JP2005271018A - Hot forming method having excellent strength after forming, and high-strength hot-formed part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot forming method having excellent strength after forming, and a high-strength hot-formed part. <P>SOLUTION: In the hot forming method having excellent strength after forming, and the high-strength hot-formed part, a steel plate having the composition consisting of, by mass, 0.05-0.55% C, 0.1 to ≤3% Mn is used, a part of a punch in contact with the steel plate is characterized in that the inner product of the advancing vector of the punch and the normal vector on the surface of the punch is positive, the steel plate is brought into contact with a die at a bottom dead center, and in some cases, cooling can be performed so that the temperature of the die is maintained at ≤300°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a member that requires strength such as that used for a structural member / reinforcing member of an automobile, and more particularly, to a method for manufacturing a part having excellent strength after high-temperature molding and a hot-formed part.

As one of the measures to improve the fuel efficiency of automobiles that originated from global environmental problems, the weight reduction of the vehicle body has been promoted, and it is necessary to increase the strength of steel plates used in automobiles as much as possible.
However, in general, when the strength of a steel sheet is increased to reduce the weight of an automobile, the elongation and the r value are lowered, and the formability is deteriorated.
In order to solve such a problem, Patent Document 1 (Japanese Patent Application Laid-Open No. 2000-234153) discloses a technique for forming the article warmly and using the heat at that time to increase the strength.
This technology aims to control the components in steel appropriately, hold and form in a temperature range of 200 to 850 ° C., and increase the strength using precipitation strengthening in this temperature range.

Patent Document 2 (Japanese Patent Laid-Open No. 2000-87183) proposes a high-strength steel sheet that has low yield strength during warm pressing and high yield strength at room temperature for the purpose of improving press forming accuracy. Yes.
However, these techniques may limit the strength that can be obtained.
For the purpose of obtaining higher strength, Patent Document 3 (Japanese Patent Laid-Open No. 2002-282951) discloses a technique for heating to a high-temperature austenite single-phase region after molding and then transforming to a hard phase in the subsequent cooling process. .
This method defines the clearance between the molds, and can introduce a refrigerant into the gap to perform quenching to obtain a component having high strength and excellent shape freezing properties. The manufacturing cost is increased by 0. Further, without using a refrigerant, the side wall shown in the example of Patent Document 3 is a mold shape perpendicular to the punch traveling direction, and the clearance is set to 1.3. When bending is performed, there is a case where the central portion of the side wall is not baked and the hardness cannot be obtained sufficiently.
This is probably because the steel plate in the side wall portion does not come into contact with the mold, so that the cooling rate is slow and quenching is insufficient. That is, there are cases where sufficient strength cannot be obtained simply by defining the mold clearance as described above.
JP 2000-234153 A JP 2000-87183 A JP 2002-282951 A

  The present invention provides a hot forming method and a hot formed part that solve the above-described problems and are excellent in strength after forming.

The present inventors conducted basic studies to solve the above problems. As a result, they found that the problem can be solved by optimizing the mold shape.
That is, for the part where strength is required, the angle between the normal direction of the surface of the punch including the side wall and the direction of travel of the punch is less than 90 °, and the die should be in contact with the steel plate at the bottom dead center. . A cooling rate sufficient for quenching can be obtained by contacting the steel plate with the mold at the bottom dead center. However, if the angle between the normal direction of the punch surface including the side wall and the direction in which the punch proceeds is 90 ° or more, it is difficult to reliably bring the steel plate into contact with the mold. It was found that the angle between the direction and the direction of punch travel should be less than 90 °. In addition, when the production number is large, it is necessary to cool the mold to keep it below the martensitic transformation end temperature of the steel sheet. Furthermore, when the strength of 1000 MPa or more is required, it is desirable to define the components of the steel sheet to be used.

That is, the gist of the present invention is as follows.
(1) Mass ratio of C: 0.05 to 0.55% and Mn: 0.1% to 3% or less of the chemical composition, the portion of the punch that contacts the steel plate is the punch direction vector and the punch surface normal vector. Is a hot forming method with excellent strength after forming, using a punch with a positive inner product and hot forming so that the steel plate contacts the mold at the bottom dead center.
(2) The hot forming method having excellent strength after forming as described in (1), wherein the mold is cooled so as to keep the temperature of the mold during hot forming at 300 ° C. or lower.
(3) A high-strength hot-formed part excellent in strength after forming, characterized by being manufactured by performing hot forming according to (1) or (2).

  According to the present invention, an automobile part having excellent strength and shape freezing property can be manufactured after hot forming, and an automobile having a lightweight vehicle body and excellent collision safety can be manufactured.

In the present invention, a hot-rolled material or a cold-rolled material having a specific chemical composition is used, but the means for producing the hot-rolled material or the cold-rolled material is not particularly limited.
Also, the hot-forming, after heating to the austenite range above Ac ₃ transformation point, to start molding at Ac ₃ transformation point or above the temperature (for example, press working), by heat removal simultaneously mold and after molding It refers to a molding process in which it is rapidly cooled, transformed into martensite and cured.

The reasons for limiting the steel components are shown below.
C is an element added to secure the material with the structure after cooling as martensite, and it is necessary to add 0.05% or more in order to secure the strength of 1000 MPa or more. However, if the addition amount is too large, it is difficult to ensure the strength during impact deformation, so the upper limit was made 0.55%.
Mn is an element that improves strength and hardenability. If it is less than 0.1%, sufficient strength at the time of quenching cannot be obtained, and even if added over 3%, the effect is saturated, so Mn is 0.1 to 3 In the range of%.
In addition, the following elements may be added as necessary.
Si is a solid solution strengthened alloy element, but if it exceeds 1%, a problem of surface scale occurs.
In addition, when plating is performed on the steel sheet surface, if the amount of Si added is large, the plating properties deteriorate, so the upper limit is preferably set to 0.5%.

Al is a necessary element used as a deoxidizer for molten steel, and is also an element that fixes N, and its amount affects the crystal grain size and mechanical properties. In order to have such an effect, a content of 0.005% or more is necessary. However, if it exceeds 0.1%, nonmetallic inclusions increase and surface defects are likely to occur in the product. For this reason, Al is desirably in the range of 0.005 to 0.1%.
S affects non-metallic inclusions in the steel, which deteriorates workability and causes increased toughness, anisotropy and reheat cracking sensitivity. For this reason, S is preferably 0.02% or less.
In addition, More preferably, it is 0.01% or less. Further, by defining S to 0.005% or less, the impact characteristics are dramatically improved.

Since P is an element that adversely affects weld cracking and toughness, P is preferably 0.03% or less.
In addition, Preferably it is 0.02% or less 0, More preferably, it is 0.015% or less.
Cr is an element that improves hardenability and has the effect of precipitating M ₂₃ C ₆ type carbide in the matrix, and has the effect of increasing the strength and miniaturizing the carbide. If it is less than 0.01%, these effects cannot be sufficiently expected, and if it exceeds 1%, the yield strength tends to increase excessively, so Cr is desirably in the range of 0.01 to 1%. More desirably, it is 0.05 to 1%.

B is added in order to improve the hardenability during press molding or cooling after press molding, but 0.0002% or more is necessary to exert this effect. However, if this amount increases excessively, there is a concern of hot cracking and the effect is saturated, so the upper limit is preferably 0.0050%.
Ti may be added for the purpose of fixing B and N to form a compound in order to effectively exhibit the effect of B.
In order to demonstrate this effect, (Ti-3.42 × N) is required to be 0.001% or more. However, if the amount of Ti increases excessively, the amount of C not bonded to Ti decreases, and sufficient strength is obtained after cooling. As the upper limit, the amount of C not bound to Ti is 0.1%.
The Ti equivalent that can be secured above, that is, 3.99 × (C−0.1)% is desirable.
Elements such as Ni, Cu and Sn that are considered to be mixed from scrap may be contained. Furthermore, Ca, Mg, Y, As, Sb, and REM may be added from the viewpoint of shape control of inclusions. Ti, Nb, Zr, Mo, V may be added for the purpose of further improving the strength. However, if these elements increase excessively, the amount of C not bonded to these elements decreases, and sufficient strength is obtained after cooling. Therefore, when added, it is desirable that C · 12 × (Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51) ≧ 0.1 is satisfied.

N is not particularly specified, but if it exceeds 0.01%, the toughness tends to deteriorate due to coarsening of nitride and age hardening due to solute N. For this reason, the N content is desirably 0.01% or less.
O is not particularly specified, but excessive addition causes generation of an oxide that adversely affects toughness and generates an oxide that becomes a starting point of fatigue fracture. Therefore, the content is preferably 0.015% or less.
In addition, even if impurities inevitably contained, no particular problem occurs.
The steel plate having the above components may be subjected to aluminum plating, aluminum / zinc plating, or galvanization. As for the production method, pickling and cold rolling may be performed by a conventional method, and thereafter, the aluminum plating step, the aluminum-zinc plating step, and the galvanizing may be performed by a conventional method.
In other words, 5 to 12% is suitable for the Si concentration in the bath for aluminum plating, and 40 to 50% for the zn concentration in the bath for aluminum-zinc plating. Moreover, even if Mg or Zn is mixed in the aluminum plating layer or Mg is mixed in the aluminum-zinc plating layer, a steel plate having the same characteristics can be produced without any particular problem.

In addition, about the atmosphere in a plating process, even if it is a continuous type plating equipment which does not have a non-oxidation furnace even if it is a continuous type plating equipment which has a non-oxidation furnace, it can plate by making it a normal condition. Since this steel plate does not require special control, productivity is not hindered. Further, as long as it is a galvanizing method, any method such as hot dip galvanizing, electrogalvanizing, alloying hot dip galvanizing may be adopted.
Under the above manufacturing conditions, metal pre-plating is not performed on the steel sheet surface before plating, but there is no particular problem even if Ni pre-plating, Fe pre-plating, or other metal pre-plating that improves plating properties is performed. Moreover, there is no particular problem even if different metal plating or a film of inorganic or organic compound is applied to the surface of the plating layer.

The inner product of the punch direction vector and the normal vector of the punch surface is positive. If this value is negative, the punch will undercut and the die will be sure to be at the bottom dead center regardless of the die shape. This is because the steel plate cannot be brought into contact with the mold. If this value is 0, the plate can be brought into contact with the die at the bottom dead center if the plate thickness and the mold clearance are the same. It is difficult to make sure that the steel plate is brought into contact with the mold at the bottom dead center by changing (for example, when the plate thickness is reduced due to elongation deformation).
In this case, the clearance may be set to be equal to or less than the plate thickness. In this case, there is a possibility that galling and wrinkles occur on the surface of the steel plate and the mold life is shortened.

If the inner product of the punch traveling direction vector and the normal vector of the punch surface is positive for the above reason, the steel plate can be reliably brought into contact with the mold at the bottom dead center.
The reason why the steel plate is in contact with the mold at the bottom dead center is that a sufficient cooling rate necessary for quenching cannot be obtained unless the steel plate is in contact at the bottom dead center.
If hot forming is performed using a mold having the above shape, it is possible to manufacture a part that is sufficiently quenched and has increased strength. However, when it is desired to provide a soft part in the part without quenching and hardening, the part may not have the mold shape shown in claim 1.

When the processing interval is short, the mold temperature rises, the cooling rate becomes slow, and the strength of the part may not be ensured without martensitic transformation. In such a case, mold cooling may be performed. In this case, the mold temperature should be 300 ° C. or lower, preferably 200 ° C. or lower. When the mold temperature is 300 ° C. or higher, martensitic transformation does not occur during hot working, and curing may be insufficient.
The cooling method is not particularly defined, but a method of water cooling piping in the mold, a method of securing the volume of the mold to increase the heat capacity, a method of cooling the mold surface with a refrigerant, and the like may be adopted.

Slabs with chemical components shown in Table 1 were cast. These slabs were heated to 1050 to 1350 ° C. and hot rolled to form hot rolled steel sheets having a finishing temperature of 800 to 900 ° C. and a winding temperature of 450 to 680 ° C. and a thickness of 4 mm.
Some hot-rolled steel sheets were cold-rolled into cold-rolled steel sheets having a thickness of 1.4 mm. Moreover, hot-dip aluminum plating, hot-dip aluminum-zinc plating, and alloying hot-dip galvanization were performed on a part of the cold-rolled sheet.
Then, after heating those cold-rolled steel sheets and surface-treated steel sheets to an austenite region of 950 ° C that is Ac ₃ point or higher by furnace heating, from a 900 ° C that is Ac ₃ point or more to a press machine having a water-cooled mold Hot forming was performed.

Figures 1, 2, 3, 4, and 5 show the mold shapes. A schematic diagram of the molded product is shown in FIG.
The die was shaped like a die with a clearance of 1.4 mm, following the punch shape, but for parts where undercut occurs, the die shapes A to D are columnar shapes, and the length of the part is 300mm. The molding conditions for the parts having shapes A to D were a blank size of 300 mm × 244 mm, a punch speed of 10 mm / s, a pressing force of 150 tons, and a holding time at the bottom dead center of 10 seconds. The part of the mold shape E has a disk shape.
The molding conditions here were a blank size of 180 mm in diameter, a wrinkle holding force of 20 tons, a punch speed of 10 mm / s, a pressing force of 150 tons, and a holding time at the bottom dead center of 10 seconds. Thereafter, the part was cut out and the structure was observed, and the martensite passed 80% or more of the structure. Vickers hardness was measured for those requiring strength of 1000 MPa or more.
After heating the steel sheet before hardness molded into the austenite region of 950 ° C. is Ac ₃ points or more, based on the hardness of when water quenched from 900 ° C. is Ac ₃ points or more, sites strength is required Was rejected if there was a part with a hardness of 70% or less. The experimental results are shown in Table 2.

実験番号1〜40は金型形状の影響を検討したものである。実験番号1〜24については側壁の角度の検討を行ったものであるが、実験番号9〜24はパンチの鋼板に接触する部分が、パンチの進行方向ベクトルとパンチ表面の法線ベクトルとの内積が負である部位があるため、鋼板が金型に接触せずに冷却速度が遅くなり、硬度が不十分な部位があった。実験番号25〜32は側壁部に凹みをつけた形状であるが、凹み部はパンチの鋼板に接触する部分が、パンチの進行方向ベクトルとパンチ表面の法線ベクトルとの内積が負であるため、鋼板が金型に接触せずに冷却速度が遅くなり、硬度が不十分な部位があった。実験番号1〜8は強度が必要とされる部分について、パンチの鋼板に接触する部分がパンチの進行方向ベクトルとパンチ表面の法線ベクトルとの内積が正であり、金型と鋼板が下死点で接触するため、部品全体が充分硬化した。 Moreover, the continuous molding test was done about several metal mold | dies. The mold temperature was changed by passing water-cooled piping through the mold and changing the amount of water. Subsequent evaluation was performed by the cross-sectional hardness and the structure observation. The results are shown in Table 3. Tables 4 and 5 show the legends on the results of microstructure observation and hardness.

Experiment numbers 1 to 40 examine the influence of the mold shape. In Experiment Nos. 1 to 24, the angle of the side wall was examined. In Experiment Nos. 9 to 24, the portion of the punch that contacts the steel plate is the inner product of the punch traveling direction vector and the punch surface normal vector. Since there is a portion where is negative, the steel plate does not contact the mold, the cooling rate is slow, and there is a portion where the hardness is insufficient. Experiment Nos. 25 to 32 have a shape with a dent on the side wall, but the dent is the portion where the punch contacts the steel plate because the inner product of the punch traveling direction vector and the punch surface normal vector is negative. The steel plate did not come into contact with the mold, and the cooling rate was slow, and there was a portion with insufficient hardness. In Experiment Nos. 1-8, the inner product of the punch traveling direction vector and the punch surface normal vector is positive for the part where strength is required, and the part that contacts the punch steel sheet is dead. The entire part was fully cured due to point contact.

Experiment Nos. 33 to 40 were formed by ball head overhanging, but the inner product of the punch traveling direction vector and the punch surface normal vector was positive at the part contacting the steel plate of the punch. Contacted at the bottom dead center, so the entire part was fully cured.
Experiment Nos. 41 to 64 examine the influence of mold temperature.
In Experiment Nos. 45, 46, 51, 52, 57, 58, 63, and 64, the mold temperature was above the limit, so there was a portion where the martensite transformation did not proceed sufficiently and the hardness was insufficient.
In other experiments, because the mold temperature was within the limits of the present invention, the entire part was fully cured.

It is a figure which shows the metal mold | die (die A) of the Example of this invention. It is a figure which shows the metal mold | die (metal mold | die B) of the comparative example of this invention. It is a figure which shows the metal mold | die (die C) of the comparative example of this invention. It is a figure which shows the metal mold | die (metal mold | die D) of the comparative example of this invention. It is a figure which shows the metal mold | die (die E) of the Example of this invention. It is a figure which shows the molded article by metal mold | die AE in this invention.

Claims (3)

  A steel plate containing chemical components of C: 0.05 to 0.55% and Mn: 0.1% to 3% or less in mass%, where the punch contact direction vector and the punch surface normal vector A hot forming method with excellent strength after forming, characterized by using a punch with a positive inner product and performing hot forming so that the steel sheet contacts the mold at the bottom dead center.   2. The hot forming method with excellent strength after forming according to claim 1, wherein cooling is performed so that the temperature of the mold during hot forming is maintained at 300 ° C. or lower.   A high-strength hot-formed part excellent in strength after forming, wherein the hot-formed part is manufactured by performing hot forming according to claim 1 or 2.

Images