JP2010121181A - Steel plate for hot press working - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel plate for hot press working having excellent productivity. <P>SOLUTION: A Zn-Ni plating is, for example, executed and the emissivity of a surface of a steel plate is changed to be ≥0.3. By increasing the emissivity of the surface of the steel plate, the temperature-rising rate of the steel plate is increased, and the in-furnace time is considerably reduced. By reducing the in-furnace time in which the steel plate is inserted in a heating furnace, the productivity is improved. The favorable range of the emissivity of the surface of the steel plate is ≥0.5. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel sheet for hot pressing that is inserted into a facility that raises the temperature of a steel sheet using radiant heat, and in particular, heat having excellent productivity by dramatically increasing the heating rate. The present invention relates to a steel plate for hot pressing.

In recent years, steel sheets for automobiles have been increased in strength from the viewpoint of reducing CO ₂ emissions and ensuring safety in automobiles. However, it is known that when the strength of the steel sheet is increased, workability represented by elongation and r value and press formability are generally lowered. Further, at the time of press working, the contact surface pressure with the mold increases, so that the mold is easily damaged, so-called mold galling, and the productivity is hindered. Thus, there are many technical problems in pressing high-strength steel sheets.
In recent years, as a method of obtaining parts using such a high-strength steel sheet, in recent years, the steel sheet is heated at the time of pressing and then subjected to press working, and then rapidly cooled by a press stand. A technology for forming the above parts, so-called hot pressing technology is promising (for example, Patent Document 1).
JP 2006-289425 A

  However, in such a hot press working technique, when the steel plate is heated to a high temperature of 800 ° C. or higher, the time for which the steel plate is left in the furnace for heating (hereinafter referred to as in-furnace time) becomes long. For this reason, the number of pressable sheets per unit time is small, which is a cause of hindering productivity.

  In view of such circumstances, an object of the present invention is to provide a hot-press steel sheet having excellent productivity.

  In order to solve the above-mentioned problems, the present inventors have intensively studied and studied means that can improve productivity by reducing the in-furnace time. As a result, it has been found that by increasing the emissivity of the steel sheet, the heating rate of the steel sheet increases and the in-furnace time is significantly reduced.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] A steel sheet for hot pressing, wherein the emissivity of the steel sheet surface is 0.3 or more.
[2] A steel sheet for hot press working according to [1], wherein the emissivity is 0.5 or more.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate for hot press work excellent in productivity is obtained.

There are various types of heating furnaces for heating a steel sheet during hot pressing, and among them, a heating furnace using radiant heat is common.
The heating rate of the steel sheet by radiant heat largely depends on the state of the steel sheet surface and varies greatly depending on the surface condition. Here, there is an emissivity as one index for evaluating the temperature rise degree of the steel sheet absorbed by the radiant heat. The emissivity is defined as 1 when the heat is completely absorbed, that is, a state generally referred to as a black body, while 0 is completely reflected.
In the case of a steel plate or metal manufactured by a general method and used for press working or the like, the emissivity is about 0.1 and heat cannot be absorbed efficiently. Therefore, the in-furnace time until it reaches the temperature required for press working becomes long, and as a result, the time required for press working per sheet becomes long. This is a cause of inhibiting productivity. Further, the cost is increased by maintaining the temperature in the furnace for a long time.

In view of the above, it is considered effective to increase the emissivity of the steel sheet surface in order to efficiently utilize the radiant heat from such a furnace body to increase the temperature of the steel sheet. As a result of the examination, it was found that when the emissivity of the steel sheet surface is 0.3 or more, the temperature rise rate of the steel sheet is increased and the in-furnace time can be reduced. More preferably, when the emissivity is 0.5 or more, a remarkable reduction effect of the in-furnace time is recognized. The effect of reducing the in-furnace time is preferably as the emissivity is higher, and closer to 1 is better. Therefore, there is no particular upper limit for emissivity.
As described above, in the present invention, the emissivity of the steel sheet surface is set to 0.3 or more. In addition, as a method of making the emissivity of the steel plate surface 0.3 or more, a method of giving a thin oxide to the steel plate surface layer, a plating treatment, a vapor deposition treatment, a coating treatment, and the like can be mentioned. Moreover, in order to improve the emissivity, it is effective to give a substance having a high emissivity on the steel sheet, and the substance for improving the emissivity is not particularly limited, but metal oxides, sulfides, ceramics Are preferably used.

It does not specifically limit about the steel plate used as the base material of the steel plate for hot press processing of this invention. The present invention is intended to increase the rate of temperature rise by improving the state of the steel sheet surface, that is, the emissivity, and therefore there is no particular limitation on the components of the steel sheet.
As a preferred embodiment of the hot-press steel sheet of the present invention, for example, when applying the hot-press steel sheet of the present invention as an automobile part, for the purpose of improving the corrosion resistance in actual use, It is preferable that the galvanized layer is provided on the steel sheet surface. Although the galvanized layer partially evaporates in a high temperature environment, the corrosion resistance can be maintained by remaining on the surface of the steel sheet as alloyed zinc or oxide.

  Next, the present invention will be described in more detail with reference to examples.

Tensile strength (TS) with a thickness of 0.8 mm: Various materials shown in Table 1 were applied to the surface of each cold-rolled steel sheet and hot-dip galvanized steel sheet having a strength of 440 MPa. In addition, the method of the surface treatment which provides various substances is as follows.
<Zn-Ni plating>
In a bath to which 350 g / L of zinc sulfate and 150 g / L of nickel sulfate were added, the electrolysis time was adjusted so as to form the thickness shown in Table 1 at a current density of 100 A / dm ² using a steel plate as a cathode. Zn-Ni plating was provided by obtaining a film in which the Ni content in the plating film was 12% by mass.
<Black nickel plating>
In a bath containing 100 g / L of nickel nitrate, the steel plate was used as a cathode, the electrolysis time was adjusted to form the thickness shown in Table 1 at a current density of 30 A / dm ² , and black nickel plating was applied.
<Oxidation treatment>
3N NaOH aqueous solution is heated to 120 ° C., and then steel plate is immersed in this heated NaOH aqueous solution, and an oxidation treatment is performed to adjust the immersion time so as to form the thickness shown in Table 1, thereby providing an iron-based oxide. did.
<Ti-O>
The deposition time was adjusted to form the thickness shown in Table 1 by using a deposition process, so-called PVD (Physical Vapor Deposition), and Ti—O (Ti oxide) was applied.
Next, the steel plate provided with various substances on the surface obtained above was cut into 30 mm × 70 mm, and a thermocouple was attached to the central part of the cut steel plate in order to investigate the temperature change after being held in the furnace. Using an infrared image furnace, the dew point in the furnace was set to −30 ° C., and heating was performed from room temperature (25 ° C.) to 900 ° C. All the steel plates had the same output, the temperature change after being held in the furnace was investigated, the emissivity was measured by the following method, and the temperature rising rate was evaluated.
(Measurement of emissivity)
The emissivity was measured using a D and S AERD emissometer manufactured by Kyoto Electronics Industry under the conditions of a measurement wavelength of 3 to 30 μm and a measurement temperature of 23 ° C.
(Temperature increase rate evaluation)
The arrival time from room temperature (25 ° C) to 900 ° C of various steel plates is measured, compared with the processing time of untreated steel plates, calculated as the in-furnace time ratio by the following formula, The rate of temperature increase was evaluated as follows. That is, the smaller the in-furnace time ratio is, the higher the temperature can be raised in a short time, and the higher the rate of temperature increase.
In-furnace time ratio = (900 ° C arrival time of various steel plates) / (900 ° C arrival time of untreated steel plates)
(Evaluation)
○: When the rate of temperature increase is 2/3 or less Δ: When the rate of temperature increase is greater than 2/3 and equal to or less than 0.9 x: Test when the rate of increase of temperature increase is greater than 0.9 The results are shown in Table 1 together with the conditions.

From Table 1, the following matters became clear.
(1) No. 1 and No. 1 No. 2 is a comparative example in which a normal cold-rolled steel sheet and a hot-dip galvanized steel sheet are applied without performing a surface treatment for changing the emissivity of the steel sheet surface. Therefore, it can be seen that the time to reach 900 ° C. is long.
(2) No. 3 and 4 are examples of the present invention in which the emissivity of the steel sheet surface is changed by changing the electroplating conditions. In either case, the time to reach 900 ° C. is shortened, and it can be seen that the rate of temperature increase is excellent.
(3) No. 5 is an example of the present invention in which the emissivity of the steel sheet surface is increased by applying an iron oxide to the cold-rolled steel sheet surface. It can be seen that the time to reach 900 ° C. is shortened and the rate of temperature increase rate is excellent.
(4) No. 6 is an example of the present invention in which the emissivity of the steel sheet surface is increased by applying Ti oxide to the steel sheet surface. It can be seen that the time to reach 900 ° C. is shortened and the rate of temperature increase rate is excellent.
(6) No. 7 is an example of the present invention in which the emissivity of the surface of the steel sheet is increased by applying Zn-Ni plating on the hot dip galvanized steel sheet. It can be seen that the time to reach 900 ° C. is shortened and the rate of temperature increase rate is excellent.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate for hot press processing which can improve productivity significantly can be provided, and it can apply in a wide field | area centering on a vehicle body use.

Claims (2)

  A steel sheet for hot pressing, wherein the steel sheet has an emissivity of 0.3 or more.   2. The hot-press steel sheet according to claim 1, wherein the emissivity is 0.5 or more.