JP2005254297A - Hot forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot forming method capable of enhancing the forming property by a die rayher than before, and manufacturing a product having excellent strength after the forming. <P>SOLUTION: After a steel plate is heated to the temperature range from A<SB>c3</SB>point to the melting point, the forming of the steel plate is started at the temperature higher than the temperature at which ferritic, pearlitic, bainitic and martensitic transformations occur by using a die 11 having a punch 14 formed of a material having the heat conductivity smaller than that of steel. The steel plate is rapidly cooled after the forming, and the forming property can be enhanced, and a product having excellent strength after the forming can be manufactured. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a hot forming method for producing a product (part) that requires strength, such as a structural member of a car, a reinforcing member, etc., and more specifically, improves moldability by a mold, The present invention relates to a hot forming method for producing a product having excellent strength after forming.

Conventionally, for example, as one of the measures for improving the fuel efficiency of automobiles originating from global environmental problems, the weight of the vehicle body has been reduced, and it is necessary to increase the strength of steel sheets used in automobiles as much as possible. .
However, in general, when the strength of a steel plate is increased in order to reduce the weight of an automobile, the elongation and r value (Rankford value) decrease, and the formability decreases.
In order to solve such a problem, for example, Patent Document 1 discloses a technique of forming a steel plate warm and using the heat at that time to increase the strength. In this technique, a steel plate in which the components in the steel are appropriately controlled is used, the steel plate is formed in a temperature range of 200 to 850 ° C., and precipitation strengthening in this temperature range is used to form the steel plate. Aims to increase strength.
Patent Document 2 proposes a high-strength steel sheet that has low yield strength during warm pressing and high yield strength at room temperature for the purpose of improving the accuracy of press forming.

However, with these techniques, there is a possibility that the obtained strength may be limited, and there is a possibility that a steel plate having strength that can be used in an automobile cannot be manufactured.
Therefore, Patent Document 3 discloses a technique of forming a steel sheet for the purpose of obtaining higher strength, and then heating the steel sheet to a high temperature austenite single phase region and transforming it into a hard phase in the subsequent cooling process. . This method limits the clearance between the molds and quenches the molded product by introducing a refrigerant into the gap, thereby obtaining a product with high strength and excellent shape freezing (shape retention) properties. It can be done.

JP 2000-234153 A JP 2000-87183 A JP 2002-282951 A

However, when a high-temperature blank (intermediate product) punched from a steel plate is pressed using the square tube deep drawing die shown in the embodiment of Patent Document 3, for example, the portion in contact with the punch during blank forming There is a problem that the temperature is lowered, the ductility of the molding portion is lowered, and the blank is often broken, and the shape that can be molded is restricted.

This invention is made | formed in view of this situation, and it aims at providing the hot forming method which can manufacture the product which improves the moldability by a metal mold | die conventionally, and is excellent in the intensity | strength after shaping | molding.

The hot forming method according to claim 1, wherein the hot forming method according to claim 1, wherein the steel plate is heated to a temperature range from the _AC3 point to the melting point, and then a die having a punch made of a material having a lower thermal conductivity than the steel is used. Using, the forming of the steel sheet is started at a temperature higher than the temperature at which any of ferrite, pearlite, bainite, and martensitic transformation occurs, and the steel sheet is rapidly cooled after the forming.

The hot forming method according to claim 2, which meets the above-mentioned purpose, is a method in which a portion in contact with a portion where it is necessary to improve the forming limit of the steel plate after the steel plate is heated to a temperature range from the AC ₃ point to the melting point. Using a mold having a punch made of a material having a lower thermal conductivity, the forming of the steel sheet is started at a temperature higher than the temperature at which any of ferrite, pearlite, bainite, and martensitic transformation occurs. Cool rapidly after molding.

3. The hot forming method according to claim 1, wherein the steel sheet contains, for example, 0.05 to 0.55 mass% carbon (C) and 0.1 to 3 mass% manganese (Mn). Can be used. In order to increase the formability of the steel sheet and the strength after forming, the steel sheet is heated to a temperature range between the _AC3 point and the melting point of which the structure becomes an austenite single phase region, and austenite is transformed. In addition, the ductility of the forming site | part which is a part in which the shaping | molding of a steel plate is performed becomes favorable by starting shaping | molding of a steel plate at the temperature higher than the temperature in which all of a ferrite, pearlite, a bainite, and a martensitic transformation occur.
Moreover, as a material having a lower thermal conductivity than steel, for example, ceramic, stainless steel, or the like can be used.
3. The hot forming method according to claim 2, wherein the forming limit of the steel sheet means a limit that can be formed without forming a crack or necking (necking phenomenon) in the steel sheet when the steel sheet is formed by a mold. .
Moreover, the site | part which needs to improve the shaping | molding limit of a steel plate means the shaping | molding site | part of a steel plate. Therefore, the portion in contact with the forming portion means, for example, the surface layer portion of the punch.
In addition, as a steel plate, in order to improve the strength, toughness, and other properties of the steel plate, aluminum (Al), silicon (Si), oxygen (O), sulfur (S), titanium (elements other than carbon and manganese) Ti), vanadium (V), chromium (Cr), nickel (Ni), copper (Cu), zirconium (Zr), niobium (Nb), molybdenum (Mo), antimony (Sb), tungsten (W), calcium ( A material in which elements such as Ca), magnesium (Mg), and rare earth elements (REM) are optimized according to the purpose, and a metal structure, precipitates, and the like are optimized.

The hot forming method according to claim 3 is the hot forming method according to claims 1 and 2, wherein the material having a lower thermal conductivity than the steel is ceramic.
In the hot forming method according to claim 3, for example, alumina (Al ₂ O ₃ ), zirconia (ZrO ₂ ), sialon or the like can be used as the ceramic.

The hot forming method according to claim 4 is the hot forming method according to claims 1 to 3, wherein the die of the mold is made of a material having a thermal conductivity equal to or larger than that of steel, The rapid cooling after forming is performed at a speed equal to or higher than a cooling rate at which martensitic transformation occurs by bringing the forming portion of the steel sheet into contact with the die and to the start temperature of martensitic transformation.
In the hot forming method according to claim 4, as a material having a thermal conductivity equivalent to that of steel, for example, mild steel, hard steel and the like can be used, and as a material having a higher thermal conductivity than steel, for example, Copper, copper alloy, aluminum, aluminum alloy, tungsten, brass, etc. can be used.
The cooling rate at which martensitic transformation occurs is, for example, 20 ° C./second or more, preferably 30 ° C./second or more.
The rapid cooling is performed up to the start temperature of the martensitic transformation (for example, about 400 ° C.) or lower. In order to increase the strength after molding more reliably, the rapid cooling is performed at the end temperature of the martensitic transformation (for example, 300 ° C.). It is preferable to carry out to the following extent.

Since the hot forming method according to claim 1 and claims 3 and 4 dependent thereon uses a die having a punch made of a material having a lower thermal conductivity than steel, the forming of the steel plate with which the punch comes into contact is performed. The temperature drop of the part can be suppressed. Thereby, compared with the case where the metal mold | die which has the punch comprised with steel is used, the moldability of a shaping | molding site | part can be improved. Then, by rapidly cooling the formed steel sheet from a high temperature state, it is possible to increase the hardness of the product and manufacture a product having excellent strength after forming.

The hot forming method according to claim 2 and claims 3 and 4 subordinate thereto is made of a material having a lower thermal conductivity than that of the steel at the portion in contact with the portion that needs to improve the forming limit of the steel plate. Since a mold having a punch is used, it is possible to suppress a temperature drop at a forming part of the steel sheet, which is a part that needs to improve the forming limit. Thereby, compared with the case where the metal mold | die which has the punch comprised with steel is used, the moldability of a shaping | molding site | part can be improved. Then, by rapidly cooling the formed steel sheet from a high temperature state, it is possible to increase the hardness of the product and manufacture a product having excellent strength after forming.

In particular, in the hot forming method according to claim 3, since the material having a lower thermal conductivity than steel is ceramic, the formability of the forming portion of the steel sheet can be improved with a simple configuration.
The hot forming method according to claim 4 uses a die having a die made of a material having a thermal conductivity equal to or larger than that of steel, contacting the forming portion of the steel plate with the die, and rapidly cooling the martensite. Since it is performed at a speed equal to or higher than the cooling rate at which site transformation occurs and up to the start temperature of martensite transformation, the hardness after forming of the steel sheet can be reliably increased, and a product with stable quality can be provided.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
Here, FIG. 1 is a partial sectional side view of a mold to which a hot forming method according to an embodiment of the present invention is applied, and FIGS. 2 to 4 are molds according to first to third modifications, respectively. FIG. 5 is a partial cross-sectional view of a punch according to a modification used for a mold.

As shown in FIG. 1, the hot forming method according to an embodiment of the present invention includes, for example, 0.05 to 0.55 mass% of carbon (C) and 0.1 to 3 mass of manganese (Mn). For example, a blank (intermediate product) 10 punched from a steel plate (also referred to as a hot press steel plate) having a thickness of 1 to 3 mm (preferably 1 to 2.6 mm) is stretched using a mold 11. And manufacturing a structural member, a reinforcing member, and the like of an automobile such as a bumper reinforcing member, a center pillar reinforcing member, and a door impact reinforcing member. This will be described in detail below.

First, the blank 10 punched from the steel plate is charged into a heating furnace (not shown), and the temperature range from the _AC3 point, which is an austenite single phase region, to the melting point (although it varies depending on the components of the blank 10, for example, 800 ˜1500 ° C., preferably 900-1200 ° C.) to cause austenite transformation.
Next, as shown in FIG. 1, the heated blank 10 is pressed with a mold 11 having a predetermined shape (for example, a shape capable of manufacturing a bumper reinforcing member, a center pillar reinforcing member, a door impact reinforcing member, etc. of an automobile). To do.

The mold 11 includes a projecting portion 12 formed of a curved surface, and has a circular cross-section punch (lower mold portion) 14 that is movable in the vertical direction with respect to the wrinkle pressing portion 13 on which the blank 10 is placed. When the mold 11 is in operation, a die 15 (also referred to as an upper mold part or a die) 15 disposed with a predetermined gap (for example, a plate thickness of the blank 10) from the upper surface of the protruding portion 12 of the punch 14 is provided. Have. The wrinkle pressing portion 13 is also movable in the vertical direction with respect to the die 15. After the blank 10 is disposed between the die 15 and the wrinkle pressing portion 13, the lower surface of the die 15 and the wrinkle pressing portion 13 are arranged. The blank 10 is pressed against the upper surface of the sheet 10 to prevent the blank 10 from being displaced during molding by the punch 14.

The punch 14 is made of a material having a lower thermal conductivity than steel (for example, about 60 W / m / K or less), for example, ceramic such as alumina, zirconia, and sialon, stainless steel such as SUS304, and the like.
The die 15 is made of a material having a thermal conductivity equivalent to that of steel, for example, mild steel, hard steel, or a material larger than steel, for example, copper, copper alloy, aluminum, aluminum alloy, tungsten, brass, or the like. It is configured. Note that a cooling pipe through which a coolant (for example, water or the like) flows can be arranged inside the die, and the cooling efficiency by the die can be increased.
As shown in FIG. 1, the mold 11 has a radius of curvature of 50 mm at the upper end of the projecting portion 12 of the punch 14, a projection height of 28 mm from the upper surface of the wrinkle holding portion 13, and a recess 16 of the die 15. The maximum diameter is 110 mm, and the radius of curvature of the boundary between the concave portion 16 of the die 15 and the flat portion 17 communicating with the concave portion 16 is 5 mm.

In addition, the shape and numerical value of the metal mold | die to be used are not limited to this, For example, the punch from the upper surface of the wrinkle pressing part 13 like the metal mold | die 22 which has the punch 20 and die | dye 21 shown in FIG. The protrusion of the punch 25 from the upper surface of the crease presser 13 such as a protrusion having a protrusion height of 20 mm and a mold 27 having a punch 25 and a die 26 shown in FIG. It is also possible to use a portion 28 having a protruding height of 35 mm.

Further, as the mold, the mold 30 shown in FIG. 4 can be used.
The mold 30 includes a punch 31 made of the same material as the punch 14 and a die 33 having an opening 32 formed in the moving direction of the punch 31. For this reason, even if the punch 31 protrudes above the wrinkle pressing portion 13, the blank forming portion does not contact the surface of the die 33. The radius of curvature of the boundary between the inner surface 34 and the lower surface 35 of the die 33 that forms the opening 32 is 5 mm.
Thereby, like the above-mentioned each metal mold | die 11,22,27, it can shape | mold a blank, without causing the temperature fall of the shaping | molding site | part of a blank at the time of shaping | molding.

In addition, as shown in FIG. 5, as punch mentioned above each metal mold | die 11,22,27,30, it contacts with the shaping | molding site | part which is a site | part which needs to improve the limit (molding limit) which can shape | mold a blank. It is also possible to use a punch 41 in which the portion, that is, the protruding portion 40 is made of a material having a lower thermal conductivity than steel (for example, a material constituting the punch 14).
The punch 41 has a structure in which a projecting portion 40 having a circular cross section is attached to an upper portion of a steel punch main body 42. The projecting portion 40 has a maximum diameter of 103 mm, a curvature radius of 50 mm at its upper end, and a projecting portion. Although the height is 35 mm, it is not limited to this shape and numerical value. In addition, attachment of the protrusion part 40 with respect to the punch main body 42 can be performed by a screw structure, a fitting structure, joining, etc., for example.

Using the mold 11 described above, the blank 10 is started to be molded at a temperature (for example, about 700 ° C. or higher) higher than the temperature at which any of ferrite, pearlite, bainite, and martensitic transformation occurs.
At this time, since the heat insulating property of the protruding portion 12 of the punch 14 that comes into contact with the molding part of the blank 10 is good, the temperature drop of the molding part of the blank 10 is suppressed, and the ductility of the molding part can be increased. And the shaping | molding of the blank 10 is complete | finished by making the punch 14 project until the shaping | molding site | part of the blank 10 contacts the recessed part 16 of the die | dye 15, and the rapid cooling after shaping | molding is performed continuously.

Since the die 15 including the recess 16 with which the molding part of the blank 10 contacts has good thermal conductivity, the molding part can be quenched and quenched. Here, rapid cooling is performed at a cooling rate or higher (for example, 20 ° C./second or higher, preferably 30 ° C./second or higher) at which martensitic transformation occurs in a state where the molding portion of the blank 10 is in contact with the concave portion 16 of the die 15. It is performed at a speed and not higher than the start temperature of the martensitic transformation (for example, about 400 ° C.), preferably not higher than the end temperature of the martensitic transformation (for example, about 300 ° C.).
Thereby, the austenite transformed blank 10 can be martensitic transformed while suppressing and further preventing transformation to a soft phase (for example, bainite, pearlite).
By continuously quenching the blank 10 with the mold 11 by the method described above, a product having a quenching strength corresponding to the reaction, for example, 0.22% C steel, having a tensile strength of about 1470 MPa or more can be produced.

In addition, when the metal mold 30 shown in FIG. 4 is used, since the molding site of the blank cannot be brought into contact with the surface of the die 33, it cannot be quenched and quenched, and the hardness of the product after molding is increased. I can't. For this reason, after performing molding with the punch 31, the molded product is molded by using the same-shaped mold that has been conventionally used, for example, hot pressing again using a steel mold for both the punch and die. It is preferable to quench the part and increase the strength of the product.
Thereby, the product from which the tensile strength equivalent to the case where other metal mold | dies 11, 22, and 27 are used can be manufactured.

Next, examples carried out for confirming the effects of the present invention will be described.
First, slabs of steel types a to c having chemical components shown in Table 1 were cast.

These slabs were heated to 1050 to 1350 ° C., and hot rolled steel sheets with a finishing temperature of 800 to 900 ° C. and a winding temperature of 450 to 680 ° C. were manufactured by hot rolling. A part of the hot-rolled steel sheet is made into a cold-rolled steel sheet having a thickness of 1.4 mm by cold rolling, and a part of the cold-rolled steel sheet is subjected to hot-dip aluminum plating (aluminum plating) and alloyed hot-dip zinc shown in Table 2. Plating and hot dip galvanizing were performed respectively.

After that, blanks punched from these cold-rolled steel sheets and surface-treated steel sheets, for example, were heated in a heating furnace to the austenite region of 950 ° C., which is higher than the _{AC 3} point (lower melting point), and then hot-formed. In addition, as a metal mold | die used for shaping | molding of a blank, the metal mold | die A, B, C, D of the shape and size of each metal mold | die 11,22,27,30 demonstrated in the said embodiment is used, respectively. As the material for the punches of the molds A to D, those shown in Table 3 are used. Here, the ceramic whose punch material is smaller in thermal conductivity than steel is that when the punch shown in FIGS. 1 to 4 is used, and the ceramic whose punch material is smaller in thermal conductivity than steel is used on the steel. The insert is the one when the punch shown in FIG. 5 is used.

Each die A to C follows the punch shape, and the shape of each die was determined with a clearance of 1.4 mm. As the forming conditions, a disc-shaped blank with a thickness of 1.4 mm and a diameter of 150 mm punched from a steel plate is used, a punch speed of 10 mm / second, a pressing force of 100 tons, a wrinkle pressing force of 30 tons, and holding at the bottom dead center. The time (time for sandwiching the molding part between the protruding part of the punch and the concave part of the die) was 5 seconds.
Here, combinations of the tests performed are shown in Tables 4 to 7 based on the symbols in Tables 1 to 3, respectively. In addition, Comparative Examples 1-26 in Tables 4-7 are conventional examples (the material of the punch is steel).

The molding results in Tables 4 to 7 are the results of taking out the molded parts from the molds A to D and visually judging. As shown in Table 8, there is no cracking (◯), necking Judgment is made based on three criteria: occurrence (Δ) and crack occurrence (×).

Moreover, the hardness in Table 4-Table 7 is the result of having measured the Vickers hardness of the components taken out from each metal mold | die AD after shaping | molding. This is shown in Table 9.

As shown in Table 9, hardness, after the steel sheet before forming was heated to the austenite region of 950 ° C. is A _C3 or more points, based on the hardness when the water quenching from 900 ° C. is A _C3 or more points, “◎” when the hardness of the part where the strength is required (molded part) is 85% or more, “◯” when 70% or more and less than 85%, “△” when 50% or more and less than 70%. The case of less than 50% was evaluated as “x”, and “Δ” and “x” of less than 70% were rejected. Here, the hardness measurement points were the side wall and the bottom part of the component, and the flange part was excluded from the measurement range.

Example 2-1 to Example 25-2 and Comparative Examples 2 to 25 use steel type b as a steel plate, and use each die A to D, each punch material, and each plating type, respectively. It is the result of examining the molding limit.
In the case of the comparative example in which the punch material is steel, when the molding height is low (28 mm: using molds A and D), that is, in Comparative Examples 2, 5, 8, 11, 14, 17, 20, and 23, molding is performed. Although the result was good (◯), the molding result deteriorated as the molding height increased (33 mm, 35 mm: using molds B, C, and D). However, in the case of the example in which the punch material is ceramic having a lower thermal conductivity than steel, or the ceramic having the lower thermal conductivity than steel is inserted on the steel, the punch material is formed in comparison with the case where the punch material is steel. The molding limit was improved when any of the molds A to D was used without being affected by the height.

In addition, when the mold D was used, even if the molded part was held in the mold D, the hardness was insufficient (x) because a part of the blank was not rapidly cooled to the martensite start temperature. However, after forming, for example, a conventionally used punch and die are both made of steel and hot-pressed again using a mold having the same shape, sufficient quenching hardness can be obtained.
Therefore, also in the case where the mold D is used, the moldability by the mold can be improved as compared with the prior art, and a sufficient quenching hardness (◎) can be obtained.
In the case of the other molds A to C, the moldability can be improved, but the quenching hardness is sufficient. In addition, as is clear from the examples, any type of plating was used, so that the molding limit was improved and a sufficient quenching hardness could be obtained.

Examples 1-1 and 1-2, Examples 26-1 and 26-2, and Comparative Examples 1 and 26 use steel types a and c as steel plates, respectively, a mold B, each punch material, and a plating type. It is the result of examining the forming limit of the blank using each CR.
In the case of an embodiment in which the punch material is ceramic having a thermal conductivity smaller than that of steel, or a ceramic having a thermal conductivity smaller than that of steel is inserted on the steel, the forming limit is smaller than that in the case where the punch material is steel. Improved.
Thus, it has been confirmed that by using the hot forming method of the present invention, it is possible to produce a product that has improved moldability with a mold and is excellent in strength after molding.

As described above, the present invention has been described with reference to one embodiment. However, the present invention is not limited to the configuration described in the above embodiment, and is described in the claims. Other embodiments and modifications conceivable within the scope of the above are also included. For example, a case where the hot forming method of the present invention is configured by combining a part or all of the above-described embodiments and modifications is also included in the scope of the right of the present invention.
In the above embodiment, for example, hot forming for manufacturing a structural member, a reinforcing member, etc. of an automobile such as a bumper reinforcing member, a center pillar reinforcing member, and a door impact reinforcing member has been described, but it is processed into a predetermined shape. Of course, it is possible to apply the hot forming method of the present invention in order to manufacture structural members such as vehicles, heavy machinery and ships, reinforcing members, etc., in the field where the high-strength steel is used.

It is a fragmentary sectional side view of the metal mold | die which applies the hot forming method which concerns on one embodiment of this invention. It is a fragmentary sectional side view of the metal mold | die which concerns on a 1st modification. It is a fragmentary sectional side view of the metal mold | die which concerns on a 2nd modification. It is a fragmentary sectional side view of the metal mold | die which concerns on a 3rd modification. It is a fragmentary sectional side view of the punch which concerns on the modification used for a metal mold | die.

Explanation of symbols

10: Blank, 11: Mold, 12: Protruding part, 13: Wrinkle pressing part, 14: Punch, 15: Die, 16: Concave part, 17: Flat part, 20: Punch, 21: Die, 22: Mold, 23: protruding portion, 25: punch, 26: die, 27: mold, 28: protruding portion, 30: mold, 31: punch, 32: opening, 33: die, 34: inner surface, 35: lower surface, 40 : Protruding part, 41: Punch, 42: Punch body

Claims (4)

After heating the steel sheet to a temperature range from the AC ₃ point to the melting point, any of ferrite, pearlite, bainite, and martensite transformation using a die having a punch made of a material having a lower thermal conductivity than steel A hot forming method characterized in that the forming of the steel sheet is started at a temperature higher than the temperature at which water is generated, and rapidly cooled after the forming. After the steel plate is heated to a temperature range from the AC ₃ point to the melting point, the portion in contact with the portion that needs to improve the forming limit of the steel plate has a punch made of a material having a lower thermal conductivity than the steel. A hot forming method characterized by using a mold to start forming the steel sheet at a temperature higher than a temperature at which any of ferrite, pearlite, bainite, and martensite transformation occurs, and quenching after the forming. 3. The hot forming method according to claim 1, wherein the material having a lower thermal conductivity than the steel is a ceramic. 4. The hot forming method according to any one of claims 1 to 3, wherein the die of the mold is made of a material having a thermal conductivity equal to or larger than that of steel, and rapid cooling after forming the steel plate is performed. A hot forming method characterized in that the forming portion of the steel sheet is brought into contact with the die and is performed at a speed equal to or higher than a cooling rate at which martensitic transformation occurs and up to the start temperature of martensitic transformation.

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