Method for Warm Forming of Super High Tensile Strength Steel Sheet

Abstract

Disclosed herein is a method for warm forming of a super high tensile strength steel sheet. The method may improve productivity and simultaneously maintain accurate strength by securing thermal structure stability and dimensional stability during heating and forming the super high tensile strength steel sheet. According to an exemplary embodiment of the present invention, the method for warm forming of a super high tensile strength steel sheet includes: heating a steel sheet having a single-phase structure at Al transformation temperature or less; pressing, in a mold, the heated steel sheet to be formed in a predetermined shape; and extracting and cooling the pressed steel sheet.

Description

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2014-0139781, filed Oct. 16, 2014, the entire contents of which is incorporated herein for all purposes by this reference.

TECHNICAL FIELD

The present invention relates to a method for warm forming of a super high tensile strength steel sheet by simultaneously improving strength and formability.

BACKGROUND

Recently, use of a super high tensile strength steel sheet has been increased with the development of vehicle industries. The super high tensile strength steel sheet refers to a steel material that may have greater tensile strength than a high tensile strength steel sheet having a tensile strength of about 50 kg/mm² and may have further improved strain hardening, quenching, annealing, tempering, normalizing, precipitation hardening, grain refinement, solid solution strengthening, and the like using various strengthening mechanisms. However, the super high tensile strength steel sheet may have reduced formability that is in inverse proportion to the improvement of the strength.

Further, in the related art, the super high tensile strength steel sheet may not be applied to parts having a complicated shape, parts having a deep forming depth, or the like due to reduced formability of the super high tensile strength steel sheet, and thus, many attempts to improve the formability of the super high tensile strength steel sheet by heating have been conducted.

However, because vehicle part generally require accurate strength and precise dimensional stability, the super high tensile strength steel sheet may not be used as the vehicle parts due to the reduction in strength , a twist of a member, and a dimensional change thereof.

Recently, in various efforts to solve the above problems, the super high tensile strength steel sheet for vehicle parts has been manufactured by a hot stamping method which may improve formability by heating a steel sheet at a temperature of about 900° C. or greater. Accordingly, the steel sheet may be formed and obtain high strength by rapidly cooling a steel sheet simultaneously with performing press forming using a special mold in which cooling water is circulated.

The hot stamping method may provide the steel sheet with an improved strength of about 150 K or greater by transforming an austenite phase of a steel sheet into a martensite phase. The hot stamping method may not cause a spring back even after extracting the steel sheet and may further prevent a dimension of parts from changing, since the parts within the mold are completely cooled in a fixed state.

However, the hot stamping method may have low productivity to a level of 2 SPM, because a specific mold in which the cooling water is circulated should be used, laser trimming of a draw panel should be performed, and the steel sheet is completely cooled within the mold.

Further, since a heating temperature of the steel sheet may increase to an A₃ transformation temperature or greater of austenite, when the super high tensile strength steel sheet is processed by the precipitation hardening, strengthening effect of the steel sheet due to precipitates may be reduced.

In other words, re-crystallization may occur by high-temperature heat applied to the steel sheet, and thus most of the strengthening effects may be reduced and a lattice structure and a lattice constant may be deformed depending on a change in a crystalline phase, and as a result, a dimension of the steel sheet may also be changed.

As such, the hot stamping method which has been currently rapidly applied may not be considered as an optimal method for manufacturing a super high tensile strength steel sheet for vehicle parts. Therefore, the present invention may provide a method to solve the above-mentioned problems.

In the related arts, a certain method for preventing formability of a steel sheet from reducing has been reported in “Steel Strip For The Automotive Reinforcement Parts And Method Of Manufacturing Thereof (KR 10-0530068)” and “Quenched Steel Sheet Having Ultra High Strength, Parts Made Of It And The Method For Manufacturing Thereof(KR 10-0878614)”.

Further, a CERTAIN method of manufacturing a steel strip for vehicle reinforcement parts has been reported in “Steel Strip For The Automotive Reinforcement Parts And Method Of Manufacturing Thereof(KR 10-0530068)”. The vehicle reinforcement parts manufactured by the method including performing homogenization treatment on aluminum killed steel at 1050 to 1300° C., hot rolling thereon at a finishing rolling temperature condition of 850 to 950 ° C. which is just above an Ar3 transformation point, winding it at a temperature range of 650 to 800 ° C., and performing cold rolling thereon at a cold reduction ratio of 30 to 80% may have improved heat treatment properties.

Further, in the related art, a quenched steel sheet, parts from the steel sheet and the method of manufacturing thereof have been developed as reported in “Quenched Steel Sheet Having Ultra High Strength, Parts Made Of It And The Method For Manufacturing Thereof(KR 10-0878614)”. The quenched steel sheet may have improved tensile strength by heat treatment hardening and then improved yield strength after suffering from painting heat treatment.

However, the above mentioned technical problems such as reduced productivity due to the use of the specific mold and the rapid cooling in the mold, reduced strengthening effect due to the re-crystallization, change in the lattice constant and the lattice structure, and the like still remain in the related art.

The above information disclosed in this background section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY

Thus, in a preferred aspect, the present invention provides a method for warm forming of a super high tensile strength steel sheet. The method may increase productivity and accurate strength, by securing thermal structure stability and dimensional stability when the super high tensile strength steel sheet is heated and formed.

According to an exemplary embodiment of the present invention, a method for warm forming of a super high tensile strength steel sheet may include: heating a steel sheet having a single-phase structure at A_l transformation temperature or less; pressing, in a mold, the heated steel sheet to be formed in a predetermined shape; and extracting and cooling the pressed steel sheet.

The steel sheet may be a steel sheet having a single-phase martensite structure or a steel sheet having a single-phase ferrite structure.

The steel sheet having the single-phase ferrite structure may have improved strength by precipitation hardening.

A temperature of the mold used in the pressing may be controlled by heat transferred from the heated steel sheet without additional heating and cooling.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an exemplary graph illustrating elongation and strength of a steel sheet having a multi-phase structure strengthened by a strengthening mechanism for each temperature in the related art;

FIG. 2 is an exemplary graph illustrating that a strength of the steel sheet is changed when the steel sheet having the multi-phase structure and an exemplary steel sheet having a single-phase structure according to an exemplary embodiment of the present invention are heated at about 400° C. to 700° C. and then cooled; and

FIG. 3 is an exemplary graph illustrating a change in temperature, a change in strength, and a change in elongation of an exemplary super high tensile strength steel sheet at the time of warm forming according to the exemplary embodiment of the present invention.

DETAILED DESCRIPTION

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles. The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about.”

Hereinafter, an exemplary embodiment of the present invention will be described with reference to the accompanying drawings and background art and reference numerals of the foregoing components are similarly applied unless specifically mentioned.

A method for warm forming of a super high tensile strength steel sheet described below is an exemplary embodiment of the present invention, and is not limited to the exemplary embodiment but may be implemented in various forms.

As generally known in the art, for improving strength of metal which is elastically and plastically deformed, atoms may be prevented from moving. As such, various strengthening mechanisms of strain hardening, quenching, annealing, tempering, normalizing, precipitation hardening, grain refinement, solid solution strengthening, and the like which restrict the movement of atoms within a lattice have been used.

When the strengthening mechanism is applied to a steel sheet, a phase change and a structure change of the steel sheet may internally occur and thus the strength of the steel sheet may be sharply increased, but the formability may be reduced in inverse proportion to the strength and have inappropriate characteristics to manufacture parts having a complicated shape. Accordingly, to apply the steel sheet manufactured by the strengthening mechanism to the parts having a complicated shape, the formability may be improved by a separate heating process. That is, parts having a predetermined shape may be manufactured by heating the steel sheet at high temperature to increase the elongation of the steel sheet and then performing the press forming thereon.

However, a phase change, a structure change, a change in a lattice structure, and the like may occur in internal structure of the steel sheet due to high heat applied to the steel sheet and thus the strengthening mechanism may be reduced and thus, the quenching may be performed simultaneously with the press forming using a mold in which cooling water flows to secure the strength of the steel sheet. Accordingly, the present invention provides a method for warm forming of a super high tensile strength steel sheet. Particularly, in the method of the present invention, the strengthening mechanism and the lattice structure may not react sensitively to temperature and additional cooling process such as quenching or water cooling may not be used.

The method for warm forming of a super high tensile strength steel according to the exemplary embodiment of the present invention may include a heating process, a press process, and a cooling process. The heating process may be a process of heating the steel sheet to improve the formability of the steel sheet and setting the heating temperature of the steel sheet to be A₁ transformation temperature or less. Mechanical physical properties of the steel sheet may be sharply changed depending on the heating temperature. For example, when the temperature of the steel sheet increases, vibration energy of the lattice may be increased and thus an elasto-plastic behavior may be changed to a viscoelastic behavior and thus the elongation may be increased as illustrated in FIG. 1.

FIG. 1 is an exemplary graph illustrating the elongation and the strength of the steel sheet which has a multi-phase structure strengthened by the strengthening mechanism at each temperature of room temperature, about 300° C., about 400° C., about 500° C., and about 600° C., respectively. As shown in FIG. 1, when the temperature of the steel sheet is room temperature, the steel sheet may have a strength of about 1200 MPa. As the temperature increases, the strength may be reduced and the elongation may be increased in inverse proportion to the strength.

The increase in elongation of the steel sheet may mean improvement of formability, and thus, when the formality is improved, the parts having a complicated shape may be easily manufactured by the press forming. However, the steel sheet having the multi-phase structure in which phases such as ferrite, austenite, and martensite are mixed may deteriorate from a phase change, a structure change, and a change in a lattice structure, and the like as temperature increases. As the temperature of the steel sheet increases, the strength may be reduced, thereby failing to solve technical problems in the related art. Therefore, in the heating process according to the exemplary embodiment of the present invention, the steel sheet having a single-phase structure, not the multi-phase structure, may be applied. For example, the steel sheet having the single-phase structure may be heated to a A₁ transformation temperature or less.

FIG. 2 is an exemplary graph illustrating that the strength of the steel sheet is changed when the steel sheet having the multi-phase structure and the steel sheet having the single-phase structure are heated at about 400° C. to 700° C. and then cooled. As shown in FIG. 2, when the steel sheet having the single-phase structure is cooled, the final strength of the cooled steel sheet may not be changed substantially compared to initial strength before the steel sheet is heated. In contrast, as the heating temperature increases, a final strength of the steel sheet having the multi-phase structure may be sharply reduced after being cooled.

In other words, when the steel sheet having the single-phase structure, not the steel sheet having the multi-phase structure, is strengthened by the strengthening mechanism and then is heated at A₁ transformation temperature or less, both the strength and the formability may be improved. In particular, heat may be applied until the A₁ transformation temperature or less of the steel sheet to prevent the dimensional stability from reducing due to the phenomenon of the phase change, the structure change, the change in the lattice structure, the change in the lattice constant, and the like which may be caused when the high heat of the A₁ transformation temperature or greater applied to the steel sheet having the single-phase structure. Further, reduction of the strength due to reduced strengthening mechanism may be prevented.

In particular, the steel sheet having the single-phase structure may have a single-phase structure of martensite or ferrite to provide improved strength required for the super high tensile strength steel sheet. The martensite may have a needle-shaped structure and substantially improved strength and thus may be suitably used for the super high tensile strength steel sheet. The ferrite may have relatively less strength than the martensite but may have improved strength by various strengthening mechanisms, and thus, may be applied to the method for warm forming of the super high tensile strength steel sheet according to the exemplary embodiment of the present invention.

Further, a steel sheet of which the strength is improved by the precipitation hardening may be applied among the steel sheets having the ferrite structure, and thus, the improved strength required for the super high tensile strength steel sheet may be obtained. Since the heating temperature is also equal to or less than the A_l transformation temperature, the precipitation hardening effect may remain without deterioration. The press process may form the steel sheet heated by the heating process in a predetermined shape. In particular, when the method for warm forming according to the exemplary embodiment of the present invention is applied, since the strength, the phase, the structure, the lattice structure, the lattice constant, and the like of the steel sheet are not changed, the quenching in the related art may not be performed.

The press process may use various methods for forming a steel sheet. However, according to an exemplary embodiment of the present invention, using a generally used mold, cooling may not be performed at the time of the press forming, and thus cooling time may not be required. Further, the temperature of the mold may be controlled by heat transferred from the steel sheet heated without additional heating and cooling, and thus the generally used mold may be used, unlike a specific mold including cooling water as used in the related art.

The air cooling process may extract and then cool the steel sheet from the press process. In the related art, the steel sheet may be extracted from inside of the mold after the steel sheet is cooled to prevent the shape of the pressed steel sheet from changing and the steel sheet from reducing. However, according to an exemplary embodiment of the present invention, extracting and cooling the steel sheet may be completed without performing additional cooling after the press process.

Further, as described above, since the strength, the phase, the structure, the lattice structure, the lattice constant, and the like of the steel sheet may not be changed, there is no need of using the cooling method such as cooling water and quenching that requires substantial period of time . For example, the air cooling may be applied effectively. For example, when the heated and pressed steel sheet is cooled, the steel sheet may be cooled by air as being immediately extracted without waiting, and as a result, a production efficiency may be substantially improved.

FIG. 3 is an exemplary graph illustrating the change in temperature, the change in strength, and the change in elongation of an exemplary super high tensile strength steel sheet according to the exemplary embodiment of the present invention during an exemplary warm forming process at a temperature range of 200° C. to 600° C. according to the exemplary embodiment of the present invention. Particularly, the temperature may be maintained less than the A₁ transformation temperature of the applied steel sheet. As shown in FIG. 3, when the temperature of the steel sheet increases, the strength may be gradually reduced to a predetermined level and the elongation may be improved in inverse proportion thereto.

However, when the heated steel sheet is conveyed and mounted in the mold, the temperature of the steel sheet may be substantially reduced. Since the additional heating apparatus is not mounted in the mold and thus the temperature of the mold is in a less state than that of the heated mold, when the heated steel sheet contacts the mold, the temperature may be sharply reduced. Further, to the contrary, the reduced strength may be increased again and thus the predetermined level may be maintained, and the elongation may be substantially reduced and then the predetermined level may be maintained.

The elongation may be substantially reduced and then maintained at the predetermined level at the time of the press process due to the temperature of the steel sheet that may be sharply reduced and then maintained at the predetermined level. As such, the elongation may be reduced and thus the pressed steel sheet may not be easily deformed and the shape of the steel sheet may be maintained.

Moreover, when the pressed steel sheet is immediately extracted without additional process and then cooled by air, the temperature of the steel sheet may be reduced to the room temperature over time, and to the contrary, the strength may be further increased and the elongation may be reduced, and thus the further forming may not be performed.

As described above, according to various exemplary methods for warm forming of a super high tensile strength steel sheet, improved strength of the steel sheet may be obtained simultaneously with improved formability when forming the parts having a complicated shape thereby improving the productivity.

The present invention may provide various advantages as follows.

First, the present invention may improve the productivity since phase change may not occur and additional cooling process may not be required.

Second, the present invention may perform the heating at the A₁ transformation temperature or less to minimize the waste of heating time and energy.

Third, the present invention may improve strength while manufacturing the parts having a complicated shape.

Fourth, the present invention may manufacture the super high tensile strength steel sheet having substantially improved dimensional stability and thermal structural stability.

Fifth, the present invention may maintain the processing hardening effect applied to the steel sheet.

The present invention is described in detail with reference to the exemplary embodiments but the scope of the present invention is not limited to the specific exemplary embodiment but needs to be construed by the accompanying claims. Further, it will be to be understood that the present invention may be variously modified and changed by those skilled in the art without departing from the scope of the present invention.

Claims

1. A method for warm forming of a super high tensile strength steel sheet, comprising:

heating a steel sheet having a single-phase structure at A1 transformation temperature or less;

pressing, in a mold, the heated steel sheet to be formed in a predetermined shape; and

extracting and cooling the pressed steel sheet.

2. The method of claim 1, wherein the steel sheet has a single-phase martensite structure or a single-phase ferrite structure.

3. The method of claim 2, wherein the steel sheet having the single-phase ferrite structure has improved strength by precipitation hardening.

4. The method of claim 1, wherein a temperature of the mold used in the pressing is controlled by heat transferred from the heated steel sheet without additional heating and cooling.

5. A super high tensile strength steel sheet manufactured by a method of claim 1.

6. A vehicle part comprising a super high tensile strength steel sheet of claim 5.