WO2019117832A2

WO2019117832A2 - Method of obtaining dual-phase parts with press hardening method

Info

Publication number: WO2019117832A2
Application number: PCT/TR2018/050342
Authority: WO
Inventors: Ersoy ERIŞIR; Volkan AKINCI; Mesut Kaya; Oğuz Gürkan BILIR
Original assignee: Coşkunöz Kalip Maki̇na Sanayi̇ Ve Ti̇caret Anoni̇m Şi̇rketi̇
Priority date: 2017-07-27
Filing date: 2018-07-03
Publication date: 2019-06-20
Also published as: WO2019117832A3

Abstract

The invention is a method of obtaining dual-phase microstructured parts by application of press-hardening method at low temperatures in automobile chassis and energy absorbing parts, characterized by comprising; to achieve the production of parts showing tensile strength in the range of 1400-2100 MPa and a total elongation between 5-10%; a process of operation that takes place at austenitization temperatures of 721-782 ° C with austenite content of at least 80%.

Description

DESCRIPTION

METHOD OF OBTAINING DUAL-PHASE PARTS WITH PRESS HARDENING

METHOD

Technical Field

The invention relates to the application of press-hardening method in automobile parts at low temperatures and to the application of press-hardening method to obtain a parts with martensitic microstructure.

Prior Art

Press hardening is applied at high temperatures (ddO^qdO'Ό) in coated steels. At said temperatures, while the part is being shaped, cooling is performed simultaneously in the mold. As a result of the conversion from austenite to martensite at high temperatures, hardening is achieved and formability is increased. Thus, even though it is in complex geometry, high strength parts can be produced. In addition, problems such as spring back and fracturing that can occur with conventional cold forming are reduced to minimum levels. In press hardening process, typically, 22MnB5 steel is used. The minimum tensile strength of a part produced by this method is 1500 MPa and 5% elongation is achieved.

The technical problems experienced in the prior art;

1 - The process is carried out at high temperatures and therefore cost increases. Unlike the cold forming method, the heat treatment process applied in press hardening increases energy costs. It requires a continuous furnace investment that is resistant to high temperatures.

2- Zn coated steel sheets cannot be used because it is applied above AC3 temperature. Coated steels can be used in order to prevent decarburization during heating of press- hardened steels and to avoid the need for controlled atmosphere. However, metallic coatings at high temperatures can be melt and the coating metal which becomes liquid during press forming moves along grain boundaries with deformation. This results in liquid-metal embrittlement. For this reason, alloys with low melting temperature such as Zn cannot be used in the press-hardening method directly. Zn coating can only remain solid up to 782^<C after the processes of galvanizin g followed by annealing of sheet steel. For this reason, it cannot be used in press-hardening processes. The most commonly applications coating for press-hardened steels is Al-based alloys. These coatings can remain solid at high temperatures. However, galvanic protection provided by Zn cannot be achieved with Al coatings.

3- When considered to be 0.2% C in 22MnB steel, hardenability is limited. This makes the mold cooling process difficult. Since a material containing 0.2% C is used in austenitic structure, cooling speed plays a critical role in press-hardening process. When the cooling speed is below critical speed during cooling in the press, the part becomes bainite/pearlite instead of martensite and the strength values have been very low than expected. However, the heating, waiting and cooling durations for the hardening to be achieved completely decrease the production amount. In order to increase hardenability, much more amount of alloy elements must be used.

Description of the Invention

The invention is to perform the production of parts which showing tensile strength in the 1400-2100 MPa range and a total elongation of 5-10%. It is similar to the hot working method with regard to apply heat treatment and press-hardening process .

The invention is about performing partial austenitization process, different from the conventional press-hardening process. For this purpose, the process is carried out at partial austenitic temperatures between 721 -910 Ό, with austenite between 40-80%. The purpose of the invention is to cut uncoated steels containing 0.1 %-0.3% C and 3-8% Mn which hot or cold rolled to desired sizes.

The purpose of the invention is to apply heat treatment to the cut parts in the industrial furnace at a temperature between AC1 -AC3 (723-782 degrees).

The mold to be used in press is water cooled, similar to the one used in conventional press hardening process. The material is pressed to decrease martensite finish temperature (at 200-300 degrees, there is no clear data for this steel). The cooling process is the same as the cooling speed used in the press hardening and is the same as the temperature at which it descends.

Another purpose of the invention is to cool the part simultaneously with the pressing. The product obtained will be composed of martensite. In the product obtained, the martensite ratio will be at least 80% and it will have a total elongation of 10% with tensile strength between 1400-2100 MPa.

Solutions for the Problems Encountered in the Prior Art:

-The current technique is processed at high temperatures (such as 950 degrees) and therefore the cost increases. The AC3 temperatures of the said steels containing between 3% and 8% of Mn and 0.1 % to 0.3% C are less than 750 degrees. Hardening temperatures are traditionally selected up to 50 *C above AC3 temperature to ensure full austenite formation. Unlike traditional press hardening process, temperatures will be 200-300 *C lower. Therefore, less energy consumptio n is expected.

-Zn coated steel sheets cannot be used because it is applied above AC3 temperature. Since the invention provides press-hardening process at temperatures between AC1 and AC3, the temperatures mentioned in the invention are less than 780*0. Therefore, it makes the use of Zn coated steels is possible.

- Considering that 22MnB steel contains 0.2% C and between 1 -1.5% Mn, hardenability is limited. This makes the mold cooling process difficult. However, the steel mentioned in the invention can turn into martensite even during air cooling. Therefore, the fact that the cooling is homogeneous in the whole part does not constitute a major concern. Despite this, in order to make the production faster, cooling can be done in the mold.

Claims

1. The invention is a method of obtaining dual-phase micro-structured parts by application of press-hardening method at low temperatures in automobile chassis and energy absorbing parts, characterized by comprising; to achieve the production of parts showing tensile strength in the range of 1400-2100 MPa and a total elongation between 5-10%;

a process of operation that takes place at austenitization temperatures of 721 - 782 °C with austenite content of at least 80%.

2. A method according to Claim 1 , characterized by comprisingjthe process of cutting hot or cold-rolled Zn coated steels having a thickness of 0,2-3 mm, containing Mn between 3-8% and 0,1 -0,3 C into the desired dimensions.

3. A method according to Claim 1 , characterized by comprising; the process of heat treatment of the cut parts in the furnace at a temperature of 723-782 degrees.

4. A method according to Claim 1 , characterized by comprising; the mold to be used in press is water-cooled, similar to the conventional press-hardening process.

5. A method according to Claim 1 , characterized by comprising; the process of lowering of the material to below martensite finish temperature in press approximately 200-300 degrees.

6. A method according to Claim 1 , characterized by comprising; the process of cooling is the same as the cooling speed used in the press hardening and is the same as the temperature at which it descends.

7. A method according to Claim 1 , characterized by comprising; the temperatures between AC1 and AC3 are lower than the AC3 temperature in the process.

8. A method according to Claim 1 , characterized by comprising; the hardening temperatures are traditionally selected up to 50 ^<C above AC3 temperature to ensure full austenite formation in process.