JP4616756B2 - Hot forming mold for high strength steel sheet and hot forming method for high strength steel sheet - Google Patents

Description

  The present invention relates to a die used in a die quench method for processing a steel plate into a predetermined shape hot, and a hot forming method for a steel plate using the die.

  In fields such as automobiles and construction machinery, efforts to reduce mass using high-strength materials are actively being made. For example, in automobiles, the use of high-strength steel sheets steadily increases with the proposition of offsetting the increase in body mass associated with ensuring collision safety and higher functionality, and further improving fuel efficiency and reducing carbon dioxide emissions. Have been doing.

  The biggest problem in the flow of expanding the use of high-strength steel sheets is the manifestation of a phenomenon called “degradation of shape freezing property” that is inevitable when the strength of the steel sheets is increased. This phenomenon is a general term that the amount of springback after molding increases with increasing strength, making it difficult to obtain the desired shape. A material that is not necessary for a material having excellent properties or a problem) is added (for example, re-striking) and a product shape is changed.

  As one method for solving such a situation, a hot forming method called a die quench method has attracted attention. This is because the steel sheet (work material) is heated to a predetermined temperature (generally the temperature at which it becomes an austenite phase) to lower the strength (that is, to facilitate forming), and then at a lower temperature than the work material. By molding with a mold (for example, room temperature), the shape is easily imparted, and at the same time, a rapid cooling heat treatment (quenching) using the temperature difference between the two is performed to ensure the strength of the molded product (molded product). That's it.

  Several techniques have been reported for the die quench method. For example, Patent Document 1 discloses an example in which the method is applied to the manufacture of an automobile collision reinforcing material called a door impact beam. Patent Document 2 shows an example in which a direct energization method is employed as a method of heating a workpiece.

  The present inventors have also paid attention to the usefulness of the die quench method from the past, and in Patent Document 3, a mold for obtaining a molded product with little variation in the shape and strength of the finished product is proposed. In No. 4, a method of obtaining composite parts having different strengths for each part from a single steel sheet was proposed. In addition, Patent Document 5 discloses a method of cooling by changing the cooling rate for each shape part of the molded product and changing the quenching hardness for each part. Further, Patent Document 6 discloses a press molding apparatus and a press molding method provided with a cooling medium jetting / discharging mechanism and a mold body cooling mechanism for the purpose of promoting cooling of the mold and the molded product. Yes.

As described above, the usefulness of the die quench method has been widely recognized, and various members have been studied for application. Among them, for example, an undercarriage part of an automobile includes not only a strength as a part but also a fatigue characteristic that is one of important important characteristics.
Japanese Patent Laid-Open No. 2002-102980 JP 2002-18531 A JP 2005-59010 A JP 2005-161366 A JP2003-328031 JP 2005-169394 A

  It is known that the fatigue properties of a steel sheet increase in proportion to the static strength, but the rising margin decreases when it exceeds a certain strength level. This is understood as a phenomenon in which the fatigue properties do not increase as the “notch sensitivity” increases. However, a steel plate or a molded product that has been strengthened by the die quench method is a steel plate that has been strengthened to the same extent by the die quench method (a high strength steel plate manufactured by controlling the chemical composition of the steel plate or the manufacturing method, the following) It was revealed that the fatigue properties were inferior when compared to a normal high-strength steel sheet) or a molded product.

  As a result of detailed studies, one of the causes is that the die quench method has the highest surface (the outermost layer), which is the most strongly reinforced, and is therefore more sensitive to notches than ordinary high-strength steel sheets. I came to the conclusion that it might be. One approach that can be considered is to reduce the quenching speed of the outermost layer in order to make the quenching degree in the plate thickness direction uniform and make the notch sensitivity comparable to that of a normal high-strength steel plate of the same strength. It is. However, in the die quench method, it is difficult to reduce the cooling rate of only the outermost layer, and if the cooling rate in the entire plate thickness direction is reduced, the ultimate strength may be reduced and the desired strength may not be obtained. It ’s not the best way. Thus, there is a need for a die quench method that can secure the required strength and can ensure the same level of fatigue characteristics as a normal high-strength steel plate of the same strength, but no technology that solves such a problem has been found.

  In the patent document 4, the present inventors have proposed a method of slowing the cooling rate of a specific part of a molded part. However, some consideration is added to the cooling rate difference in the thickness direction of a minute part. There wasn't. The same applies to the method of Patent Document 5. Patent Document 6 proposes a mold having a plurality of convex portions that satisfy a predetermined condition on the molding surface, which is cooled on the molding surface from an ejection hole communicating with a cooling medium supply pipe. It is a metal mold used for a method of pressing while ejecting a medium, and cannot be a means for solving the problems of the conventional die quench molded product that the outermost surface layer is most strongly hardened and hardened.

  The present invention has been made to overcome such a current situation, and obtains a press-formed product having the same strength as that of a conventional high-strength steel sheet having the same strength as that of a conventional die-quenched molded product. The present invention provides a hot forming mold for high-strength steel sheet that can be used and a hot forming method for high-strength steel sheet using the mold.

  The present inventors have intensively studied to solve the above-described problems. As a result, the inventors have conceived of providing a plurality of spherical depressions that form an air layer at the time of pressing on the surface in contact with the workpiece. In addition, if the diameter of the recess and the distance between the centers satisfy a predetermined condition, the steel sheet is hot-formed without substantially reducing the average quenching speed of the entire molded product. Found that it would be possible. This ensures the target strength, especially when hot-forming high-strength steel sheets, and at the same time, significantly suppresses the phenomenon that only the outermost layer protrudes and strengthens, and succeeds in suppressing deterioration of fatigue properties. did.

The present invention has been made on the basis of these findings, and is a mold that does not have a cooling mechanism for cooling a molded product and a mold, particularly a cooling medium jetting mechanism, and between the molded product and the mold during pressing. By using the fine air layer that is formed as a weak heat insulation layer, a part with a small quenching speed difference in the thickness direction can be dispersed in the surface layer of the molded product to obtain a die quench molded product without impairing fatigue characteristics And a die quench method using the mold, and the following is a summary.
(1) A hot forming mold that operates in a vertical direction with a pair of upper and lower parts, and an air layer between a part and all of a surface of the upper and lower molds in contact with at least one work material. A hot forming mold for a high-strength steel sheet, having a plurality of spherical depressions that form a diameter r [mm] of the depressions and a center distance d [mm] satisfying the following equations.

2r + 0.2 ≧ d ≧ 2r−0.6
And 1.1 ≧ r ≧ 0.6
And d ≧ 1.2
Here, the depth h of the dent may be set so as to satisfy the condition of h ≦ r / 2.
(2) By mass%, C: 0.1 to 0.4%, Si: 0.5% or less, Mn: 0.2 to 4%, and a steel plate made of the remainder Fe and inevitable impurities is 800 to A high-strength steel sheet hot forming method characterized by heating to 1100 ° C. and pressing with the hot forming die described in (1) above.

  By using the hot forming die for high strength steel sheet of the present invention, the strength of the die-quenching method is improved, without sacrificing the high strength capability of the press-formed product and easy shape imparting performance. Fatigue characteristics equivalent to those of high strength steel sheets can be obtained. Thereby, it can be expected that the applicable range of the die-quenched molded product will be extended to applications where fatigue characteristics are required.

  The present invention will be described in detail with reference to the drawings. FIG. 1 schematically shows a part of a vertical cross section when a steel sheet is pressed by a hot forming die which is an example of the present invention consisting of a pair of upper and lower sides. In the drawings, 11 is a workpiece (steel plate), 12a and 12b are an upper mold and a lower mold, respectively. Reference numerals 13a and 13b denote depressions provided on 12a and 12b, respectively. The size (depth and width) of the recess is exaggerated for the purpose of facilitating the illustration, and the ratio of the thickness of the workpiece 11 to the thickness of the workpiece 11 does not reflect the actual mold. .

  When a steel plate heated to a predetermined temperature in the range of 800 to 1100 ° C. is pressed as shown in FIG. 1, a portion where the workpiece 11 and the molds 12a and 12b are in direct contact, between p1 and q1, r1 Compared to between -s1, p2-q2, and r2-s2, there is cooling between the molds 12a, 12b and the part q1-r1 through the air layer between the workpiece 11 and between q2-r2. Start slightly later. As a result, the portion where the molds 12a, 12b and the workpiece 11 are in direct contact with each other corresponding to the arrangement of the depressions of the molds 12a, 12b, and the portion where the cooling rate of the outermost layer is slightly lower than that. Is formed on the workpiece 11. Thus, it can be presumed that the effect of lowering the notch sensitivity is produced by the two-dimensional distribution of regions having different cooling rates of the outermost layer, and the fatigue characteristics are improved. Reflecting the high thermal conductivity of the steel plate as the work material 11, there is no difference in strength inside the surface layer from several micrometers or more, and therefore the strength of the entire molded product is not impaired.

  On the other hand, when the size of the recesses 13a and 13b is too large, and when the interval between the recesses is too large or too small, the desired effect is not recognized. As shown in the examples, the appropriate range thereof is a range that satisfies the following expression, where r [mm] is the diameter of the recess and d [mm] is the distance between the centers of the adjacent recesses.

2r + 0.2 ≧ d ≧ 2r−0.6
And 1.1 ≧ r ≧ 0.6
And d ≧ 1.2
The reason why such a range exists is not necessarily clear, but when the diameter r of the dent is too large, a convex shape is formed on the surface of the workpiece 11, which itself acts as a notch to reduce fatigue characteristics. It is estimated that Further, when the center distance d is too small, it is considered that the flatness of the surface is lowered and the fatigue characteristics are lowered. On the other hand, when the center distance d is too large, the ratio of the area where the quenching speed of the outermost layer is partially reduced is insufficient, so that a desired effect is not obtained.

  The depths of the recesses 13a and 13b are not necessarily limited, but it is preferable that the depth be less than the diameter of the recesses in consideration of the difficulty of production and the effect on the strength reduction of the mold. Is ½ or less of the diameter r of the depression. The depressions 13a and 13b can be produced by any method, and etching, laser processing, or the like can be selected. The shape of the recess is optimally spherical, but it may not necessarily be spherical depending on processing conditions. In such a case, the diameter r of the dent may be calculated by measuring the area of the dent on the molding surface of the mold and assuming the shape of the dent as a circle. Further, the center distance d may be obtained centering on the position of the center of gravity obtained by numerical calculation.

  Further, as in the example of FIG. 1, the depressions do not have to be located on the same line at the top and bottom of the workpiece, and the diameter r of the depressions on the upper surface and the lower surface is within the above range. There is no problem even if the distance d between the centers is different. When the load repeatedly applied to the molded product is in one direction, a method of providing a depression only in the mold corresponding to the side receiving the tensile load can also be selected. Furthermore, the dent may be provided only in the mold part corresponding to the part where fatigue fracture is expected to occur, or may be provided in the whole.

  When the steel sheet to be formed is a high-strength steel sheet with excellent hardenability, the effect of improving fatigue characteristics is increased. The workpiece 11 contains, for example, mass%, C: 0.1 to 0.4%, Si: 0.5% or less, and Mn: 0.2 to 4%. The impurities P and S are preferably P ≦ 0.1% and S ≦ 0.05%. Further, one or more of Al, N, Ti, Nb, Cr, and Mo may be included as appropriate. The balance of these elements is iron and inevitable impurities. Further, zinc, aluminum, magnesium or an alloy thereof may be plated on the surface, or may be sprayed instead of plating.

  When a high-strength steel plate is heated to 800 to 1100 ° C. and pressed with a die quench die according to the present invention, high strength can be obtained after cooling and a portion with a small quenching speed difference in the thickness direction can be dispersed. And a molded article having excellent fatigue characteristics can be obtained.

  The invention is further illustrated by the examples.

  A 120 × 240 mm rectangular test piece was die-quenched using a press molding apparatus and a mold illustrated in FIG. The workpiece is a steel plate that contains 0.2%, 0.3%, and 1.2% by mass% of C, Si, and Mn as main components, respectively, and the balance iron and inevitable impurities. The thickness is 1.2 mm.

  As shown in FIG. 3A for the lower mold, the depression is arranged in a rectangular range having a width direction of 120 mm and a length direction of 240 mm symmetrically with respect to the center line in the width direction and the length direction. It is. The upper mold was also provided with a recess in the same range. In addition, it processed by the etching so that the depth of a hollow might be 1/2 of the diameter of a hollow. FIG. 3B shows an enlarged view of the thick rectangular portion in FIG. 3A, and FIG. 3C shows an enlarged view of a part of the 31-32 cross section.

  The workpiece (steel plate) was heated to 950 ° C., and 10 steel plates were pressed for each condition using a mold provided with a recess corresponding to the diameter r and the center-to-center distance d shown in Table 1. . In Table 1, No. 1 is a metal mold | die which does not provide a hollow. Among the 10 pressed steel plates, 2 No. 5 tensile test pieces were collected from 1 sheet according to JIS Z 2201, and 18 fatigue test pieces shown in FIG. 4 were collected from the remaining 9 sheets. Each test piece was collected by electrical discharge machining from a position corresponding to a range in which a depression of the mold was provided.

A tensile test was performed in accordance with JIS Z 2241 to determine the tensile strength σ _B (average value of two). The plane bending fatigue test was performed according to JIS Z 2275 using 18 collected fatigue test pieces, and the time strength σ _W was determined 1 × 10 ⁷ times. Test conditions are a stress ratio of −1 and a repetition rate of 5 Hz. From the tensile strength σ _B and the 1 × 10 ⁷ time strength σ _W obtained by the tensile test and the plane bending fatigue test, the fatigue limit ratio σ _W / σ _B was determined.

The results are shown in Table 1. The fatigue limit ratio σ _W / σ _B (Nos. 2 to 13 in Table 1) of a molded product pressed with a mold provided with a dent is replaced with the fatigue limit ratio σ _W / of the molded product pressed with a mold provided with no dent. FIG. 5 shows the case where the value (fatigue limit ratio improvement effect amount) divided by σ _B (No. 1 in Table 1) is 1.2 or more as ◯ and less than 1.2 as Δ. The horizontal axis in FIG. 5 is the diameter r of the recess, and the vertical axis is the distance d between the centers of the recesses.
2r + 0.2 ≧ d ≧ 2r−0.6
And 1.1 ≧ r ≧ 0.6
And d ≧ 1.2
As a result, it was found that the fatigue characteristics were improved.

  A mold having a recess with a diameter r of 1.0 mm, a center distance d of 1.6 mm, and a depth of 0.5 mm provided on only the upper side and a mold provided on both upper and lower sides is prepared. The steel plate was die-quenched under the same conditions. The indentation installation range and test quantity are the same as in Example 1.

The tensile strength σ _B was determined, and a fatigue test with a stress ratio of 0.1 was performed so that the upper die side at the time of pressing was the tensile side, and the time strength σ _W was determined 1 × 10 ⁷ times. σ _W / σ _B showed almost the same value regardless of the presence or absence of the depression on the lower mold side. Thus, when the applied load is unidirectional, the desired effect can be obtained even if the depression is provided only on the mold surface corresponding to the surface receiving the tensile load.

  A mold provided with a recess having a recess diameter r of 1.0 mm, a center-to-center distance d of 1.6 mm, and a depth of 0.5 mm was prepared. However, as shown in FIG. 6 (a), there are two types of indentation installation ranges (shaded portions): one that includes the entire fatigue test piece (a) and one corresponding to the range shown in FIG. 6 (b) (b). It was. The steel plate used and other conditions were the same as in Example 1.

A fatigue test with a stress ratio of −1 was performed to obtain a time strength σ _W of 1 × 10 ⁷ times. The values were almost the same regardless of (A) and (A). Thus, the desired effect can be obtained even if the depression is provided only in the mold part corresponding to the part where fatigue failure is expected to occur.

  In the above embodiment, the flat plate mold shown in FIGS. 2 and 3 is used so that the test piece can be easily collected. However, even if the mold has irregularities according to the shape of the molded product, If a recess satisfying the scope of the invention is provided, the same fatigue limit ratio improvement effect as that of the above-described embodiment can be obtained.

It is a schematic diagram which shows the state by which the to-be-processed material was pressed with the metal mold | die for hot forming of this invention. It is a schematic diagram which shows the press molding apparatus and metal mold | die used for the Example. (A) It is a schematic diagram which shows the arrangement | positioning range of the hollow on a lower side metal mold | die. (B) Enlarged view of thick line rectangular part (c) Enlarged view of part of section 31-32 It is a figure which shows a fatigue test piece. It is a graph which shows the improvement effect amount of a fatigue limit ratio in the combination of the diameter of a hollow and the center distance of a hollow. It is a schematic diagram which shows the arrangement | positioning range of the hollow on a metal mold | die on a fatigue test piece. (A) Whole fatigue test piece (b) Central part of fatigue test piece

Explanation of symbols

11: Work material 12a: Upper mold 12b: Lower mold 13a: Recess 13b provided on the upper mold 13b: Recesses p1 to s1 provided on the lower mold 1: Points indicating positions on the upper mold p2 to s2: Points indicating the position on the lower mold

Claims (3)

A hot molding die that operates in a vertical direction with a pair of upper and lower sides, wherein an air layer is formed between the workpiece and a part or all of the surface in contact with at least one workpiece of the upper and lower die. A hot forming mold for high-strength steel sheets, having a plurality of substantially spherical recesses, wherein the recesses have a diameter r [mm] and a center-to-center distance d [mm].
2r + 0.2 ≧ d ≧ 2r−0.6
And 1.1 ≧ r ≧ 0.6
And d ≧ 1.2   The hot forming mold for high-strength steel sheets according to claim 1, wherein a depth h of the depression satisfies a condition of h≤r / 2. % By mass
C: 0.1 to 0.4%
Si: 0.5% or less,
Mn: 0.2-4%
A high-strength steel sheet hot forming method, comprising heating a steel plate containing the balance Fe and inevitable impurities to 800 to 1100 ° C. and pressing with a hot forming die according to claim 1.

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