steel sheet may be made of a coated steel sheet by performing a plating process of forming a coating film on the surface of the cold-rolled steel sheet to impart corrosion resistance.

The plating process may be one of hot-dip galvanizing, electro-galvanizing, and hot-dip galvanizing.

In order to process the cold-rolled steel sheet into final shape parts, hot press forming can be performed.

After the steel sheet is heated at a temperature equal to or higher than Ac3, mold press molding can be performed.

The die press molding may be performed at a temperature of 600 ° C to 800 ° C.

The cooling of the steel sheet formed at the high temperature can be performed simultaneously with the mold press.

In the cooling step, the average cooling rate may be 30 [deg.] C / s to 100 [deg.] C / s.

If the average cooling rate is less than 30 캜 / s, the fraction of ferrite in the steel sheet for a hot press member increases and a tensile strength of 550 MPa or more can not be secured. If the average cooling rate exceeds 100 ° C / s, the fraction of martensite may increase in the steel sheet for hot press members and the elongation may be lowered.

The hot press member manufactured by the method of manufacturing a hot press member of the present disclosure may have a microstructure in which at least 75% of ferrite and not more than 25% of martensite or not more than 20% of bainite are mixed.

By having the hot press member having the microstructure, the elongation and the impact toughness of the member can be ensured.

The average grain size of the ferrite grains may be less than 10 占 퐉, thereby improving the impact toughness of the member.

The hot press members of the present disclosure exhibit easy welding performance in conventional welding processes such as resistance welding, arc welding and laser welding, and can be selectively welded with a different steel sheet having the same or different thickness and composition.

(Example)

The slabs having the composition shown in the following Table 1 were prepared, hot rolled to a thickness of 3 mm, cold rolled to a thickness of 1.5 mm, annealed at 780 DEG C, A steel sheet was produced.

Further, the cold-rolled steel sheet was Al-plated and cooled at 780 캜 to produce an Al-coated steel sheet.

The cold-rolled steel sheet and the Al-coated steel sheet were heated at 930 ° C for 5 minutes using a hot press facility at a temperature of Ac3 or higher, and then subjected to press forming and quenching in a flat plate mold to produce hot pressed members.

The properties of the hot press member thus prepared were evaluated, and the results are shown in Table 2 below.

The tensile properties (yield strength, tensile strength, and elongation) of the following Table 2 were evaluated by performing an impression test at room temperature using ISO6892 Type I (50 mm Gage Length ASTM Type) test pieces.

The impact toughness shown in the following Table 2 was evaluated by performing a shock toughness test at room temperature using a 3 mm thick subsize Charpy impact test specimen. At this time, the impact toughness test was carried out at a thickness of 1.5 mm Two thin steel sheets were stacked on a Charpy impact tester and tested.

division Content of components (% by weight) C Si Mn P S Al Mo Ti N Other Comparative River 1 0.069 0.025 1.571 0.015 0.002 0.039 0.002 0.073 0.0062 Nb: 0.045
Cr: 0.025
Ni: 0.015
Cu: 0.013
V: 0.009 Comparative River 2 0.128 0.543 1.627 0.010 0.002 0.034 0.004 0.018 0.0034 - Comparative Steel 3 0.056 0.575 0.551 0.011 0.002 0.033 0.041 0.003 0.0033 - Inventive Steel 1 0.064 0.595 1.212 0.011 0.002 0.032 0.043 0.069 0.0038 - Invention river 2 0.061 0.382 1.211 0.011 0.002 0.032 0.043 0.069 0.0038 - Invention steel 3 0.061 0.588 1.206 0.011 0.003 0.039 0.018 0.075 0.0032 Nb: 0.021 Inventive Steel 4 0.059 0.557 1.294 0.010 0.002 0.039 0.020 0.075 0.0033 V: 0.023 Invention steel 5 0.063 0.564 1.310 0.011 0.002 0.037 0.019 0.071 0.0033 Zr: 0.027 Invention steel 6 0.061 0.822 1.013 0.011 0.003 0.034 0.039 0.072 0.0032 Cr: 0.032
Ni: 0.016
Cu: 0.022

division Steel grade Presence or absence of plating Microstructure area fraction (%) Average
Cooling rate
(° C / s) Characteristics of steel sheet after hot pressing ferrite Martensite Bay knight Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Impact toughness
(J / cm ² ) Comparative Example 1 Comparative River 1 Nonplating 84 15 One 50 396 611 18 68 Comparative Example 2 Comparative River 1 Al plating 82 16 2 50 413 632 16 59 Comparative Example 3 Comparative River 2 Al plating 57 32 11 50 537 795 12 52 Comparative Example 4 Comparative Steel 3 Al plating 91 9 0 45 320 507 23 114 Comparative Example 5 Inventive Steel 1 Al plating 67 27 6 105 466 694 13 75 Inventory 1 Inventive Steel 1 Nonplating 89 10 One 50 389 609 23 107 Inventory 2 Inventive Steel 1 Al plating 87 12 One 50 411 627 22 93 Inventory 3 Invention river 2 Al plating 83 15 2 50 395 602 19 89 Honorable 4 Invention steel 3 Al plating 86 13 One 40 418 638 22 105 Inventory 5 Inventive Steel 4 Al plating 79 18 3 50 410 631 20 88 Inventory 6 Invention steel 5 Al plating 85 14 One 45 403 628 21 84 Honorable 7 Invention steel 6 Al plating 80 11 9 50 397 616 23 103

Examples 1 to 7 are examples satisfying the range of the content of the content of the present disclosure, with reference to Table 1 and Table 2 above.

It was confirmed that Inventive Examples 1 to 7 produced by hot forming the inventive steels 1 to 6 can produce a hot press member having a tensile strength of 550 MPa or more, an elongation of 15% or more, and an impact toughness of 70 J / cm ² or more at room temperature.

1 is an optical microscope photograph showing the microstructure of a hot press member according to one embodiment of the present disclosure;

1, Example 2 has a microstructure in which not less than 75% of ferrite and not more than 25% of martensite or not more than 20% of bainite are mixed, and the mean grain size of ferrite grains is less than 10 μm .

In Comparative Example 5, it was confirmed that the elongation was less than 15% due to an increase in the martensite fraction due to rapid cooling of the steel sheet formed at high temperature as compared with Inventive Example 2. [

2 is an electron probe micro-analysis (EPMA) photograph showing Mn segregation formed in the matrix of Comparative Example 2. Fig.

Referring to FIG. 2, Mn segregation is shown in red.

In Comparative Examples 1 and 2, the Mn content was 1.571%, which resulted in severe Mn segregation. Compared with Examples 1 to 7, the low impact toughness of Comparative Examples 1 and 2 is due to Mn segregation strongly formed in the matrix.

That is, in Comparative Examples 1 and 2, it was confirmed that the content of Mn exceeded 1.5%, and a large amount of Mn segregation was formed in the matrix.

In Comparative Example 4, the content of Mn was less than 0.6%, indicating that the tensile strength could not be sufficiently secured.

In Comparative Example 3, it was confirmed that Ti, which is a carbide-forming element for suppressing crystal grain growth, was not sufficiently contained and had an impact toughness of less than 70 J / cm 2.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. And that it should be interpreted on the basis of.

Claims

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.04 to 0.12% of C, 0.35 to 1.3% of Si, 0.6 to 1.5% of Mn, 0.01 to 0.08% of Al, 0.03 to 0.1% of Ti, 0.015 to 0.1% 0.01% or less of S, and 0.01% or less of N,
At least one selected from the group consisting of Cr: not more than 0.1%, Cu: not more than 0.1%, V: not more than 0.1%, Zr: not more than 0.1%, Nb: not more than 0.1%, and Ni: not more than 0.1% / RTI >
A hot press member excellent in elongation and impact toughness having a microstructure in which not less than 75% of ferrite and not more than 25% of martensite or not more than 20% of bainite are mixed.

The method according to claim 1,
The steel sheet according to any one of claims 1 to 3, wherein the steel contains 0.05 to 0.09% of C, 0.35 to 1.0% of Si, 0.6 to 1.3% of Mn, 0.02 to 0.05% of Al, 0.05 to 0.08% of Ti, 0.015 to 0.07% , An S content of not more than 0.005% and N content of not more than 0.01%, and excellent impact toughness.

The method according to claim 1,
Wherein the ferrite grain has an average grain size of less than 10 占 퐉 and excellent elongation and impact toughness.

6. The method of claim 5,
The steel sheet has a tensile strength of 550 MPa or more at room temperature, an elongation of 15% or more, and an impact tensile strength of 70 J / cm ² or more, and is excellent in elongation and impact toughness.

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.04 to 0.12% of C, 0.35 to 1.3% of Si, 0.6 to 1.5% of Mn, 0.01 to 0.08% of Al, 0.03 to 0.1% of Ti, 0.015 to 0.1% 0.1% or less of Cu, 0.1% or less of Cu, 0.1% or less of V, 0.1% or less of Zr, 0.1% or less of Nb and 0.1% or less of Nb % Or less, the balance Fe and other unavoidable impurities;
Heating the steel sheet at a temperature equal to or greater than Ac3; And
And a step of pressing and cooling the heated steel sheet by press molding and cooling the steel sheet.

6. The method of claim 5,
The step of preparing the steel sheet may include:
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.04 to 0.12% of C, 0.35 to 1.3% of Si, 0.6 to 1.5% of Mn, 0.01 to 0.08% of Al, 0.03 to 0.1% of Ti, 0.015 to 0.1% 0.1% or less of Cu, 0.1% or less of Cu, 0.1% or less of V, 0.1% or less of Zr, 0.1% or less of Nb and 0.1% or less of Nb %, At least one selected from the group consisting of Fe and other unavoidable impurities;
Reheating the slab at a temperature of at least 1100 占 폚;
Hot-rolling the reheated slab to a finish rolling temperature of Ar3 to Ar3 + 50 占 폚 to produce a hot-rolled steel sheet;
Winding the hot-rolled steel sheet at a temperature of 400 ° C to 700 ° C; And
And cold rolling and annealing the hot-rolled steel sheet at a reduction ratio of 50% to 60% to produce a cold-rolled steel sheet, wherein the elongation and impact toughness are excellent.

The method according to claim 6,
Forming a plated steel sheet on the surface of the cold-rolled steel sheet to produce a plated steel sheet; and further comprising an elongation ratio and impact toughness.

6. The method of claim 5,
Wherein the steel sheet is one selected from the group consisting of a hot-rolled steel sheet, a cold-rolled steel sheet and a coated steel sheet.

9. The method of claim 8,
Wherein the coated steel sheet is a coated steel sheet having a plated layer formed on a surface of a hot-rolled steel sheet or a cold-rolled steel sheet, the elongation and the impact toughness being excellent.

6. The method of claim 5,
Wherein the mold cooling is excellent in elongation and impact toughness with an average cooling rate of 30 DEG C / s to 100 DEG C / s.

6. The method of claim 5,
Wherein the die press molding is performed at a temperature of 600 캜 to 800 캜 and is excellent in elongation and impact toughness.