JP4317506B2 - Manufacturing method of high strength parts - Google Patents

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 particularly relates to a component having excellent strength after high-temperature molding and a method for manufacturing the same.

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. H10-228561 discloses a technique for forming the article warmly 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 by utilizing precipitation strengthening in this temperature range.
Patent Document 2 proposes a high-strength steel sheet in which the yield strength at the forming temperature is significantly lower than the yield strength at room temperature for the purpose of improving press forming accuracy. However, these techniques may limit the strength that can be obtained. On the other hand, Patent Document 3 proposes a technique for heating to a high temperature austenite single phase region after molding and transforming to a hard phase in the subsequent cooling process for the purpose of obtaining higher strength.
However, since heating and rapid cooling after forming may cause problems in shape accuracy, as a technique to overcome this drawback, the steel sheet is heated to the austenite single-phase region and then cooled in the press forming process. Techniques are disclosed in Non-Patent Document 1 and Patent Document 4.
JP 2000-234153 A JP 2000-87183 A JP 2000-38640 A JP 2001-181833 A JP 2003-328031 A SAE, 2001-01-0078

As described above, high strength steel sheets used in automobiles and the like have the above-mentioned problems of formability as the strength is increased, and particularly in high strength materials exceeding 1000 MPa, hydrogen embrittlement is conventionally known. There is an essential problem of conversion (sometimes called cracking or delayed destruction). When used as a steel sheet for hot pressing, although the residual stress due to pressing at high temperature is small, hydrogen penetrates into the steel during heating before pressing, and the sensitivity to hydrogen embrittlement increases due to residual stress in post-processing. . 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, it is only necessary that the strength of the portion to be post-processed is reduced. Patent Document 5 discloses a technique for reducing the cooling rate of a part to be post-processed 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, when this method is used, the mold structure becomes complicated, which is considered to be economically disadvantageous. Furthermore, this method makes no mention of hydrogen embrittlement, and even when the strength of the steel sheet is slightly reduced by this method and the residual stress after post-processing is reduced to some extent, hydrogen is not contained in the steel. If it remains, the possibility of hydrogen embrittlement cannot be denied.
The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a high-strength part excellent in hydrogen embrittlement resistance that can obtain 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, the atmosphere in the heating furnace before forming is controlled to reduce the amount of hydrogen in the steel, and subsequently the site where shearing is performed is heated to lower the strength. Found that it was effective.
That is, the gist of the present invention is as follows.
(1) Using a steel sheet containing chemical components of C: 0.05 to 0.55% and Mn: 0.1% to 3% by mass%, the volume fraction of hydrogen is 10% or less (including 0%), and the dew point is 30 After heating the steel plate to an Ac3 to melting point in an atmosphere of ℃ or less, start forming at a temperature higher than the temperature at which ferrite, pearlite, bainite, martensite transformation occurs, and cool in the mold after forming After high-strength parts are quenched and hardened, the temperature of a part of the parts is increased to 400 ° C. or more and less than Ac3, and then the parts are sheared. Production method.
(2) The method for producing a high-strength part according to (1), wherein the temperature of a part of the part is raised to 400 ° C. or more and Ac3 or less, and then the part is cooled and sheared. .
(3) The chemical composition of the steel sheet is% by mass, C: 0.05 to 0.55%, Mn: 0.1% to 3%, Si: 1.0% or less, Al: 0.005 to 0.1%, S: 0.02% or less, P: 0.03 % Or less, Cr: 0.01 to 1%, B: 0.0002% to 0.0050%, Ti: (3.42 * N + 0.001)% or more, 3.99 × (C-0.1)% or less, N: 0.01% or less, O: 0.015% The method for producing a high-strength part according to (1) or (2), comprising: a balance Fe and inevitable impurities.
(4) The method for producing a high-strength part according to any one of (1) to (3), wherein the steel sheet is subjected to any one of aluminum plating, aluminum-zinc plating, and galvanization.
(5) The method for producing a high-strength component according to any one of (1) to (4), wherein a laser beam is used as a method for increasing the temperature of a part of the component.
(6) The method for manufacturing a high-strength component according to any one of (1) to (4), wherein high-frequency heating is used as a method for increasing the temperature of a part of the component.
(7) The method for producing a high-strength component according to any one of (1) to (4), wherein a high-temperature tool is brought into contact as a method for increasing the temperature of a part of the component.

  According to the present invention, when a high-strength part is manufactured by cooling in a die after molding and manufacturing a high-strength part, a vehicle having a light body and excellent in collision safety can be manufactured. It is.

The present invention is described in detail below. The features of the present invention are as follows: (1) Strictly define the heating atmosphere before high-temperature molding, (2) High-temperature molding After the subsequent shearing process, prior to the shearing part, the temperature of the part to be processed is raised to a range where no transformation occurs. Hydrogen embrittlement cracking occurs at the site where hydrogen penetrates into the steel and the residual stress due to processing remains, so the hydrogen source in the heating atmosphere is reduced and the strength of the site where shearing is performed is increased by increasing the temperature. By lowering, it becomes possible to suppress the occurrence of cracks.
First, the conditions for high temperature molding will be described. The amount of hydrogen in the atmosphere when heating the steel sheet was set to 10% or less (including 0%) in terms of volume fraction. If the amount of hydrogen exceeded this amount, a large amount of hydrogen entered the steel sheet during heating. This is because the hydrogen embrittlement resistance is reduced. The reason why the dew point in the atmosphere is set to 30 ° C. or less is that if the dew point exceeds this, the amount of hydrogen that enters the steel sheet during heating will also increase, and the hydrogen embrittlement resistance will deteriorate.
The reason for setting the heating temperature of the steel sheet to Ac3 or more and the melting point or less is to keep the structure of the steel sheet to be austenite in order to strengthen the quenching after forming. Further, when the heating temperature exceeds the melting point, press molding is impossible.
The reason why the molding start temperature is set to a temperature higher than the temperature at which ferrite, pearlite, bainite, and martensite transformation occurs is because the hardness after molding is insufficient when molding at a temperature lower than the temperature at which these transformations occur. .

After heating the steel sheet under the above conditions, the steel sheet is formed using a press method or the like, and after forming, it is cooled in a mold and quenched to produce a high-strength part. The above conditions may be ordinary methods.
Next, conditions after high temperature molding will be described. The reason for raising the temperature of some parts of the part to 400 ° C or higher is that when heated at a temperature lower than this temperature, the strength of the part remains high, and cracking may occur during subsequent shearing Because. The reason why the temperature is less than Ac3 is that, when heated to a temperature higher than this temperature, a low-temperature transformation phase is generated during subsequent cooling, and the hardness of the heated part may be increased.
As a method for heating a part of the component, any method may be used, or industrially, heating by laser light, high-frequency heating, or a method of contacting a heated tool may be used.
The reason for cooling after raising the temperature of a part of the part to 400 ° C. or more and less than Ac 3 is that it is easy to handle hot-worked products when the part is cooled. The cooling rate and the cooling temperature are not particularly limited.
In the present invention, the portion is sheared after the heating or heating / cooling described above. In the shearing process, a process for forming the shape of the part to be manufactured, such as drilling or cutting, is performed by a conventional method.

Next, the components of the steel sheet used in the present invention will be described.
C is an element added to secure the material with the cooled structure as martensite, and it is desirable to add 0.1% 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 is preferably 0.55%.
Mn is an element that improves strength and hardenability. If it is less than 0.2%, sufficient strength at the time of quenching cannot be obtained, and even if added over 3%, the effect is saturated, so 0.2 to 3% A range is desirable.
In addition, the following elements may be added as necessary.
Si is a solid solution strengthened alloy element, but if it exceeds 1.0%, a problem of surface scale occurs. In particular, when plating is performed on the surface of the steel sheet, the plating performance deteriorates if the amount of Si added is large, 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 preferably in the range of 0.005 to 0.1%.

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 preferably 0.02% or less. More preferably, it is 0.01% or less. Further, by limiting S to 0.005% or less, 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. Further, it is more preferably 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 preferably added in order to improve the hardenability during press molding or cooling after press molding. In order to exert this effect, 0.0002% or more must be added. 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 exhibit this effect, 0.001% or more of Ti that is not bonded to N is required, that is, (3.42 * N + 0.001) or more is required. However, if the amount of Ti is increased excessively, it is not bonded to Ti. Since the amount of C decreases and sufficient strength cannot be obtained after cooling, the upper limit is Ti equivalent that can secure 0.1% or more of C not bonded to Ti, that is, 3.99 × (C-0.1)% Is good.

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.
Although there is no particular restriction on O, excessive addition causes generation of an oxide that adversely affects toughness and generates an oxide that becomes a starting point of fatigue fracture. Therefore, its content is preferably 0.015% or less.
In addition, even if impurities inevitably contained, no particular problem occurs. Elements such as Ni, Cu and Sn which are considered to be mixed from scrap may be contained. Further, Ca, Mg, Y, As, Sb, and REM may be added to control the shape of inclusions. In addition, Ti, Nb, Zr, Mo, V may be added for the purpose of further improving the strength, but if it is increased unnecessarily, the amount of C not bonded to these elements decreases, and sufficient strength is obtained after cooling. It can no longer be obtained.

A steel slab comprising the above components is hot-rolled or further pickled, cold-rolled, and annealed to obtain a steel plate. Any of these process conditions may be a conventional method.
The steel plate may be used as it is, or may be subjected to aluminum plating, aluminum-zinc plating, or galvanization. There is no problem with the conventional method for the plating process.
For example, when aluminum plating is performed, the Si concentration in the plating bath is suitably 5 to 12%, and in the case of aluminum-zinc plating, the Zn concentration in the bath is suitably 40 to 50%. 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. Also, the atmosphere in the plating process can be plated under normal conditions, whether it is 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 impede productivity.
Further, as long as it is a galvanizing method, any method such as galvanizing, electrogalvanizing, alloying galvanizing, etc. may be used. Further, there is no particular problem even if Ni pre-plating, Fe pre-plating, or other metal pre-plating for improving 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.

Slabs having 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. Subsequently, pickling was performed, and a cold-rolled steel sheet having a thickness of 1.6 mm was formed by cold rolling. A part of the cold-rolled steel sheet was subjected to hot-dip aluminum plating, hot-dip aluminum-zinc plating, alloyed hot-dip galvanizing, and hot-dip galvanizing. Table 2 shows the plating types for each experiment. Here, the symbols are “CR” for no plating, “AL” for aluminum plating, “GA” for galvannealed alloy plating, and “GI” for hot dip galvanizing.
Thereafter, the cold-rolled steel sheet and the surface-treated steel sheet were heated in an austenite region at 950 ° C., which is higher than the Ac3 point, by furnace heating, and then hot-formed. The atmosphere of the heating furnace changed the amount of hydrogen and the dew point. Table 2 shows the conditions.
A cross section of the mold shape is shown in FIG. The shape of the punch viewed from above is shown in FIG. The shape of the die viewed from below is shown in FIG. The mold was determined to be a die shape with a clearance of 1.6 mm, following the punch shape. The blank size was 1.6 mm thickness x 300 mm x 500 mm. 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. A schematic diagram of the molded product is shown in FIG.

After molding, a part of the molded product was heated by laser light, high-frequency heating, or a heated tool. The method is also shown in Table 2. The symbol in the table is “R” for heating with laser light, “I” for high frequency heating, “T” for a heated tool, and “N” for no heating. FIG. 5 shows the shape of the molded product viewed from above, and shows the heated part and the shape of the heated tool.
The heating temperature is also shown in Table 2. Moreover, cooling after heating performed water cooling and air cooling. In addition, piercing was also carried out at a high temperature without cooling. The cooling method is also shown in Table 2.
The hydrogen embrittlement resistance was evaluated by piercing and whether hydrogen embrittlement occurred at the site. 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 component was set at the center of the heating unit shown in FIG. The evaluation standard was to observe the entire pierced hole one week after piercing and to determine the presence or absence of cracks. Observation was performed with a magnifying glass or an electron microscope. The determination results are shown in Table 2.
Experiment Nos. 1 to 249 are the results of studying the effects of steel type, plating type, hydrogen concentration in the atmosphere, and dew point when heating is performed with laser light, but within the scope of the present invention, after piercing No cracking occurred. Experiment Nos. 250 to 333 examined the influence of the heating method, but cracks did not occur after piercing if within the scope of the present invention. Experiment numbers 334 to 337 examined the influence of the heating temperature when heating a part of the part, and experiment numbers 338 and 339 examined the cooling method after heating a part of the part, but within the scope of the present invention No cracks occurred after piercing.

It is a figure which shows the cross section of the metal mold | die shape used for this invention. It is a figure which shows the shape which looked at the punch used for this invention from upper direction. It is a figure which shows the shape which looked at the dice | dies used for this invention from the downward direction. It is a schematic diagram of the molded product in the present invention. It is a figure which shows the shape which looked at the molded article in this invention from upper direction.

Explanation of symbols

1: Die 2: Punch 3: Molded product 4: Heating part

Claims (7)

  Using steel sheet containing chemical components of C: 0.05-0.55% and Mn: 0.1% -3% by mass%, hydrogen volume fraction is 10% or less (including 0%), and dew point is 30 ° C or less In a certain atmosphere, after heating the steel sheet to Ac3 ~ melting point, start molding at a temperature higher than the temperature at which ferrite, pearlite, bainite, martensite transformation occurs, and after molding, quench in the mold A method for producing a high-strength part, comprising: producing a high-strength part, raising the temperature of a part of the part to 400 ° C. or more and less than Ac 3, and then shearing the part.   2. The method of manufacturing a high-strength part according to claim 1, wherein the temperature of a part of the part is raised to 400 ° C. or more and Ac3 or less and then cooled, and the part is sheared.   The chemical composition of the steel sheet is% by mass, C: 0.05 to 0.55%, Mn: 0.1% to 3%, Si: 1.0% or less, Al: 0.005 to 0.1%, S: 0.02% or less, P: 0.03% or less, Cr: 0.01-1%, B: 0.0002% -0.0050%, Ti: (3.42 * N + 0.001)% or more, 3.99 × (C-0.1)% or less, N: 0.01% or less, O: 0.015% or less The method for producing a high-strength component according to claim 1 or 2, wherein the balance Fe and unavoidable impurities are included.   The method for producing a high-strength part according to any one of claims 1 to 3, wherein the steel sheet is subjected to any one of aluminum plating, aluminum-zinc plating, and galvanization.   The method for manufacturing a high-strength component according to any one of claims 1 to 4, wherein a laser beam is used as a method for increasing the temperature of a part of the component.   The method for manufacturing a high-strength component according to any one of claims 1 to 4, wherein high-frequency heating is used as a method of increasing the temperature of a part of the component.   The method for producing a high-strength component according to any one of claims 1 to 4, wherein a high-temperature tool is brought into contact as a method for increasing the temperature of a part of the component.

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