Patent Grant
9469891
The present invention relates to a method of manufacturing a hot press-forming product that needs a strength used in a structure member of an automobile component and a facility used for the manufacturing method. Particularly, the present invention relates to a method of manufacturing a press-forming product capable of obtaining a predetermined strength by performing a heat treatment and a shaping process when a preliminarily heated metal sheet (blank) is formed in a predetermined shape and a facility used for the manufacturing method. More particularly, the present invention relates to a press-forming product manufacturing method of manufacturing a press-forming product with high productivity without causing a breakage or a crack during a press-forming process and a facility used for the manufacturing method.
As one of countermeasures for the improvement of fuel efficiency of an automobile originated from a global environmental problem, a vehicle body has been decreased in weight. For this reason, there is a need to improve the strength of a metal sheet used in the automobile as much as possible. However, when the strength of the metal sheet is improved generally in order to decrease the weight of the automobile, an elongation EL or a value r (Lankford value) decreases, and hence a shape freezing property or press-formability is degraded.
In order to solve such a problem, a hot press-forming method (a so-called “hot pressing method”) that ensures a strength after a forming process is employed to manufacture a component, and the hot press-forming method is performed in a manner such that a metal sheet (blank) is heated to a predetermined temperature (for example, an austenite-phase temperature) so as to decrease the strength (that is, to facilitate the forming process), and is formed by a forming tool having a low temperature (for example, a room temperature) compared to the metal sheet (the processing target), thereby performing a shaping process and a supper-cooling heat treatment (quenching) using a temperature difference therebetween (for example, Patent Document 1).
According to such a hot pressing method, since the metal sheet is formed in a low-strength state, spring-back decreases (with a satisfactory shape freezing property), and a tensile strength becomes 1500 MPa by the quenching process. Furthermore, such a hot pressing method is called various names such as a hot forming method, a hot stamping method, a hot stamp method, and a die-quench method other than the hot pressing method.
A hot press-forming facility including a press-forming machine having the above-described forming tool configuration is disclosed in, for example, Non-Patent Document 1. The facility includes a heating furnace that heats and softens a metal sheet, a device that conveys the heated metal sheet, a press-forming machine that press-forms the metal sheet, and a device that performs a trimming process (a correction process for obtaining a final shape by a laser or the like) on the forming product (see
When a hot pressing process (for example, a deep drawing process) is performed by using such a forming tool, the forming process starts while the blank (the metal sheet) 4 is heated and softened (a direct method). That is, the metal sheet 4 is pressed into a hole (between the dies 2 of
However, in the hot pressing method introduced so far, the pressing process was generally performed near 700 to 900° C., and the metal sheet was cooled to about 200° C. inside the forming tool so as to be quenched. For this reason, there was a need to hold the forming tool at the forming bottom dead center (the time point at which the front end of the punch was located at the deepest portion) for a certain time, and hence the time necessary for the die-quenching was long. For this reason, the number of pressing operations for 1 minute (spm: stroke/minute) was small as two to six times. As a result, the forming tool operation efficiency was low, and the productivity was poor.
For this reason, a so-called indirect method is proposed in which a metal sheet is formed in a near net state (a state where the metal sheet substantially becomes a forming product) by a cold pressing machine and is heated and die-quenched. However, in this method, the number of forming processes increases, and hence there is a demerit that the forming time is extended. Accordingly, there has been demanded a technique that further improves the productivity according to a direct method having a small number of forming processes.
The present invention is made in view of such circumstances, and an object thereof is to provide a method of manufacturing a press-forming product having a desired strength with high productivity without causing a breakage or a crack during a press-forming process and to provide a press-forming facility suitable for the manufacturing method.
In order to attain the above-described object, according to the present invention, there is provided a press-forming product manufacturing method of manufacturing a forming product by press-forming a metal sheet using a press-forming tool, including: heating the metal sheet to a transformation temperature Ac1 or more; cooling the metal sheet to 600° C. or lower; forming the metal sheet by a forming tool; ending the forming process at a martensite transformation start temperature Ms or more; taking out the metal sheet from the forming tool; and cooling the metal sheet.
According to an embodiment of the present invention, the forming process may be performed by a mechanical press-forming process or a hydraulic press-forming process having a pressing speed of 100 mm/second or more, and the cooling process to 600° C. or less may be performed by clamping the metal sheet between metal members or ejecting a gas and/or a mist.
Further, in order to attain the above-described object, according to the present invention, there is provided a press-forming facility that includes a heating furnace and a press-forming machine and is used to manufacture a forming product in a manner such that a metal sheet is heated to a transformation temperature Ac1 or more in the heating furnace, and the metal sheet is press-formed by a press-forming machine, wherein a cooling unit that rapidly cools the heated metal sheet is provided inside the heating furnace or between the heating furnace and the press-forming machine, and the press-forming machine is a mechanical pressing machine or a hydraulic pressing machine having a pressing speed of 100 mm/second or more.
The present invention also includes a press-forming product that is obtained by the press-forming facility.
According to the present invention, the metal sheet is heated, the metal sheet is cooled to a predetermined temperature, the metal sheet is subjected to the press-forming process, the press-forming process ends at the martensite transformation start temperature Ms or more, the metal sheet is taken out from the forming tool, and the metal sheet is subjected to the cooling process. For this reason, the forming tool operation efficiency may be improved, and hence the press-forming product may be manufactured with high productivity. Accordingly, the manufacturing cost of the hot stamped component may be reduced.
Further, according to the present invention, the cooling unit that rapidly cools the heated metal sheet is provided inside the heating furnace or between the heating furnace and the press-forming machine, and the mechanical press-forming machine or the high-speed hydraulic pressing machine is provided. For this reason, when the press-forming process is performed on the blank that is cooled to 600° C. or lower before the press-forming process by the facility, the forming tool operation efficiency may be improved, and hence the press-forming product may be manufactured with high productivity.
According to the present invention, it is possible to provide the satisfactory press-forming product having a desired strength with high productivity without causing a breakage or a crack during the forming process.
The present inventors have conducted various examinations in order to manufacture a satisfactory press-forming product with high productivity by heating and press-forming a metal sheet.
First, the present inventors were interested in a press-forming process. In the related art, since the metal sheet was formed and cooled by a quenching process inside a forming tool, the metal sheet needed to be held at a forming bottom dead center for a predetermined time. For example, in Patent Document 2, a punch was stopped at the forming bottom dead center after the press-forming process, and the temperature of the metal sheet was decreased by emitting the heat of the metal sheet to the forming tool (a cooling process at the bottom dead center). For this reason, the forming tool operation efficiency was poor, and the productivity was also poor.
When the metal sheet is extracted from the forming tool for the cooling process without directly performing the quenching process on the metal sheet formed in the forming tool, the metal sheet does not need to be held at the forming bottom dead center, and hence the time (the forming tool occupying time) necessary for the pressing process is shortened. Accordingly, the forming tool operation efficiency may be improved, and the productivity may be improved. Therefore, the present inventors have more carefully examined the forming condition.
As a result, when a forming process is performed in a manner such that a metal sheet is heated, is rapidly cooled to a temperature zone of 600° C. or lower, and is formed by a forming tool instead of a forming process in which a metal sheet (blank) is heated and is directly formed, the forming process ends at the temperature of a martensite transformation start temperature Ms or more, and a cooling process is performed after the metal sheet is taken out from the forming tool, the productivity may be drastically improved while maintaining a satisfactory formability without causing a crack or the like. In this way, the present invention is contrived. Hereinafter, the background of the present invention will be described in detail.
The present inventors first heated a metal sheet having a chemical composition shown in Table 1 below at 900° C. (where the metal sheet has a transformation temperature Ac1: 718° C., a transformation temperature Ac3: 830° C., and a martensite transformation start temperature Ms: 411° C.), rapidly cooled the metal sheet to 600° C. or lower, and drew the metal sheet by using a forming tool (a mechanical pressing machine) shown in
In the embodiment of the present invention, there is a need to facilitate the forming process by heating the metal sheet to the transformation temperature Ac1 or more. Furthermore, the transformation temperature Ac1 or more may be the temperature of the two-phase region of the transformation temperature Ac1 to the transformation temperature Ac3 or may be the temperature of the single region of the transformation temperature Ac3 or more. It is desirable that the upper limit of the heating temperature be about 1000° C. When the upper limit becomes higher than 1000° C., oxidized scales are noticeably generated (for example, 100 μm or more), and hence there is a concern that the sheet thickness of the (de-scaled) forming product may become thinner than a predetermined thickness.
Incidentally, an existing hot press line generally has a configuration (a facility configuration) shown in
In the present invention, the forming process is performed on the metal sheet after the metal sheet is rapidly cooled to 600° C. or lower instead of the configuration in which the metal sheet is heated to a predetermined temperature by the heating furnace and is directly conveyed to the press-forming machine so as to perform the forming process thereon. When the forming start temperature exceeds 600° C., the quenching time after the forming process is extended. Accordingly, the productivity is degraded, and the sufficient strength may not be obtained without a quenching process. Further, since the formability is degraded, it is difficult to perform a drawing process or form a product with a complex shape. The desirable forming start temperature is 580° C. or lower and more desirably 550° C. or lower. Meanwhile, when the forming start temperature is decreased too much, the metal sheet is hardened already at the forming start step, and hence satisfactory formability may not be exhibited. Accordingly, the forming start temperature is set to be higher than the point Ms. More desirably, the forming start temperature is set to be equal to or higher than a temperature of a value (the point Ms+30° C.). The cooling speed (the average cooling speed) until the heated metal sheet is cooled to 600° C. or lower needs to the cooling ability of 30° C./second or more in that the sufficient strength may not be ensured or the productivity may be degraded at the slow cooling speed. It is desirable to cool the metal sheet at 80° C./second or more.
In a case where the metal sheet is heated and is cooled to 600° C. or lower, for example, the facility configuration shown in
(1) A heat-emitting process is performed by providing a unit (for example, a cooling unit that is configured to clamp a metal sheet by a metal member such as a metal plate or a metal roll) that contacts metal as a cooling medium (for example,
(2) A gas-jet cooling process is performed by providing a gas cooling unit.
(3) A cooling process is performed by providing a mist cooling unit (for example,
(4) A cooling process is performed by providing a dry ice shot-blasting unit (the cooling process is performed by causing granule dry ice to collide with a blank material).
In the cooling using the cooling facility (the cooling unit) of the present invention, it is desirable to control the atmosphere along with the cooling process. When the atmosphere is controlled (so that the atmosphere becomes, for example, the atmosphere of nitrogen or argon), the surface oxidization of the metal sheet may be prevented. Further, when the temperature is set to be comparatively low, the surface oxidization may be suppressed.
The forming process may be performed after the cooling process is performed to a predetermined temperature in the above-described cooling unit (where the cooling process is completed until the forming process starts). However, the forming process may be continuously performed while being cooled by the forming tool even after the forming process starts.
Further, the press-forming process may be performed while being divided into a plurality of times. For example, as shown in
In the present invention, it is desirable that the press-forming machine 13 that performs a press-forming process on the metal sheet be configured as a machine press (hereinafter, referred to as a mechanical press) that performs a press-forming process by a mechanical driving force generated by a pressure generating mechanism in that the mechanical press has a fast pressing speed (for example, 100 mm/second or more), does not need to be held at the bottom dead center, and has cheap installation cost from the viewpoint in which the pressing time is shortened. However, a liquid-pressure press (for example, a hydraulic press) that uses a liquid pressure generated by a pressure generating mechanism or a hydraulic pressing machine having a pressing speed of 100 mm/second or more may be used. In a hydraulic pressing machine with such a pressing speed, the forming tool is not substantially held at the bottom dead center, and hence the forming tool operation efficiency may be improved.
In the related art, since the quenching process is performed on the metal sheet inside the forming tool, a liquid-pressure press is needed as a unit that holds the forming tool at the forming bottom dead center. However, in the present invention, since the cooling process is performed after the metal sheet is taken out from the forming tool, the liquid-pressure press having a comparatively slow pressing speed used in the related art does not need to be used. In a case where the mechanical press or the hydraulic press having a pressing speed of 100 mm/second or more is used, the time necessary for the pressing process may be shortened. Further, in the present invention, since the forming tool is not held at the forming bottom dead center for the quenching process, the number of pressing operations for 1 minute (spm: stroke/minute) may be improved, and hence the forming tool operation efficiency is satisfactory.
As the mechanical pressing machine, various slide driving mechanisms may be used. For example, a crank press, a knuckle press, a link press, a friction press, or the like may be used. Further,
The forming end temperature is set to the martensite transformation start temperature Ms or more. This is because the formability may be degraded when the martensite transformation occurs during the forming process. Accordingly, the forming end temperature is the point Ms or more and more desirably a value (the point Ms+10° C.) or more.
The quenching method after the end of the forming process is not particularly limited. For example, the formed steel sheet may be cooled after being extracted from the forming tool or the formed steel sheet may be cooled by various cooling units of (1) to (4) while the cooling speed is controlled (for example, 10 to 200° C./second). From the viewpoint of ensuring a desired strength by the quenching process, a method is desirable in which the formed steel sheet is extracted from the forming tool and is cooled by various cooling units of (1) to (4) at 30° C./second or more.
The hot press-forming product manufacturing method of the present invention may be applied to not only the case where a hot press-forming product having a simple shape is manufactured as shown in
The effect of the method of the present invention is noticeably exhibited in a case where the forming process (that is, the drawing process) is performed by using the forming tool having a folding force. However, the method of the present invention is not limited to the drawing process using the folding pressure, but includes a case where a normal press-forming process (for example, a stretch forming process) is performed. Even in a case where the forming product is manufactured according to such a method, the effect of the present invention is attained.
According to the present invention, it is possible to manufacture a satisfactory press-forming product having a predetermined strength without causing a breakage or a crack during a forming process.
Hereinafter, the effect of the present invention will be described in more detail by examples. However, the examples below do not limit the present invention, and all modifications in design are included in the technical scope of the present invention.
Priority is claimed on Japanese Patent Application No. 2011-218348, filed on Sep. 30, 2011. The entire content disclosed in Japanese Patent Application No. 2011-218348, filed on Sep. 30, 2011 is incorporated herein by reference.
A metal sheet (a circular blank having a thickness of 1.0 mm and a diameter of 100 mm) having a chemical composition shown in Table 1 was heated to 900° C. (where the steel sheet has a transformation temperature Ac1 of 718° C., a transformation temperature Ac3 of 830° C., and a martensite transformation start temperature Ms of 411° C.) by a press-forming facility including a cooling facility (a cooling unit or a cooling zone) shown in
Furthermore, the “quenching speed” of the “quenching condition of the cooling facility” was calculated in a manner such that the cooling curve of each quenching method was measured in advance and the speed was calculated based on the measurement value. Further, the pressing start temperature was adjusted by controlling the quenching time in which the metal sheet was extracted from the heating furnace and was subjected to the press-forming process based on the cooling curve. The measurement of the cooling curve was performed in a manner such that a change in temperature with time was measured while the metal sheet having a thermocouple attached thereto was rapidly cooled according to each quenching method without the press-forming process.
<Conveying Condition>
The conveying time from the heating furnace to the cooling unit (the cooling zone) and the conveying time from the cooling unit (the cooling zone) to the pressing-dedicated forming tool are respectively set to 3 seconds.
<Quenching Condition of Cooling Facility>
Quenching speed (gas jet): 85° C./second (using He gas)
Quenching speed (metal clamping): 160° C./second (using copper alloy for cooling forming tool)
Quenching speed (mist ejection): 310° C./second (mixture of air and water)
<Press-Forming Condition>
Folding force: 3 tons
Die shoulder radius rd: 5 mm
Punch shoulder radius rp: 5 mm
Clearance CL between punch and die: 0.15/2+1.0 (steel sheet thickness) mm
Forming height: 25 mm
Pressing machine: mechanical press (manufactured by AIDA Corporation. 80t crank press)
Furthermore, the transformation temperature Ac1, the transformation temperature Ac3, and the point Ms are obtained based on the following equations (1) to (3) (for example, see “Heat treatment” 41(3), 164 to 169, 2001 Kunitake stand wax and “Prediction by empirical formula of transformation temperatures Ac1, Ac3, and Ms”).
Transformation temperature Ac1(° C.)=723+29.1×[Si]−10.7×[Mn]+16.9×[Cr]−16.9×[Ni] (1)
Transformation temperature Ac3(° C.)=−230.5×[C]+31.6×[Si]−20.4×[Mn]−39.8×[Cu]−18.1×[Ni]−14.8×[Cr]+16.8×[Mo]+912 (2)
Ms(° C.)=560.5−{407.3×[C]+7.3×[Si]+37.8×[Mn]+20.5×[Cu]+19.5×[Ni]+19.8×[Cr]+4.5×[Mo]} (3)
Here, [C], [Si], [Mn], [Cr], [Mo], [Cu], and [Ni] respectively indicate the contents (mass %) of C, Si, Mn, Cr, Mo, Cu, and Ni. Further, in a case where the elements shown in the respective terms of Equations (1) to (3) are not included, the calculation is performed without the term.
After the press-forming process was performed, the metal sheet was air-cooled after being extracted from the forming tool (the “cooling speed after the press-forming process”). The result is show in Table 2.
In the test Nos. 1 to 3, the operation efficiency of the forming tool (the pressing machine) was controlled by the conveying time and the quenching time of the metal sheet. That is, since the press-forming process on the precedent metal sheet ends within the conveying time of the subsequent metal sheet, there is no need to consider the press-forming time as in the related art. In this example, since the conveying operation from the heating furnace to the cooling facility (the cooling unit or the cooling zone) and the conveying operation from the cooling facility to the pressing machine are synchronized with each other, the operation efficiency (the time necessary for manufacturing one press-forming product) of the forming tool (the pressing machine) was set to a value obtained by adding the conveying time (3 seconds) to the quenching time.
Further, since the temperature of the steel sheet before the pressing process may be controlled by setting the quenching time of the cooling facility before the press-forming process like the gas-jet method (4 seconds), the metal clamping method (2 seconds), and the mist method (1 second), the number of the pressing operations for 1 minute (the “number of times of component forming process for 1 minute”) may be set to each of 8.6 times, 12 times, and 15 times (spm).
According to the test Nos. 1 to 3, satisfactory formability may be obtained, and hence the deep drawing process may be performed at the forming bottom dead center (the state shown in
According to the comparison with the following reference example (No. 4 in Table 2), in the test Nos. 1 to 3 that satisfies the condition of the present invention, the number of the pressing operations for 1 minute is excellent, and the time (spm) necessary for the press-forming process may be shortened, so that the forming tool operation efficiency may be improved. Thus, according to the present invention, it is possible to manufacture a satisfactory press-forming product having a desired strength with high productivity without causing a breakage or a crack during the forming process.
The metal sheet having the shame chemical composition as that of Example 1 was heated to 900° C. by the press-forming facility of the related art shown in
<Press-Forming Condition>
Folding force: 3 tons
Die shoulder radius rd: 5 mm
Punch shoulder radius rp: 5 mm
Clearance CL between punch and die: 0.15/2+1.0 (steel sheet thickness) mm
Forming height: 20 mm
Pressing machine: hydraulic press (manufactured by Kawasaki oil Industry Co., Ltd., 300 t hydraulic press)
The holding time until the quenching process ended after the metal sheet subjected to the press-forming process was stopped at the forming bottom dead center was 22 seconds. Accordingly, the number of times of the pressing operations for 1 minute was about 2.7 times [2.7 spm (stroke/minute)], the forming tool operation efficiency was poor, and the productivity was low. The result is shown in Table 2.
According to the present invention, it is possible to manufacture a press-forming product having a desired strength with high productivity without causing a breakage or a crack during a press-forming process in a manner such that a metal sheet is heated to a transformation temperature Ac1 or more, the metal sheet is cooled to 600° C. or lower, the metal sheet is formed by a forming tool, the forming process ends at a martensite transformation start temperature Ms or more, the metal sheet is taken out from the forming tool, and the metal sheet is cooled.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-218348 | Sep 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2012/074571 | 9/25/2012 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2013/047526 | 4/4/2013 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20050257862 | Asai et al. | Nov 2005 | A1 |
| 20100095733 | Salamon et al. | Apr 2010 | A1 |
| 20130136945 | Charest et al. | May 2013 | A1 |
| Number | Date | Country |
|---|---|---|
| 101674901 | Mar 2010 | CN |
| 2002 102980 | Apr 2002 | JP |
| 2005-288528 | Oct 2005 | JP |
| 2005 329449 | Dec 2005 | JP |
| 2007 275937 | Oct 2007 | JP |
| 2010 520058 | Jun 2010 | JP |
| Entry |
|---|
| Aida, K., “New Automobile Production Line—Hot Press Forming and Laser Beam Machining”, Forum on Laser Material Processing, pp. 42-49, (2010) (with English translation). |
| International Search Report Issued Dec. 11, 2012 in PCT/JP12/074571 filed Sep. 25, 2012. |
| Written Opinion of the International Searching Authority Issued Dec. 11, 2012 in PCT/JP12/074571 filed Sep. 25, 2012. |
| Extended European Search Report issued May 4, 2015 in Patent Application No. 12837594.6. |
| Number | Date | Country | |
|---|---|---|---|
| 20140338802 A1 | Nov 2014 | US |
