JP2011031254A - Method of hot-press-forming steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain the improvement of press formability without such an additional process, although it is necessary that forced cooling process with another device is added after pressing or the forced cooling is added before the pressing when attempting to improve the press formability by a hot pressing in the background technology. <P>SOLUTION: In the method of hot-press-forming a steel sheet, by which the steel sheet 10 is heated to ≥850°C, the hot-press-forming is carried out from the forming starting temperature of >650°C by using dies 1, 2 and the dies are held in the position of the bottom dead center after finishing the forming, the average forming speed is defined as ≥100 mm/s. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a hot press forming method for a steel sheet. The product molded by this method can be preferably used for automobile frame parts and the like.

As background art of the present invention, Patent Documents 1 to 3 are listed.
In Patent Document 1, as a means for ensuring high productivity comparable to that of a cold press even with a hot press, a thin steel plate containing C: 0.2% by mass or more is heated to 850 ° C. or more and then press-formed. In this case, the hot pressing method in which the molded product is taken out from the pressed die within 5 seconds after reaching the bottom dead center and cooled at a cooling rate of 30 ° C./s or more, and the hardness Hv of the molded product is set to 400 or more. Is described.

  In Patent Document 2, as a hot press method in which the height is low and the compact design is excellent in high-speed operation and the stationary holding at the press bottom dead center is possible, a hot steel plate is placed between a pair of upper and lower molds. When pressing into a predetermined shape, the upper die driven by a high-speed lifting device (drive source: servomotor) is lowered to the bottom dead center of the press, and the steel plate is pressed into a predetermined shape between a pair of upper and lower molds, and then the low speed A method is described in which a lower mold driven by a high load lifting device (drive source: hydraulic cylinder) is lifted and held stationary and quenched.

  In Patent Document 3, as a technology for hot pressing a steel plate, which can be applied to drawing deformation and overhang deformation that require blank pressing, C: a steel plate containing 0.15% by mass or more is heated to 850 ° C. or more, While cooling at a cooling rate of 10 ° C./s or more, press forming is performed by setting the press forming start temperature Tp [° C.] to 600 ° C. or less and the press forming end temperature to be the martensite transformation start temperature Ms [° C.] or more of the steel sheet, Thereafter, a hot pressing method is described in which cooling is performed at a cooling rate of 10 ° C./s or higher to 300 ° C. or lower.

JP 2005-288528 A JP 2006-026637 A JP 2007-275937 A

  In the technique of Patent Document 1, a cooling device is required separately from the press device, and the press device needs to be shielded from the refrigerant of the cooling device. The technique of Patent Document 2 uses only a press device, but does not mention improvement of press formability. In the technique of Patent Document 3, it is necessary to add a step of cooling the steel sheet at a cooling rate of 10 ° C./s or higher from a heating temperature of 850 ° C. or higher to a press start temperature of 650 ° C. or lower, or the tact time is increased. There is a problem that productivity decreases.

  In other words, in the background art, in order to improve the press formability in hot pressing, it is necessary to reduce productivity, add a forced cooling process by a separate device after pressing, or add a forced cooling process before pressing. However, without such additional steps, improvement in press formability cannot be expected, which is a problem.

The present invention solves the above-mentioned problem, and in the hot press forming process, without special forced cooling before pressing, and after the pressing, heat removal from the mold (maintaining contact with the mold to the mold) The present invention provides means capable of achieving improvement in press formability without any special forced cooling other than (cooling by promoting heat conduction), and the gist configuration is as follows.
(1) Heat the steel plate to 850 ° C. or higher, perform hot press forming from a forming start temperature exceeding 650 ° C. using a mold, and hold the mold at the bottom dead center position after forming. In the press forming method, a hot press forming method of a steel sheet, wherein an average forming speed is 100 mm / s or more.
(2) The hot press forming method for a steel sheet according to (1) above, wherein a servo press is used as a press for driving the mold.

  According to the present invention, in the hot press forming process, without special forced cooling before pressing, and after the pressing, heat removal from the mold (maintaining contact with the mold to promote heat conduction to the mold) Improvement of press formability, particularly stretch formability, can be achieved without any special forced cooling other than cooling.

Sectional view showing an example of a press The figure which shows one example of the average shaping speed dependence of shaping limit height (LDH)

  The steel plate as the raw material is preferably a steel plate having a composition containing C: 0.15% or more by mass%. If C is less than 0.15%, quenching by heat removal from the mold may be insufficient. Other elements are not particularly limited, and basic elements such as Si, Mn, P, and S may be included as appropriate, and Cr, Mo, Ti, B, and the like may be added to improve hardenability. Good.

The heating temperature is 850 ° C. or higher in order to sufficiently austenite the material. Note that the heating temperature is preferably 1050 ° C. or less from the viewpoint of rigidity during conveyance.
The material plate thickness is preferably 0.8 to 3.0 mm. If it is less than 0.8 mm, it is difficult to ensure the rigidity of the molded product. In the case of more than 3.0 mm, it is difficult to quench and harden to the center of the plate thickness.

The molding start temperature is over 650 ° C. When molding is started at 650 ° C. or lower, cooling from the completion of heating to the start of molding by air cooling (natural cooling) may result in ferrite forming in the material and leading to insufficient strength, and productivity is reduced. It is necessary to add a forced cooling (cooling at a cooling rate faster than air cooling) process before pressing, and the problem cannot be solved.
Average forming speed (from the start of steel plate forming (when either a fixed or movable die first contacts the forming target part of the steel plate) to the end of forming (when the movable die reaches bottom dead center) The amount obtained by dividing the moving distance of the movable mold by the time) is 100 mm / s or more. The inventors conducted a ball head overhang forming experiment, which is the main forming mode of hot forming, and investigated the relationship between the forming limit height (Limit Dome Hight; LDH) and the average forming speed. As a result, LDH increased as the average molding speed increased. Looking at the upward trend, the increase in LDH was small when the average molding speed was less than 100 mm / s, but the average molding speed was When it becomes 100 mm / s or more, the upward trend of LDH increases rapidly. Based on this knowledge, in the present invention, the average molding speed is limited to 100 mm / s or more.

  In the experiment, the distribution curve of the plate thickness reduction rate measured from the top of the punch molded from LDH to the die shoulder (relative distance from the origin is 1) is relative to the origin. It has a steep peak at a position where the distance is about 2/3, indicating that this position is a crack-prone area. And this distribution curve shows that when the average forming speed is 100 mm / s or more, the base near the origin increases and the strain distribution tends to become uniform. On the other hand, according to the cooling curve from the start of molding, the top of the punch of the material cools near 100 ° C. in a few seconds. Further, according to the high-temperature deformation strength data, the difference in deformation strength when there is a temperature difference of 100 ° C. is very large. From these facts, the reason why the LDH greatly increases by setting the average forming speed to 100 mm / s or more is that the top of the punch of the material is not cooled very much, that is, the temperature of the top of the punch that is the lowest temperature of the material and other parts. It is considered that the molding proceeds while the temperature difference with respect to the difference, that is, the deformation strength difference is small, and the local concentration of strain to the crack-prone area is alleviated.

  By the way, there are three types of press machines for driving a mold (movable mold among them): a crank press machine, a hydraulic press machine, and a servo press machine. The crank press has the fastest molding speed, but cannot be used in the present invention because it cannot hold the mold at the bottom dead center. Although the hydraulic press machine can easily hold the mold at the bottom dead center, the molding speed is limited as compared with the crank press machine, and it is not completely unusable for the present invention, but it is not preferable. On the other hand, the servo press machine can easily hold the mold at the bottom dead center, can achieve an average molding speed of 100 mm / s or more with a large margin, and has excellent control of the molding speed. It is preferable to use a servo press for carrying out the present invention.

  A hot press molding experiment was conducted using a press machine whose cross-sectional view is shown in FIG. In this press machine, the material 10 is placed on the cushion 3, the die (movable mold) 2 is lowered by the servo motor drive 9, and the edge of the material 10 is pushed down while being clamped between the die 2 and the cushion 3. This is a servo press machine of a type in which a molding target part is pressed against a spherical head punch (fixed mold) 1 for molding. The main molding mode is stretch molding. A steel plate having a thickness of 1.6 mm having the composition shown in Table 1 was used as the material. The material is heated to 900 ° C., air-cooled until the start of molding, molding starts when it reaches 750 ° C., the average molding speed is variously changed within the range of 30 to 120 mm / s, and the die is lowered after the molding is completed. The mold was removed and cooled (quenched) while being held at the dead center position. The lubrication conditions were no lubrication and the cushion load was 50 tons. For each condition where the average molding speed was changed, the molding height was changed at a pitch of 0.5 mm, and the molding limit height (LDH) was determined. Note that LDH is a value obtained by subtracting one pitch (0.5 mm) from the molding height value when cracking occurs. The results are shown in Table 2 and FIG. From FIG. 2, it is clear that the increase in LDH due to the increase in average molding speed is markedly greater in the inventive example than in the comparative example, and the press formability is remarkably improved.

1 Ball head punch (fixed mold)
2 Die (movable mold)
3 Cushion 9 Servo motor drive 10 Material (steel plate)

Claims (2)

  A method for hot press forming a steel plate, in which a steel plate is heated to 850 ° C. or higher, hot press forming is performed using a mold from a forming start temperature exceeding 650 ° C., and the mold is held at a bottom dead center position after the forming is completed. The method of hot press forming a steel sheet, wherein the average forming speed is 100 mm / s or more.   The hot press forming method for a steel sheet according to claim 1, wherein a servo press is used as a press for driving the mold.

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