Differential heat shaping and hardening using infrared light   

FIELD OF THE INVENTION

The present invention relates to method of making a workpiece with regions of different ductility. More particularly this invention concerns an alloy-steel workpiece.

BACKGROUND OF THE INVENTION

In the field of vehicle construction, more and more vehicle parts made of high-strength and ultra-high-strength steel are being employed in order to be able to satisfy criteria for light-weight construction. This also applies to car body construction where, in order to meet weight goals and safety requirements, for instance structural and/or safety elements such as door intrusion beams, A and B columns, bumpers, side rails, and cross rails are increasingly being produced from thermoshaped and press-hardened steel having tensile strengths greater than 1000 Mpa.

A method is known from GB 1,490,535 for press-shaping and hardening a steel sheet that is relatively thin and of good dimensional stability in which a sheet made of boron-alloyed steel is heated to a temperature above its AC3 point and then in less than 5 seconds is pressed into the final shape between two indirectly cooled tools that change its shape significantly, and, while still in the press is subjected to rapid cooling such that a martensitic or bainitic structure is obtained. Using these measures produces a finished product with good shape accuracy, good dimensional stability, and high strength, and that is well suited for structural and safety elements in vehicle construction. This process is hereinafter referred to as thermoshaping and press-hardening. Both preshaped parts and even flat panels can be thermoshaped and press-hardened. In preshaped parts, the shaping process can also be limited to shaping of a small percentage of the final geometry or to calibration.

In different applications of motor vehicle engineering, shaped parts are to have a high strength in certain regions while in other regions they are to have higher ductility relative thereto. In addition to reinforcing with additional sheet or joining parts that have different strengths, also already known is treating a part using heat treatments such that local regions have higher strength or higher ductility.

It is known from US 2004/0060623 for instance to produce a hardened metal part having at least two regions with different ductility. A plate or preshaped shaped part is heated to an austenitization temperature in a heater and then transported along a transport path to a hardening process. During transport, first regions of the plate or shaped part that have higher ductility properties in the final part are cooled. The method is optimized for mass production in that the first regions are quenched from a predetermined cooling start temperature that is greater than the γ-α transformation temperature and in that the quenching is terminated when a predetermined cool temperature is attained, specifically prior to any transformation into ferrite and/or perlite taking place or after an only slight transformation into ferrite or perlite has taken place. Then the workpiece is maintained approximately under an isothermal condition for converting the austenite to ferrite and/or perlite. During this, the hardening temperature (THE) in the second region, which has lower ductility properties in the final part in comparison, is just high enough for sufficient martensite formation in the second regions during a hardening process. Then the hardening process is performed. In this method, more thermal energy is added to the first regions of the plate or shaped part than is necessary, and then thermal energy is removed in a second process step, which is also linked to expenditure of energy. The method therefore has a relatively poor energy balance.

DE 101 08 926 C1 discloses a thermal treatment process for changing the physical properties of a metal article. The article is irradiated, at least in a predetermined surface section, with electromagnetic radiation from an emitter having a radiator temperature of 2900 K or more in the near infrared range with a high power density. The material of the surface layer absorbs predetermined treatment temperature as a function of the material parameters. Then the irradiated surface region is actively cooled and thus hardened and tempered. However, completely heating an article that has a large surface area from room temperature to hardening temperature using this method would be too uneconomical for an industrial thermoshaping line.

U.S. Pat. No. 7,540,993 discloses a method for producing a shaped part that has at least two regions with different ductility from a semifinished product made of hardenable steel by heating in a continuous furnace followed by a hardening process. During transport through a continuous furnace the semifinished product to be heated simultaneously passes through at least two zones in the continuous furnace that are adjacent one another in the travel direction and that have different temperature levels and thus are heated differently so that in a subsequent hardening process at least two structural regions are created that have different ductility. The continuous furnace here is consequently provided with at least two longitudinally extending zones that are mutually adjacent in the longitudinal workpiece-advance direction, that are separated from one another by a partition such that a workpiece passing through the furnace has parts in both zones so separate temperature control is possible in each zone. However, this multizone furnace is a special furnace for parts that are to be heated zone-wise.

It is therefore an object of the present invention to provide an improved differential heating method for steel-alloy workpieces.

Another object is the provision of such an improved differential heating method for steel-alloy workpieces that overcomes the above-given disadvantages, in particular that can use a conventional thermoshaping line as economically as possible in the press cycle for producing a differentially hardened part.

SUMMARY

A shaped part having at least two structural regions of different ductility is made from one unitary blank of a hardenable steel alloy by first heating the entire blank to an elevated temperature below an AC3 point of the alloy. Then heating only first regions of the blank by a plurality of infrared lamps to above the AC3 point of the alloy while maintaining the rest of the blank below the AC3 point of the alloy. Finally the first regions are hardened in a thermoshaping and hardening tool.

The first heater according to the invention preferably comprises a conventional pass-through or tunnel-type furnace. Differentially hardened parts can be produced in a conventional thermoshaping line in this manner using the inventive method. Both preshaped parts and even flat plates can be heated using the inventive method, both being referred to hereinafter as blanks. In the case of preshaped parts, the shaping process can also be limited to shaping a small percentage of the final geometry or to calibration of the shape.

In thermoshaping and press-hardening, a defined amount of heat must be applied to the blank. All of the regions that are to undergo as close to a complete structural change to martensite as possible due to the hardening must be heated in advance to a temperature that is greater than or equal to the AC3 point of the alloy. These are referred to as first regions hereinafter. Regions that are not to be hardened or are not to be completely hardened, referred to as second regions hereinafter, must not be heated to a temperature above the AC3 point. For the press-hardening process it would be sufficient if the second regions were room temperature. This would also be the best variant in terms of energy, but at room temperature steel is significantly less malleable than heated steel. Therefore, at least for more complex deep-drawn parts, it is necessary for the shaping process that the steel be heated even in the second regions, especially since common thermoshaping steel springs back after cold-shaping, which has a negative effect on the tolerances that are to be maintained. In addition, there is the fact that if the temperature gradient between the first regions and the second regions is too great, stresses are produced in the transition region after hardening. In order to prevent formation of martensite in the second regions after hardening, in one preferred embodiment the second regions are heated to a temperature up to a maximum of the AC1 point of the alloy. Once the AC1 point has been exceeded, a partial structural change begins that after hardening can also lead to partial martensite formation, which is not desired. Conversely, however, heating with the infrared lamps should not last too long. Therefore the start temperature for the lamp heating using infrared should be as high as possible. Consequently the entire part is preferably heated to a homogeneous temperature up to a maximum of the AC1 point of the alloy in a continuous furnace and then is transferred to the infrared lamp field in order to heat first regions to above the AC3 point. While this is happening, the second regions are not irradiated with infrared at all or are merely maintained at their temperature. In this manner heating by means of infrared is performed rapidly enough to ensure the production sequence in the press cycle. If heating the first regions by means of infrared to above the AC3 point is slower than the press cycle, two or more infrared lamp fields must be used. It is therefore an advantage of the inventive method that it is possible to retain the conventional continuous furnaces in a conventional production line for the thermoshaping and to be able to simply and economically retrofit the conventional line for production of an only differentially hardened part. In addition, in an existing production line, it is possible to construct the heating furnace simpler and more economically overall if the furnace only has to withstand reach temperatures up to AC1 and not above the AC3 point in continuous operation.

In another preferred embodiment, the blank is heated overall to a homogenous temperature below the AC3 point, but greater than the AC1 point of the alloy and is then transferred to an infrared lamp field in which the first regions are heated to above the AC3 point. Then after hardening a mixed structure occurs in the second areas, and this mixed structure settles between the properties of the initial structure and the properties of the hard structure. This mixed structure can be advantageous for certain purposes. The part parameters can therefore be flexibly adjusted as needed by controlling the power of the infrared lamps.

The method is suitable in particular for thermoshaping an uncoated boron-alloy steel that constitutes, as expressed in weight percent: Carbon (C) 0.18% to 0.3% Silicon (Si) 0.1% to 0.7% Manganese (Mn) 1.0% to 2.5% Phosphorus (P) maximum 0.025% Chromium (Cr) up to 0.8% Molybdenum (Mo) up to 0.5% Sulfur (S) maximum 0.01% Titanium (Ti) 0.02% to 0.05% Boron (B) 0.002% to 0.005% Aluminum (Al) 0.01% to 0.06% and remainder iron and impurities resulting from melting.

A blank made of this steel is first heated homogeneously to at least 400° C., preferably to about 700° C., and then is heated in the first regions to a temperature of about 930° C. by means of infrared lamps. The second regions are maintained at approximately 700° C. while this heating takes place. Immediately following the heating, the blank is fitted to a thermoshaping and hardening tool and shaped and hardened in first regions. A differentially hardened, dimensionally accurate, thermoshaped part with defined properties in the respective regions is thus obtained.

However, the method can also be employed for a coated metal workpiece such as for instance thermoform steel coated with aluminum or zinc. However, in particular a thermoform steel coated with a layer containing aluminum must initially be heated to a temperature above the AC3 point of the alloy and fully alloyed in order to form a so-called intermetal phase. In order to use the inventive method described herein in a cost-effective manner, a thermoform steel coated with aluminum must therefore first be fully alloyed in a separate work step. It would be best for this work step to be performed by the steel manufacturer when the coil is produced.

The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a schematic diagram of an inventive thermoshaping line for an uncoated steel workpiece;

FIG. 2 is a schematic diagram of an inventive thermoshaping line for a coated steel workpiece;

FIG. 3 is a large-scale view of a detail of the infrared lamp heater of FIGS. 1 and 2;

FIG. 4 shows the hardness distribution in an B-column according to the invention;

FIG. 5 is a schematic top view of the infrared lamp station 70; and

FIG. 6 shows a heating curve for the first region.

As seen in FIG. 1 a thermoshaping line 1 has a workpiece-supply coil 2 of uncoated thermoform steel-alloy sheet that is unwound and cut to create a shaped blank 4 at a cutting station 3. The shaped blank 4 can be selectively cold preshaped in a molding station 5 and/or can be cut. As a rule, cold-shaping is deep-drawing at room temperature, and trimming is done as close to the final contours as possible. The shaping station 5 is optional and depends on the complexity of the part\'s shape, and there may be no shaping station 5 at all.

Then the shaped blank 4 is transferred in a travel direction that is left to right in the drawing directly to a heating station 6. In the heating station 6, the shaped blank 4 is homogeneously heated to a temperature that is below the AC3 point and is then immediately transferred to an infrared lamp station 7. The infrared lamp station 7 is shown as a separate station here. However, the infrared lamps can also for instance be integrated into the heating station 6, for instance at its downstream end. In the infrared lamp station 7, a first region of the shaped blank 4 is heated to a temperature above the AC3 point of the steel alloy to produced a differentially heat-treated B-column 41. Second regions remain at a temperature that is below the AC3 point.

In the illustrated embodiment in FIG. 1, the second regions are at each end of the treated blank 41 and the first region is in the center of the shaped blank 4. The shaped blank 41 preheated in this manner is then advanced to a force-cooled shaping and hardening tool 8 and is there thermoshaped and differentially hardened.

FIG. 2 shows an embodiment of a thermoshaping line 10 for coated steel sheet. A coil 20 of thermoform steel coated with an alloy containing aluminum is continuously unwound and rewound after moving through a heater 9. In the heater 9, the coated thermoform steel is homogeneously heated to a temperature above the AC3 point so that the coating is completely alloyed and forms with the base metal a so-called intermetal phase. The heated coated steel is not quenched at this point, however, so that it does not harden, because then its resistance to deformation would be too high for further processing. When it leaves the heater 9, the completely alloyed coated steel is re-wound onto a second coil 21.

The coated steel is then continuously unwound from this coil 21 and cut into a coated shaped blank 40 at a cutting station 3. The molding station 5 for cold preshaping is not used because the intermetal phase that occurred during the complete alloying process cannot be cold shaped without cracking. Therefore the shaped blank 40 is transferred directly to the heating station 6. In the heating station 6, the coated shaped blank 40 is homogeneously heated to a temperature that is below the AC3 point and is then immediately transferred to the infrared lamp station 7. The infrared lamp station 7 is shown as a separate station here. However, the infrared lamps can also for instance be integrated into the heating station 6, for instance in the end area. In the infrared lamp station 7, the first region of the shaped blank 40 is heated to a temperature above the AC3 point of the alloy. The second regions remain at a temperature below the AC3 point. In the illustrated embodiment in FIG. 2, the second regions are at each end of the shaped blank 40 and the first region is in the center of the shaped blank 40.

The shaped blank 40 preheated in this manner is then supplied to a force-cooled shaping and hardening tool 8 and is thermoshaped and differentially hardened in the station 8.

FIG. 3 provides a detailed view of the infrared lamp station 7 in FIGS. 1 and 2. Rod-shaped infrared lamps 71 are attached to an overhead support 75. The infrared lamps 71 are controlled in the temperature fields 72 and 74 such that they maintain the two end regions of the preshaped and preheated part 41 that are on a support plate 76 at 700° C. In the temperature field 73, the rod-shaped infrared lamps are controlled such that they heat the center of the part 41 to 930° C. In this FIG. 3, the temperature fields 72, 73, and 74 are separated from one another by shields or partitions 77 and 78. The shields 77 and 78 make it easier to control the temperature distribution in the part 41 and to adjust the hardness values more precisely in the finished part.

As shown in FIG. 4 after the thermoshaping and hardening, a differentially hardened B column 42 has been created from the blank 41 in FIG. 3. The B column 42 is relatively ductile in the head area 43 and in the foot 44. The B column has been hardened in the center region 47 and a mixed structure was created in the transition regions 45 and 46 from the hardened region to the unhardened regions.

FIG. 5 is a schematic top view of another embodiment 70 of an infrared lamp station. The heated shaped blank 4 is positioned beneath spot-like infrared lamps 710. The shaped blank 4 is maintained at a temperature of 700° C. in the head region 43 and in the foot region 44, both constituting the “second” regions. The shaped blank 4 is heated to 930° C. in the center region 47 constituting the “first” region. The temperature drops from 930° C. to 700° C. in the transition regions 45 and 46.

FIG. 6 shows a heating curve 110 for a first region in a sheet. The temperature is shown in ° C. over time in seconds. The curve area 11 shows the continuous heating of the sheet in a continuous furnace. The entire sheet is homogeneously heated from room temperature to about 700° C. in just under 200 seconds. Then at curve point 12 the sheet is transferred to a position beneath an infrared lamp field and within about 30 seconds it is heated to just under 1000° C. Heating concludes at point 13