JP2016003389A - Steel plate for hot pressing, hot pressing molding using steel plate and production method of hot pressing molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel plate for hot pressing which realizes excellent ductility after hot pressing without addition of large amounts of expensive elements, such as Ni, or an additional step in cooling after press forming with a die.SOLUTION: A steel plate for hot pressing comprises 0.10-0.4% of C, 2.0% or less of Si, 0.5% or less of Mn, 0.8-2.0% of Cr, 0.015 or less of P, 0.01% or less of S, 0.001-0.1% of Al, 0.01-0.1% of Ti, 0.0005-0.005% of B and N in a content giving [Ti]/[N] of 3.3 or higher and has a carbon equivalent Ceq of 0.43-0.7% and an Ms point of 415°C or higher. (1) Ceq=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14; and (2) Ms(°C)=550-361[C]-39[Mn]-10[Cu]-17[Ni]-20[Cr]-5[Mo]+30[Al].

Description

  The present invention relates to a steel sheet for hot pressing, a hot press-formed product using the steel sheet, and a method for producing the same. Below, although it demonstrates centering on the steel plate for motor vehicles which is a representative example of the said steel plate for hot presses, this invention is not limited to this.

  In recent years, it has been demanded to increase the strength of automobile steel plates as much as possible in order to achieve both improved fuel economy and improved collision safety. However, when the strength of the steel plate is increased, the Rankford value indicating the elongation and the deep drawability of the plate material decreases, and the press formability and the shape freezeability deteriorate.

  In order to solve such a problem, a hot press molding method is employed. The hot press forming method is a method in which a steel plate is heated to a temperature at which it becomes an austenite single phase to reduce the strength and press forming with a mold in a state in which forming is facilitated. Specifically, for example, the shape of the steel sheet is imparted by molding with a mold at a lower temperature than that of the steel sheet, for example, room temperature, and at the same time, quenching using the temperature difference between the two is performed to increase the strength of the part after press forming. Secure.

  Thus, since the parts manufactured by the hot press molding method exhibit high strength, application to collision parts such as bumper beams and door beams is being promoted. Collision parts are required to have ductility and toughness in addition to high strength. Therefore, research and development are being conducted to increase the toughness of parts after hot pressing using a hot press forming method.

  For example, Patent Document 1 proposes a steel sheet for hot pressing that can improve the hardenability and impact characteristics after hot pressing by containing Ni in an amount of 1% by mass over 1%.

  Patent Document 2 includes, in mass%, C: 0.33% to 0.40%, Ni: 2.0 to 5.0%, Mn: 0.01% to less than 0.5%. In addition, a hot press-molded product with improved ductility is proposed by making it a steel structure consisting of martensite containing Cr as a selective component in a range of 0.5% or less and having a prior austenite average particle size of 5 μm or more. Has been.

  However, the techniques of Patent Documents 1 and 2 require a large amount of expensive Ni, which increases the cost.

In Patent Document 3, a steel sheet is heated to a temperature not lower than the Ac ₃ transformation point, press-molded with a mold heated to a temperature not lower than 400 ° C. and not higher than 550 ° C., and 5 at a temperature not lower than 400 ° C. and not higher than 550 ° C. There has been proposed a hot pressing method that improves toughness by holding for more than a second and then cooling to room temperature at a rate less than air cooling.

Patent Document 4 discloses that a steel sheet is heated to a temperature not lower than the AC ₃ transformation point, press-molded with a mold heated to a temperature not lower than 400 ° C. and not higher than 550 ° C., and brought to a temperature range of 400 ° C. to 550 ° C. Hold for 5 seconds or more, heat to 550 ° C or more and 590 ° C or less, perform soft nitriding, and then cool to room temperature at a rate of 20 ° C / second or more to improve toughness and fatigue properties A pressing method has been proposed.

  However, the techniques disclosed in Patent Documents 3 and 4 have a problem that the mold cooling rate decreases and the productivity decreases because the mold is heated to a temperature of 400 ° C. or higher and 550 ° C. or lower. Furthermore, in the techniques of Patent Documents 3 and 4 above, after maintaining at 400 ° C. or higher and 550 ° C. or lower for 5 seconds or longer, cooling to room temperature at a rate of air cooling or lower, or soft nitriding at a temperature of 550 ° C. or higher and 590 ° C. or lower. Therefore, productivity is extremely reduced.

  In Patent Document 5, the total of one or more of Ni, Cu, and Sn is 0.005 to 2%, and the total of one or more of Ca, Mg, Y, As, Sb, and REM is added. By adding 0.0005 to 0.05, a steel sheet for hot pressing that can improve impact characteristics and delayed fracture characteristics after hot pressing has been proposed.

  Patent Document 6 contains C: 0.25 to 0.45% and Mn + Cr: 0.5 to 3.0%, Si: 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less, containing a chemical composition consisting of Fe and impurities, steel structure has a prior austenite average grain size of 10 μm or less A hot press-formed product with improved toughness and a steel plate for hot press that is the basis of the hot-tempered martensite and tensile strength of 1.8 GPa or more have been proposed. .

  According to the techniques of Patent Documents 5 and 6, although a certain improvement effect can be expected by controlling inclusions and making the particle size finer, sufficient ductility cannot be ensured, so further improvement is necessary.

Japanese Patent Laid-Open No. 2004-211197 JP 2012-1802 A JP 2005-177805 A JP 2006-224162 A JP-A-2005-139485 JP 2006-152427 A

  The present invention has been made paying attention to the above circumstances, and the object thereof is to add an additional step without adding a large amount of expensive elements such as Ni and at the time of cooling after press molding in a mold. It is another object of the present invention to provide a hot-press steel sheet, a hot-press formed product, and a method for producing the same that can realize excellent ductility after hot pressing.

  In order to solve the above-mentioned problems, the present inventors have studied mainly on the composition of a steel sheet for hot pressing. As a result, if the amount of Mn in the hot-press steel sheet is reduced to 0.5% or less, and if the amount of Mn is supplemented with Cr and a large amount of Cr is added to 0.8% or more, We found that ductility was improved.

  That is, in order to ensure the hardenability of the steel sheet by die cooling during hot pressing, Mn is conventionally used. For example, as shown in the conventional material of steel type 1 in Table 1 to be described later, Mn compared to Cr A large amount of was added. However, the hot press-molded product using the above-mentioned conventional material has a test No. in Table 2. As shown in FIG. 1, the ductility such as the bending angle was inferior, and there was room for improvement.

  Therefore, in order to achieve the above-mentioned problems, the present inventors pay attention to Cr which has the same hardenability improving effect as Mn and can suppress the formation of ferrite, bainite and the like, and is excellent in the effect of suppressing the coarsening of carbides. did. The biggest difference between Mn and Cr lies in the decrease in the Ms point at which martensite formation starts. The Ms point is defined by the following formula (2). In the above formula (2), the coefficient of Mn is −39, whereas the coefficient of Cr is −20. Compared to Mn, Cr is The decrease width of the Ms point is small. Therefore, when a large amount of Cr is added compared with Mn, the Ms point can be increased while suppressing the formation of ferrite and bainite. If the Ms point is low, the amount of dissolved C increases, which adversely affects the deformability of the steel material, and in particular the ductility decreases. On the other hand, when the Ms point rises, the temperature at which martensite is formed increases, and C dissolved in the steel material precipitates as carbide during the mold cooling of the generated martensite, so-called self-tempering. Promoted.

  And in this invention, since the carbide | carbonized_material coarsening suppression effect with a large amount is contained, the growth rate of a carbide | carbonized_material is suppressed significantly. As a result, coarsening of the carbide formed by self-tempering is prevented, and the carbide that becomes the starting point of fracture can be refined, so that the strength and toughness of the hot press-formed product are improved while increasing the ductility of the hot press-formed product. Can also be secured.

  The present invention finds that the intended purpose can be achieved by determining the Mn amount and the Cr amount based on the above viewpoint, and controlling the Ms point and the Ceq amount that is an index of hardenability to an appropriate range, The present invention has been completed.

That is, the steel sheet for hot pressing according to the present invention that has solved the above-mentioned problems has a component composition of mass%, C: 0.10 to 0.4%, Si: 0% or more and 2.0% or less, Mn : 0% to 0.5%, Cr: 0.8 to 2.0%, P: more than 0% to 0.015% or less, S: more than 0% to 0.01% or less, Al: 0.001 to 0 0.1%, Ti: 0.01 to 0.1%, B: 0.0005 to 0.005%, N: [Ti] / [N] each including a content of 3.3 or more, the balance being The carbon equivalent Ceq defined by the following formula (1) is 0.43 to 0.7%, and the Ms point defined by the following (2) is 415 ° C. or higher. It has a gist at some point.
Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
Ms (° C.) = 550-361 × [C] −39 × [Mn] −10 × [Cu] −17 × [Ni] −20 × [Cr] −5 × [Mo] + 30 × [Al]. (2)
However, [X] means content of the element X and a unit is the mass%.

  In a preferred embodiment of the present invention, the steel sheet for hot pressing is further mass%, Mo: more than 0% and 0.5% or less, Cu: more than 0% and 0.5% or less, and Ni: more than 0% and 0%. And at least one selected from the group consisting of 5% or less.

  In a preferred embodiment of the present invention, the hot-press steel sheet further includes at least one of mass%, V: more than 0% and 0.1% or less, and Nb: more than 0% and 0.1% or less.

  In a preferred embodiment of the present invention, the steel sheet for hot pressing further includes at least one of Ca: more than 0% and 0.005% or less and REM: more than 0% and 0.005% or less.

Moreover, the manufacturing method of the hot press-formed product according to the present invention that has solved the above-mentioned problems is a hot press steel sheet according to any one of the above, with Ac ₃ or more defined by the following formula (3): Then, the steel sheet is press-molded with a mold, and after the press molding is started, the range up to the Ms point defined by the following formula (2) is 20 to 300 ° C. in the mold. It has a gist in that it is cooled at an average cooling rate of / s.
Ms (° C.) = 550-361 × [C] −39 × [Mn] −10 × [Cu] −17 × [Ni] −20 × [Cr] −5 × [Mo] + 30 × [Al]. (2)
Ac ₃ (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V] − 11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15.2 × [Ni] (3)
However, [X] means content of the element X and a unit is the mass%.

  Moreover, the hot press-formed product of the present invention that has solved the above-mentioned problems has the component composition described in any of the above, and martensite: 95% or more in area ratio with respect to the entire structure, the balance: ferrite, and bainite And having a structure in which the average particle size of the carbide dispersed in the steel is 1 μm or less.

  According to the present invention, a part of Mn is replaced with relatively inexpensive Cr, and a hot-press steel sheet containing a larger amount of Cr than Mn is used. Therefore, a large amount of expensive elements such as Ni are contained. Hot press forming that can realize excellent ductility after hot pressing by a method with excellent productivity without adding, and without requiring additional steps when cooling after press forming in the mold Goods can be provided.

First, the hot press steel sheet of the present invention will be described. As described above, the steel sheet for hot pressing of the present invention has a component composition of C: 0.10 to 0.4%, Si: 0% to 2.0%, Mn: 0% to 0.5%. Cr: 0.8 to 2.0%, P: more than 0% and 0.015% or less, S: more than 0% and 0.01% or less, Al: 0.001 to 0.1%, Ti: 0.01 -0.1%, B: 0.0005-0.005%, N: [Ti] / [N] each include a content of 3.3 or more, the balance is made of iron and inevitable impurities, The carbon equivalent Ceq defined by the following formula (1) is 0.43 to 0.7%, and the Ms point defined by the following formula (2) is 415 ° C. or higher.
Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
Ms (° C.) = 550-361 × [C] −39 × [Mn] −10 × [Cu] −17 × [Ni] −20 × [Cr] −5 × [Mo] + 30 × [Al]. (2)

  Hereinafter, the component composition of the steel sheet for hot pressing according to the present invention will be described in detail.

C: 0.10 to 0.4%
C is an important element for ensuring the strength at the time of quenching during hot pressing. In particular, it is an essential element for producing martensite and achieving high strength of a hot press-formed product. In order to obtain the desired strength of 1180 MPa or more, the lower limit of the C content is 0.10% or more, preferably 0.15% or more, more preferably 0.2% or more. However, when C is excessively contained, not only the strength is increased more than necessary, but the hot workability is deteriorated, but also the weldability is deteriorated, so the upper limit of the C amount is 0.4% or less, preferably 0.3% or less.

Si: 0% or more and 2.0% or less Si has a high solid solution strengthening ability and suppresses coarsening of cementite, but also tends to suppress formation of fine carbides useful for improving ductility. Further, Si inhibits the self-tempering action of martensite at the time of mold quenching and causes a decrease in ductility. Therefore, it is desirable that the amount of Si is as small as possible. The amount of Si is 2.0% or less, preferably 0.5% or less, more preferably 0.1% or less. The amount of Si may be 0%.

Mn: 0% or more and 0.5% or less Mn is an element that greatly reduces the Ms point, while the effect of improving hardenability is very large. Therefore, when Mn is contained excessively, self-tempering does not occur sufficiently and ductility is lowered. Therefore, in the present invention, the upper limit of the amount of Mn is limited to the minimum content necessary for ensuring hardenability. The amount of Mn is 0.5% or less, preferably 0.3% or less. In addition, the minimum with the preferable amount of Mn is 0.05% or more, More preferably, it is 0.1% or more.

Cr: 0.8 to 2.0%
Cr has the effect of improving hardenability like Mn, but since the decrease in Ms point is smaller than that of Mn, it is necessary to achieve both improvement in ductility by promoting self-tempering and increase in strength by improving hardenability. Can do. Furthermore, Cr has a high effect of suppressing the coarsening of carbides formed during tempering, and is an element useful for improving ductility by minimizing carbides that are the starting point of fracture in hot press-formed products to prevent fracture. In order to effectively exhibit these effects, the Cr content is 0.8% or more, preferably 1.0% or more, more preferably 1.2% or more. However, even if Cr is contained excessively, the above effect is saturated and the cost increases, so the upper limit of Cr content is 2.0% or less, preferably 1.8% or less, more preferably 1.5%. The following.

  In addition, in Claim 1 of the patent document 6 mentioned above, although the hot-pressed steel plate member containing Mn + Cr: 0.5-3.0% is prescribed | regulated, it differs from this invention by the following points. . First, in the above-mentioned Patent Document 6, Mn and Cr are only recognized as synergistic elements having an effect of improving hardenability, and a part of Mn amount is replaced with Cr amount as in the present invention, and ductility after press forming is performed. There is no idea of improving the quality. Therefore, in the said patent document 6, reducing the upper limit of Mn to 0.5% or less like this invention, and adding 0.8% or more of Cr is not indicated. Actually, Table 1 of the above-mentioned Patent Document 6 discloses only an example including an amount of Mn of 1.10% or more. Even in the case of the Cr content, most of the above examples are 0.48% or less, and even in the case of containing only 0.80% Cr, the Mn content is as high as 1.10%, which satisfies the requirements of the present invention. Not done.

P: more than 0% and not more than 0.015% P is a harmful element that causes grain boundary segregation in steel and adversely affects hot workability and press formability. Therefore, the P content is 0.015% or less, preferably 0.010% or less, more preferably 0.005% or less.

S: more than 0% and 0.01% or less S forms sulfide inclusions such as MnS and can cause cracks. Therefore, the S content is 0.01% or less, preferably 0.005% or less, more preferably 0.002% or less.

Al: 0.001 to 0.1%
Al is useful as a deoxidizing material for molten steel, and is also an element that fixes N, which inhibits the effect of improving hardenability by adding B described later. In order to effectively exhibit such an effect, the lower limit of the Al content is 0.001% or more, preferably 0.01% or more, more preferably 0.02% or more. However, if Al is contained excessively, non-metallic inclusions increase and surface defects are likely to occur. Therefore, the upper limit of Al content is 0.1% or less, preferably 0.07% or less, more preferably 0. 0.04% or less.

Ti: 0.01 to 0.1%
Ti binds to N in steel in preference to B and becomes TiN to fix N, thereby suppressing B from being wasted as BN and improving the hardenability improvement effect by B . In order to effectively exhibit such an effect, the lower limit of the Ti amount is set to 0.01% or more, preferably 0.015% or more. However, if Ti is excessively contained, Ti-based precipitates are produced in a large amount in the steel and the toughness is deteriorated, so the upper limit of the Ti amount is 0.1% or less, preferably 0.06% or less. More preferably, it is 0.04% or less.

B: 0.0005 to 0.005%
B is an element useful for improving hardenability. For this reason, the lower limit of the B amount is set to 0.0005% or more, preferably 0.001% or more. However, when B is contained excessively, there is a concern of hot cracking and the above effect is saturated. Therefore, the upper limit of the B amount is 0.005% or less, preferably 0.004% or less, and more preferably 0.003% or less.

N: Content at which [Ti] / [N] is 3.3 or more N, as described above, binds to B to become BN, wastes B, and deteriorates the effect of improving hardenability by B. Therefore, the atomic% of N is limited so as not to exceed the atomic% of Ti so that N can be fixed as TiN in preference to B. From the above viewpoint, N is a content such that [Ti] / [N] is 3.3 or more. Note that 3.3 is an atomic weight ratio of Ti and N. A preferable N amount is an amount such that [Ti] / [N] is 3.5 or more, and a more preferable N amount is an amount such that [Ti] / [N] is 3.8 or more. The upper limit is not particularly limited from the above viewpoint, but considering that [Ti] / [N] is too large, coarse TiN is formed and the bendability is deteriorated. The upper limit of N] is 10 or less, more preferably 7 or less.

Carbon equivalent Ceq: 0.43-0.7%
Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
As will be described later, in the present invention, die quenching is performed in a hot pressing process. In the mold quenching, for example, the quenching speed is low as compared with general quenching methods such as water quenching and oil quenching. Therefore, in order to obtain sufficient hardness after hot pressing by mold quenching, it is necessary to sufficiently secure the hardenability of the steel sheet for hot pressing. Even if there is no decrease in hardness, if a small amount of ferrite is formed, the ferrite becomes a crack propagation path and local ductility deteriorates. Also from such a viewpoint, it is necessary to ensure hardenability. Therefore, in the present invention, the carbon equivalent Ceq, which is an index of hardenability, is 0.43% or more, preferably 0.45% or more, more preferably 0.46% or more. However, if the hardenability is too high, it becomes too hard and the ductility deteriorates, so the upper limit of Ceq is 0.7% or less, preferably 0.67% or less, more preferably 0.65% or less. The above equation (1) is based on JIS G 0203.

Ms point: 415 ° C. or higher Ms (° C.) = 550-361 × [C] −39 × [Mn] −10 × [Cu] −17 × [Ni] −20 × [Cr] −5 × [Mo] + 30 × [Al] (2)
As described above, the cooling rate of the mold quenching in the hot pressing process is smaller than that of a general quenching method. Therefore, promoting self-tempering of martensite in the cooling process after the formation of martensite is an effective means for promoting tempering while still being hot pressed. In order to promote self-tempering and increase ductility, the temperature at which martensite is formed during cooling, that is, the Ms point, is increased by lowering the cooling rate and securing the self-tempering time. It is effective to use self-tempering at Thereby, characteristics, such as ductility, can be improved, without causing productivity deterioration. Therefore, in the present invention, the lower limit of the Ms point is set to 415 ° C. or higher, preferably 420 ° C. or higher, more preferably 425 ° C. or higher. The upper limit of the Ms point is not particularly limited from the above viewpoint, but if the Ms point is increased, self-tempering is promoted too much and the carbide becomes coarse and there is a concern that the bendability is deteriorated. Yes, more preferably 500 ° C. or less.

  The steel sheet for hot pressing of the present invention basically contains the above components, and the balance is iron and inevitable impurities. Furthermore, the steel sheet for hot pressing of the present invention can further selectively contain the following permissible components as long as the effects of the present invention are not impaired.

At least one element selected from the group consisting of Mo: more than 0% to 0.5% or less, Cu: more than 0% to 0.5% or less, and Ni: more than 0% to 0.5% or less. In order to suppress transformation, pearlite transformation, and bainite transformation, it effectively prevents the formation of ferrite, pearlite and bainite during cooling in the mold after heating, and effectively acts to secure martensite contributing to high strength. . These elements may be added alone or in combination of two or more. In order to effectively exhibit such an action, the lower limit of each element is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.05% or more. Considering only the above action, it is better that the content of each element is large. However, since the cost increases, the upper limit of each element is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably. Is 0.3% or less.

At least one of V: more than 0% and 0.1% or less and Nb: more than 0% and 0.1% or less V and Nb have the effect of forming fine carbides and making the structure fine by the pinning effect. In order to effectively exhibit such an effect, the lower limit of any element is preferably 0.001% or more. However, when the content of these elements becomes excessive, coarse carbides are formed, and the ductility is deteriorated by becoming the starting point of destruction. Therefore, the upper limit of any element is preferably 0.1% or less, more preferably 0.08% or less, and still more preferably 0.05% or less.

Ca: more than 0% to 0.005% or less, and REM: more than 0% to 0.005% or less Ca and REM (rare earth elements) both have the effect of refining the form of inclusions in steel. It is an element effective for preventing cracking during hot pressing due to inclusions. In order to effectively exhibit such an effect, the lower limit of any element is preferably 0.0002% or more, more preferably 0.0005% or more. However, even if these elements are contained excessively, the above effect is saturated and only the cost is increased. Therefore, the upper limit of any element is preferably 0.005% or less, more preferably 0.004% or less, and still more preferably 0.003% or less.

  The surface form of the steel sheet for hot pressing according to the present invention is not particularly limited, and a hot-rolled material and a cold-rolled material which are bare materials whose surfaces are not plated; Both plated materials applied are included.

  The manufacturing method of the said steel plate for hot presses is not specifically limited. For example, a steel sheet for hot pressing can be obtained by melting a steel having the above component composition and casting it into a steel slab by a conventional method, hot rolling the steel slab and processing it into a hot rolled material. If necessary, it may be further cold-rolled and processed into a cold-rolled material. The hot-press steel sheet of the present invention can be made into a hot press-formed product by using the hot-rolled sheet and the cold-rolled sheet obtained as described above as in the bare material of Table 1 described later. . Alternatively, the hot-press steel sheet according to the present invention is made of, for example, Zn plating or Al-Si plating on these hot-rolled sheets and cold-rolled sheets, such as Zn plated materials and Al-Si plated materials shown in Table 1 described later. Plating may be performed. However, the type of plating is not limited to this.

  Next, a method for producing a hot press-formed product according to the present invention will be described.

As described above, in the method for producing a hot press-formed product according to the present invention, the steel sheet for hot pressing is heated to a temperature of Ac ₃ or higher, and then press forming of the steel sheet is started by a mold. It is characterized by cooling after the start of molding to the range of Ms point defined by the following formula (2) while securing an average cooling rate of 20 to 300 ° C./s in the mold.
Ac ₃ (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V] − 11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15.2 × [Ni] (3)

  Hereinafter, each step will be described.

<Heating steel sheet for hot pressing to a temperature of Ac ₃ points or higher>
In order to make the structure of the hot press-molded product mainly composed of martensite, the heating temperature is set to Ac ₃ points or more which is an austenite single phase region. Preferably it is Ac ₃ point +15 degreeC or more, More preferably, it is Ac ₃ point +30 degreeC or more. The upper limit is not particularly limited, and even if the heating temperature is high, there is no problem in the characteristics of the obtained steel sheet for hot pressing, but the fuel cost and the like increase and the cost increases. Therefore, the upper limit of the heating temperature is preferably 1000 ° C. or less.

  In the production method of the present invention, it is important to control the heating temperature as described above, and the average heating rate up to the heating temperature is not particularly limited. However, it is desirable that it is not too slow from the viewpoint of productivity and not too fast from the viewpoint of controllability. In consideration of these, the average heating rate up to the heating temperature is, for example, preferably 1 to 200 ° C./s, more preferably 2 to 150 ° C./s.

  Further, the heating and holding time after reaching the heating temperature is not particularly limited, but for example, it is preferable to hold for 1 s or longer. More preferably, it is 3 s or more. The upper limit of the heating and holding time is not particularly limited, and even if the heating and holding time is long, there is no problem with the characteristics of the obtained hot-press steel sheet. However, if the heating and holding time is too long, the fuel cost increases and the cost increases. Therefore, the heating and holding time is preferably 1800 seconds or less. More preferably, it is 1200 s or less.

<Start press molding with a mold, and cool to below the Ms point while ensuring an average cooling rate of 20 to 300 ° C./s in the mold during and after press molding>
In the present invention, press molding is started with a mold. The temperature of the mold before press molding is usually room temperature, which is at most about several tens of degrees Celsius.

  In the present invention, since the austenite formed in the heating step is a martensite-based structure while preventing the formation of ferrite and bainite, the average cooling rate in the range from the start of press molding to the Ms point is 20 ° C. / S or higher, preferably 30 ° C./s or higher, more preferably 40 ° C./s or higher. On the other hand, if the average cooling rate in the above range is too large, sufficient time cannot be secured to promote self-tempering after martensitic transformation, and ductility is lowered. Therefore, in this invention, the upper limit of the said average cooling rate shall be 300 degrees C / s or less, Preferably it is 200 degrees C / s or less, More preferably, you may be 100 degrees C / s or less.

  The method for controlling the cooling rate within the above range is not particularly limited. For example, a method in which the mold is sandwiched by a mold at the time of press molding and is cooled by heat removal by the mold as it is; A method of cooling by blowing a coolant such as water is used.

  As described above, in the present invention, it is important to cool by controlling the average cooling rate in the range from the start of press molding to at least the Ms point within the above range, and the cooling rate after the Ms point is not particularly limited. . Generally, the press molding end temperature is lower than the Ms point. For example, the range from the start of press molding to the press molding end temperature may be cooled at the above cooling rate. Preferably, the range of 300 ° C. from the start of press molding is cooled at the above cooling rate. More preferably, the range of 200 ° C. from the start of press molding is cooled at the above cooling rate.

  After cooling as described above, the range up to room temperature is generally cooled at an average cooling rate of 0.1 to 300 ° C./s. In this way, the hot press-formed product of the present invention is obtained.

  Next, the hot press-formed product of the present invention will be described. As described above, the hot press-formed product of the present invention has the same component composition as that of the above-described hot-press steel plate, martensite: 95% or more in area ratio with respect to the entire structure, and the balance: at least one of ferrite and bainite. And having a structure in which the average particle size of carbides dispersed in the steel is 1 μm or less.

  Among these, since the said component composition was demonstrated in detail in the column of the steel plate for hot press mentioned above, description is abbreviate | omitted. Hereinafter, the structure characterizing the hot press-formed product of the present invention will be described.

<Martensite: 95% or more in area ratio to the entire structure, remaining: at least one of ferrite and bainite>
The hot press-molded product of the present invention has a martensite single-phase structure as much as possible in order to ensure ductility by minimizing the interface between the hard phase and the soft phase that becomes the starting point of fracture during deformation while achieving high strength. It is desirable to do. The area ratio of martensite with respect to the whole structure is 95% or more, preferably 98% or more, and more preferably 100%. The remaining structure other than martensite is at least one of ferrite and bainite.

  Each of the above-mentioned structures is mirror-polished on the surface of the hot press-molded product, then subjected to nital corrosion and observed with an optical microscope (magnification 1000 times) to identify each structure of martensite, ferrite, bainite, and pearlite. What is necessary is just to calculate the area ratio of each structure | tissue.

<Average particle size of carbide is 1 μm or less>
As described above, the carbide formed by self-tempering becomes a starting point of fracture when the hot press-molded product is deformed, and thus needs to be refined. The refined carbide contributes to the improvement of ductility. Therefore, the average particle size of the carbide is 1 μm or less, preferably 0.7 μm or less, more preferably 0.4 μm or less. The lower limit of the average particle size of the carbide is not particularly limited, but is generally 0.01 μm or more.

  The average particle size of the carbide can be measured as follows. First, carbides precipitated by the extraction replica method are extracted, and a 1 μm × 1 μm region is observed and photographed with a transmission electron microscope at a magnification of 150,000 times. Of the observed carbides, those having a diameter equivalent to a circle of 2 nm or more are subjected to image analysis to determine the area of each carbide particle. From the area, the equivalent circle diameter is obtained to calculate an average value, which is defined as the average particle size of the carbide.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples, and can be implemented with modifications within a range that can meet the purpose described above and below. They are all included in the technical scope of the present invention.

A steel material having the composition shown in Table 1 was melted and subjected to hot rolling and cold rolling by a conventional method, and then a cold-rolled steel plate having a thickness of 1.4 mm was produced. In Table 1, “-” means no addition. In this example, the following three types of hot-press steel plate samples having different surface states were used.
-Bare material that is subjected to hot pressing as it is with the cold-rolled steel sheet-Zn plating material that has been hot dip galvanized with a plating simulator-Al-Si plating material that has been provided with Al-Si plating.

  The Zn plating material is heated at 750 ° C. for 60 seconds in an inert atmosphere, cooled to 480 ° C., immersed in a 480 ° C. Zn-0.1% Al plating bath, taken out, and heated to 550 ° C. And kept for 10 seconds, and then cooled to produce.

  The Al—Si plating material was heated at 750 ° C. for 60 seconds in an inert atmosphere, then cooled to 650 ° C., immersed in a Zn-10% Si plating bath at 650 ° C., taken out, and then cooled. Manufactured.

  Subsequently, using each of the hot-press steel plate samples described above, a hot-press molded product sample was obtained under the manufacturing conditions shown in Table 2 assuming hot press-forming. Specifically, each steel sheet for hot pressing is charged into an atmospheric furnace or energized and heated to a predetermined heating temperature at the average heating rate shown in Table 2, and then at the heating temperature. Heated and held for a predetermined time. Next, it is sandwiched between cooling molds and press molding is started. After the press molding is started, the temperature is cooled to the temperature below the Ms point at the average cooling rate shown in Table 2, and then the temperature is allowed to cool to the room temperature. Then, it was cooled at an average cooling rate of about 2 ° C./s to obtain a hot press molded product sample.

  About each hot press-molded product sample thus obtained, the area ratio of each structure and the average particle size of carbides were measured by the method described above. In “Martensite area ratio” in Table 2 described later, only the martensite area ratio is described, but the remaining structure other than martensite is ferrite or bainite.

  Next, in order to evaluate the strength and ductility of each hot press-formed product sample, a tensile test and a bending test were performed as follows.

  The tensile test was performed using a JIS No. 5 test piece at a strain rate of 10 mm / s, and the tensile strength was measured as an evaluation index of strength. In this example, those having a tensile strength of 1180 MPa or more were accepted.

  The bending test was performed under the conditions specified in VDA standard 238-100, and the bending angle was measured as an evaluation index of ductility. Since the bending angle strongly depends on the surface condition of the steel sheet, in this example, the ductility is improved for each of the same steel sheet surface conditions; that is, for each of the bare material, the Zn plating material, and the Al-Si plating material. The effect was relatively evaluated. Specifically, test No. 1 using steel type 1 of Table 1 having a typical composition as a conventional steel sheet for hot pressing. No. 1 bare material as reference 1; Test No. 1 using the above steel type 1 No. 2 Zn plating material 2; test No. 2 using the above steel type 1 3 with the Al-Si plating material of 3 as the standard, and those with the bending angle increased by 10% or more with the respective bending angles as the standard, were regarded as acceptable. In the column of bending angle in Table 2, a column “Comparison with each reference value” is provided, and the rate of increase with respect to each reference value is also shown.

  These measurement results are also shown in Table 2. In Table 2, TS means tensile strength. In Table 2, “-” described in the column of “average particle size of carbide” means that the average particle size of the carbide was not measured because no clear carbide was observed at the time of observation.

  Test No. in Table 2 4-7, 11-13, 15-17, 22-31 all use the steel sheet for hot pressing that satisfies the requirements of the present invention, and the hot press-formed product of the present invention under the production conditions of the present invention. This is a manufactured example. The hot press-formed product of the present invention thus obtained satisfies both acceptance criteria for both tensile strength TS and bending angle. Therefore, according to the present invention, it is excellent in strength and ductility without adding a large amount of expensive elements such as Ni, and without requiring an additional step in cooling after press molding with a mold. It was confirmed that a hot press-formed product could be obtained efficiently.

  On the other hand, the test No. in Table 2 produced without satisfying any of the production conditions defined in the present invention. As for 8-10, 14, 18-21, at least any one among the tensile strength TS and the bending angle does not satisfy the acceptance criteria.

  For details, see Test No. in Table 2. No. 8 is a steel grade No. in Table 1 that satisfies the requirements of the present invention. However, since the heating temperature at the time of hot pressing was low, the area ratio of martensite was reduced and the bendability was lowered.

  Test No. in Table 2 9 is the above-mentioned No. 9. As with No. 8, the steel type No. 1 in Table 1 satisfying the requirements of the present invention. However, since the average cooling rate after heating was slow, the area ratio of martensite was reduced, and both the tensile strength and the bendability were reduced.

  Test No. in Table 2 10 is the above-mentioned No. 10. As with No. 8, the steel type No. 1 in Table 1 satisfying the requirements of the present invention. However, since the average cooling rate after heating was fast, the bendability deteriorated.

  Test No. in Table 2 No. 14 is a steel type No. 1 in Table 1 with a large amount of Si. 6 was used, the area ratio of martensite decreased, and the bendability deteriorated.

  Test No. in Table 2 No. 18 is a steel type No. 1 in Table 1 with a small amount of Cr. Since 10 was used, the average particle diameter of the carbides became coarse and the bendability was lowered.

  Test No. in Table 2 No. 19 has a high Mn content and a low Ms point. 11 was used, the self-tempering was insufficient, the ductility of the base material was deteriorated, and the bendability was lowered.

  Test No. in Table 2 No. 20 is a steel grade No. in Table 1 with a small amount of C and a small Ceq. 12 was used, the martensite area ratio decreased and the tensile strength decreased.

  Test No. in Table 2 No. 21 is a steel grade No. in Table 1 having a small Ceq. Since 13 was used, the area ratio of martensite decreased and the bendability deteriorated.

Claims (6)

Ingredient composition is mass%,
C: 0.10 to 0.4%,
Si: 0% or more and 2.0% or less,
Mn: 0% to 0.5%,
Cr: 0.8 to 2.0%,
P: more than 0% and 0.015% or less,
S: more than 0% and 0.01% or less,
Al: 0.001 to 0.1%,
Ti: 0.01 to 0.1%,
B: 0.0005 to 0.005%,
N: [Ti] / [N] each includes a content of 3.3 or more, and the balance is made of iron and inevitable impurities,
The carbon equivalent Ceq defined by the following formula (1) is 0.43 to 0.7%, and
A steel sheet for hot pressing, wherein the Ms point defined by the following formula (2) is 415 ° C or higher.
Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
Ms (° C.) = 550-361 × [C] −39 × [Mn] −10 × [Cu] −17 × [Ni] −20 × [Cr] −5 × [Mo] + 30 × [Al]. (2)
However, [X] means content of the element X and a unit is the mass%. Ingredient composition is further mass%,
Mo: more than 0% and 0.5% or less,
The steel sheet for hot pressing according to claim 1, comprising at least one selected from the group consisting of Cu: more than 0% and 0.5% or less, and Ni: more than 0% and 0.5% or less. Ingredient composition is further mass%,
The steel sheet for hot pressing according to claim 1 or 2, comprising at least one of V: more than 0% and 0.1% or less and Nb: more than 0% and 0.1% or less. Ingredient composition is further mass%,
The steel sheet for hot press according to any one of claims 1 to 3, which contains at least one of Ca: more than 0% and 0.005% or less, and REM: more than 0% and 0.005% or less. The steel sheet for hot pressing according to any one of claims 1 to 4 is heated to a temperature of Ac ₃ or more defined by the following formula (3), and then press forming of the steel sheet is started by a mold. Then, after the start of press molding, the hot press is cooled to an Ms point range defined by the following formula (2) at an average cooling rate of 20 to 300 ° C./s in the mold. Manufacturing method of molded products.
Ms (° C.) = 550-361 × [C] −39 × [Mn] −10 × [Cu] −17 × [Ni] −20 × [Cr] −5 × [Mo] + 30 × [Al]. (2)
Ac ₃ (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V] − 11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15.2 × [Ni] (3)
However, [X] means content of the element X and a unit is the mass%. The component composition according to any one of claims 1 to 4,
Martensite: 95% or more in area ratio for all tissues,
The balance: at least one of ferrite and bainite,
A hot press-formed product having a structure in which an average particle size of carbides dispersed in steel is 1 µm or less.