JP5304678B2 - HOT PRESSING METHOD AND METHOD FOR PRODUCING MOLDED ARTICLE - Google Patents

Description

  The present invention relates to a hot pressing method for efficiently obtaining a high-strength molded product excellent in shape freezing property, and a method for producing the molded product.

  In the automotive field, in order to achieve both improved collision safety and lighter weight, it is a hot press-formed steel material that provides ultra-high strength with a tensile strength exceeding 1370 MPa, centering on automotive parts such as bumpers and door beams. Adoption is progressing.

  Hot pressing heats the material to an austenite region of about 900 ° C. and presses it with a mold, and at the same time, cools the material from room temperature to about 200 ° C. by removing the heat by bringing a cooling means into contact with the mold and quenching. It is a technique to perform. In the hot press, since the raw material is molded in a high-temperature soft state, a press load can be suppressed low, and a molded product excellent in shape freezing property can be obtained. Further, since quenching is performed at the same time as molding, the strength of the product can be as high as 1370 MPa. However, since the cooling means is brought into contact with the mold and the heat is removed from the mold and cooled, it is necessary to hold the press at the bottom dead center for about 15 to 30 seconds. As a result, the number of presses per minute is limited to about 1 to 2 times at most, and it must be said that the productivity is remarkably low as compared with 8 to 15 times / minute of the cold press.

  Patent Document 1 discloses a technique in which a die is specialized in the function of molding only, and quenching is performed in a separate process, based on the attention that the productivity reduction of the hot press is caused by quenching in the die. Yes. That is, in Patent Document 1, when a thin steel plate is heated to 850 ° C. or higher and then formed at high temperature with a press die, the molded product is taken out from the press die and cooled at a cooling rate of 30 ° C./second or more within 5 seconds. A hot pressing method is disclosed.

  Further, Patent Document 2 discloses a hot press technique using a transfer press apparatus provided with a molding die and a quenching die, thereby enabling high-speed processing.

JP 2005-288528 A JP 2007-144495 A

  However, in the method described in Patent Document 1, since the steel material is taken out from the mold at a temperature equal to or higher than the martensite transformation start temperature (Ms point), the austenite to ferrite and bainite are transported from the mold to the next quenching process. There is a risk that a shape change occurs due to the transformation to a shape and a defective shape occurs. Moreover, since the apparatus described in Patent Document 2 also performs molding and quenching with different molds, there is a risk that a shape defect that also causes a shape change due to transformation during conveyance occurs as in Patent Document 1. . In addition, the transfer press apparatus has a problem that the equipment cost increases and the cost increases.

  Accordingly, the present invention provides a hot pressing method capable of efficiently producing high strength parts with a good shape, and a method for producing a molded product by the method, which solves the problems of the above-described conventional techniques. Is an issue.

As a result of intensive studies, the present inventors obtained the following knowledge and completed the present invention.
FIG. 5 is a schematic diagram for explaining a temperature history in a conventional hot pressing method. Production of a high-strength member by hot pressing is performed by obtaining a martensite structure by rapidly cooling the steel material to be formed from the austenite region to the Ms point or less. That is, in order to secure sufficient hardness while suppressing the shape change accompanying martensitic transformation, as shown in FIG. 5, the steel material to be formed is restrained and held at the bottom dead center of the press mold for 15 to 30 seconds ( It is also referred to as “restraint” or “holding”), cooled to about 100 to 200 ° C. in the mold, and then taken out.

The present inventors examined the manufacturing process by hot pressing in three stages. That is,
(1) Processing for forming a high temperature material into a predetermined shape using a mold (reference numeral 101 in FIG. 5)
(2) Processing for quenching by quenching from the austenite region (reference numeral 102 in FIG. 5)
(3) Processing to restrain and cool in order to prevent a decrease in dimensional accuracy due to thermal shrinkage after quenching (reference numeral 103 in FIG. 5)
It is.

  It is important to perform the treatment (2) in the mold in order to suppress the shape change due to the transformation accompanying quenching. On the other hand, the process of (3) above is a cooling process after quenching, and it is only necessary to prevent shape deformation accompanying thermal contraction due to cooling, and it is not necessary to cool using a mold used for quenching. . Therefore, in the process (3), if a cooling means different from the mold used for molding and quenching is used, the mold used for the molding and quenching can be used for the process (1). Productivity can be increased.

  Next, the present inventors obtain by holding and holding temperature when holding the molded product at the bottom dead center of the mold (hereinafter sometimes referred to as “bottom dead center holding end temperature”) and quenching. We paid attention to the relationship with the ultimate hardness of the steel. The holding end temperature is also referred to as restraint end temperature.

Specifically, a steel plate heated to 900 ° C. having a thickness of 1.6 mm, a length of 280 mm, and a width of 70 mm was placed in a normal temperature mold and formed into a hat-shaped cross-sectional member having a height of 70 mm. . At the time of molding, the hat-shaped cross-section member (molded product) was taken out after holding at the molding bottom dead center for a predetermined time, and the Vickers hardness (HV) of the vertical wall portion of the molded product was measured. Here, the used steel plate contains chemical components of C: 0.21% by mass, Si: 0.25%, Mn: 1.20%, and the Ac3 point is 823 ° C.
At this time, a thermocouple was attached to the central part of the vertical wall on the side of the plate, and the temperature history from heating to forming was measured. The bottom dead center retention end temperature is T1. The clearance between the upper mold and the lower mold in the vertical wall portion of the mold was adjusted to a steel plate thickness +0.1 mm. The temperature of the molded product at the start of holding the bottom dead center was about 500 ° C., and the average cooling rate from the start of press molding to the end of holding the bottom dead center was 70 to 100 ° C./second.
The press used for the test is a crank press having a function of stopping motion by cutting the clutch at the bottom dead center. In addition, when the bottom dead center is not maintained, the time required for one molding (the time for one crank motion rotation) is about 1 second.

  FIG. 4 shows the relationship between the bottom dead center retention end temperature T1 obtained from the measurement results and the Vickers hardness (HV) of the vertical wall portion of the molded product described above. The ultimate hardness is almost converged at 450 HV. Moreover, as can be seen from FIG. 4, it has an inflection point in the vicinity of T1 = 300.degree. The Ms point of this steel sheet is about 420 ° C. at a cooling rate of 30 ° C./second or more. When T1 is close to the Ms point, sufficient hardness cannot be obtained, and it is sufficient to set T1 to (Ms−120) ° C. It was found that the ultimate hardness was obtained.

  Next, the present inventors examined the shape change of the molded product after it was taken out from the mold after the bottom dead center was maintained. If the bottom dead center holding end temperature T1 is (Ms-120) ° C. or lower, the molded product is taken out from the mold, and if it is quickly restrained to a regular shape and allowed to cool, it is equivalent to that held at the bottom dead center for a long time. It was found that the dimensional accuracy was ensured.

  That is, the bottom dead center holding end temperature in the mold is set to a temperature at which the reached hardness converges, and after taking out from the mold, the mold is allowed to cool while being restrained to a normal shape using a restraining jig or the like. The processing time for molding and quenching can be shortened. In addition, since the hardness and dimensional accuracy of the obtained product can be ensured, productivity can be improved.

  The present invention will be described below.

  In the invention according to claim 1, in mass%, C is 0.08% or more and 0.45% or less, the total of Mn and Cr is 0.5% or more and 3.0% or less, and the balance is C, Mn, This is a method of press-molding a steel plate made of an optional additive other than Cr, Fe, and a chemical component that is an inevitable impurity and heated to a temperature of Ac3 point or higher with a mold from a press start temperature of Ar3 point or higher. Then, while restraining the steel plate at the bottom dead center of the mold, the steel plate is cooled to a temperature of 200 ° C. or higher (Ms−120) ° C. or lower at a cooling rate higher than the critical cooling rate of the steel plate, and then the steel plate is taken out from the die. The process of starting restraint by means different from the mold before the temperature drop of the steel plate after the end of restraint at the bottom dead center by the mold reaches 15 ° C. or more and cooling to a temperature of less than 200 ° C. It is a hot press molding method characterized by including.

  Here, “Ms” means the Ms point and is the martensitic transformation start temperature. Further, “the total of Mn and Cr being 0.5% or more and 3.0% or less” includes that either Mn or Cr is 0%. The same applies hereinafter.

  The invention according to claim 2 is characterized in that, in the hot press forming method according to claim 1, the hardness of the steel material cooled to below 200 ° C. is 420 HV or more.

  In the invention according to claim 3, in mass%, C is 0.08% or more and 0.45% or less, the total of Mn and Cr is 0.5% or more and 3.0% or less, and the balance is C, Mn, A steel plate made of any additive other than Cr, Fe, and chemical components that are unavoidable impurities, and heated to a temperature higher than the Ac3 point by pressing with a die from a pressing start temperature higher than the Ar3 point. And cooling the steel plate to a temperature of 200 ° C. or higher (Ms−120) ° C. or lower at a cooling rate higher than the critical cooling rate of the steel plate while restraining the steel plate at the bottom dead center of the mold, and Following this step, the steel plate is removed from the mold, and the restraint is started by means different from the mold before the temperature drop of the steel plate after the end of restraint at the bottom dead center by the mold reaches 15 ° C. or more. Hot press-molded product including a process of cooling to a temperature below ℃ It is a manufacturing method.

  According to the present invention, the bottom dead center retention time can be greatly shortened compared to the conventional case, and a molded product having high strength and excellent dimensional accuracy can be efficiently manufactured.

It is a schematic diagram showing the temperature transition in one embodiment. It is a schematic diagram showing the temperature transition in an Example. It is a figure explaining the shape freezing property evaluation of the molded article of an Example. It is a graph which shows the relationship between the bottom dead center holding | maintenance completion temperature obtained from the measurement result, and the ultimate hardness of a molded article. It is a schematic diagram showing the temperature transition in the conventional press molding.

  The above-mentioned operation and gain of the present invention will be clarified from the following embodiments for carrying out the invention. However, the present invention is not limited to these embodiments.

<Steel plate>
The chemical composition of the steel sheet is specified as follows. In addition,% which prescribes | regulates the composition of a steel plate means the mass%.
The C content is 0.08% or more and 0.45% or less. C is an important element that enhances the hardenability of the steel sheet and determines the strength of the molded product. If the C content is less than 0.08%, the effect is not sufficient, and if the C content exceeds 0.45%, the toughness and weldability may deteriorate. Desirable C content is 0.3% or less.
The total content of Mn and Cr is 0.5% or more and 3.0% or less. Here, the range is a concept including that either Mn or Cr may be 0%. Mn and Cr are effective elements for improving the hardenability of the steel sheet and ensuring the strength of the molded product stably. However, if the total content of Mn and Cr is less than 0.5%, the effect is not sufficient, and if it exceeds 3.0%, the effect is saturated, and it is difficult to ensure stable strength. A desirable total content is 0.8% or more and 2.0% or less.
From the viewpoint of ensuring hardenability, the contents of C, Mn, and Cr described above may be ensured.

In order to further increase the strength or to secure the strength more stably, Si is 0.5% or less, P is 0.05% or less, S is 0.05% or less, Al is 1% or less, and N May be added as an optional additive in an amount of 0.01% or less. The balance is Fe, C, Mn, Cr and unavoidable impurities.
Further, from the viewpoint of improving hardenability and improving toughness, B is 0.0002% to 0.01%, Ni is 2% or less, Cu is 1% or less, Mo is 1% or less, and V is 1%. In the following, one or more of Ti 1% or less and Nb 1% or less may be contained as optional additives. The balance is Fe, C, Mn, Cr and unavoidable impurities.
It is also possible to use a galvanized steel plate or an aluminum plated steel plate in which a zinc or aluminum plating layer is formed on the surface of the steel plate containing the chemical component. These plated steel sheets do not generate scale during pressing and do not require subsequent scale removal.

  If the thickness of the steel sheet is too thick, it is difficult to perform sufficient quenching by cooling with a mold, so it is desirable that the thickness is 2.6 mm or less. The lower limit of the plate thickness is not particularly limited, but can be 1.0 mm.

  Next, a hot pressing method according to one embodiment and a manufacturing method of a molded product obtained thereby will be described. FIG. 1 is a schematic diagram showing temperature transition in the embodiment, in which the horizontal axis represents elapsed time and the vertical axis represents temperature (° C.).

<Heating>
The steel sheet is heated to a temperature of Ac3 point or higher. A steel sheet heated to a temperature of Ac3 point or higher becomes an austenite single phase. In order to ensure austenitization, the heating temperature is preferably (Ac3 + 20) ° C. or higher. On the other hand, if the heating temperature is too high, there is a problem in terms of energy saving, and in the case of a bare material, there is a concern that excessive scale may be generated. Therefore, it is desirable to set it to (Ac3 + 120) ° C. or lower. If it can heat to the said temperature range, a heating method will not be specifically limited. However, when a plated steel sheet is used as the steel sheet, there is a concern that the plating may disappear, and thus rapid heating such as current heating is not suitable. When using a bare material, it is desirable to use a furnace whose atmosphere is controlled to a low oxygen concentration in order to prevent scale.

<Conveyance, press molding (press down), bottom dead center maintenance>
The steel sheet heated to the austenite single-phase region is promptly conveyed to the mold, and pressing is started with the mold from a temperature of the Ar3 point or higher at which transformation to ferrite or bainite starts, and is formed into a predetermined shape ( Press molding, press down). And it is hold | maintained at a bottom dead center, and is rapidly cooled and hardened with a metal mold | die. More specifically, the bottom dead center holding end temperature T1 is (Ms−120) ° C. or lower and 200 ° C. or higher at a bottom dead center at a cooling rate higher than the critical cooling rate by heat removal by contact with the mold. It is kept and quenched until it reaches the temperature range of. Thereby, sufficient ultimate hardness (for example, 420 HV) as described above can be obtained.
When the bottom dead center retention end temperature T1 exceeds (Ms−120) ° C., the transformation from austenite to martensite becomes incomplete, and the untransformed austenite is transformed into ferrite and bainite by subsequent cooling, so that sufficient hardness is obtained. I can't. On the other hand, when the bottom dead center holding end temperature T1 is less than 200 ° C., the strength is saturated, the holding time is lengthened, and the productivity is lowered. Desirably, the bottom dead center retention end temperature T1 is 250 ° C. or higher and (Ms−120) ° C. or lower.

  The faster the cooling rate is, the higher the productivity is. It is desirable to keep the mold temperature sufficiently low by bringing the cooling means into contact with the mold for cooling and to ensure a cooling rate of 60 ° C./second or more. In addition, although the critical cooling rate of the steel sheet described above is about 30 ° C./second, when a steel material for hot mold such as SKD11 or SKD61 is used as a mold, the mold temperature is kept at about room temperature. The critical cooling rate can be obtained relatively easily. The material of the mold is not particularly limited as long as the molded product can be sufficiently cooled by heat removal as described above. However, it is preferable that hot wear does not cause excessive wear, deformation, or damage. In addition, when the number of moldings increases, the mold accumulates heat and becomes hot, and if there is a risk that the cooling effect required for quenching the molded product may not be expected, a water pipe is placed inside the mold to circulate the refrigerant. It is desirable to prevent heat storage in the mold by such a method.

<Press lift / conveyance / secondary restraint>
After the bottom dead center holding end temperature T1 reaches a temperature of (Ms−120) ° C. or lower, the bottom dead center holding is finished, and the molded product is quickly taken out from the mold (press rise). Before the temperature drop amount of the steel sheet reaches 15 ° C. or more, the restraint of holding and restraining the molded product in a regular shape is started, and cooling to less than 200 ° C. is performed in the state of keeping and restraining. That is, the temperature drop amount of the steel sheet from “end of bottom dead center holding” to “start of secondary restraint” in FIG. Thereby, the shape change accompanying heat contraction can be suppressed small, and it becomes possible to obtain a molded product excellent in dimensional accuracy.
The martensitic transformation is sufficiently completed during the above-mentioned bottom dead center holding, and after the molded product is taken out from the mold, the shape change due to the transformation does not occur. However, if the temperature drop of the steel sheet from taking out the molded product from the mold to holding it in a regular shape and restraining it (secondary restraint) is too large, the shape will change due to the thermal stress caused by cooling during non-restraining. This causes dimensional accuracy to deteriorate. Accordingly, the amount of temperature drop from “end of bottom dead center holding” to “start of secondary restraint” is set to less than 15 ° C. Note that the cooling rate of the steel sheet from the end of holding the bottom dead center to the start of secondary restraint is about 1.5 ° C./second in the case of air cooling. It is desirable to initiate restraint.
In addition, although the cooling rate in secondary restraint is not prescribed | regulated, it is desirable to raise a cooling rate by forced cooling etc. from a viewpoint of the efficiency in secondary cooling.

The shape of the jig used for the secondary restraint is not limited as long as it can hold a portion requiring dimensional accuracy at a normal position. That is, a jig capable of restraining only a part of the part other than the jig capable of restraining all parts of the cross section of the molded product may be used.
In addition, if the temperature at which the secondary restraint ends T3 is too high, a shape change occurs in the cooling after the end of the secondary restraint, which may deteriorate the dimensional accuracy. Therefore, the secondary cooling end temperature T3 is preferably less than 200 ° C. On the other hand, the lower limit value of T3 is not particularly limited, but it is not preferable to restrain the temperature to a very low temperature from the viewpoint of the use efficiency of the restraining jig.

  Molded products obtained by the above hot pressing methods include bumper beams, door beams and other impact members, side members, floor members, floor cross members, roof cross members, side sill structural members, pillars, roofs and other reinforcing members, sheets It is suitably used for automotive parts such as skeletons.

In the examples, hot press molding was performed by changing the conditions in the temperature transition as shown in FIG. This will be described in detail below.
<Steel plate>
The steel sheet contained 0.21% by mass of C, 0.25% of Si, and 1.20% of Mn, and had a plate thickness of 1.6 mm, a length of 280 mm, and a width of 70 mm. The steel material has an Ac3 point of 823 ° C and an Ms point of 420 ° C when the cooling rate is 30 ° C / second or more. That is, (Ms−120) ° C. is 300 ° C. Although the Ar3 point varies depending on the cooling rate, for example, when the thickness is 1.6 mm, it becomes about 610 ° C. when air-cooled.

Heating was performed by holding the steel plate for 240 seconds in an electric furnace in an air atmosphere set at 900 ° C. The ultimate temperature of the steel plate at this time was Tf (= 900 ° C.).
Immediately after leaving the furnace, it was charged into a normal temperature mold and press-molded into a hat shape having a height of 70 mm, and held at the bottom dead center for a time t1. The press start temperature at this time was represented by Tp. Further, the time from the start of pressing until reaching the bottom dead center (press lowering time) was 0.5 seconds, and the bottom dead center holding end temperature was T1. Here, the average cooling rate during the bottom dead center retention time t1 is 30 ° C./second or more. The clearance between the upper mold and the lower mold in the vertical wall portion of the mold was adjusted to a steel plate thickness +0.1 mm.

  After holding the bottom dead center, the press was raised and the molded product was transported, and was secondarily restrained by a restraining jig having a room temperature substantially the same as that of the molding die and air-cooled until the temperature reached T3. Here, the time from the end of holding the bottom dead center to the start of the secondary restraint is t2, the time from the start of the secondary restraint to the end of the secondary restraint is t3, and the secondary restraint start temperature is T2. The temperature of the steel sheet was measured by attaching a thermocouple to the central part of the vertical wall on the side surface of the steel sheet, and also using a thermography or contact thermometer.

The molded product thus obtained was evaluated for its hardness and shape freezing property. Hardness was measured with a Vickers hardness tester (load 1 kgf), and 420 HV at which 1370 MPa was obtained in terms of tensile strength was used as a reference, with a higher value being good (◯) and a lower value being bad (x).
Further, the shape freezing property was evaluated by measuring the flange angle θ of the part of the molded product 10 shown in FIG. A flange angle of less than 1.5 ° was evaluated as good (◯), and more than that was determined as poor (×). In addition, in this press molding, the so-called spring go tendency that the vertical wall enters inward under all conditions shows that the larger the flange angle is, the larger the spring go is, and the case of 0 ° is a normal shape. .
Productivity was evaluated by the time required from the start of press molding to the end of holding the bottom dead center. Based on the conventional example of No. 8, the shorter one was defined as good productivity (◯) and the longer one as poor productivity (×). The time from the start of press molding to the start of holding the bottom dead center is constant and is 0.5 seconds.
Table 1 shows the conditions and results.

  As can be seen from Table 1, no. 1-No. No. 3 had a bottom dead center retention time (t1) of 5 seconds or less, a hardness of 420 HV or more, a flange angle of less than 1.5 °, and good productivity, hardness and dimensional accuracy. No. 4-No. In all cases, the flange angle exceeded 2 ° and the shape was poor. No. In No. 7, the bottom dead center retention end temperature T1 was 350 ° C., and the hardness was less than 420 HV. No. 8 and no. 9 is a conventional example in which secondary restraint is not performed, and the hardness and dimensional accuracy are good, but the bottom dead center holding time t2 needs to be 10 seconds or more, and the productivity is low.

  From the above, no. 4-No. In the example of No. 7, dimensional accuracy or hardness is insufficient. 8 and no. The conventional example of 9 is low in productivity. In contrast, no. 1-No. The example No. 3 had high productivity and good hardness and dimensional accuracy.

10 Molded product T1 Bottom dead center retention end temperature

Claims (3)

In mass%, C is 0.08% or more and 0.45% or less, the total of Mn and Cr is 0.5% or more and 3.0% or less, and the balance is any additive other than C, Mn, and Cr, A method of pressing a steel plate made of Fe and a chemical component that is an inevitable impurity and heated to a temperature of Ac3 point or higher with a mold from a pressing start temperature of Ar3 point or higher,
While the steel plate is restrained at the bottom dead center of the mold, the steel plate is cooled to a temperature of 200 ° C. or more (Ms−120) ° C. or less at a cooling rate that is higher than the critical cooling rate of the steel plate. Take out from the mold, start restraint by means different from the mold before the temperature drop of the steel plate from the end of the restraint at the bottom dead center by the mold becomes 15 ℃ or more, to a temperature below 200 ℃ A hot press molding method comprising a cooling process.   The hot press molding method according to claim 1, wherein the steel material cooled to less than 200 ° C has a hardness of 420 HV or more. In mass%, C is 0.08% or more and 0.45% or less, the total of Mn and Cr is 0.5% or more and 3.0% or less, and the balance is any additive other than C, Mn, and Cr, A method of manufacturing a molded product by pressing a steel plate made of Fe and a chemical component that is an unavoidable impurity and heated to a temperature of Ac3 point or higher with a mold from a press start temperature of Ar3 point or higher,
Subsequent to the step of cooling the steel plate to a temperature of 200 ° C. or higher (Ms−120) ° C. or lower at a cooling rate equal to or higher than the critical cooling rate of the steel plate while restraining at the bottom dead center of the mold, The steel plate is taken out from the mold, and restraint is started by means different from the mold before the temperature drop amount of the steel plate from the end of restraint at the bottom dead center by the mold reaches 15 ° C. or more. A method for producing a hot press-formed product, which comprises a step of cooling to a temperature below.