JP2013040390A - Method for manufacturing hot-pressed member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a hot-pressed member having more excellent toughness than before.SOLUTION: A steel material has a chemical composition which includes 0.15-0.45% of C, 0.5-3.0% of Mn+Cr, and further one or two or more of 0.05% or less of P, 0.03% or less of S, 0.5% or less of Si, 3% or less of Ni, 1% or less of Cu, 1% or less of V and 1% or less of Al with the balance comprising Fe and impurities. The steel material is subjected to hot-pressing after being held in a temperature region of the Acpoint to (Acpoint+100°C) for 10 minutes or shorter. Then, the steel material is subjected to quenching treatment to be cooled to an Mf point at a cooling rate equal to or higher than an upper critical cooling rate, obtaining a tensile strength of 1.2 GPa or higher, and then subjected to heat treatment to be held in a temperature region of 150-200°C for 10 minutes or longer, thereby manufacturing the hot-pressed member.

Description

  The present invention relates to a method for manufacturing a hot press member that is suitably used for, for example, machine structural parts such as automobile body structural parts and underbody parts.

  In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel materials and reduce the weight used. And in the steel plate widely used for a motor vehicle, since press formability falls with the increase in intensity | strength, it has become difficult to manufacture a complicated shape. Specifically, there arises a problem that the ductility is lowered and the rupture occurs at a portion where the degree of processing is high, or the dimensional accuracy is deteriorated due to an increase in springback and wall warpage. Accordingly, it is not easy to produce a formed member by pressing a steel sheet having high strength, particularly a tensile strength of 780 MPa class or higher (hereinafter also referred to as “TS”).

  On the other hand, as shown in Patent Document 1, in a forming method called a hot press method in which a heated steel plate is press-formed, the steel plate is soft and highly ductile at high temperatures during press forming, and thus has a complicated shape. However, it is possible to mold with dimensional accuracy. Furthermore, by increasing the strength of the steel sheet by martensitic transformation, the steel sheet is heated in the austenite region and rapidly cooled (quenched) in the press die.

  Patent Document 2 discloses a pre-press quench method that simultaneously achieves high strength and formability of a steel sheet by forming into a predetermined shape at room temperature and then heating to an austenite region and quenching in a mold. The invention which concerns is disclosed.

Such a hot press method and a pre-press quench method are excellent molding methods that can simultaneously ensure high strength and excellent moldability of the molded member.
The microstructure of the member thus obtained is a martensite single-phase structure and is generally said to have poor toughness. Therefore, Patent Document 3 proposes improving toughness by controlling the cooling rate during hot pressing.

British Patent Publication No. 1490535 Japanese Patent Laid-Open No. 10-96031 JP 2004-353026 A

  The invention described in Patent Document 3 is an excellent invention that effectively improves the toughness of a hot pressed member. However, due to the recent need for further toughness improvement, a method for producing a hot press member having further excellent toughness is required.

  The present invention has been made in view of such needs for further improvement in toughness in recent years, and an object of the present invention is to provide a method for manufacturing a hot-pressed member that is more excellent in toughness than before.

  As a result of intensive studies to further improve the toughness of the hot pressed member, the present inventors have dramatically improved the toughness of the hot pressed member by applying an appropriate heat treatment to a steel material having an appropriate chemical composition. I found that it was improved. The present invention completed based on the knowledge is as follows.

(1) By mass%, C: 0.15 to 0.45%, Mn + Cr: 0.5 to 3.0%, P: 0.05% or less, S: 0.03% or less, Si: 0.00. A steel material containing one or more of 5% or less, Ni: 3% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less, and having a chemical composition composed of the balance Fe and impurities. , After holding for 10 minutes or less in a temperature range of Ac ₃ points or more (Ac ₃ points + 100 ° C.) A method for producing a hot pressed member, characterized in that after the tensile strength is set to 1.2 GPa or more, heat treatment is performed for 10 minutes or more in a temperature range of 150 ° C. or more and 200 ° C. or less.

(2) The method for producing a hot-pressed member according to (1), wherein the chemical composition contains B: 0.01% or less in mass% instead of part of Fe.
(3) The chemical composition contains one or two selected from the group consisting of Nb: 1.0% or less and Mo: 1.0% or less in mass%, instead of part of Fe. The manufacturing method of the hot press member as described in (1) or (2) characterized by these.

(4) The heat according to any one of (1) to (3), wherein the chemical composition contains Ti in an amount satisfying the following formula (1) instead of a part of Fe: A manufacturing method of an intermediate press member.
3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5 (1)
Here, Ti and N in a formula show content (unit: mass%) of each element in steel.

  (5) The chemical composition according to any one of (1) to (4), wherein the chemical composition contains, by mass%, Ca: 0.005% or less instead of a part of Fe. Manufacturing method of hot press member.

  According to the present invention, it is possible to provide a method for manufacturing a hot-pressed member that is more excellent in toughness than before.

  Next, the reason why the present invention is limited to each range will be described. In the following description, “%” for alloy elements represents “mass%”.

(I) Chemical composition About the chemical composition of the base steel materials in this invention, it prescribes | regulates as follows.

[C: 0.15-0.45%]
C is a very important element that enhances the hardenability of the steel material and mainly determines the strength after quenching. If the C content is less than 0.15%, it is difficult to ensure a TS of 1.2 GPa or more in strength after quenching. Therefore, the C content is 0.15% or more. Preferably it is 0.20% or more. On the other hand, if the C content exceeds 0.45%, the strength after quenching becomes too high, and the deterioration of toughness becomes remarkable. Therefore, the C content is set to 0.45% or less. Preferably it is 0.33% or less.

[Mn + Cr: 0.5 to 3.0%]
Both Mn and Cr are elements that are very effective for enhancing the hardenability of the steel material and stably securing the strength after quenching. However, this effect is not sufficient when the total content of Mn and Cr (hereinafter also referred to as “(Mn + Cr) content”) is less than 0.5%. Therefore, the (Mn + Cr) content is 0.5% or more. Preferably it is 0.8% or more. On the other hand, if the (Mn + Cr) content exceeds 3.0%, the effect is saturated, but it is difficult to secure a stable strength. Therefore, the (Mn + Cr) content is 3.0% or less. Preferably it is 2.0% or less.

[P: 0.05% or less, S: 0.03% or less, Si: 0.5% or less, Ni: 3% or less, Cu: 1% or less, V: 1% or less, and Al: 1% or less Species or 2 or more types]
These elements are elements that are effective in enhancing the hardenability of the steel material and stably securing the strength after quenching. However, since the effect is small even if it contains more than an upper limit, and it causes a cost increase unnecessarily, the content of each element is set to the above-described range.

[B: 0.01% or less]
B is effective for enhancing the hardenability of the steel material and further enhancing the effect of stably securing the strength after quenching. It is also an important element in that it segregates at grain boundaries to increase grain boundary strength and improve toughness. Furthermore, the effect of suppressing the grain growth of austenite at the time of heating at the time of subjecting to hot pressing is also high, and it also has the effect of improving toughness. Therefore, in the present invention, it is preferable to contain B as an optional element. However, if the B content exceeds 0.01%, the above effect is saturated, leading to an increase in cost. Therefore, when it contains B, the content shall be 0.01% or less. Preferably it is 0.0030% or less. In order to obtain the above effect more reliably, the B content is preferably 0.0001% or more, and more preferably 0.0010% or more.

[Nb: 1.0% or less, Mo: 1.0% or less]
Nb and Mo both form fine carbides, and suppress the coarsening of austenite grains and keep them finer when the steel material used for hot pressing is heated to Ac ₃ or higher, greatly improving toughness. Has the effect of Furthermore, since Nb has a strong action of suppressing recrystallization, the above effect is even greater. Therefore, in the present invention, it is preferable to contain Nb and / or Mo as optional elements. However, when the content of any element exceeds 1.0%, the effect is saturated, and the cost is unnecessarily increased. Therefore, when Nb and / or Mo are contained, each content is made 1.0% or less. The Nb content is preferably 0.15% or less, and more preferably 0.10% or less. The Mo content is preferably 0.20% or less, and more preferably 0.15% or less. In order to obtain the above effect more reliably, the Nb content is preferably 0.02% or more, and more preferably 0.04% or more. Further, the Mo content is preferably 0.01% or more, and more preferably 0.04% or more.

[3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5]
Ti forms fine carbides and suppresses recrystallization, thereby suppressing the coarsening of austenite grains when heating the steel material to be subjected to hot pressing to Ac ₃ points or more, and keeps it fine. Has the effect of greatly improving Therefore, in the present invention, it is preferable to contain Ti as an optional element. In order to obtain the above effect more reliably, the Ti content is preferably set to (3.42N + 0.001) or more. On the other hand, when the Ti content exceeds (3.42N + 0.5), the effect is saturated and the cost is unnecessarily increased. A more desirable Ti content is 3.42N + 0.02 ≦ Ti ≦ 3.42N + 0.08.

[Ca: 0.005% or less]
Ca has the effect of reducing the inclusions in the steel and improving the toughness after quenching. Therefore, in the present invention, it is preferable to contain Ca as an optional element. However, when the Ca content exceeds 0.005%, the effect is saturated and the cost is increased. Therefore, the Ca content is 0.005% or less. Preferably it is 0.004% or less. In order to obtain the above effect more reliably, the Ca content is preferably 0.001% or more, and more preferably 0.002% or more.

  The balance other than those described above is Fe and impurities.

(Ii) Manufacturing conditions [Hot press]
The steel material having the above-described chemical composition is held in a temperature range of Ac ₃ points or more (Ac ₃ points + 100 ° C.) for 10 minutes or less and then hot pressed.

When the holding temperature of the steel material to be subjected to hot pressing is less than Ac ₃ points, it does not become an austenite single phase, so that it is difficult to obtain a strength of 1.2 GPa or more even if a subsequent quenching treatment is performed. On the other hand, if the holding temperature of the steel material subjected to hot pressing exceeds (Ac ₃ points + 100 ° C.), the austenite grains become coarse and the toughness deteriorates remarkably, or the scale loss increases remarkably. Therefore, the holding temperature of the steel material subjected to hot pressing is set to Ac ₃ points or more (Ac ₃ points + 100 ° C.).

  Moreover, the holding time in the said temperature range of the steel materials used for a hot press shall be 10 minutes or less in order to prevent the cost required for a heating increasing unnecessarily. The lower limit of the holding time is not particularly required, but is preferably 1 minute or more from the viewpoint of homogenizing the structure before quenching and stabilizing the product characteristics.

[Quenching treatment]
The steel material subjected to hot pressing is subjected to a quenching process for cooling to the Mf point at a cooling rate equal to or higher than the upper critical cooling rate.

  If the cooling rate in the quenching process is lower than the upper coastal cooling rate or the cooling end temperature is higher than the Mf point, a substantial amount of phases and structures other than martensite exceeding the level of inevitably mixed are generated. , It may be difficult to ensure a strength of 1.2 GPa or more. Therefore, the quenching process is performed under the conditions described above.

  In addition, about the steel materials which have the said chemical composition, it is enough if the cooling rate in a hardening process shall be 60 degrees C / sec or more. Moreover, it is sufficient that the cooling end temperature in the quenching process is 100 ° C.

[Heat treatment]
The hot-pressed member that has been subjected to the quenching treatment is subjected to a heat treatment that is held in a temperature range of 150 ° C. or higher and 200 ° C. or lower for 10 minutes or longer. This heat treatment dramatically improves toughness.

  If the holding temperature in the heat treatment is less than 150 ° C., the effect of improving toughness may not be sufficiently obtained. Therefore, the holding temperature in the heat treatment is set to 150 ° C. or higher. Preferably it is 170 degreeC or more. On the other hand, if the holding temperature in the heat treatment exceeds 200 ° C., the strength of the hot pressed member may be significantly reduced. Therefore, the holding temperature in the heat treatment is set to 200 ° C. or less. Preferably it is 190 degrees C or less.

  Furthermore, if the holding time in this heat treatment is less than 10 minutes, the effect of improving toughness may not be sufficiently obtained. Therefore, the holding time in the heat treatment is 10 minutes or more. Preferably it is 20 minutes or more. The upper limit of the holding time is not particularly limited, but if it exceeds 3 hours, it is disadvantageous in terms of cost, and it is preferably 3 hours or less. More preferably, it is 120 minutes or less, Most preferably, it is 60 minutes or less. Any method may be used as the heating method when the heat treatment is performed, and examples thereof include furnace heating, high-frequency heating, and electric heating.

  This improvement in toughness by heat treatment is particularly effective for hot-pressed members with higher C content and higher strength. This is mainly because the strength of the member having a high C content (approximately 0.3% C or more) is slightly reduced by the heat treatment of the present application. On the other hand, in a member with a low C content (about 0.2% C), the strength tends to increase due to this heat treatment, but as the strength increases, the toughness does not deteriorate but conversely toughness. It is also a feature of this heat treatment that is further improved. Another characteristic of the heat treatment is that the yield ratio, which is a value obtained by dividing the yield strength by the tensile strength, greatly increases (approximately 0.75 or more).

Since the steel material to be subjected to hot pressing is press-molded after being maintained at a temperature of Ac ₃ point or higher, the mechanical properties at room temperature are not important. Regarding the metal structure before being subjected to hot pressing, There is no need to specify in particular. Therefore, when the steel material to be subjected to hot pressing is a steel plate, either a hot rolled steel plate or a cold rolled steel plate may be used, and the surface of the steel plate may be plated. Moreover, it is not necessary to specifically limit the manufacturing method.

  The present embodiment described above makes it possible to manufacture a hot-pressed member having excellent toughness. For example, machine structural parts such as automobile body structural parts and undercarriage parts are reliably provided. It becomes possible.

Furthermore, the present invention will be described more specifically with reference to examples.
Steel type No. shown in Table 1 A steel plate (plate thickness: 2.6 mm) having a chemical composition of 1 to 7 was used as a test material.

  These steel plates were heated at 1250 ° C. for 30 minutes and then hot-rolled at 900 ° C. or higher to obtain a steel plate having a thickness of 4 mm. After hot rolling, after water spray cooling to 600 ° C., charging in a furnace, holding at 600 ° C. for 30 minutes, and then gradually cooling to room temperature at 20 ° C./hour to simulate a hot rolling winding process. . The obtained hot rolled sheet was made to have a thickness of 2.6 mm by cold rolling after removing the scale by pickling.

The steel plate produced in this way is heated in the gas furnace set to an air-fuel ratio of 1.1 under the conditions shown in Table 2, and then removed from the heating furnace, and immediately after that, a flat steel mold is used. Then, hot pressing was performed. In addition, the “holding time” in Table 2 refers to the time from the point when Ac ₃ is reached after charging into the furnace to the removal from the furnace.

Thereafter, using an oil bath, heat treatment was performed under the conditions shown in Table 2, followed by water cooling. Various test pieces were collected from the finally obtained steel plate and subjected to a tensile test (JIS No. 5 test piece) and a Charpy test. In addition, about the Charpy impact test, after grinding the 2.6t steel plate to 2.5t, the V notch test piece was produced and it used for the Charpy impact test. The toughness was evaluated as follows. A value E ₀ obtained by dividing the Charpy impact value at a test temperature of −120 ° C. by the tensile strength of the hot pressed member before the heat treatment was obtained. Next, a value E _h obtained by dividing the Charpy impact value at the test temperature of −120 ° C. by the tensile strength of the hot pressed member after the heat treatment was obtained. Here we define the E _h as E value divided by the E _0, and a case where the E value is equal to or greater than 1.1 Pass. In Table 2, the yield ratio YR, which is a value obtained by dividing the yield strength by the tensile strength, is also described.

  Example No. which is an example of the present invention. The toughness improving effect at 1 to 10 is clear. On the other hand, Example No. which is a comparative example. 11 and 13 do not satisfy the scope of the present invention, and thus the toughness improving effect is not recognized. In addition, Example No. No. 12 is insufficient in strength because the range of the present invention is not satisfied.

Claims (5)

In mass%, C: 0.15-0.45%, Mn + Cr: 0.5-3.0%, P: 0.05% or less, S: 0.03% or less, Si: 0.5% or less , Ni: 3% or less, Cu: 1% or less, V: 1% or less and Al: containing one or more of 1% or less, a steel having a chemical composition the balance being Fe and impurities, Ac ₃ Tensile strength by applying a hot press after holding for 10 minutes or less in a temperature range of ₃ points or more (Ac ₃ points + 100 ° C.) and then cooling to Mf point at a cooling rate higher than the upper critical cooling rate A method for producing a hot pressed member, characterized in that after heat treatment is set to 1.2 GPa or more, a heat treatment is performed in a temperature range of 150 ° C. to 200 ° C. for 10 minutes or more.   The method for producing a hot-pressed member according to claim 1, wherein the chemical composition contains B: 0.01% or less in mass% instead of part of Fe.   The chemical composition contains one or two selected from the group consisting of Nb: 1.0% or less and Mo: 1.0% or less in mass%, instead of a part of Fe. The manufacturing method of the hot press member of Claim 1 or 2 to do. The method for producing a hot-pressed member according to any one of claims 1 to 3, wherein the chemical composition contains Ti in an amount satisfying the following formula (1) instead of a part of Fe.
3.42N + 0.001 ≦ Ti ≦ 3.42N + 0.5 (1)
Here, Ti and N in a formula show content (unit: mass%) of each element in steel.   The method for producing a hot-pressed member according to any one of claims 1 to 4, wherein the chemical composition contains Ca: 0.005% or less in mass% instead of part of Fe. .