Patent Grant
9315876
The present invention pertains to the filed of producing thin steel sheet formed products to be applied mainly to automobile bodies, and more specifically, the present invention relates to a method for producing press-formed products by heating a steel sheet (blank) as their material to a temperature not lower than an austenite temperature (Ac3 transformation point) thereof and then press-forming the steel sheet into a prescribed shape, in which the steel sheet can be given the shape and at the same time hardened to have prescribed hardness, as well as to press-formed products and others obtained by such a production method. In particular, the present invention relates to a method for producing press-formed products, which makes it possible to achieve favorable forming without causing fracture, crack, or any other defects during the press-forming, as well as to press-formed products and others.
From the viewpoint of global environment protection, automobile lightening has strongly been desired for the purpose of making fuel-efficient automobiles. When a steel sheet is used for parts composing a vehicle, lightening has been attempted by applying a high-strength steel sheet and reducing the sheet thickness of this steel sheet. On the other hand, to improve the collision safety of automobiles, further strengthening has been required for automobile parts, such as pillars, and there has been an increasing need for ultrahigh-strength steel sheets having higher tensile strength.
However, when thin steel sheets are made to have higher strength, the elongation EL or r value (Lankford value) thereof is lowered, resulting in the deterioration of press formability or shape fixability.
Under these circumstances, to realize high-strength structural parts for automobiles, a hot pressing method (a so-called “hot press method”) has been proposed (e.g., Patent Document 1), in which both press-forming and improving the strength of parts by hardening are achieved at the same time. This technique is a method in which a steel sheet is heated up to an austenite (γ) region not lower than an Ac3 transformation point thereof and then hot press-formed, during which the steel sheet is simultaneously hardened by being brought into contact with a press tool at ordinary temperature, to realize ultrahigh strengthening.
According to such a hot pressing method, the steel sheet is formed in a state of low strength, and therefore, the steel sheet exhibits decreased springback (favorable shape fixability), resulting in the achievement of a tensile strength in the 1500 MPa class by rapid cooling. In this regard, such a hot pressing method has been called with various names, in addition to a hot press method, such as a hot forming method, a hot stamping method, a hot stamp method, and a die quenching method.
When a steel sheet is hot pressed (e.g., subjected to hot deep drawing) with such a press tool, the forming is started in a state where a blank (steel sheet 4) is softened by heating to a temperature not lower than an Ac3 transformation point thereof. That is, steel sheet 4 is pushed into a cavity of die 2 (between the parts indicated by reference numerals 2 and 2 in
In the conventional hot pressing, a steel sheet is heated up to an austenitic region (e.g., about 900° C.) not lower than an Ac3 transformation point thereof, and the steel sheet is then cooled by a press tool for press-forming while being kept in a high-temperature state. Therefore, the steel sheet may easily have a temperature difference between its portion coming into contact with, and its portion not coming into contact with, the press tool composed of a punch and a die, so that strain may be concentrated on its portion becoming relatively high temperature, or so that, for example, in deep drawing, a shrink flange becomes unshrinkable by cooling, both resulting in the deterioration of formability, and in particular, thereby making it difficult to achieve deep drawing.
In view of such problems, a so-called indirect method has been proposed, in which a steel sheet is formed into a near net (in a state close to a formed product) by cold pressing and the near net is then heated and die-quenched. This method, however, has a defect that the forming time is lengthened because of its increased forming steps. Therefore, presently there has been a demand for some technique in which deep drawing can be made by the so-called direct method not including so many forming steps.
Furthermore, in the hot pressing, a steel sheet is cooled while being press-formed with a press tool, and therefore, the cooling rate may vary in the blank depending on the state of its contact with the press tool. This may cause a variation in the hardness distribution (uneven hardening) of a portion that has undergone hot pressing, resulting in a problem in quality.
The present invention has been made in view of the above-described circumstances, and its object is to provide a method for producing a useful method for producing press-formed products without causing disadvantages such as hardness variation, which products have favorable formability in a level so as to be able to be produced by deep drawing, as well as press-formed products obtained by such a production method.
The method of the present invention for producing a press-formed product, which method was able to achieve the object described above, is characterized in that when a formed product is produced by press-forming a thin steel sheet with a punch and a die, the thin steel sheet is heated to a temperature not lower than an Ac3 transformation point of the this steel sheet, and the thin steel sheet is then cooled at a rate not lower than a critical cooling rate, during which the thin steel sheet is formed into the formed product, wherein the forming is started from a temperature higher than a martensitic transformation start temperature Ms thereof, the cooling rate is kept to be 10° C./sec. or higher during the forming, and the forming is finished in a temperature range not higher than the martensitic transformation start temperature Ms.
In the method of the present invention, when the thin steel sheet is cooled before the start of the forming, there may be adopted, for example, a) gas-jet cooling or b) bringing the thin steel sheet into contact with a cooled metal roll. In addition, the cooling rate of the thin steel sheet before the start of the forming may be 25° C./sec. or higher. Furthermore, the cooling rate during the forming may preferably be 30° C./sec. or higher.
The finish temperature of the forming may preferably be set to a temperature higher than a martensitic transformation finish temperature Mf thereof. In addition, the method of the present invention is particularly effective when the forming is carried by drawing with a blank holder. Even if such a forming method is adopted, favorable formability can be secured without causing fracture or crack. The press-formed product obtained by the method of the present invention may have a Vickers hardness Hv of 450 or higher.
According to the present invention, it became possible the production of press-formed products in high productivity without causing fracture, crack, or any other defects during the forming because a steel sheet is cooled at a rate not lower than a critical cooling rate, during which the steel sheet is formed into the formed product, wherein the forming is started from a temperature higher than a martensitic transformation start temperature Ms thereof, the cooling rate is kept to a prescribed cooling rate during the forming, and the forming is finished in a temperature range not higher than the martensitic transformation start temperature Ms.
The present inventors have studied from various angles to produce press-formed products having favorable formability without causing disadvantages such as hardness variation when a thin steel sheet is heated to a temperature not lower than an Ac3 transformation point thereof and then press-formed As a result, they have found that favorable formability can be secured without causing disadvantages such as hardness variation, if a thin steel sheet is heated to a temperature not lower than an Ac3 transformation point thereof, and then press-forming is not immediately started, but the thin steel sheet is cooled at a rate not lower than a critical cooling rate, during which the thin steel sheet is formed into the formed product, wherein the press-forming is started from a temperature higher than a martensitic transformation start temperature Ms thereof, the cooling rate is kept to a prescribed cooling rate during the forming, and the forming is finished in a temperature range not higher than the martensitic transformation start temperature Ms, thereby completing the present invention. The following will specifically explain the present invention along the background of how the present invention has been completed.
The present inventors have made a square tube drawing experiment in which a steel sheet with a chemical element composition shown in Table 1 below is first heated to 900° C. (this steel sheet has an Ac3 transformation point of 830° C., a martensitic transformation start temperature Ms of 411° C., and a martensitic transformation finish temperature Mf of 261° C.) and then subjected to square cup drawing by the above-described procedure with a press tool shown in
The Ac3 transformation point described above means an austenite transformation completion temperature Ac3 when a steel sheet is heated, and it can be calculated by formula (1) below. In addition, the martensitic transformation start temperature Ms and martensitic transformation finish temperature Mf are values calculated by formulae (2) and (3), respectively (see, e.g., “Heat Treatment,” 41(3), 164-169, 2001, Tatsuro KUNITAKE, “Prediction of Ac1, Ac3, and Ms Transformation Points of Steel by Empirical Formulae”).
Ac3 transformation point (° C.)=−230.5×[C]+31.6×[Si]−20.4×[Mn]−39.8×[Cu]−18.1×[Ni]−14.8×[Cr]+16.8×[Mo]+912 (1)
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]} (2)
Mf(° C.)=Ms−150.0 (3)
where [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] indicate C, Si, Mn, Cu, Ni, Cr, and Mo contents (wt %), respectively.
In the convention hot forming, it has been considered as the common general technical knowledge to start the forming at as high a temperature as possible. In contrast, a steel sheet is once heated and then rapidly cooled down to a temperature higher than a martensitic transformation start temperature Ms thereof at a rate not lower than a critical cooling rate to put it into a state liable to cause martensitic transformation, after which press-forming is started and the forming is finished in a temperature range not higher than the martensitic transformation start temperature Ms, resulting in the improvement of drawing formability. This seems to be because the occurrence of martensitic transformation during the press-forming causes transformation plasticity phenomenon to make deformation strain small.
To clarify the mechanism of the present invention, the following simulations (tensile tests) were carried out to study the influence of martensitic transformation on deformation behavior in the deformation process. The heat-treatment pattern at that time is shown in
As shown in
When press-forming is carried out under the conditions as described above, mechanical material characteristics during the press-forming become uniform material strength, while keeping a high n value, and material ductibility can also be secured; therefore, deep drawing formability can also be improved. In addition, the press-forming start temperature can also be set to a relatively low temperature, so that holding time at the lower dead point in the forming can be shortened, thereby making it possible to improve productivity.
The method of the present invention applies the fundamentals that a steel sheet is heated up to a temperature not lower than an Ac3 transformation point thereof and then rapidly cooled down to a prescribed temperature whereby the steel sheet is put into a state liable to cause martensitic transformation before forming and make effective progress in the martensitic transformation during the forming. To allow the steel sheet to exhibit such an effect, the cooling rate after heating up to a temperature not lower than the Ac3 transformation point should be set to a rate (25° C./sec. or higher for the steel sheet shown in Table 1) not lower than a critical cooling rate (i.e., lower critical cooling rate). That is, depending on the kind of steel, when the cooling rate becomes lower than the critical cooling rate, martensitic transformation itself hardly occurs and it becomes difficult to effectively exhibit the effect (press formability improvement effect) due to martensitic transformation. In addition, the upper limit of the cooling rate during the rapid cooling is not particularly limited, but it may preferably be set to be 450°/sec. or lower from the viewpoint of securement of temperature uniformity in the blank.
To secure favorable formability by allowing the steel sheet to cause martensitic transformation during the forming, the cooling rate should be secured to be 10° C./sec. or higher, more preferably 30° C./sec. or higher, even during the forming.
By the way, the conventional hot press line (equipment structure) generally has a structure as shown in
In the present invention, a thin steel sheet is heated to a temperature not lower than an Ac3 transformation point thereof, and then the forming is not immediately started, but the thin steel sheet is rapidly cooled down to a temperature higher than a martensitic transformation start temperature Ms thereof, so that the thin steel sheet is put into a state liable to cause martensitic transformation, after which press-forming is started. When such cooling is carried out, an equipment structure may be adopted, such as shown in
(1) Gas-jet cooling is carried out with a gas cooling means.
(2) Heat is removed with a means for bringing the steel sheet into contact with a metal as a cooling medium (e.g., water-cooled metal roll).
(3) Cooling is carried out with a mist cooling means.
(4) Cooling is carried out with a dry ice shot means (the blank material is cooled by allowing dry ice granules to impinge thereon).
The steel sheet is cooled down to a prescribed temperature in cooling zone 15 as described above and then moved to press-forming machine 13, in which the steel sheet may be formed, while being cooled with a press tool, subsequently to the start of the forming.
When the method of the present invention is carried out, a thin steel sheet should first be heated to a temperature not lower than an Ac3 transformation point thereof. The upper limit of the heating temperature may preferably not be allowed to exceed approximately 1000° C. When the heating temperature becomes higher than 1000° C., the formation of oxide scales becomes significant (e.g., 100 μm or greater), and therefore, formed products (after descaling) are likely to have smaller sheet thickness than the prescribed one.
In the present invention, the forming should be started from a temperature higher than a martensitic transformation start temperature Ms of a steel sheet, and the forming should be finished in a temperature range not higher than the martensitic transformation start temperature Ms. With respect to the forming finish temperature, since formability rather becomes worse, if martensitic transformation is completely finished during the forming, this temperature (forming finish temperature) may preferably be set to a temperature higher than a martensitic transformation finish temperature Mf thereof.
The method of the present invention can achieve the above-described object by appropriately controlling the forming start temperature, forming finish temperature, and cooling rates (before forming and during the forming). Such an effect becomes prominently exhibited when formed products having complicated shapes are formed (i.e., formed by deep drawing) with a press tool having a blank holder. In this regard, however, the method of the present invention is not limited to drawing with a blank holder, but includes the case where ordinary press-forming (e.g., stretch forming) is carried out, and the effect of the present invention can be achieved even in the case where formed products are produced by such a method.
The following will describe the present invention in detail by way of Examples, but the present invention is not limited to the Examples described below. The present invention can be put into practice after appropriate modifications or variations within a range capable of meeting the gist described above and below, all of which are included in the technical scope of the present invention.
Steel with a chemical element composition shown in Table 1 above was cold-rolled to have a thickness of 1.4 mm by an ordinary means. This steel sheet was punched out into round blanks having diameters (blank diameters) of from 90 mm to 110 mm for experiments (therefore, these blanks had an Ac3 transformation point of 830° C., a martensitic transformation start temperature Ms of 411° C., and a martensitic transformation finish temperature Mf of 261° C.).
The round blanks were subjected to square cup drawing with a press tool, in which the head shape of a punch was square (45 mm on a side), (i.e., a square cup die and a square cup punch), (see
The forming experiments were carried out with a press tool shown in
In experiments Nos. 1 to 7, the forming time was set in such a manner that the blanks came to have temperatures not higher than the martensitic transformation start temperature Ms after the finish of the forming. In experiments Nos. 8 to 17, the forming time was set in such a manner that the blanks came to have temperatures higher than the martensitic transformation start temperature Ms after the finish of the forming. The respective forming times (pressing times) were set on the basis of the cooling rate (50° C./sec.) of the press tool separately calculated. The blanks were cooled at a cooling rate of 25° C./sec. by blowing cold air from the heating temperature to the forming start temperature. The other press-forming conditions were as described below.
(The Other Press-Forming Conditions)
Blank holding force: 3 tons
Die shoulder radius rd: 5 mm
Punch shoulder radius rp: 5 mm
Clearance CL between punch and die: 1.32/2+1.4 (steel sheet thickness) mm
Forming height: 37 mm
The results are shown in Table 2. As can be seen from these results, it was confirmed that when the forming was started from a temperature higher than the martensitic transformation start temperature Ms and the forming was finished in a temperature range not higher than the martensitic transformation start temperature Ms (experiment Nos. 1 to 7), the forming was able to be made to a larger blank diameter (formable blank diameter) and favorable formability was exhibited.
The appearance configuration of a formed product which could have undergone favorable forming is schematically shown in
The method of the present invention includes heating a thin steel sheet to a temperature not lower than an Ac3 transformation point thereof and then cooling the thin steel sheet at a rate not lower than a critical cooling rate, during which the thin steel sheet is formed into a press-formed product, wherein the forming is started from a temperature higher than a martensitic transformation start temperature Ms thereof, the cooling rate is kept to be 10° C./sec. or higher during the forming, and the forming is finished in a temperature range not higher than the martensitic transformation start temperature Ms. Thus, the method of present invention makes it possible to produce press-formed products without causing disadvantages such as hardness variation, which product has favorable formability in a level so as to be able to be produced by deep drawing.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-222943 | Sep 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/072672 | 9/30/2011 | WO | 00 | 3/28/2013 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2012/043837 | 4/5/2012 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 7137201 | Brodt et al. | Nov 2006 | B2 |
| 7559998 | Nishibata et al. | Jul 2009 | B2 |
| 8021497 | Fleischanderl et al. | Sep 2011 | B2 |
| 20030129444 | Matsuoka et al. | Jul 2003 | A1 |
| 20040096352 | Ozaki et al. | May 2004 | A1 |
| 20050257862 | Asai et al. | Nov 2005 | A1 |
| 20060185774 | Nishibata et al. | Aug 2006 | A1 |
| 20130136945 | Charest et al. | May 2013 | A1 |
| 20140338802 | Okita et al. | Nov 2014 | A1 |
| Number | Date | Country |
|---|---|---|
| 1698993 | Nov 2005 | CN |
| 101619383 | Jan 2010 | CN |
| 1642991 | Apr 2006 | EP |
| 61-243127 | Oct 1986 | JP |
| 2002-102980 | Apr 2002 | JP |
| 2004-124221 | Apr 2004 | JP |
| 2006-224162 | Aug 2006 | JP |
| 2007-275937 | Oct 2007 | JP |
| Entry |
|---|
| International Search Report from the International Bureau, in corresponding International Application No. PCT/JP2011/072672, mailed Jan. 10, 2012, pp. 1-2 and Written Opinion of International Searching Authority, pp. 1-3. |
| T. Kunitake, “Prediction of Ac1, Ac3 and Ms Transformation Points of Steel by Empirical Formulae”, Heat Treatment, 41(3), 164-169, 2001, 2 pages. |
| Number | Date | Country | |
|---|---|---|---|
| 20130180635 A1 | Jul 2013 | US |
