JP5866871B2 - Hot press forming method for hot rolled steel sheet - Google Patents

Description

  The present invention relates to a hot press forming method in which a hot-rolled steel sheet is heated to an austenite region temperature and then press-formed in an austenite region to obtain a predetermined shape and the press-formed product is quenched.

  Conventionally, structural members used in automobiles and the like have been manufactured by press-forming steel sheets having a predetermined strength. In recent years, the application of high-strength steel sheets has been increasing as structural members for vehicle bodies in order to achieve both weight reduction and collision safety performance of automobiles. On the other hand, when the strength of the steel plate is increased, the workability deteriorates and it becomes difficult to process into a predetermined part shape. In particular, when a high-strength steel sheet is cold-formed, when the product is removed from the press mold, a springback that is elastically deformed and loses its shape tends to occur, and it is difficult to improve the dimensional accuracy of the product.

  Therefore, a hot press method (press quench method) in which a heated steel plate is press-formed has been proposed as a means of simultaneously satisfying both the high strength, workability, and product accuracy of the steel plate. For example, Patent Document 1 is known as a conventional technique of a hot press method (hot press forming method). In this hot press method, a steel sheet is heated to an austenite region and then press-formed, and at the same time, quenching is performed by cooling after forming, thereby obtaining a high-strength material. In the hot press method, since the steel sheet is soft and highly ductile at high temperatures, workability such as cracking at the time of forming is improved, and a product having good product accuracy can be manufactured.

Japanese Patent Laid-Open No. 10-96031

  Generally, a hot-rolled steel sheet is heated at 1100 to 1300 ° C. in a heating furnace, hot-rolled at a temperature in the austenite region, cooled on a hot run table, and wound at 500 to 700 ° C. Manufactured. In order to obtain predetermined mechanical characteristics, the slab heating temperature, rolling finishing temperature, and winding temperature are strictly controlled.

  In the conventional hot rolling process of hot-rolled steel sheet, temperature control such as slab heating temperature, rolling finishing temperature, coiling temperature is necessary from the viewpoint of material control of hot-rolled steel sheet. This hindered productivity, resulting in decreased productivity and increased costs.

  The present invention has been made in view of the circumstances as described above, and when performing hot press forming of a hot-rolled steel sheet, it does not require control such as temperature management when manufacturing a hot-rolled steel sheet to be used. It is an object of the present invention to provide a hot press forming method for hot rolled steel sheets, which can be easily scheduled in the hot rolling process and can improve the productivity and cost reduction of hot rolling.

  As a result of diligent research to solve the above problems, the present inventor has performed hot pressing using a hot-rolled steel sheet manufactured by performing hot rolling while changing temperature conditions within the range of normal hot rolling conditions. With respect to the press-formed product that has been molded, the inventors have found that the same material can be obtained under any conditions, and completed the present invention.

  That is, the present invention has the following features.

  [1] A hot rolling process in which a hot rolled steel sheet is manufactured by hot rolling without controlling properties other than the shape, and a heating process in which the hot rolled steel sheet manufactured in the hot rolling process is heated to an austenite region; The press forming step of obtaining a press-formed product by press-forming the hot-rolled steel sheet heated in the heating step in the austenite region, and the quenching step of quenching the press-formed product obtained in the press forming step, in that order. A hot press forming method for hot-rolled steel sheets, characterized by comprising:

  [2] The hot press forming method of a hot-rolled steel sheet according to [1], wherein the slab heating performed prior to hot rolling is performed in a transition temperature range in the middle of changing the temperature of the heating furnace. .

  [3] Immediately after the completion of hot finish rolling, the hot rolled steel strip temperature is measured or a ferrite transformation sensor is used to determine whether ferrite is formed during finish rolling. The hot-rolled steel sheet according to the above [1] or [2], wherein the hot-rolled steel sheet produced by is used.

  [4] Hot press forming of hot-rolled steel sheet according to any one of [1] to [3], wherein finish rolling is performed at a maximum speed determined from specifications such as rolling load and mill power of a rolling mill. Method.

  In the present invention, when performing hot press forming of a hot-rolled steel sheet, since a hot-rolled steel sheet manufactured without managing properties other than the shape is used, control such as temperature control is performed when manufacturing the hot-rolled steel sheet to be used. As compared with the prior art, scheduling in the hot rolling process is easy, and productivity improvement and cost reduction of hot rolling can be achieved.

It is a figure which shows the manufacturing process in one Embodiment of this invention. It is the schematic of the hot press molded product in the Example of this invention. It is sectional drawing of the hot press molded product which shows the hardness measurement position in the Example of this invention. It is a figure which shows the hardness distribution in the Example of this invention.

  An embodiment of the present invention will be described with reference to FIG.

  In one embodiment of the present invention, hot-rolled steel sheets manufactured without controlling the properties other than the shape within the range of normal hot rolling conditions are pickled and then heated to the austenite region to perform hot pressing. Then, a press-molded product having a required strength is obtained by quenching.

  That is, as shown in FIG. 1, in one embodiment of the present invention, a hot rolling process for producing a hot-rolled steel sheet by hot rolling without controlling properties other than the shape, and the hot rolling process. A pickling process for pickling the manufactured hot-rolled steel sheet, a heating process for heating the hot-rolled steel sheet pickled in the pickling process to the austenite area, and the hot-rolled steel sheet heated in the heating process for the austenite area The hot press forming step for obtaining a press molded product by press molding and the quenching step for quenching the press molded product obtained in the hot press forming step are performed in that order.

  In a hot rolling process for producing a normal hot-rolled steel sheet, the slab is heated to 1100 to 1300 ° C., followed by rough rolling and finish rolling to obtain a hot-rolled steel sheet. When the rolled hot-rolled steel sheet passes through the hot run table, cooling water is injected and cooled to a predetermined temperature, and then wound by a coiler. Usually, from the viewpoint of material control, the rolling finishing temperature immediately after finishing rolling is controlled to be equal to or higher than the ferrite transformation start temperature. The coiling temperature, which is the temperature of the hot-rolled steel sheet just before winding on the coiler, is strictly controlled from the viewpoint of phase transformation and precipitate control.

  However, since the temperature of the heating furnace can only be changed slowly, the heating furnace temperature is the most difficult part in schedule management of the hot rolling process. The hot-rolled steel sheet used in this embodiment may have any heating temperature in the heating temperature range of the normal hot rolling process, and this embodiment relaxes the scheduling of the heating furnace in the hot rolling process. The For example, when the temperature of the heating furnace is changed from a low temperature to a high temperature, a slab to be a hot-rolled steel sheet used in this embodiment can be incorporated in a time zone corresponding to the transition temperature range. Therefore, according to this embodiment, it is possible to reduce the operating cost and improve the productivity of the heating furnace.

  When ferrite transformation is performed during hot rolling, the generated ferrite grains become coarse and the material of the hot-rolled steel sheet deteriorates. Therefore, the rolling finishing temperature, which is the temperature immediately after the end of rolling, is controlled to be equal to or higher than the ferrite transformation start temperature. Due to this restriction, the minimum manufacturable thickness of the hot-rolled steel sheet is 1.2 mm. In addition, the coil front end and rear end of the hot-rolled steel sheet having a thickness of about 1.2 mm have a low ferrite finishing temperature, and thus have undergone ferrite transformation, resulting in a decrease in yield. On the other hand, in the case of the hot-rolled steel sheet used in this embodiment, since it is heated to the austenite region during hot pressing, there is no problem even if ferrite is generated during rolling, and the yield of the hot-rolled steel sheet according to this embodiment. Will improve. Moreover, when using in this embodiment, the steel plate less than 1.2 mm manufactured by cold rolling can also be manufactured by hot rolling, and the cost of a cold rolling process can be reduced.

  Usually, the coiling temperature is strictly controlled from the viewpoint of phase transformation control and precipitate control. Since the cooling capacity of the hot run table is determined, the rolling speed may be lowered to lower the winding temperature. On the other hand, since the hot-rolled steel strip used in this embodiment is heated to the austenite region during hot pressing, there is no problem even if the winding temperature is any number of times, and the rolling speed of finish rolling can be reduced by the rolling load or Productivity can be improved by increasing the maximum speed determined by specifications such as mill power of the rolling mill.

  In this embodiment, immediately after the hot finish rolling is completed, the hot-rolled steel strip temperature is measured, or by using a ferrite transformation sensor, it is determined whether ferrite is generated during the finish rolling, A hot-rolled steel sheet in which ferrite is generated during finish rolling may be used in the present invention.

  And the hot press process of this embodiment may be the same as a general hot press process. That is, a blank material is produced by blanking from a coil of a hot-rolled steel sheet, and heated by a heating means until the blank material is completely austenite. The heated blank material is conveyed to a press device and subjected to press working to obtain a predetermined shape. A predetermined strength is obtained by holding at the bottom dead center of the press as it is, quenching the molded product between press dies, and transforming the martensite. Alternatively, a cooling device can be installed in addition to the press device, and quenching can be performed after press molding. The molded article may be made into a product through finishing steps such as removal of surface oxide by shot blasting, piercing, and laser cutting.

  Thus, in this embodiment, when performing hot press forming of a hot-rolled steel sheet, since a hot-rolled steel sheet manufactured without managing properties other than the shape is used, the hot-rolled steel sheet to be used is manufactured. Therefore, control such as temperature management is not required, scheduling in the hot rolling process is easier than in the prior art, and hot rolling productivity improvement and cost reduction can be achieved.

  In addition, in FIG. 1 mentioned above, including pickling (Zn, Al, Al-Si, Zn-Ni, etc.) and surface treatment processes, such as antioxidant coating, between pickling and heating to an austenite region, It falls within the scope of the present invention.

  In addition, it is also possible to carry out heating and hot pressing after the above-described hot-rolled steel sheet is used as a cold-rolled steel sheet, or to perform heating and hot-pressing after using the above-described hot-rolled steel sheet as a surface-treated cold-rolled steel sheet. Can be considered within the range.

  As an example of the present invention, in order to obtain a hat-shaped press-formed product as shown in FIG. 2, hot press forming of a hot-rolled steel sheet is performed as follows based on the above-described embodiment of the present invention. It carried out (Invention Examples 1 to 8).

  In the experimental furnace, the chemical components were C: 0.23 mass%, Si: 0.07 mass%, Mn: 1.5 mass%, P: 0.02 mass%, S: 0.002 mass%, Al: 0.04 mass%, The remainder was ingot made of Fe and inevitable impurities and hot-rolled with a model rolling mill to produce a material with a plate thickness of 20 mm. The material was reheated in a heating furnace at a predetermined temperature for 1 hour and hot-rolled with a model rolling mill to obtain a hot-rolled steel sheet having a thickness of 1.2 mm. The rolling finishing temperature was adjusted to a predetermined temperature by adjusting the time between rolling passes. Furthermore, in order to simulate the temperature history after winding, the hot rolled steel sheet cooled to a predetermined temperature (equivalent to the winding temperature) is inserted into an electric furnace heated to the same temperature (equivalent to the winding temperature) and held for 3 hours. After that, the electric furnace was turned off and gradually cooled in the electric furnace to produce a hot press blank. Table 1 shows the reheating temperature, rolling finishing temperature, and winding equivalent temperature. The range of temperature conditions for normal hot rolling is almost covered.

  The microstructure of the sample produced in Example 5 of the present invention was a structure composed of ferrite coarse grains and pearlite obtained by stretching the ferrite produced during rolling. The other samples had a structure composed of approximately equiaxed ferrite grains and pearlite.

  The sample was heated to 950 ° C. in a heating furnace and held for 5 minutes to form a complete austenite structure, then removed from the furnace and pressed into a hat shape as shown in FIG. Quenching was performed by holding for 2 seconds.

  The hat molded product was cut at the center in the longitudinal direction, and Vickers hardness was measured at 17 points on the cut surface shown in FIG. The measurement results are shown in FIG. As shown in FIG. 4, in any of the inventive examples, the Vickers hardness was good over the reference value of 430 Hv, which is the lower limit value, at all measured positions. When the microstructure was observed on the cut surface, it was confirmed that the entire surface was martensite in any sample.

  The present invention can be used for press forming of steel sheets, facilitates scheduling in the hot rolling process, and can be expected to improve hot rolling productivity and reduce costs.

Claims (2)

Slab heating prior to hot rolling is performed in the transition temperature range in the middle of changing the temperature of the heating furnace , and immediately after completion of hot finish rolling, the hot rolled steel strip temperature is measured, or ferrite By using a transformation sensor, it was determined whether or not ferrite was produced during finish rolling, and a hot rolling process for producing a hot-rolled steel sheet in which ferrite was produced during finish rolling, and the hot rolling process was used. The hot-rolled steel sheet is pickled, and after the pickling, the heating process of heating the surface-treated cold-rolled steel sheet plated with Zn-Ni as a cold-rolled steel sheet to the austenite region, and the hot-rolled steel sheet heated in the heating step in the austenite region A hot-rolled steel comprising a press forming step of press forming to obtain a press-formed product, and a quenching step of quenching the press-formed product obtained in the press forming step in that order. Hot press molding method. The hot press forming method for hot-rolled steel sheets according to claim 1, wherein finish rolling is performed at a maximum speed determined from specifications such as rolling load and mill power of a rolling mill.

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