Method and device for forming and hardening steel materials

Abstract

The invention relates to a method for internal high-pressure forming and hardening of galvanized pipes made of sheet steel in which a pre-fabricated pipe is used; the pipe has at least one inlet opening (5) and a cavity (4); the pipe is heated to a temperature above the austenitization temperature (AC.sub.3) of the respective steel alloy and after the achievement of a desired degree of austenitization, is inserted into an internal high-pressure forming tool and acted on with a pressurized medium, which is forced into the cavity (4) through the at least one inlet opening (5) until the pipe fills a predetermined mold (2) of the tool, characterized in that the forming tool is heated to a temperature between 400 and 650° C., in particular 450-550° C., and the pressurized medium is likewise heated and has a temperature of 400-650° C.; after the austenitization, the pipe is allowed to passively cool or is actively cooled to a temperature of 400-600° C., but a temperature above the martensite starting temperature (Ms) of the selected steel alloy, and the cooling of the pipe for hardening purposes only takes place after the removal from the mold.

Claims

1. A method for internal high-pressure forming and hardening of galvanized pipes made of sheet steel in which a partially-shaped pipe is used; the pipe having at least one inlet opening (5) and a cavity (4), comprising: heating the entire pipe to a temperature above the austenitization temperature (AC.sub.3) of a respective steel alloy and, after the achievement of a desired degree of austenitization, which varies depending on a desired final hardness level, inserting the pipe into an internal high-pressure forming tool where the pipe is acted on with a pressurized medium, which is forced into the cavity (4) through the at least one inlet opening (5) until the pipe fills a hot mold (2) of the tool, the forming tool having been heated to a temperature between 400 and 650° C. and the pressurized medium having been also heated and having a temperature of 400-650° C.; and, after the austenitization, cooling the pipe to a temperature of 400-600° C., but a temperature above the martensite starting temperature (Ms) of the respective steel alloy; and removing the pipe from the hot mold before further cooling of the pipe for hardening purposes, wherein after heating the entire pipe and before inserting the pipe into the internal high-pressure forming tool, the entire pipe is actively cooled by being blown or sprayed with a cooling medium to cool the pipe at a cooling speed of more than 20 K/s.

2. The method according to claim 1, further comprising: after removing the pipe from the hot mold, allowing the entire pipe to passively cool in the air.

3. The method according to claim 1, further comprising: after removing the pipe from the hot mold, transferring the pipe into a cold mold where a cavity of the cold mold corresponds to the outer contour of the pipe after removal from the tool.

4. The method according to claim 1, further comprising: after forming the pipe in the hot mold, retaining the pipe in the tool and rinsing the entire pipe with a cold cooling medium.

5. The method according to claim 3, wherein the temperature of the cold mold is at least 50° C. below the martensite starting temperature of the pipe when the pipe is transferred into the cold mold.

6. The method according to claim 1, wherein after removing the pipe from the hot mold, the entire pipe is passively cooled by cooling in natural air if the pipe sheet thickness is 1 mm or less, and the entire pipe is actively cooled by being blown or sprayed with a cooling medium if the pipe sheet thickness is 1.5 mm or more.

7. The method according to claim 1, wherein the pipe comprises a hardenable boron/manganese steel.

8. The method according to claim 1, wherein the steel material comprises a metallic coating including a zinc layer, a zinc alloy layer, an aluminum layer, an aluminum alloy layer, or a zinc/iron layer.

Description

(1) FIG. 1 shows a process flow of a method for internal high-pressure forming and hardening of a galvanized pipe, according to an embodiment of the subject invention.

(2) FIG. 2 shows a process flow of a method for internal high-pressure forming and hardening of a galvanized pipe, according to an embodiment of the subject invention.

(3) The object of the invention is to create a method for forming and hardening galvanized steel pipes that can reliably produce crack-free hardened steel pipes.

(4) This object is attained with a method having the features of claim 1.

(5) Up to this point, internal high-pressure forming methods have been unable to form and harden galvanized pipes without producing microcracks. If such galvanized pipes or piping components are subjected to internal high-pressure forming, this always results in a very large amount of microcracking so that by contrast with other forming methods, it has not been possible to use the press hardening method or form hardening method for piping components.

(6) The inventors have discovered that the microcrack-free forming of piping components is possible if a special temperature control and process control are carried out.

(7) According to the invention, piping components of this kind are prefabricated and, analogously to the known internal high-pressure forming method, are pre-bent, pre-quenched, or pre-formed in some other way.

(8) Then, these pipes are austenitized, which means that they are brought to a temperature above AC.sub.3 and kept at this temperature until a desired degree of austenitization is achieved.

(9) According to the invention, the pipe is then allowed to cool passively or is forcibly cooled actively to temperatures between 400 and 650° C.

(10) This cooling can be carried out in that the component is transferred into the internal high-pressure forming tool and in the process, is passively cooled in the air or possibly, after the austenitization furnace, the tool is actively cooled for example by being blown or sprayed with suitable cooling mediums and is then transferred into the internal high-pressure forming tool.

(11) Such an active cooling takes place at a cooling speed >5 K/sec, preferably >10 K/sec, particularly preferably >20 K/sec.

(12) Then the pipe undergoes final forming in which a pressurized medium is forced into the pipe so that an intrinsically known internal high-pressure forming is achieved.

(13) According to the invention, however, this forming is carried out with a temperature-controlled medium. In this case, the medium has a temperature of 400-650° C., for example. According to the invention, it has specifically turned out that when a medium that is too cold is used, parts of the pipe already undergo a hardening before the final forming has taken place. This means that a complete removal from the mold is hindered. Consequently, the forming is carried out with a temperature-controlled medium; the temperature-controlled medium preferably has a temperature that corresponds to the temperature of the pipe, which is to be formed, and is at least high enough that the martensite starting temperature (Ms) of the steel alloy used is exceeded.

(14) According to the invention, the hardening then takes place; the hardening according to the invention can be carried out in different ways.

(15) In a first variant according to the invention, the internal high-pressure forming takes place in a hot tool with the hot pressurized forming medium. Then the component that has been formed in this way is removed from the tool and allowed to passively cool in the air if the cooling in the air is enough to reach the critical cooling speed of the steel material so that a martensitic hardening is assured. This process is illustrated in FIG. 2.

(16) This passive cooling primarily depends on the sheet thickness with thinner sheet thicknesses of approximately 1 mm, a passive cooling in the air can be enough to reach the critical cooling speed.

(17) With a sheet thickness of 3 mm, for example, an active cooling with suitable cooling means can be required in order reach this cooling speed.

(18) In a second variant according to the invention, the internal high-pressure forming once again takes place in a hot tool with the hot pressurized forming medium and then the pipe is transferred into a cold form hardening tool. In this cold form hardening tool, the contour of the tool cavity corresponds exactly to the outer contour of the pipe so that when the tool is closed, the tool rests against the entire surface of the pipe on all sides and a quench hardening is achieved as a result. For the purposes of the invention, “cold” means that the temperature is at least 50° C. below the martensite starting temperature of the chosen steel material, i.e. Ms−50° C. This process is illustrated in FIG. 1.

(19) In another variant according to the invention, the forming takes place in the hot tool with the aid of the hot pressurized forming medium, but after the forming is completed, a cold medium is conveyed through the pipe so that the cooling with the cold medium achieves the martensitic hardening by exceeding the critical cooling speed. In this case, it is technologically possible to carry out the internal high-pressure forming procedure with a pressurized, hot gaseous medium and to carry out the quenching procedure with a cold gaseous medium or also with a liquid cold medium. Here, too, the temperature of the cold medium is preferably below the martensite starting temperature of the material, i.e. Ms−50° C.

(20) To achieve this, the pipes generally have an inlet and an outlet.

(21) For purposes of the invention, pipes are understood not only to be cylindrical pipes, but also to be any form of elongated hollow bodies made of sheet steel, in particular structural components, longitudinal members, reinforcing members, rocker panels, and similar structural components, particularly of motor vehicles.

(22) According to the invention, a material is used, which, like the materials of the prior art, is hardenable and in particular, is a hardenable boron/manganese steel such as a steel material of the 22MnB5 or 20MnB8 type or the like.

(23) Sheet steels of this kind can be provided with a zinc layer, a zinc alloy layer, and in particular, a zinc/iron layer.

(24) In particular, a so-called galvannealed coating is provided, i.e. a zinc coating on a sheet steel, which coating is pre-reacted by means of tempering, consists of zinc/iron phases, and can also withstand the blowing-in by means of a pressurized medium.

(25) The invention will be explained by way of example based on FIGS. 1 and 2. FIGS. 1 and 2 show the process sequence with the two variants of the method.

(26) Referring to FIGS. 1 and 2, an austenitized pipe 1 is inserted into a mold 2; for example, the pipe 1 is assembled from two sheets 3; in the region of a gas inlet and gas outlet of a cavity 4 that is formed by the sheets, the sheets each have a corresponding inlet opening 5. After temperature-controlled gas, for example a gas whose temperature has been adjusted to 400-650° C., has been dispensed into the cavity 4, the pipe 1 expands into the mold 2 so that the fully pre-formed blank is produced. The invention has the advantage of being able to reliably produce microcrack-free tubular components out of hardenable steel with a zinc coating.