JP5717024B2 - Thin-walled steel product and heat treatment method thereof - Google Patents

Description

  The present invention relates to a thin-walled steel product obtained by pressing thin-walled steel into a predetermined shape and a heat treatment method thereof.

  For example, seat frames for transportation equipment such as automobiles and airplanes are strongly required to be lighter from the viewpoints of improving fuel efficiency and regulating carbon dioxide emissions. For this reason, it is necessary to increase the strength of steel materials forming the seat frame. ing. On the other hand, the seat frame is required not only to increase the strength, but also to have a portion having high ductility (elongation characteristics) from the viewpoint of impact absorption due to deformation. Also, from the viewpoint of notch sensitivity and durability, it is required to reduce or eliminate the tensile residual stress or to obtain a compressive residual stress. Conventionally, for example, high strength steel sheets disclosed in Patent Documents 1 to 3 are known.

  The high-strength steel sheets disclosed in these are all premised on controlling the addition amount of alloying elements other than carbon. For example, Mn, Mo, Cr, etc. are contained in a predetermined amount or more to obtain a predetermined hardness. To ensure ductility. And since it is finally cold-rolled to 1.2 mm for use as a steel material for automobiles, etc., the heat treatment performed in the process before cold-rolling is hot-rolling the steel slab to a thickness of 3.2 mm. To do. In other words, since a steel sheet having a thickness of several millimeters or more is obtained, it is necessary to make the microstructure uniform in the steel sheet including the thickness direction in the heat treatment. Is an important element.

  On the other hand, Patent Documents 4 to 5 disclose techniques for increasing the strength of ordinary low carbon steel. In Patent Document 4, since the hardenability of ordinary low carbon steel is poor in the previous technology, when martensite is used as the starting structure, a heterogeneous mixed grain structure is generated during annealing, and a predetermined high strength and high ductility are obtained. It was made in order to solve the problem that a steel material could not be obtained. For this reason, in Patent Document 4, after a normal low carbon steel is quenched to obtain a martensite phase of 90% or more, cold rolling and annealing with a total reduction of 20% or more and less than 80% are performed. An ultrafine grain ferrite structure of 0 μm or less is obtained. Patent Document 5 is a technique proposed by the present applicant, and performs processing that increases internal stress such as press molding, and by heat treatment, the metal structure of the low-carbon steel is refined and mixed to increase the grain size. It is strengthened.

Japanese Patent No. 4005517 JP 2005-213640 A JP 2008-297609 A Japanese Patent No. 4189133 JP 2008-13835 A

  For automobile seat frames and the like, demands for cost reduction and resource recyclability will increase in the future due to energy saving and response to environmental problems. Therefore, it is desired that it can be achieved by using ordinary steel (particularly low carbon steel) having higher recyclability than the high strength and high ductility by alloying as in the techniques of Patent Documents 1 to 3. In addition, these are methods that are mainly used by steel material manufacturers to create predetermined high-strength, high-toughness steel from steel slabs, and can be used by processing manufacturers that process seat frames using commercially available steel. It's not technology. As a processing manufacturer, instead of purchasing and using high-strength, high-toughness steel (high-tensile steel) that steel material manufacturers sell in this way, ordinary steel that is cheap and easy to form from steel material manufacturers If it is possible to increase the strength and ductility of the ordinary steel at the necessary locations when necessary, the cost of the seat frame will be reduced.

  On the other hand, when high strength high toughness steel (high tensile steel) having a tensile strength of 980 MPa or more is used, it has been pointed out that it is extremely difficult to process into a predetermined shape by pressing. This is because it is difficult to achieve a balance with formability (stretch flange characteristics, bending characteristics, etc.) when the tensile strength is increased. A steel material with a tensile strength of 980 MPa is composed of two phases, martensite and ferrite, to improve ductility, but when it is subjected to press working, it is microscopically at the interface between ferrite and martensite. It is also known that cracks occur and cracks occur based on them.

  The technique of Patent Document 4 is a technique for obtaining desired strength and ductility by using ordinary low carbon steel as an acceptor for heat treatment. After the whole steel material is martensitic, it is cold-rolled and homogenized. It is necessary to make it finer. Accordingly, equipment having a rolling function is required, and there are problems in terms of equipment cost and manufacturing cost. As is apparent from the fact that an ordinary low carbon steel material having a thickness of 2 mm is exemplified in the example of Patent Document 4, in order to increase the strength and ductility of a steel having a certain thickness, This is because uniform refinement in the sheet thickness direction is necessary, and therefore a cold rolling process under a predetermined condition after the martensite formation is essential.

  In the case of the technique of Patent Document 5, in the examples, a cold rolled steel sheet and a hot rolled steel sheet made of thin-walled low carbon steel having a thickness of 1.2 mm and 1.0 mm are heat-treated and refined to increase the strength. Although it is possible to increase the strength of the low-carbon steel by performing press work in advance, it only describes that one member is controlled to a certain tensile strength.

  By the way, a seat frame for automobiles requires strength and rigidity as a structure that safely supports people, and in order to protect people from impact forces such as collisions, it increases the shock absorption energy while maintaining a predetermined rigidity. There is a need to. However, if this depends only on the material characteristics of the steel material for forming the seat frame, there is a limit in terms of the difficulty of forming described above. Further, it is necessary to improve the durability. However, by increasing the strength, notch sensitivity and residual stress cause deterioration in durability.

  In view of this, the present invention has a configuration in which a single member has a plurality of regions of physical characteristics by pressing into a predetermined shape and then performing a heat treatment, thereby balancing strength, durability, rigidity, shock absorption characteristics, and the like. An object of the present invention is to provide a thin-walled steel processed product suitable for a seat frame and a heat treatment method thereof.

  In order to solve the above-mentioned problems, the thin-walled steel processed product of the present invention is a thin-walled steel processed product obtained by pressing thin-walled steel into a predetermined shape, and a plurality of regions having different physical characteristics are included in one member. It is formed by heat treatment, and the tensile strength of a part of the plurality of regions is 980 MPa or more.

  The plurality of regions can be controlled to have a single phase structure of martensite, a two-phase structure of martensite and ferrite, or a mixed grain structure having a plurality of crystal grains having different particle sizes by changing the heat treatment conditions for each region. It is preferable to be obtained. Further, it is preferable that at least one physical characteristic of the tensile strength, ductility, and tensile residual stress is different between the plurality of regions. The thickness of the thin steel is preferably 1.0 mm or less. The thickness of the thin steel is more preferably 0.8 mm or less. A heat treatment is performed on the overlapping portion formed by hemming of the thin steel, and the folded portion and the facing portion in the overlapping portion are welded at the same time as increasing the strength by the heat treatment. . It is preferable that the member is any plate-like frame forming a seat frame.

  The present invention is also a heat treatment method for a thin-walled steel product obtained by pressing thin-walled steel into a predetermined shape, and rapidly heats from room temperature to a temperature higher than the A1 transformation point at a heating rate of 400 ° C./second or more. A rapid heating step and a rapid cooling step of rapid cooling at a cooling rate of 800 ° C./second or more after the rapid heating step, adjusting the heating temperature of the rapid heating step for the member to be heat-treated, or the rapid cooling A plurality of regions having different physical properties are formed depending on whether the rapid cooling start temperature of the process is higher than the A1 transformation point or lower than the A1 transformation point, and a part of the region is controlled to have a tensile strength of 980 MPa or more. Provided is a heat treatment method for a thin-walled steel product.

  It is preferable to heat-treat by applying the rapid heating step and the rapid cooling step to an overlapping portion where a plurality of the thin steels overlap. As the overlapping portion, heat treatment can be performed on a portion formed by hemming. It is preferable that the folded portion and the facing portion in the overlapping portion formed by the hemming are welded simultaneously with increasing strength by the rapid heating step and the rapid cooling step. In addition, a protrusion protruding outward is formed in a portion adjacent to the facing portion, and the protrusion is melted by the rapid heating step and enters between the folded portion and the facing portion. can do. The heating rate in the rapid heating step is preferably 1000 ° C./second or more, and the cooling rate in the rapid cooling step is preferably 1000 ° C./second or more.

  According to the present invention, a thin-walled steel product obtained by pressing thin-walled steel into a predetermined shape, and a plurality of regions having different physical properties are formed in one member by heat treatment. The tensile strength of the area of the part is 980 MPa or more. That is, by subjecting the pressed thin-walled steel product to a heat treatment, a region having a tensile strength of 980 MPa or more is set in part. When a high strength high toughness steel (high tensile steel) with a tensile strength of 980 MPa or more is used, the formability by press working is poor as described above, and the conventional method relies only on the material properties. As a result, such a high-strength portion cannot be provided in a portion with a high degree of processing (for example, a portion subjected to hemming), but according to the present invention, a portion having a tensile strength of 980 MPa or more even in a portion with a high degree of processing. Can be provided, and the rigidity of the thin-walled steel product can be increased.

  On the other hand, by adopting a configuration having different characteristics in any of tensile strength, ductility and tensile residual stress, a portion having high rigidity as described above, a portion that performs an energy absorption function when subjected to an impact, The part which improves durability can be set to one member. For this reason, it is particularly suitable as a vehicle seat frame.

  Further, according to the heat treatment method of the present invention, characteristics such as tensile strength can be partly easily made depending on whether the rapid cooling start time is set to be equal to or higher than the A1 transformation point or less than the A1 transformation point depending on the part to be heat treated. It can be adjusted to a predetermined value. Further, residual stress in the tensile direction accompanying press working or the like can be removed by performing heat treatment.

  Moreover, it is also possible to simultaneously perform welding for integrating a plurality of sheets by heat-treating an overlapping portion where a plurality of thin-wall steels are overlapped by press working or the like. In particular, when the folded portion formed by hemming and the facing portion are heat-treated and simultaneously welded, the mechanical coupling force at the portion can be increased.

FIG. 1 is a perspective view showing a seat frame that is a thin-walled steel product according to one embodiment of the present invention. FIG. 2 is a side view of FIG. 3A and 3B are cross-sectional views taken along line C in FIG. FIGS. 4A to 4D are views for explaining heat treatment in the case where a protrusion is provided in a hemmed portion. FIG. 5 is a diagram showing an example of a heat treatment facility. FIG. 6 is a view showing a high-frequency electrode of the heat treatment facility. FIG. 7 is a target temperature history curve in the heat treatment of the test example. FIG. 8 is a diagram showing the shape of a test piece subjected to a tensile test in a test example. FIG. 9 is a diagram showing a stress-strain curve of SPCC / t1.0 used in the test example. FIG. 10 is a view showing a stress-strain curve of SPHC / t1.2 used in the test example. FIG. 11 is a diagram showing the microstructure of each sample of SPCC / t1.0 and SPHC / t1.2, where (a) is a material, (b) is induction-hardened, and (c) is electricity. It is the figure of what performed furnace hardening. FIG. 12 is a diagram showing the temperature history of the heat treatment of SPCC / t0.5 used in the test example. FIG. 13 is a diagram showing a stress-strain curve of SPCC / t0.5 used in the test example. FIG. 14 is a diagram showing the microstructure of each material of SPCC / t0.5 used in the test example. (A) shows sample A, (b) shows sample B, and (c) shows sample C. FIG. FIG. 15 is a diagram showing a temperature history of the heat treatment of SPFC / t0.6 used in the test example. FIG. 16 is a diagram showing a stress-strain curve of SPFC / t0.6 used in the test example. FIG. 17 is a diagram showing the microstructure of each SPFC / t0.6 material used in the test example, where (a) shows sample A, (b) shows sample B, and (c) shows sample C. FIG. FIG. 18A is a perspective view of an automobile seat frame including a side frame manufactured using SPCC / 0.5t and SPFC440 / 0.6t thin plates, and FIG. 18B is a perspective view of FIG. FIG. 18C is a side view taken along the line AA in FIG. 18B, and FIG. 18D is a cross-sectional view taken along the line BB in FIG. 18B. FIG. 19 is a diagram showing the deformation behavior of the seat frame when a downward load is applied. (A) shows a state before addition, and (b) shows a state after addition. FIG. 20 is a diagram showing the analysis results of reaction force and displacement from the frame.

  In the present invention, a thin steel product to be heat-treated is processed into a predetermined shape by pressing using a thin-walled product (hereinafter referred to as “thin steel”). As thin-walled steel, rolled steel sheets that are inexpensive and have good workability are suitable for use in automobile seat frames, and include both cold-rolled steel sheets and hot-rolled steel sheets. The thickness is 1.2 mm or less. When steel is thicker than this, a large heat source and a large-scale cooling facility are required for rapid heating / cooling, or heat treatment takes time. In consideration of increasing the strength of a desired part of a thin-walled steel product by rapid and rapid heating and quenching heat treatment processes and considering the weight reduction of the product, a thin-wall steel having a thickness of 1.0 mm or less is preferable. Thin wall steel of 8 mm or less is more preferable, and thin wall steel of 0.5 mm or less is further preferable.

  As the thin-walled steel, an inexpensive material can be used to reduce the manufacturing cost of a seat frame or the like, so that the carbon content is low. It is preferably 1% or less, and more preferably 0.05% or less. Moreover, it is preferable that content of Mn is 1.5% or less by mass%, and it is more preferable that it is 0.5% or less. Although the lower limit of the carbon content is not limited, in the test examples described later, even the 0.002% content is enhanced by the method of the present invention. However, in the present invention, heat treatment is performed on a portion that has already been subjected to press working or the like, and the thin-walled steel may be high-tensile steel, and has a higher carbon content and alloy element content. Those are also subject to the heat treatment of the present invention. For example, cold rolled steel sheets of 590 MPa class, 780 MPa class, and 980 MPa class or higher have a two-phase structure of ferrite + martensite. This is a two-phase structure in order to provide a ductility of a predetermined level or more for use in press working or the like, but in the case of a two-phase structure, microcracks at the interface are likely to occur. However, after manufacturing a thin-walled steel product using such a cold-rolled steel sheet, when the heat treatment of the present invention is applied, the two-phase structure becomes a martensitic single-phase structure and microcracks are less likely to occur. There are advantages.

  However, the present invention is limited to thin-walled pressed products, so that the strength can be increased even if the carbon content is low, and the balance with ductility can also be achieved. Since it is not essential to perform addition, etc., it is excellent in recyclability. In addition, plate-shaped thing is object as thin-walled steel which forms the thin-walled steel processed goods of this invention.

  Heat treatment of the desired part of the thin-walled steel product is performed by the following rapid heating process and rapid cooling process. In the rapid heating step, the thin steel is heated from room temperature to a temperature equal to or higher than the A1 transformation point (usually about 750 to 1250 ° C.) at a heating rate of 400 ° C./second or more using a high-frequency electrode. The heating rate is preferably 1000 ° C./second or more, more preferably 2000 ° C./second or more. Furthermore, since the desired part to be heated moves relative to the high-frequency electrode, the time from when it is first heated by the high-frequency electrode until it passes the position facing the high-frequency electrode is within 0.5 seconds, preferably Is heated from room temperature to a temperature equal to or higher than the A1 transformation point within 0.2 seconds, more preferably within 0.1 seconds. As described above, since the thin steel product is extremely thin, it can be heated to the A1 transformation point or higher in such a very short time.

  In the rapid cooling step, after heating to a predetermined temperature not lower than the A1 transformation point, rapid cooling is performed by water cooling. The cooling rate is 800 ° C./second or more, preferably 1000 ° C./second or more, more preferably 2000 ° C./second or more. Furthermore, when the desired part that has been heated moves relative to the cooling equipment, water starts to cool within 0.5 seconds, preferably within 0.2 seconds, more preferably within 0.1 seconds. Reduce the temperature from the current temperature to near room temperature. Also in this case, since the thickness of the thin steel product is extremely thin, it can be rapidly cooled in such a short time.

  When the quenching start temperature with water is not less than the “A1 transformation point”, a structure in which the martensite phase occupies most, such as a structure composed of a single martensite phase, is formed. When the quenching start temperature with water is less than the “A1 transformation point”, for example, a single-phase mixed grain structure of ferrite in which fine crystals having a crystal grain size of 1 to 5 μm and crystals of 10 μm or more are mixed is formed. . Therefore, when quenching from the A1 transformation point or higher, the tensile strength increases, but the ductility decreases. When quenched from below the A1 transformation point, the tensile strength is lower than that of the martensite single-phase structure, but the ductility is increased.

In the present invention, heat treatment is performed on a thin steel product obtained by processing thin steel into a predetermined shape by press working. Examples of the thin-walled steel processed product include a seat frame of a vehicle seat such as an automobile. In the present invention, a plurality of physical characteristics are set in one member such as a side frame. For example, in the seat frame 1 shown in FIG. 1 and FIG. 2, when heat treatment is performed on an overlapping portion in which a plurality of thin steels overlap, the thin steels that overlap in that portion are welded together. In particular, as shown in FIG. 1, two pieces of thin steel (inner frame 10 and outer frame 20 (see FIG. 3)) used as the side frame 2 are overlapped, and the edge of the inner frame 10 is folded back 180 degrees. It is preferable to heat-treat the overlapping portion ( overlapping portion 2a) by performing hemming.

  As shown in FIG. 3, heat treatment is performed on the folded portion 11 where the edge of the inner frame 10 is folded and the facing portion 21 of the outer frame 20 facing the folded portion 11. The distance from the high frequency electrode 30 is closer to the folded portion 11 than to the facing portion 21. However, as shown in FIG. 3A, the flange portion including the folded-back portion 11 and the facing portion 21 is not easily raised in temperature because three plates are stacked. However, as shown in FIG. 3B, if the gap is provided on the back side of the folded portion 11 (that is, between the folded portion 11 and the facing portion 21), the temperature of the flange portion is likely to increase. . Therefore, when both are heated at the same time and then rapidly cooled, the temperature at the start of the rapid cooling is higher in the folded portion 11 having a gap on the back side. For this reason, the folded portion 11 having a gap on the back side has higher tensile strength and lower ductility than the facing portion 21. At this time, if the temperature at the time of the rapid cooling start of the folded part 11 having voids on the back side is equal to or higher than the A1 transformation point, the folded part 11 has a structure in which the martensite phase occupies most (for example, a single phase of the martensite phase). Organization). If the temperature at the start of quenching of the facing part 21 is less than the A1 transformation point, for example, a single-phase mixed grain structure of ferrite in which fine crystals with a crystal grain size of 1 to 5 μm and crystals with a diameter of 10 μm or more are mixed is obtained. Become higher. That is, the strength of the seat frame can be improved by forming the folded portion 11 with a single-phase structure of martensite phase. On the other hand, since the facing portion 21 of the outer frame 20 has a predetermined ductility, when it receives an impact, it deforms while sticking, which is useful for impact absorption characteristics.

  The folded portion 11 of the inner frame 10 and the facing portion 21 of the outer frame 20 are subjected to the heat treatment as described above, so that the folded portion 11 having a gap on the back side melts and is caused by gravity or pressure (air pressure or water pressure). It is preferably welded to the facing portion 21 of the outer frame 20. Thereby, the mechanical coupling force of the site | part which carried out hemming process can be raised. In addition, according to the present invention, the modification and welding of the surface structure by heat treatment can be achieved in one process, so that the work process can be simplified and the manufacturing cost can be reduced.

  In order to perform welding of the folded portion 11 and the facing portion 21 at another portion, as shown in FIGS. 4A and 4C, the outer frame 20 has a portion adjacent to the facing portion 21 outward. The protruding protrusion 22 can also be formed. The protrusion 22 is preferably provided so as to be the same height as or higher than the folded portion 11 of the inner frame 10. As a result, the distance from the high-frequency electrode is reduced, and the protrusion 22 is melted during heating (see FIGS. 4B and 4D), and enters the gap between the folded portion 11 and the facing portion 21 to form the filler material. The function is achieved, and the folded portion 11 and the facing portion 21 can be reliably welded. The shape of the protrusion 22 is not limited. As shown in FIG. 4A, the protrusion 22 may be protruded in a curved shape adjacent to the facing portion 21, or as shown in FIG. As described above, the entire portion adjacent to the facing portion 21 may be bulged outward. At the time of melting, the protruding portion 22 in FIG. 4A melts and deforms, for example, as shown in FIG. 4B, and enters the gap between the folded portion 11 and the facing portion 21, and the protruding portion 22 in FIG. 4C. For example, as shown in FIG. 4D, it melts and deforms and enters the gap between the folded portion 11 and the facing portion 21.

  Here, as shown in FIG. 5, the heat treatment facility 100 includes a high-frequency electrode 30 and a water-cooling nozzle 40 and a robot arm 50 that grips a workpiece and controls its position. As shown in FIG. 6, the high-frequency electrode 30 is preferably formed in a substantially U shape when viewed from above. And the heat processing object part of a workpiece | work is operated so that it may move relatively along the opposing direction (arrow direction of FIG. 6) of the two opposing side parts 31 and 32 of a substantially U shape. Thereby, the heat treatment target part is heated in the first stage by the opposing side part 31 that passes first, and is heated in the second stage by the opposing side part 32 that passes next, so that the two opposing side parts 31 are heated. , 32 are heated from room temperature to the A1 transformation point or higher at a stretch. Further, the high frequency electrode 30 (opposing side portions 31 and 32) is set to have a separation distance of several mm or less (for example, 1 to 3 mm) with respect to the heat treatment target portion for rapid heating.

  Further, according to the present invention, the tensile residual stress generated in the stretch flange portion by press working can be reduced by heat treatment to the part, or it can be made zero, or a state in which compressive residual stress is generated can be achieved. Thereby, the durability of the product can be improved.

(Test example)
(sample)
As test materials, automotive mild steel sheet SPCC / t1.0, SPCC / t0.5, SPHC / t1.2 and automotive low carbon sheet SPFC440 / t0.6 were used. Table 1 shows chemical components by the mill sheet. The carbon content is 0.002 mass% for SPCC / t1.0, 0.050 mass% for SPHC / t1.2, 0.040 mass% for SPCC / t0.5, and 0.120 mass% for SPFC440 / t0.6.

(Heat treatment)
A general-purpose device with a capacity of 30 kW was used as the high-frequency power source. For SPCC / t1.0 and SPHC / t1.2, electric furnace quenching was also performed for comparison. FIG. 7 shows a target temperature history curve obtained by heating and cooling a thin steel sheet for automobiles. The temperature was measured using thermography as shown in FIG. Table 2 shows the heat treatment conditions.

(Tensile test)
SPCC / t1.0, SPCC / t0.5, SPHC / t1.2, and SPFC440 / t0.6 samples were cut by wire electrical discharge machining and the test of JIS-Z2201 regulation 13B shown in FIG. It was a single shape. Using a precision universal testing machine (AG-250kNG, manufactured by Shimadzu Corp.), a tensile test was performed under the test conditions shown in Table 3, and the tensile strength and elongation at break of each sample were evaluated.

(Microstructure observation)
Since an oxide film adhered to the heat-treated sample surface, it was removed with water-resistant abrasive paper, and the sample surface was finished to a mirror surface. Thereafter, etching was performed by immersion for about 10 to 30 seconds using picral (alcohol: picric acid = 100: 5) as a corrosive solution.

(Experimental result)
-About SPCC / t1.0 and SPHC / t1.2 FIG. 9 shows the stress-strain curve of SPCC / t1.0, and FIG. 10 shows the stress-strain curve of SPHC / t1.2. By performing induction hardening for both SPCC and SPHC, the tensile strength has increased by about twice from about 300 MPa to about 600 MPa compared to the material. On the other hand, the elongation at break is reduced by about 1/2 from about 40% to less than 20%. From these results, it is possible to increase the strength by heat treatment even in the ultra mild steel sheet for automobiles with a carbon content of 0.1% or less (Mn1.5% or less) and a carbon content of 0.05% or less (Mn0.5% or less). It can be said that there is.

  For the electric furnace quenching SPCC, the tensile strength was about 400 MPa, which was lower than that of the induction-quenched sample. On the other hand, the tensile strength of SPHC is the same as that of induction hardening, but the elongation at break is less than 10% and the ductility is extremely reduced. The difference in tensile strength and elongation at break in electric furnace quenching is thought to be due to the carbon content of SPCC having a carbon content of 0.002 mass%, while SPHC has a carbon content of 0.05 mass%, and the carbon content is different. It is done.

  FIG. 11 shows the observation results of the microstructure of each sample of SPCC / t1.0 and SPHC / t1.2. (A) is a raw material, (b) is induction-hardened, and (c) is electric furnace-hardened, both of which are SPCC in the left column and SPHC in the right column.

  It can be seen that both SPCC and SPHC have a typical ferrite structure with an average grain size of about 10 μm. By subjecting the material to induction hardening, both SPCC and SPHC have a mixed grain structure in which fine crystal grains having a crystal grain size of 1 to 3 μm are dispersed at a grain boundary of about 10 μm. On the other hand, in the electric furnace quenching, the SPHC has a lath martensite structure in which packets, blocks, and lath are recognized in grain boundaries of several tens of μm over almost the entire region.

  From the observation results of the microstructure, it is estimated that by induction hardening, fine crystal grains with a grain size of 1 to 3 μm changed to a mixed grain structure dispersed around a crystal with a size of about 10 μm, which is related to strength improvement. it is conceivable that. From the above, it was confirmed that even an ultra mild steel sheet for automobiles that is not normally subject to quenching can be strengthened by quenching if it is a thin sheet with a small heat capacity that can sufficiently increase the cooling rate.

  Next, SPCC / t0.5 prepared a sample with a temperature history as shown in FIG. In each case, the temperature is higher than the A1 transformation point (723 ° C.) in 1 to 2 seconds by rapid heating. The temperature immediately before quenching with the water-cooled nozzle due to the difference in the maximum temperature was less than the A1 transformation point (723 ° C.) in the sample B and more than the A1 transformation point (723 ° C.) in the sample C.

  FIG. 13 shows the stress-strain curve and FIG. 14 shows the microstructure of these samples and the material (sample A). From the tensile test results, compared with the material (sample A) (Fig. 14 (a)) with a tensile strength of 320MPa and elongation at break of 45%, sample B (Fig. 14 (b)), which was quenched immediately below the A1 transformation point, was The strength is about double at 590MPa and about 1/2 at 20% elongation at break. Furthermore, Sample C (FIG. 14 (c)) rapidly cooled from directly above the A1 transformation point shows about 2.7 times at a tensile strength of 840 MPa and about 1/4 at a breaking elongation of 20%. Examining the microstructure of these samples, as in the results of SPCC and SPHC described above, the material (sample A) showing a ferrite structure with an average crystal grain size of about 10 μm is fine in sample B with a crystal grain size of 1 to 5 μm. In the sample C, the structure changed to a martensite structure.

  SPFC440 / t0.6 prepared a sample with a temperature history as shown in FIG. In each case, the temperature is higher than the A1 transformation point (723 ° C.) in 1 to 2 seconds by rapid heating. The temperature immediately before quenching with the water-cooled nozzle due to the difference in time was less than the A1 transformation point (723 ° C.) in the sample B and more than the A1 transformation point (723 ° C.) in the sample C.

  FIG. 16 shows a stress-strain curve and FIG. 17 shows a microstructure of these samples and the material (sample A). From the tensile test results, compared with the material (sample A) (Fig. 17 (a)) with a tensile strength of 450MPa and an elongation at break of 35%, the sample B (Fig. 17 (b)) quenched immediately below the A1 transformation point has a tensile strength. The strength is about double at 830MPa and about 1/2 at 17% elongation at break. Further, Sample C (FIG. 17C) rapidly cooled from directly above the A1 transformation point shows about 2.7 times when the tensile strength is 1220 MPa and about 1/4 when the elongation at break is 10%. When the microstructure of these samples is examined, a material (sample A) showing a ferrite structure having an average crystal grain size of about 6 μm is refined to an average crystal grain size of 2 μm in sample B. On the other hand, Sample C is refined to an average crystal grain size of 3 μm, and a martensite structure is partially observed. From this, it is understood that a thin-walled steel product having a tensile strength of 980 MPa or more can be obtained by applying the heat treatment of the present invention after processing into a predetermined shape by pressing.

  As is clear from the above test, the carbon content of SPCC / t1.0, SPCC / t0.5 and SPHC / t1.2 is 0.05% or less, and alloy elements such as Mn that improve hardenability are specially used. Does not contain. On the other hand, SPFC440 has a carbon content of 0.12 mass% and manganese of 1.06 mass%, and alloy elements that improve hardenability compared to SPCC and SPHC are not specially added, but hardenability is increased. it is conceivable that. The conventional theory is that these ultra-soft steel sheets for automobiles and low-carbon steel sheets for automobiles are difficult to strengthen even when subjected to normal quenching. However, when low-carbon steel sheets such as SPCC / t1.0, SPCC / t0.5, SPHC / t1.2, and SPFC440 / t0.6 are induction-quenched under the conditions described above, a mixed grain structure and a fine structure are obtained. It was revealed that the strength could be improved without extreme embrittlement.

  In particular, in the above-described test, as shown in the temperature history of FIG. 12 and FIG. 15 by high-frequency induction heating and immediately after water cooling, it is rapidly heated to 800 to 1,000 ° C. in 1 to 2 seconds, and 0.5 seconds from the vicinity of the A1 transformation point. It is rapidly cooled to below 100 ° C. When quenched from a relatively low temperature (below the A1 transformation point), a single-phase mixed grain structure or microstructure is obtained. When quenched from a relatively high temperature (above the A1 transformation point), martensite A single-phase structure or a mixed grain structure in which two phases of a martensite phase and a ferrite phase with different particle diameters are mixed is obtained. These structural changes are considered to be caused by the extremely high cooling rate of about 1000 ° C./s. Therefore, heat treatment conditions are partially changed by subjecting one member to heat treatment under partially different conditions, that is, a region where quenching is performed from below the A1 transformation point, and a region where quenching is performed from above the A1 transformation point. Thus, a member having a plurality of physical characteristics can be provided.

(Application to car seat frames)
SPCC / 0.5t and SPFC440 / 0.6t thin plates are hemmed as shown in FIGS. 18 (c) and 18 (d) to form a closed cross section, and the automotive seat shown in FIGS. 18 (a) and 18 (b) I made a side frame of the frame. The hemmed portion was heat treated. The hemming-processed portion is subjected to heat treatment, the folded portion 11 has a single-phase structure of martensite phase, and the facing portion 21 has a single-phase ferrite phase in which fine crystals having a grain size of 1 to 5 μm and crystals of 10 μm or more are mixed. It was a mixed grain structure (see FIG. 3). With respect to this automobile seat frame, a load was applied downward at a position indicated by an arrow A in FIG. 19A by a model imitating a human body. FIG. 19 (b) is a diagram showing a state in which buckling occurs. FIG. 20 shows the analysis results of the reaction force and displacement from the frame. From FIG. 20, it can be seen that the buckling phenomenon is eliminated by the heat treatment, and a reaction force of more than twice is generated. From the above, for example, an automobile is constituted by subjecting a press-formed product made of a mild steel plate having good formability to induction hardening and setting partially different physical characteristics in one member (for example, a side frame). It has been found that various structural parts can be reduced in weight without losing strength.

Claims (9)

A thin-walled steel product obtained by pressing thin-walled steel into a predetermined shape,
In one member, a plurality of regions having different physical properties are formed by heat treatment,
The tensile strength of a part of the plurality of regions is 980 MPa or more ,
And,
The member has an overlapping portion formed by hemming the thin steel, and heat treatment is performed to form the plurality of regions in the overlapping portion, and the folded portion and the facing portion in the overlapping portion are the heat treatment. Thin-walled steel product characterized by being strengthened and welded by The thin-walled steel workpiece according to claim 1 , wherein the plurality of regions differ in at least one physical property of tensile strength, ductility, and tensile residual stress . The thin- walled steel product according to claim 1 or 2, wherein a thickness of the thin-walled steel is 1.0 mm or less . The thin steel processed product according to any one of claims 1 to 3, wherein the member is any plate-shaped frame forming a seat frame of a vehicle seat . A heat treatment method for a thin-walled steel product obtained by pressing thin-walled steel into a predetermined shape,
A rapid heating step of rapid heating from room temperature to a temperature higher than the A1 transformation point at a heating rate of 400 ° C./second or more;
A quenching step of quenching at a cooling rate of 800 ° C./second or more after the rapid heating step;
Have
Applying the rapid heating step and the rapid cooling step to the overlapping portion of the thin steel in the member to be heat treated, adjusting the heating temperature of the rapid heating step, or the rapid cooling start temperature of the rapid cooling step Thin-walled steel processing characterized by forming a plurality of regions having different physical properties depending on whether the A1 transformation point or higher or less than the A1 transformation point, and controlling a part of the region to a tensile strength of 980 MPa or more Product heat treatment method . The method for heat-treating a thin-walled steel product according to claim 5, wherein the overlapping portion is formed by hemming . The method for heat treatment of a thin-walled steel product according to claim 6, wherein the folded portion and the facing portion in the overlapping portion formed by the hemming are welded simultaneously with increasing strength by the rapid heating step and the rapid cooling step . Protrusions projecting outward are formed in a part adjacent to the facing part, and the projecting part is melted by the rapid heating step and enters between the folded part and the facing part. Item 8. A heat treatment method for a thin-walled steel product according to Item 7 . The thin-walled steel processing according to claim 7, wherein the hemming is performed so that a gap is provided between the folded portion constituting the overlapping portion and the facing portion, and then the rapid heating step and the rapid cooling step are performed. Product heat treatment method.