JP2006224162A - Hot press forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot press forming method for manufacturing parts, such as reinforcing parts and suspension parts of an automobile, in which high strength, high toughness and high fatigue strength are required. <P>SOLUTION: In this manufacturing method of hot press-formed articles excellent in toughness and fatigue strength, a steel sheet is heated to a temperature not lower than the transformation point A<SB>C3</SB>, press-formed with a die heated to a temperature of 400-550°C and held for ≥5 s in the range of 400-550°C. Soft nitriding treatment is performed by heating the sheet to a temperature of 550-590°C and succeedingly the parts are cooled to the room temperature at the rate of ≥20°C/s. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hot press molding method for manufacturing parts that require high strength, high toughness, and high fatigue strength, such as automobile reinforcement parts and undercarriage parts.

  In recent years, in order to cope with environmental problems, it is desired to reduce the weight of automobiles for the purpose of reducing carbon dioxide emissions and reducing fuel consumption. Increasing the strength of steel is an effective means for reducing the weight of automobiles. In general, the higher the strength of steel, the worse the formability and the problem of cracking during molding. In addition, the higher the strength of the steel, the greater the amount of springback after molding, and the deterioration of the shape freezing property is a major problem.

As a method for solving such problems, the hot press molding method has recently been attracting attention. This steel plate was heated above A _C3 transformation point, and press-molded at austenite single-phase state, by placing tempered by quenching using a heat removal to the mold, good dimensional accuracy high strength molding Product. For example, Patent Document 1 discloses a method in which after heating to 950 ° C., press forming, and maintaining the bottom dead center for a time corresponding to the hardenability of the steel sheet.

  However, since the oxide is formed on the surface because it is heated at high temperature in the atmosphere, it has to be removed in a later process, resulting in a problem that productivity is lowered. Even if the steel sheet is heated in a non-oxidizing atmosphere, there is a similar problem because oxides are generated on the surface while the steel sheet is taken out of the heating furnace and set in a press.

  As means for solving such a problem, Patent Document 2 proposes a technique of coating the surface with zinc, a zinc base alloy compound, a zinc-iron base alloy compound, or a zinc-iron-aluminum base alloy compound.

  These technologies are improving oxide problems.

  On the other hand, in the conventional hot press forming method, the structure of the hot press formed product is mainly a martensite structure, and there is a problem that the toughness decreases when the content of alloy elements such as C is increased to increase the strength. In order to improve toughness, a tempering process may be performed in a subsequent process, but there is a problem that the cost increases. In addition, fatigue strength is important when applying hot press to undercar parts for automobiles, but there have been few proposals for improving the fatigue characteristics of hot press molded products.

  In addition to hot pressing, a warm press forming method is disclosed in Patent Document 3 in which a metal plate is heated to 80 to 600 ° C. and then subjected to press forming to obtain a product with good dimensional accuracy. None of these are described.

As described above, it has been difficult to manufacture hot press molded products having high strength and excellent toughness and fatigue characteristics with the conventional technology.
JP 2002-248525 A JP 2001-353548 A JP 2002-143935 A

  The present invention is intended to solve the above-described problems, and an object of the present invention is to provide a method for producing a hot press molded product having high strength and excellent toughness and fatigue characteristics.

  The inventors of the present invention have conducted research on various materials having different components from both the component and manufacturing methods on methods for improving the toughness and fatigue properties of molded articles after hot press molding.

The present invention is configured based on such knowledge, and the gist thereof is as follows.
(1) The steel sheet is heated to a temperature not lower than the _AC3 transformation point, press-molded with a mold heated to a temperature not lower than 400 ° C and not higher than 550 ° C, and held in a temperature range of 400 ° C to 550 ° C for 5 seconds or longer. , Nitrocarburizing treatment by heating to a temperature of 550 ° C. to 600 ° C., followed by cooling to room temperature at a rate of 20 ° C./second or more.
(2) The hot press forming method as described in (1) above, wherein the steel plate is a galvanized steel plate, an aluminum plated steel plate or a zinc-aluminum alloy plated steel plate.

  According to the present invention, a hot press molded product having high strength and excellent toughness and fatigue characteristics can be obtained.

  Below, the significance of each requirement in the present invention and the reason for limitation will be specifically described.

First, the reasons for limiting the hot press molding method will be described.
In the method for producing a hot press molded product excellent in toughness and fatigue characteristics of the present invention, the steel plate is heated to a temperature not lower than the _AC3 transformation point, and press molding is performed with a mold heated to a temperature not lower than 400 ° C. and not higher than 550 ° C. Hold at a temperature range of 400 ° C or higher and 550 ° C or lower for 5 seconds or more, heat to a temperature of 550 ° C or higher and 590 ° C or lower, perform soft nitriding, and then cool to room temperature at a rate of 20 ° C / second or higher .

If the heating temperature of the steel sheet is less than the _AC3 transformation point, the structure before quenching becomes a two-layer structure of austenite and ferrite, so that it cannot be made a bainite-based structure after quenching, ensuring high strength and high toughness at the same time. Therefore, the heating temperature of the steel sheet is set to the _AC3 transformation point or higher.

  If the mold temperature before press molding is less than 400 ° C., the bainite transformation does not proceed sufficiently, martensite is generated in the cooling process after pressing, and the toughness deteriorates. On the other hand, if the mold temperature exceeds 550 ° C., ferrite transformation proceeds and sufficient strength cannot be obtained. Therefore, the mold temperature was limited to 400 ° C. or more and 550 ° C. or less.

  After the press forming, in order to sufficiently advance the bainite transformation, it is kept in a temperature range of 400 ° C. to 550 ° C. for 5 seconds or more. More desirably, hold for 10 seconds or more. The holding temperature is set to 400 ° C. or more and 550 ° C. or less for the same reason that the mold temperature is limited to the same temperature.

  The reason why the holding temperature is set to 5 seconds or more is to sufficiently advance the bainite transformation to obtain a bainite-based structure. More desirably, hold for 10 seconds or more. In addition, the method of holding at a temperature of 400 ° C. or more and 550 ° C. or less is not particularly specified such as holding in a mold after pressing, inserting into a furnace, or covering with a heat insulating cover.

  After maintaining at a temperature range of 400 ° C. or more and 550 ° C. or less for 5 seconds or more, heating is performed at a temperature of 550 ° C. or more and 590 ° C. or less to perform soft nitriding, and then cooling to room temperature at a rate of 20 ° C./second or more. A soft nitriding method may be a normal soft nitriding method such as a salt bath soft nitriding method or a gas soft nitriding method.

The salt bath used in the salt bath soft nitriding may be a commonly used one such as a salt bath containing cyanate as a main component. The gas in the case of performing the gas nitrocarburizing, RX gas mixing (CO: 20% by mass + H _2:: 40 wt% + N ₂ 40 wt% gas composition) and ammonia gas (1: 1) was generally used such as a gas What is used may be used. When soft nitriding is performed, a nitride layer of about 10 to 20 μm is formed on the surface layer portion, and a nitrogen diffusion layer of about 0.8 mm is formed inside (on the inside of the nitride layer). The internal diffusion layer is effective for improving the fatigue strength because compressive residual stress is generated when nitrogen is supersaturated in ferrite.

  If the nitrocarburizing temperature is less than 550 ° C., the diffusion of nitrogen does not proceed sufficiently. On the other hand, if it exceeds 590 ° C., transformation to austenite occurs and the effect of soft nitriding cannot be obtained sufficiently. Therefore, the soft nitriding temperature is set to 550 ° C. or higher and 590 ° C. or lower. When the cooling rate after soft nitriding is less than 20 ° C./second, nitrogen dissolved in supersaturation in the internal nitrogen diffusion layer is precipitated as iron nitride, so that compressive residual stress is reduced and fatigue characteristics are lowered. Therefore, cool to room temperature at a rate of 20 ° C / second or more. The upper limit need not be specified. In addition, room temperature means 0 degreeC-40 degreeC.

Next, the structure of the hot press molded product according to the present invention will be described.
As for the structure of the hot press-molded article excellent in toughness and fatigue characteristics of the present invention, the content of bainite is desirably 70% or more in terms of area ratio. When the content of bainite is less than 70% in area ratio, sufficient strength cannot be obtained when the balance is mainly ferrite, and sufficient toughness cannot be obtained when the balance is mainly martensite. is there.

  Further, there is no problem even if one or more of martensite, tempered martensite, retained austenite, ferrite and pearlite are present in an area ratio of less than 30%.

  In the present invention, the area ratio of each structure of martensite, tempered martensite, bainite, ferrite, retained austenite, and pearlite is the C section (width direction section) t / 4 part (t is the plate thickness) of the steel sheet. Or it defines as an average value at the time of 10-field observation by 200-1000 times with a scanning electron microscope.

Next, the raw steel plate of the hot press-formed product in the present invention will be described.
The material steel plate of the hot press-formed product of the present invention may be a galvanized steel plate, an aluminum plated steel plate, or a zinc-aluminum alloy plated steel plate in addition to a hot rolled steel plate and a cold rolled steel plate. Furthermore, there is no problem even if galvanizing is alloyed. By using these surface-treated steel sheets, the formation of oxides can be suppressed during high-temperature heating in the atmosphere, and post-processes such as shot blasting to remove the oxides can be omitted. It is preferable because it can exhibit good corrosion resistance.

  There are no particular restrictions on the components of the steel sheet, but the C content is 0.2% by mass or more in order to ensure the strength of the steel sheet for hot press molded products with high strength and excellent toughness and fatigue properties. It is desirable to do.

  Moreover, in order to ensure hardenability, Si: 3.0 mass% or less, Mn: 0.01-3.0 mass%, Cr: 0.05-3.0 mass%, Mo: 0.1-0.1% It is desirable to contain 1 type (s) or 2 or more types of 3.0 mass%, Ni: 0.05-5.0 mass%, B: 0.0003-0.05 mass%. Moreover, since P and S reduce toughness, it is desirable that P: 0.02% by mass or less and S: 0.02% by mass or less.

  Moreover, Ti: 0.005-0.3 mass%, Nb: 0.005-0.3 mass%, V: 0.02-0.5 mass%, depending on the desired strength level and other necessary characteristics , Al: 0.005 to 0.1 mass%, N: 0.001 to 0.05 mass%, Cu: 0.05 to 2.0 mass%, W: 0.05 to 3.0 mass%, Ca : 0.001 to 0.01% by mass, Mg: 0.0005 to 0.01% by mass, Zr: 0.001 to 0.05% by mass, REM: 0.001 to 0.05% by mass or You may contain 2 or more types.

  Next, characteristic values will be described. In the present invention, the tensile strength of the hot press-molded article having excellent toughness and fatigue characteristics is determined by considering ductile brittle transition temperature (vTrs) -40 in consideration of characteristics required for automobile reinforcement parts and suspension parts that require high strength. Desirably, the tensile strength is 1000 ° C. or lower.

Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
Steel A having the composition shown in Table 1 was hot-rolled to produce a hot-rolled steel sheet having a thickness of 3.0 mm.
Table 1 also shows the _AC3 transformation point and Ms point (martensitic transformation start point). In addition, a part of this hot-rolled steel sheet was galvanized to obtain steel B. The amount of galvanized adhesion was 45 g / m ² .

These steel plates A and B were hot press molded under the conditions shown in Table 2, bent into a hat shape, and further subjected to soft nitriding.

  Here, the blank shape was 100 mm wide × 300 mm long, the punch die radius = 5 mm, and the die die radius = 5 mm.

  Soft nitriding was performed by salt bath soft nitriding. After molding, the toughness of the press-formed product was examined by Charpy test. In the Charpy test, a 2.5 mm thick sub-sized sample was prepared and tested, and the ductile brittle transition temperature was measured.

The Vickers hardness of the press-molded product was examined and converted to tensile strength. In addition, the fatigue characteristics of press-formed products were evaluated. Fatigue properties were determined the identified fatigue strength ratio fatigue limit at 10 ⁷ times perform fatigue test of bending Reversed plane (fatigue limit / tensile strength).

  The fatigue test was performed by the method described in JIS Z 2275. Furthermore, the structure was observed with an optical microscope and a scanning electron microscope, and the area ratio of each structure was measured. The test results are shown in Table 2. In each of the inventive examples (Nos. 1 and 4), the bainite content is 92% by area ratio and 90% or more by 70% or more, and a ductile brittle transition with respect to tensile strengths of 1026 MPa and 1013 MPa (converted value). As for temperature, -50 degreeC and a favorable value are obtained.

  In addition, both fatigue strength ratios are as high as 0.6. On the other hand, in the comparative example (No. 2) in which the mold temperature is less than 400 ° C., the area ratio of martensite is high (88%), so the tensile strength is as high as 1457 MPa, but the ductile brittle transition The temperature is 10 ° C., the toughness is low, and the fatigue strength ratio is a low value of 0.45.

  Further, in the comparative example (No. 3) in which the mold temperature is higher than 550 ° C. (650 ° C.), the area ratio of ferrite is high (65%), so the tensile strength is low at 663 MPa, and the required strength is obtained. Not.

  From the above, it is apparent that a hot press molded product having high strength and excellent toughness and fatigue characteristics can be obtained by manufacturing under the conditions shown in the present invention.

Claims (2)

The steel sheet is heated to a temperature not lower than the _AC3 transformation point, press-molded with a mold heated to a temperature of 400 ° C. or higher and 550 ° C. or lower, held in a temperature range of 400 ° C. or higher and 550 ° C. or lower for 5 seconds or longer, and 550 ° C. A hot press molding method characterized by performing a soft nitriding treatment by heating to a temperature of 590 ° C. or lower and then cooling to room temperature at a rate of 20 ° C./second or higher. 2. The hot press forming method according to claim 1, wherein the steel plate is a galvanized steel plate, an aluminum plated steel plate, or a zinc-aluminum alloy plated steel plate.