METHOD OF HOT-SHAPING AND HARDENING A SHEET STEEL BLANK

Abstract

A sheet steel blank is heated to the austenite range and formed in a cooled tool pair that rapidly cools the formed product until the product's temperature drops somewhat below the temperature Ms for the start of the formation of martensite. The cooling is rapidly interrupted and the product's temperature is raised until it exceeds Ms and is maintained there until the material comes to contain more than 50% by volume bainite. The short time under the Ms temperature favours the formation of bainite and shortens the holding time.

Description

FIELD OF THE INVENTION

The invention relates to a method of hot forming and hardening a sheet steel blank by forming the material heated to the austenite range in a tool pair that rapidly cools the shaped product.

BACKGROUND OF THE INVENTION

In the automotive industry products of high strength steel are being used more and more that are formed and hardened with press-hardening technology, that is, a sheet steel blank of hardenable boron steel is heated to the austenite range and is formed in a cooled tool pair and maintained in the tool pair with the tool pair as a fixture for several seconds so that the formed product obtains a martensitic structure. This process yields a tensile strength of above 1400 MPa. The subsequent working, e.g., punching or laser cutting of edges is made on hardened material but sometimes a rapid cooling of portions that are to be subsequently worked is prevented in order to avoid the working of fully hardened material.

A steel with bainite structure can have approximately the same high tensile strength as the one obtained with the press-hardening process but the bainite structure is tougher and more ductile than the martensitic structure. U.S. Pat. No. 6,149,743 describes a method for bainite hardening. The material is rapidly cooled from the austenite range to just above Ms (the temperature for the start of the formation of martensite) and this temperature is maintained for approximately 10 hours and, toward the end of the holding time, the temperature is raised in order to shorten the hardening time. This process can be used for roller bearings.

PURPOSE OF THE INVENTION

It is an object of the invention to produce, in an economical manner, products that have better material properties than those that are produced with conventional press hardening. Another object is to create the possibility of subsequent working before the product fully hardens.

BRIEF DESCRIPTION OF THE INVENTION

The formed product is rapidly cooled until the product's temperature drops below the temperature for the start of the formation of martensite and the product's temperature is then raised until it exceeds the temperature for the start of the formation of martensite and is maintained there until bainite is formed. The times and the temperatures for the formation of martensite and the formation of bainite can be selected so that the material comes to contain more than 50% by volume bainite. The forming and the rapid cooling take a few seconds while maintaining the heat takes a few minutes. The time for maintaining the heat (the holding time) can be utilized for subsequent working, e.g. edge cutting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a time-temperature diagram for an example of a process in accordance with the invention.

FIG. 2 is a block diagram that shows the process.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a block diagram with an austenitic phase in a furnace 11, a forming phase in a cooled forming tool 12, and a subsequent temperature holding in a furnace 13.

The temperature curve in FIG. 1 for an example of a process in accordance with the invention shows a first part 20 that is the heating in a furnace of a blank to the austenite range. The moving of the material to a cooled forming tool pair (curve part 21 in the temperature curve) normally takes 5-10 seconds and entails a small reduction of temperature. The forming in the cold tool pair is in the order of magnitude of one or a few seconds but the product must remain in the tools until it is rapidly cooled to somewhat below Ms (the temperature for the start of a formation of martensite). The formation of martensite is instantaneous and is a function of the temperature but is not a function of the time. This cooling is accordingly carried out with the tool pair as fixture. The forming and rapid cooling is designated as 22. The product is rapidly taken out of the tool pair and the started formation of martensite generates heat and entails an elevation of temperature 23 that may exceed Ms. Additional heat can be used, e.g., induction heat or radiant heat in order to rapidly bring the temperature up above Ms. The product is then moved to a furnace where the product is maintained at a rather even temperature somewhat above Ms for a number of minutes represented by the curve part 24. The product is thereafter cooled down in the air, which is represented by the dashed curve part 25 or is speeded up as is represented by the curve part 26.

The start of formation of martensite favours the formation of bainite and the holding time for the formation of bainite is shortened, generally to less than half an hour or below 10 minutes or even below 1 min. This is a prerequisite for an economical process The amount of martensite can be predetermined by selecting how much the temperature is lowered below Ms. The temperature for the formation of bainite can be made to vary by a few tens of degrees and therefore the holding time can be used for subsequent working, e.g. edge cutting, which is advantageous to carry out before the material reaches its full hardness, since this reduces the wear on the tool and also reduces the risk for the initiation of fissures and the following formation of fissures when the product is used. The subsequent working can also be integrated into the shaping tool and be integrated in the curve part 22, i.e. it can be carried out before or during the formation of martensite.

In a modified process the product can be formed in a first tool pair and cooled down to just over Ms and the formed product can then be moved to a second tool pair that makes the fixture and cools the product down to somewhat below Ms. This second tool pair can carry out the subsequent work at the same time, e.g. edge cutting, before the product is moved to the furnace for maintaining the heat. Alternatively, the subsequent working can be integrated in the first tool pair. The two tool pairs can work simultaneously, which shortens the cycle time.

Boron steel is used in conventional press hardening, that is, a carbon-manganese steel with boron, that completely hardens to martensite. A carbon-silicon-manganese steel can be suitably used in a process in accordance with the invention. The steel can have a carbon content of 0.2-0.3 weight %, a manganese content of 1-2 weight % and a silicon content of 1-2 weight %. In addition, chromium and other customary alloy substances with a total content less than 1 weight % may be present. Silicon prevents the separation of cementite and creates the desired microstructure.

Claims

1. A method of hot shaping and hardening a sheet steel blank by forming the blank, heated to the austenite range, in a tool pair that rapidly cools the formed product,

characterized in

that the formed product is cooled until the product's temperature drops below the temperature for the start of the formation of martensite, Ms, and the product's temperature is then raised until it exceeds the temperature for the start of the formation of martensite and is maintained hot so that bainite is formed.

2. The method according to claim 1, characterized in that the raised heat is maintained for a holding time of 1-20 minutes.

3. The method according to claim 1, characterized in that temperatures are selected so that the material comes to contain more than 50% by vol. bainite.

4. The method according to claim 1, characterized in that the formed product is subsequently worked directly in the tool pair.

5. The method according to claim 1, characterized in that the material is cooled and shaped in a first tool pair down to a temperature exceeding Ms and the shaped blank is then moved to a second tool pair and cooled further to a temperature below Ms with this tool pair as a fixture.

6. The method according to claim 5, characterized in that the formed product is subsequently worked in the first tool pair.

7. The method according to claim 5, characterized in that the formed product is subsequently worked in the second tool pair.

8. The method according to claim 2, characterized in that the formed product is subsequently worked directly in the tool pair.

9. The method according to claim 3, characterized in that the formed product is subsequently worked directly in the tool pair.

10. The method according to claim 2, characterized in that the material is cooled and shaped in a first tool pair down to a temperature exceeding Ms and the shaped blank is then moved to a second tool pair and cooled further to a temperature below Ms with this tool pair as a fixture.

11. The method according to claim 3, characterized in that the material is cooled and shaped in a first tool pair down to a temperature exceeding Ms and the shaped blank is then moved to a second tool pair and cooled further to a temperature below Ms with this tool pair as a fixture.

12. The method according to claim 4, characterized in that the material is cooled and shaped in a first tool pair down to a temperature exceeding Ms and the shaped blank is then moved to a second tool pair and cooled further to a temperature below Ms with this tool pair as a fixture.

13. The method according to claim 10, characterized in that the formed product is subsequently worked in the first tool pair.

14. The method according to claim 11, characterized in that the formed product is subsequently worked in the first tool pair.

15. The method according to claim 12, characterized in that the formed product is subsequently worked in the first tool pair.

16. The method according to claim 10, characterized in that the formed product is subsequently worked in the second tool pair.

17. The method according to claim 11, characterized in that the formed product is subsequently worked in the second tool pair.

18. The method according to claim 12, characterized in that the formed product is subsequently worked in the second tool pair.