阅读说明：本技术 热冲压部件及其制造方法 (Hot-stamped component and method for producing same ) 是由 孔帝烈 金志永 朴映喆 金惠真 刘炳吉 陆玩 尹承采 林气鹤 张正焕 于 2019-12-26 设计创作，主要内容包括：根据本发明的实施方案的热冲压部件包括作为基层的钢,所述钢包含：0.28重量％至0.38重量％的碳(C)；0.1重量％至0.4重量％的硅(Si)；1.2重量％至2.0重量％的锰(Mn)；大于0重量％至0.020重量％的磷(P)；大于0重量％至0.003重量％的硫(S)；0.1重量％至0.5重量％的铬(Cr)；0.0015重量％至0.0040重量％的硼(B)；0.025重量％至0.05重量％的钛(Ti)；以及余量的铁(Fe)和不可避免的杂质,其中基层的微观结构为全马氏体。(A hot stamped component according to an embodiment of the invention comprises as a base layer a steel comprising: 0.28 to 0.38% by weight of carbon (C); 0.1 to 0.4 wt% of silicon (Si); 1.2 to 2.0 wt.% manganese (Mn); more than 0 to 0.020% by weight of phosphorus (P); greater than 0 to 0.003 weight percent sulfur (S); 0.1 to 0.5% by weight of chromium (Cr); 0.0015 to 0.0040 wt% of boron (B); 0.025 to 0.05 wt% titanium (Ti); and the balance of iron (Fe) and inevitable impurities, wherein the microstructure of the base layer is fully martensitic.)

1. A hot-stamped component comprising, as a base layer, a steel material whose composition comprises 0.28 to 0.38% by weight of carbon (C), 0.1 to 0.4% by weight of silicon (Si), 1.2 to 2.0% by weight of manganese (Mn), more than 0% but not more than 0.020% by weight of phosphorus (P), more than 0% but not more than 0.003% by weight of sulfur (S), 0.1 to 0.5% by weight of chromium (Cr), 0.0015 to 0.0040% by weight of boron (B), 0.025 to 0.05% by weight of titanium (Ti), and the balance iron (Fe) and unavoidable impurities,

wherein the microstructure of the base layer is full martensite.

2. The hot stamped component of claim 1, further comprising an Al-Si coating on the base layer,

wherein the part of the Fe-Al-Si-based intermetallic compound layer in the coating is less than 40%.

3. The hot-stamped component according to claim 1, wherein the steel material has a Tensile Strength (TS) of 1700MPa or more, a Yield Strength (YS) of 1200MPa or more, and an Elongation (EI) of 6% or more.

4. A method of manufacturing a hot stamped component, the method comprising the steps of:

(a) preparing an ingot including 0.28 to 0.38% by weight of carbon (C), 0.1 to 0.4% by weight of silicon (Si), 1.2 to 2.0% by weight of manganese (Mn), greater than 0% by weight but not more than 0.020% by weight of phosphorus (P), greater than 0% by weight but not more than 0.003% by weight of sulfur (S), 0.1 to 0.5% by weight of chromium (Cr), 0.0015 to 0.0040% by weight of boron (B), 0.025 to 0.05% by weight of titanium (Ti), and the balance of iron (Fe) and inevitable impurities;

(b) heating the billet;

(c) forming a shaped body by hot stamping the heated blank in a die; and

(d) the hot stamped part is formed by cooling the shaped body.

5. The method of claim 4, wherein step (b) comprises: if the thickness of the ingot is 1.2mm, the ingot is heated in a preheating furnace for a certain time to a temperature defined by graphs I-II-III-IV whose heating time-temperature coordinates as vertices are I (175 seconds, 880 ℃), II (210 seconds, 820 ℃), III (745 seconds, 820 ℃) and IV (455 seconds, 880 ℃); if the thickness of the billet is reduced by 0.1mm from 1.2mm, the time to heat the billet is reduced by 12 seconds from the time defined in charts I-II-III-IV; and if the thickness of the billet increases by 0.1mm from 1.2mm, the time to heat the billet increases by 12 seconds from the time defined in graphs I-II-III-IV.

6. The method according to claim 5, wherein the heating condition for heating the billet is set to 6.0 x 10 per unit thickness of the billet⁵[. degree.C.. sec/mm]Or smaller.

7. The method of claim 6, wherein the cooling of the shaped body in step (d) is performed at a rate of at least 10 ℃/sec.

Technical Field

The present invention relates to a hot-stamped component and a method for manufacturing the same, and more particularly to a hot-stamped component having excellent weldability, high delayed fracture resistance and high tensile strength, and a method for manufacturing the same.

Background

Environmental and fuel economy regulations and safety standards are becoming more and more stringent in the current automotive industry. Therefore, the utility of ultra-high strength steels and hot stamping steels is steadily increasing. In particular, in the case of hot stamped steels (including conventional 1.5G hot stamped steels), research and development have been conducted to improve toughness and strength. The hot stamping process typically includes heating, forming, cooling, and trimming, and uses phase transformation and microstructural changes of the material during the process.

The related art includes korean patent application laid-open No. 10-2018-0095757 (entitled "method of manufacturing hot-stamped parts").

Disclosure of Invention

Technical problem

An object of the present application is to provide a hot stamped part having excellent weldability, high delayed fracture resistance and high tensile strength, and a method for manufacturing the same.

Technical scheme

A hot stamped component according to an embodiment of the present application for achieving the above object includes a steel material as a base layer, a composition of which contains 0.28 to 0.38 wt% of carbon (C), 0.1 to 0.4 wt% of silicon (Si), 1.2 to 2.0 wt% of manganese (Mn), more than 0 wt% but not more than 0.020 wt% of phosphorus (P), more than 0 wt% but not more than 0.003 wt% of sulfur (S), 0.1 to 0.5 wt% of chromium (Cr), 0.0015 to 0.0040 wt% of boron (B), 0.025 to 0.05 wt% of titanium (Ti), and the balance of iron (Fe) and inevitable impurities, wherein a microstructure of the base layer is fully martensitic.

The hot-stamped component may further include an Al-Si based coating on the base layer, wherein the fraction of the Fe-Al-Si based intermetallic compound layer in the coating may be less than 40%.

In the hot-stamped part, the steel material may have a Tensile Strength (TS) of 1700MPa or more, a Yield Strength (YS) of 1200MPa or more, and an Elongation (EI) of 6% or more.

A method of manufacturing a hot-stamped component according to an embodiment of the present application for achieving the above object includes the steps of: (a) preparing an ingot including 0.28 to 0.38% by weight of carbon (C), 0.1 to 0.4% by weight of silicon (Si), 1.2 to 2.0% by weight of manganese (Mn), greater than 0% by weight but not more than 0.020% by weight of phosphorus (P), greater than 0% by weight but not more than 0.003% by weight of sulfur (S), 0.1 to 0.5% by weight of chromium (Cr), 0.0015 to 0.0040% by weight of boron (B), 0.025 to 0.05% by weight of titanium (Ti), and the balance of iron (Fe) and inevitable impurities; (b) heating the billet; (c) forming a shaped body by hot stamping the heated blank in a die; and (d) forming the hot stamped part by cooling the shaped body.

In the method of manufacturing a hot-stamped component, step (b) may include: if the thickness of the ingot is 1.2mm, the ingot is heated in a preheating furnace for a certain time to a temperature defined by graphs I-II-III-IV whose heating time-temperature coordinates as vertices are I (175 seconds, 880 ℃), II (210 seconds, 820 ℃), III (745 seconds, 820 ℃) and IV (455 seconds, 880 ℃); if the thickness of the billet is reduced by 0.1mm from 1.2mm, the time to heat the billet is reduced by 12 seconds from the time defined in charts I-II-III-IV; and if the thickness of the billet increases by 0.1mm from 1.2mm, the time to heat the billet increases by 12 seconds from the time defined in graphs I-II-III-IV.

In the method of manufacturing the hot-stamped component, the heating condition for heating the billet may be set to 6.0 × 10 per unit thickness of the billet⁵[. degree.C.. sec/mm]Or smaller.

In the method of manufacturing a hot-stamped component, the cooling of the shaped body in step (d) may be carried out at a rate of at least 10 ℃/s.

Advantageous effects

According to the embodiments of the present application, a hot stamped part having excellent weldability, high delayed fracture resistance, and high tensile strength, and a method for manufacturing the same can be provided. It should be understood that the scope of the present application is not limited by this effect.

Drawings

Fig. 1 is a process flow diagram illustrating a method of manufacturing a hot-stamped component according to an embodiment of the present application.

Fig. 2 is a process flow diagram illustrating a step of preparing a blank for hot stamping in the method of manufacturing a hot stamped part according to an embodiment of the present application illustrated in fig. 1.

Fig. 3 is a graph showing the time and temperature at which a billet is heated in the method of manufacturing a hot-stamped component according to the embodiment of the present application.

Fig. 4 is a graph showing the temperature change with time of the steps of heating a blank, forming a formed body by hot stamping the heated blank in a press mold, and cooling the formed body in the method of manufacturing a hot stamped part according to the embodiment of the present application.

Fig. 5a and 5b are sectional photographs of samples for comparing the parts of the Fe-Al-Si based intermetallic compound layer in the coating layers in the experimental examples of the present application.

Fig. 6a and 6b are photographs showing visual observation of a final part obtained by a hot stamping process according to an experimental example of the present application.

Fig. 7a and 7b are photographs of the microstructure of the final part obtained by the hot stamping process according to the experimental example of the present application.

Detailed Description

Hereinafter, a method of manufacturing a steel material for a line pipe according to an embodiment of the present application will be described in detail. The terms used herein are terms appropriately selected in consideration of the functions of the present application. Therefore, the definition of terms should be made based on the entire contents of the present specification.

Steel material

One aspect of the present application relates to a hot stamped component, which is a steel material obtained by a hot stamping process. In one embodiment, a hot stamped component according to an aspect of the present application includes 0.28 to 0.38 wt% of carbon (C), 0.1 to 0.4 wt% of silicon (Si), 1.2 to 2.0 wt% of manganese (Mn), greater than 0 wt% but not more than 0.020 wt% of phosphorus (P), greater than 0 wt% but not more than 0.003 wt% of sulfur (S), 0.1 to 0.5 wt% of chromium (Cr), 0.0015 to 0.0040 wt% of boron (B), 0.025 to 0.05 wt% of titanium (Ti), and the balance iron (Fe) and inevitable impurities.

The functions and contents of the respective components contained in the steel according to the present application will now be described.

Carbon (C)

Carbon (C) is a main element determining the strength and hardness of the steel, and is added in order to secure the tensile strength of the steel after the hot stamping (or hot pressing) process. In one embodiment, carbon (C) is preferably added in an amount of 0.28 to 0.38 wt.%, based on the total weight of the steel. If the carbon (C) is added in an amount of less than 0.28 wt%, it may be difficult to achieve the mechanical strength of the present application, and if the carbon (C) is added in an amount of more than 0.38 wt%, the toughness of the steel material may be reduced, and it may be difficult to control the brittleness of the steel material.

Silicon (Si)

The purpose of the addition of silicon (Si) is to ensure a softer low temperature phase during the heat treatment. Silicon (Si) is preferably added in an amount of 0.1 to 0.4 wt%, based on the total weight of the steel material of the present application. If the silicon (Si) is added in an amount of less than 0.1 wt%, it may be difficult to secure a softer low-temperature phase during the heat treatment, and if the silicon (Si) is added in an amount of more than 0.4 wt%, there may be a problem in that coating properties of the steel material are deteriorated.

Manganese (Mn)

Manganese (Mn) is added for the purpose of improving hardenability and strength during heat treatment. Manganese (Mn) is preferably added in an amount of 1.2 to 2.0 wt%, based on the total weight of the steel according to the present application. If manganese (Mn) is added in an amount of less than 1.2 wt%, its grain refining effect may be insufficient. On the other hand, if manganese (Mn) is added in an amount of more than 2.0 wt%, the following problems may occur: since the segregation of central manganese occurs, the toughness of the steel is deteriorated and there is a disadvantage in cost.

Phosphorus (P)

Phosphorus (P) is an element that tends to segregate and impair the toughness of steel. Phosphorus (P) is preferably added in an amount of more than 0 wt% but not more than 0.020 wt%, based on the total weight of the steel material of the present application. When phosphorus is added in an amount within the above range, deterioration of toughness of the steel material can be prevented. If phosphorus is added in an amount of more than 0.020% by weight, the martensite grain boundary may be broken, cracks may occur during the process, and an iron phosphide compound may be formed, thereby causing deterioration of toughness of the steel.

Sulfur (S)

Sulfur (S) is an element that impairs processability and physical properties. Sulfur (S) is preferably added in an amount of more than 0 wt% but not more than 0.003 wt%, based on the total weight of the steel according to the present application. If sulfur is added in an amount of more than 0.003 wt%, the martensite grain boundaries may be broken, the hot workability may be reduced, and surface defects such as cracks may occur due to the formation of macro inclusions.

Chromium (Cr)

Chromium (Cr) is added for the purpose of improving hardenability and strength of the steel. Chromium (Cr) is preferably added in an amount of 0.1 to 0.5 wt.%, based on the total weight of the steel according to the present application. If chromium (Cr) is added in an amount of less than 0.1 wt%, the effect of improving hardenability and strength may be insufficient. On the other hand, if chromium (Cr) is added in an amount of more than 0.5 wt%, there may be a problem that the toughness of the steel material is deteriorated.

Boron (B)

The purpose of adding boron (B) is to ensure hardenability of the softer martensite and to refine the grains. Boron (B) is preferably added in an amount of 0.0015 to 0.0040 wt.%, based on the total weight of the steel according to the present application. If boron (B) is added in an amount of less than 0.0015% by weight, the effect of improving hardenability may be insufficient. On the other hand, if boron (B) is added in an amount of more than 0.0040 wt%, there may be a problem that there is an increased risk of embrittlement and a decreased risk of elongation.

Titanium (Ti)

The purpose of adding titanium (Ti) is to increase hardenability and improve material properties through precipitation formation after hot stamping heat treatment, and the purpose of adding titanium (Ti) is to increase strength and toughness by reducing martensite bundle size. Titanium (Ti) is preferably added in an amount of 0.025 to 0.05 wt.%, based on the total weight of the steel according to the present application. If titanium (Ti) is added in an amount of less than 0.025 wt%, precipitate formation may be insufficient and the effect of refining grains may be insufficient. On the other hand, if titanium (Ti) is added in an amount of more than 0.05 wt%, the risk of a decrease in elongation may increase, and the toughness of the steel may deteriorate.

The hot-stamped component includes a steel material having the above composition as a base layer, and the microstructure of the base layer is composed of full martensite. The steel material in the hot-stamped part may have a Tensile Strength (TS) of 1700MPa or more, a Yield Strength (YS) of 1200MPa or more, and an Elongation (EI) of 6% or more. The hot-stamped component may further include an Al-Si based coating on the base layer, wherein the fraction of the Fe-Al-Si based intermetallic compound layer in the coating may be less than 40%.

Hereinafter, a method of manufacturing a hot-stamped part using the above-described steel material of the present application will be described in detail.

Method for producing hot-stamped components

Another aspect of the present application relates to a method of manufacturing a hot-stamped component using a steel material having the above composition.

Fig. 1 is a process flow diagram illustrating a method of manufacturing a hot stamped part according to an embodiment of the present application, and fig. 2 is a process flow diagram illustrating a step of preparing a blank for hot stamping in the method of manufacturing a hot stamped part according to the embodiment of the present application illustrated in fig. 1.

Referring to fig. 1, a method of manufacturing a hot stamped part according to an embodiment of the present application includes the steps of: (a) preparing a blank for hot stamping in the form of a steel material having the above composition (S110); (b) heating the blank (S120); (c) forming a molding body by hot-stamping the heated blank in a die (S130); and (d) forming the hot-stamped component by cooling the molded body (S140).

A step (S110) of preparing a blank for hot stamping

The step of preparing a blank for hot stamping (S110) is a step of forming a blank by cutting a sheet for forming a hot stamped part into a desired shape depending on the intended use.

As shown in fig. 2, the method of forming a billet may include a hot rolling step (S210), a cooling/winding step (S220), a cold rolling step (S230), and an annealing step (S240).

In the method of manufacturing a hot-stamped part according to the present application, a semi-product slab sheet to be subjected to a process of forming a slab includes 0.28 to 0.38 wt% of carbon (C), 0.1 to 0.4 wt% of silicon (Si), 1.2 to 2.0 wt% of manganese (Mn), more than 0 wt% but not more than 0.020 wt% of phosphorus (P), more than 0 wt% but not more than 0.003 wt% of sulfur (S), 0.1 to 0.5 wt% of chromium (Cr), 0.0015 to 0.0040 wt% of boron (B), 0.025 to 0.05 wt% of titanium (Ti), and the balance of iron (Fe) and unavoidable impurities.

For hot rolling, a step of reheating slab sheets is performed. In the slab reheating step, a slab sheet obtained by the continuous casting process is reheated at a Slab Reheating Temperature (SRT) within a predetermined first temperature range, thereby re-dissolving components segregated in the casting process. If the Slab Reheating Temperature (SRT) is below the lower limit of the predetermined first temperature range, the following problems may occur: the components segregated in the casting process are not sufficiently re-dissolved, so that it is difficult to achieve a significant effect of the homogeneous alloy element and a significant effect of dissolving titanium (Ti). Higher Slab Reheating Temperatures (SRT) are more favorable for homogenization. However, if the Slab Reheating Temperature (SRT) is higher than the upper limit of the predetermined first temperature range, austenite grain size may increase, so that it is difficult to secure strength, bake hardenability and aging resistance may also decrease, and production costs of steel sheets may increase due to an excessive heating process.

In the hot rolling step (S210), the reheated slab sheet is finish hot rolled at a finish rolling exit temperature (FDT) within a predetermined second temperature range. In this case, if the finish rolling outlet temperature (FDT) is lower than the lower limit of the predetermined second temperature range, the following problem may occur: it is difficult to ensure workability of a steel sheet due to the occurrence of a mixed grain structure caused by two-phase zone rolling, workability is reduced due to non-uniformity of a microstructure, and a passing property of a sheet is reduced due to rapid phase change during hot rolling. Like SRT, higher finish rolling exit temperature (FDT) is more favorable for homogenization, and is determined by SRT and pass number. However, if the finish rolling exit temperature (FDT) is higher than the upper limit of the predetermined second temperature range, austenite grains may coarsen, and result in a reduction in bake hardenability and aging resistance.

In the cooling/winding step (S220), the hot rolled sheet is cooled to a winding temperature (CT) within a predetermined third temperature range and wound at the winding temperature (CT). The winding temperature affects the redistribution of the carbon (C). If the winding temperature is lower than the lower limit of the predetermined third temperature range, the following problem may occur: due to the supercooling, the fraction of low-temperature phase increases, resulting in an increase in strength and a significant increase in rolling load during cold rolling, and rapid deterioration in ductility. On the other hand, if the winding temperature is higher than the upper limit of the predetermined third temperature range, there may be a problem that moldability and strength are deteriorated due to two-phase grain growth or excessive grain growth.

In the cold rolling step (S230), the wound sheet is unwound, pickled, and then cold rolled. In this case, the pickling is performed for the purpose of removing an oxide scale from the wound sheet (i.e., a hot-rolled coil produced by a hot rolling process).

The annealing step (S240) is a step of annealing the cold-rolled sheet. In one embodiment, annealing includes the steps of heating the cold rolled sheet and cooling the heated cold rolled sheet at a predetermined cooling rate.

Meanwhile, in a hot stamping step (S130) of fig. 1 described later, a blank to be formed is softened by heating at a high temperature, pressed, and then cooled. Therefore, since the steel material is softened by heating at a high temperature, it is easily pressed, and the mechanical strength of the steel material is improved by cooling quenching after forming. However, oxidation of iron (Fe) on the surface of the steel material generates oxides (scale) due to heating of the steel material at a high temperature of 800 ℃ or higher. Accordingly, in one embodiment of the present application, a predetermined coating layer may be formed on the cold-rolled steel sheet after annealing. Specifically, an aluminum (Al) -based metal coating, such as an aluminum (Al) -silicon (Si) -based coating, having a higher melting point than an organic coating or a zinc (Zn) -based coating may be formed. The aluminum (Al) -silicon (Si) -coated cold-rolled steel sheet may be prevented from being corroded, and may be prevented from forming scale on the surface of the hot steel sheet during moving to the press. Specifically, the adhesive is used at both surfaces with 100g/m²To 180g/m²The Al-Si coated Mn-B steel sheet.As mentioned above, the composition of the base layer consists of: 0.28 to 0.38 wt% of carbon (C), 0.1 to 0.4 wt% of silicon (Si), 1.2 to 2.0 wt% of manganese (Mn), more than 0 wt% but not more than 0.020 wt% of phosphorus (P), more than 0 wt% but not more than 0.003 wt% of sulfur (S), 0.1 to 0.5 wt% of chromium (Cr), 0.0015 to 0.0040 wt% of boron (B), 0.025 to 0.05 wt% of titanium (Ti), and the balance iron (Fe).

Step of heating the billet (S120)

Fig. 3 is a graph showing the time and temperature at which a billet is heated in the method of manufacturing a hot-stamped component according to the embodiment of the present application.

Referring to fig. 3, if the thickness of the ingot is 1.2mm, the ingot is heated in a preheating furnace for a certain time to a temperature defined by graphs I-II-III-IV whose heating time-temperature coordinates as vertexes are I (175 seconds, 880 c), II (210 seconds, 820 c), III (745 seconds, 820 c) and IV (455 seconds, 880 c). That is, the time and temperature at which the billet is heated include the time and temperature corresponding to the inner region and boundary line of the graphs I-II-III-IV. Meanwhile, if the thickness of the ingot is reduced by 0.1mm from 1.2mm, the time for heating the ingot is reduced by 12 seconds from the time defined in the graphs I-II-III-IV. Accordingly, the graph having the heating time-temperature coordinate as a vertex may be moved in the direction of the first arrow 310. Further, if the thickness of the ingot is increased by 0.1mm from 1.2mm, the time for heating the ingot is increased by 12 seconds from the time defined in FIGS. I-II-III-IV. Accordingly, the graph having the heating time-temperature coordinate as a vertex may be moved in the direction of the second arrow 330.

Fig. 4 is a graph showing the temperature change with time of the steps of heating a blank, forming a formed body by hot stamping the heated blank in a press mold, and cooling the formed body in the method of manufacturing a hot stamped part according to the embodiment of the present application. 'heating' corresponds to a step of heating the blank before hot stamping, and 'cooling' corresponds to a step of forming the heated blank by hot stamping and cooling the formed body.

With reference to FIG. 4, for addingThe heating conditions of the hot billet may be set to 6.0X 10 per unit thickness of the billet⁵[. degree.C.. sec/mm]Or smaller. That is, although there is no limitation on the heating method and heating rate of the ingot, the ratio of the area per unit thickness of the ingot under the heating curve of fig. 4 is limited to 6.0 × 10⁵[. degree.C.. sec/mm]Or smaller. If the blank is heated to more than necessary at a heating rate higher than this value, the desired weldability and hydrogen embrittlement resistance cannot be obtained when the blank is applied to a vehicle body member.

A hot stamping step (S130) and a cooling step (S140)

The blank heated under the above conditions was transferred into a press mold. After the blank is formed into the shape of the final part in a die for hot stamping, the obtained formed body is cooled to form the final product. The die may include cooling channels through which a refrigerant is circulated. The heated blank may be rapidly cooled via circulation of a refrigerant provided through the cooling channel. In this case, in order to prevent the spring back phenomenon of the steel material while maintaining a desired shape, rapid cooling may be performed while pressing the die in a state where the die is closed. In the method of shaping and cooling the heated material, the heated material may be cooled to the martensite finish temperature at an average cooling rate of at least 10 ℃/sec. If the cooling rate is less than the above cooling rate, ferrite or bainite may be generated, and thus, mechanical properties (e.g., tensile strength of 1700MPa or more) may not be satisfied.

The use of the above-described components and process conditions enables to obtain a hot-stamped part (member) having excellent weldability and delayed fracture resistance. The microstructure of the base layer of the hot-stamped component has an all-martensite structure, and satisfies a tensile strength of 1700MPa or more and an elongation of 6% or more.

Environmental and fuel economy regulations and safety standards are becoming more and more stringent in the current automotive industry. Therefore, the utility of ultra-high strength steels and hot stamping steels is steadily increasing. In particular, in the case of hot stamped steels (including conventional 1.5G hot stamped steels), research and development have been conducted to improve toughness and strength. The hot stamping process typically includes heating, forming, cooling, and trimming, and uses phase transformation and microstructural changes of the material during the process.

The present application aims to improve the strength of conventional hot stamped steels by applying a hot stamping process suitable for the altered material composition to ensure the desired properties. However, when the conventional 1.5G hot stamping process conditions are applied to the hot stamped component steel grade according to the embodiment of the present application as described above (1.8G hot stamped steel grade), various problems may occur due to excessive heating. Namely the following problems: the Austenite Grain Size (AGS) is coarsened and not uniform, resulting in deterioration of mechanical properties, change in appearance color and reduction in welding performance, and increased susceptibility to hydrogen embrittlement due to an increase in the amount of hydrogen added.

In this regard, conventional 1.5G hot stamping process conditions include the following steps: heating the blank between 20 ℃ and 700 ℃ at an average heating rate of 4 ℃/s to 12 ℃/s; ② if the thickness of the steel sheet is 0.7mm to 1.5mm, the slab is heated in the preheating furnace for a certain time to a temperature defined by the graphs A-B-C-D having heating time-temperature coordinates of A (3 minutes, 930 ℃), B (6 minutes, 930 ℃), C (13 minutes, 880 ℃) and D (4.5 minutes, 880 ℃) as vertexes, and if the thickness of the steel sheet is 1.5mm to 3.0mm, the slab is heated in the preheating furnace for a certain time to a temperature defined by the graphs E-F-G-H having heating time-temperature coordinates of E (4 minutes, 940 ℃), F (8 minutes, 940 ℃), G (13 minutes, 900 ℃) and H (6.5 minutes), 900 ℃); and thirdly cooling the blank to 400 ℃ at an average cooling rate of at least 30 ℃/sec.

In contrast, when the hot stamped part composition and the process conditions according to the embodiments of the present application are applied, a hot stamped part having excellent weldability and delayed fracture resistance and having high tensile strength, and a method of manufacturing the same can be realized.

For example, according to one embodiment of the present application, a final part obtained by a hot stamping process has an average Prior Austenite Grain Size (PAGS) of 25 μm or less to ensure delayed fracture resistance, and exhibits mechanical properties including Tensile Strength (TS) of 1700MPa or more, Yield Strength (YS) of 1200MPa or more, and Elongation (EI) of 6% or more. Thus, the final part can overcome the problem of deterioration of mechanical properties due to coarsening and non-uniformity of Austenite Grain Size (AGS).

Furthermore, according to one embodiment of the present application, the visually observed color of the final part obtained by the hot stamping process is not reddish. This indicates that iron (Fe) in the base layer is prevented from excessively diffusing into the coating layer due to unnecessary heating. Further, weldability of the final part can be ensured by controlling the fraction of the Fe — Al-Si based intermetallic compound layer in the coating layer to less than 40%. Therefore, the final part can overcome the problems of change in the appearance color and deterioration in the weldability.

Further, according to one embodiment of the present application, when the delayed fracture resistance of the final part obtained by the hot stamping process was evaluated by the 4-point bending test, it was confirmed that no fracture occurred within 100 hours. Thus, the final part can overcome the problem of increased hydrogen embrittlement sensitivity due to an increase in the amount of hydrogen added.

Experimental examples

The constitution and effect of the present application will be described in more detail below with reference to experimental examples. However, these experimental examples are presented as preferred examples of the present application and are not to be construed as limiting the scope of the present application in any way. In addition, since a person skilled in the art can sufficiently and technically deduce what is not described herein, the description thereof will be omitted.

[ Table 1]

[ Table 2]

Table 1 above shows the component system ingredients according to the experimental examples of the present application. Referring to table 1 above, component system 1 satisfies the ingredients comprising: 0.28 to 0.38 wt% of carbon (C), 0.1 to 0.4 wt% of silicon (Si), 1.2 to 2.0 wt% of manganese (Mn), more than 0 wt% but not more than 0.020 wt% of phosphorus (P), more than 0 wt% but not more than 0.003 wt% of sulfur (S), 0.1 to 0.5 wt% of chromium (Cr), 0.0015 to 0.0040 wt% of boron (B), 0.025 to 0.05 wt% of titanium (Ti), and the balance iron (Fe). On the other hand, unlike component system 1, component system 2 does not satisfy a composition containing 0.1 to 0.4 wt% of silicon (Si) and 1.2 to 2.0 wt% of manganese (Mn), but further contains nickel (Ni), niobium (Nb), and molybdenum (Mo).

Table 2 above shows the steel material composition and hot stamping heating conditions of the hot stamped parts according to the experimental examples of the present application.

Referring to table 2 above and fig. 3, each of examples 1 to 7 had ingredients satisfying component system 1 of ingredients comprising: 0.28 to 0.38 wt% of carbon (C), 0.1 to 0.4 wt% of silicon (Si), 1.2 to 2.0 wt% of manganese (Mn), more than 0 wt% but not more than 0.020 wt% of phosphorus (P), more than 0 wt% but not more than 0.003 wt% of sulfur (S), 0.1 to 0.5 wt% of chromium (Cr), 0.0015 to 0.0040 wt% of boron (B), 0.025 to 0.05 wt% of titanium (Ti), and the balance iron (Fe). Further, each of examples 1 to 5 corresponds to the case where the blank thickness was 1.2mm as the hot stamping heating condition. In examples 1 to 5, the ingot was heated in a preheating furnace for a certain period of time to a temperature defined by graphs I-II-III-IV whose heating time-temperature coordinates as vertices were I (175 seconds, 880 ℃ C.), II (210 seconds, 820 ℃ C.), III (745 seconds, 820 ℃ C.) and IV (455 seconds, 880 ℃ C.).

Example 6 corresponds to a billet thickness of 1.0mm as the hot stamping heating conditions. In example 6, if the thickness of the ingot is reduced by 0.1mm from 1.2mm, the time to heat the ingot is reduced by 12 seconds from the time defined in graphs I-II-III-IV. That is, the heating conditions for hot stamping are determined from graphs having heating time-temperature coordinates of I (151 seconds, 880 ℃ C.), II (186 seconds, 820 ℃ C.), III (721 seconds, 820 ℃ C.) and IV (431 seconds, 880 ℃ C.) as vertexes. Example 7 corresponds to a billet thickness of 1.8 mm. In example 7, if the thickness of the ingot was increased from 1.2mm by 0.1mm, the time to heat the ingot was increased from the time defined in charts I-II-III-IV by 12 seconds. That is, the heating conditions for hot stamping are determined from graphs having heating time-temperature coordinates of I (247 seconds, 880 ℃ C.), II (282 seconds, 820 ℃ C.), III (817 seconds, 820 ℃ C.) and IV (527 seconds, 880 ℃ C.) as vertexes.

Unlike these examples, in each of comparative examples 1 to 4, if the thickness of the billet as the hot stamping heating condition was 1.2mm, the billet was not heated in a preheating furnace for a certain period of time to the temperature defined by the graphs I-II-III-IV whose heating time-temperature coordinates as vertexes were I (175 seconds, 880 ℃ C.), II (210 seconds, 820 ℃ C.), III (745 seconds, 820 ℃ C.) and IV (455 seconds, 880 ℃ C.).

Unlike component system 1, the composition of each of comparative examples 5 to 7 does not satisfy the composition including 0.1 to 0.4 wt% of silicon (Si) and 1.2 to 2.0 wt% of manganese (Mn), but further includes nickel (Ni), niobium (Nb), and molybdenum (Mo). Each of comparative examples 5 to 7 did not satisfy the following condition if the blank thickness as a heating condition for hot stamping was 1.2 mm: the ingot is heated in a preheating furnace for a certain time to a temperature defined by the graphs I-II-III-IV whose heating time-temperature coordinates as vertices are I (175 seconds, 880 ℃), II (210 seconds, 820 ℃), III (745 seconds, 820 ℃) and IV (455 seconds, 880 ℃).

[ Table 3]

Table 3 below shows mechanical properties, microstructures and application properties of the hot-stamped parts according to the experimental examples of the present application.

Referring to fig. 3, it can be confirmed that each of examples 1 to 7 satisfies all mechanical properties including: a Tensile Strength (TS) of 1700MPa or more, a Yield Strength (YS) of 1200MPa or more, and an Elongation (EI) of 6% or more, the microstructure thereof being all martensite, the final parts obtained by the hot stamping process each having an average Prior Austenite Grain Size (PAGS) of 25 μm or less to ensure delayed fracture resistance, no fracture occurred in the delayed fracture test, and the fraction of the Fe-Al-Si based intermetallic compound layer in the coating layer being less than 40%.

On the other hand, it was confirmed that each of comparative examples 1 to 5 did not satisfy mechanical properties including: a Tensile Strength (TS) of 1700MPa or more and a Yield Strength (YS) of 1200MPa or more, and its microstructure is not fully martensitic but is composed of ferrite and martensite. It can be confirmed that comparative example 2 does not satisfy the mechanical properties including: a Tensile Strength (TS) of 1700MPa or more, a Yield Strength (YS) of 1200MPa or more, and an Elongation (EI) of 6% or more, and its microstructure is not fully martensitic, but is composed of ferrite and martensite. It was confirmed that, in comparative example 3, the final part obtained by the hot stamping process had an average Prior Austenite Grain Size (PAGS) of more than 25 μm, fracture occurred in the delayed fracture test, and the fraction of the Fe-Al-Si based intermetallic compound layer in the coating was more than 40%. It was confirmed that in comparative example 4, fracture occurred in the delayed fracture test, and the fraction of the Fe-Al-Si based intermetallic compound layer in the coating layer was more than 40%. It was confirmed that the fraction of the Fe-Al-Si based intermetallic compound layer in the coating layers was more than 40% in comparative examples 6 and 7.

FIG. 5 shows a cross-sectional photograph of a sample for comparing the parts of Fe-Al-Si based intermetallic compound layers in the coatings in the experimental examples of the present application. Fig. 5(a) is a photograph of a cross section of a sample of example 3 corresponding to table 3, and fig. 5(b) is a photograph of a cross section of a sample of comparative example 3 corresponding to table 3.

As shown in fig. 5(a), the cross-section of the coating of the final part obtained by the hot stamping process should have a Fe-Al-Si based intermetallic compound layer fraction corresponding to less than 40% of the total fraction of the coating to ensure the desired weldability. It was confirmed that the above-mentioned range of parts can be satisfied when the process conditions in the ingredient system 1 in table 1 and table 2 are satisfied. On the other hand, referring to fig. 5(b), when the Fe — Al-Si based intermetallic compound layer becomes thick and exceeds 40% in fraction, welding resistance thereof increases, so that the possibility of occurrence of spatters or iron burrs increases, resulting in deterioration of weldability.

Fig. 6 shows a photograph showing visual observation of a final part obtained by a hot stamping process according to an experimental example of the present application. Fig. 6(a) is a photograph of a cross section of a sample of example 2 corresponding to table 3, and fig. 6(b) is a photograph of a cross section of a sample of comparative example 6 corresponding to table 3.

As shown in fig. 6(a), the visually observed color of the final part obtained by the hot stamping process should not be reddish. It was confirmed that the visually observed color was not reddish when the process conditions in ingredient system 1 in table 1 and table 2 were satisfied. That is, in fig. 6(a), the appearance color is pale blue. On the other hand, in fig. 6(b), it can be confirmed that the color of visual observation is reddish. Although the apparent color is not directly related to the Fe-Al-Si based intermetallic compound layer in the coating layer, it is understood that Fe in the base layer has sufficiently diffused into the coating layer due to unnecessary heating as shown in fig. 6 (b). In this case, if the fraction of the Fe-Al-Si based intermetallic compound layer in the coating layer is increased to 40% or more, weldability is deteriorated.

Fig. 7 shows a photograph of the microstructure of the final part obtained by the hot stamping process according to an experimental example of the present application. Fig. 7(a) is a photograph of a microstructure of a sample (average PAGS of 12.2 μm) corresponding to example 2 of table 3, and fig. 7(b) is a photograph of a microstructure of a sample (average PAGS of 26.2 μm) corresponding to comparative example 3 of table 3.

As shown in fig. 7(a), the final part obtained by the hot stamping process should have an average PAGS of 25 μm or less to ensure delayed fracture resistance. It was confirmed that the above PAGS range was satisfied when the process conditions in ingredient system 1 in table 1 and table 2 were satisfied. On the other hand, referring to fig. 7(b), it can be confirmed that, due to additional heating after completion of austenite phase transformation during heating, austenite grain growth occurs, so that austenite grains are gradually coarsened and become non-uniform. Since austenite grain size is coarse and uneven, the delayed fracture resistance of the part is reduced after completion of the martensitic transformation caused by the cooling operation. In the embodiments of the present invention, one of the main objects of the present application is to obtain the corresponding properties through the preferred hot stamping operation without much reliance on elements for grain refinement, such as Nb, Mo, and V.

Although the present application has been described above with reference to the embodiments thereof, various changes or modifications may be made by those skilled in the art. These changes and modifications are to be considered within the scope of the present application as long as they do not depart from the scope of the present application. Accordingly, the scope of the present application should be determined by the claims described below.