JP5257062B2 - High strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance and method for producing the same - Google Patents

Description

  The present invention relates to hot stamping in which a steel plate heated to high temperature is subjected to press forming and quenching simultaneously using heat removal by a mold, and in particular, hot stamping forming excellent in strength, toughness and hydrogen embrittlement resistance. Product and a manufacturing method thereof.

  In recent years, in the automobile field, in order to reduce the thickness of a member and reduce the weight of an automobile, the strength of a steel plate as a material has been increased. However, since the ductility and workability of a steel plate decrease with increasing strength, members to which the high strength steel plate can be applied are limited by the shape. Further, when the strength of the steel plate is increased, there is a problem that the spring back becomes larger and the accuracy of the shape of the molded product is lowered.

  In response to such problems, development of high-strength steel sheets with improved formability and development of forming methods that enable processing of high-strength steel sheets with inferior formability are underway. In particular, in recent years, a technique for producing a high-strength steel part by pressing a steel sheet hot and quenching it almost simultaneously with a mold has been proposed (for example, see Patent Documents 1 to 4). ).

  In the method proposed in Patent Document 1, hot stamping is performed using a steel plate with a finer average grain size of ferrite as a raw material to suppress variation in hardness of a molded product. However, since the method described in Patent Document 1 does not make the heating temperature at the time of hot stamping molding an appropriate condition, the balance between the strength and toughness of the molded product is insufficient.

  Patent Documents 2 and 3 propose a method for improving the hydrogen embrittlement resistance by controlling the prior austenite grain size of the hot stamping molded product. However, these methods described in Patent Documents 2 and 3 do not make the heating temperature at the time of hot stamping molding an appropriate condition, so that the structure is not sufficiently refined.

On the other hand, Patent Document 4 performs hot stamping by heating the steel sheet to a relatively low temperature of _Ac 3 ~Ac 3 + ₁₀₀ ℃, a method of the prior austenite grain size of the molded article to 10μm or less has been proposed. However, in this method, Cr and Mo are added in order to improve hardenability, and carbides of Ti and Nb are used for fine graining, which has a problem of high cost and insufficient improvement in toughness. there were.
JP 2006-265583 A JP-A-2005-97725 JP 2006-70346 A JP 2006-152427 A

  The present invention has been made in view of the above problems, and suppresses the addition amount of high-cost alloy elements Cr, Mo, Ti, Nb, is inexpensive, and has strength, toughness, and hydrogen embrittlement resistance. An object is to provide an excellent hot stamping molded article and a method for producing the same.

As a result of intensive research conducted by the present inventors to solve the above problems, the addition amount of C, Si, and Mn is within the optimum range, and Al is added to increase the crystal grain size. By using hot stamping with the heating temperature of the steel sheet at a relatively low temperature condition, the prior austenite grain size of the hot stamped molded product becomes finer, and the toughness and resistance are reduced without reducing the strength. The present inventors have found that hydrogen embrittlement characteristics can be improved and completed the present invention.
That is, the gist of the present invention is as follows.

[1] By mass%, C: 0.15 to 0.35%, Si: 0.1 to 1.0%, Mn: more than 1.3% to 1.8% or less, Al: 0.010 to 0. 0. 100% , N: 0.001 to 0.010%, each content of P and S is limited to P: 0.030% or less, S: 0.020% or less, and Cr, In the case of including one of Mo, or the total content in the case of including both, less than 0.2%, in the case of including one of Ti, Nb, or both The total content of the steel is limited to less than 0.10%, the balance is composed of steel components composed of Fe and inevitable impurities, the Ac ₃ transformation point is less than 900 ° C., and the metal structure is composed of martensite. And high strength hot with excellent toughness and hydrogen embrittlement resistance, characterized in that the prior austenite grain size is 15 μm or less Tamping moldings.
[ 2 ] The high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance according to [1 ] above, wherein the mass is limited to Ti: 0.040% or less.
[ 3 ] The high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance according to [1] or [2] , wherein the metal structure is composed of tempered martensite.

[ 4 ] A method for producing a high-strength hot stamped molded article having excellent toughness and hydrogen embrittlement resistance according to [1] or [2] , comprising the components according to claim 1 or [2]. the steel sheet, Ac ₃ transformation point to Ac ₃ transformation point + 100 ° C., and then heated to less than 900 ° C., was then pressed at Ar ₃ transformation point or more of the temperature, as it is possible to perform the quenching by contact with the mold A method for producing a high-strength hot stamping molded article having excellent toughness and hydrogen embrittlement resistance.
[ 5 ] High strength hot stamped molded article having excellent toughness and hydrogen embrittlement resistance produced by the method described in [ 4 ] above, and further dehydrating for 1 to 100 minutes in a temperature range of 100 to 300 ° C A method for producing a high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance, characterized by performing a heat treatment.
[ 6 ] A method for producing a high strength hot stamping molded article having excellent toughness and hydrogen embrittlement resistance according to [3] ,
The toughness and hydrogen embrittlement resistance characterized by further subjecting the hot stamped molded product molded by the method described in [4] above to a tempering treatment that is held at a temperature range of 400 to 700 ° C. for 1 to 100 minutes. To produce high strength hot stamping molded products with excellent resistance.

According to the high strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance of the present invention, it is a prescribed steel component, the Ac ₃ transformation point is less than 900 ° C., the metal structure is composed of martensite, and With a structure in which the prior austenite grain size is 15 μm or less, high toughness and hydrogen embrittlement resistance are achieved without reducing the strength while suppressing the addition amount of high-cost alloy elements Cr, Mo, Ti, Nb. It can be secured. Therefore, it is possible to provide a high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance at a low cost, and the industrial contribution becomes extremely remarkable.

Further, according to the method for producing a high strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance according to the present invention, a steel plate having a specified component is subjected to an Ac ₃ transformation point to an Ac ₃ transformation point + 100 ° C., and It is a method of heating to below 900 ° C., then press-molding at a temperature not lower than the Ar ₃ transformation point, and then performing quenching by contact with the mold as it is, so that it has excellent toughness and hydrogen embrittlement resistance as described above. A strength hot stamping molded product can be manufactured at low cost.

  Hereinafter, an embodiment of a high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance and a manufacturing method thereof according to the present invention will be described. In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

The high-strength hot stamping molded product excellent in toughness and hydrogen embrittlement resistance of the present invention (hereinafter sometimes abbreviated as hot stamping molded product) is mass%, C: 0.15 to 0.35%, Si: 0.1 to 1.0%, Mn: more than 1.3% to 1.8% or less, Al: 0.010 to 0.100% N: 0.001 to 0.010%, P, Each content of S is limited to P: 0.030% or less, S: 0.020% or less, and further, the content when one of Cr and Mo is included, or when both are included The total content is less than 0.2%, the content in the case where one of Ti and Nb is included, or the total content in the case where both are included is limited to less than 0.10%, the balance being Fe and It has a steel component consisting of inevitable impurities, has an Ac ₃ transformation point of less than 900 ° C., and has a metal structure of martenser. And a prior austenite grain size of 15 μm or less.

Here, the hot stamping forming described in the present invention is a temperature at which the structure of the steel sheet becomes austenite, that is, an Ac ₃ transformation point (hereinafter sometimes simply referred to as Ac ₃ ), and is formed by a mold. Then, it is a method of quenching as it is using heat removal by a mold. Therefore, the structure of the hot stamping molded product is martensite as it is quenched. When tempering is performed after hot stamping molding, the structure of the hot stamping molded product is tempered martensite.

  In order to increase the strength and secure toughness by setting the structure of the hot stamped molded article to martensite or tempered martensite, it is necessary to control the contents of C, Si, and Mn to an appropriate range. Furthermore, in order to refine the prior austenite grain size, it is necessary to add Al to produce AlN.

  On the other hand, Mo and Cr, which have been conventionally added to improve hardenability, and Ti and Nb that form precipitates and reduce the particle size, if added excessively, reduce the toughness of hot stamping molded products. I understood it. Therefore, it is necessary to limit the contents of Mo, Cr, Ti, and Nb, and by limiting the contents of these expensive elements, it is possible to simultaneously achieve cost reduction and toughness improvement. It becomes possible.

Furthermore, when the steel sheet structure is martensite or tempered martensite as in the hot stamping molded product according to the present invention, it is effective to refine the prior austenite grain size in order to improve toughness. For that purpose, it is necessary to lower Ac ₃ [° C.] and to suppress the upper limit of the heating temperature during hot stamping. Here, Ac ₃ [° C.] can be obtained by the following equation (1) based on the content [mass%] of C, Mn, Si, P, Al, and Ti among the steel components.
Ac ₃ = 910-203C ^1/2 -30Mn + 44.7Si + 700P + 400Al + 400Ti (1)
In the above formula (1), when Ti which is a selective element is not contained, it is calculated as 0.

  Hereinafter, the high-strength hot stamping molded article excellent in the toughness and hydrogen embrittlement resistance of the present invention and the production method thereof will be described in detail.

[Steel component (chemical component composition)]
First, the reason for limiting the chemical composition of the steel plate base material in the hot stamped molded product according to the present invention will be described. In addition, in the following description, all the addition amounts of each element are represented by mass%.

“C: Carbon” 0.15 to 0.35%
C is an element necessary for ensuring the strength by setting the metal structure of the hot stamped molded article to martensite, and in order to obtain such an effect, addition of 0.15% or more is necessary. C is an element that lowers Ac ₃ [° C.], and is preferably added in an amount of 0.20% or more in order to lower the heating temperature of hot stamping. On the other hand, if the amount of C is excessively contained, the toughness of the hot stamped molded product is lowered, so the upper limit is made 0.35%.

"Si: silicon" 0.1-1.0%
Si is an element that increases the strength of a hot stamping molded product by solid solution strengthening, and in order to obtain such an effect, addition of 0.1% or more is necessary. Moreover, since Si suppresses precipitation of cementite and contributes to improvement in strength and toughness, addition of 0.3% or more is more preferable. On the other hand, if the amount of Si exceeds 1.0%, the hot stamped molded product is cured and the toughness is impaired, so the upper limit is made 1.0%. In addition, when plating a hot stamping molded product or a steel plate as a material, the Si content is preferably 0.7% or less.

“Mn: Manganese” more than 1.3% to 1.8% or less Mn is an element necessary for improving the hardenability and making the metal structure of the hot stamped molded article martensite. Further, Mn is an element that lowers Ac ₃ [° C.], and in order to lower the heating temperature of hot stamping, it is necessary to add more than 1.3%. On the other hand, if Mn is added in excess of 1.8%, MnS becomes coarse and becomes a starting point of fracture due to hydrogen embrittlement, or the strength increases so much that the toughness is impaired, so the upper limit is 1.8% or less. And

"Al: Aluminum" 0.010 to 0.100%
Al is a deoxidizing element. In the present invention, Al is an important element that contributes to the refinement of the prior austenite grain size by forming AlN. In order to make the prior austenite grain size of the hot stamped product fine, it is necessary to add Al in an amount of 0.010% or more. On the other hand, if the Al content exceeds 0.100%, inclusions increase and the toughness is impaired, so the upper limit is made 0.100%.

“N: Nitrogen” 0.001 to 0.010%
N is an element inevitably contained in the steel, but in the present invention, 0.0010% or more is contained in order to refine the prior austenite grain size of the hot stamped product by forming AlN. It is necessary. On the other hand, if the N content exceeds 0.0100%, nitrides increase and the toughness is impaired, so the upper limit is made 0.0100%.

“P: Phosphorus” 0.030% or less P is an impurity element inevitably mixed in, and its content is limited to 0.030% or less in order to reduce toughness. Further, a more preferable upper limit of P is 0.015% or less.

“S: Sulfur” 0.020% or less S is an impurity element which is inevitably mixed in the same manner as P. In order to impair workability and toughness, the upper limit is made 0.020% or less. A more preferable upper limit of S is 0.005% or less.

  In the present invention, it is necessary to limit the content of Cr, Mo, which improves the strength of the base material, and Ti, Nb, which is used to refine the prior austenite grain size, for the reasons described below. It is.

"Content when one of Cr and Mo is included, or total content when both are included"
Cr and Mo are elements that enhance the hardenability, but when added in excess, toughness is reduced. Therefore, in the present invention, when one of Cr and Mo is added alone, the upper limit of each is added, and when both Cr and Mo are added together, the total amount is less than 0.2%. Limit to. Moreover, since Cr and Mo are expensive elements, it is more preferable to limit the upper limit of the total amount of one or both to less than 0.1%.

"Content when including one of Ti and Nb, or total content when including both"
Ti and Nb are elements that form fine precipitates and contribute to the refinement of the prior austenite grain size. When these elements are added excessively, the toughness is lowered. Therefore, in the present invention, when one of Ti and Nb is added alone, the upper limit of each is added, and when both Ti and Nb are added together, the total amount is less than 0.10%. Limit to. In particular, in order to increase toughness, it is more preferable to limit the upper limit of the total amount of one or both of Ti and Nb to less than 0.05%.

  Next, the reason for limiting the addition range of the selected component element in the present invention will be described below. In the present invention, B may be further added in order to improve the hardenability with the contents of the essential elements and the elements inevitably contained within the above ranges. In this case, in order to suppress the generation of BN, it is preferable to add Ti in the following range within a range that satisfies the above upper limit and within the range described below.

“B: Boron” 0.0050% or less B is an element having an effect of greatly improving hardenability in a small amount. In order to obtain such an effect, 0.0003% or more of B is preferably added. In particular, in order to improve the tensile strength of the hot stamping molded product, it is preferable to add B in an amount of 0.0005% or more. On the other hand, even if containing B exceeding 0.0050%, the effect of improving the hardenability is saturated and the molded product may become brittle, so the upper limit is preferably made 0.0050%.

“Ti: Titanium” 0.040% or less Ti is effective to combine with N in steel to generate TiN and suppress the formation of BN. In order to effectively act B added for improving the hardenability, 0.001% or more of Ti is preferably added. On the other hand, when the strength of the hot stamped molded product is high, if more than 0.040% of Ti is added, TiN serves as a starting point of fracture and may impair toughness. Therefore, it is more preferable to limit the total amount of one or both of Ti and Nb to less than 0.10% and to set the upper limit of the Ti content to 0.040% or less. As described above, in the present embodiment, it is preferable that the total content of both Ti and Nb is less than 0.10% while the content of Ti is limited to 0.040% or less.

[Metal structure]
Next, the metal structure of the hot stamping molded product according to the present invention will be described.
The present invention is a hot-rolled steel sheet or a cold-rolled steel sheet whose chemical component composition is defined in the above range, and is press-molded by heating to Ac ₃ [° C.] or higher under the conditions specified by the production method described below. The metal structure of the steel sheet is a martensite structure. When tempering is performed after hot stamping molding, the metal structure of the hot stamping molded product is tempered martensite.

  Generally, when the crystal grain size in steel is refined, the propagation resistance of cracks is increased and the toughness is improved. On the other hand, when hydrogen intrudes into steel, dislocations easily move and cracks due to strain concentration are likely to occur. In addition, the penetration resistance of hydrogen reduces crack propagation resistance and deteriorates toughness, resulting in a hydrogen embrittlement phenomenon. That is, by making the crystal grain size finer, dislocations hardly move and the toughness is improved, so that the hydrogen embrittlement resistance is also improved. In steels with a martensitic metal structure, toughness and hydrogen embrittlement resistance are improved when the prior austenite grain size is refined. This is because the martensite particle size is approximately proportional to the prior austenite particle size.

  The prior austenite grain size of the hot stamped molded product according to the present invention needs to be 15 μm or less in order to improve toughness and hydrogen embrittlement resistance. In addition, the toughness and hydrogen embrittlement resistance of the hot stamped molded article are improved by the refinement of the prior austenite grain size, so that the prior austenite grain size is preferably 10 μm or less, more preferably 5 μm or less. The lower limit of the prior austenite grain size is not particularly specified, but it is difficult to make it less than 3 μm by hot stamping molding.

  The particle size of the prior austenite is measured using an optical microscope and an image processing apparatus by etching the base material using a picric acid alcohol solution to reveal the prior austenite grain boundary of the hot stamped molded product. At this time, the area of each crystal grain of prior austenite is obtained, and the diameter of a circle (equivalent circle) having the same area as each crystal grain is defined as the grain size of each crystal grain. In this way, the average value of the crystal grain size defined as the equivalent circular diameter is obtained and used as the grain size of prior austenite.

  The hot stamping molded product according to the present invention may be subjected to a surface treatment such as galvanization or aluminum plating. Moreover, you may give surface treatments, such as galvanization and aluminum plating, to the surface of the steel plate which is a raw material previously.

[Ac ₃ transformation point]
When the hot stamping molded product according to the present invention is manufactured by hot stamping molding to be described later, the upper limit of the heating temperature of the steel sheet is set to Ac ₃ or higher and Ac ₃ + 100 ° C. or lower and lower than 900 ° C., so that Ac ₃ is 900 It is necessary to make it less than ° C. Thereby, it becomes possible to control the metal structure of the hot stamping molded article as the structure.

According to the high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance according to the present invention as described above, the steel component is defined, the Ac ₃ transformation point is less than 900 ° C., and the metal structure is High toughness without reducing the strength while suppressing the addition amount of high-cost alloy elements Cr, Mo, Ti, Nb by the composition consisting of martensite and the prior austenite grain size of 15 μm or less It becomes possible to ensure hydrogen embrittlement resistance. Therefore, it is possible to provide a high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance at a low cost, and the industrial contribution becomes extremely remarkable.

[Production method]
Below, the reason for limitation is explained about the manufacturing method of the high strength hot stamping molded article excellent in the toughness and hydrogen embrittlement resistance according to the present invention.
The production conditions for the hot stamped molded article of the present invention are not particularly defined, but are preferably the conditions described below.

The method for producing a hot stamping molded product described in the present embodiment is a method for producing a high-strength hot stamping molded product having excellent toughness and hydrogen embrittlement resistance as described above, and a steel plate having the above chemical composition. the, Ac ₃ transformation point to Ac ₃ transformation point + 100 ° C., and then heated to less than 900 ° C., then, Ar ₃ transformation point after press molding at a temperature (hereinafter, simply is sometimes referred to as Ar ₃₎ or more, as This is a method of quenching by contact with a mold.

In the method for producing a hot stamping molded product according to the present invention, first, a steel piece obtained by melting and casting by a conventional method is reheated or cast and then hot-rolled as it is. In this case, the reheating temperature of the steel slab is preferably 1000 to 1300 ° C. Further, the rolling end temperature of the hot rolling is preferably set to Ar ₃ transformation point or more. Moreover, it is preferable that the steel sheet after hot rolling is cooled and wound at a temperature of 550 ° C. or higher.
Furthermore, pickling and cold rolling may be performed on the steel sheet or plating may be performed. These steps may be carried out in a conventional manner, and the plating may be any of hot dip aluminum plating, hot dip galvanizing, galvannealed alloying, and electrogalvanizing.

Next, a hot stamping molding method in the manufacturing method according to the present invention will be described.
First, in order to press-form and rapidly cool a steel sheet heated to a high temperature to transform the metal structure into martensite, it is necessary to change the metal structure of the heated steel sheet to austenite. Therefore, it is necessary to set the heating temperature of hot stamping to Ac ₃ or higher.
When the temperature of the steel plate becomes Ac ₃ or higher, the metal structure of the steel plate becomes austenite and grain growth occurs. In particular, in the present invention, in order to limit the addition amount of Ti and Nb, which has the effect of suppressing grain growth in order to improve toughness, if the heating temperature is too high, the austenite grain growth suppressing effect by AlN is reduced, Austenite grains become coarse.
Further, since the prior austenite grain size of the hot stamped molded article is the grain size of austenite at the time of heating, the upper limit of the heating temperature of the steel sheet in hot stamping is extremely important in order to refine this. In order to make the prior austenite grain size of the hot stamped molded article 15 μm or less, it is necessary to limit the upper limit of the heating temperature to (Ac ₃ +100) ° C. or less and less than 900 ° C.

As a method of heating the steel sheet during hot stamping, any of a heating furnace, induction heating, electric heating, etc. may be used. In addition, in order to heat a steel plate uniformly, it is preferable to use a heating furnace.
Further, when the steel sheet is cooled to a temperature lower than the Ar ₃ transformation point, the ferrite transformation starts. Therefore, it is necessary to perform press molding in hot stamping at Ar ₃ or higher. When the press molding temperature is lower than Ar ₃ , since it is not rapidly cooled before molding, ferrite is generated, a part of the metal structure of the hot stamped molded product does not become martensite, and the strength decreases. This Ar ₃ transformation point can be determined by the following equation (2) based on the content [mass%] of C, Mn, Si, P, and Al among the components of the steel.
Ar ₃ = 901-325C-92Mn + 33Si + 287P + 40Al (2)

  During press forming, the steel sheet comes into contact with the mold, and therefore, after press forming, the heat removal by the mold is used as it is for quenching. Further, usually, in press forming, a mold having a sufficiently larger heat capacity than that of a steel plate is used, so that the cooling rate is 80 ° C./s or more. Further, a coolant such as water may be used to increase the cooling rate. In this case, the coolant may be circulated inside the mold, or the coolant may be ejected from the mold.

In the manufacturing method according to the present invention, a dehydrogenation treatment or a tempering treatment may be further performed on the hot stamping molded product obtained by the hot stamping molding of the above procedure.
Dehydrogenation is a heat treatment that dissipates hydrogen that has entered the steel into the atmosphere during steel plate manufacturing, surface treatment, or hot stamping, and this is a type of delayed fracture. Can be prevented. Thus, in order to dissipate hydrogen from the hot stamping molded product into the atmosphere, it is necessary to set the temperature of the dehydrogenation treatment to 100 ° C. or higher. On the other hand, when the dehydrogenation treatment is performed at a temperature higher than 300 ° C., a low temperature temper embrittlement phenomenon occurs, and the toughness may be lowered. Therefore, the dehydrogenation treatment is preferably performed at 100 to 300 ° C. The dehydrogenation time is preferably 1 minute or longer in order to release hydrogen. Note that even if the dehydrogenation treatment is performed for a long time, the structure and characteristics of the hot stamped molded product are not adversely affected. However, in consideration of productivity, it is preferably set to 100 minutes or less.

Further, the tempering treatment can precipitate the carbon in the martensite as fine cementite and the tempered martensite to improve the toughness of the hot stamping molded product. Further, the tempering treatment increases the propagation resistance against cracks and improves the embrittlement characteristics. As a result, the hydrogen embrittlement resistance can be improved. When such a tempering process is performed at less than 400 ° C., toughness may be reduced. On the other hand, when the tempering process is performed at over 700 ° C., a decrease in strength may be a problem. Therefore, the tempering treatment is preferably performed in a temperature range of 400 to 700 ° C.
In addition, the tempering time is preferably 1 minute or longer in order to improve toughness. On the other hand, if the time for tempering exceeds 100 minutes, the coarsening of precipitates and the decrease in strength may become a problem, so the upper limit is preferably set to 100 minutes or less.

As described above, according to the method for producing a high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance according to the present invention, a steel plate having a specified component is converted into an Ac ₃ transformation point to an Ac ₃ transformation. It is a method of heating to a point + 100 ° C. and less than 900 ° C., then press-molding at a temperature equal to or higher than the Ar ₃ transformation point, and then performing quenching by contact with the mold as it is. A high-strength hot stamping molded product having excellent characteristics can be manufactured at a low cost.

  Hereinafter, examples of the high strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance according to the present invention and a method for producing the same will be given to explain the present invention more specifically. The present invention is not limited to the examples, and can be carried out with appropriate modifications within a range that can be adapted to the purpose described above and below, all of which are included in the technical scope of the present invention. .

[Manufacture of steel sheets]
In the steel making process, steel having the chemical composition shown in Table 1 below was melted and cast into steel pieces. Then, after reheating these steel pieces to 1050 to 1250 ° C., hot rolling was performed, and pickling and cold rolling were further performed to obtain a steel plate having a thickness of 1.2 mm. In addition, after the steel plate was manufactured, some steel plates were plated (see Table 1 below). In the case of hot dip aluminum plating, the plating treatment was performed with a basis weight of 120 g / m ² , and in the case of hot dip galvanization, the weight of basis was 90 g / m ² .

A test piece having a width of 240 mm and a length of 500 mm was collected from the steel plate obtained by the above procedure. Subsequently, the test piece was held in a heating furnace at a predetermined heating temperature for 5 minutes, extracted and then subjected to press molding (hot stamping molding) in which bending was performed immediately, followed by rapid cooling with a mold. At this time, the temperature of the extracted from the heating furnace steel was measured by a radiation thermometer, it was confirmed that the temperature before the start of press forming is Ar ₃ temperature or more of each steel sheet. Subsequently, after each steel plate was press-formed, various heat treatments were performed on the formed product according to the heat treatment type (dehydrogenation heat treatment or tempering heat treatment) and heat treatment temperature shown in Table 2.

  And the test piece of a tensile test and a delayed fracture test was extract | collected from this press molded product (hot stamping molded product), and each evaluation demonstrated below was performed. In addition, the press-formed product manufactured in this example has a hat shape with a bottom portion of 70 mm and a height of 60 mm.

[Evaluation test]
The following evaluation tests were performed on each press-formed product manufactured by the above method.

(Tensile test)
The tensile test was conducted in accordance with JIS Z 2241 by collecting JIS Z 2201 No. 5 test piece from the bottom of each press-formed product. The results are shown in Table 2 below as the strength of the press-formed product.

(Charpy impact test)
In order to evaluate the toughness of the press-formed product, a Charpy specimen was collected and a Charpy impact test was performed. The shape of the test piece was the same as the V-notch test piece of JIS Z 2242. However, since the test piece collected from the press-molded product was thin, about 10 test pieces were overlapped and fixed with screws so that the total thickness was about 10 to 12 mm. The test method is based on the method of JIS Z 2242. The absorbed energy is measured at a test temperature of −120 ° C., and the impact energy [J / cm ² ] is obtained by dividing the absorbed energy by the cross-sectional area of the notch bottom. The results are shown in Table 2 below.

(Hydrogen embrittlement resistance)
The hydrogen embrittlement resistance was evaluated by the following procedure.
First, 80 mm × 30 mm strip-shaped pieces were collected by shear cutting from the bottom of the press-formed product to form strip-shaped test pieces, and holes for passing bolts were provided at both ends of the test pieces. Next, the test piece was subjected to U-bending with a bending radius of 10 mm. Next, a water-resistant strain gauge was attached to the outer periphery of the bent portion. Thereafter, bolts were passed through holes at both ends of the test piece, and strain was applied to the bent portion while tightening with nuts.

Then, the amount of strain measured by a strain gauge was multiplied by 205800 MPa, which is the Young's modulus of general steel, to calculate the load stress, and the amount of strain was adjusted so that the load stress was 590 MPa. Thereafter, the sample was immersed in 0.5 mol / l sulfuric acid and subjected to electrolytic charging at a current density of 40 mA / cm ² . The electrolytic charging time at this time was 120 minutes at the maximum, and the presence or absence of cracks was evaluated. The occurrence of cracking was determined by a change in strain during electrolytic charging with a strain gauge attached to the bent portion. The hydrogen embrittlement resistance is shown in Table 2 below, where x indicates that cracking occurred during electrolytic charging, and ○ indicates that cracking did not occur.

A list of chemical composition of the steel sheet, Ac ₃ transformation point, Ar ₃ transformation point and plating type in this example is shown in Table 1 below, and a list of manufacturing conditions and evaluation results of the press molded product (hot stamping molded product) is shown. It is shown in Table 2 below.

[Evaluation results]
The examples of the symbols A1, A2-1, A3-1, A4 to A8, A9-1, A9-2, and A10 to A11 in Table 2 use steel sheets whose components are within the specified range of the present invention. It is the example of this invention which performed hot stamping on the manufacturing conditions within the regulation range of invention, and heat-processed as needed. As shown in Table 2, it is clear that the molded products of these inventive examples have good strength and hydrogen embrittlement resistance.

  On the other hand, in the examples of symbols A2-2, A3-2, A3-3, and B1 to B7 in Table 2, the steel plate components are out of the specified range of the present invention, or out of the specified range of the present invention. This is a comparative example in which hot stamping and heat treatment are performed under manufacturing conditions. In the examples indicated by reference signs A2-2 and A3-3, since the heating temperature of hot stamping is high, the prior austenite grain size exceeds 15 μm. Moreover, code | symbol A3-2 is an example which performed the heat processing of the molded article at unsuitable temperature, and the hydrogen embrittlement resistance property has deteriorated.

  In addition, the examples of reference signs B1 to B10 are comparative examples in which any one or more of the strength, toughness, and hydrogen embrittlement resistance deteriorates because the component is outside the specified range of the present invention. Here, the symbol B1 is an example in which the Al amount is small and the prior austenite grain size exceeds 15 μm, and the symbol B2 is an example in which inclusions increase because the Al amount is excessive. Hydrogen embrittlement resistance is degraded.

Further, the code B3 has too much C amount, the code B5 has too much total amount of Cr and Mo to improve the hardenability, the code B4 has too much content of Si and Mn, and the codes B6 and B7 have This is an example in which the contents of Nb and Ti are too high and the toughness is lowered.
Reference B8 is an example in which the Si content is low and the strength, toughness, and hydrogen embrittlement resistance are reduced. Further, the symbol B9 is an example in which the Mn content is low, the hardenability is lowered, the ferrite phase appears, and the toughness and the hydrogen embrittlement resistance are improved, but the strength is lowered.

Reference B10 is an example in which the Ac ₃ transformation point is 900 ° C. or higher, and the heating temperature is increased to make the structure austenite during heating. In the code | symbol B10, since the prior-austenite particle size is over 15 micrometers, toughness and hydrogen embrittlement resistance are falling.

  From the results of the examples described above, the high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance of the present invention suppresses the addition amount of high-cost alloy elements Cr, Mo, Ti, and Nb. However, it can be seen that high toughness and hydrogen embrittlement resistance can be secured without reducing the strength. Therefore, it is clear that by applying the high-strength hot stamping molded product of the present invention to automobile parts, etc., it is possible to reduce the manufacturing cost as well as to reduce the weight and safety of the automobile.

Claims (6)

% By mass
C: 0.15-0.35%,
Si: 0.1 to 1.0%,
Mn: more than 1.3% and 1.8% or less,
Al: 0.010~0.100%,
N: 0.001 to 0.010%
And the contents of each of P and S are
P: 0.030% or less,
S: limited to 0.020% or less,
The content when containing one of Cr and Mo, or the total content when containing both is less than 0.2%,
The content in the case where one of Ti and Nb is included, or the total content in the case where both are included is limited to less than 0.10%, and the balance has steel components composed of Fe and inevitable impurities. Become
High strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance, characterized in that Ac ₃ transformation point is less than 900 ° C., metal structure is martensite, and prior austenite grain size is 15 μm or less . % By mass
Ti: 0.040% or less,
The high-strength hot stamping molded article having excellent toughness and hydrogen embrittlement resistance according to claim 1, wherein The high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance according to claim 1 or 2 , wherein the metal structure is composed of tempered martensite. A method for producing a high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance according to claim 1 or 2 ,
The steel plate comprising the component according to claim 1 or claim 2 is heated to an Ac ₃ transformation point to an Ac ₃ transformation point + 100 ° C and less than 900 ° C,
Next, a method for producing a high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance, characterized by performing press molding at a temperature equal to or higher than the Ar ₃ transformation point and then performing quenching by contact with a mold as it is. A high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance produced by the method according to claim 4 is further subjected to a dehydrogenation heat treatment that is maintained for 1 to 100 minutes in a temperature range of 100 to 300 ° C. A method for producing a high-strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance, characterized by A method for producing a high-strength hot stamping molded article having excellent toughness and hydrogen embrittlement resistance according to claim 3 ,
The hot stamping molded product molded by the method according to claim 4 is further subjected to tempering treatment for 1 to 100 minutes in a temperature range of 400 to 700 ° C. Manufacturing method for excellent high-strength hot stamping molded products.