JP2005205453A - Pressure forming method of steel sheet for hot pressing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pressure forming method of a steel sheet for hot pressing by which the productivity of products is raised than that of a conventional method and which is excellent in workability. <P>SOLUTION: In this pressure forming method of the steel sheet for hot pressing, the steel sheet 10 is heated in the temperature range of 800-1,200°C and quenched and formed while pressing this steel sheet 10 by using a die 11 under the conditions which satisfy the next formula: Y ≥ 22×h+0.04×T-43. In the formula, Y is the time (s) necessary for quenching, h is the thickness (mm) of the steel sheet and T is the temperature (°C) of the steel sheet when starting pressing. In this way, the steel sheet is quenched and formed by performing proper cooling under a definite criterion and the manufacturing time of the products is shortened. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a pressure forming method of a steel sheet for hot pressing, for example, for manufacturing a product that requires strength, such as a structural member or a reinforcing member of an automobile, and more specifically, to maintain a shape after high temperature forming. The present invention relates to an excellent hot pressing steel plate forming method.

Conventionally, for example, automobiles have applied high-tensile steel (also referred to as high tensile steel) to structural members, reinforcing members, etc. in order to realize both reduction in vehicle weight for improving fuel efficiency and safety in the event of a collision. .
However, in order to produce this product, when high-tensile steel plate is press-molded using a mold, the processed product is subject to springback (deformation to return to its original state), wall warpage, etc. The shape freezing (shape retention) property after molding was poor, and it was difficult to produce a product with good quality.
For this reason, for example, the shape of the mold is corrected in advance in consideration of the amount of springback or the like, and the number of processing steps for correcting the shape is increased. However, the cost is increased and it is not economical.
Therefore, the heated steel plate is hot-pressed with a mold (also called hot pressing, hot stamping, die quenching), so that the steel plate is made into high-tensile steel by quenching with a mold and has good shape freezing property. A method for manufacturing a simple product is disclosed (for example, see Patent Document 1).

JP 2002-282951 A

However, in the above-described conventional method, the end of quenching of the steel sheet by the mold is predicted, and the cooling time that enables the worker to increase the strength of the steel sheet is predicted based on the heating temperature of the steel sheet heated in the heating furnace. However, this was done by actually measuring the steel plate temperature after the predicted time had elapsed.
For this reason, the time required for cooling the steel sheet is unnecessarily long due to the reliable quenching of the steel sheet, resulting in a problem that the productivity of the product is lowered. At this time, in order to increase the productivity of the product, it is possible to shorten the prediction time and repeatedly measure the temperature of the steel sheet to determine whether or not the cooling is finished, but the workability is not good.
As described above, performing the quenching time of the steel sheet without providing a clear criterion may cause a decrease in product productivity and a deterioration in workability.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a pressure forming method for a hot-press steel sheet that is more productive than the prior art and has good workability.

The press forming method of the steel sheet for hot press according to claim 1 that meets the above-mentioned purpose, the steel sheet is heated within a temperature range of 800 to 1200 ° C., and the steel sheet is used under a condition that satisfies the following formula. Harden while pressing.
Y ≧ 22 × h + 0.04 × T-43
Here, Y is the time (seconds) required for quenching, h is the thickness (mm) of the steel sheet, and T is the temperature (° C.) of the steel sheet at the start of pressing.

In the press forming method of the steel sheet for hot press according to claim 1, as the steel sheet, for example, a component having a carbon (C) content of 0.4 mass% or less and a manganese (Mn) content of 2.5 mass% or less. You can use what you have.
Moreover, a metal mold | die is comprised with the material which can be hardened when the heated steel plate contacts, For example, things, such as steel, can be used.

Since this steel sheet is made into a high-strength steel after forming, the steel sheet is first heated to a temperature equal to or higher than the A _C1 point (the point at which the structure of the steel sheet becomes austenite) and transformed to austenite, and then cooled under conditions that cause martensitic transformation. There is a need. For this reason, the structure of a steel plate can be made into an austenite structure by heating a steel plate to 800 degreeC or more. In addition, when the steel plate temperature exceeds 1200 ° C., the steel plate is heated excessively more than necessary, so that the time required for cooling becomes long and the product productivity decreases when the steel plate is quenched. At this time, the heat energy cost required for heating the steel sheet is excessive, which increases the manufacturing cost of the product and is not economical.
Therefore, in order to increase the productivity of the product and to manufacture a product composed of high-strength steel economically, it is preferable that the heating temperature of the steel plate is 850 to 1100 ° C, and further 850 to 1000 ° C. It is preferable that
In addition, although the upper limit value of the time required for quenching is not set, considering the increase in productivity, for example, it is preferable to set the time required for quenching to about Y + 10 (seconds).

The hot pressing steel plate press forming method according to claim 2 is the hot press steel plate press forming method according to claim 1, wherein the quenching of the steel plate has a quenching end temperature of 300 ° C. The cooling rate at the time of quenching in the mold is 30 ° C./second or more.
3. The hot pressing steel sheet press forming method according to claim 2, wherein the quenching conditions of the steel sheet are austenite by setting the quenching temperature in the mold to 300 ° C. or lower and the cooling rate to 30 ° C./second or higher. A high-tensile steel can be produced by reliably transforming the transformed steel sheet into martensite.

The press forming method of a hot press steel plate according to claim 3 is the hot press steel plate press forming method according to claim 1 or 2, wherein the steel plate has a thickness of 1 to 3 mm.
In the press forming method of the hot press steel plate according to claim 3, the steel plate having a plate thickness of 1 to 3 mm indicates the thickness of a conventionally used hot press steel plate. In addition, the plate | board thickness of the steel plate especially used is about 1-2.6 mm.

The press forming method of the hot press steel plate according to claim 4 is the press forming method of the hot press steel plate according to claims 1 to 3, wherein the time Y required for the quenching is a press time t1 by the mold. And a cooling time t2 for holding and cooling the pressed steel plate in the mold as it is, and the cooling time t2 is longer than the pressing time t1.
In the pressure forming method of the steel sheet for hot press according to claim 4, the press time t1 means the time during the forming of the steel sheet by the mold, and the contact surface of the mold with the steel sheet is still on the entire surface of the steel sheet. Indicates the time of no contact. Moreover, the cooling time t2 means the time which hold | maintains and cools the steel plate processed into the product shape in a metal mold | die, and shows the time when the contact surface with respect to the steel plate of a metal mold is contacting the whole surface of a steel plate. .

Since the pressure forming method of the steel sheet for hot pressing according to claims 1 to 4 is quench-molding while pressing the steel sheet using a mold under a predetermined condition, appropriate cooling is performed under a clear criterion. Can be quenched and molded. Thereby, since the manufacturing time of the product can be shortened as compared with the conventional case, the productivity of the product can be improved, and the workability at the time of manufacturing is also improved.
In addition, since the cooling time required for quenching can be determined from the thickness of the steel sheet and the temperature of the steel sheet, it is possible to set an appropriate cooling time according to each condition, and quench forming without causing excessive cooling or insufficient cooling. it can. As a result, a product with stable quality can be manufactured with a reduction in manufacturing time as compared with the prior art, so that the yield and productivity of a product with good quality can be increased as compared with the prior art.

In particular, the method for press forming a hot-press steel sheet according to claim 2 ensures that the heated steel sheet is ensured because the end temperature of quenching in the mold is 300 ° C. or lower and the cooling rate is 30 ° C./second or higher. The target high-tensile steel can be reliably manufactured.

The method for pressure forming a hot-press steel sheet according to claim 3 uses a steel sheet having a thickness of 1 to 3 mm, so that a steel sheet having a thickness that is conventionally used as a hot-press steel sheet is used. Products with quality can be manufactured with high productivity and high yield.

According to the method for press-forming a hot-press steel sheet according to the fourth aspect of the invention, the cooling time t2 can be made longer than the pressing time t1, so that the cooling time with enhanced cooling effect can be lengthened, so that the productivity can be further improved.

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 an explanatory view showing the time required for quenching in the hot press steel plate pressure forming method according to one embodiment of the present invention, and FIG. Explanatory drawing of the mold to which the forming method is applied, (B) is an explanatory view of the transition of the mold temperature during use, and FIG. 3 is for quenching when the steel plate thickness is 1.0 mm and the steel plate temperature is 850 ° C. FIG. 4 is an explanatory diagram showing the time required, FIG. 4 is an explanatory diagram showing the time required for quenching when the plate thickness is 1.6 mm and the steel plate temperature is 950 ° C., and FIG. 5 is an embodiment of the present invention. It is explanatory drawing of the quenching conditions in the pressure forming method of the steel plate for hot press which concerns.

As shown in FIG. 2 (A), the hot-press steel sheet according to one embodiment of the present invention has, for example, a carbon (C) content of 0.4 mass% or less and a manganese (Mn) content. A steel plate (also referred to as a hot press steel plate) 10 having a component of 2.5% by mass or less and a plate thickness of 1 to 3 mm (preferably 1 to 2.6 mm) is pressed using a mold 11. It is a method of 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 by quenching and forming. This will be described in detail below.

First, the steel plate 10, is charged into a heating furnace (not shown), A _C1 temperature above, i.e. 800 to 1200 ° C. (preferably, 850 to 1000 ° C.) is heated to austenite transformation within a temperature range of.
Next, as shown in FIG. 2A, the heated steel plate 10 is molded into a mold 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). 11 to press.

The mold 11 is made of steel, and includes an upper mold part 13 having a protruding portion 12 that can move in the vertical direction, and a lower surface of the upper mold part 13 and a predetermined gap when the mold 11 is operated. (For example, about 0.8 to 3 mm) and the lower mold part 14 arranged. In addition, a water cooling pipe 16 is provided at a position of, for example, 100 mm from the upper lowermost surface 15 of the lower mold part 14 inside the lower mold part 14 to enhance the cooling effect of the mold 11.
Using this mold 11, a steel plate 10 that maintains the temperature (cooling start temperature: for example, 800 ° C. or higher) before transformation into a soft phase (bainite, pearlite, etc.) is pressed.

The pressing of the steel plate 10 is performed under conditions that satisfy the following formula.
Y ≧ 22 × h + 0.04 × T-43 (1)
Here, Y is the time (seconds) required for quenching, h is the thickness (mm) of the steel sheet, and T is the temperature (° C.) of the steel sheet at the start of pressing.
Here, the reason why the conditional expression for quenching while pressing using the mold 11 is set to the expression (1) will be described.
First, as shown in FIG. 3, when the thickness h of the steel plate 10 to be heated is 1.0 mm and the temperature T of the steel plate 10 is 850 ° C., the time Y required for quenching, that is, the press time t1 by the mold 11 It can be seen that a total time of 13 seconds or more with the mold cooling time t2 held in the mold 11 after pressing is required. Moreover, as shown in FIG. 4, when the thickness h of the steel plate 10 to be heated is 1.6 mm and the temperature T of the steel plate 10 is 950 ° C., it is understood that the time Y required for quenching is 31 seconds or more. .

Thus, the thickness h of the steel plate 10 to be heated is, for example, 1 mm, 1.2 mm, 1.6 mm, etc., and the heating temperature T is, for example, 850 ° C., 900 ° C., 950 ° C., etc. FIG. 1 shows the results obtained by determining the time required for each heating temperature T, the time Y required for quenching for each heating temperature T on the vertical axis, and the plate thickness h on the horizontal axis.
As is apparent from FIG. 1, at each heating temperature, a relational expression “Y = 22 × h + 0.04 × T-43” showing the relationship between the time Y required for quenching and the plate thickness h of the steel sheet is obtained. By using this relational expression, the minimum time required for quenching is obtained from each heating temperature and the thickness of each steel plate.
From the above, the time required for this quenching is expressed by equation (1).

Accordingly, the time Y required for quenching is a predetermined time (when the steel plate thickness is 1.0 mm and the heating temperature is 850 ° C .: 13 seconds, when the steel plate thickness is 1.6 mm and the heating temperature is 950 ° C .: 31 seconds) or longer, the intended quenching can be performed.
Here, the press time t1 is a time during which the contact surface of the mold 11 with the steel plate 10 is not in contact with the entire surface of the steel plate 10, and the cooling time t2 is a time when the contact surface of the mold 11 is the surface of the steel plate 10. It shows the total contact time. For this reason, by making cooling time t2 longer than press time t1, the cooling time which heightened the cooling effect can be lengthened, and the productivity of a product can further be improved.

Further, the expression (1) satisfies the conditions that the quenching of the steel sheet 10 is 300 ° C. or lower and the cooling rate at the time of quenching in the mold 11 is 30 ° C./second or higher. To do.
Here, description will be made with reference to FIG. 5 showing a continuous cooling transformation diagram (CCT curve) of a steel type applicable to the steel sheet 10, for example, steel containing 0.24 mass% of carbon (C) and 20 ppm of boron (B). To do. In FIG. 5, the temperature of the steel plate is taken on the vertical axis and the cooling time of the steel plate is taken on the horizontal axis, the steel plate at 950 ° C. is set at a cooling rate of 1 to 100 ° C./second, and the steel plate temperature is changed for each elapsed time. It shows the temperature. The numbers in the lower rectangular frame in FIG. 5 indicate Vickers hardness (Hv), and the three broken lines that cross the curve at a cooling rate of 30 ° C./second or higher indicate the amount of martensite (mass%) in the steel sheet. It is.

By setting the end temperature of quenching in the mold 11 to 300 ° C. or less and the cooling rate to 30 ° C./second or more, 90 mass% or more of the austenite transformed structure is martensitic transformed. This is because the cooling rate is less than 30 ° C./sec (15 ° C./sec) and the hardness of the steel sheet is Hv 391 or less, whereas the cooling rate is 30 ° C./sec. Is also obvious.
That is, since 300 ° C. is the end temperature of the martensite transformation, if the end temperature of quenching is 300 ° C. or less, the austenite transformed steel sheet can be martensitic transformed. Moreover, if the cooling rate at the time of quenching in the mold is 30 ° C./second or more, transformation to the soft phase (for example, bainite, pearlite) of the austenitic transformed steel sheet can be suppressed until the martensitic transformation is completed. .
The same applies to steel types applicable to the steel sheet 10, for example, steel containing 0.22% by mass of carbon (C) and 20 ppm of boron (B).

With the above-described method, as shown in FIG. 2A, the steel sheet 10 is continuously quenched and formed with a mold 11 to produce a product.
In addition, when the steel plate 10 is continuously pressed using the mold 11 repeatedly, as is apparent from the result of the heat transfer analysis shown in FIG. Since the (steel plate temperature: about 230 ° C.) is stabilized, the steel plate 10 can be continuously quenched and molded using the die 11 without forced cooling from the surface of the die 11. Here, in the heat transfer analysis, for example, the thickness h of the steel plate is 1.0 mm, 1.2 mm, 1.6 mm, the temperature T of the steel plate at the start of pressing is 850 ° C., 900 ° C., 950 ° C., the initial stage of the mold The temperature was set to 25 ° C., and the heat transfer coefficient between the mold and the steel plate was set to 4500 kcal / m ² / Hr / ° C. (T ≧ 500 ° C.), 1200 kcal / m ² / Hr / ° C. (T <500 ° C.). It is used for the dotted line portion in FIG.
However, in order to further enhance the cooling effect of the mold, for example, a plurality of cooling pipes can be provided inside the mold, or the flow rate of the cooling water flowing through the cooling pipe can be increased.
By quenching and forming the steel plate 10 by the above method, for example, a product made of high-tensile steel having a hardness of about Hv 470 (about 1500 MPa) can be manufactured.

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 pressure forming method for a hot-press steel sheet according to the present invention is configured by combining some or all of the above-described embodiments and modifications is also included in the scope of the present invention.
Moreover, in the said embodiment, the press forming method of the steel plate for hot press which manufactures a structural member, a reinforcement member, etc. of a motor vehicle like a bumper reinforcement member, a center pillar reinforcement member, and a door impact reinforcement member is demonstrated, for example. did. However, in the field of using high-strength steel processed into a predetermined shape, for example, structural members such as vehicles, heavy machinery and ships, reinforcing members, etc., the method for pressure forming a hot-press steel sheet according to the present invention Of course, it is also possible to apply.

It is explanatory drawing which shows the time required for hardening with the pressure forming method of the steel plate for hot press which concerns on one embodiment of this invention. (A) is explanatory drawing of the metal mold | die which applies the pressure forming method of the steel plate for hot presses, (B) is explanatory drawing of transition of the metal mold temperature at the time of use. It is explanatory drawing which shows time required for hardening when the plate | board thickness of a steel plate is 1.0 mm and the temperature of a steel plate is 850 degreeC. It is explanatory drawing which shows time required for hardening when the plate | board thickness of a steel plate is 1.6 mm and the temperature of a steel plate is 950 degreeC. It is explanatory drawing of the quenching conditions in the pressure forming method of the steel plate for hot press which concerns on one embodiment of this invention.

Explanation of symbols

10: Steel plate, 11: Mold, 12: Projection part, 13: Upper mold part, 14: Lower mold part, 15: Upper lower surface, 16: Water-cooled tube

Claims (4)

The steel sheet is heated in a temperature range of 800 to 1200 ° C., and the steel sheet is subjected to quench forming while being pressed using a mold under the conditions satisfying the following expression. Method.
Y ≧ 22 × h + 0.04 × T-43
Here, Y is the time (seconds) required for quenching, h is the thickness (mm) of the steel sheet, and T is the temperature (° C.) of the steel sheet at the start of pressing. 2. The hot-press steel sheet press forming method according to claim 1, wherein the quenching of the steel sheet has a quenching end temperature of 300 ° C. or less and a cooling rate of 30 ° C. during quenching in the mold. / Pressure forming method of a steel sheet for hot press characterized by being at least. The pressure forming method of a hot press steel plate according to any one of claims 1 and 2, wherein the steel plate has a thickness of 1 to 3 mm. The pressure forming method for a hot-press steel sheet according to any one of claims 1 to 3, wherein the time Y required for the quenching is the same as the press time t1 by the mold and the pressed steel sheet as it is. And a cooling time t2 for holding and cooling in the mold, and the cooling time t2 is longer than the pressing time t1.

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