KR102110679B1 - Hot stamping product and method of manufacturing the same - Google Patents

Abstract

The present invention is carbon (C): 0.1 to 0.35% by weight, silicon (Si): 0.01 to 0.5% by weight, manganese (Mn): 1.0 to 3.0% by weight, phosphorus (P): more than 0 and 0.1% by weight or less, sulfur ( S): more than 0 and less than 0.01% by weight, titanium (Ti): 0.025 to 0.05% by weight, chromium (Cr): 0.1 to 2.0% by weight, boron (B): 0.0001 to 0.005% by weight and the rest of iron (Fe) and others Made of unavoidable impurities, the final microstructure provides a hot stamping part made of tempered martensite.

Description

Hot stamping parts and its manufacturing method {HOT STAMPING PRODUCT AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a component and a method for manufacturing the same, and more particularly, to a hot stamping component and a method for manufacturing the same.

The automotive industry is leading the development of ultra-high-strength steel for automobiles by reflecting the demands of the automobile market in accordance with the trend of weight reduction and collision law enforcement. Currently, there is a demand for such steel types in the manufacture of automobile parts for the purpose of enhancing collision properties such as bumper back beams of ultra-high-strength steel of thin plates having a tensile strength of 1500 MPa or more having weldability, moldability, paintability and hydrogen-delay fracture resistance. . Carbon content is the most important factor in determining the maximum tensile strength of martensitic steel, and martensitic alloys having a maximum tensile strength of 1500 MPa have been developed. In the case of heat-treated hardened steel, it has 60K grade pearlite and ferrite before hot stamping, but after hot forming and mold cooling at high temperature, it is a steel type having a maximum tensile strength of 1500 to 2200 MPa, elongation of 5% or more, all with a martensite structure. However, in the case of high-strength martensitic steel, it has been reported that there is concern about the occurrence of hydrogen delay destruction by hydrogen introduced into the steel. Lecture development is necessary.

A related prior art is Korean Patent Application No. 10-2015-0059516.

In order to solve the above problems, the technical problem to be achieved by the present invention is to provide a hot stamping component and a method of manufacturing the same as heat-hardened hardened steel for reducing diffusive hydrogen and improving hydrogen delay destruction performance.

Hot stamping parts according to an embodiment of the present invention for achieving the above object is carbon (C): 0.1 ~ 0.35% by weight, silicon (Si): 0.01 ~ 0.5% by weight, manganese (Mn): 1.0 ~ 3.0% by weight , Phosphorus (P): more than 0 to 0.1% by weight, sulfur (S): more than 0 to 0.01% by weight, titanium (Ti): 0.025 to 0.05% by weight, chromium (Cr): 0.1 to 2.0% by weight, boron (B ): 0.0001 to 0.005% by weight and the remaining iron (Fe) and other inevitable impurities, but the final microstructure is made of tempered martensite.

Hot stamping parts according to another embodiment of the present invention for achieving the above object is carbon (C): 0.1 ~ 0.35% by weight, silicon (Si): 0.01 ~ 0.5% by weight, manganese (Mn): 1.0 ~ 3.0% by weight , Phosphorus (P): more than 0 to 0.1% by weight, sulfur (S): more than 0 to 0.01% by weight, titanium (Ti): 0.02 to 0.05% by weight, niobium (Nb): more than 0 to 0.05% by weight, chromium ( Cr): 0.1 to 2.0% by weight, boron (B): 0.0001 to 0.005% by weight, and the remaining iron (Fe) and other inevitable impurities, but the final microstructure consists of tempered martensite.

In the hot stamping part, the yield strength is 1100 ~ 1400MPa, the tensile strength is 1600 ~ 2000MPa, the hardness is 480 ~ 580Hv, the elongation may be 5% or more.

Method for manufacturing a hot stamping part according to an embodiment of the present invention for achieving the above object is a step of reheating the steel having the composition of the above-described component system at 1150 ~ 1300 ℃; (b) hot rolling the reheated steel under conditions of a finish rolling temperature of 800 to 950 ° C., and then cooling at a cooling rate of 5 to 50 ° C./s to wind at 600 to 800 ° C .; (c) after cold-rolling the steel, annealing heat treatment at 700 to 900 ° C and cooling to a cooling end temperature of 100 to 300 ° C at a cooling rate of 5 to 100 ° C / s; (d) performing a hot stamping process on the steel material; It includes.

In the method of manufacturing the hot stamping part, the step (d) may include maintaining the steel at an austenizing temperature of 700 to 1000 ° C. for 1 to 20 minutes; Cooling the steel material at a cooling rate of 5 to 100 ° C / s; It may include sequentially; the step of heat-treating the steel material.

In the method of manufacturing the hot stamping part, the microstructure of the steel material subjected to step (b) includes ferrite and pearlite, and the microstructure of the steel material subjected to step (c) includes ferrite and pearlite, and (d) The microstructure of the steel material that has been subjected to step may include tempered martensite.

According to an embodiment of the present invention, it is possible to implement a hot stamping component and a method of manufacturing the heat-hardened hardened steel for reducing diffusive hydrogen and improving hydrogen delay destruction performance. Of course, the scope of the present invention is not limited by these effects.

1 is a flowchart schematically showing a method of manufacturing a hot stamping part according to an embodiment of the present invention.
Figure 2 is a graph showing the comparison of the solid concentration of carbon in the matrix in the specimens according to the experimental example of the present invention.
Figure 3 is a graph showing the comparison of the carbide production concentration in the specimens according to the experimental example of the present invention.
Figure 4 is a photograph of the behavior of the precipitates through the TEM Replica technique in the specimens according to the experimental example of the present invention and the observation of the increase pattern of fine carbide.
5 is a photograph observing the behavior of the precipitate through the TEM thin-foil analysis and the precipitation pattern of the fine carbide in the specimens according to the experimental example of the present invention.
6 is an optical micrograph and SEM photograph of the martensitic microstructure observed in specimens according to the experimental examples of the present invention.
FIG. 7 is an optical micrograph of a microstructure of aprior austenite in specimens according to an experimental example of the present invention.
8 is a graph showing the results of measuring the hydrogen reduction current density in the specimens according to the experimental example of the present invention.
9 is a graph showing the results of measuring the change in the hydrogen reduction voltage according to the cycle in the specimens according to the experimental example of the present invention.
10 is a graph showing the transition of the maximum tensile strength according to the hydrogen injection time in the specimens according to the experimental example of the present invention.
11 is a graph showing the transition of elongation according to the hydrogen injection time in specimens according to the experimental examples of the present invention.
12 is a graph showing hydrogen susceptibility showing the elongation reduction rate after injecting hydrogen for 1 hour in specimens according to the experimental examples of the present invention.

Hereinafter, a hot stamping part and a method of manufacturing the same according to an embodiment of the present invention will be described in detail. Terms to be described later are terms appropriately selected in consideration of functions in the present invention, and definitions of these terms should be made based on contents throughout the present specification.

Hot stamping parts

The hot stamping part according to an embodiment of the present invention is implemented with heat-treated hardened steel for reducing diffusive hydrogen and improving hydrogen delay destruction performance. The heat-treated hardened steel can be understood as a hot stamping steel, carbon (C): 0.1 to 0.35% by weight, silicon (Si): 0.01 to 0.5% by weight, manganese (Mn): 1.0 to 3.0% by weight, phosphorus (P): more than 0 to 0.1% by weight, sulfur (S): more than 0 to 0.01% by weight, titanium (Ti): 0.025 to 0.05% by weight, chromium (Cr): 0.1 to 2.0% by weight, boron (B): 0.0001 to 0.005% by weight and the rest of iron (Fe) and other unavoidable impurities. Or, carbon (C): 0.1 to 0.35% by weight, silicon (Si): 0.01 to 0.5% by weight, manganese (Mn): 1.0 to 3.0% by weight, phosphorus (P): more than 0 and 0.1% by weight or less, sulfur (S ): More than 0 and less than 0.01% by weight, titanium (Ti): 0.02 to 0.05% by weight, niobium (Nb): more than 0 and less than 0.05% by weight, chromium (Cr): 0.1 to 2.0% by weight, boron (B): 0.0001 to It consists of 0.005% by weight and the remaining iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the hot stamping part according to an embodiment of the present invention will be described.

Carbon (C)

Carbon (C) is the most effective and important element for increasing the strength of steel. In addition, it is dissolved in austenite by the addition of carbon to form a martensite structure upon quenching. Furthermore, it forms a carbide by combining with elements such as iron, chromium, molybdenum, and vanadium to improve strength and hardness. Carbon (C) may be added in a content ratio of 0.1 to 0.35% by weight of the total weight of the hot stamping part according to an embodiment of the present invention. When the content of carbon is less than 0.1% by weight of the total weight, the above-described effect cannot be realized and difficulty in securing sufficient strength may follow. Conversely, when the carbon content exceeds 0.35% by weight of the total weight, the hydrogen embrittlement resistance is deteriorated, which may cause deterioration of toughness and weldability of the hot stamping part.

Silicon (Si)

Silicon (Si) is a solid solution strengthening element that contributes to strengthening of the steel and is an effective element for improving ductility. In addition, it plays a role of suppressing the formation of cementite, which is a starting point for cracking due to hydrogen embrittlement. Furthermore, it is well known as a ferrite stabilizing element and is known as an element that increases ductility by increasing the ferrite fraction during cooling. On the other hand, silicon is added as a deoxidizer for removing oxygen in the steel in the steelmaking process together with aluminum, and may also have a solid solution strengthening effect. The silicone may be added in a content ratio of 0.01 to 0.5% by weight of the total weight of the hot stamping part according to an embodiment of the present invention. When the content of silicone is less than 0.01% by weight of the total weight, the above-described effect of adding silicone cannot be properly exhibited. Conversely, when the content of silicon exceeds 0.5% by weight of the total weight, when added in large amounts, the toughness of the base material and the toughness of the weld heat-affected zone are impaired, the plastic workability is deteriorated, the weldability of the steel is reduced, and during reheating and hot rolling. Creating a red scale can cause problems with surface quality.

Manganese (Mn)

Manganese (Mn) is an element that can improve the quenching property, and is an element that improves the strength of the steel sheet by compensating for the reduced hardenability by the low carbon content. A part of manganese is employed in the steel, and some is combined with sulfur contained in the steel to form a non-metallic inclusion, MnS. This MnS is ductile and elongated in the machining direction during plastic working. However, the formation of MnS reduces the sulfur content in the steel, making the grains vulnerable and suppressing the formation of the low-melting compound FeS. It inhibits the acid and oxidation resistance of the steel, but the pearlite becomes fine and ferrite is solidified to improve the yield strength. Manganese may be added in a content ratio of 1.0 to 3.0% by weight of the total weight of the hot stamping part according to an embodiment of the present invention. When the content of manganese is less than 1.00% by weight, the above-described effect of adding manganese cannot be sufficiently exhibited. In addition, when the content of manganese exceeds 3.0% by weight, segregation occurs, resulting in tissue nonuniformity, lowering toughness, causing quenching cracks or deformation, lower weldability, MnS inclusions and center segregation ) May cause the ductility of the hot stamping parts to deteriorate and the corrosion resistance to deteriorate.

Phosphorus (P)

Phosphorus (P) is an element that causes intergranular segregation in the base material and welds, and it needs to be actively reduced to prevent the problem of embrittlement of the steel, but in order to reduce phosphorus to the limit, the load of the steelmaking process is intensified. However, if the content of phosphorus is 0.1% by weight or less, the above problem does not occur significantly, so it is preferable to limit the upper limit to 0.1% by weight. Furthermore, it is possible to strictly limit the upper limit to 0.05% by weight or less, and more strictly to 0.03% or less.

Sulfur (S)

Sulfur (S) promotes hydrogen absorption into steel in a corrosive environment, forms the same sulfide as MnS, which is the starting point for cracking by hydrogen embrittlement, and prevents the problem of embrittlement of steel as an element that causes red-hot embrittlement of steel. In order to reduce the sulfur to the limit, the load of the steelmaking process is intensified. If the content of sulfur is less than 0.01% by weight, the above problem does not occur significantly, so it is preferable to limit the upper limit to 0.01% by weight. Furthermore, the upper limit can be strictly limited to 0.005% by weight or less.

Titanium (Ti)

Titanium (Ti) is an effective element for improving toughness of a steel sheet by forming a nitride together with nitrogen and preventing coarsening of austenite grains in a slab heating step. For example, it is an element that suppresses the formation of AlN through the formation of high temperature TiN and has the effect of miniaturizing grain size through formation of Ti (C, N). Hot stamping parts according to an embodiment of the present invention, when not containing niobium (Nb), the titanium may be added in a content ratio of 0.025 to 0.05% by weight of the total weight. When the content of titanium is less than 0.025% by weight of the total weight, the above-described effect cannot be realized, and when the content of titanium exceeds 0.05% by weight of the total weight, the nitride is coarsened to lower the toughness of the base material, and The carbon content may decrease, resulting in a problem that the properties of the steel are deteriorated.

Niobium (Nb)

Niobium (Nb) is an element that improves the strength of the base material and the weld by depositing in the form of NbC or Nb (C, N). The niobium may be added in a content ratio of more than 0 to 0.05% by weight or less of the total weight of the hot stamping part according to an embodiment of the present invention. When the content of niobium exceeds 0.05% by weight of the total weight, a brittle crack occurs when a large amount is added. When the hot stamping part according to an embodiment of the present invention contains niobium (Nb), the aforementioned titanium may be added in a content ratio of 0.02 to 0.05% by weight of the total weight of the hot stamping part.

Chrome (Cr)

Chromium (Cr) is a ferrite stabilizing element that, when added to C-Mn steel, delays the diffusion of carbon due to a solute interference effect and affects particle size refinement. The chromium may be added in a content ratio of 0.1 to 2.0% by weight of the total weight of the hot stamping part according to an embodiment of the present invention. When the chromium content is less than 0.1% by weight of the total weight, the above-described effect of adding chromium cannot be properly exhibited. Conversely, when the content of chromium exceeds 2.0% by weight of the total weight, when added in large quantities, the toughness is lowered and the workability and machinability may deteriorate.

Boron (B)

Boron (B) is an element effective in improving the quenchability and improving the strength of steel materials. Boron may be added in a content ratio of 0.0001 to 0.005% by weight of the total weight of the hot stamping part according to an embodiment of the present invention. When the content of manganese is less than 0.0001% by weight, the effect of adding boron cannot be sufficiently exhibited. In addition, when the boron content exceeds 0.005% by weight, hot workability deteriorates.

As described above, the final microstructure of the hot stamping part according to an embodiment of the present invention having an alloy element composition is made of tempered martensite. The hot stamping part having such a composition and microstructure may have a yield strength of 1100 to 1400 MPa, a tensile strength of 1600 to 2000 MPa, a hardness of 480 to 580 Hv, and an elongation of 5% or more.

Hereinafter, a method of manufacturing a hot stamping component according to an embodiment of the present invention having the above-described alloy element composition will be described.

Method of manufacturing hot stamping parts

1 is a flowchart schematically showing a method of manufacturing a hot stamping part according to an embodiment of the present invention.

Referring to Figure 1, the method of manufacturing a hot stamping part according to an embodiment of the present invention (a) reheating the steel having the above-described component system at 1150 ~ 1300 ℃ (S100); (b) hot rolling the reheated steel under the conditions of a finish rolling temperature of 800 to 950 ° C., and then cooling at a cooling rate of 5 to 50 ° C./s to wind at 600 to 800 ° C. (S200); (c) cold rolling the steel and annealing heat treatment at 700 to 900 ° C., followed by cooling at a cooling rate of 5 to 100 ° C./s to a cooling end temperature of 100 to 300 ° C. (S300); (d) performing a hot stamping process on the steel (S400).

The hot stamping steel is carbon (C): 0.1 to 0.35% by weight, silicon (Si): 0.01 to 0.5% by weight, manganese (Mn): 1.0 to 3.0% by weight, phosphorus (P): more than 0 and 0.1% by weight or less , Sulfur (S): more than 0 and less than 0.01% by weight, titanium (Ti): 0.025 to 0.05% by weight, chromium (Cr): 0.1 to 2.0% by weight, boron (B): 0.0001 to 0.005% by weight and the rest of iron (Fe ) And other inevitable impurities, or carbon (C): 0.1 to 0.35 wt%, silicon (Si): 0.01 to 0.5 wt%, manganese (Mn): 1.0 to 3.0 wt%, phosphorus (P) : More than 0 and 0.1% by weight or less, sulfur (S): more than 0 and less than 0.01% by weight, titanium (Ti): 0.02 to 0.05% by weight, niobium (Nb): more than 0 and less than 0.05% by weight, chromium (Cr): 0.1 to 2.0% by weight, boron (B): 0.0001 to 0.005% by weight, and may have a second component system composed of the remaining iron (Fe) and other inevitable impurities.

In one embodiment, the steel material may be reheated at a temperature of 1150 ~ 1300 ℃. When the steel is reheated at the above-mentioned temperature, components segregated during the continuous casting process may be re-used. When the reheating temperature is lower than 1150 ° C, the austenite structure is not sufficiently recrystallized, resulting in uneven microstructure, which may cause deterioration of mechanical properties. In addition, the solid solution may not be sufficiently employed, and there may be a problem that segregated components are not evenly distributed during the continuous casting process. When the reheating temperature exceeds 1300 ° C., niobium (Nb), which is used as a grain refinement element, is completely dissolved in the matrix structure, and austenite grains become coarse, which may ultimately lead to a decrease in toughness. Very coarse austenite grains can form, making it difficult to secure strength. In addition, when it exceeds 1300 ° C, heating cost increases and process time is added, which may lead to an increase in manufacturing cost and a decrease in productivity.

Subsequently, the reheated steel is hot-rolled under the conditions of a finish rolling temperature of 800 to 950 ° C, and then cooled at a cooling rate of 5 to 50 ° C / s, and wound up at 600 to 800 ° C (S200). The total rolling reduction of the hot rolling may be 30 to 60%. The microstructure of the steel material after performing step S200 may include ferrite and pearlite.

Subsequently, after cold-rolling the steel material subjected to step S200, annealing heat treatment is performed at 700 to 900 ° C, followed by cooling at a cooling rate of 5 to 100 ° C / s to a cooling end temperature of 100 to 300 ° C (S300). To perform. The microstructure of the steel material after performing step S300 may include ferrite and pearlite.

Subsequently, a step (S400) of performing a hot stamping process is performed on the steel material that has performed the step (S300). The step (S400) is a step of charging a steel material for hot stamping in a heating furnace and heating it up at a heating rate of 5 to 100 ° C / s; Maintaining the steel material at an austenizing temperature of 700 to 1000 ° C. for 1 to 20 minutes; Performing plastic working on the malleable steel; Cooling the steel material at a cooling rate of 5 to 100 ° C./s in a hot stamping mold; It may include sequentially; the step of heat-treating the steel material. The post-heat treatment may be performed for 20 minutes or more. The microstructure of the steel material after performing step S400 may include tempered martensite.

The hot stamping part according to an embodiment of the present invention implemented by performing the above-described process has a yield strength of 1100 to 1400 MPa, a tensile strength of 1600 to 2000 MPa, a hardness of 480 to 580 Hv, and an elongation of 5% or more. .

According to the above-described method of manufacturing a hot stamping part of the present invention, fine precipitation was induced by adding Ti and Nb elements to the composition and annealing at high temperatures and winding at high temperatures. In particular, securing of fine precipitates and homogeneous structures is effective for hydrogen embrittlement. Hydrogen embrittlement resistance is improved by the formation of Ti, Nb, V, and Cr carbides. In addition, Ti, Nb, Si, and Mn contents are factors that contribute to the refinement of the average grain size of the crystal grains. In order to form ferrite and pearlite microstructures, recrystallization of ferrite must be suppressed. This can be suppressed by an increase in the recrystallization temperature of ferrite. This is because when the phase transformation enters the two-phase region, austenite is generated in the process of heating the steel sheet after cold rolling, and recrystallization of ferrite is very suppressed. First, in order to increase the recrystallization temperature of ferrite, the addition of Ti or V is effective. Accordingly, the steel material for hot stamping of the present invention induces fine precipitation by adding Ti and Nb, and increases the recrystallization temperature of ferrite and lowers the cold rolling rate to induce tissue uniformity. By lowering the cold rolling ratio, the accumulated strain energy is reduced, so that the driving force of recrystallization is reduced, and the recrystallization temperature can be increased. After the final cold rolling, according to the conditions of the hot stamping process, the hot rolled material having ferrite and pearlite as a microstructure is heated to an austenite formation temperature, maintained for a certain period of time, and then cooled by a mold to perform hot stamping with tempered martensite as a microstructure. Parts were prepared.

The hot stamping part and its manufacturing method according to an embodiment of the present invention provide a configuration for inducing an irreversible hydrogen trap of fine precipitated carbide according to the amount of fine precipitated elements added and controlling deterioration characteristics of the surface oxide layer. In the present invention, the hydrogen retardation resistance is improved by controlling the structure uniformity and fine precipitate control through optimization of the additive element and hot rolling conditions and cold rolling conditions for the ultra-high-strength, non-plated heat-treated hardened steel of 1800 MPa or more. In addition, hot stamping parts with excellent hydrogen retardation resistance and a method of manufacturing the same were implemented by limiting the inflow of hydrogen by reducing the inflow of diffusive hydrogen, which is a problem for hydrogen delay destruction, through surface control according to an increase in the amount of Ti and Nb added. .

Experimental Example

Hereinafter, preferred experimental examples are provided to help understanding of the present invention. However, the following experimental examples are only to help understanding of the present invention, and the present invention is not limited by the following experimental examples.

1. Preparation of Specimen

In this experimental example, specimens having the composition of Table 1 are provided.

C
(wt%) Si
(wt%) Mn
(wt%) P
(ppm) S
(ppm) B
(ppm) Cr
(wt%) Ti
(wt%) Nb
(wt%) Comparative example 0.290 0.191 1.41 0.012 0.0016 0.002 0.208 0.020 - Example 1 0.293 0.187 1.39 0.012 0.0012 0.0020 0.200 0.020 0.047 Example 2 0.288 0.192 1.43 0.012 0.0013 0.0020 0.203 0.031 -

The composition of Example 1 in Table 1 is carbon (C): 0.1 to 0.35% by weight, silicon (Si): 0.01 to 0.5% by weight, manganese (Mn): 1.0 to 3.0% by weight, phosphorus (P): more than 0 0.1 Weight% or less, sulfur (S): more than 0 and 0.01% by weight or less, titanium (Ti): 0.02 to 0.05% by weight, niobium (Nb): more than 0 and 0.05% by weight or less, chromium (Cr): 0.1 to 2.0% by weight, Boron (B): satisfies the composition of the second component system composed of 0.0001 to 0.005% by weight and the remaining iron (Fe) and other inevitable impurities, and the composition of Example 2 in Table 1 is carbon (C): 0.1 to 0.35% by weight, Silicon (Si): 0.01 to 0.5% by weight, manganese (Mn): 1.0 to 3.0% by weight, phosphorus (P): more than 0 to 0.1% by weight, sulfur (S): more than 0 to 0.01% by weight or less, titanium (Ti) : 0.025 to 0.05% by weight, chromium (Cr): 0.1 to 2.0% by weight, boron (B): 0.0001 to 0.005% by weight, and the composition of the first component system composed of the remaining iron (Fe) and other inevitable impurities. On the other hand, the composition of the comparative example in Table 1 does not satisfy the composition of the first component system or the composition of the second component system described above in terms of titanium and niobium.

On the other hand, the hot rolling and cold rolling processes were applied under the same process conditions to the hot stamping steel according to the comparative examples and examples described above, and the same heat treatment conditions corresponding to the hot stamping conditions were applied. For example, the annealing heat treatment after cold rolling was performed at 800 ° C., and the same heat treatment conditions corresponding to the hot stamping conditions were performed with an austenizing treatment maintained at a temperature of 930 ° C. for 300 seconds and 120 minutes at a temperature of 180 ° C. The post-heat treatment was maintained for a while.

2. Tensile test results

Table 2 shows the tensile test results for the specimens to which the composition and process conditions of Table 1 were applied.

division Comparative example
(YS / TS / EL) Example 1
(YS / TS / EL) Example 2
(YS / TS / EL) Before hot stamping 339/696 / 20.5 508/792 / 17.7 428/788 / 17.5 After hot stamping 1422/1798 / 6.1 1393/1788 / 6.4 1402/1777 / 6.7

Referring to Table 2, the yield strength (YS) and the tensile strength (TS) of the specimens subjected to the heat treatment corresponding to the hot stamping condition are significantly increased compared to the specimens not subjected to the heat treatment corresponding to the hot stamping condition, but the elongation decreases. can confirm. For example, the yield strength of Example 1, which did not undergo the corresponding heat treatment under the hot stamping condition, was 508 MPa, while the yield strength of Example 1, which performed the corresponding heat treatment under the hot stamping condition, increased significantly to 1393 MPa, but the elongation was at 17.7%. It was found to decrease to 6.4%.

However, it can be confirmed that the significant difference between the comparative example and the example is not great under the same conditions. For example, the comparative examples that did not undergo the corresponding heat treatment under the hot stamping conditions, and the yield strengths of Examples 1 and 2 were 339 MPa, 508 MPa, and 428 MPa, respectively, so that there was no significant difference, and the comparative example that performed the corresponding heat treatment under the hot stamping condition , Example 1, Example 2, the yield strength was 1422MPa, 1393MPa, 1402MPa, respectively, and the difference was not significant.

3. Fine carbide precipitation behavior

2 is a graph showing the comparison of the solid solution concentration of the carbon in the matrix in the specimens according to the experimental example of the present invention, and FIG. 3 is a graph showing the comparison of the carbide production concentration in the specimens according to the experimental example of the present invention. 4 is a photograph of the behavior of the precipitates through the TEM Replica technique in the specimens according to the experimental example of the present invention and the observation of an increase pattern of fine carbide. 5 is a photograph observing the behavior of the precipitate through the TEM thin-foil analysis and the precipitation pattern of the fine carbide in the specimens according to the experimental example of the present invention. The item '0.30C' divided in the drawing corresponds to the comparative example in Table 1, the item '0.30CNb' corresponds to Example 1 in Table 1, and the item '0.30CTi' corresponds to Example 2 in Table 1 .

2 to 5, it can be confirmed that the concentration of fine carbides precipitated in the matrix in the specimen according to the comparative examples of the present invention is relatively higher. It was confirmed that these carbides (TiC, NbC) serve as irreversible hydrogen trapping and thus can reduce the amount of diffusible hydrogen.

4. Microstructure size behavior

6 is an optical micrograph and SEM photograph of the martensitic microstructure observed in specimens according to the experimental examples of the present invention. The photo on the top is an optical micrograph, and the photo on the bottom is an SEM photo. In addition, the photographs on the left correspond to the specimen according to the comparative example in Table 1, the photographs on the right correspond to the specimen according to Example 2 in Table 1, and the intermediate photographs correspond to the specimen according to Example 1 in Table 1 do.

Referring to FIG. 6, it can be seen that the final microstructure of the hot stamping part includes tempered martensite, but the size of the microstructure implemented in the specimens of the Examples compared to the comparative example is smaller. Specifically, the martensite microstructure of Comparative Example is the largest, and the martensite microstructure of Example 1 is the smallest. The smaller the microstructure size of martensite, the final microstructure of a hot stamping part, may have the effect of reducing the amount of diffusible hydrogen.

FIG. 7 is an optical micrograph of a microstructure of aprior austenite in specimens according to an experimental example of the present invention. The photograph on the left corresponds to the specimen according to the comparative example in Table 1, the photograph on the right corresponds to the specimen according to Example 2 in Table 1, and the intermediate photograph corresponds to the specimen according to Example 1 in Table 1.

Referring to FIG. 7, the average size of the fryer austenite microstructure in the specimen according to the comparative example in Table 1 is 23.5 μm, and the average size of the fryer austenite microstructure in the specimen according to Example 1 in Table 1 is 15.3 μm. , In the specimen according to Example 2 of Table 1, the average size of the fryer austenite microstructure is 22.5 μm. Therefore, the fryer austenite microstructure of the comparative example is the largest, and the fryer austenite microstructure of the example 1 is the smallest. It can be understood that the amount of diffusible hydrogen is reduced in embodiments where the size of the fryer austenite microstructure of the hot stamping component is relatively small, in that the fryer austenite microstructure corresponds to the microstructure of the final microstructure martensite.

5. Behavior of hydrogen penetration through surface oxide film

8 is a graph showing the results of measuring the hydrogen reduction current density in the specimens according to the experimental example of the present invention, Figure 9 is a change in the hydrogen reduction voltage according to the cycle in the specimens according to the experimental example of the present invention It is a graph showing the results. The item '0.30C' divided in the drawing corresponds to the comparative example in Table 1, the item '0.30CNb' corresponds to Example 1 in Table 1, and the item '0.30CTi' corresponds to Example 2 in Table 1 .

Referring to Figures 8 and 9, the surface oxide properties are relatively better according to the addition effect of titanium or niobium in the specimens of the Examples compared to the comparative examples, reducing the incoming hydrogen and effectively inhibiting the internal penetration of hydrogen. I can understand. For example, in the case of the comparative example of FIG. 9, as the cycle increases, the change width of the hydrogen reduction voltage is relatively large, and thus the tendency for internal penetration of hydrogen is high and the surface oxide film does not inhibit the penetration of hydrogen. In the case of the embodiment, as the cycle increases, the hydrogen reduction increases. It can be seen that the change width of the voltage is relatively small, so the tendency for internal penetration of hydrogen is low, and the surface oxide film suppresses the penetration of hydrogen.

6. Evaluation of hydrogen embrittlement performance

10 is a graph showing the transition of the maximum tensile strength according to the hydrogen injection time in the specimens according to the experimental example of the present invention, Figure 11 is the transition of the elongation according to the hydrogen injection time in the specimens according to the experimental example of the present invention 12 is a graph showing hydrogen susceptibility showing an elongation reduction rate after injecting hydrogen for 1 hour in specimens according to an experimental example of the present invention. The item 'Comparative Example' divided in the drawing corresponds to the comparative example in Table 1, and the item 'Example 2' corresponds to Example 2 in Table 1.

Referring to FIG. 10, it can be confirmed that the maximum tensile strength according to the hydrogen injection time in the specimen of Example 2 is relatively higher than the maximum tensile strength according to the hydrogen injection time in the specimen of the comparative example.

Referring to FIG. 11, it can be seen that the elongation according to the hydrogen injection time in the specimen of Example 2 is relatively higher than the elongation according to the hydrogen injection time in the specimen of the comparative example.

Referring to FIG. 12, the elongation reduction rate of the vertical axis corresponds to a value obtained by dividing the elongation fluctuation range before and after hydrogen injection by the elongation before hydrogen injection. According to this, it can be confirmed that the elongation reduction rate in the specimen of Example 2 is relatively lower than that of the comparative example.

Summarizing the results of FIGS. 10 to 12, it can be understood that the hydrogen embrittlement resistance in the experimental example is relatively better than the comparative example of the present invention.

In the above, although the description has been mainly focused on the embodiment of the present invention, various changes or modifications can be made at the level of those skilled in the art. It can be said that such modifications and variations belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention should be judged by the claims set forth below.

Claims (6)

delete Carbon (C): 0.1 to 0.35% by weight, silicon (Si): 0.01 to 0.5% by weight, manganese (Mn): 1.0 to 3.0% by weight, phosphorus (P): more than 0 and 0.1% by weight or less, sulfur (S): More than 0 and less than 0.01% by weight, titanium (Ti): 0.02 to 0.05% by weight, niobium (Nb): more than 0 and less than 0.05% by weight, chromium (Cr): 0.1 to 2.0% by weight, boron (B): 0.0001 to 0.005% % And the rest of iron (Fe) and other inevitable impurities, including non-plated heat-treated hardened steel,
The final microstructure of the non-plated heat-treated hardened steel is made of tempered martensite,
The final microstructure of the non-plated heat-treated hardened steel serves as irreversible hydrogen trapping and thus includes Ti carbide and Nb carbide to reduce the amount of diffusible hydrogen,
It characterized in that the average size of the primary austenite (prior austenite) microstructure of the non-plated heat-treated hardened steel is 15.3㎛ 22.5㎛,
Hot stamping parts comprising non-plated heat treated hardened steel.
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