METHOD OF HEAT TREATING A MANGANESE STEEL PRODUCT AND MANGANESE STEEL PRODUCT WITH A SPECIAL ALLOY

Abstract

The en-bloc heat treatment of a manganese steel product whose alloy has a carbon fraction (C) in the following range 0.02≦C≦0.35% by weight, and a manganese content (Mn) in the following range of 3.5% by weight≦Mn≦6% by weight. The en-bloc annealing method has the following substeps: heating (E1) the steel product to a first holding temperature (T1) which is in the range of 820° C.±20° C., first holding (H1) of the steel product during a first holding period (δ1) at the first holding temperature (T1), faster first cooling (A1) of the steel product to a second holding temperature (T2) which is in the range between 350° C. and 450° C., second holding (H2) of the steel product during a second holding period (δ2) in the range of the second holding temperature (T2), performing a slower second cooling (A2).

Claims

1. A method for producing a manganese steel product, the method comprising the following steps: providing an alloy with a carbon fraction (C) in the following range 0.02≦C≦0.35% by weight, and ’a manganese content (Mn) in the following range of 3.5% by weights≦Mn≦6% by weight, carrying out an en-bloc annealing process with the following substeps, wherein the en-bloc annealing process is a continuously conducted temperature treatment without interruption, after which the steel product must be reheated: heating (E1) the steel product to a first holding temperature (T1) which is in the range of 820° C.±20° C., first holding (H1) of the steel product during a first holding period (61) at the first holding temperature (T1), faster first cooling (A1) of the steel product to a second holding temperature (T2) which is in the range between 350° C. and 450° C., second holding (H2) of the steel product during a second holding period (δ2) in the range of the second holding temperature (T2), performing a slower second cooling (A2), wherein the faster first cooling (A1) is performed at a cooling rate higher than the cooling rate of the slower second cooling (A2).

2. A method according to claim 1, characterized in that the carbon content (C) lies in one of the following ranges: a) 0.05≦C≦0.22% by weight, or b) 0.09≦C≦0.18% by weight.

3. A method according to claim 1, characterized in that the manganese content (Mn) lies in the range of 4% by weights≦Mn≦6 .

4. A method according to claim 1, characterized in that the manganese steel product is wound during the slower second cooling (A2).

5. A method according to claim 1, characterized in that the second cooling (A2) has a curve-shaped, preferably an asymptotic progression whose asymptote (Asy) is preferably at 100° C.

6. A method according to claim 1, characterized in that the temperature of the manganese steel product is constant during the second holding (H2) in the range of the second holding temperature (F2) or decreases with time.

7. Method according to claim 1, characterized in that when providing the alloy the following admixtures are carried out: Al plus Si contents≦4% by weight, and/or P content≦0.03% by weight, and/or Cu content≦0.1% by weight.

8. A method according to claim 1, characterized in that the first holding period (δ1) has a duration of at most 10 minutes and the second holding period (δ2) has a respective maximum duration of 15 minutes, wherein preferably the following applies: δ1≦5 min and δ2≦10 min.

9. A method according to claim 1, characterized in that the manganese steel product has bainitic laths having a width between 10 and 350 nm, wherein the laths preferably have a width between 10 nm and 100 nm.

10. A method according to claim 1, characterized in that the manganese steel product concerns a medium-manganese steel product which has a bainitic microstructure whose content is greater than 5% by volume of the steel product, wherein the content of the bainitic microstructure is preferably in the range from 10 to 80% by volume and particularly advantageously in the range from 20 to 40% by volume.

11. A manganese steel product manufactured by means of a method according to claim 1, wherein the manganese steel product comprises: a proportion of a bainitic microstructure which lies between 5 and 80% by volume and preferably between 10 and 80% by volume of the steel product, and wherein the steel product has a tensile strength greater than 1200 MPa and a minimum elongation at break between 10% and 20%.

12. A manganese steel product according to claim 11, comprising: a residual austenite content of less than 30% by volume of the steel product, wherein the residual austenite content is preferably less than 10% by volume of the steel product, a proportion of an austenitic microstructure which is in the range of 5 to 20% by volume of the steel product, and a volume fraction of austenite grains, which is preferably less than 5% of the total volume of the steel product.

13. A manganese steel product according to claim 11, which comprises bainitic laths having a width between 10 and 350 nm, wherein the laths preferably have a width between 10 nm and 100 nm.

Description

DRAWINGS

[0073] Embodiment examples of the invention are described in more detail below with reference to the drawings.

[0074] FIG. 1 is a highly schematic diagram in which the elongation at break is plotted as a percentage over the tensile strength in MPa for various steels;

[0075] FIG. 2 is a schematic diagram of the unique temperature treatment employed as part of the manufacture of a steel product of the invention.

DETAILED DESCRIPTION

[0076] According to the invention, the subject matter concerns ultrafine multi-phase medium-manganese steel products comprising martensite, ferrite and residual austenite regions or phases, as well as optionally also bainite microstructures. This means that the steel products of the invention are characterized by a special structure constellation, which is also referred to as a multiphase structure.

[0077] The following partly refers to steel (intermediate) products when it comes to emphasizing that not the finished steel product is concerned but a preliminary or intermediate product in a multi-stage production process. The starting point for such production processes is usually a melt. In the following, the alloy composition of the melt is given, since on this side of the production process the alloy composition can be influenced relatively precisely (e.g. by adding constituents such as silicon). In the normal case, the alloy composition of the steel product differs only slightly from the alloy composition of the melt.

[0078] The term “phase” is defined among other things by its composition of fractions of the components, enthalpy content and volume. Different phases are separated from each other in the steel product by phase boundaries.

[0079] The “components” or “constituents” of the phases can be either chemical elements (such as Mn, Ni, Al, Fe, C, . . . etc.) or neutral, molecule-like aggregates (such as FeSi, Fe.sub.3C, SiO.sub.2, etc.) or charged, molecule-like aggregates (such as Fe.sup.2+Fe.sup.3+, etc.).

[0080] Specifications on quantities or proportions are made here in percent by weight (in short % by weight), unless otherwise stated. If specifications are given on the composition of the alloy or of the steel product, the composition, in addition to the explicitly listed materials or matters, comprises iron (Fe) as the base material and so-called unavoidable impurities, which always occur in the molten bath, and which also show up in the resulting steel product. All % by weight specifications are therefore always to be supplemented to 100% by weight and all % by volume specifications are always to be supplemented to 100% of the total volume.

[0081] The medium-manganese steel products of the invention all have a manganese content which is in the range of 3.5 and 6% by weight, wherein the stated limits belong to the range, i.e. the manganese content is in the range 3.5% by weights Mn .sup.s 6% by weight. The manganese content in all embodiments is preferably in the range 4% by weights≦Mn≦6% by weight.

[0082] In addition, the carbon content C in the following range is 0.02≦C≦0.35% by weight.

[0083] When preparing a manganese steel product, the following steps are carried out, among other things, wherein these steps do not necessarily have to follow one another immediately.

[0084] In the course of the provision of the alloy according to the invention, a carbon fraction C in the following range of 0.02≦C≦0.35% by weight, and a manganese content Mn in the following range 3.5% by weight≦Mn≦6% by weight are added to a starting amount of iron. The corresponding procedure is sufficiently known.

[0085] Within the framework of further processing of the alloy thus obtained, a particularly efficient annealing process (called en-bloc temperature treatment) follows. The word en-bloc is used herein to emphasize that, in contrast to numerous alternative approaches, no two-step annealing or temperature treatment is required.

[0086] When carrying out the en-bloc annealing process, the following partial steps are carried out (in this connection reference is made to FIG. 2): [0087] heating E1 of the steel (intermediate) product to a first holding temperature T1, which is in the range of 820° C.±20° C., [0088] first holding H1 of the steel (intermediate) product during a first holding period δ1 at the first holding temperature T1, [0089] fast first cooling A1 of the steel (intermediate) product to a second holding temperature T2, which lies in the range between 350° C. and 450° C., [0090] second holding H2 of the steel (intermediate) product during a second holding period δ2 in the range of the second holding temperature T2, [0091] performing a slow second cooling A2.

[0092] The first interim holding phase H1 has preferably in all embodiments a maximum duration of 5 minutes. The second interim holding phase H2 has preferably in all embodiments a maximum duration of 10 minutes.

[0093] The holding phase H2 can in all embodiments be carried out in a salt bath.

[0094] Particularly preferred embodiments are those in which the following applies: δ1+δ2<15 min and δ1<δ2.

[0095] The first cooling A1 can be effected in all embodiments in an air stream or by using a cooling fluid. In all embodiments, the second cooling A2 can take place in an air stream. However, the steel product of the invention can also be placed in a separate environment (e.g. in an annealing unit) in order to be held there for a longer period of time (at 300 to 450° C. for example). In this case, the time δ2 is extended correspondingly.

[0096] The phase of the rapid cooling A1 preferably has a cooling rate of more than −30 K/sec in all embodiments. Particular preference is given to the cooling rates A1, which are greater than −50 K/sec. These rapid cooling rates have an advantageous effect on the microstructure of the steel product of the invention.

[0097] It can be seen in FIG. 2 that the faster first cooling A1 takes place with a cooling rate which is higher than the cooling rate of the slower second cooling A2. Preferably, the second cooling takes place in all embodiments along an asymptotic curve A2*, which approximates the asymptote Asy (see FIG. 2). Preferably, the steel product coils are left in all embodiments to themselves after the slower second cooling A2 or A2*, so that they can cool down slowly on their own.

[0098] According to the invention, preference is given to steel products which comprise, as a proportion, the following admixtures: [0099] Al plus Si contents≦4% by weight, and/or [0100] Nb content≦0.4% by weight, and/or [0101] Ti content≦0.2% by weight, and/or [0102] V content≦0.1% by weight, and/or [0103] P content≦0.03% by weight, and/or [0104] Cu content≦0.1% by weight.

[0105] According to the invention, steel products are preferred which comprise a proportion of a bainitic microstructure which is greater than 5% by weight of the steel product, wherein the proportion of the bainitic microstructure is preferably in the range from 10 to 70% by volume of the steel product. The proportion of the microstructure is particularly preferably in the range from 20 to 40% by volume.

[0106] According to the invention, steel products are preferred which comprise a residual austenite content which is less than 30% by volume of the steel product, wherein the residual austenite content is preferably less than 10% by volume of the steel product.

[0107] According to the invention, steel products are preferred which have a proportion of an austenitic microstructure, which is in the range from 5 to 20% by volume of the steel product, in particular from 2 to 10% by volume.

[0108] According to the invention, steel products are preferred which comprise a volume content of austenite grains which preferably amounts to less than 5% of the total volume of the steel product. These austenitic grains preferably have a maximum size which is less than 1 μm.

LIST OF REFERENCE NUMERALS

[0109]

TABLE-US-00001 Medium-manganese steels 1 TRIP steels 2 HD tempering 3 First cooling A1 Second cooling A2 Asymptote Asy First holding period δ1 Second holding period δ2 Heating E1 First holding H1 Second holding H2 First holding temperature T1 Second holding temperature T2