WO2019188622A1

WO2019188622A1 - Steel plate for hot stamping

Info

Publication number: WO2019188622A1
Application number: PCT/JP2019/011606
Authority: WO
Inventors: 晴香荒木; 紗江濱本; 浅井　達也
Original assignee: 株式会社神戸製鋼所
Priority date: 2018-03-27
Filing date: 2019-03-19
Publication date: 2019-10-03
Also published as: RU2766947C1; MX2020009944A; CN115404409A

Abstract

A steel plate for hot stamping according to the present invention contains, in terms of % by mass, 0.25% to 0.4% C, 1.05% to 1.4% Si, 0% to 1.4% Mn, 0.6% to 3.0% Cr, 0% to 0.03% P, 0% to 0.02% S, 0.01% to 1% Al, 0% to 0.01% N, 0.0005% to 0.005% B, and 0.005% to 0.1% Ti, the remainder being iron and unavoidable impurities. By satisfying the relational expression [C] + 2/9[Si] + 7/9[Mn] + 8/9[Cr] – 7/4 > 0, this steel plate for hot stamping has excellent hardness stability in addition to a balance between strength and toughness.

Description

Steel sheet for hot stamping

The present invention relates to a steel sheet for hot stamping.

In recent years, there has been a demand for improved collision safety of automobiles, and accordingly, even higher strength is required for hot stamping steel sheets used in parts that require rigidity in automobiles. However, when the strength of the steel sheet is improved, the low-temperature toughness deteriorates, so that the balance between strength and toughness is lacking. In contrast, Non-Patent Document 1 proposes improving the balance between strength and toughness of a steel sheet by refining old austenite grains after hot stamping.

In the case of hot stamping, the cooling rate inside the steel sheet may decrease due to an increase in the mold temperature and the clearance between the mold and the steel sheet. When the cooling rate of the steel sheet is equal to or lower than the critical cooling rate, a soft phase such as ferrite and bainite precipitates, and the hardness of the steel sheet is reduced. In particular, the autotemper is promoted by a decrease in the cooling rate below the Ms point, which causes a decrease in the hardness of the steel sheet.

Non-Patent Document 2 discusses the change in cooling rate when the clearance between the mold and the steel plate is changed. When this clearance is 0.4 mm, the cooling rate is up to about 15 ° C./s. It has been shown to decline.

As described in Non-Patent Document 1, there is a method of refining the crystal grains of steel as a general structure design technique for hot stamping steel plates, thereby obtaining a steel plate with an excellent balance between strength and toughness. it can. As a method of refining crystal grains, there is a method of adding an element such as Nb, Ni, Ti, etc. In this case, the economic efficiency of the steel sheet is deteriorated. Moreover, since the hardenability deteriorates, the steel plate which refined the crystal grain lacks hardness stability.

In response to this problem, it is also conceivable to improve process problems that cause a decrease in hardness, such as an increase in mold temperature and a clearance between the mold and the steel sheet. However, in that case, it is necessary to modify the mold repeatedly or to prepare a special mold, which requires a great deal of labor and cost. Therefore, the conventional hot stamping steel plate has a problem that it is difficult to obtain a member (molded product) having an excellent balance between strength and toughness and excellent hardness stability without increasing labor and cost.

An object of the present invention is to provide a steel sheet for hot stamping capable of obtaining a molded product excellent in hardness stability in addition to a balance between strength and toughness while suppressing an increase in labor and cost in the hot stamping process. It is.

The steel sheet for hot stamping according to one aspect of the present invention is
% By mass
C: 0.25% or more, 0.4% or less,
Si: 1.05% or more, 1.4% or less,
Mn: 0% or more, 1.4% or less,
Cr: 0.6% or more, 3.0% or less,
P: 0% or more, 0.03% or less,
S: 0% or more, 0.02% or less,
Al: 0.01% or more, 1% or less,
N: 0% or more, 0.01% or less,
B: 0.0005% or more and 0.005% or less, and Ti: 0.005% or more and 0.1% or less, with the balance being iron and inevitable impurities. This hot stamping steel sheet has the following relational expression (1) when the C content is [C], the Si content is [Si], the Mn content is [Mn], and the Cr content is [Cr]. By satisfying the above, in addition to the balance between strength and toughness, hardness stability is excellent.

According to the present invention, there is provided a hot stamping steel plate capable of obtaining a molded product having excellent hardness stability in addition to a balance between strength and toughness while suppressing an increase in labor and cost in the hot stamping process. Can do.

It is a graph which shows the relationship between the absorption energy in the Charpy impact test when quenching a flat plate using a metal mold, and the hardness when quenched at a cooling rate of 10 ° C./s. It is a figure which shows the process of a hot stamp typically. It is a schematic diagram which shows each dimension of the test piece used for the Charpy pendulum type impact test. It is a schematic diagram which shows each dimension of the test piece used for the hardness test.

Hereinafter, the hot stamping steel plate according to the embodiment of the present invention will be described in detail.

(Steel plate for hot stamping)
The steel sheet for hot stamping according to this embodiment is
% By mass
C: 0.25% or more, 0.4% or less,
Si: 1.05% or more, 1.4% or less,
Mn: 0% or more, 1.4% or less,
Cr: 0.6% or more, 3.0% or less,
P: 0% or more, 0.03% or less,
S: 0% or more, 0.02% or less,
Al: 0.01% or more, 1% or less,
N: 0% or more, 0.01% or less,
B: 0.0005% or more and 0.005% or less, and Ti: 0.005% or more and 0.1% or less, with the balance being iron and inevitable impurities. This hot stamping steel sheet has the following relational expression (1) when the C content is [C], the Si content is [Si], the Mn content is [Mn], and the Cr content is [Cr]. By satisfying the above, in addition to the balance between strength and toughness, hardness stability is excellent.

In order to obtain a steel sheet for hot stamping excellent in both strength and toughness balance and hardness stability, the present inventors have conducted intensive research on the composition of the steel sheet. According to the description of Non-Patent Document 2, in the hot stamping process, the normal member is cooled in the range of 30 ° C./s to 10 ° C./s due to the clearance between the die and the steel plate and the rise of the die temperature. Speed fluctuations were expected to occur. For this reason, in addition to the balance between strength and toughness, the present inventors have focused on suppressing variation in hardness even if fluctuations in the cooling rate occur, and details on the component system of the steel sheet to achieve this The examination was done. As a result, the present inventors adjust the balance of the contents of C, Si, Mn and Cr so that each component composition in the steel sheet satisfies the above range and the relational expression (1) is satisfied. Thus, the inventors have newly found that it is possible to achieve both a balance between strength and toughness and hardness stability, and have arrived at the present invention.

First, each component composition in the steel sheet for hot stamping according to the present embodiment will be described in detail.

[C (carbon): 0.25 mass% or more, 0.4 mass% or less]
C content determines the intensity | strength of the steel plate after metal mold | die cooling. In order to obtain sufficient strength of the steel sheet, the C content is 0.25% by mass or more, preferably 0.255% by mass or more, and more preferably 0.260% by mass or more. .

However, when the C content is excessive, the strength of the steel sheet after hot rolling is increased, which may cause cracking during cold rolling and deterioration of weldability. For this reason, C content is 0.4 mass% or less, it is preferable that it is 0.38 mass% or less, and it is more preferable that it is 0.36 mass% or less.

[Si (silicon): 1.05 mass% or more, 1.4 mass% or less]
Si contributes to the hardness stability of the steel sheet by increasing the temper softening resistance. Si also has an effect of preventing scale peeling after cooling the mold when the surface of the steel sheet is not plated. In order to exert these effects, the Si content is 1.05% by mass or more.

On the other hand, Si facilitates the formation of retained austenite (γ) and promotes a decrease in yield strength (YS) and segregation of Mn. For this reason, Si content is 1.4 mass% or less, and it is preferable that it is 1.35 mass% or less.

[Mn (manganese): 0% by mass or more and 1.4% by mass or less]
Mn is one of the important elements contained in the steel sheet for hot stamping according to the present embodiment, and contributes to increasing the strength of the steel sheet after cooling the mold by enhancing the hardenability of the steel sheet. In order to exert this effect, the Mn content is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and further preferably 0.8% by mass or more. .

On the other hand, in the study to balance the strength and toughness of the steel sheet after cooling the mold, if Mn is excessive, coarse carbides were precipitated during the mold cooling, and impact stress was applied in a low temperature environment. In this case, it was confirmed that it causes a brittle fracture. For this reason, Mn content is 1.4 mass% or less, it is preferable that it is 1.35 mass% or less, and it is more preferable that it is 1.30 mass% or less.

In addition, since Mn is an element inevitably mixed in the steel sheet, it is difficult to make the content 0 mass%.

[Cr (chromium): 0.6 mass% or more, 3.0 mass% or less]
Cr is one of important elements in the hot stamping steel plate according to the present embodiment. In the study of balancing the strength and toughness of the steel sheet after mold cooling, Cr contributes to securing hardness at a low cooling rate (for example, 10 ° C./s) and precipitates coarse carbides during mold cooling. It was confirmed that brittle fracture was suppressed when impact stress was applied in a low-temperature environment. In order to exhibit these effects, the Cr content is 0.6% by mass or more, preferably 0.8% by mass or more, and more preferably 1.05% by mass or more.

On the other hand, if Cr is excessively contained in the steel sheet, the strength of the steel sheet after hot rolling increases, which causes cracking of the steel sheet during cold rolling and deterioration of pickling properties after hot rolling. For this reason, Cr content is 3.0 mass% or less, and it is preferable that it is 2.5 mass% or less.

[P (phosphorus): 0 mass% or more, 0.03 mass% or less]
P needs to prescribe | regulate the upper limit of content from a viewpoint of the weldability of a member, toughness, and surface flaw prevention. For this reason, P content is 0.03 mass% or less, it is preferable that it is 0.025 mass% or less, and it is more preferable that it is 0.02 mass% or less.

In addition, since P is an element inevitably mixed in the steel sheet, it is difficult to make the content 0 mass%.

[S (sulfur): 0 mass% or more, 0.02 mass% or less]
S produces MnS, thereby lowering the uniformity of the Mn concentration distribution and degrading the weldability of the steel sheet. For this reason, S content is 0.02 mass% or less, it is preferable that it is 0.018 mass% or less, and it is more preferable that it is 0.015 mass% or less.

In addition, since S is an element inevitably mixed in the steel plate like P, it is difficult to make the content 0 mass%.

[Al (aluminum): 0.01 mass% or more, 1 mass% or less]
Al is an element that acts as a deoxidizer. In order to exhibit this effect, the Al content is 0.01% by mass or more, and preferably 0.015% by mass or more.

However, when Al is excessively contained in the steel sheet, the hardness after cooling the mold is lowered, and the low temperature toughness is deteriorated due to excessive generation of Al ₂ O ₃ . For this reason, Al content is 1 mass% or less, it is preferable that it is 0.8 mass% or less, and it is more preferable that it is 0.1 mass% or less. In addition, Al content here means content of Al (sol.Al) of a solid solution state.

[N (nitrogen): 0% by mass or more and 0.01% by mass or less]
N is an element inevitably mixed in the steel sheet. If N is excessively contained in the steel sheet, N forms boride, thereby reducing the amount of solute B in the steel sheet, resulting in deterioration of hardenability. For this reason, N content is 0.01 mass% or less, it is preferable that it is 0.008 mass% or less, and it is more preferable that it is 0.005 mass% or less.

[B (boron): 0.0005 mass% or more, 0.005 mass% or less]
B is an important element for improving the hardenability of the steel sheet. By adding an appropriate amount of B to the steel sheet to enhance the hardenability, the strength of the steel sheet after cooling the mold can be stably increased. In order to exhibit this effect, the B content is 0.0005% by mass or more, preferably 0.0010% by mass or more, and more preferably 0.0015% by mass or more.

On the other hand, if B is excessively contained in the steel sheet, a coarse iron boron compound is precipitated, resulting in a decrease in toughness. For this reason, B content is 0.0050 mass% or less, it is preferable that it is 0.0045 mass% or less, and it is more preferable that it is 0.0030 mass% or less.

[Ti (titanium): 0.005 mass% or more, 0.1 mass% or less]
Ti reduces the amount of BN produced in the steel sheet by producing TiN. Thereby, the quantity of the solid solution B in a steel plate increases, and the effect of the hardenability improvement by B can be heightened. In order to exert this effect, the Ti content is 0.0050% by mass or more, preferably 0.010% by mass or more, and more preferably 0.015% by mass or more.

On the other hand, if Ti is excessively contained in the steel plate, carbides are precipitated at the grain boundaries, and the hardenability of the steel plate is deteriorated. For this reason, Ti content is 0.1 mass% or less, it is preferable that it is 0.08 mass% or less, and it is more preferable that it is 0.06 mass% or less.

The steel sheet for hot stamping according to this embodiment may further contain one or more selected from the group consisting of Mo, Nb and V in addition to the above component composition, or from the group consisting of Cu and Ni. One or more selected may further be contained. The range of these component compositions will be described below. These elements are not essential elements in the hot stamping steel sheet of the present invention, and may not be added.

[Mo (molybdenum): 0% by mass or more and 1.0% by mass or less]
Mo is an element that contributes to improving the hardenability of the steel sheet. In order to exhibit this effect, the Mo content is preferably 0.01% by mass or more. However, when Mo is excessively contained in the steel sheet, the strength of the steel sheet before hot forming is increased. In order to prevent this, the Mo content is preferably 1.0% by mass or less.

[Nb (niobium), V (vanadium): 0 mass% or more, 0.1 mass% or less]
Nb and V have the effect of forming fine carbides and refining the steel structure by the pinning effect. V also has the effect of secondary curing by precipitating during tempering. In order to exert these effects, the Nb and V contents are each preferably 0.0008% by mass or more.

However, when Nb and V are excessively contained in the steel sheet, coarse carbides are formed, which becomes a starting point of fracture and causes deterioration of toughness. Accordingly, the Nb and V contents are each preferably 0.1% by mass or less, more preferably 0.08% by mass or less, and further preferably 0.07% by mass or less.

[Cu (copper), Ni (nickel): 0 mass% or more, 0.5 mass% or less]
Cu and Ni are preferably added when it is necessary to improve the delayed fracture characteristics of the member. However, if Cu and Ni are excessively contained in the steel plate, it may cause wrinkles on the surface of the steel plate, and finally on the surface of the member. For this reason, it is preferable that each content of Cu and Ni is 0.5 mass% or less, and it is more preferable that the total content is 0.5 mass% or less.

The steel sheet for hot stamping according to this embodiment is excellent in hardness stability in addition to the balance of strength and toughness by satisfying the relational expression (1) below by adjusting the balance of the contents of C, Si, Mn and Cr. It has become. In this relational expression (1), [C] represents the C content (mass%) of the hot stamping steel plate. [Si] indicates the Si content (mass%) of the steel sheet for hot stamping. [Mn] indicates the Mn content (% by mass) of the steel sheet for hot stamping. [Cr] indicates the Cr content (mass%) of the steel sheet for hot stamping.

When each component composition satisfies the component range of the claims and satisfies the relational expression (1), the steel sheet for hot stamping according to the present embodiment has an excellent balance between strength after quenching by mold cooling and low temperature toughness. In addition to being a steel plate, it has excellent hardness stability. Specifically, the absorbed energy in a Charpy impact test at −40 ° C. when quenching a flat plate using a mold is A (J / cm ² ), and the steel sheet for hot stamping is heated to the austenite region and then 10 ° C./s. Hardness when quenched to room temperature at a cooling rate of B (Hv), hardness when quenched and quenched to room temperature at a cooling rate of 30 ° C / s after heating the steel sheet for hot stamping to the austenite region When C (Hv) is satisfied, the following relational expressions (2), (3), and (4) are all satisfied.

The above relational expression (2) is an index of the balance between strength and toughness of the steel plate newly devised by the present inventors, and is an important concept in considering the balance between strength and toughness of the steel plate for hot stamping. is there. In examining the balance between strength and toughness, the present inventors paid attention to the hardness when the cooling rate was 10 ° C./s and the toughness after cooling the flat plate. Flat plate mold cooling takes into account ideal cooling conditions in which no clearance is generated between the mold and the steel plate in the hot stamping process. By using the relational expression (2), it is possible to more faithfully evaluate the balance between strength and toughness when the hot stamping steel plate is processed into a member (formed product).

The graph of FIG. 1 shows the absorbed energy A in a Charpy impact test at −40 ° C. when a flat plate is quenched using a mold (horizontal axis) and the hardness B of the steel plate when quenched at a cooling rate of 10 ° C./s. (Vertical axis), and the straight line (1) in the graph corresponds to the relational expression (2). A straight line (2) in the graph corresponds to an equation of B = 516.

The horizontal axis (A) in the graph of FIG. 1 assumes the toughness of the most brittle part in the member after mold cooling. That is, when the flat plate is die-cooled, the die and the steel plate come into contact with each other in an ideal state, so that the cooling rate is increased. For this reason, the strength after cooling increases, but on the other hand it becomes very brittle. That is, this horizontal axis has a meaning as toughness in the most brittle part when the hot stamping steel plate is formed into a member (molded product).

On the other hand, the vertical axis (B) of the graph of FIG. 1 assumes the hardness of the most softened portion of the member after cooling the mold. As described above, in the hot stamping process, a clearance may be generated between the mold and the steel plate, and the mold temperature may increase. For this reason, the member after mold cooling has a portion with low hardness (strength) cooled at a low cooling rate. From the description of Non-Patent Document 2, the minimum cooling rate during mold cooling is assumed to be about 10 ° C./s. Therefore, this vertical axis has the meaning as the hardness (strength) at the most softened portion of the member (molded product) after cooling the mold. Therefore, by using these two axes, the toughness of the weakest part when impact stress is applied to the molded member and the strength of the weakest part when static stress is applied to the member Can be evaluated.

Usually, in the hardness region where B is 516 Hv or more, the strength and toughness of the steel sheet are in a trade-off relationship, and therefore the toughness tends to deteriorate when the strength of the steel sheet is improved. That is, it is difficult to improve both the strength and toughness of the steel sheet, and it is normal that the A / B distribution exists in the region below the straight line (1) in the graph of FIG.

Also, the straight line (2) is one index indicating hardness stability. During continuous operation of the mold in the hot stamping process, the temperature of the mold rises and there may be a clearance between the mold and the steel plate. Due to these factors, the cooling rate of the steel plate during quenching decreases, and the hardness of the steel plate after quenching decreases as the cooling rate decreases. Usually, even a steel sheet whose balance between strength and toughness is improved by refining crystal grains, it is difficult to satisfy the hardness range of 516 Hv or higher when quenched in a low cooling rate region (10 ° C./s). . Therefore, even in a steel sheet in which the balance between strength and toughness is improved by refining crystal grains, it is normal that the distribution of A and B exists in the region below the straight line (2) in FIG. .

On the other hand, as a result of intensive studies by the present inventors, in the hot stamping steel sheet satisfying the relational expression (1), the distribution of A and B has a straight line (1) and a straight line (2) in FIG. It became clear that it is located in the upper region. Therefore, the hot stamping steel plate according to this embodiment has excellent hardness stability in addition to the balance between strength and toughness. That is, this hot stamping steel sheet has an excellent balance between strength and toughness satisfying the above relational expression (2), and has a certain hardness or more even when cooled at a cooling rate of 10 ° C./s. It is feasible.

The above relational expression (4) is an index of hardness stability of another steel plate. When the mold temperature rises in the hot stamping or a clearance is generated between the mold and the steel sheet, the cooling rate of the steel sheet may decrease, and the hardness of the steel sheet after quenching may become unstable. Further, as described above, since the hardness stability decreases when the crystal grains are refined, it is usually difficult to satisfy the relational expression (4).

On the other hand, as a result of intensive studies by the present inventors, the steel sheet for hot stamping satisfying the relational expression (1) and satisfying the component ranges of the claims has a cooling rate of 10 ° C./s. It was clarified that hardness after quenching exceeding 516 Hv can be obtained even in the low cooling rate region, and the hardness difference between the cooling rate of 30 ° C./s and 10 ° C./s can be suppressed to 35 Hv or less. . 30 ° C./s is an ideal cooling rate at the time of mold cooling confirmed by experiments and the like, while 10 ° C./s is the minimum cooling rate expected as described above. That is, the relational expression (4) is an index indicating that the hardness difference (variation) after quenching is small between the upper and lower limits of the cooling rate assumed in the hot stamp. According to the steel sheet for hot stamping according to the present embodiment, the hardness of the steel sheet after quenching to the extent that the above relational expression (4) is satisfied regardless of the temperature rise of the mold and the occurrence of clearance between the mold and the steel sheet. Can be stabilized.

The steel sheet for hot stamping of the present invention may be a base steel sheet that has not been subjected to plating treatment, or a plated steel sheet that has been subjected to plating treatment.

(Method for manufacturing hot stamping steel sheet)
Next, a method for manufacturing the hot stamping steel plate according to the present embodiment will be described.

First, the slab manufacturing process is performed. In this step, a slab is obtained by melting steel according to a conventional method, pouring molten steel into a mold and continuously casting the steel. In this step, the component composition of the steel is adjusted during melting so that each component composition contained in the slab satisfies the above range and the contents of C, Si, Mn, and Cr satisfy the relational expression (1).

Next, a hot rolling process is performed. In this step, first, the slab obtained in the above step is placed in a heating furnace, heated to a predetermined temperature (eg, 1200 ° C.), and held at the heating temperature for a predetermined time (eg, 30 minutes).

Next, the heated slab is placed upstream of the hot rolling line. And the said slab is rolled into the steel plate which has predetermined | prescribed plate | board thickness by flowing toward the downstream, passing the said slab between the rolls of the rolling stand of a rough rolling mill and a finish rolling mill sequentially. And after the steel plate after hot rolling is cooled to predetermined temperature in a cooling device, it is wound up with a coiler.

Next, a cold rolling process is performed. In this step, first, the scale (iron oxide) generated on the surface of the steel plate in the hot rolling step is washed off with acid (pickling), and then the hot-rolled steel plate is further rolled so that the plate thickness is further reduced. Process. Specifically, the hot-rolled steel sheet after pickling is passed between rolls of a rolling stand so that the hot-rolled steel sheet is further thinned. The cold-rolled steel sheet obtained by the above process is the hot stamping steel sheet according to the present embodiment.

(Hot Stamp)
Next, the hot stamp using the steel plate manufactured by the said process is demonstrated with reference to FIG. First, the hot stamping steel plate 1 manufactured by the above process is heated in the predetermined heating furnace 2 to the austenite transformation temperature or higher. Then, the heated hot stamping steel plate 1 is placed between the dies 3 and 4, and the hot stamping steel plate 1 is press-formed into a desired shape by the dies 3 and 4. At this time, the hot stamping steel plate 1 is cooled by coming into contact with the

molds

3 and 4, and quenching is performed simultaneously with the forming. And the steel plate after hardening is taken out from the metal mold | dies 3 and 4 as the molded article 5 (molded member).

The molded product 5 has the same component composition as the hot stamping steel plate 1 according to this embodiment described above, and the balance of the contents of C, Si, Mn and Cr is adjusted so as to satisfy the relational expression (1). It has been done. Therefore, the molded product 5 is excellent in hardness stability in addition to the balance between strength and toughness, and can be used for various applications including automobile members.

The outline of the above embodiment is as follows.

The steel sheet for hot stamping according to the above embodiment is
% By mass
C: 0.25% or more, 0.4% or less,
Si: 1.05% or more, 1.4% or less,
Mn: 0% or more, 1.4% or less,
Cr: 0.6% or more, 3.0% or less,
P: 0% or more, 0.03% or less,
S: 0% or more, 0.02% or less,
Al: 0.01% or more, 1% or less,
N: 0% or more, 0.01% or less,
B: 0.0005% or more and 0.005% or less, and Ti: 0.005% or more and 0.1% or less, with the balance being iron and inevitable impurities. This hot stamping steel sheet has the following relational expression (1) when the C content is [C], the Si content is [Si], the Mn content is [Mn], and the Cr content is [Cr]. By satisfying the above, in addition to the balance between strength and toughness, hardness stability is excellent.

The hot stamping steel plate
% By mass
Mo: 0% or more, 1.0% or less,
Nb: 0% or more, 0.1% or less, and V: 1% or more selected from the group consisting of 0% or more, 0.1% or less may be contained.

The hot stamping steel plate
% By mass
Cu: 0% or more, 0.5% or less, and Ni: 1% or more selected from the group consisting of 0% or more, 0.5% or less may be contained.

Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited by the following examples, and can be implemented with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. To be included in the scope.

<Manufacture of steel sheets for hot stamping>
First, No. 1 in Table 1 below. Slabs were produced by melting steel having the composition shown in 1 to 17 (the balance being iron and inevitable impurities). The molten slab was heated to 1200 ° C. and held for 30 minutes, and then hot rolled. The finishing temperature was 900 ± 20 ° C., and the finishing plate thickness was 2.8 mm. Thereafter, the hot-rolled steel sheet was cooled to a coiling temperature (CT temperature) at a cooling rate of 20 to 30 ° C./s, held at 650 ° C. for 30 minutes, and then cooled in a furnace. Thereafter, the hot-rolled steel sheet was pickled, and the steel sheet was processed by cold rolling so that the sheet thickness became 1.4 mm.

<Charpy impact test>
First, the cold-rolled steel sheet produced by the above procedure was cut and quenched. Quenching was performed under the following conditions using a die quench method using a flat plate simulating a mold (testing machine: JIS Charpy impact testing machine (300J)).

[Hardening conditions]
Steel plate dimensions before quenching: 1.4mm x 70mm x 150mm
Steel plate temperature: 900 ° C
Steel plate temperature holding time after the steel plate reaches 900 ° C .: 100 seconds Cooling time: about 15 seconds Die quench start temperature: 700 ° C.
Die quench load: 2000kgf
Bottom dead center retention time: 30 seconds

Next, a Charpy pendulum impact test was performed using the cold-rolled steel sheet after quenching. This test was performed in accordance with “Charpy impact test method for metal materials” of JIS 2242 except for the dimensions of the test pieces. The dimensions of the test pieces used in this test are as follows. In addition, the code | symbol which shows each dimension respond | corresponds to the code | symbol shown in FIG.

[Test specimen dimensions]
Test piece height h1: 10 mm ± 0.05 mm
Test piece length L: 55 mm ± 0.6 mm
Test piece width b: 1.4 mm ± 0.05 mm
Notch shape: V notch Notch angle: 45 ° ± 2 °
Notch bottom radius: 0.25 mm ± 0.025 mm
Notch height h2: 8mm ± 0.05mm
Angle between specimen longitudinal direction and notch symmetry plane: 90 ° ± 2 °
Angle between adjacent surfaces to remove fracture surface: 90 ° ± 2 °

The test piece having the above dimensions was placed in liquid nitrogen adjusted to a temperature of −40 ° C. ± 1 ° C. and held for at least 10 minutes. Thereafter, the test piece was taken out of the liquid nitrogen and placed on a support table, and an impact was applied to the test piece. At this time, after putting a test piece on a support stand, time until giving an impact was made into 5 seconds or less.

JIS Charpy impact tester (300J) was used as a tester, and an impact blade having a radius of 2 mm was used. The number of test pieces was two, and evaluation was performed using the average value of the two measured values.

<Evaluation of scale adhesion>
After quenching by the die quench method under the same conditions as in the Charpy impact test, the adhesion of the scale was evaluated by visually confirming the degree of scale peeling on the surface of the steel sheet. When the area ratio at which scale peeling occurred on the surface of the steel sheet was 14% or less, it was evaluated as “◯”, and when the area ratio exceeded 14%, it was evaluated as “x”.

<Hardness test>
First, the cold-rolled steel sheet produced by the above procedure was processed into a test piece having the shape shown in FIG. In FIG. 4, L1 is 10 mm, L2 is 2 mm, L3 is 1.4 mm, L4 is 0.7 mm, L5 is 3 mm, and L6 is 1 mm. Using this test piece, quenching was performed under the following conditions.

[Hardening conditions]
Rate of temperature rise when austenitizing: 10 ° C / s
High temperature hold: hold at 900 ° C. for 100 seconds Cooling rate: constant cooling from 900 ° C. to room temperature at 10 ° C./s or 30 ° C./s

The hardness test based on the “Vickers hardness test method” defined in JIS Z 2244 was performed using the specimen after quenching. In this test, five-point measurement was performed at a test load of 9.8 N at a position 1/4 of the plate thickness from the surface of the test piece, and the average value thereof was used for evaluation.

Tables 1 and 2 below show No. For each of the steel plates 1 to 17, the composition (mass%), the absorbed energy A (J / cm ² ) in the Charpy impact test at −40 ° C., and the Vickers hardness B (Hv when the cooling rate is 10 ° C./s) ), Vickers hardness C (Hv) when the cooling rate is 30 ° C./s, hardness difference (Hv) when the cooling rate is 10 ° C./s and 30 ° C./s, and the above relational expression ( The value of the left side of 1), the value when the right side is subtracted from the left side of the relational expression (2), and the evaluation of scale adhesion are shown.

In the graph of FIG. Each data of steel plates 1 to 17 is plotted. No. The data of 1 to 9 and 14 to 17 are marked with black circles. The data of 10 to 13 are marked with white circles.

<Discussion>
Based on Table 1 and Table 2 above, the following can be considered.

No. 1 to 9 and 14 to 17 are the contents of C, Si, Mn, Cr, P, S, Al, N, B and Ti in the steel sheet satisfy the scope of the present invention, respectively, and C, Si, Mn and The Cr content satisfied the relational expression (1). In this case, the value of “B + 4A−627” was a positive value and satisfied the relational expression (2), so that the steel sheet had an excellent balance between strength and toughness. Moreover, no. In 1 to 9 and 14 to 17, “B ≧ 516” and “CB ≦ 35” were satisfied, and the above relational expressions (3) and (4) were satisfied, so that the steel sheet was excellent in hardness stability. This is because in the graph of FIG. It is clear from the fact that the data (black circles) 1 to 9 and 14 to 17 are present in the region above the straight lines (1) and (2). In addition, all the evaluations of scale adhesion were “◯”.

On the other hand, No. that does not meet the requirements defined in the present invention. In 10 to 13, a steel plate excellent in both strength and toughness balance and hardness stability was not obtained as described below. As shown in the graph of FIG. Data 10 to 13 (white circles) were all present in the area below the straight lines (1) and (2).

No. 10, the Si content is less than 1.05% by mass, and the value of [[C] +2/9 [Si] +7/9 [Mn] +8/9 [Cr] −7/4] is a negative value. Therefore, the value of “B + 4A−627” became a negative value, and the balance between strength and toughness was inferior. In addition, the hardness B when the cooling rate is 10 ° C./s is less than 516 Hv, and the difference in hardness between the cooling rate of 10 ° C./s and 30 ° C./s exceeds 35 Hv, which also improves the hardness stability. inferior. The evaluation of scale adhesion was also “x”.

No. 11 to 13, the Cr content is less than 0.6% by mass, and the value of “[C] +2/9 [Si] +7/9 [Mn] +8/9 [Cr] −7/4” is negative. Therefore, the value of “B + 4A−627” was a negative value, and the balance between strength and toughness was inferior. In addition, the hardness B when the cooling rate is 10 ° C./s is less than 516 Hv, and the difference in hardness between the cooling rate of 10 ° C./s and 30 ° C./s exceeds 35 Hv, which also improves the hardness stability. inferior.

It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

Claims

% By mass
C: 0.25% or more, 0.4% or less,
Si: 1.05% or more, 1.4% or less,
Mn: 0% or more, 1.4% or less,
Cr: 0.6% or more, 3.0% or less,
P: 0% or more, 0.03% or less,
S: 0% or more, 0.02% or less,
Al: 0.01% or more, 1% or less,
N: 0% or more, 0.01% or less,
B: 0.0005% or more, 0.005% or less, and Ti: 0.005% or more, 0.1% or less, the balance being iron and inevitable impurities,
When the C content is [C], the Si content is [Si], the Mn content is [Mn], and the Cr content is [Cr], by satisfying the relational expression (1) below, the strength and A steel sheet for hot stamping characterized by excellent hardness stability in addition to a balance of toughness.
% By mass
Mo: 0% or more, 1.0% or less,
2. The hot stamp according to claim 1, comprising at least one selected from the group consisting of Nb: 0% or more and 0.1% or less and V: 0% or more and 0.1% or less. Steel plate.
% By mass
It contains 1 or more types chosen from the group which consists of Cu: 0% or more and 0.5% or less, and Ni: 0% or more and 0.5% or less, The Claim 1 or 2 characterized by the above-mentioned. Steel sheet for hot stamping.