Steel Plate for Hot Stamping

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

In recent years, there has been a demand for improvement in collision safety of motor vehicles, and in association with this, there has been a demand for a further increase in strength of steel plates for hot stamping used in parts required to exhibit rigidity of motor vehicles. However, when the strength of steel plate is improved, the low temperature toughness deteriorates and the balance between strength and toughness is thus lost. In order to cope with this problem, Non Patent Literature 1 proposes that the balance between strength and toughness of a steel plate is improved by refining the former austenite grains after hot stamping.

In hot stamping, the cooling velocity inside the steel plate may decrease by an increase in the die temperature and the clearance between the die and the steel plate. When the cooling velocity of the steel plate is equal to or lower than the critical cooling velocity, soft phases such as ferrite and bainite precipitate and the hardness of the steel plate thus decreases. In particular, as the cooling velocity at a temperature equal to or lower than the Ms point decreases, auto tempering is promoted and this causes a decrease in hardness of the steel plate.

In Non Patent Literature 2, a change in cooling velocity when changing the clearance between the die and the steel plate is examined and it has been indicated that the cooling velocity decreases to about 15° C./s when this clearance is 0.4 mm.

As described in Non Patent Literature 1, there is a method in which the crystal grains of steel are refined as a general structure design technology of steel plates for hot stamping and this method makes it possible to obtain a steel plate having an excellent balance between strength and toughness. As a method for refining the crystal grains, there is a method in which elements such as Nb, Ni, and Ti are added, but the economical efficiency of the steel plate becomes poor in this case. A steel plate having refined crystal grains exhibits poor hardenability and thus lacks hardness stability.

In order to solve this problem, it is also considered to improve process problems that cause a decrease in hardness, such as an increase in die temperature and clearance between the die and the steel plate. However, in that case, it is required to repeatedly modify the die and prepare a special die, and this requires a great deal of labor and cost. Hence, in the conventional steel plates for hot stamping, there is 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.

Non Patent Literature 1: Kazuo Hikida et al., “Development of TS 1800 MPa Grade Hot Stamping Steel Sheet” Materia Vol. 52, No. 2, 2013, pp. 68-70

Non Patent Literature 2: Katsuji Nakashima, “Hardening Technology of Steel by Die Quenching and Application to Body Parts” CAMP-ISIJ Vol. 17 2004, pp. 980-983

An object of the present invention is to provide a steel plate for hot stamping which can provide a molded product which exhibits excellent hardness stability in addition to the balance between strength and toughness while suppressing increases in labor and cost in the hot stamping process.

A steel plate for hot stamping according to an aspect of the present invention contains,

According to the present invention, it is possible to provide a steel plate for hot stamping which can provide a molded product which exhibits excellent hardness stability in addition to the balance between strength and toughness while suppressing increases in labor and cost in the hot stamping process.

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

The steel plate for hot stamping according to the present embodiment contains,

This steel plate for hot stamping exhibits excellent hardness stability in addition to a balance between strength and toughness as a following relational expression (1) is satisfied, where [C] denotes a C content, [Si] denotes a Si content, [Mn] denotes a Mn content, and [Cr] denotes a Cr content.

In order to obtain a steel plate for hot stamping which is excellent in both the balance between strength and toughness and the hardness stability, the present inventors have conducted extensive studies on the component composition of steel plate. From the description in Non Patent Literature 2, it has been expected that the cooling velocity of a normal member fluctuates in a range of 30° C./s to 10° C./s in the hot stamping process due to the clearance between the die and the steel plate and an increase in the die temperature. For this reason, the present inventors have focused on suppression of the variation in hardness even when the cooling velocity fluctuates in addition to the balance between strength and toughness, and conducted detailed investigations on the component system of steel plate for achieving this. As a result, the present inventors have newly found out that the balance between strength and toughness and hardness stability can be both achieved by adjusting the balance among the contents of C, Si, Mn, and Cr so that the relational expression (1) is satisfied as well as each component composition in a steel plate satisfies the above range, and thus conceived the present invention.

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

The C content determines the strength of the steel plate after die cooling. In order to obtain sufficient strength of the steel plate, the C content is 0.25% by mass or more, preferably 0.255% by mass or more, more preferably 0.260% by mass or more.

However, when the C content is excessive, the strength of the steel plate after hot rolling may increase and this may lead to cracking during cold rolling and deterioration in weldability. Hence, the C content is 0.4% by mass or less, preferably 0.38% by mass or less, more preferably 0.36% by mass or less.

Si contributes to the hardness stability of the steel plate by increasing the temper softening resistance. Si also has an effect of preventing scale peeling off after die cooling when the surface of the steel plate 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 generation of retained austenite (Y) and promotes a decrease in yield strength (YS) and segregation of Mn. Hence, the Si content is 1.4% by mass or less, preferably 1.35% by mass or less.

Mn is one of the important elements contained in the steel plate for hot stamping according to the present embodiment and contributes to an increase in the strength of the steel plate after die cooling by enhancing the hardenability of the steel plate. 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, still more preferably 0.8% by mass or more.

On the other hand, in the investigations to achieve both the strength and toughness of the steel plate after die cooling, it has been confirmed that when Mn is excessive, coarse carbides precipitate during die cooling and brittle fracture is caused when shocking stress is applied to the steel plate in a low temperature environment. Hence, the Mn content is 1.4% by mass or less, preferably 1.35% by mass or less, more preferably 1.30% by mass or less.

Mn is an element that is inevitably mixed into the steel plate and it is thus difficult to set the Mn content to 0% by mass.

Cr is one of the important elements in the steel plate for hot stamping according to the present embodiment. In the investigations to achieve both the strength and toughness of the steel plate after die cooling, it has been confirmed that Cr contributes to securing of the hardness at a low cooling velocity (for example, 10° C./s) as well as suppression of coarse carbide precipitation during die cooling and thus suppresses brittle fracture when shocking stress is applied to the steel plate in a low temperature environment. In order to exert these effects, the Cr content is 0.6% by mass or more, preferably 0.8% by mass or more, more preferably 1.05% by mass or more.

On the other hand, when Cris excessively contained in the steel plate, the strength of the steel plate after hot rolling increases and this leads to cracking of the steel plate during cold rolling and deterioration in pickling property after hot rolling. Hence, the Cr content is 3.0% by mass or less, preferably 2.5% by mass or less.

From the viewpoint of weldability of the member, toughness, and prevention of surface flaws, it is required to regulate the upper limit of P content. Hence, the P content is 0.03% by mass or less, preferably 0.025% by mass or less, more preferably 0.02% by mass or less.

P is an element that is inevitably mixed into the steel plate and it is thus difficult to set the P content to 0% by mass.

S forms MnS to decrease the uniformity of the Mn concentration distribution and also deteriorate the weldability of the steel plate. Hence, the S content is 0.02% by mass or less, preferably 0.018% by mass or less, more preferably 0.015% by mass or less.

S is an element that is inevitably mixed into the steel plate as P and it is thus difficult to set the S content to 0% by mass.

Al is an element that acts as a deoxidizer. In order to exert this effect, the Al content is 0.01% by mass or more, preferably 0.015% by mass or more.

However, when Al is excessively contained in the steel plate, the hardness after die cooling decreases and excessive generation of AlOdeteriorates the low temperature toughness. Hence, the Al content is 1% by mass or less, preferably 0.8% by mass or less, more preferably 0.1% by mass or less. The Al content here means the content of Al (sol. Al) in a solid solution state.

N is an element that is inevitably mixed into the steel plate. When Nis excessively contained in the steel plate, the amount of solid solution B in the steel plate decreases as N forms a boride and this leads to deterioration in hardenability. Hence, the N content is 0.01% by mass or less, preferably 0.008% by mass or less, more preferably 0.005% by mass or less.

B is an important element for improving the hardenability of the steel plate. By adding an appropriate amount of B to the steel plate, the hardenability is enhanced and this makes it possible to stably increase the strength of the steel plate after die cooling. In order to exert this effect, the B content is 0.0005% by mass or more, preferably 0.0010% by mass or more, more preferably 0.0015% by mass or more.

On the other hand, when B is excessively contained in the steel plate, a coarse iron-boron compound precipitates and this leads to deterioration in toughness. Hence, the B content is 0.0050% by mass or less, preferably 0.0045% by mass or less, more preferably 0.0030% by mass or less.

Ti decreases the amount of BN generated in the steel plate by forming TIN. This increases the amount of solid solution B in the steel plate and makes it possible to enhance the hardenability improving effect by B. In order to exert this effect, the Ti content is 0.0050% by mass or more, preferably 0.010% by mass or more, more preferably 0.015% by mass or more.

On the other hand, when Ti is excessively contained in the steel plate, a carbide precipitates at the crystal grain boundaries and the hardenability of the steel plate deteriorates. Hence, the Ti content is 0.1% by mass or less, preferably 0.08% by mass or less, more preferably 0.06% by mass or less.

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

Mo is an element that contributes to the improvement in hardenability of the steel plate. In order to exert this effect, the Mo content is preferably 0.01% by mass or more. However, when Mo is excessively contained in the steel plate, the strength of the steel plate before hot molding is increased. In order to prevent this, the Mo content is preferably 1.0% by mass or less.

Nb and V form fine carbides and have the effect of refining the structure of steel by the pinning effect. V also has a secondary hardening action by being precipitated during tempering. In order to exert these effects, the Nb and V contents are both preferably 0.0008% by mass or more.

However, when Nb and V are excessively contained in the steel plate, coarse carbides are formed and this becomes the starting point of fracture to lead to deterioration in toughness. Hence, the Nb and V contents are both preferably 0.1% by mass or less, more preferably 0.08% by mass or less, still more preferably 0.07% by mass or less.

Cu and Ni are preferably added when it is required to improve the delayed fracture properties of the member. However, when Cu and Ni are excessively contained in the steel plate, flaws may be generated on the surface of the steel plate and finally on the surface of the member. Hence, it is preferable that the Cu and Ni contents are each 0.5% by mass or less and it is more preferable that the sum of the Cu and Ni contents is 0.5% by mass or less.

The steel plate for hot stamping according to the present embodiment exhibits excellent hardness stability in addition to the balance between strength and toughness as the following relational expression (1) is satisfied by adjustment of the balance among the contents of C, Si, Mn, and Cr. In this relational expression (1), [C] denotes the C content (% by mass) in the steel plate for hot stamping. [Si] denotes the Si content (% by mass) in the steel plate for hot stamping. [Mn] denotes the Mn content (% by mass) in the steel plate for hot stamping. [Cr] denotes the Cr content (% by mass) in the steel plate for hot stamping.

As the relational expression (1) is satisfied as well as the respective component compositions satisfy the component ranges in the claims, the steel plate for hot stamping according to the present embodiment exhibits excellent hardness stability as well as is a steel plate having an excellent balance between the strength after hardening by die cooling and the low temperature toughness. Specifically, the following relational expressions (2), (3), and (4) are all satisfied where, A (J/cm) denotes the absorbed energy in a Charpy impact test at −40° C. when a flat plate is hardened using a die, B(Hv) denotes the hardness when the steel plate for hot stamping is heated to the austenite region, then cooled to room temperature at a cooling velocity of 10° C./s, and hardened, and C(Hv) denotes the hardness when the steel plate for hot stamping is heated to the austenite range, then cooled to room temperature at a cooling velocity of 30° C./s, and hardened.

The relational expression (2) is an index of the balance between the strength and toughness of the steel plate newly devised by the present inventors and is an important concept when considering the balance between the strength and toughness of the steel plate for hot stamping. In the course of investigations on the balance between strength and toughness, the present inventors have focused on the hardness when the cooling velocity is 10° C./s and the toughness after die cooling of a flat plate. In the die cooling of a flat plate, ideal cooling conditions in which a clearance is not generated between the die and the steel plate in the hot stamping process are taken into consideration. By using the relational expression (2), it is possible to more faithfully evaluate the balance between strength and toughness when the steel plate for hot stamping is processed into a member (molded product).

The graph ofillustrates the relation between the absorbed energy A (horizontal axis) in a Charpy impact test at −40° C. when a flat plate is hardened using a die and the hardness B (vertical axis) of the steel plate when being hardened at a cooling velocity of 10° C./s. The straight line (1) in this graph corresponds to the relational expression (2). The straight line (2) in this graph corresponds to an equation of B=516.

The horizontal axis (A) of the graph ofassumes the toughness at the most brittle portion of the member after die cooling. In other words, when a flat plate is subjected to die cooling, the die and the steel plate are in contact with each other in an ideal state and the cooling velocity is thus high. For this reason, the strength after cooling is high but, on the other hand, the flat plate is extremely brittle. In other words, this horizontal axis has a meaning as toughness at the most brittle portion when the steel plate for hot stamping is molded into a member (molded product).