Tailor-Rolled Blank for Use in Hot Stamping Automotive Parts, and Method of Making Hot Stamped Automotive Parts

The information provided in this section is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

This disclosure relates to hot stamped automotive parts, and in particular to a tailor-rolled blank for use in hot stamping automotive parts, and method of making hot stamped automotive parts.

Many structural parts for automobiles are made by hot stamping or press hardening. A steel typically used in this process is 22MnB5, which has a nominal composition of 0.22 wt % carbon, 1.59 wt % manganese, 0.25 wt % silicon, 0.006 wt % chromium, 0.002 wt % copper, 0.001 wt % sulfur, 0.015 wt % phosphorous, 0.082 wt % aluminum, 0.026 wt % titanium, and 0.0027 wt % boron. However, this alloy is prone to oxidation during high temperature processing, and thus is usually provided with a protective aluminum-silicon coating.

While effective at reducing oxidation, the coating interferes with pre-hot stamping processing such as a cold rolling. The amount of cold rolling reduction of the coated alloy must be limited (both in absolute and relative terms) to avoid undue thinning of the coating.

Embodiments of this disclosure provide a tailor-rolled blank for use in hot stamping automotive parts, a process for making tailor-rolled blanks, and automotive parts made from the tailor-rolled blanks.

According to a first embodiment of this disclosure, a tailor-rolled blank is provided for use in hot stamping automotive parts. The blank is made from an alloy comprising between from about 0.05 to about 0.35 wt % carbon, about 0.5 to about 5.0 wt % manganese, about 0.5 to about 2.0 wt % silicon, about 0.6 to about 4.0 wt % chromium, optionally about 0.02-0.05 wt % niobium, and optionally about 0.03 to about 0.3 wt % yttrium and/or cerium, and the balance being iron and impurities. This blank does not require a coating to resist excessive oxidation during subsequent hot stamping, and thus it can be reduced in thickness by a greater amount and by a greater percentage than coated alloys. Moreover, the alloy can be hardened at a slow cooling rate, allowing sharper transitions in thickness because it is not necessary that the surface of the blank remain in contact with the dies during hot stamping. The alloy is cold rolled to form a tailored-rolled blank having one or more of these geometrical features: (a) a maximum thickness difference between the thinnest area of the blank and the thickest area of the blank can be about 0.8 mm up to about 2.0 mm; (b) the thickest gage is between 1.4 and 3.2 mm, and the thinnest gage is between 0.8 and 1.5 mm, and the thickness ratio (thickest/thinnest) is between 1.2 and 2.5; and (c) at least one transition zone between adjacent areas of the blank with different thicknesses that has a length less than 80 times the difference in thickness between the areas, but greater than 10 times the difference in thickness between the areas.

In some versions of the first embodiment, the tailor-rolled blank optionally comprises about 0.02-0.05 wt % niobium, and in other versions of the first embodiment the blank optionally comprises about 0.03 to about 0.3 wt % yttrium and/or cerium. In still other versions of the first embodiment the tailor rolled blank optionally comprises both about 0.02-0.05 wt % niobium and about 0.03 to about 0.3 wt % yttrium and/or cerium.

According to a second embodiment of this disclosure, A structural part for an automobile is provided by hot stamping a tailor-rolled blank according to any of the versions of the first embodiment.

According to a third embodiment of this disclosure a method of making tailor-rolled blank for use in hot stamping structural parts for automobiles. Generally, methods of this third embodiment comprise forming a tailor-rolled blank suitable for use in hot stamping automotive parts, by cold rolling a blank made from an alloy comprising between from about 0.05 to about 0.35 wt % carbon, about 0.5 to about 5.0 wt % manganese, about 0.5 to about 2.0 wt % silicon, about 0.6 to about 4.0 wt % chromium, optionally about 0.02-0.05 wt % niobium, and optionally about 0.03 to about 0.3 wt % yttrium and/or cerium to have one or more of these geometrical features: (a) a maximum thickness difference between the thinnest area of the blank and the thickest area of the blank greater than about 0.8 mm and about 1.5 mm; (b) a maximum ratio of the thinnest area of the blank to the thickest area of the blank about 1.2 and less than about 2.5, inclusive; and (c) at least one transition zone between adjacent areas of the blank with different thickness being less than 80 times the difference in thickness between the areas, but greater than 10 times the difference in thickness between the areas; and hot stamping the tailor-rolled blank to form the structural part for an automobile.

The resulting automotive parts of the second embodiment, and the automotive parts resulting from the third embodiment can be less expensive to manufacture and lighter weight than conventionally heat stamped parts. Because of the greater variance in thickness permitted with the tailor-rolled blanks of the embodiments of this disclosure, the blanks can be made thinner in less critical areas of the part while maintaining appropriate thickness in critical areas of the finished part, reducing the amount of material used and the weight of the final part. In addition, the increased ability to vary the thickness of the tailor-rolled blank eliminates the need to reinforce the areas of the blank. Further the transitions between areas of different thicknesses can be much shorter and still achieve desirable strength and hardness properties because die contact is not critical to achieving a cooling profile that achieves desirable properties.

In the drawings, reference numbers may be reused to identify similar and/or identical elements.

Embodiments of this disclosure provide a tailor-rolled blank for use in hot stamping automotive parts, a process for making tailor-rolled blanks, and automotive parts made from the tailor-rolled blanks.

According to a first embodiment of this disclosure, a tailor-rolled blank is provided for use in manufacturing an automotive part by hot stamping. The tailor-rolled blank is made by cold rolling an alloy comprising between from about 0.05 to about 0.35 wt % carbon, about 0.5 to about 5.0 wt % manganese, about 0.5 to about 2.0 wt % silicon, about 0.6 to about 4.0 wt % chromium, optionally about 0.02-0.05 wt % niobium, and optionally about 0.03 to about 0.3 wt % yttrium and/or cerium, and the balance being iron and impurities.

The cold rolling process creates a smoother surface (e.g., an Ra of about 0.3) than does hot rolling (e.g., an Ra of about 2.5). Furthermore, because alloy does not have a protective aluminum-silicon coating to resist excessive oxidation during subsequent hot stamping, a blank of this alloy can be cold rolled to greater thickness reductions, both absolute and as a ratio. For example, the alloy can be cold rolled to thickness differences greater than 0.8 mm and up to 2.0 mm, and to thickness difference ratios (thickest/thinnest) of between 1.2 about 2.5, inclusive, because there is no surface coating that thins or disrupts (e.g., by inversion or breaking off).

Furthermore, it is difficult for the die to match the transition zone perfectly during hot stamping. Air gaps between the die and the transition can reduce the cooling rate. The alloy of the present disclosure can be hardened at a much slower cooling rate than conventional alloys, such as coated 22MnB5, allowing sharper transitions between areas of different thicknesses because it is not necessary that the surface of the blank remain in contact with die during cooling. To achieve desired physical properties conventional alloys such as 22MnB5 requires a cooling rate of 30° C./s for hardening, while the current alloy achieves satisfactory hardening at a cooling rate of between about 5° C./s and about 20° C./s. Furthermore, after hot stamping, the alloy of the current disclosure can achieve a yield strength between about 900 and about 1400 MPa and an ultimate tensile strength of about 1300-1900 MPa, while coated 22MnB5 has an ultimate tensile strength of 1350-1550 MPa. Further, the microhardness of the heat stamped alloy of the present disclosure is between 400 and 560 HV, including transition zones and corners on the hot stamped component.

Surface oxidation can form during hot stamping. This oxidation is thicker at thinner gage and thinner at thicker gage. The variation is greater than 0.1 times of thickness ratio.

Thus, the alloy of the present disclosure can be cold rolled to achieve one or more of geometries beneficial to the ultimate heat stamped product, and which could not be achieved with prior coated alloys such as coated 22MnB5, including one or more of (a) a maximum thickness difference between the thinnest area of the blank and the thickest area of the blank greater than about 0.8 mm and less than about 2.0 mm; (b) a maximum ratio of the thinnest area of the blank to the thickest area of the blank between about 1.2 and about 2.5, inclusive; and (c) at least one transition zone between adjacent areas the blank of different thickness areas of the blank with a length less than 80 times the difference in thickness between the areas, but greater than 10 times the difference in thickness between the areas.

The ability to form a blank with greater thickness reductions, and greater thickness reduction ratios, allows the tailor-rolled blank to be made thinner in less critical areas of the part while maintaining appropriate thickness in critical areas of the part, reducing the amount of material used and the weight of the final part. Furthermore, the wider variations permitted in the thickness of the blanks means that reinforcements and other post manufacture modifications to the blanks are unnecessary. Similarly, allowing shorter transitions between areas of different thickness also means that the blanks can be made with less material, and can result in lighter weight parts.

In some versions of the first embodiment, the tailor-rolled blank comprises about 0.02-0.05 wt % niobium, and in other versions of the first embodiment the blank comprises about 0.03 to about 0.3 wt % yttrium and/or cerium. In still other versions of the first embodiment the tailor rolled blank comprises both about 0.02-0.05 wt % niobium and about 0.03 to about 0.3 wt % yttrium and/or cerium.

According to a second embodiment of this disclosure, a structural part for an automobile is provided by hot stamping a tailor-rolled blank according to any of the versions of the first embodiment. The tailor-rolled blank, and the resulting structural part, is made of an alloy that does not require coating to protect it from excessive oxidation during the hot stamping process. This allows the blank to be cold rolled to provide greater variations in thickness (between about 0.8 mm and about 2.0 mm) and greater ratios of thickness variation (thickest/thinnest) (up to 2.5 or more), reducing the amount of material used, and reducing the weight of both the tailor-rolled blank, and the resulting structural part. Further the ability of the alloy to harden during relatively slow cooling allows shorter transitions between adjacent areas of different thickness, because die contact is not critical to cooling. Thus, while traditional alloys such as coated 22mnB5 require transition zones with lengths larger than 80 times the change in thickness, the alloys used in the present disclosure can have transition zones ranging from 10 times the change in thickness to less than 80 times the change in thickness.

According to a third embodiment of this disclosure a method of making tailor-rolled blank for use in hot stamping structural parts for automobiles. Methods of this third embodiment as indicated generally asin, and atincludes forming a tailor-rolled blank suitable for use in hot stamping automotive parts, by cold rolling a blank made from an alloy comprising between from about 0.05 to about 0.35 wt % carbon, about 0.5 to about 5.0 wt % manganese, about 0.5 to about 2.0 wt % silicon, about 0.6 to about 4.0 wt % chromium, optionally about 0.02-0.05 wt % niobium, and optionally about 0.03 to about 0.3 wt % yttrium and/or cerium to have one or more of these geometrical features: (a) a maximum thickness difference between the thinnest area of the blank and the thickest area of the blank greater than about 0.8 mm and less than about 2.0 mm; (b) a maximum ratio of the thickest area of the blank to the thinnest area of the blank is between about 1.2 and about 2.5; and (c) at least one transition zone between adjacent areas the blank of different thickness areas of the blank with a length of less than 80 times the difference in thickness between the areas, but greater than 10 times the difference in thickness between the areas. Then, at, hot stamping the tailor-rolled blank to form the structural part for an automobile.

The resulting automotive parts of the second embodiment, and the automotive parts resulting from the methods of the third embodiment can be less expensive to manufacture and lighter weight than conventionally heat stamped parts. Because of the greater variance in thickness permitted with the tailor-rolled blanks of the embodiments of this disclosure, the blanks can be made thinner in less critical areas of the part while maintaining appropriate thickness in critical areas of the part, reducing the amount of material used and the weight of the final part. In addition, the increased ability to vary the thickness of the tailor-rolled blank eliminates the need to reinforce the areas of the blank. Further the transitions between areas of different thicknesses can be much shorter and still achieve desirable strength and hardness properties because die contact is not critical to achieving a cooling profile that achieves desirable properties. The resulting part can have an ultimate tensile strength greater than about 1500 MPa and/or a hardness above about 45 HRC (between about 400 and 560 HV).

The present disclosure can be applied to any vehicle body structures, including center pillars, door beams, and floor rails, for cost reduction, weight reduction and crash resistance.

The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. Further, although each of the embodiments is described above as having certain features, any one or more of those features described with respect to any embodiment of the disclosure can be implemented in and/or combined with features of any of the other embodiments, even if that combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with one another remain within the scope of this disclosure.