KR20180011004A - Steel for hot stamping molding, manufacturing method for steel for hot stamping molding, hot stamping product and manufacturing method for hot stamping product - Google Patents

Abstract

The present invention relates to a manufacturing method for a steel material for hot stamping. The manufacturing method for a steel material for hot stamping can minimize deviation of a material caused by a process variable during a hot stamping process. According to a specific embodiment of the present invention, the manufacturing method for a steel material for hot stamping includes: a step of reheating steel slab; a step of hot-rolling the reheated steel slab; a step of manufacturing a hot-rolled coil by winding the steel slab hot-rolled in a condition at 620-660 C of the winding temperature (CT); a step of manufacturing a cold-rolled plate by uncoiling the hot-rolled coil and performing cold rolling; and a step of annealing and heat-treating the cold-rolled plate. The steel slab contains: 0.04-0.06 wt% of carbon (C); 0.2-0.4 wt% of silicon (Si); 1.6-2.0 wt% of manganese (Mn); 0-0.018 wt% (not including 0) of phosphorus (P); 0-0.003 wt% (not including 0) of sulfur (S); 0.01-0.1 wt% of titanium (Ti); 0.01-0.1 wt% of niobium (Nb); and residues composed of iron (Fe) and inevitable impurities.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel material for hot stamping, a method of manufacturing the hot stamping material, a hot stamping part, and a manufacturing method therefor. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a steel for hot stamping, a method for manufacturing hot stamped steel, a hot stamping part and a method for manufacturing hot stamping part.

B-pillar, which is an important component for the collision member of an automobile, mainly uses heat treated steel of 150K or more in grade. This plays a very important role in ensuring the survival space of the driver in the side collision. In addition, the brittle steel member used as the collision member causes a brittle fracture phenomenon that threatens the safety of the driver in the side collision. Therefore, the collision absorbing ability is improved by connecting the low-strength steel member to the lower end portion of the brittle B- pillar. These steel members are referred to as steels for Taylor Welded Blank (TWB). The steel material for TWB is produced through a hot pressing process such as hot stamping after cold rolling and hot rolling.

BACKGROUND ART [0002] The background art relating to the present invention is disclosed in Korean Registered Patent Publication No. 1304621 (the name of the invention: a method of manufacturing press-molded articles having different strengths in different regions).

According to one embodiment of the present invention, there is provided a method of manufacturing a hot stamping steel material capable of minimizing a material deviation according to process variables during a hot stamping process.

According to one embodiment of the present invention, there is provided a method of manufacturing a hot stamping steel material having excellent mechanical properties such as impact performance, tensile strength and elongation.

According to an embodiment of the present invention, there is provided a hot stamping steel produced by the above-described manufacturing method.

According to one embodiment of the present invention, there is provided a method of manufacturing a hot stamping component using the steel material for hot stamping and a hot stamping component manufactured thereby.

One aspect of the present invention relates to a method of manufacturing a steel material for hot stamping. In one embodiment, the method for manufacturing a hot stamping steel includes 0.04 to 0.06 weight% of carbon (C), 0.2 to 0.4 weight% of silicon (Si), 1.6 to 2.0 weight% of manganese (Mn) (S): more than 0 wt% to 0.003 wt% or less, titanium (Ti): 0.01 to 0.1 wt%, niobium (Nb): 0.01 to 0.1 wt% ) And unavoidable impurities at a slab reheat temperature (SRT) of 1220 ° C to 1250 ° C; Hot rolling the reheated steel slab at a finishing rolling temperature (FDT) of 860 캜 to 900 캜; Rolling the hot-rolled steel slab at a winding temperature (CT) of 620 to 660 ° C to produce a hot-rolled coil; Uncoiling the hot rolled coil, and cold rolling to produce a cold rolled sheet; And heating and annealing the cold-rolled sheet material at 760 to 850 ° C.

In one embodiment, the steel slab may further include chromium (Cr): not less than 0% by weight and not more than 0.3% by weight based on the total weight of the steel slab.

One aspect of the present invention relates to a method of manufacturing a hot stamping component. The method of manufacturing a hot stamping component includes the steps of preparing a first blank by cutting a hot stamping steel manufactured by an embodiment of the present invention; Preparing a second blank by cutting a steel material provided separately from the first blank and having different mechanical properties from the first blank; Welding the first and second blanks in a Taylor-welded blank manner to form a bonded steel; Hot-stamping the bonded steel material with a press mold to form a formed body; And cooling the shaped body at a rate of 30 to 120 DEG C / s.

In one embodiment, the step of forming the shaped body comprises heating the bonded steel to a temperature of 850 to 950 캜; And transferring the heated bonded steel material to the press mold at a transfer time of 7 to 13 seconds.

In another embodiment, the steel material corresponding to the first blank among the above-mentioned formed bodies may be a microstructure composed of 83% to 92% of ferrite, 3% to 10% of martensite and 3 to 10% of bainite at a cross- .

Another aspect of the invention relates to the hot stamping component. The hot stamping component comprises 0.04 to 0.06 wt% of carbon (C), 0.2 to 0.4 wt% of silicon (Si), 1.6 to 2.0 wt% of manganese (Mn) (S): not less than 0% by weight but not more than 0.003% by weight, titanium (Ti): 0.01 to 0.1% by weight, niobium (Nb): 0.01 to 0.1% by weight, and the balance iron (Fe) and unavoidable impurities , A microstructure composed of 83% to 92% of ferrite in a sectional area fraction, 3% to 10% of martensite, and 3 to 10% of bainite.

In one embodiment, it may further include chromium (Cr): not less than 0 wt% and not more than 0.3 wt% with respect to the total weight of the hot stamping component.

In one embodiment, the hot stamping component may have a tensile strength (TS) of 600 MPa or more, a yield strength (YS) of 400 MPa or more, and an elongation (El) of 20% or more.

INDUSTRIAL APPLICABILITY The hot stamping steel material produced by the hot stamping steel material manufacturing method of the present invention is excellent in mechanical properties such as impact performance, tensile strength and elongation, is capable of disposing of expensive expensive elements, Material variations such as tensile strength and elongation ratio can be minimized according to process variables during the hot stamping process, and the economical efficiency can be excellent.

1 shows a method of manufacturing a steel material for hot stamping according to one embodiment of the present invention.
FIG. 2 is a graph showing changes in tensile strength with respect to a hot stamping transfer time at a portion corresponding to an embodiment according to the present invention and a comparative steel material according to the present invention. FIG.
FIG. 3 is a graph showing changes in elongation according to the hot stamping transfer time at the portion corresponding to the steel according to the embodiment of the present invention and the comparative steel according to the present invention.
4 is a graph showing a change in the final microstructure of the portion corresponding to the steel material of the comparative example according to the present invention as a function of the hot stamping die transfer time.
FIG. 5 is a graph showing a change in the final microstructure according to the transfer time of the hot stamping mold in the region corresponding to the example steel according to the present invention.
FIG. 6 is a graph showing the surface texture of the hot-stamping mold according to the embodiment of the present invention.
FIG. 7 is a view showing the surface texture of the hot-stamping mold according to the embodiment of the present invention.
8 is a graph showing the surface texture of the mold part corresponding to the comparative steels according to the present invention with respect to the time of hot stamping mold transfer.

Hereinafter, the present invention will be described in detail. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to be exemplary, self-explanatory, allowing for equivalent explanations of the present invention.

Manufacturing method of hot stamping steel

One aspect of the present invention relates to a method of manufacturing a steel material for hot stamping. 1 shows a method of manufacturing a steel material for hot stamping according to one embodiment of the present invention. Referring to FIG. 1, the method for manufacturing a hot stamping steel material comprises: (S10) a steel slab reheating step; (S20) hot rolling step; (S30) a winding step; (S40) cold rolling; And (S50) an annealing step.

More specifically, the method for manufacturing a hot stamping steel material comprises (S10) a method of producing a steel material for hot stamping which comprises 0.04 to 0.06% by weight of carbon (C), 0.2 to 0.4% by weight of silicon (Si), 1.6 to 2.0% (P): not less than 0% by weight but not more than 0.018% by weight, sulfur (S): not less than 0% by weight but not more than 0.003% by weight, titanium (Ti): 0.01 to 0.1% by weight, niobium (Nb) Reheating a steel slab containing negative iron (Fe) and unavoidable impurities; (S20) hot-rolling the reheated steel slab; (S30) winding the hot-rolled steel slab at a winding temperature (CT) of 620 DEG C to 660 DEG C to manufacture a hot-rolled coil; (S40) uncoiling the hot rolled coil and cold rolling to produce a cold rolled sheet; And (S50) annealing the cold-rolled sheet material.

Hereinafter, a method of manufacturing a steel material for hot stamping according to the present invention will be described step by step.

(S10) Steel slab reheating step

Wherein said step comprises: 0.04 to 0.06 weight percent of carbon (C), 0.2 to 0.4 weight percent of silicon (Si), 1.6 to 2.0 weight percent of manganese (Mn) And a steel slab containing sulfur (S): more than 0% by weight and not more than 0.003% by weight, titanium (Ti): 0.01 to 0.1% .

Hereinafter, the role and content of the components contained in the steel slab will be described in detail.

Carbon (C)

The carbon (C) is a main element that determines the strength and hardness of the steel, and is added for the purpose of securing the tensile strength of the steel after hot stamping (or hot pressing).

In one embodiment, the carbon is included in an amount of 0.04 to 0.06% by weight based on the total weight of the steel slab. When the carbon content is less than 0.04 wt%, it is difficult to achieve the mechanical strength of the present invention. When the carbon content is more than 0.06 wt%, the toughness of the steel material may be deteriorated.

Silicon (Si)

The silicon (Si) serves as an effective deoxidizing agent and is included as a major element in reinforcing the ferrite in the matrix.

In one embodiment, the silicon is included in an amount of 0.2 to 0.4 wt% based on the total weight of the steel slab. When the amount of silicon is less than 0.2% by weight, the effect of addition is insignificant. When the amount of silicon is more than 0.4% by weight, the toughness of the steel is deteriorated and the formability is lowered.

Manganese (Mn)

The manganese (Mn) is added for the purpose of increasing the incombustibility and strength at the time of heat treatment.

In one embodiment, the manganese is present in an amount of 1.6-2.0 wt% based on the total weight of the steel slab. When the content of manganese is less than 1.6 wt%, the properties and strength may be lowered. If the content of manganese exceeds 2.0 wt%, the ductility and toughness due to manganese segregation may be deteriorated.

In (P)

Phosphorus (P) is an element that segregates well and inhibits the toughness of steel. In one embodiment, the phosphorus (P) is included in an amount of more than 0 wt% to 0.018 wt% or less based on the total weight of the steel slab. When the content is in the above range, deterioration in toughness can be prevented. When the phosphorus is contained in an amount exceeding 0.018% by weight, cracks are generated in the process, and the iron phosphate compound is formed and the toughness may be lowered.

Sulfur (S)

The sulfur (S) is an element which hinders workability and physical properties. In one embodiment, the sulfur may be present in an amount greater than 0% by weight and less than 0.003% by weight based on the total weight of the steel slab. When the sulfur is contained in an amount exceeding 0.003 wt%, the hot workability is deteriorated, and surface defects such as cracks may occur due to the formation of large inclusions.

Titanium (Ti)

The titanium (Ti) is added for the purpose of enhancing the entrapment property by the formation of the precipitate after the hot stamping heat treatment and the material upward. In addition, a precipitate phase such as Ti (C, N) is formed at a high temperature, thereby effectively contributing to miniaturization of austenite grains.

In one embodiment, the titanium is included in an amount of 0.01 to 0.1% by weight based on the total weight of the steel slab. When the amount of titanium is less than 0.01% by weight, the effect of addition is insignificant. When the amount of titanium exceeds 0.1% by weight, poor performance occurs, and it is difficult to secure the physical properties of the steel material and the elongation rate is lowered. And a crack may be generated on the surface of the steel material. For example, 0.02 to 0.1% by weight.

Niobium (Nb)

The niobium (Nb) is added for the purpose of increasing the strength and toughness according to the decrease of the martensite packet size.

In one embodiment, the niobium is contained in an amount of 0.01 to 0.1% by weight based on the total weight of the steel slab. If the content of niobium is less than 0.01% by weight, the effect of grain refinement of the steel material is insignificant in the hot rolling and cold rolling processes, and when the niobium is contained in excess of 0.1% by weight, stiff coarse precipitates may be generated, , Which is disadvantageous in terms of cost. For example, 0.05 to 0.09% by weight.

Chromium (Cr)

In one embodiment, the steel slab may further comprise chromium (Cr). The chrome (Cr) is added for the purpose of improving the incombustibility and strength of the steel. In one embodiment, the chromium is present in an amount greater than 0 wt% and less than 0.3 wt% based on the total weight of the steel slab. When it is contained in the above amount, it is possible to obtain excellent extrudability and strength of the steel.

In one embodiment, the steel slab may be heated at a slab reheating temperature (SRT) of 1,220 ° C to 1,250 ° C. At the temperature of the steel slab reheating temperature, the homogenizing effect of the alloy element component is advantageous. When the steel slab is reheated at a temperature lower than 1,220 DEG C, the homogenizing effect of the alloy element component is lowered, and the process cost may be increased when reheating exceeds 1,250 DEG C.

(S20) Hot rolling step

Wherein the reheating steel slab is hot rolled. In one embodiment, the hot rolling may be carried out at a finishing rolling temperature (FDT) of 860 to 900 占 폚 in the reheated steel slab. When the hot rolling at the finish rolling temperature is performed, the rigidity and the formability of the steel material are excellent at the same time, and the hot-rolled coil can be prevented from being crushed.

(S30)

The step is a step of winding the hot-rolled steel slab to produce a hot-rolled coil. In one embodiment, the winding is performed at a coiling temperature (CT) of 620 캜 to 660 캜. In one embodiment, the hot-rolled steel slab may be cooled to a winding temperature within the above range and wound. Carbon can be easily redistributed under the above coiling temperature condition, and the low temperature phase fraction due to supercooling is increased to prevent the increase in strength due to addition of Nb, and the rolling load during cold rolling can be prevented.

In one embodiment, the cooling may be cooled by a shear quenching method. When the coiling temperature is lower than 620 캜, the low temperature phase fraction due to supercooling increases, and the rolling load during cold rolling may increase. Further, when the coiling temperature is higher than 660 DEG C, the uniformity of the microstructure is lowered, and the mechanical strength such as elongation of the steel may be lowered.

In one embodiment, the microstructure of the wound hot-rolled coil may include ferrite and perlite composite.

(S40) Cold rolling step

The step is a step of uncoiling the hot rolled coil and cold rolling to produce a cold rolled sheet. In one embodiment, the hot-rolled coil may be uncoiled, pickled, and then cold-rolled. The pickling can be carried out for the purpose of removing scale formed on the hot-rolled coil surface. In one embodiment, the cold rolling may be performed at a reduction of 50% to 85%. The deformation ratio of the hot-rolled steel sheet during cold rolling is small at the reduction ratio, and the elongation and formability can be excellent. In one embodiment, a zinc plated layer may be formed on the surface of the cold-rolled sheet. When the zinc plated layer is formed, corrosion resistance can be improved.

(S50) Annealing step

The step of annealing the cold-rolled sheet is a step of heat-treating the cold-rolled sheet. In one embodiment, the annealing heat treatment is performed by heating the cold-rolled sheet to 760 to 850 캜; And cooling the heated cold rolled sheet at a cooling rate of 10 to 50 DEG C / s. When the cold-rolled sheet is heated in the above-mentioned range, the process efficiency, the strength and the formability of the steel can be simultaneously excellent. For example, it can be heated to 780 to 820 캜.

When the cold-rolled sheet is cooled at a cooling rate of less than 10 ° C / s, the productivity of the steel decreases, and it may be difficult to secure a uniform microstructure of the steel when cooled at a cooling rate exceeding 50 ° C / s. For example, at a cooling rate of 25 to 40 DEG C / s.

hot For stamping  Hot produced by the method of manufacturing steel For stamping  Steel

Another aspect of the present invention relates to a steel material for hot stamping produced by the above hot stamping steel material producing method. In one embodiment, the steel material for hot stamping includes 0.04 to 0.06% by weight of carbon (C), 0.2 to 0.4% by weight of silicon (Si), 1.6 to 2.0% by weight of manganese (Mn) (S): more than 0 wt% to 0.003 wt% or less, chromium (Cr): more than 0 wt% to 0.3 wt%, titanium (Ti): 0.01 to 0.1 wt%, niobium (Nb) : 0.01 to 0.1% by weight, and the balance iron (Fe) and unavoidable impurities. The components and content of the steel material for hot stamping are the same as those of the steel slab, and thus a detailed description thereof will be omitted.

The steel material for hot stamping of the present invention can be manufactured as a hot stamping part through a hot stamping process. Specifically, the hot-stamping steels of the present invention are bonded to a steel material having different tensile strengths to produce bonded steels. The bonded steels are heated to 910 ° C to 950 ° C, then transferred to a mold for hot stamping, So that it can be provided as a hot stamping part. The steel material for hot stamping of the present invention can be applied to a hot stamping part applied to a vehicle B-pillar.

On the other hand, a B-pillar, which is an important component for a collision member of a vehicle, can be manufactured by welding two steel materials in a form in which steel materials having different strengths are coupled to the impact support portion at the upper portion and the impact absorption portion at the lower portion. have. The TWB (Tailor Welded Blank) method, which is mainly used, is a method of cutting a different type of steel sheet having different thicknesses, strengths, and materials to a desired shape and welding it. Thereafter, a series of processes of hot-pressing the welded jointed steel to produce the components is performed. The different kinds of steel can be made to have the required properties for each part. For example, 120 to 150K ultra high strength steel is used for the collision support part of the upper part of the B-pillar and a shock absorbing member is applied to the lower end part of the stress-concentrated B-pillar to improve the shock absorption ability I have to. The steel material for hot stamping of the present invention can be applied to the impact absorbing portion of the B-pillar.

According to one embodiment, a method of manufacturing a hot stamping component comprises the steps of: (a) preparing a first blank by cutting a steel material for hot stamping according to an embodiment of the present invention, and providing a steel plate separately provided from the first blank Preparing a second blank by cutting; (b) welding the first and second blanks by a Taylor welded blank method to form a bonded steel (c) hot stamping the bonded steel with a press mold to form a formed body; And (d) cooling the shaped body to form a hot stamped part. At this time, the second blank may be an ultra-high strength steel. As an example, it may be a cold rolled steel sheet having a tensile strength of 1,200 to 1,500 MPa.

In one embodiment, the hot stamping step comprises heating the bonded steels to a temperature of 850 to 950 占 폚, transferring the heated bonded steels to the press mold at a transfer time of 7 to 13 seconds, and then performing hot stamping . The cooling of the molded body may be carried out at a speed of 30 to 120 ° C / s.

In the case of manufacturing a hot stamping part using the steel material for hot stamping according to the present invention, as described later, after the bonded steel material is heated to a temperature of 850 to 950 캜 during the hot stamping step, It is possible to minimize variations in materials such as tensile strength and elongation according to process variables such as the time for transferring the material, and hence the economical efficiency can be excellent.

In one embodiment, the steel material corresponding to the first blank among the hot stamping parts may have a tensile strength (TS) of 600 MPa or more, a yield strength (YS) of 400 MPa or more, and an elongation (El) of 20% or more. For example, tensile strength (TS): 600 to 750 MPa, yield strength (YS): 400 to 560 MPa and elongation (El): 20 to 29%.

In one embodiment, the steel material corresponding to the first blank among the hot stamping parts may have a composite structure of ferrite, martensite, and bainite. As an example, the microstructure of the hot stamping steel may be composed of 83% to 92% of ferrite, 3% to 10% of martensite and 3 to 10% of bainite in a cross sectional area fraction.

As described above, the steel material corresponding to the first blank of the hot stamping parts manufactured by the manufacturing method according to one embodiment of the present invention is excellent in mechanical properties such as tensile strength and elongation, and in particular, It is excellent in performance, and it is possible to discharge costly inconceivable elements such as molybdenum (Mo) and the like, so that the cost reduction effect can be excellent.

The hot stamping component to which the hot stamping steel is applied may be applied not only to the hot stamping component applied to the vehicle B-pillar but also to components of other component parts, thereby improving the overall toughness of the component parts for the impact member.

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Examples and Comparative Examples

Example  1-4

A steel slab containing the components shown in Table 1 and the balance of iron (Fe) and other unavoidable impurities was reheated at a slab reheating temperature of 1,220 占 폚, hot rolled at a finish rolling temperature of 880 占 폚, : 630 占 폚 to obtain a hot-rolled coil. The hot-rolled coil was uncoiled, cold-rolled to produce a cold-rolled plate, the cold-rolled plate was heated to 810 ° C, and then annealed at a cooling rate of 33 ° C / s to produce a hot- .

Comparative Example  1-4

The same hot rolling, cold rolling and annealing steps were carried out in the same manner as in Examples 1 to 4 except that a steel slab containing the same components as in Table 1 and residual iron (Fe) and other unavoidable impurities was applied, Respectively.

division
Unit: wt% C Si Mn P S Nb Ti Cr B Mo Example 1 0.05 0.2 1.8 0.01 0.003 0.09 0.02 0.2 - - Example 2 0.05 0.2 1.8 0.01 0.003 0.05 0.1 - - - Example 3 0.04 0.2 1.8 0.01 0.003 0.05 0.1 - - - Example 4 0.06 0.2 1.8 0.01 0.003 0.05 0.1 - - - Comparative Example 1 0.08 - 1.8 0.01 0.003 0.05 0.065 - - 0.17 Comparative Example 2 0.07 0.2 1.8 0.01 0.003 0.05 0.01 - - - Comparative Example 3 0.08 0.2 1.8 0.01 0.003 0.05 0.01 - - - Comparative Example 4 0.10 0.2 1.8 0.01 0.003 0.05 0.01 - - -

hot Stamping  Parts Manufacturing

The steels of Examples 1 to 4 and Comparative Examples 1 to 4 and the 150K grade steels were jointed by laser welding to produce bonded steels. The bonded steel material was heated at 930 캜 for 5 minutes and then the heated bonded steel material was transferred to a hot stamping mold at a transfer time of 9 seconds and hot stamped to produce an intermediate molded body, s < / RTI >

Further, the bonded steels obtained by bonding the steels of Examples 1 and 2 and Comparative Example 1 to the 150 kg steels were heated at 930 캜 for 5 minutes, and then the time for feeding them to the hot stamping mold was changed from 7 to 13 seconds And after hot stamping molding, an intermediate molded body was produced, and the intermediate molded body was cooled at a cooling rate of 100 DEG C / s to prepare a molded body.

Mechanical properties evaluation

The tensile strength (MPa), the yield strength (MPa) and the elongation percentage (%) of the hot stamping parts were measured for the parts to which the steels of Examples 1 to 2 and Comparative Example 1 were applied, Respectively.

division Tensile Strength (MPa) Yield strength (MPa) Elongation (%) Example 1 615 430 22 Example 2 625 420 26 Comparative Example 1 700 427 13

(TS): 600 MPa or more, yield strength (YS): 400 MPa, and 20% or less. The tensile strength (TS) In particular, in the case of the portion to which the steel material of Comparative Example 1 including molybdenum (Mo) was applied, the tensile strength and the yield strength were increased, but the elongation was lowered to less than 20% I could.

2 is a graph showing changes in tensile strength with time of transfer to a hot stamping mold at a portion to which a steel according to Examples 1 to 2 and Comparative Example 1 is applied. FIG. 5 is a graph showing changes in elongation according to a hot stamping mold transfer time at a region to which the steel according to Comparative Example 1 is applied. FIG.

2 and 3, the steels of Examples 1 and 2 exhibited less change in tensile strength and elongation according to the transfer time to the mold than the steels of Comparative Example 1, It was found that the steel material for hot stamping was a component system insensitive to changes in the mold transfer time.

Fig. 4 shows the final microstructural change of the formed body corresponding to the steel material of Comparative Example 1 in accordance with the feeding time of the hot stamping die. Fig. 5 shows the results of the hot stamping of the molded body corresponding to the steels of Examples 1 and 2 The final microstructural changes over time are shown.

4 and 5, the component steel material of Comparative Example 1 is finally produced from the steel material of Examples 1 and 2 to the hot stamping mold after heating of the bonded steel material It can be predicted that the organization can change sensitively. Specifically, in the case of the steel material of Comparative Example 1, the martensite and ferrite fractions were abruptly changed in accordance with the time of transferring to the hot stamping mold, And it was found that it was unsuitable for use as a collision member part of a vehicle.

FIG. 6 is a graph showing the surface texture of the formed body portion corresponding to the steel material of Example 1 in accordance with the feeding time of the hot stamping mold, and FIG. 7 is a graph showing the surface texture of the molded body portion corresponding to the steel material of Example 2 Fig. 8 shows the surface texture of the molded body portion corresponding to the steel material of Comparative Example 1 in accordance with the hot stamping transfer time. Fig. 6 to 8, ferrites, bainites, and martensite microstructures were produced in the steels of Examples 1 and 2 of the present invention, and the material deviation was small in accordance with the mold transfer time, , Ferrite, and martensite were formed. As a result, it was found that the bainite structure could not be stably secured, and material deviations due to the mold transfer time were significantly larger than those of the steels of Examples 1 and 2.

In general, the steels according to the first and second embodiments are made of titanium (Ti) and niobium (Nb) to prevent material variations in the formed body caused by process parameters such as the transfer time to the hot stamping die, ) Is added to ensure a ferrite region, the incombustibility is increased, the amount of carbon (C) added is reduced to reduce the martensite fraction, and the bainite content is reduced within the range of the hot press process parameter (hot press mold transfer time) (Bainite) structure can be stably secured, and it is possible to prevent the material deviation of each part of the molded article. It was also found that even when expensive molybdenum (Mo) was excluded, the toughness was superior to that of the steel material of Comparative Example 1 and the economical efficiency was excellent.

Microstructure assessment

The structures and elongation rates after hot stamping of the steels of Examples 2 to 4 and Comparative Examples 2 to 4 showing differentiated carbon contents were evaluated and shown in Table 3.

Microstructure (%) Elongation (%) ferrite Martensite Bay knight Example 2 87.5 6.5 6 26 Example 3 88.5 6 5.5 24 Example 4 86.5 7.5 6 25 Comparative Example 2 68 26 6 15 Comparative Example 3 64 30 6 14 Comparative Example 4 53.5 40.5 6 11

Referring to Table 1, in the case of Examples 2 to 4 and Comparative Examples 2 to 4, the content of alloying elements other than the carbon content is substantially the same. Changes in microstructure and elongation due to changes in carbon content can be observed.

In the case of Examples 2 to 4 in which carbon contents of 0.5 wt%, 0.4 wt% and 0.6 wt% were added, respectively, in the cross sectional area fraction, 86.5% to 88.5% of ferrite, 6% to 7.5% of martensite and 5.5 to 6 % Of bainite.

On the contrary, in the case of Comparative Examples 2 to 4, it can be confirmed that the ferrite fraction and the relatively high martensite fraction are relatively lower than those of Examples 2 to 4. As a result, it can be confirmed that the elongation ratios of Examples 2 to 4 are relatively higher than those of Comparative Examples 2 to 4. In the case of Comparative Examples 2 to 4, it can be seen that as the carbon content increases, the martensite fraction increases and the elongation tends to decrease.

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.

-

Claims (8)

(C): 0.04 to 0.06 wt%, silicon (Si): 0.2 to 0.4 wt%, manganese (Mn): 1.6 to 2.0 wt%, phosphorus (P): more than 0 wt% ) Of a steel slab containing more than 0 wt% to 0.003 wt% or less, 0.01 to 0.1 wt% of titanium (Ti), 0.01 to 0.1 wt% of niobium (Nb), and the balance of iron (Fe) and unavoidable impurities Temperature (SRT): reheating at 1220 ° C to 1250 ° C;
Hot rolling the reheated steel slab at a finishing rolling temperature (FDT) of 860 캜 to 900 캜;
Rolling the hot-rolled steel slab at a winding temperature (CT) of 620 to 660 ° C to produce a hot-rolled coil;
Uncoiling the hot rolled coil, and cold rolling to produce a cold rolled sheet; And
And heating and annealing the cold-rolled sheet material at 760 to 850 ° C.
The method according to claim 1,
Wherein the steel slab further comprises chromium (Cr): 0 to 0.3% by weight based on the total weight of the steel slab.
(a) preparing a first blank by cutting a hot stamping steel produced by the manufacturing method of claim 1;
(b) preparing a second blank by cutting a steel material provided separately from the first blank and having different mechanical properties from the first blank;
(c) welding the first and second blanks in a Taylor-welded blank manner to form a bonded steel;
(d) hot-stamping the bonded steel material with a press mold to form a formed body; And
(e) cooling the shaped body at a rate of 30 to 120 DEG C / s
A method of manufacturing a hot stamping component.
The method of claim 3,
(d)
(d1) heating the bonded steel material to a temperature of 850 to 950 占 폚; And
(d2) transferring the heated bonded steel material to the press mold at a transfer time of 7 to 13 seconds
A method of manufacturing a hot stamping component.
The method of claim 3,
After step (e)
The steel material corresponding to the first blank of the formed body has a microstructure composed of ferrite of 83% to 92%, martensite of 3% to 10% and bainite of 3 to 10% at a cross-sectional area fraction
A method of manufacturing a hot stamping component.
(C): 0.04 to 0.06 wt%, silicon (Si): 0.2 to 0.4 wt%, manganese (Mn): 1.6 to 2.0 wt%, phosphorus (P): more than 0 wt% (Ti): 0.01 to 0.1 wt%, niobium (Nb): 0.01 to 0.1 wt%, and the balance of iron (Fe) and unavoidable impurities, Having a microstructure composed of% to 92% of ferrite, 3% to 10% of martensite and 3 to 10% of bainite
Hot stamping parts.
The method according to claim 6,
(Cr): not less than 0 weight% and not more than 0.3 weight% of chromium (Cr) relative to the total weight.
The method according to claim 6,
A tensile strength (TS) of 600 MPa or more, a yield strength (YS) of 400 MPa or more, and an elongation (El) of 20% or more.

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