JP2014024074A - Hot press formed product having low strength part and manufacturing method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot press formed product having different strength parts, i.e., a high strength part and a low strength part.SOLUTION: A hot press formed product made of a steel plate having a chemical composition including C:0.10-0.33%, Mn and Cr: 0.5-2.0% in total, B:0.0010-0.010%, and Ti: 0.01-0.10%, comprises: a low strength part which is mainly composed of at least one of a ferrite structure and a bainite structure; and a high strength part which is composed of a martensite structure. A nitrogen content of the low strength part is 50 ppm or less, and a nitrogen content of the high strength part is 70 ppm or more. Further, hardness of the low strength part is 350 Hv or less in Vickers hardness, and hardness of the high strength part is 420 Hv or more in Vickers hardness.

Description

  The present invention relates to a hot press-formed product having a low-strength portion and a method for producing the same, specifically, a hot press-formed product having different strengths, that is, both a low-strength portion and a high-strength portion, and the hot press-formed product. The present invention relates to a manufacturing method capable of manufacturing a press-formed product with high accuracy.

  The structural member for automobiles, which is a molded product of a steel plate, is required to have higher strength and lighter weight from the viewpoint of both improving safety and improving fuel consumption. The application of high-strength steel sheets is expanding for both of them. However, the cold press forming method of a high-strength steel sheet has problems such as a decrease in formability such as cracks and wrinkles in the steel sheet and a decrease in dimensional accuracy due to a shape defect such as a spring back.

  In order to cope with these problems in the cold press forming method of high-strength steel sheets, it is promoted to apply a hot press forming method in which quenching is performed simultaneously with press forming. According to the hot press forming method, press forming is performed when the steel sheet is soft at high temperature, so there is little problem of change in dimensional accuracy, and the steel sheet can be formed at high temperature and high ductility. Excellent properties. Furthermore, since the steel sheet is quenched and quenched at the same time as forming from a high temperature state, it has a feature that an ultra-high strength press-formed product having a tensile strength of 1200 MPa or more can be obtained.

  However, a hot press-molded member manufactured by a hot press-molding method has high strength but poor ductility, and thus may be unexpectedly broken when subjected to a large deformation due to a collision.

  For example, B pillars, side sills, A pillars, roof rails, bumpers, door impact beams, floor members, front side members, etc., which are important members constituting the body side of an automobile, are all flanges provided at the edges. It is required to have a characteristic of absorbing impact energy at the time of collision by being plastically deformed without being completely broken at the time of automobile collision. .

  However, when severe impact tests such as the SUV side impact test of the American Road Safety Insurance Association (IIHS) and the Euro NCAP pole side impact test are performed, spot welds and arcs on the above-mentioned flanges in these hot press-formed members Starting from geometrical or metallurgical stress concentration parts such as welds, ridges, punched holes, vertical walls, and suddenly deformed parts, hot press-formed members break unexpectedly and design target performance cannot be obtained. Sometimes.

  Therefore, by forming a low-strength portion having a low strength and a high ductility partly in a part of such a hot press-formed member, it is possible to positively control the deformation mode and break position at the time of the hot press-formed member, A hot press-molded product excellent in impact energy absorption is desired.

  As a method of forming low strength in a part of a hot press-formed product, Patent Document 1 discloses a hot press using a tailored blank in which a steel plate having high hardenability and a steel plate having low hardenability are integrated by welding. A method of manufacturing a molded article is disclosed.

  In Patent Document 2, the temperature of each part of the material steel plate is controlled by partial contact heat removal with the conveying device during conveyance after heating, and the heat is obtained by varying the quenching start temperature according to the part of the material steel plate. A method is disclosed in which the quenching hardness is different for each part of the intermediate press-formed product.

  In Patent Document 3, in the local softening method of high-strength steel materials (for example, high-strength steel plates), arc or laser beam is irradiated using a gas containing 5% by volume or more of oxygen as a shielding gas, and formability is required. A method of partially softening a portion to be taken is disclosed.

  Furthermore, Non-Patent Document 1 discloses a method of manufacturing a hot press-formed product in which ductility is partially recovered by partially tempering the hot press-formed product by high-frequency induction heating.

JP 2004-58082 A JP 2006-212690 A Japanese Patent Publication No. 9-87737

Tempering of hot-formed steel using induction heating (http://publications.lib.charmers.se/cpl/record/index.xsql?pubid=144308)

  However, as in the method disclosed in Patent Document 1, the hot press forming method using a tailored blank joins and welds a plurality of steel plates, so that the strength in a wider area than the part where the strength is originally desired to be changed. Therefore, there is a problem that the strength of the entire hot press-formed product is unnecessarily lowered. In addition, although it is possible to increase the location where the strength was partially changed by increasing the number of welded portions between the steel plates in the tailored blank, the manufacturing cost is significantly increased in order to increase the number of welded locations.

  As in the method disclosed in Patent Document 2, in the method of adjusting the hardness for each part by controlling the heat removal part during conveyance, a large transition area is unavoidable between the quenched area and the unquenched area. Occur. In this transition region, strength characteristics are likely to fluctuate, which may lead to variations in the performance of hot press-formed products.

  Even if the method of irradiating an arc or a laser beam using a gas containing 5% by volume or more of oxygen as a shielding gas, as in the method disclosed in Patent Document 3, is applied to a hot press molded product, The molded article cannot be partially softened. That is, in the hot press-formed product, the softening effect due to the addition of oxygen is hardly recognized. Further, in this method, since the hot press molded product inevitably undergoes thermal distortion, the shape of the hot press molded product changes, and the dimensional accuracy of the hot press molded product may be reduced.

  Further, as in the method disclosed in Non-Patent Document 1, when the tempering is performed by high-frequency induction heating after assembling the hot press-formed product, as in the invention disclosed in Patent Document 3, hot press-forming is performed. Due to the thermal strain generated in the product, the dimensional accuracy of the hot press-formed product may be reduced.

  The present invention has been made in view of these problems of the prior art, and is a hot press-formed product having different strength portions, that is, both a low-strength portion and a high-strength portion, and this hot press-formed product. An object of the present invention is to provide a production method capable of producing a product with high accuracy.

  The production of high-strength members by hot pressing generally heats a steel material having high hardenability for hot pressing, raises the temperature to the austenite region, and from the austenite region to the martensite transformation start temperature (Ms point) or lower. This is performed by a hot pressing process for rapid cooling. The present inventors diligently studied a method for partially changing the strength of a hot press-formed product in the production of such a hot press-formed product.

  As a result, the inventors have stated that, "Before the hot pressing process, a part of the steel sheet material for hot pressing is heated and partially melted and solidified by using a nitrogen-containing gas as a shielding gas. When the pretreatment is carried out, the critical cooling rate of the pretreated portion is lowered, the austenitization is less likely to occur at the heating temperature of hot press molding, and the hardness is lowered by bainite transformation and ferrite pearlite transformation. The present invention has been made on the basis of this novel finding, and the following hot press-formed products and their production methods are listed below.

(1) C: 0.10 to 0.33%, Mn and Cr: 0.5 to 2.0% in total, B: 0.0010 to 0.010%, Ti: 0.01 to 0.10% A part of the material steel plate having a chemical composition is heated using a nitrogen-containing gas as a shielding gas to perform a pretreatment for melting and solidifying a part, and after performing this pretreatment, By heating the raw steel plate to a temperature of Ac ₃ points or higher and performing hot press molding at a press starting temperature of Ar ₃ points or higher of the raw steel plate, a hot press-formed product having a low strength portion and a high strength portion is obtained. A method for producing a hot press-formed product, characterized by producing the product.

  (2) The method for producing a hot press-formed product described in the item (1), wherein the cooling rate in the hot press molding is equal to or higher than the critical cooling rate in the remaining part and less than the critical cooling rate in the part. .

(3) A hot press-formed product manufactured by the manufacturing method described in (1) or (2),
The low-strength part has a structure mainly composed of at least one of ferrite and bainite in which a part of the part is hot-press formed.
The high-strength portion is a hot press-formed product characterized in that the remaining portion is a hot-press formed portion and has a structure composed of martensite.

  (4) The hot press-molded article described in the item (3), wherein the hardness of the low strength portion is Vickers hardness of Hv 350 or less and the hardness of the high strength portion is Vickers hardness of Hv 420 or more.

  (5) The hot press-molded article described in (3) or (4), wherein the nitrogen content in the low-strength part is 50 ppm or less and the nitrogen content in the high-strength part is 70 ppm or more.

  (6) The low-strength portion is formed in all or part of the thickness direction of the steel plate constituting the hot press-formed product, any one of the items (3) to (5) The hot press-formed product described in 1.

  (7) The hot press-formed product has a groove bottom, a ridge line continuous to the groove bottom, a vertical wall continuous to the ridge line, a curved line continuous to the vertical wall, and a flange continuous to the curved line. From the section (3), characterized in that it has a cross-sectional shape of the mold, and the low-strength portion is formed in a ridge line portion, a flange, a planned welding portion, a punching scheduled portion, a vertical wall portion, or the vicinity thereof. The hot press-formed product described in any one of items up to (6).

(8) C: 0.10 to 0.33%, Mn and Cr: 0.5 to 2.0% in total, B: 0.0010 to 0.010%, Ti: 0.01 to 0.10% It is a hot press-formed product made of a steel plate having a chemical composition having
A low-strength portion mainly composed of at least one of ferrite or bainite structure, and a high-strength portion composed of a martensite structure,
While the nitrogen content of the low strength part is 50 ppm or less, the nitrogen content of the high strength part is 70 ppm or more,
A hot press-formed product characterized in that the hardness of the low strength portion is Vickers hardness of Hv 350 or less and the hardness of the high strength portion is Vickers hardness of Hv 420 or more.

  (9) A structural member for an automobile having the hot press-formed product described in any one of items (3) to (8) as a constituent member.

  According to the present invention, a high-strength portion and a partial low-strength portion can be mixed in one hot press-formed product, and a hot press-formed product excellent in dimensional accuracy can be manufactured. . Thereby, for example, it becomes possible to manufacture a structural member for automobiles excellent in occupant protection performance at the time of collision with light weight and high dimensional accuracy.

Fig.1 (a)-FIG.1 (f) are explanatory drawings which show typically the manufacturing method which concerns on this invention. FIG. 2 is an explanatory view showing the hardness distribution of the hot press-formed product according to the present invention, and FIG. 2 (a) is a hardness distribution when melting and solidification (melting in the plate thickness direction) by laser is performed. Fig. 2 (b) shows the hardness distribution when melting and solidification (partial melting) is performed by laser, and Fig. 2 (c) shows melting and solidification (complete melting in the plate thickness direction) by plasma. The hardness distribution when broken. FIG. 3 is an explanatory view showing an example in which the present invention is applied to various automotive structural members, FIG. 3 (a) shows a situation where the present invention is applied to a B pillar reinforcement, and FIG. 3 (b) shows a B pillar reinforcement. 3 (c) shows the situation applied to the vicinity of the joint portion of the roof rail with the A pillar, and FIG. 3 (d) shows the situation applied to the vicinity of the joint portion of the side sill with the A pillar. Furthermore, FIG.3 (e) shows the condition applied to the tow hook attachment part in bumper reinforcement. FIG. 4 is an explanatory view showing various automotive structural members to which the present invention is applied.

  The present invention will be described with reference to the accompanying drawings. In the following description, “%” relating to chemical composition means “mass%” unless otherwise specified.

First, the chemical composition, type, and thickness of the raw steel plate for hot press-formed products will be described.
(C: 0.10 to 0.33%)
C is an important element that enhances the hardenability of the steel sheet material and determines the strength of the hot press-formed product. If the C content is less than 0.10%, this effect cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.33%, the toughness and weldability of the hot press-formed product may be deteriorated. Therefore, the C content is 0.10% or more and 0.33% or less. The C content is desirably 0.17% or more and 0.30% or less.

(Mn and Cr: 0.5 to 2.0% in total)
Both Mn and Cr are effective elements for enhancing the hardenability of the raw steel sheet and for ensuring the strength of the hot press-formed product stably. However, if the total content of Mn and Cr is less than 0.5%, the effect of improving the hardenability cannot be sufficiently obtained. On the other hand, if the total content exceeds 2.0%, the hardenability becomes high. Pass. Therefore, the total content of Mn and Cr is 0.5% or more and 2.0% or less. Either Mn or Cr may not be included.

(B: 0.0010 to 0.010%)
B is an effective element for ensuring the hardenability of the raw steel plate. If the B content is less than 0.0010%, the hardenability of the raw steel plate is insufficient, and the desired hot press-formed product is desired. It is impossible to stably secure the high strength of the steel. On the other hand, if the B content exceeds 0.010%, the production cost of the hot press-formed product increases. Therefore, the B content is set to 0.0010% or more and 0.010% or less.

(Ti: 0.01-0.10%)
When the Ti content is less than 0.01%, the effect of improving the hardenability of B is not sufficiently exhibited, and the hardenability of the raw steel plate is insufficient. On the other hand, if the Ti content exceeds 0.10%, the toughness of the melted and solidified portion in the hot press-formed product is lowered. Therefore, the Ti content is set to 0.01% or more and 0.10% or less.

  Furthermore, the material steel plate has an Si content of 0.5% as an optional additive element in order to increase the strength of the hot press-formed product or to ensure the strength of the hot press-formed product more stably and toughness. % Or less, P is 0.05% or less, S is 0.05% or less, Nb is 0.2% or less, sol. You may contain 0.1% or less of Al, respectively.

The balance other than the above is Fe and impurities.
As the material steel plate containing the chemical component, a non-plated bare steel plate can be used, and a zinc-based plated steel plate having a zinc-based plating layer formed on the surface thereof can also be used. Examples of the zinc-based plated steel sheet include a pure zinc-plated steel sheet, an iron-zinc alloy-plated steel sheet, and a zinc-nickel-plated steel sheet. These galvanized steel sheets can enhance sacrificial corrosion resistance by the action of zinc.

  The plate thickness of the material steel plate is desirably 3.2 mm or less because it is difficult to perform sufficient quenching by cooling with a mold if it is too thick. The lower limit of the thickness of the raw steel plate is not particularly limited, but 0.7 mm frequently used as the lower limit of the thickness of the automobile structural member is exemplified.

Next, a method for producing a hot press-formed product will be described.
Fig.1 (a)-FIG.1 (f) are explanatory drawings which show typically the manufacturing method of the hot press-formed product 1 which concerns on this invention.

  As shown in FIG. 1 (f), the hot press-formed product 1 includes a groove bottom portion 1a, ridge line portions 1b, 1b continuous to the groove bottom portion 1a, vertical wall portions 1c, 1c continuous to the ridge line portions 1b, 1b, It has a substantially hat-shaped cross-sectional shape having curved portions 1d, 1d continuous with the vertical wall portions 1c, 1c and flanges 1e, 1e continuous with the curved portions 1d, 1d. In the following description, the case where the hot press-formed product 1 has this cross-sectional shape is taken as an example. This is an example for facilitating the understanding of the present invention, and the present invention uses this cross-sectional shape. It is not limited to the case where it has, of course, when other components are added to these components in the hot press-formed product 1, even if these components are not at all, The present invention applies equally.

  In the method for manufacturing a hot press-formed product 1 according to the present invention, the ridge line portions 1b and 1b and the flange portions 1e and 1e that are parts required to be ductile in the hot press-formed product 1 are portions 2b that are press-formed. 2b and flange portions 2e and 2e (in FIG. 1B, groove bottom portion 1a, ridge portions 1b and 1b, vertical wall portions 1c and 1c, curved portions 1d and 1d, curved portions 1e and 1e in the hot press-formed product 1 In order to make the portions of the material steel plate 2 press-formed into 2a, 2b, 2b, 2c, 2c, 2d, 2d, 2e, and 2e) as softened low-strength portions, FIG. As shown in FIG. 1 (a) and FIG. 1 (b), a shield gas supply device using a part of portions 2b, 2b, 2e, and 2e of the raw steel plate 2 before hot press forming as a shielding gas with a gas containing nitrogen. 4 to heat a part 2b, b, 2e, thereby melting and solidifying the 2e.

  For the heat source 3, an arc or a laser is used. As the arc, a TIG arc using a non-consumable electrode such as tungsten or a plasma arc can be used. Further, a consumable electrode type arc may be used. As the laser, a carbon dioxide laser, a YAG laser, a fiber laser, a disk laser, and a diode laser can be used.

  Such laser heating is desirably used because of its high energy density and excellent controllability of the irradiated region. Irradiate these arcs or laser beams to the parts 2b, 2b, 2e, 2e of the material steel plate 2 that is press-formed into the parts 1b, 1b, 1e, 1e to be locally reduced in strength in the hot press-formed product 1. And then melt.

  The output and speed of the heat source 3 may be appropriately adjusted according to the melting area. When processing a wide area, it is also effective to melt the laser beam by vibrating it.

  The melting depth of the raw steel plate 2 depends on the required properties, and may be partial melting only on the surface of the raw steel plate 2 or through melting that completely melts the back surface of the raw steel plate 2.

  When the part 2b, 2b, 2e, 2e of the material steel plate 2 is heated by irradiating an arc or laser beam as the heat source 3, the shield gas supplied from the shield gas supply device 4 contains nitrogen. It is essential. Although the mechanism of the hardenability reduction by nitrogen is not clear, it is estimated that the effect of improving the hardenability by B disappears when nitrogen combines with boron (B) in the raw steel plate 2 to become BN.

  The concentration of the nitrogen gas contained in the shield gas need not be specified, but is preferably 5% by volume or more. This is because the higher the amount of nitrogen, the higher the effect of reducing hardenability. Examples of components other than nitrogen to be contained in the shield gas include inert gases such as argon and helium, carbon dioxide, oxygen, and the like. As the amount of nitrogen increases, the hardenability in the hot pressing process can be lowered. Therefore, the ratio with other components may be adjusted as necessary. In terms of cost, compressed air (80% by volume of nitrogen and 20% by volume of oxygen) may be used.

  Pure nitrogen gas is the most effective for suppressing the hardenability, but when an arc or a carbon dioxide laser is used, blow holes due to nitrogen may be generated. In this case, the nitrogen concentration may be lowered by using the mixed gas.

The shield gas is exemplified by spraying the molten pool from the front, the same axis, or the rear of the heat source 3 using the shield gas supply device 4. When a laser is used, stable construction is possible regardless of the method of spraying the shielding gas, but when using an arc or plasma arc, the arc may become unstable if nitrogen is added to the coaxial gas. Therefore, the coaxial gas may be mainly composed of Ar with CO ₂ , O ₂ , and H ₂ added thereto, and nitrogen gas may be blown into the molten pool from the rear. The flow rate to be sprayed is not particularly defined, but if it is weak, the effect of lowering the hardenability cannot be obtained, and if it is too strong, the melted part blows off due to the gas pressure. Generally, the spraying amount is 3 to 100 l / min. Moreover, in order to make the hardness after hot press molding differently in the same hot press-formed product, the gas flow rate and composition may be changed.

Thus, nitrogen is used as the shielding gas, partially melted and solidified steel sheet 2 was is being charged into the heating furnace 5, as shown in FIG. 1 (c), the Ac ₃ point or more in temperature Heated. Ac The raw steel plate 2 heated to a temperature of ₃ or more points becomes an austenite single phase. The heating time of the raw steel plate 2 is desirably maintained at a temperature of Ac ₃ or higher for 60 seconds or longer to perform sufficient austenitization. However, from the viewpoint of productivity, the retention time of Ac ₃ or more is preferably 10 minutes or less.

Thereafter, as shown in FIG. 1 (d), the raw steel plate 2 is cooled simultaneously with the press molding to a temperature below the Ms point by a water-cooled mold from a press start temperature of Ar ₃ or higher.

  The cooling rate at this time is equal to or higher than the critical cooling rate in the remaining portions 2a, 2c, and 2d and lower than the critical cooling rate in some portions 2b and 2e. This is because if the cooling rate is too slow, the remaining portions 2a, 2c and 2d will not be burned, whereas if the cooling rate is too fast, some portions 2b and 2e will also be burned. The cooling rate varies depending on the material, but is generally 30 to 150 degrees / second.

  By this step, the remaining portions 2a, 2c, and 2d undergo martensitic transformation and become a hard structure mainly composed of martensite having a Vickers hardness of Hv420 or more. On the other hand, some portions 2b and 2e have a soft structure of Hv 350 or less due to bainite transformation and ferrite pearlite transformation.

  Thereafter, as shown in FIG. 1 (e), the press-formed raw steel plate 2 is taken out from the mold 6, and when the raw steel plate 2 is a non-plated bare steel plate, the oxide scale produced on the surface is removed. Shot blasting is performed for the purpose of removal, and the oxide scale is removed.

  Thus, the hot press-formed product 1 as the final product shown in FIG. 1 (f) is manufactured.

  FIG. 2 is an explanatory view showing the hardness distribution of the hot press-formed product according to the present invention, and FIG. 2 (a) is a hardness distribution when melting and solidification (melting in the plate thickness direction) by laser is performed. Fig. 2 (b) shows the hardness distribution when melting and solidification (partial melting) is performed by laser, and Fig. 2 (c) shows melting and solidification (complete melting in the plate thickness direction) by plasma. The hardness distribution when broken.

  As shown in the graph of the right figure of FIG. 2A showing the relationship between the position and hardness at the hardness measurement position shown in the left figure of FIG. 2A, the hardness of the low strength portion in the hot press-formed product is The hardness of the high-strength portion is about 470 Hv while it is about 300 Hv.

  As shown in the lower graph of FIG. 2 (b) showing the relationship between the position and hardness at the hardness measurement position shown in the upper diagram of FIG. 2 (b), the hardness of the low-strength portion in the hot press-formed product is approximately Whereas it is 270 to 300 Hv, the hardness of the high strength portion is approximately 440 to 500 Hv.

  Furthermore, as shown in the graph of the right figure of FIG.2 (c) which shows the relationship between the position and hardness in the hardness measurement position shown in the left figure of FIG.2 (c), the low intensity | strength part in a hot press molded product is shown. The hardness is approximately 300 Hv, whereas the hardness of the high strength portion is approximately 460 to 500 Hv.

  Thus, according to the present invention, the remaining portions 2a, 2c, and 2c are formed by performing hot press forming using the hot-press material steel plate 2 that has been subjected to pretreatment that partially reduces the hardenability. 2d is a high strength portion formed by hot press forming, a groove bottom portion 1a, a vertical wall portion 1c, a curved portion 1d, and a ridge line portion 1b that is a low strength portion in which some portions 2b and 2e are hot pressed. A hot press-formed product 1 having a flange 1e can be manufactured.

  Further, in the hot press-formed product 1, the amount of nitrogen in the groove bottom portion 1a, the vertical wall portion 1c, and the curved portion 1d, which are high strength portions, is 50 ppm or less, and the amount of nitrogen in the ridge portion 1b, flange 1e, which is a low strength portion. It is 70 ppm or more, and has a portion where the amount of nitrogen is different in the hot press-formed product 1.

  That is, in the hot press-formed product 1 according to the present invention, C: 0.10 to 0.33%, Mn and Cr: 0.5 to 2.0% in total, B: 0.0010 to 0.010% , Ti: a material steel plate 2 having a chemical composition having 0.01-0.10%, low strength portions 1b, 1e mainly composed of at least one of ferrite or bainite structure, and a high strength composed of a martensite structure. Parts 1a, 1c, and 1d, the low-strength parts 1b and 1e have a nitrogen content of 50 ppm or less, and the high-strength parts 1a, 1c, and 1d have a nitrogen content of 70 ppm or more, and the low-strength parts The hardness of 1b and 1e is Hv 350 or less in terms of Vickers hardness, and the hardness of the high strength portion is Hv 420 or more in terms of Vickers hardness.

  The low strength portions 1b and 1e may be low strength portions penetrating from the front surface to the back surface in the plate thickness direction, or may be local low strength portions in the plate thickness direction. That is, as illustrated in FIG. 2 (a), the low strength portions 1b and 1e melted and solidified to the back surface may be used, or as illustrated in FIG. 2 (b), only the surface of the steel plate 2 is melted. The strength portions 1b and 1e may be used. It is more preferable to use the low-strength portions 1b and 1e only on the surface because the decrease in the collision performance of the hot press-formed product 1 is small and the limit of breakage against local deformation can be increased. For example, in the ridge line portion 1b of the hot press-formed product 1, it is particularly desirable to form a low-strength portion only on the inner surface of the ridge line portion 1b from the viewpoint of suppressing breakage due to bending back due to collision.

  FIG. 3 is an explanatory view showing an example in which the present invention is applied to various automotive structural members 7 to 11, and FIG. 3 (a) shows a situation where the present invention is applied to the B pillar reinforcement 7, and FIG. 3 (b). 3 shows another situation applied to the B pillar reinforcement 8, FIG. 3C shows a situation applied to the vicinity of the joint portion 9a of the roof rail 9 with the A pillar 12, and FIG. The situation applied to the vicinity of the joint portion 10a with the pillar 12 is shown, and FIG. 3 (e) shows the situation applied to the tow hook attaching portion 10a in the bumper reinforcement 11.

  3B shows the B pillar reinforcement 8 'of the conventional example, and the right figure shows the pillar reinforcement 8 of the present invention. Moreover, the code | symbol 13 in FIG.3 (e) shows a crash box, and the code | symbol 14 shows a side member.

  As shown in FIGS. 3 (a) to 3 (e), the application site of the hot press-formed product 1 according to the present invention is B pillar reinforcement 7, 8, roof rail 9, A pillar 12, side sill 10, bumper. Reinforce is exemplified.

  Specifically, the low-strength portion in the hot press-formed product 1 according to the present invention includes a ridge line portion 1b, a flange 1e, a planned welding portion (not shown) (spot welded portion, arc welded portion, laser welded portion, projection welded portion, etc. ), And are formed in the unillustrated punched portion, the vertical wall portion 1c, and their vicinity (within about 20 mm). By forming a low-strength portion in these stress-concentrated portions due to the shape and metallurgical stress-concentrated portions due to the HAZ softening of the weld, the stress concentration can be alleviated, and the hot press-formed product 1 is in the event of a collision. Breaking unexpectedly can be suppressed, and the deformation mode can be controlled by artificially forming a low-strength portion.

  As an example of relaxation of the stress concentration, spot welding of the flange 7a is performed by disposing a low strength portion on the flange 7a (black portion in FIG. 3A) of the B pillar reinforcement 7 shown in FIG. The effect of softening the HAZ due to can be mitigated, and breakage at the flange 7a at the time of a side collision can be suppressed.

  In the B pillar reinforcement 7 shown in FIG. 3A, the low-strength portion is formed only on a part of the flange 7a where the stress is increased, but it may be formed on the entire length of the flange 7a. Moreover, as shown to Fig.3 (a), by forming a low intensity | strength part in the ridgeline 7b of the lower part of B pillar reinforcement 7, it is possible to suppress that the ridgeline 7b is fractured | ruptured by the bending return by a collision. It becomes. Furthermore, by forming a low strength portion around the hole 7c formed in the middle of the height direction of the B pillar reinforcement 7, the stress concentration around the hole 7c can be alleviated.

  As an example of the control of the break mode, as shown in FIG. 3 (b), a hole below the hot press B pillar reinforcement 8 ′ that has been formed so far for the origin of deformation of the B pillar. The present invention is applied in place of the shape stress concentration portion such as 8a, and the vertical wall portion 8b (the black painted portion in the right view of FIG. 3B) of the hot press B pillar reinforcement 8 is applied. By forming the low-strength portion, the hole 8a can be eliminated, and transmission of noise from the lower portion of the vehicle can be suppressed.

  Furthermore, the present invention can also be utilized when mechanically punching holes in the hot press-formed product 1. For example, in general, in the process of mechanically punching a punched hole in the hot press-formed product 1, the hot press-formed product 1 has a high hardness, so that the mold is likely to be worn out, and the punched hole has a minute delayed fracture. There is a problem of generating cracks. However, by punching the low-strength portion softened in advance according to the present invention, the life of the mold is improved and the occurrence of delayed fracture at the punched portion is also suppressed.

  Further, as shown in FIG. 3 (c), by arranging a low strength portion on the flange portion 9a (blacked portion in FIG. 3 (c)) which is a joint portion with the A pillar 12 in the roof rail 9, the Euro NCAP In the pole side collision test, breakage starting from the HAZ softened portion of spot welding can be suppressed.

  Similarly, as shown in FIG. 3 (d), by forming a low-strength portion in the flange portion 10a (the black-painted portion in FIG. 3 (d)) that is a joint portion of the side sill 10 with the A pillar 12, collision occurs. In this case, breakage starting from the spot breakage portion can be suppressed.

  Further, as shown in FIG. 3 (e), in the bumper reinforcement 11, the tow hook is attached and the bumper reinforcement 11 may break from the arc welded portion 10a of the nut, but the vicinity of the arc welded portion 10a (FIG. 3). By forming the low-strength portion in the black-coated portion in (e), local stress concentration on the arc welded portion 10a can be alleviated, and breakage of the bumper reinforcement 11 can be suppressed.

  The hot press-formed product 1 according to the present invention is not the one whose strength is changed by a heat treatment such as tempering after the hot press forming, but the strength is partially changed by the pretreatment performed before the hot press forming. Therefore, the hot press-formed product 1 has a high shape accuracy and is suitable as a constituent material of a structural member for automobiles that requires high assembly accuracy.

FIG. 4 is an explanatory view showing various automotive structural members to which the present invention is applied.
As structural members for automobiles, for example, as shown in FIG. 4, as shown in FIG. 4, A pillar 15, B pillar 16, side sill 17, roof rail 18, bumper 19, front side member 20, door beam 21, floor member 22, rear side member 23. And the like.

  In this example, a test for manufacturing a hot press-formed product having a low strength portion and a high strength portion was performed. This will be described in detail below.

  The material steel plate contains C0.21%, Si0.25%, Mn1.20%, Cr0.2%, Ti0.02% and B0.0018% by mass%, and the balance is Fe and impurities, and is not plated. A steel plate having a thickness of 1.6 mm, a length of 40 mm, and a width of 200 mm was used.

  The conditions of the melting and solidification treatment of the raw steel plate before hot press forming and the cooling rate of the quenching treatment in hot press forming were changed and examined.

  A YAG laser, a direct diode laser, and a plasma arc heat source were used as a heat source for melting and solidifying the steel plate. Further, nitrogen, oxygen, argon, carbon dioxide gas and a mixed gas thereof were used as a shielding gas for melting and solidifying the raw steel plate.

  During the laser beam irradiation, a shielding gas was flowed coaxially with the laser beam. When irradiating a plasma arc, a double nozzle is used to prevent wear of the electrode, argon is used as the plasma gas flowing around the electrode, and a mixed gas of argon and nitrogen is used as a side shield gas from the outside. Shed.

With these heat sources, an irradiated part having a width of 1 to 4 mm and a length of 200 mm was formed.
Then, the raw steel plate was charged into a 900 ° C. furnace and held for about 4 minutes, and cooled to a temperature of 300 ° C. or lower with a cooling device to obtain a quenched product.

  The hardness of the low-strength part and the high-strength part of the obtained quenched product is measured with a Vickers hardness meter, and the hardness of the high-strength part is Hv420 or more and the hardness of the low-strength part is Hv350 or less. investigated.

  Table 1 shows the measurement results of the hardness of the low-strength part and the high-strength part of the quenched product along with the test conditions.

  Test numbers 1 to 6 are the results of changing the cooling rate. Test No. 1 is a comparative example in which cooling was performed very slowly, and cooling was performed at a cooling rate slower than the critical cooling rate of both a part of the raw steel plate and the remaining part. For this reason, in the test number 1, both a part and the remaining part softened to Hv 350 or less.

  Test Nos. 2 to 5 are examples of the present invention, and since cooling was performed at an appropriate cooling rate, the remaining portion had a high strength of Hv420 or higher, and a part of the strength became a low strength of Hv350 or lower.

  Test No. 6 was a case where the cooling was performed very rapidly in the comparative example, and cooling was performed at a high cooling rate exceeding the critical cooling rate of both the remaining part and a part of the part. For this reason, not only the remaining part but also a part of the part was cured to Hv420 or higher.

  Test numbers 7 to 10 are the results of changing the type of shield gas. In the test numbers 7 to 9 using the shielding gas containing nitrogen which is an example of the present invention, the remaining part was Hv420 or higher, and a part was softened to Hv350 or lower.

  On the other hand, in the shielding gas containing no nitrogen of test number 10 which is a comparative example, the remaining part was Hv420 or higher, but a part was not softened to Hv350 or lower.

  Test numbers 11 to 14 are results using a direct laser. In the test numbers 11 to 13, which are examples of the present invention, the remaining part was Hv420 or higher, and part of the part was softened to Hv350 or lower. On the other hand, in test number 14 which is a comparative example, softening of Hv 350 or less was not observed in a part.

  Test Nos. 15 to 16 are examples of the present invention using a plasma heat source, and the remaining part was Hv420 or higher, and a part was softened to Hv350 or lower.

In this test, the amount of nitrogen was measured under some conditions.
In the case of the present invention, the nitrogen content in the high-strength portion in which the remaining portion was press-molded was 33 ppm by weight, the nitrogen content in the low-strength portion in which a portion was press-molded was wt%, and the test number 4 was 82 ppm and Test No. 7 was 72 ppm.

  On the other hand, in the comparative example of test number 10, the nitrogen content in the high-strength portion in which the remaining portion was press-molded was 33 ppm by weight, and the nitrogen content in the low-strength portion in which a portion was press-molded was 39 ppm. there were.

DESCRIPTION OF SYMBOLS 1 Hot press molding product 1a Groove bottom part 1b Edge line part 1c Vertical wall part 1d Curved part 1e Flange 2 Material steel plate 2a Part 2b shape | molded in a groove bottom part 2c Shaped in a vertical wall part Part 2d Part 2e Curved part 2e Flange part 3 Heat source 4 Shield gas supply device 5 Heating furnace 6 Mold 7 B pillar reinforcement 7a Flange 7b Edge line 7c Hole 8, 8 'B pillar reinforcement 8a Hole 8b Vertical wall part 9 Roof rail 9a Flange part 10 Side sill 10a Flange part 11 Bumper reinforcement 11a Tow hook attaching part 12 A pillar 13 Crash box 14 Side member 15 A pillar 16 B pillar 17 Side sill 18 Roof rail 19 Bumper 20 Front side member 21 Door beam 22 Floor member 23 rear side members

Claims (9)

In mass%, C: 0.10 to 0.33%, Mn and Cr: 0.5 to 2.0% in total, B: 0.0010 to 0.010%, Ti: 0.01 to 0.10 A part of the steel sheet having a chemical composition having a% is heated using a nitrogen-containing gas as a shielding gas to perform a pretreatment for melting and solidifying the part, and the pretreatment was performed. Later, by heating the raw steel plate to a temperature of Ac ₃ points or higher and performing hot press forming at a press start temperature of Ar ₃ points or higher of the raw steel plate, a hot steel having a low strength portion and a high strength portion is obtained. A method for producing a hot press-formed product, characterized by producing a press-formed product.   The method for producing a hot press-formed product according to claim 1, wherein a cooling rate in the hot press molding is equal to or higher than a critical cooling rate in the remaining portion and is less than a critical cooling rate in the partial portion. A hot press-formed product manufactured by the manufacturing method according to claim 1 or 2,
The low-strength portion has a structure mainly composed of at least one of ferrite and bainite in which the part of the portion is hot-press formed.
The high-strength portion is a hot press-formed product characterized in that the remaining portion is a hot-press formed portion and has a structure made of martensite.   The hot press-molded product according to claim 3, wherein the hardness of the low-strength part is Vvs hardness of Hv 350 or less, and the hardness of the high-strength part is Vvs hardness of Hv 420 or more.   The hot press-formed product according to claim 3 or 4, wherein the nitrogen content in the low-strength portion is 50 ppm or less, and the nitrogen content in the high-strength portion is 70 ppm or more.   The low-strength portion is formed on all or part of a thickness direction of a steel plate constituting the hot press-formed product, and is described in any one of claims 3 to 5. Hot press molded product.   The hot press-molded product has a groove bottom, a ridge line continuous with the groove bottom, a vertical wall continuous with the ridge line, a curved part continuous with the vertical wall, and a flange continuous with the curved part. It has a substantially hat-shaped cross-sectional shape, and the low-strength portion is formed in the ridgeline portion, the flange, a planned welding portion, a punching planned portion, the vertical wall portion, or the vicinity thereof. The hot press-formed product according to any one of claims 3 to 6. In mass%, C: 0.10 to 0.33%, Mn and Cr: 0.5 to 2.0% in total, B: 0.0010 to 0.010%, Ti: 0.01 to 0.10 %, A hot press-formed product made of a steel plate having a chemical composition having
A low-strength portion mainly composed of at least one of ferrite or bainite structure, and a high-strength portion composed of a martensite structure,
While the nitrogen content of the low-strength part is 50 ppm or less, the nitrogen content of the high-strength part is 70 ppm or more,
A hot press-molded product, wherein the low strength portion has a Vickers hardness of Hv 350 or less and the high strength portion has a Vickers hardness of Hv 420 or more.   An automotive structural member comprising the hot press-formed product according to any one of claims 3 to 8 as a constituent member.