JP6852273B2 - Manufacturing method of tailored blank molding material - Google Patents

Description

The present invention relates to a blank used for manufacturing a member requiring strength such as a structural member of an automobile, in which a plurality of steel plates are pre-welded and then pressed at a high temperature (so-called hot stamping) to obtain the required strength. It relates to a tailored blank molding material processed into a member having.

In recent years, from the viewpoint of protecting the global environment, weight reduction of automobile bodies has become an important issue in order to reduce _{CO 2 gas emissions.} As one of the measures, the application of high-strength steel sheets is under consideration, and the strength of steel sheets is increasing more and more.

However, increasing the strength of the steel sheet increases the pressing force required for press forming, which increases the equipment cost due to the increase in size of the equipment and the difficulty of forming due to the increasing strength of the steel sheet. It has been pointed out that there are various problems that bring about an increase in cost, such as correction cost, maintenance cost of the mold itself, and deterioration of productivity due to wrist-like to improve shape freezing.

Hot stamping is attracting attention as one way to solve this problem. Hot stamping is a technique for heating a steel sheet to a high temperature and pressing it in a high temperature range. Hot stamping is characterized in that a steel sheet is heated to an Ac3 transformation temperature or higher and pressed hot, and is rapidly cooled by heat removal from a die to cause transformation in a state where press pressure is applied. This is a technology capable of producing a pressed product having high strength and excellent shape freezing property.

On the other hand, there is a tailored blank as a method for realizing weight reduction and high functionality of parts. The tailored blank is a method of using a tailored blank material which is a press material in which end faces of a plurality of steel plates are joined by laser welding or the like, depending on the purpose. Tailored blank material can freely change the plate thickness and strength in one part by joining dissimilar steel plates, so in addition to being able to reduce the weight and enhance the functionality of the part, it is integrated. There is a merit that the number of parts can be reduced due to the conversion.

In recent years, with the aim of further weight reduction and high functionality, there have been some members that combine the above two technologies, that is, to manufacture a welded tailored blank material by hot pressing.

On the other hand, zinc-based plated steel sheets are used for many of the parts that require corrosion resistance, such as automobile members. When hot stamping a blank using a galvanized steel sheet, the blank is heated to 700 to 1000 ° C. Since this temperature is close to the boiling point of zinc, zinc evaporates during heating and the corrosion resistance is impaired. Therefore, for the blank for hot stamping, it is desirable to use a steel sheet that has been subjected to Al-based plating, which has a higher boiling point than zinc-based plating, that is, a so-called aluminum-plated steel sheet.

However, when a tailored blank material obtained by welding an aluminum-plated steel plate is hot-pressed, the Al-plated layer on the surface of the steel plate melts into the weld metal and the hardenability of the weld metal is reduced, so that sufficient joint strength is obtained after hot stamping. It is known that it cannot be done.

Patent Document 1 discloses an aluminum alloy-plated steel in which an aluminum-plated layer is removed from a region located around a plate.

Patent Document 2 states that the product of the hardness of the weld metal after hot stamping and the thickness of the thinnest portion of the weld metal is higher than the product of the hardness of the steel plate on the low strength side after hot stamping and the plate thickness. , A tailored blank in which steel plates to be butt welded are combined and welded is disclosed.

Japanese Patent No. 5237263 Japanese Patent No. 5316664

Since the technique disclosed in Patent Document 1 requires removing the plating layer of the portion to be welded by a brush or laser ablation before manufacturing the tailored blank material, an increase in man-hours and equipment for that purpose are required. There are issues such as.

The gist of the technique disclosed in Patent Document 2 is to adjust the component of the weld metal by adding a wire and to secure the quenching hardness of the welded portion. According to the technique of Patent Document 2, a tailored blank material for hot stamping having high joint strength can be obtained, but as a result of examination by the present inventors, Al is concentrated at the weld bead end (near the melting boundary). It was found that the welded layer may be formed and the joint strength may vary.

The present invention is a tailored blank molding material that improves the above-mentioned variation in joint strength, forms an aluminum-plated steel plate by butt laser welding as it is without removing the plating layer of the portion to be welded, and processes it by hot pressing. Therefore, it is an object of the present invention to provide a tailored blank molding material having sufficient stable joint strength.

In the tailored blank material for hot stamping, the steel plate base material is hardened by the hot stamping and becomes hard. In order to secure the required joint strength after hot stamping, the hardness of the weld metal after hot stamping is at least equal to or higher than the hardness of the steel plate base material on the low strength side of the steel plates forming the joint. At the same time, it is necessary not to generate an Al-enriched region at the weld bead end.

When a blank made of an aluminum-plated steel plate is welded to obtain a tailored blank material, the present inventors form a weld metal portion having a hardness equal to or higher than that of a steel plate base material portion on the low strength side after hot stamping. In addition, the above-mentioned problems have been solved by examining means for preventing the Al-concentrated region from being generated at the weld bead end, and as a result, finding the conditions necessary for forming such a weld metal portion.

The gist of the present invention made in this way is as follows.

(1) A method for manufacturing a tailored blank molding material, in which a plurality of aluminum-plated steel sheets are butted against each other, irradiated with a laser beam, and welded. A tailored blank material manufactured by welding while injecting gas is processed by hot stamping, and at the welded portion of the tailored blank molded material.
The average concentration of aluminum in the weld metal of the weld is 1.5% by mass or less, the structure of the weld metal contains martensite in an area ratio of 80% or more, and the hardness of the weld metal and that of the weld metal The value of the product of the thickness of the thinnest part is higher than the value of the product of the product of the hardness and the thickness of the aluminum-plated steel sheet having the lower strength among the above two aluminum-plated steel sheets, and the value of the hardness of the weld metal and the weld metal The value of the product of the thickness of the thinnest part of the metal is 682.5 or more, and the region of the weld metal within 0.3 mm from the molten boundary and within 1/4 of the thickness of the steel plate from the surface of the weld. A method for producing a tailored blank molding material, which comprises an aluminum concentration of less than 1.5% by mass.
(2) The method for producing a tailored blank molding material according to (1), wherein the direction of injecting the gas crosses directly above the molten pool.
(3) The method for producing a tailored blank molded material according to (1) or (2), wherein the angle of the gas with respect to the surface of the steel sheet is 0 degrees or more and 30 degrees or less.
(4) The method for producing a tailored blank molded material according to any one of (1) to (3) above, wherein the injection speed of the gas is higher around the molten pool than immediately above the molten pool.

According to the present invention, it is possible to provide a tailored blank molded material having high joint strength in which aluminum-plated steel sheets are butt-welded and laser-welded without removing Al plating at the end of the steel sheet.

It is a figure which showed each structure of the welding apparatus of this invention schematically. It is a figure which shows the example which the nozzle and a jet are tilted upward or downward. It is a figure for demonstrating the nozzle provided in the welding apparatus of this invention. It is a figure for demonstrating the relationship between the jet velocity distribution and a molten pool in the welding method of this invention.

First, the contents examined by the present inventors will be described. In the following description, a material for hot stamping in which aluminum-plated steel plates are butt-welded is referred to as a tailored blank material, and a structural member obtained by processing a tailored blank material by hot stamping is referred to as a tailored blank molding material.

The steel sheet for hot stamping has a high C content, and the weld metal formed by butt laser welding is hardened by cooling after welding and becomes harder than the base metal. However, when the aluminum-plated steel plate is butt-welded by butt laser welding, the Al of the molten plating layer is mixed with the steel base material almost evenly in the weld metal due to the direct irradiation of the laser beam, and the Al in the weld metal is mixed. Depending on the concentration, the hardenability may decrease and a welded metal having sufficient hardness may not be obtained after welding.

On the other hand, Al around the laser spot (laser irradiation diameter) that is not melted by direct irradiation of laser light melts by heat transfer from the formed molten pool, penetrates into the molten pool being solidified, and reaches the end of the weld bead. Since it is thickened, the hardenability is similarly reduced.

The weld metal undergoes austenite transformation and martensite transformation due to melting and cooling during laser welding, and becomes a martensite-based structure. By being heated in the subsequent hot stamping process, it is transformed into austenite again, and by cooling by contact with the mold during hot stamping, it is transformed into martensite.

However, in the component mixed with the molten Al plating layer, the temperature at the Acc3 point rises, and the austenite transformation does not occur sufficiently at the heating temperature of the hot stamp (for example, 900 ° C.). Tissues that do not undergo austenite transformation will not be burned and will be burned back.

Further, in a tailored blank for hot stamping, one of aluminum-plated steel sheets to be butt-welded to each other is often a high-strength steel sheet, and the other is a combination of lower-strength steel sheets. In a tailored blank formed by laser welding aluminum-plated steel plates having different strengths, even if the formed weld metal is tempered in the hot stamping process, the hardness of the steel plate on the lower strength side after hot stamping is hard. As long as it has a hardness equal to or higher than that of the welded joint, the required strength of the welded joint can be maintained.

When the aluminum-plated steel plate is laser-welded to produce a tailored blank material, the present inventors suppress the Al-concentrated layer generated at the weld bead end and form a weld metal having the quenching characteristics required in the hot stamping process. If possible, from the viewpoint that welding is possible without removing the aluminum plating at the end of the butt joint, the conditions of the weld metal and aluminum-plated steel plate required for that purpose were examined.

Due to the distribution pattern of the Al-concentrated region at the end of the weld bead, the Al plating outside the laser irradiation range was melted by the heat of the molten pool formed by laser irradiation, and invaded during the solidification process of the molten pool. It was speculated that it was formed on the edge. Therefore, it is difficult to adjust the components by the wire disclosed in Cited Document 2. Therefore, in order to prevent the invasion of the molten Al plating layer, the present inventors have investigated the conditions for suppressing the invasion by applying gas.

On the other hand, in order to ensure the hardenability of the portion where Al is mixed, at least the weld metal formed during laser welding undergoes austenite transformation in the cooling process after welding, and is further hardened in the cooling process to become sufficiently hard. It is necessary. In addition, even if the hardened welded part is transformed into austenite and hardened again in the hot stamping process, or tempered without being sufficiently transformed, the hardness is equal to or higher than that of the steel sheet on the lower strength side. It is necessary to be able to maintain the strength.

In butt laser welding of aluminum-plated steel sheets, the composition of the weld metal is determined by the composition of the steel sheet to be welded, the thickness of the steel sheet, and the amount of melt in the Al-plated layer. Therefore, if the conditions of the weld metal to be hardened are clarified, the present inventors can perform welding without removing the Al plating at the end of the butt portion, the steel plate composition, the steel plate plate thickness, and the Al plating layer. It was thought that the combination of thicknesses could be determined.

As a result of the study by the present inventors, the molten Al plating layer is formed into the molten pool by ejecting gas from the welding progress direction so as not to directly hit the molten pool toward the molten pool formed by irradiation with a laser beam. In addition to suppressing intrusion, the average concentration of Al in the weld metal formed by butt-welding aluminum-plated steel plates is 0.3% by mass or more and 1.5% by mass or less, and the Ac 3 points of the welding metal. When the temperature is set to 1250 ° C or lower, the value of the product of the hardness of the weld metal after hot stamping and the thickness of the thinnest part of the weld metal is higher than the value of the product of the hardness of the low-strength steel plate after hot stamping and the plate thickness. The tailored blank molding material of the present invention, which is welded by combining the steel plates to be butt-welded so as to be high, has been reached.

Hereinafter, the necessary conditions and preferable conditions of the present invention will be sequentially described.

(Suppression of molten Al intrusion into the bead end)
The present inventors have conducted various studies considering that a gas ejection device is used to prevent the Al plating melted by heat transfer from the molten pool formed during laser welding from entering the bead end. As a result, it was found that it is effective to install a gas ejection device on the front and back surfaces of the welding in front of the welding progress direction and eject the gas in the welding line direction directly above the molten pool.

Since the melting and intrusion of Al plating due to heat transfer in the molten pool occurs on both the front and back surfaces of the weld, it is necessary to provide gas ejection devices on both the front and back surfaces of the weld. Further, in order to suppress the disturbance of the molten pool due to the ejected gas having a high flow velocity and the outflow from the molten pool, it is desirable that the ejected gas is applied directly above the molten pool.

For gas injection, it is preferable to inject gas across the directly above the molten pool so that the vertical distance between the extension line along the lower end of the injection port of the gas injection means and the molten pool is within a range of 3 mm or less. FIG. 1 is a diagram schematically showing the configuration of the welding apparatus of the present invention, and shows a cross section of the welded portion in the plate thickness direction. Laser welding proceeds from right to left on the paper surface, and gas injection by the nozzle is performed from the front in the direction of travel of the laser. In FIG. 1, the gas ejection device on the back surface side is omitted.

The "extension line along the lower end of the injection port" of the gas injection means is determined by the shape of the lower end of the injection port. For example, the lower end of the injection port is parallel to the upper surface of the steel plate 1 as in the example shown in FIG. If so, the extension line Ak is also parallel to the steel plate 1. Further, for example, when the lower end of the injection port has a divergent shape, an extension line is formed so as to extend the divergent lower end. By reducing the vertical distance between the extension line along the lower end of the nozzle and the molten pool to 3 mm or less, it is possible to efficiently prevent the infiltration of molten Al.

Further, the jet is preferably horizontal to the upper surface of the molten pool, but is not limited to this, and may have an angle in a direction away from or closer to the upper surface of the molten pool. The angle is not particularly limited, but if the jet directly hits the molten pool, molten metal may flow out of the molten pool, so both θ'and θ'' shown in FIG. 2 are 0 to 30 degrees. It is preferable that there is, and in particular, θ'' is preferably 0 to 20 degrees.

The shape of the nozzle is not particularly limited. For example, as shown in FIG. 3, the welding quality is expected to be improved by injecting gas from a nozzle having two injection ports in the width direction and increasing the injection speed around the nozzle as compared with the one directly above the molten pool (Fig. 4). Will be done.

The type of gas to be injected is not particularly limited. Compressed air, an inert gas, oxygen, nitrogen, or the like can be used, but the inert gas (Ar) is preferable when there is a concern that it will react with the molten metal.

(Al concentrated area at the end of the weld bead)
As described above, when aluminum-plated steel plates are butt-welded, Al around the laser spot (laser irradiation diameter), which is not melted by direct irradiation of laser light, is melted by heat transfer from the formed molten pool and is solidifying. Invades the molten pool and forms an Al-enriched region at the end of the weld bead.

In the present invention, the invasion of molten Al into the weld bead end is suppressed by the method described above, and the Al concentration at the weld bead end where Al is likely to be concentrated is set to less than 1.5%. Here, the weld bead end means a region within 0.3 mm from the melting boundary and within 1/4 of the plate thickness of the steel plate from the surface of the welded portion. The melt boundary can be easily confirmed by cutting the weld bead in a cross section perpendicular to the weld line, polishing and etching.

(Welded metal structure)
The weld metal of the tailored blank molding must have sufficient weld hardness to ensure strength. Therefore, the structure of the weld metal needs to contain martensite in an area ratio of 80% or more. Here, martensite includes both non-tempered fresh martensite and tempered martensite.

The area ratio of fresh martensite can be observed as a whitened phase by an optical microscope if the martensite is subjected to repeater etching. In addition, the area ratio of tempered martensite can be obtained by microstructure analysis using SEM-EBSD.

(Ac3 point temperature of weld metal)
If the Al concentration of the weld metal is 1.5% by mass or less and the Ac3 point of the weld metal defined by the following formula (1) is 1250 ° C. or less, the weld metal is sufficiently hardened after laser welding. It is advantageous to obtain the above-mentioned structure. As a result, a tailored blank material having sufficient weld hardness can be produced, so that sufficient strength can be maintained even if the weld is backfired by the hot stamping process.

^{Ac3 = 910-230C 0.5 -15.2Ni + 44.7Si +} 104V
+ 31.5Mo + 13.1W-30Mn-11Cr-20Cu + 700P
+ 400Al + 120As + 400Ti ... (1)

Here, in the formula (1), the element symbol represents the content (mass%) of each element. In addition, elements that are not contained are calculated with a content of 0.

The formula of Ac3 is a well-known formula in the literature (Leslie, W.C., translated by Shigeyasu Koda / translated by Maruzen (1985), p.273).

Depending on the chemical composition of the steel material, such as when the content of C and Mn is low and the content of Si is high, even if the Al concentration of the weld metal is 1.5 wt% or less, it will be sufficiently baked after laser welding. It may not fit. The present inventors have experimentally confirmed that if the Ac 3-point temperature estimated by the formula (1) is 1250 ° C. or lower, sufficient quenching is performed by laser welding.

(Hardness of weld metal after hot stamping)
After manufacturing a structural member that is a tailored blank molding material by hot stamping a tailored plank material, even when the member is incorporated into an automobile and undergoes large deformation due to a collision, it does not break at the weld bead and is good. It must exhibit deformability, energy absorption characteristics, and proof stress.

For that purpose, the strength of the welded portion of the tailored blank molded material after hot stamping needs to be at least equal to or higher than the strength after hot stamping of the steel sheet on the lower strength side.

That is, the product of the hardness Hv (WM) of the weld metal after hot stamping and the thickness t (WM) of the thinnest portion of the weld metal is as low as the hardness Hv (LBM) of the low-strength side steel plate after hot stamping. It is higher than the value of the product of the plate thickness t (LBM) of the strength side steel plate, that is,
Hv (WM) x t (WM)> Hv (LBM) x t (LBM)
It is necessary that the steel plate and the weld metal on the high-strength side and the low-strength side are combined and welded so as to be.

Here, the hardness of the weld metal is measured at 5 points in the plate thickness direction at the center of the bead width in the cross section in the longitudinal direction of the weld bead, and 3 points are measured excluding the highest measurement value and the lowest measurement value. Let the values be averaged.

The thickness of the thinnest part of the weld metal means the length of the shortest part of the weld metal from the front surface of the weld metal to the back surface of the weld metal in the plate thickness direction perpendicular to the plate. And.

When the Ac3 point temperature rises, the austenite transformation is not sufficiently performed at the heating temperature of the hot stamp, and the welded portion is backfired by the hot stamp. Even in such a case, it is possible to select a combination of steel sheets to be welded so as to satisfy the condition of Hv (WM) × t (WM)> Hv (LBM) × t (LBM). , It was confirmed that the hot stamped member exerts a sufficient function as a structural member of an automobile.

Whether or not the tailored blank molded material to be manufactured satisfies such a condition is basically manufactured and confirmed. It is also possible to predict in advance whether or not the tailored blank satisfies the above conditions by predicting the hardness Hv (BM) of the low-strength side steel plate after hot stamping and the hardness Hv (BM) of the weld metal.

Hv (WM) estimates the C amount of the weld metal from the chemical composition and plate thickness of both steel plates, and the hardness Hv (WM) when the weld metal is martensite is calculated by the following formula (HM) based on the estimated C amount. Calculated from 2), the value obtained by subtracting 100 from the calculated hardness is estimated as the lower limit of the weld metal. Note that 100 is an experimentally determined number.

Hv (M) = 884C (1-0.3C2) +294 ... (2)

Since Al is contained in the weld metal, Ac1 and Ac3 increase. Depending on the amount of Al, the hot stamping process does not completely transform into austenite, but only in the two-phase region or simply reprints. It becomes the softest when it is simply baked. It was empirically confirmed that the hardness was about Hv (M) -100.

Hv (BM) is calculated by using the Hv (M) value calculated from the formula (2) and the element content (mass%) of the steel sheet.
+ Cu / 2 + Ni / 4)}
The lower value of is adopted as the estimated value.

However, when the B content ≥ 0.0004 mass%, f (B) = 0.03, and when the B content <0.0004 mass%, f (B) = 0.

The hardness predicted as described above, the thickness of the steel plate to be combined, and the thickness of the weld metal are set to the steel plate thickness to 0.9 × the steel plate thickness, and it is determined whether or not the above formula is satisfied, and the tailored blank is formed. It is possible to predict in advance the possibility of combining the constituent steel sheets.

(Thickness of weld metal)
In butt laser welding of the end face of a steel sheet as it is shear-cut, the surface of the weld bead is usually welded in a recessed state (thin state) with respect to the surface of the steel sheet due to the cutting accuracy of the end face.

However, if the wall thickness of the weld metal becomes too small than the plate thickness of the base steel plate, the strength of the welded joint decreases, so the wall thickness of the thinnest part of the weld metal is the steel plate plate thickness (steel plate thickness). If they are different, it is preferable that the thickness is not less than 90% of the thinner plate thickness).

If the wall thickness is less than 90%, even if the weld metal has a composition that is easily hardened, it may break at the welded part during hot stamping at a high temperature, or the product strength may decrease even if it does not break. , It is preferable to weld using a filler wire to supplement the thin meat.

On the other hand, not only when the wall thickness of the welded portion is thin, welding may be performed using a filler wire for adjusting the Al concentration of the weld metal.

When welding using a filler wire, the strength of the welded portion can be ensured by raising the front and back surfaces of the weld bead with respect to the front and back surfaces of the steel plate to increase the wall thickness of the weld metal. However, if the raised height of the weld metal becomes excessive, the contact between the steel plate and the mold becomes poor near the welded portion at the time of hot stamping, and quenching of the steel plate becomes insufficient.

Therefore, the front and back surfaces of the weld metal do not protrude outward beyond 300 μm based on the extension line of the front and back surfaces of the aluminum-plated steel sheet (or the front and back surfaces of the thicker steel sheet when the thickness of the steel sheet is different). To do so. When the protrusion amount is 300 μm or less, the steel sheet can be sufficiently hardened by using a die, particularly a direct water-cooled die. The direct water cooling mold is a mold that cools a steel sheet by ejecting cooling water from the mold.

(Aluminum galvanized steel sheet)
The aluminum-plated steel sheet used for the tailored blank material in the present invention is not limited to a specific one. Hereinafter, an example of the steel plate base material and the plating layer that can be applied to the tailored blank molding material of the present invention will be described.

[Base material of aluminum-plated steel sheet]
As the base steel sheet, the steel sheet on the high strength side is hardened by the hot stamping process and has high mechanical strength (for example, tensile strength, yield point, elongation, drawing, hardness, impact value, fatigue strength, etc.) It is desirable to use a steel sheet with a composition that has creep strength, etc.).

Examples of such steel sheets are, in terms of mass%, C: 0.15 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.8 to 3.0%, Cr: 0. 01-0.5%, B: 0.1% or less (including 0%), and a steel component consisting of the balance Fe and unavoidable impurities, or based on this steel, Ti, Mo, Examples of steels in which one type or two or more types of Nb are added in an amount of 1.0% or less are exemplified.

As an example of a steel sheet to be combined with such a steel sheet, an aluminum-plated steel sheet having a composition capable of obtaining a tensile strength of 270 to 1800 MPa class as a cold-rolled steel sheet can be exemplified. However, in the steel sheet for hot stamping, the strength of the steel sheet before hot stamping itself is not particularly specified.

The plate thickness of the steel plate base material can be, for example, in the range of 0.8 to 4 mm according to the present invention.

[Al plating layer]
The Al plating layer prevents corrosion of the steel sheet and prevents the formation of scale (iron oxide) generated by oxidation of the surface of the steel sheet heated to a high temperature when the steel sheet is processed by hot stamping. .. Since the Al plating layer has a higher boiling point than the plating coating with an organic material or the plating coating with another metal material (for example, Zn), it is possible to process at a high temperature when molding by the hot stamping method. It is advantageous for baking the weld metal. For these actions, it is desirable that the Al plating layer is formed on both sides of the steel sheet.

The Al plating layer is formed on the surface of the steel sheet by, for example, a hot dip galvanizing method. The present invention can be applied as long as Al is contained as a component of the plating layer. The components other than Al are not particularly limited. For example, Si may be added in an amount of 3 to 15% by mass.

By adding Si, it is possible to control the alloy layer formed during hot dip galvanizing metal coating. In order to obtain the effect, it is preferable to contain 3% or more of Si. If the Si content exceeds 15%, the processability and corrosion resistance of the plating layer may decrease. Therefore, the Si content is preferably less than 15%.

(Welding method for manufacturing tailored blank material)
The tailored blank material used to obtain the tailored blank molded material of the present invention is obtained by welding an aluminum-plated steel sheet by laser welding. In the tailored blank material of the aluminum-plated steel plate, the Al of the plating layer melts into the weld metal, so that the temperature at the Ac3 point of the weld metal rises, and the hot stamping process cannot sufficiently austenite. Therefore, when manufacturing a tailored blank, it is necessary to sufficiently harden the welded portion by cooling after welding, so welding by laser welding with a high cooling rate after welding is required.

The laser welding method is not particularly limited to the type of laser oscillator and the like, and welding may be performed with a laser output according to the thickness of the steel plate used. At that time, as described above, the filler wire can be supplied and welded.

The tailored blank molding material of the present invention is not limited to the one composed of two aluminum-plated steel plates. It may include three or more aluminum-plated steel plates and two or more welded portions. In that case, the value of the product of the hardness of the weld metal of each weld and the thickness of the thinnest part of the weld metal is that of the aluminum-plated steel plate with the lowest strength among the two aluminum-plated steel plates welded by the weld. It may be set to be higher than the value of the product of hardness and plate thickness.

A steel sheet having a tensile strength of 1470 MPa class by hot stamping (steel type HS) and a steel sheet having a tensile strength of 270 MPa, 440 MPa, and 590 MPa (steel type 270r, 440, 590) before hot stamping were joined by laser welding. The thickness of the steel plate used was in the range of 1.0 mm to 1.8 mm. Further, a welding wire (YGW12, YM-80) having a diameter of 0.9 mm was appropriately supplied during welding.

Prototype To separately form the Al average concentration of weld metal, out of the free steel sheet Al plating, Al coating weight is, per side 40Gr / ^{m 2} on both surfaces, and the steel sheet per surface 80 gr / ^{m 2} on both surfaces did.

These steel sheets were butted in a shear-cut state and welded by a fiber laser. The focused spot diameter of the laser was 0.6 mm.

Further, for the purpose of preventing the intrusion of molten Al, a round tube adjacent nozzle having two round tubes having an inner diameter of 4 mm and an outer diameter of 6 mm adjacent to each other in the horizontal direction was used. As a result, it was possible to obtain a velocity distribution with a slow center velocity in the cross section of the jet as shown in FIG.

The nozzle was placed on the front and back surfaces of the weld at a position where the tip position was 12 mm away from the laser optical path, aiming at 3 mm directly above the molten pool, and Ar gas was injected under the conditions of a pressure of 0.5 MPa and 100 L / min. The welding speed was kept constant at 4 m / min, and the laser output was adjusted in the range of 2 kW to 4 kW according to the plate thickness.

The tailored blank material obtained by welding was then hot stamped to obtain a hot stamped molding material. Hot stamping was performed by heating the blank material to 900 ° C. by heating in a furnace and sandwiching it with a mold to finish it as a flat plate.

After laser welding, the average Al concentration in the weld metal was analyzed and determined. In addition, in order to confirm the quality of the welded portion after laser welding, cross-section observation and bead thickness measurement of the welded portion in the plate thickness direction were carried out.

Further, in order to confirm the quenching state by hot stamping, the hardness of the base metal portion and the weld bead portion after hot stamping was measured. In addition, as a member performance evaluation after hot stamping in which the strength was partially divided, a tensile test and a close contact bending test in which a load was applied orthogonal to the welding bead were performed.

Table 1 shows the steel sheets used and the results of various measurements after welding and hot stamping. Example When two numbers are described in the thickness of the weld metal, for example, No. 12 means that the thickness of the weld metal was measured as the plate thickness of the steel plate on the thin side +0.20 mm (front side of the weld metal) +0.15 mm (back side of the weld metal).

1 Material to be welded 17 Nozzle 27a Nozzle 27b Spout 27c Spout C Welded part

Claims (4)

A method for manufacturing tailored blank molding materials.
When two aluminum-plated steel sheets are butted against each other, irradiated with a laser beam, and welded, a tailored blank material manufactured by welding while injecting gas onto both the front and back surfaces of the steel sheet in a direction facing the welding progress direction is produced. , Processed by hot stamping, and
In the welded part of the tailored blank molding material
The average concentration of aluminum in the weld metal of the weld is 1.5% by mass or less.
The structure of the weld metal contains martensite in an area ratio of 80% or more.
The value of the product of the hardness of the weld metal and the thickness of the thinnest portion of the weld metal is higher than the value of the product of the hardness and the thickness of the aluminum-plated steel sheet having the lower strength among the two aluminum-plated steel sheets, and , The value of the product of the hardness of the weld metal and the thickness of the thinnest part of the weld metal is 682.5 (HV · mm) or more.
Tailored blank molding characterized in that the aluminum concentration in the region of the weld metal within 0.3 mm from the molten boundary and within 1/4 of the plate thickness of the steel sheet from the surface of the welded portion is less than 1.5% by mass. Material manufacturing method. The method for producing a tailored blank molding material according to claim 1, wherein the direction of injecting the gas crosses directly above the molten pool. The method for producing a tailored blank molded material according to claim 1 or 2, wherein the angle of the gas with respect to the surface of the steel sheet is 0 degrees or more and 30 degrees or less. The method for producing a tailored blank molded material according to any one of claims 1 to 3, wherein the injection speed of the gas is higher around the molten pool than immediately above the molten pool.

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