JP6958214B2 - Manufacturing method of processed steel parts - Google Patents

Description

The present invention relates to processed steel parts and methods for manufacturing the same.

Social demands for improving the collision safety of automobiles are further increasing, and steel materials having excellent energy absorption characteristics in the event of a collision during traveling are used for the impact absorbing members of automobiles due to their increased strength. On the other hand, in general, as the strength increases, the deformability of the steel sheet decreases, so that the parts are cracked during molding.

Cracks generated in the molding of high-strength steel sheets include cracks due to insufficient ductility and cracks due to hole expansion that occur on the sheared surface of blanking and punching. All of these properties decrease with increasing strength.

As a high-strength steel material with improved workability of a steel sheet, a work-induced transformation type (TRIP type) steel is known. TRIP steel is a steel in which austenite remains in the steel material to improve ductility. When a force is applied at the time of collision, austenite changes to hard martensite, and the strength of the deformed part is locally increased to improve ductility. can.

On the other hand, since TRIP steel undergoes martensitic transformation even in blanking and punching, the interface between the hard martensite and the soft ferrite becomes the starting point of cracking in the hole expanding, and the hole expanding property is low. DP steel, which also has high ductility, contains a ferrite phase and a martensite phase in its original structure, and has low hole expandability for the same reason.

Patent Document 1 discloses a high-strength cold-rolled steel sheet having excellent ductility and stretch flangeability, in which the steel structure has retained austenite in an area% of 3% or more.

Patent Document 2 discloses a two-phase steel sheet having an improved impact resistance, which is composed of a ferrite phase and a martensite phase.

Patent Document 3 describes a high-strength steel sheet having a predetermined composition and structure and excellent elongation and stretch flangeability by warm working, and the steel sheet being warm-worked at 100 to 400 ° C., preferably 150 to 250 ° C. The high-strength member or high-strength component obtained by the above is disclosed.

Japanese Unexamined Patent Publication No. 2015-161023 Japanese Unexamined Patent Publication No. 10-147838 Japanese Unexamined Patent Publication No. 2004-190050

In a high-strength steel sheet, as in Patent Document 1, it is not sufficient to ensure both ductility and stretch flangeability with the steel sheet alone.

As described above, the two-phase steel sheet disclosed in Patent Document 2 has high ductility, but has low hole expansion property because it contains a ferrite phase and a martensite phase.

Although it is conceivable to reduce the press formability of the part by devising the shape, the method of suppressing the consumption of deformability by changing the shape is becoming the limit from the viewpoint of the degree of freedom of the shape.

Further, when the steel sheet is warmly processed as in Patent Document 3, the formability of the steel sheet is generally improved in the warm state, so that both ductility and hole widening property (elongation flange property) can be improved. There are problems in the processing between the molds, such as severe damage to the mold and biting of the scale, which tends to cause defects in the parts.

In view of the above circumstances, it is an object of the present invention to provide a method for manufacturing a processed steel part that can improve the hole expandability after shearing a steel sheet and suppress the occurrence of cracks during the subsequent press working. And.

The present inventors have diligently studied a method for improving hole expandability (ductility) after shearing and preventing cracking due to press working in a method for manufacturing steel processed parts that are sheared and then pressed. .. As a result, it was found that by performing an appropriate heat treatment after the shearing process, the characteristics deteriorated by the shearing process can be recovered, the hole expandability can be improved, and cracking due to the press process can be prevented.

The present inventors further studied and completed the present invention. The summary is as follows.

(1) A method for manufacturing a processed steel part, which comprises shearing a steel sheet, heat-treating the processed steel sheet at 200 to 600 ° C. for 5 minutes or less, and pressing the heat-treated steel sheet.

(2) The method for manufacturing a processed steel part according to (1), wherein the heat treatment is a treatment for locally heating a sheared portion.

(3) The method for manufacturing a processed steel part according to (2), wherein the locally heating treatment heats a portion where cracks are likely to occur in the press working after the heat treatment.

(4) The method for manufacturing a processed steel part according to any one of (1) to (3) above, wherein the heat treatment is furnace heating.

(5) The method for manufacturing a processed steel part according to (2) or (3), wherein the locally heating treatment is a heat treatment using a laser.

(6) The method for manufacturing a steel processed part according to any one of (1) to (5) above, wherein the shearing process is a punching process and / or a hole punching process.

(7) The method for producing a processed steel part according to any one of (1) to (6) above, wherein the structure of the steel sheet contains austenite in an area ratio of 5% or more.

(8) The chemical composition of the steel sheet is C: 0.05 to 0.50%, Si: 0.10 to 3.00%, Mn: 1.00 to 8.00%, P: 0 in mass%. .100% or less, S: 0.010% or less, Al: 0.005 to 2.00%, N: 0.010% or less, O: 0.010% or less, Nb: 0 to 0.10%, Ti : 0 to 0.20%, Cr: 0 to 0.50%, and Mo: 0 to 0.50%, and the balance is Fe and unavoidable impurities. 7) A method for manufacturing a processed steel part.

According to the method for manufacturing a processed steel part of the present invention, it is possible to process a difficult-to-manufacture part by improving the hole expanding property after shearing the steel sheet and suppressing the occurrence of cracking during the subsequent press working. It will be possible.

The method for manufacturing a processed steel part of the present embodiment is characterized in that a steel sheet is subjected to shearing such as punching and drilling, and then heat treatment is performed after the shearing in a method for manufacturing a processed steel part by press working. do. Specifically, the heat treatment is performed in an environment of 200 ° C. or higher and 600 ° C. or lower, preferably 400 ° C. or higher and 600 ° C. or lower, in a time of 5 minutes or less.

When a steel sheet containing austenite is sheared, the vicinity of the cut surface undergoes strong processing, and the austenite in the steel sheet is transformed into martensite by work-induced transformation, which becomes the starting point of cracking during press working.

In the present embodiment, the above heat treatment recovers and tempers the martensite phase, or reverse-transforms martensite into austenite. When the martensite phase is tempered, the toughness is improved and cracking due to press working can be suppressed. In addition, when martensite is reverse-transformed into austenite, the austenite structure, which is a crystal lattice with good elongation, is introduced into the steel machined parts, which improves hole expansion and ductility and suppresses cracking due to press working. Can be done.

In the heat treatment of the present embodiment, in the low temperature range, the effects of recovery, tempering of the martensite phase, and recovery of the processed structure are the main effects, but when the temperature is high, the effect of reverse transformation of martensite to austenite is obtained. It becomes the center, and the crack improvement effect appears more prominently. The former effect can be obtained even in a steel sheet containing a martensite phase such as DP steel, and both effects can be obtained in a steel containing an austenite phase.

The heating rate during the heat treatment is preferably 1 ° C./s or higher. When the heating rate is slow, cementite is formed and the reverse transformation from martensite to austenite is suppressed. The heating rate is more preferably 10 ° C./s or higher. The upper limit of the heating rate is not particularly limited, but in reality, it is about 500 ° C./s.

The heat treatment of the present embodiment aims to control the structure of a portion that has been strongly processed by shearing. Therefore, the entire steel sheet may be heated, but only the portion that has undergone strong processing may be locally heated. Further, if a portion where cracking is likely to occur in the subsequent press working is known in advance, only that portion may be locally heated.

In general, recovery of the processed structure, tempering of martensite, and regeneration of austenite by reverse transformation reduce the strength. Therefore, when heat is applied to the entire steel sheet, it is inferior in terms of component strength. In addition, if the heating area is wide, the productivity can be lowered. Therefore, local heating is desirable from the above viewpoint.

The heating method is not particularly limited, and known methods such as laser heating and furnace heating can be used. In the case of local heating by laser heating, if the laser is arranged so that heating can be performed immediately after processing, local heating can be started immediately without installing a furnace and without moving the steel plate to the furnace. It is advantageous in terms of sexuality.

The cooling rate and cooling method after heating do not affect the characteristics of the processed steel parts, and are not particularly limited.

According to the heat treatment of the present embodiment, the austenite area ratio of the portion subjected to the strong processing is preferably restored to 60% or more of the austenite area ratio before the shearing process.

Recovery occurs in a low temperature range due to the heat treatment of the present embodiment, but this can also occur with general steel, so the target steel material is not particularly limited, and in addition to this for steel sheets containing a martensite phase. Then, the effect of tempering martensite is added.

In addition to the above, reverse transformation can occur in the high temperature region. This is a phenomenon that is more likely to occur in a structure containing an austenite phase, but it is also a phenomenon that can occur in general steel, and the target steel material is not particularly limited.

In order to cause reverse transformation at low temperature and in a short time, it is preferable that austenite is present in the steel material before cold press working. Since the reverse transformation temperature is lowered by the presence of austenite, it is possible to more effectively cause the reverse transformation of the tissue.

The austenite of the steel material before press working is preferably 5% or more in area ratio. 10% or more is more preferable, and 15% or more is further preferable. For example, a TRIP steel containing 5% or more of retained austenite is suitable as the steel material of the present embodiment.

Here, the area fraction of retained austenite can be measured by, for example, an electron backscatter diffraction (EBSD) method or an X-ray diffraction method. When measuring by the X-ray diffraction method, the diffraction intensity of the (111) plane of ferrite (α (111)) and the diffraction intensity of the (200) plane of retained austenite (γ (200)) are measured using Mo-Kα rays. , The diffraction intensity of the (211) plane of ferrite (α (211)) and the diffraction intensity of the (311) plane of retained austenite (γ (311)) were measured, and the area fraction (fA) of retained austenite was calculated from the following equation. ) Can be calculated.

fA = (2/3) {100 / (0.7 × α (111) / γ (200) +1)}
+ (1/3) {100 / (0.78 x α (211) / γ (311) +1)}

As described above, the reverse transformation of the structure of the present embodiment can occur in general steel, and therefore the chemical composition is not particularly limited. Hereinafter, an example of the composition of steel suitable for this embodiment will be described.

[C: 0.050 to 0.500%]
C adjusts the contents of ferrite, bainite, martensite, and austenite to secure the amount of austenite required in the present embodiment. The content of C is preferably 0.050% to 0.500%, more preferably 0.100 to 0.400%.

[Si: 0.10 to 3.00%]
Si has the effect of improving the strength and ductility of steel by solid solution strengthening, and suppressing the formation of carbides (cementite) to retain austenite. The Si content is preferably 0.1 to 3.00%, more preferably 0.50 to 2.00%.

[Mn: 1.00 to 8.00%]
Mn controls the transformation behavior and controls the amount and hardness of the transformation phase. Considering the dynamic strength, the Mn content is preferably 1.00 to 8.00%, more preferably 1.50 to 3.00%.

[P: 0.100% or less]
P is contained in the steel as an impurity and segregates at the grain boundaries to embrittle the steel. The smaller the P content, the more preferable, and 0.100% or less is preferable. More preferably, it is 0.050% or less.

[S: 0.010% or less]
S is contained in steel as an impurity to form sulfide-based inclusions and reduce ductility. The smaller the S content, the more preferable, and 0.010% or less is preferable. More preferably, it is 0.005% or less.

[Al: 0.005-2.000%]
Al is used to deoxidize molten steel. Further, like Si, it has the effect of suppressing the formation of cementite and retaining austenite. The Al content is preferably 0.005 to 2.000%, more preferably 0.050 to 1.000%.

[N: 0.0100% or less]
N is contained in steel as an impurity and deteriorates ductility. The smaller the N content, the more preferable, and 0.0100% or less is preferable. More preferably, it is 0.0050% or less.

[O: 0.0100% or less]
O forms an oxide and deteriorates elongation. The smaller the O content, the more preferable, and 0.0100% or less is preferable. More preferably, it is 0.0050% or less.

[Nb: 0 to 0.100%]
Nb contributes to the increase in the strength of the steel sheet by strengthening the precipitate, strengthening the fine grains by suppressing the growth of ferrite crystal grains, and strengthening the dislocations by suppressing the recrystallization, and may be contained as necessary. If Nb is present even in a small amount, it contributes to an increase in strength. In order to increase the strength more effectively, it is preferably contained in an amount of 0.005% or more. If the content is too large, the precipitation of carbonitride increases and the moldability deteriorates. Therefore, the content is preferably 0.100% or less, more preferably 0.050% or less.

[Ti: 0 to 0.200%]
Ti contributes to the increase in the strength of the steel sheet by strengthening the precipitate, strengthening the fine grains by suppressing the growth of ferrite crystal grains, and strengthening the dislocations by suppressing the recrystallization, and thus Ti may be contained as necessary. If Ti is present even in a small amount, it contributes to an increase in strength. In order to increase the strength more effectively, it is preferably contained in an amount of 0.005% or more. If the content is too large, the precipitation of carbonitride increases and the moldability deteriorates. Therefore, the content is preferably 0.200% or less, more preferably 0.050% or less.

[Cr: 0 to 0.500%]
Cr is a reinforcing element and improves hardenability, so it may be contained if necessary. If even a small amount of Cr is present, these effects can be obtained. In order to obtain the effect of the content more effectively, the content is preferably 0.050% or more. If the content is too large, the manufacturability is adversely affected, so the content is preferably 0.500% or less.

[Mo: 0 to 0.500%]
Mo is a reinforcing element and improves hardenability, so it may be contained if necessary. These effects can be obtained if even a small amount of Mo is present. In order to obtain the effect of the content more effectively, the content is preferably 0.050% or more. If the content is too large, the manufacturability is adversely affected, so the content is preferably 0.500% or less.

The rest of the chemical composition of steel is Fe and unavoidable impurities.

Using the steel sheets A to H having the chemical components shown in Table 1, punching or punching is performed, the processed steel sheet is heat-treated, and the processed and heat-treated steel sheet has holes conforming to JIS Z 2256. An expansion test and an evaluation of press cracking by press working were performed. The heat treatment was carried out in two ways: laser heating and furnace heating.

Table 2 shows the materials of the steel sheet before shearing and pressing. The amount of γ in Table 2 is the austenite area ratio, which was calculated from the peak area ratio of bcc and fcc by performing X-ray diffraction using a surface having a thickness of 1/4 of the steel sheet as an observation surface. TS is the tensile strength and λ is the hole expansion rate. In this example, steel sheets having different amounts of austenite and mechanical properties were produced from slabs having the same composition by changing the production method, and were subjected to processing.

Table 3 shows an experimental example in which local heating or total heating was performed by laser heating after shearing, and Table 4 shows an experimental example in which local heating or total heating was performed by furnace heating after shearing. .. In the experimental example subjected to the heat treatment of the present invention, it was confirmed that the hole expansion rate λ increased and press cracking did not occur.

Claims (7)

Shear the steel sheet
The processed steel sheet is heat-treated at 200 to 600 ° C. for 5 minutes or less, and then heat-treated.
It is a manufacturing method of steel processed parts that press-processes a steel sheet after heat treatment.
A method for producing a processed steel part, wherein the structure of the steel sheet before shearing contains austenite in an area ratio of 5% or more. The method for manufacturing a processed steel part according to claim 1, wherein the heat treatment is a treatment for locally heating a sheared portion. The method for manufacturing a processed steel part according to claim 2, wherein the locally heating treatment heats a portion where cracks are likely to occur in the press working after the heat treatment. The method for manufacturing a processed steel part according to any one of claims 1 to 3, wherein the heat treatment is furnace heating. The method for manufacturing a processed steel part according to claim 2 or 3, wherein the locally heating treatment is a heat treatment using a laser. The method for manufacturing a processed steel part according to any one of claims 1 to 5, wherein the shearing process is a punching process and / or a hole punching process. The chemical composition of the steel sheet is mass%.
C: 0.050 to 0.500%,
Si: 0.10 to 3.00%,
Mn: 1.00 to 8.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.005-2.000%,
N: 0.0100% or less,
O: 0.0100% or less,
Nb: 0 to 0.10%,
Ti: 0-0.20%,
Cr: 0 to 0.50%, and Mo: 0 to 0.50%
The method for producing a processed steel part according to any one of claims 1 to 6 , wherein the method contains Fe and the balance is Fe and unavoidable impurities.