DE102006001198A1 - Method and device for setting specific property combinations in multiphase steels - Google Patents

Abstract

Compared to conventional steel grades, multi-phase steels have a significantly improved combination of strength and ductility and are therefore becoming increasingly important, particularly for the automotive industry. The z. The most important steel groups for automotive engineering are dual-phase steels and TRIP steels. The production of different multi-phase steel strength classes to be carried out for different requirements directly on the hot strip requires a very extensive process know-how as well as a corresponding adjustment of the alloying elements in advance. According to the invention, it is proposed to carry out an annealing treatment (30) with variable annealing temperature and annealing duration after the actual production of multi-phase steels with a standard analysis and a standard process control, whereby almost any combination of different materials or combinations of properties (amount of yield strength, level of tensile strength) can be set.

Description

The The invention relates to a method and a device for adjustment Targeted combinations of properties in hot-rolled multiphase steels, the multiphase structure comprises at least 30% ferrite and at most 50% martensite, For example, dual-phase and TRIP steels on a conventional Hot rolling mill, a thin slab casting mill or corresponding narrow and medium strip streets or a wire line with a Standard analysis and standard process management.

Multi-phase steels have across from conventional steel grades a significantly improved combination of strength and ductility on and win therefore - in particular for the Automotive industry - more and more in importance. The z. Z. most important steel groups for the automotive industry are dual phase steels and TRIP steels.

by virtue of the significantly lower production costs offers the variant the production directly as hot strip while economic benefits and thus has a very large one Potential for the future.

Characteristic for dual phase steels a low yield ratio, which is usually between 50 and 70%. Compared to HSLA steels (high-strength low-alloy), d. H. high strength, low alloy structural steels in addition to the lower yield strength at the same level of tensile strength achieved significantly better elongation values. For some applications (for example Pipes) it may be desired that the yield ratio must be set to defined values, but still the elongation at break as big as possible is.

There the production of different strength classes directly on the hot strip a very extensive process know-how is required It is state of the art, for every single material either the chemical analysis or else the litigation adapt, being trip steels in principle have a slightly higher yield ratio compared to dual-phase steels.

From the EP 1 108 072 B1 is a method for the production of dual-phase steels is known in which after the finish rolling with a two-stage cooling, a two-phase structure of 70 to 90% ferrite and 30 to 10% martensite is achieved. The first (slow) cooling is carried out in a cooling section in which the hot strip is cooled by spaced water cooling stages at a cooling rate of 20-30 K / s. The cooling is adjusted so that the cooling curve enters the ferrite region with an even high temperature, so that ferrite formation can take place quickly. This first cooling is continued until at least 70% of the austenite has been converted to ferrite, followed by further (rapid) cooling immediately and without holding time.

Of the special effect of TRIP steels (transformation induced plasticity) with a structure of for example 40-70 % Ferrite, 15-40 % Bainite and 5-20 Restaustenite is the transformation of metastable retained austenite too Martensite, if an external plastic Deformation occurs. These with an increase in volume and a plasticization The transformation associated with the ferritic matrix, which is not alone austenite, but also from the surrounding Gefügebe components is born has a higher Solidification entails and leads overall to higher plastic strains. The result is an extraordinary for such steels produced Combination of high strength and high ductility, which is why she is especially for suitable for use in the automotive industry.

For the production of a pearlite-free hot-rolled steel strip with TRIP properties is from the EP 1 396 549 A1 a method is known in which a molten steel, in addition to iron and unavoidable impurities at least one of the elements Ti or Nb as an essential component and optionally one or more of the elements max. 0.8% Cr, max. 0.8% Cu, max. 1.0% Ni, is cast into thin slabs, which are annealed with a 850 to 1050 ° C entry temperature in an annealing furnace for an annealing time of 10 to 60 minutes at 1000 to 1200 ° C. After descaling, the thin slabs are then finish hot rolled in the range of 750 to 1000 ° C and then to a coiler temperature of 300 to 530 ° C in two stages with a controlled first stage cooling rate of at least 150 K / s and a cooling pause of 4 to Cooled for 8 seconds. In addition to the prescribed process control, the presence of Ti and / or Nb is important, since these elements remain in solution until the start of hot rolling and improve their subsequent separation, inter alia, the grain size of the hot strip, an increase in Austenitgehaltes and its stability.

Finally, out of the EP 1 394 279 B1 a method of producing a low-carbon steel of high strength and high ductility with a tensile strength of greater than 800 MPa, a uniform elongation of greater than 5% and an elongation at break of greater than 20% known. Starting from a hardened or tempered starting material, a steel with 0.20% C, 1.60% Mn and admixtures of boron and a Martensitphasenanteil greater than 90% is after a cold rolling of greater than 20% of the total rolling an annealing treatment at a temperature between 500 and 600 ° C, wherein a microstructure having an ultrafine, crystalline, granular ferrite structure of 100 to 300 nm is obtained with ferrite-deposited iron carbides.

From Starting from this prior art, it is an object of the invention to provide a method and a device with which multiphase steels, the produced with a standard analysis and a standard process management were, to steel grades with almost any property combinations can be converted.

The Asked task is procedurally with the characterizing Characteristics of claim 1 solved by the fact that following the Cooling from hot rolling or a later one Manufacturing step, for example in the manufacture of components, by a subsequent or intermediate annealing treatment with variable annealing temperature and variable annealing time the desired Combinations of strength and yield strength ratios on the multiphase steels be set. An apparatus for carrying out the method is by the features of claim 8 characterized. Advantageous embodiments The invention are specified in the subclaims.

By the actual production downstream, adapted according to the invention and easy to perform annealing on multiphase steels with a standard analysis and standard process management almost any combination of different materials or property combinations (height of yield strength, tensile strength level) to adjust. The production of different multiphase steel strength classes directly on the hot strip, however, requires a very extensive Process know-how and in advance a corresponding adjustment of the alloying elements.

According to the invention annealing with a variable annealing temperature of ≤ 600 ° C and one as well variable annealing time from ≤ 120 s so happens, that the resulting texture from a basic ferritic matrix and tempered martensite or bainite with 10 to 50% of the area fraction. By the annealing temperature is primarily the height the yield strength by finely divided precipitates of carbides influenced on the grain boundaries of martensite or bainite and by the annealing time is the tensile strength level adjustable.

The execution the annealing treatment can according to the invention, adapted to existing conditions, independently from upstream or downstream process stages offline in one continuous annealing done or online in the existing process line, for example as part of a strip galvanizing in the heating stage of a galvanizing line before entering the zinc bath.

It is according to the invention still possible that the annealing treatment on already finished components (frame constructions, wheels, fasteners u. a.) Is carried out, whereby these components in retrospect their mechanical properties are improved. Advantage of this The procedure is that the transformation to the component at a good cold-workable material with low yield ratio good elongation can be made and thus the tool wear comparatively is kept low. The subsequent annealing treatment strengthens The components increased to values that were otherwise difficult to set can be because then the pressing force of the forming machines would not be enough.

Next the holistic annealing treatment a component is according to the invention, the application of a zonal annealing at local limited locations of a component specifically possible. Goal is here the partial replacement of welded tailor blanks. The tailor blanks are targeted At certain points of components, steels of higher strength are welded in order to set desired component stiffnesses. On this welding could but be waived if instead then to the concerned Make a zonal annealing treatment is made.

A device for setting specific combinations of properties in hot-rolled multiphase steels by an annealing treatment, according to the invention characterized by a arranged at a freely selectable location within the production facility or production line thermal plant in which a Annealing up to an annealing temperature of ≤ 600 ° C and up to an annealing time of ≤ 120 s is feasible. This thermal plant can be a continuous annealing device in which the annealing of components is performed offline, for example, or it is arranged online in an existing process line, for example in the context of strip galvanizing, in the heating stage of a galvanizing line prior to entry into the zinc bath.

The mode of operation of the annealing treatment according to the invention is clear, inter alia, from the following example. Dual phase steels exhibit partially anisotropic toughness properties in the rolling direction and transversely thereto. In the case of a short annealing treatment carried out for 60 seconds at 500 ° C. on a dual-phase steel with 980-1035 N / mm ² tensile strength produced as a hot strip, this anisotropy of properties could be uniformed in both directions (isotropic properties). As the following table shows, the untreated hot strip (annealing time 0 s) has a significantly different formation of the elongations at break in roll longitudinal and transverse rolling direction. Due to the short annealing treatment (annealing time 1 min.), The tensile strength decreases slightly, but the values for the elongation at break altogether rise to a higher level:

These apply the example of the dual phase steel shown relationships in the same way also for TRIP steels.

Further Details of the possible execution The above-described annealing treatment according to the present invention will be described below to flow diagrams shown in schematic drawing figures explained in more detail.

It demonstrate:

1 a flow chart of the annealing of strip material,

2 a flow diagram of the annealing of wire material,

3 a flow chart of the annealing of components.

In the 1 to 3 are in the form of flow images used for the inventive annealing of strip material ( 1 ), of wire material ( 2 ) and components ( 3 ) required individual process steps, wherein the respective process path is marked with numbered direction arrows. Common to all the flow charts listed is that as a starting point, first a hot rolling takes place, followed by a controlled cooling from the hot rolling to achieve a multi-phase structure. The other possible process steps and the timing of the performed annealing in the various materials are described below.

In 1 are possible procedural paths 1 . 2 for an annealing treatment of strip material prior to further processing. In the process 1 will after hot rolling 10 and the controlled cooling 20 an annealing treatment 30 carried out and then the strip material for further processing to the finished product 80 guided. The annealing treatment 30 can be carried out online, for which a corresponding continuous furnace in the existing process line is to be arranged.

When marked process way 2 finds, for example, a strip galvanizing 40 the hot strip, so before online a continuous annealing 30 in the heating stage of the galvanizing line can be carried out. Following the strip galvanizing 40 then the further processing takes place to the finished product 80 of the band material.

In the 2 are possible procedural paths 1 . 2 . 3 shown for an annealing of wire material. In the illustrated process path 1 takes place after hot rolling 10 and the subsequent controlled cooling 20 the annealing treatment 30 , which can be done online like the band material here. To the annealing treatment 30 then closes the processing directly to the finished product 80 at.

According to the procedure 2 takes place after the also here possible online carrying out the annealing treatment 30 yet another processing step, the pressing 50 of fasteners before the wire material of further processing to the finished product 80 is supplied.

Alternatively, this pressing 50 of fasteners already before the annealing treatment 30 be carried out as the procedure 3 shows. The resulting process steps arranged one behind the other are then: hot rolling 10 , controlled cooling 20 , Pressing 50 of fasteners, annealing 30 and finally the further processing to the finished product 80 ,

In 3 are possible procedural paths 1 . 2 . 3 for an annealing of components, wherein for all three process paths after the controlled cooling 20 first with the production of a blank 60 a further process step takes place.

In the process 1 , The production of components with adjusted mechanical properties, takes place after the production of the blank 60 the pressing of the components 70 , The entire component is then subjected to an annealing treatment 30 and then the further processing to the finished product 80 fed.

In the process 2 , the manufacture of components with prior local annealing of the blank, takes place after the production of the blank 60 a zonal annealing treatment 35 , which is why the pressing of the components 70 must be made on already locally heat treated blank and thus on a blank with locally changed mechanical properties.

Alternative to the procedure 2 will be in the process 3 the production of components with a subsequent local change in mechanical properties through a zonal annealing treatment 35 of the pressed component, whereby the pressing of the components 70 can be made with advantage on the still untreated blank. After this zonal annealing treatment 35 can then the locally in its mechanical strength changed component of the further processing to the finished product 80 be supplied.

1, 2, 3

process route

10

hot rolling

20

controlled Cooling

30

annealing of the entire workpiece

35

zonal annealing

40

Bandverzinkung

50

Press of fasteners

60

manufacturing of the blank

70

Press of the components

80

processing to the finished product

Claims (11)

Method for setting specific combinations of properties in hot-rolled multiphase steels whose multi-phase structure comprises at least 30% ferrite and at most 50% martensite, for example dual-phase and TRIP steels, on a conventional hot rolling mill, a thin slab caster rolling mill or corresponding narrow and medium strip lines or a Wire rod are prepared with a standard analysis and a standard process management, characterized in that following the cooling from the hot rolling ( 10 ) or a later manufacturing step, for example the production of components, by a subsequent or intermediate annealing treatment ( 30 . 35 ) with variable annealing temperature and variable annealing time the desired combinations of strengths and stretch limit ratios are set on the multiphase steels. Method according to claim 1, characterized in that the annealing treatment ( 30 . 35 ) is carried out so that the resulting structure consists of a ferritic matrix and martensite or bainite tempered with 10 to 50% of the surface portion, wherein the annealing temperature primarily the level of yield strength by finely divided precipitates of carbides on the grain boundaries of martensite or bainite is affected and the duration of the tensile strength is adjustable by the annealing time. Method according to claim 1 or 2, characterized in that the annealing treatment ( 30 . 35 ) is performed with an annealing temperature of ≤ 600 ° C and an annealing time of ≤ 120 s. Method according to claim 1, 2 or 3, characterized in that the annealing treatment ( 30 . 35 ) is performed offline in a continuous firing device. Method according to claim 1, 2 or 3, characterized in that the annealing treatment ( 30 ) online as part of a strip galvanizing ( 40 ) is carried out in the heating stage of a galvanizing line before entering the zinc bath. Method according to claim 1, 2 or 3, characterized in that the annealing treatment ( 30 . 35 ) is carried out on already finished components. Method according to claim 1, 2, 3 or 6, characterized in that the annealing treatment ( 35 ) Zonal, ie at localized points of a component, is made specifically. Apparatus for setting specific combinations of properties in hot-rolled multiphase steels whose multi-phase structure comprises at least 30 ferrite and at most 50% martensite, for example dual-phase and TRIP steels, on a conventional hot rolling mill, a thin slab casting mill or equivalent narrow and medium strip lines or a Wire are prepared with a standard analysis and a standard process management, in particular for carrying out the method according to one or more of the preceding claims 1 to 7, characterized in that within the production plant or production line at a freely selectable location, a thermal system is arranged, in which Annealing treatment ( 30 . 35 ) with a variable annealing temperature of ≤ 600 ° C and a variable annealing time of ≤ 120 s is feasible. Device according to claim 8, characterized in that that the thermal plant is online in a galvanizing line arranged continuous furnace is. Device according to claim 8, characterized in that that the thermal plant is an offline operated continuous heating device is. Apparatus according to claim 8, characterized in that the thermal system is formed so that a zonal annealing ( 35 ) at localized points of a component before or after its actual production as a finished product is feasible.