JP2008195999A - Steel sheet for hot press, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for a hot press in which the production of scale can be prevented upon a hot press, and whose formability can be improved as well, and to provide a method for producing the same. <P>SOLUTION: Disclosed is a steel sheet for a hot press coated with a plating layer comprising, by mass ≥50% aluminum and 3 to 38% graphite. Its oxidation is prevented by the aluminum plating layer, and further, cracks upon its forming is prevented by the solid lubrication effect of the graphite incorporated into the plating layer. Further, the graphite is incorporated in such a manner that the cleavage face of the graphite is preferentially oriented parallelly to the steel sheet surface in the plating layer, and the solid lubrication effect is more effectively exhibited. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is a hot-press steel plate suitable for a forming method called hot press in which a steel plate heated to about 900 ° C. is formed with a mold at room temperature. In particular, scale formation is prevented during hot pressing and formability is improved. The present invention relates to a hot-press steel plate and a method for producing the same.

  For example, a hot press (also referred to as die quench) proposed in Patent Document 1 is a forming method in which a steel sheet is formed with a metal mold at room temperature in a state where the steel sheet is heated to about 900 ° C. in an austenite region. In this method, since the steel sheet can be rapidly cooled at the same time as forming, a molded product having a strength of 1500 MPa can be produced by forming a hardened phase such as a martensite phase. Since the steel sheet temperature at the time of forming is high, the yield strength of the steel sheet is greatly reduced. Therefore, there are many advantages such as a very small forming load and the difficulty of generating residual stress in the formed product.

  However, since molding is performed in the austenite region, there is a problem that a preferable texture cannot be developed as in the case of molding in the ferrite region, and the formability is decreased because the friction coefficient cannot be reduced by the lubricating oil.

  Furthermore, in the hot press, the steel sheet is heated to about 900 ° C. in the atmosphere, so an iron oxide film called scale is generated on the steel sheet surface, causing the following problems. In other words, when hot-pressing automobile parts, etc., which require peeling or scattering during molding to damage the mold or require complicated cleaning, the generated scale is taken into account when welding and electrodeposition coating during assembly. Is removed by shot blasting or pickling, but a process for that is required, which increases the manufacturing cost. When removing the scale by shot blasting, the molded product may be deformed. Therefore, it is desirable to prevent scale generation as much as possible.

As means for preventing the generation of scale, Patent Document 2 proposes a method of heating using a non-oxidation furnace, and Patent Document 3 proposes a method of using an aluminum-plated steel sheet. However, in the method described in Patent Document 2, since oxidation is performed between the non-oxidizing furnace and the press apparatus, it is difficult to completely prevent scale generation. Further, in the method described in Patent Document 3, scale generation can be prevented, but cracking tends to occur during molding because the friction coefficient cannot be sufficiently reduced.
JP 2002-282951 A JP 2006-110713 A JP 2000-038640 A

  An object of this invention is to provide the steel plate for hot presses which can prevent the production | generation of a scale at the time of hot press, and can improve a moldability, and its manufacturing method.

  This object is achieved by a hot press steel sheet coated with a plating layer containing 50 mass% or more of aluminum and 3 mass% or more and 38 mass% or less of graphite.

  More preferably, the cleaved surface of graphite is preferentially oriented so as to be parallel to the steel plate surface.

  The steel sheet for hot pressing of the present invention can be produced by a method of subjecting a steel sheet to hot-dip plating using a plating bath containing 50 mass% or more of aluminum and 3 mass% or more and 38 mass% or less of graphite.

  According to the present invention, it is possible to produce a hot-press steel plate that can prevent scale formation during hot pressing and can improve formability. The steel sheet for hot pressing according to the present invention can be applied to the Indirect method in which preforming is performed at room temperature, and then heating is performed in an austenite region and then re-striking and rapid cooling are performed using a mold at room temperature. That is, cracks are not generated in the plating layer during the preliminary molding, and scales are not generated by the subsequent heat treatment.

  The steel sheet for hot pressing according to the present invention is a steel sheet coated with an aluminum plating layer containing graphite. By incorporating graphite into the aluminum plating layer effective for preventing oxidation, a solid inherent to graphite can be obtained. The friction coefficient can be reduced due to the lubricating effect, and no cracking occurs during molding.

  Here, in order to reliably prevent oxidation of the plating layer, it is necessary to contain 50 mass% or more of aluminum. In addition, the amount of graphite contained in the plating layer needs to be 3 mass% or more and 38 mass% or less. This is because if it is less than 3 mass%, a sufficient lubricating effect cannot be obtained, and if it exceeds 38 mass%, the appearance of the molded product is impaired and the paintability is also adversely affected.

  In addition, the inclusion of the cleaved surface of graphite in the plating layer so that it is preferentially oriented in parallel with the surface of the steel sheet can express the solid lubrication effect of graphite more effectively, further reducing the friction coefficient. It is more desirable because it can be expected. This is because, in the case of graphite, the solid lubrication effect is highest at the peel interface. Here, the preferential orientation means not non-oriented. As shown in FIG. 1, in order to orient the cleavage plane parallel to the surface of the steel sheet, the graphite in the plating layer has a thin flake shape in the cleavage plane direction (the 001 direction because graphite is hexagonal). It is preferable.

  In order to manufacture the steel plate for hot pressing as described above, graphite may be introduced into an aluminum plating bath in order to contain graphite in the plating layer. In general, in hot dipping, the concentration in the plating bath is the same as the concentration in the plating layer. do it.

  The plating bath preferably contains about 10 mass% Si in order to suppress the formation of dross. In addition, the plating bath contains about 2 mass% Fe by elution from the steel sheet to be immersed, but other than that, Mg, Ti, Sn, Sb, Zn, etc. without departing from the effect of the present invention These elements can also be contained.

  The conditions other than those described above are the same as in the case of the aluminized steel sheet for hot press described in Patent Document 3.

A steel with the composition shown in Table 1 was melted in a laboratory to form a 50 kg steel ingot. This steel ingot was heated to 1200 ° C. and then hot-rolled into a steel plate with a thickness of 4 mm. A steel plate having a thickness of 0.8 mm was produced by hot rolling. Then, using a non-oxidizing furnace-reduction type hot dipping simulator, the steel sheet is minimum length 2 μm or more, maximum length 10 μm or less, thickness 0.3 μm or more as shown in FIG. 2a (plan view) and b (side view). Immerse a flake graphite of 2μm or less in a plating bath consisting of 10mass% Si, 2mass% Fe and the remaining aluminum added with 0, 5, 30, 38 mass% for 1 second, and deposit the plating amount to 120 g / mm ² by gas wiping It was adjusted.

  The steel plate coated with an aluminum plating layer containing graphite thus produced was punched into a 105 mm diameter disc, heated in the atmosphere at 950 ° C. for 5 minutes, and then placed in a 50 mm diameter cup with a room temperature mold. Drawing was performed, and the maximum cup height that could be molded, that is, the limit molding height was measured.

  The results are shown in Table 2. Any of the examples of the present invention in which the grphite content in the plating layer is 5 mass% or more is squeezed out, and no cracks are generated.

  FIG. 3 shows the result of X-ray diffraction (Cu source) of the steel sheet of the present invention. As the diffraction peak (G) of graphite, only the {001} plane, which is a cleavage plane, is detected, and cleavage is performed. It can be seen that the plane is oriented parallel to the steel plate surface.

It is a figure which shows typically the presence state of the graphite in the plating layer of the steel plate for hot presses which is this invention. It is a figure which shows typically the shape of the graphite used for the present Example. It is a figure which shows the X-ray-diffraction result of the steel plate for hot presses which is this invention.

Claims (3)

  A steel sheet for hot pressing coated with a plating layer containing aluminum of 50 mass% or more and graphite of 3 mass% or more and 38 mass% or less.   2. The hot-press steel sheet according to claim 1, wherein the cleavage plane of graphite is preferentially oriented parallel to the steel sheet surface.   A method for producing a steel sheet for hot pressing, wherein a steel sheet is hot-plated using a plating bath containing 50 mass% or more of aluminum and 3 mass% or more and 38 mass% or less of graphite.

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