JP2006104527A - Method for producing high strength component and high strength component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a member requiring strengths used for automobile structural members and reinforcing members, to provide a component having excellent strengths particularly after high temperature forming, and to provide a production method therefor. <P>SOLUTION: In the subject method for producing a high strength component, at the time when a steel sheet having a composition comprising, by mass, 0.05 to 0.55% C, ≤2% Si, 0.1 to 3% Mn, ≤0.1% P and ≤0.03% S, and the balance Fe with inevitable impurities composed of Al, Cr, N, Ti, B, or the like, is heated to Ac<SB>3</SB>to a melting point in an atmosphere having a hydrogen content of ≤10% in a volume fraction and a dew point of ≤30°C, thereafter, press forming is started at a temperature higher than a temperature at which ferritic, pearlitic and martensitic transformations occur, and, after the forming, in a die, it is cooled and quenched, so as to produce a component having high strengths, the cooling rate in a part of the component is reduced, thus the part having strengths lower than those of the other part is provided, and the part is subjected to shearing working. The high strength component is produced by this method. <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 or a reinforcing member of an automobile, and particularly relates to a component having excellent strength after high-temperature molding and a manufacturing method thereof.

  In order to reduce the weight of automobiles that originate in global environmental problems, it is necessary to increase the strength of steel sheets used in automobiles as much as possible. Generally, as steel sheets are increased in strength, the elongation and r value decrease. However, the moldability deteriorates. In order to solve such a problem, Japanese Patent Application Laid-Open No. 2000-234153 (Patent Document 1) discloses a technique for forming a product warm and using the heat at that time to increase the strength. This technique aims to appropriately control the components in the steel, heat in the ferrite temperature range, and increase the strength using precipitation hardening in this temperature range.

  Japanese Patent Application Laid-Open No. 2000-87183 (Patent Document 2) proposes a high-strength steel sheet that lowers the yield strength at the forming temperature to be higher than the yield strength at room temperature for the purpose of improving the press forming accuracy. However, these techniques may limit the strength that can be obtained. On the other hand, for the purpose of obtaining higher strength, Japanese Patent Laid-Open No. 2000-38640 (Patent Document 3) proposes a technique of heating to a high-temperature austenite single-phase region after molding and then transforming to a hard phase in the subsequent cooling process. ing.

  However, if heating and rapid cooling are performed after molding, there may be a problem in shape accuracy. As a technique for overcoming this drawback, a technique in which a steel sheet is heated to an austenite single-phase region and then cooled in a press forming process is disclosed in the literature (SAE, 2001-01-0078) (Non-patent Document 1) and Japanese Patent Laid-Open No. 2001. No. 181833 (Patent Document 4).

JP 2000-234153 A JP 2000-87183 A JP 2000-38640 A JP 2001-181833 A JP 2003-328031 A Literature (SAE, 2001-01-0078)

  Thus, as the strength of high-strength steel sheets used in automobiles and the like increases, hydrogen embrittlement as previously known in the above-mentioned problems of formability and particularly in high-strength materials exceeding 1000 MPa. There is an essential problem (sometimes called cracking or delayed fracture). When used as a hot-press steel sheet, although there is little residual stress due to pressing at high temperatures, hydrogen penetrates into the steel during heating before pressing, and it is highly susceptible to hydrogen embrittlement due to residual stress in the subsequent process. Become. Therefore, simply pressing at a high temperature does not solve the essential problem, and it is necessary to optimize the process conditions in the integrated process from the heating process to the post-processing.

  In order to reduce the residual stress at the time of post-processing such as shearing, the strength of the portion to be post-processed should be lowered. Japanese Patent Application Laid-Open No. 2003-328031 (Patent Document 5) discloses a technique for reducing the cooling rate of a part where a post-process is performed to make quenching insufficient and reducing the strength of the part. According to this method, the strength of a part of the component is reduced, and post-processing such as shearing can be easily performed. However, this method does not mention hydrogen embrittlement at all, and even if the steel sheet strength is slightly reduced by this method and the residual stress after post-processing is reduced to some extent, If hydrogen remains, the possibility of hydrogen embrittlement cannot be denied. The present invention has been made to solve the above-described problems, and it is an object of the present invention to provide a high-strength part excellent in hydrogen embrittlement resistance and capable of obtaining a strength of 1200 MPa or more after high-temperature molding and a method for producing the same.

The present inventors have conducted various studies to solve the above problems. As a result, in order to suppress hydrogen embrittlement, it is necessary to reduce the cooling rate of the part where post-processing is performed to make quenching insufficient and to control the atmosphere in the heating furnace before molding. I found out.
That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.05 to 0.55%, Si: 2% or less, Mn: 0.1 to 3%, P: 0.1% or less, S: 0.03% or less Then, using a steel plate made of the remainder Fe and inevitable impurities such as Al, Cr, N, Ti, B, in an atmosphere where the hydrogen content is 10% or less in volume fraction and the dew point is 30 ° C. or less, Ac ₃ After heating the steel plate to the melting point, start press forming at a temperature higher than the temperature at which ferrite, pearlite, bainite, martensite transformation occurs, and after forming, cool and quench in the mold to form high-strength parts A method for producing a high-strength component, comprising: lowering a cooling rate of a part of a component to provide a lower-strength portion than the other portion, and shearing the portion.

(2) In the method described in (1) above, as a method for reducing the cooling rate of a part of the component, a gap is provided in the same position of the upper mold and the lower mold in a part of the mold. To manufacture high-strength parts.
(3) In the method described in (1) above, a method for manufacturing a high-strength component, wherein a heat insulating material is incorporated in a part of a mold as a method for reducing the cooling rate of a part of the component.
(4) The high-strength part manufactured by the method described in (1) to (3) above.

  As described above, according to the present invention, when a high-strength part is manufactured by cooling in a mold after molding and manufacturing a high-strength part, an automobile having a lightweight vehicle body and excellent collision safety can be manufactured. Social contribution is great.

Below, the limited range of this invention is demonstrated in detail.
The amount of hydrogen was set to 10% or less in the volume fraction. When the amount of hydrogen was more than the limit, the amount of hydrogen entering the steel sheet during heating became large, and the hydrogen embrittlement resistance was deteriorated. Because. The reason why the dew point in the atmosphere is 30 ° C. or lower is that when the dew point is higher than this, the amount of hydrogen that enters the steel sheet during heating becomes large, and the hydrogen embrittlement resistance is deteriorated. The reason why the heating temperature Ac _{3 of the} steel sheet is set to be not lower than the melting point and not higher than the melting point is to keep the structure of the steel sheet to be austenite for strengthening by quenching after forming. Moreover, it is because press molding is impossible when heating temperature is more than melting | fusing point.

  This is because when the molding start temperature is set to a temperature higher than the temperature at which ferrite, pearlite, bainite, and martensite transformation occurs, when molding is performed at a temperature lower than that temperature, the hardness after molding is insufficient. The reason why the cooling rate of a part of the component is reduced is that when the cooling rate decreases, the martensite transformation rate decreases, or when the cooling rate is slower, ferrite, bainite, and pearlite transformation occur. This is because the strength is reduced as compared with the mold quenching portion.

  Any method may be used as a method of reducing the cooling rate of a part of the component, but industrially, the method shown in claims 2 and 3 may be used. The reason why the gap is provided in the same position of the upper mold and the lower mold in a part of the mold is that the cooling rate is lowered because the mold does not contact with the mold during hot forming. However, when no gap is provided at the same position in the upper mold and the lower mold, the cooling rate does not decrease and the strength is not sufficiently decreased. The reason why the heat insulating material is incorporated into a part of the mold in claim 3 is that even when the heat insulating material is hot-molded as a tool and then the mold is quenched, the cooling rate is reduced. This is because the strength decreases.

The restrictions on the material will be described below.
C: 0.05-0.55%
C is an element added to secure the material with the cooled structure as martensite. To ensure a strength of 1000 MPa or more, 0.05% or more, preferably 0.1% or more may be added. desirable. However, if the addition amount is too large, it is difficult to ensure the strength during impact deformation, so the upper limit is made 0.55%. Therefore, the range was made 0.05 to 0.55%.

Si: 2% or less Si is a solid solution strengthening type alloy element. However, if it exceeds 2%, a problem of surface scale occurs, so it is limited to 2% or less. In addition, when plating is performed on the steel sheet surface, if the amount of Si added is large, the plateability deteriorates, so the upper limit is desirably set to 0.5%.
Mn: 0.1 to 3%
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. It is limited to a range of 0.1 to 3%.

P: 0.1% or less Since P is an element that adversely affects weld cracking and toughness, P is limited to 0.1% or less. In addition, Preferably it is 0.02% or less. Further, it is more preferably 0.015% or less.
S: 0.03% or less S affects non-metallic inclusions in steel and degrades workability and causes toughness deterioration, anisotropy and increased reheat cracking sensitivity. For this reason, S is limited to 0.03% or less. In addition, More preferably, it is 0.01% or less. Moreover, by restricting S to 0.005% or less, impact characteristics are dramatically improved.

In addition, the following elements may be added as necessary.
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%. However, Al is not limited as a deoxidizing element, and other deoxidizing elements such as Si may be used. In that case, the amount of Al added may be 0.005% or less.

Cr is an element that improves hardenability, and has the effect of precipitating M ₂₃ C ₆ type carbide into the matrix, and has the effect of increasing the strength and miniaturizing the carbide. However, if less than 0.01%, these effects cannot be expected sufficiently, 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 to improve the hardenability during press molding or cooling after press molding, but 0.0002% or more of addition 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 that forms a compound in order to effectively exhibit the effect of B. In order to exert this effect, 0.001% or more of (Ti-3.42 × N) is necessary. However, if the amount of Ti is increased unnecessarily, the amount of C that is not bonded to Ti decreases and is sufficient after cooling. Therefore, the upper limit is preferably the Ti equivalent that can secure 0.1% or more of the amount of C not bonded to Ti, that is, 3.99 × (C−0.1)%. .

  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 inclusion morphology control. Furthermore, Ti, Nb, Zr, Mo, and V may be added for the purpose of improving the strength. However, if these elements increase unnecessarily, the amount of C that is not bonded to these elements decreases and is sufficient after cooling. A sufficient strength cannot be obtained.

N is not particularly restricted, 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 restricted, 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, there is no problem even if impurities inevitably included. 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. That is, 5 to 12% of the Si concentration in the bath is suitable for aluminum plating, and 40 to 50% of the Zn concentration in the bath is suitable for aluminum-zinc plating. 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 manufactured without any particular problem.

  As for the atmosphere in the plating process, it is possible to perform plating under normal conditions in either a continuous plating facility with a non-oxidizing furnace or a continuous plating facility without a non-oxidizing furnace, and only this steel plate needs special control. It does not hinder productivity. Moreover, as long as it is a galvanization method, what kind of methods, such as hot dip galvanization, electrogalvanization, and alloying hot dip galvanization, may be taken. Under the above manufacturing conditions, metal pre-plating is not performed on the steel plate 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. Further, there is no particular problem even if different metal plating or a coating of inorganic or organic compound is applied to the surface of the plating layer.

Hereinafter, the present invention will be specifically described with reference to examples.
Slags of 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. Then, after pickling, it was set as the cold steel plate of 1.6 mm thickness by cold rolling. Further, a part of the cold-rolled plate was subjected to hot-dip aluminum plating, hot-dip aluminum-zinc plating, alloyed hot-dip galvanizing, and hot-dip galvanizing. Table 2 shows the legend of plating types. Thereafter, the cold-rolled steel sheet and the surface-treated steel sheet were heated in an austenite region at 950 ° C., which is at least Ac ₃ point, by furnace heating, and then hot-formed. The atmosphere of the heating furnace changed the amount of hydrogen and the dew point. The conditions are shown in Tables 3 to 9. Molding was performed using mold A and mold B. The hydrogen embrittlement resistance in the piercing process using the mold A and the trim process using the mold B was evaluated.

  FIG. 1 is a view showing a cross section of a mold. Reference numeral 1 shown in FIG. 1 indicates a press molding die, 2 indicates a press molding punch, 3 indicates a gap, or a heat insulating material. FIG. 2 is a top view showing the shape of the punch. FIG. 3 is a bottom view showing the shape of the die. The mold was determined as the shape of the forming die with a clearance of 1.6 mm, following the shape of the forming punch. On the punch bottom of the mold, an air cooling part and a part where a heat insulating material was inserted were provided. Ceramic was used for the heat insulating material. For comparison, an experiment was also conducted on a mold having the same shape as the mold shown in FIGS. The blank size was 1.6 mm thick × 300 × 500. The molding conditions were a punch speed of 10 mm / s, a pressing force of 200 tons, and a holding time at the bottom dead center of 5 seconds. FIG. 4 is an overall schematic view of the molded product. The slow cooling methods are also shown in Tables 3-9. As a legend, “air-cooled” was provided with a gap, “insulation” was inserted with ceramic, and “none” was indicated when there was no gap or insulation inserted.

  The hydrogen embrittlement resistance was evaluated based on whether hydrogen embrittlement occurred at the site after piercing. Piercing was performed within 30 minutes after hot working. A punch with a diameter of 10 mmφ was used, and a die with a diameter of 10.5 mm was used. The processing position of the part was set to the center of the air cooling part shown in FIGS. The evaluation standard was that one week after piercing, the entire pierced hole was observed and the presence of cracks was determined. Observation was performed with a magnifying glass or an electron microscope. The determination results are shown in Tables 3-9.

  FIG. 5 is a view showing a cross section of the mold B. FIG. Reference numeral 4 shown in this figure is a distance piece, and reference numeral 5 indicates a wrinkle presser. FIG. 6 is a view showing the upper surface shape of the die punch. Moreover, FIG. 7 is a figure which shows the die die lower surface shape. The mold was determined to be a die shape with a clearance of 1.6 mm following the punch shape. By using a distance piece, a 2.0 mm clearance was provided in the flange portion. The blank size was 1.6 mm thick × 300 × 500. The molding conditions were a punch speed of 10 mm / s, a wrinkle pressing force of 20 tons, a pressing force of 200 tons, and a holding time at the bottom dead center of 5 seconds. FIG. 8 is a schematic diagram of the molding process. The molded product has the same shape as the mold A and is shown in FIG.

  The hydrogen embrittlement resistance was evaluated based on whether or not hydrogen embrittlement occurred at the site after trimming the flange. Piercing was performed within 30 minutes after hot working. Trimming was performed so that the flange remained 20 mm from the die R portion. The evaluation standard was that the pierced holes were observed all around one week after trimming to determine the presence or absence of cracks. Observation was performed with a magnifying glass or an electron microscope. The determination results are also shown in Tables 3 to 9.

  No. 3 to Table 9 1 to 249 are the results of studying the influence of steel type, plating type, hydrogen concentration in the atmosphere, and dew point when a gap is provided in the punch bottom of the mold A, but within the scope of the present invention. For example, no cracks occurred after piercing. No. shown in Tables 7-9. For 250 to 305, the method of slowly cooling the bottom of the punch was examined for the mold A, but cracking did not occur after piercing if it was within the scope of the present invention. No. shown in Table 9 306 to 323 are the results of studying the influence of steel type, plating type, hydrogen concentration in the atmosphere, and dew point when a gap is provided in the punch bottom of the mold B, but within the scope of the present invention. For example, no cracks occurred after piercing.

It is a figure which shows the cross section of a metal mold | die. It is a top view which shows the shape of a punch. It is a bottom view which shows the shape of dice | dies. It is the whole molded article schematic. 2 is a view showing a cross section of a mold B. FIG. It is a figure which shows the upper surface shape of a metal mold | die punch. It is a figure which shows the die die bottom surface shape. It is a schematic diagram of a forming process.

Explanation of symbols

1 Press Molding Dies 2 Press Molding Punch 3 Gap or Heat Insulating Material 4 Distance Piece 5 Wrinkle Presser


Patent applicant: Nippon Steel Corporation
Attorney Attorney Shiina and others 1

Claims (4)

% By mass
C: 0.05 to 0.55%,
Si: 2% or less,
Mn: 0.1 to 3%
P: 0.1% or less,
S: A steel sheet containing 0.03% or less and the balance Fe and inevitable impurities such as Al, Cr, N, Ti, B is used, the hydrogen content is 10% or less in volume fraction, and the dew point is 30 ° C. After heating the steel sheet to Ac ₃ to melting point in the following atmosphere, press molding is started at a temperature higher than the temperature at which ferrite, pearlite, bainite, martensite transformation occurs, and the mold is cooled in the mold after molding. When manufacturing high-strength parts by quenching, the cooling rate of a part of the part is reduced to provide a lower-strength part than the other part, and the part is sheared. Production method. 2. The method according to claim 1, wherein as a method of reducing the cooling rate of a part of the part, a gap is provided in the same position of the upper mold and the lower mold in a part of the mold. Manufacturing method. The method according to claim 1, wherein a heat insulating material is incorporated into a part of the mold as a method of reducing the cooling rate of a part of the part. A high-strength part manufactured by the method according to claim 1.

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