JP4867320B2 - High strength steel member and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel member and a method for manufacturing the same. In particular, the present invention relates to a high-strength steel member excellent in absorbed energy during bending deformation and a method for producing the same.

  In recent years, in order to improve the fuel efficiency of automobiles and the safety of passengers in the event of a collision, it has been actively studied to apply high-strength steel sheets with a tensile strength of 780 MPa or more to automobile parts, mainly reinforcing members. . However, a high strength steel sheet having a tensile strength of 780 MPa or more has a problem that since the formability is poor, cracks and shape defects are likely to occur during forming and the material cost is high.

  Therefore, in recent years, high strength has been achieved by securing formability using a low strength steel plate of 440 MPa level and performing induction hardening or the like. For example, Non-Patent Document 1 discloses an invention that secures a desired high strength by induction hardening using low strength steel plates of 440 MPa and 390 MPa levels for the center pillar reinforcement and the front cross member, respectively. ing.

Patent Document 1 discloses that a steel sheet that is heated to an austenite region to be softened and highly ductile is press-formed into a complex shape with high dimensional accuracy and rapidly cooled (quenched) in a mold. Discloses an invention for enhancing the strength of a steel sheet by martensitic transformation.
JP 2002-102980 A Materia, Vol. 37, No. 6 (1998) 525-527

  The invention disclosed in Non-Patent Document 1 and Patent Document 1 aims to improve impact characteristics by increasing the strength of the entire member, that is, to improve the absorbed energy during bending deformation. However, since these inventions focus only on the strength of the entire member, there is still room for improvement in improving the impact characteristics.

  The subject of this invention is providing the high strength steel member which is excellent in the absorbed energy at the time of bending deformation, and has the tensile strength of 780 Mpa or more, and its manufacturing method.

  As a result of intensive investigations on improving the absorbed energy at the time of bending deformation of a high-strength steel member having a tensile strength of 780 MPa or more, the present inventor has obtained chemical composition, surface properties, surface layer portion and inner layer portion structure. In other words, by optimizing the composition of the raw materials and the heat treatment conditions during production, the absorbed energy during bending deformation is dramatically improved while maintaining the tensile strength of 780 MPa or more. As a result, the present invention has been completed.

  In the present invention, C: 0.08% or more and 0.45% or less (in this specification, “%” means “mass%” unless otherwise specified), Si: 0.5% or less, Mn + Cr : 0.5% or more and 3.0% or less, P: 0.05% or less, S: 0.05% or less, Al: 1% or less, N: 0.01% or less, from the balance Fe and impurities Martensite having a steel composition of which the structure fraction of ferrite in the surface layer portion from the surface to a depth of 15 μm is 80% or more, and the inner layer portion excluding the surface layer portion has a prior austenite average particle size of 25 μm or less And the product of the center line average roughness Ra of this surface and the number PPI per 25.4 mm of a pair of a peak of 0.3175 μm or more and a valley of 0.3175 μm or more from the center line of the surface roughness curve Tensile, characterized in that is 200 or more Saga is 780MPa or more high strength steel member.

The high strength steel member according to the present invention preferably further contains Ni: 2% or less and / or Cu: 1% or less.
These high-strength steel members according to the present invention preferably further contain B: 0.01% or less.

  These high-strength steel members according to the present invention are further selected from the group consisting of Nb: 1.0% or less, Ti: 1.0% or less, Mo: 1.0% or less, and V: 1.0% or less. It is desirable to contain one kind or two or more kinds.

From another viewpoint, the present invention heats a steel material having the above steel composition to a temperature range of Ac ₃ points or more (Ac ₃ points + 200 ° C.) at an average temperature rising rate of 1 to 100 ° C./s, and this The method for producing a high-strength steel member according to the present invention is characterized in that after being kept in a temperature range for 5 minutes or less, quenching is performed at a cooling rate equal to or higher than the upper critical cooling rate.

  In the manufacturing method of the high strength steel member according to the present invention, the steel material has a surface centerline average roughness Ra, a peak of 0.3175 μm or more and a valley of 0.3175 μm or more from the centerline of the surface roughness curve. The product of the number PPI per 25.4 mm of the pair is preferably 50 or more.

  According to the present invention, it is possible to provide a high-strength steel member having excellent tensile energy of 780 MPa or more and a method for producing the same while being excellent in absorbed energy during bending deformation.

BEST MODE FOR CARRYING OUT THE INVENTION A best mode for carrying out a high strength steel member and a method for producing the same according to the present invention will be described in detail with reference to the accompanying drawings.
The reason for limiting the composition of the high-strength steel member of the present embodiment and the steel material that is the material thereof will be described.

C: 0.08% or more and 0.45% or less C is a very important element that enhances hardenability and determines strength after quenching. In order to ensure a tensile strength of 780 MPa or more, at least 0.08% or more is contained. On the other hand, when the C content exceeds 0.45%, the strength becomes too high, the toughness is significantly deteriorated, and the absorbed energy at the time of bending deformation is reduced. Therefore, in the present embodiment, the C content is limited to 0.08% or more and 0.45% or less. A more desirable lower limit of the C content is 0.15%, and an upper limit is 0.33%.

Si: 0.5% or less Si is an element that enhances hardenability and is effective for ensuring stable strength after quenching. However, even if the Si content exceeds 0.5%, such an effect cannot be expected, and the cost is unnecessarily increased. Therefore, in the present embodiment, the Si content is limited to 0.5% or less.

Mn + Cr: 0.5% or more and 3.0% or less Mn and Cr are very effective elements for improving the hardenability and ensuring the strength after quenching stably. If the total content of Mn and Cr (hereinafter also referred to as “(Mn + Cr) content”) is less than 0.5%, such an effect cannot be sufficiently obtained, while the (Mn + Cr) content is 3. If it exceeds 0%, the effect is saturated, and it is difficult to secure stable strength. Therefore, in this embodiment, the (Mn + Cr) content is limited to 0.5% to 3.0%. The lower limit of the more desirable (Mn + Cr) content is 0.8%, and the upper limit is 2.0%.

P: 0.05% or less When the P content exceeds 0.05%, the toughness of the steel member is greatly deteriorated. Therefore, in the present embodiment, the P content is limited to 0.05% or less.

S: 0.05% or less When the S content exceeds 0.05%, the toughness of the steel member is greatly deteriorated. Therefore, in the present embodiment, the S content is limited to 0.05% or less.

Al: 1% or less Al is an element that enhances hardenability and is effective for ensuring stable strength after quenching. However, when the Al content exceeds 1%, such an effect is saturated, and the cost is unnecessarily increased. Therefore, in the present embodiment, the Al content is limited to 1% or less.

N: 0.01% or less N is an element that enhances hardenability and is effective for ensuring stable strength after quenching. However, when the N content exceeds 0.01%, such an effect is saturated and the cost is unnecessarily increased. Therefore, in the present embodiment, the N content is limited to 0.01% or less.
Further, in the present embodiment, the following elements may be included as optional addition elements, and therefore these optional addition elements will be described.

Ni: 2% or less and / or Cu: 1% or less Ni and Cu are optional addition elements in the present embodiment, and are necessary because they are elements effective for enhancing hardenability and ensuring stable strength after quenching. 1 type or 2 types can be contained according to. However, even if Ni is contained in excess of 2% and Cu is contained in excess of 1%, the effect is small and the cost is unnecessarily increased. Therefore, when Ni and Cu are contained, the content is desirably Ni: 2% or less and / or Cu: 1% or less. A more desirable lower limit of the B content is 0.01% for Ni and 0.01% for Cu.

B: 0.01% or less B is an optional additive element in the present embodiment, and is an element that enhances hardenability and is effective in ensuring the stability of strength after quenching. It is also an important element in that it segregates at the grain boundaries to increase grain boundary strength, improve toughness, and increase the energy absorbed during bending deformation. Furthermore, the effect of suppressing the growth of austenite grains at the time of temperature rise before quenching is also high. However, when the B content exceeds 0.01%, such an effect is saturated and the cost is increased. Therefore, when B is contained, the content is desirably limited to 0.01% or less. A more desirable lower limit of the B content is 0.001%, and an upper limit is 0.0030%.

One or more selected from the group consisting of Nb: 1.0% or less, Ti: 1.0% or less, Mo: 1.0% or less, and V: 1.0% or less Nb, Ti, Mo and V is an optional additive element, and when heated to Ac ₃ or more points, V suppresses recrystallization and forms fine carbides to make the austenite grains fine, thus greatly improving toughness, Since it has the effect of improving the absorbed energy at the time of bending deformation, one or more kinds are contained. However, even if the content of each element exceeds 1.0%, such an effect is saturated and the cost is unnecessarily increased. Therefore, when Nb, Ti, Mo or V is contained, the content is Nb: 1.0% or less, Ti: 1.0% or less, Mo: 1.0% or less, or V: 1.0%. It is desirable to limit to the following.

  A desirable lower limit of the Nb content is 0.01% and an upper limit is 0.2%, and a more desirable lower limit of the Nb content is 0.02% and an upper limit is 0.15%. Further, the lower limit of the desirable Ti content is 0.005%, the upper limit is 0.2%, the more desirable lower limit is 0.01%, and the upper limit is 0.15%. The desirable lower limit of the Mo content is 0.01%, the upper limit is 0.2%, the more desirable lower limit is 0.02%, and the upper limit is 0.15%. Furthermore, the lower limit of the desirable V content is 0.01%, the upper limit is 0.2%, the more desirable lower limit is 0.02%, and the upper limit is 0.15%.

Compositions other than those described above are Fe and impurities.
The high-strength steel member of the present embodiment and the steel material that is the material thereof have the steel composition described above.

Next, the manufacturing method of the high strength steel member of this embodiment will be described.
In the present embodiment, the advanced steel member of this embodiment is a steel material having the above-described steel composition at a temperature of Ac ₃ points or more (Ac ₃ points + 200 ° C.) at an average temperature rising rate of 1 to 100 ° C./s. It is manufactured by heating to a zone and holding at this temperature zone for 5 minutes or less, followed by quenching at a cooling rate equal to or higher than the upper critical cooling rate.

  The advanced steel member according to the present embodiment is manufactured using a steel material having the above-described steel composition as a raw material, and the (Ra × PPI) value of the obtained steel member is further increased to improve the absorbed energy at the time of bending deformation. For this purpose, it is desirable to perform a quenching treatment using a steel material having the above-described steel composition and having a (Ra × PPI) value of 50 or more as a raw material. The (Ra × PPI) value of this steel material is desirably 100 or more, and more desirably 150 or more.

Further, in order to make the inner layer portion of the steel member martensite, it is necessary to heat the steel material to Ac ₃ point or higher and to cool at the upper critical cooling rate or higher. As for the heating temperature, if heating exceeds (Ac ₃ points + 200 ° C.), the amount of scale generated becomes too large, or the prior austenite grains become coarse and the toughness of the whole member becomes severely deteriorated (Ac ₃ points + 200 ° C.). The following. A more desirable range is Ac ₃ points or more (Ac ₃ points + 150 ° C.) or less, and a further desirable range is Ac ₃ points or more (Ac ₃ points + 100 ° C.).

  When the average heating rate of the steel material is 1 ° C./s or more and 100 ° C./s or less, the heating time in the ferrite region becomes long, so the decarburization reaction of the surface layer is promoted, and a ferrite layer is formed on the surface layer during quenching cooling. It becomes easy. If the average heating rate is less than 1 ° C./s, the thickness of the ferrite main layer exceeds 15 μm or more. On the other hand, if the average heating rate exceeds 100 ° C./s, the ferrite phase is not sufficiently formed. A desirable range is 1 ° C./s to 50 ° C./s, and a more desirable range is 1 ° C./s to 20 ° C./s.

In order to refine the prior austenite grains of the martensite in the inner layer, the holding time after the temperature rise is set to 5 minutes or less. Desirably 3 minutes or less, more desirably 1 minute or less. Here, the holding time means the elapsed time from the point when Ac ₃ points are reached.

  The cooling rate is set to a cooling rate equal to or higher than the upper critical cooling rate in order to achieve martensite formation of the inner layer portion of the steel member. As the means, any means such as water cooling, oil cooling, and cooling by a mold may be used.

Next, the surface properties of the high-strength steel member of the present embodiment manufactured as described above, and the structure of the surface layer portion and the inner layer portion will be described.
Structure fraction of ferrite in the surface layer part from the surface to a depth of 15 μm: 80% or more By making the surface layer part of the steel member mainly composed of ferrite, the absorbed energy at the time of bending deformation can be improved. If the ferrite structure fraction is less than 80%, the effect of improving the absorbed energy is not sufficient, so the ferrite structure fraction in the surface layer portion is set to 80% or more. It is desirably 85% or more, and more desirably 90% or more. In addition, when the thickness of the surface layer portion, which is a structure mainly composed of ferrite, exceeds 15 μm, the strength of the entire steel member is lowered. Therefore, the surface layer portion has a depth of 15 μm from the surface.

Inner layer portion excluding the surface layer portion: Old martensite average particle size of 25 μm or less old martensite-based structure By reducing the old austenite average particle size of martensite in the inner layer portion of the steel member excluding the surface layer portion described above, during bending deformation The absorbed energy can be improved. If the prior austenite average particle diameter exceeds 25 μm, the effect is not sufficient, so the average austenite particle diameter is set to 25 μm or less. The thickness is desirably 20 μm or less, and more desirably 15 μm or less. Even if the martensite in the present invention contains less than 10% by volume of retained austenite and bainite, the properties are not substantially affected, so that it is allowed.

The product of the surface centerline average roughness Ra and the number PPI per 25.4 mm of a pair of a peak of 0.3175 μm or more and a valley of 0.3175 μm or more from the centerline of the surface roughness curve (hereinafter referred to as Ra × (Expressed as PPI value): 200 or more By increasing the (Ra × PPI) value on the surface of a steel member, the absorbed energy at the time of bending deformation is improved. However, if the (Ra × PPI) value is less than 200, such an effect is not sufficient. Desirably 250 or more, more desirably 300 or more.

Tensile strength of the high-strength steel member of this embodiment : 780 MPa or more The tensile strength of the high-strength steel member of this embodiment is 780 MPa or more. For this reason, this high-strength steel member is extremely desirable as a material for high-strength parts for automobiles such as a center pillar reinforcement and a front cross member.

Furthermore, the present invention will be described more specifically with reference to examples.
A steel plate having a chemical composition, Ac ₃ points and (Ra × PPI) value shown in Table 1 and having a plate thickness of 2.3 mm was used as a steel material. These steel plates are steel plates manufactured by cold rolling by pickling hot-rolled or hot-rolled slabs in a laboratory. In addition, skin pass rolling was performed to adjust the surface roughness of the steel sheet.

  Samples having a thickness of 2.3 mm, a width of 70 mm, and a length of 160 mm were cut from these steel plates, and the conditions shown in Table 2 (temperature increase rate, heating temperature, holding time) were performed in a heating furnace in an air atmosphere. Then, it was taken out from the heating furnace, and immediately after that, it was quenched by water cooling or pressing using a flat steel mold. A thermocouple was attached to the steel plate, and the cooling rate was also measured. Table 2 also shows the cooling rate during quenching.

About the obtained steel member, it used for the cross-sectional structure | tissue observation, the old austenite particle size measurement by the cutting method, the tension test (JIS No. 5 test piece), and the static three-point bending test.
The ferrite structure fraction of the surface layer portion was calculated by performing image analysis from an optical microscope observation image or an electron microscope observation image of the cross section.

The _three Ac points and the upper critical cooling rate of each steel type were measured by the following method. That is, a cylindrical test piece having a diameter of 3.0 mm and a length of 10 mm shown in FIG. 1 is cut out from the hot-rolled steel sheet, heated to 900 ° C. at a temperature increase rate of 10 ° C./s, and at that temperature, 5 After holding for a minute, it was cooled to room temperature at various cooling rates. The Ac ₃ point and Ms point were measured by measuring the thermal expansion change of the test piece during heating and cooling at that time. Moreover, the Vickers hardness measurement (load 49N, the number of measurements: 3) and structure | tissue observation of the obtained test piece were performed, and the upper critical cooling rate was estimated from those results.

About Ra and PPI regarding surface roughness, it measured based on JIS-B0601 and SAEJ911.
The static three-point bending test conditions were a punch diameter R6.5, a stroke speed of 10 mm / min, and a span interval of 140 mm. The absorbed energy was evaluated by the absorbed energy up to the maximum load.

  The results are summarized in Table 2. As shown in Table 2, when the members (steel types No. 1-6, 2-7, 3-8, 4-9, 5-10) having the same tensile strengths of the present invention example and the comparative example are compared, Even if the strength is substantially the same, the effect of the present invention is clear since the example of the present invention is higher than the comparative example by about 10% with respect to the absorbed energy.

  Further, in the high strength steel member of the present invention, the absorbed energy at the time of bending deformation is 0.28 (J / J) in the ratio of absorbed energy at the time of bending deformation to the tensile strength {(absorbed energy) / (tensile strength)}. MPa) or more, and excellent tensile strength of 780 MPa or more. In addition, as a typical example of comparison, steel type No. 3 and no. The load-stroke curve for 8 is shown graphically in FIG. From the results shown in FIG. 2, the steel type No. which is an example of the present invention regarding the absorbed energy. 3 is a steel type No. 3 as a comparative example. It is clear that it is better than 8.

It is explanatory drawing which shows the test piece for _AC3 point and an upper critical cooling rate. It is a load-stroke curve.

Claims (6)

In mass%, C: 0.08 to 0.45%, Si: 0.5% or less, Mn + Cr: 0.5 to 3.0%, P: 0.05% or less, S: 0.05% or less, Al: not more than 1%, N: not more than 0.01%, having a steel composition consisting of the remainder Fe and impurities, and having a structure fraction of ferrite of 80% or more in the surface layer portion from the surface to a depth of 15 μm Furthermore, the inner layer portion excluding the surface layer portion is composed of martensite having a prior austenite average particle size of 25 μm or less, and the center line average roughness Ra of the surface is 0.3175 μm or more from the center line of the surface roughness curve. A high-strength steel member having a tensile strength of 780 MPa or more, characterized in that a product of a number PPI per 25.4 mm of a pair of a mountain and a valley of 0.3175 μm or more is 200 or more. The high-strength steel member according to claim 1, further comprising, by mass%, Ni: 2% or less and / or Cu: 1% or less. Furthermore, the high-strength steel member described in Claim 1 or Claim 2 which contains B: 0.01% or less by mass%. Furthermore, by mass%, Nb: 1.0% or less, Ti: 1.0% or less, Mo: 1.0% or less, and V: 1.0% or less selected from the group consisting of 1.0% or less The high-strength steel member according to any one of claims 1 to 3, comprising: The steel having the steel composition described in any one of claims 1 to 4, at an average heating rate 1 to 100 ° C. / s Ac ₃ point or more (Ac ₃ point + 200 ° C.) below the temperature range 5, and after maintaining at the temperature range for 5 minutes or less, a quenching treatment is performed at a cooling rate that is equal to or higher than the upper critical cooling rate. Manufacturing method of high strength steel members. The steel material has a product of the surface center line average roughness Ra and the number PPI per 25.4 mm of a pair of a peak of 0.3175 μm or more and a valley of 0.3175 μm or more from the center line of the surface roughness curve. The method for producing a high-strength steel member according to claim 5, which is 50 or more.

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