Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/041—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing involving a particular fabrication or treatment of ingot or slab
  • C21D8/0415—Rapid solidification; Thin strip casting
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
  • C21D8/0426—Hot rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
  • C21D8/0436—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the invention relates to a method for manufacturing flat steel products, such as strips or sheet metal blanks, from high-tensile, boron microalloyed steels.
• Such steels belong to the group of multi-phase steels. These are usually steels, the properties of which are determined by type, quantity and alignment of the phases of the microstructure. Therefore at least two phases exist in the microstructure (ferrite, martensite, bainite for example). As a result, they have a superior strength/formability combination compared to conventional steels.
• multi-phase steels are of major interest for automotive construction, since due to their high strength on the one hand they allow the use of smaller material thicknesses and consequently at the same time a reduction in the vehicle weight and on the other hand improve the safety of the vehicle body in the event of a collision (crash behavior).
• multi-phase steels with at least equal strength of the overall body permit a reduction in the sheet metal thickness of a component made from such multi-phase steels compared to a body made from conventional steels.
• multi-phase steels are melted in a converter steel mill and cast on a continuous casting machine into slabs or thin slabs, which are then hot-rolled into hot-rolled strip and coiled.
• the mechanical properties of the hot-rolled strip can be varied by selectively controlled cooling of the hot-rolled strip after hot-rolling with the aim of adjusting certain microstructural fractions.
• the hot-rolled strip can also be cold-rolled into cold-rolled strip in order to also obtain thinner sheet metal thicknesses (EP 0 910 675 B1, EP 0966547 B1, EP 1 169486 B1, EP 1 319725 B1, EP 1 398390 A1).
• a problem with manufacturing flat products from high-tensile multi-phase steels with a tensile strength of more than 800 MPa is that high rolling forces must be applied when rolling such steels. This requirement has the consequence that normally with the production machines at present generally available, high-tensile hot-rolled strip made from steel of the type under discussion can often only be manufactured in a width and thickness, which no longer fully meet the requirements demanded today by the automotive industry. In particular, strip of narrow thickness with sufficient width cannot be produced very well in conventional installations. Also, with conventional methods it is shown in practice that it is difficult to manufacture cold-rolled strip with a strength of more than 800 MPa from multi-phase steel.
• the cast strip is subsequently hot-rolled in-line into a hot-rolled strip in one or more passes, the deformation degree ranging between 25% and 70%.
• the final hot-rolling temperature in this case is above the Ar 3 temperature.
• the obtained hot-rolled strip is then cooled down in two steps. In the first step of this cooling a cooling rate of 5-100° C. per second is maintained until a temperature ranging between 400-550° C. is reached.
• the hot-rolled strip is then held at this temperature for a dwell time, which is needed in order to allow bainitic transformation of the steel with a residual austenite content greater than 5%.
• the formation of pearlite in this case is to be avoided.
• the transformation process is interrupted by the beginning of the second cooling step, wherein the hot-rolled strip is brought to a temperature below 400° C., in order then to wind it into a coil at a coiling temperature below 350° C.
• an aspect of the invention is to provide a method, which allows high-tensile flat steel products to be manufactured with less effort in a wide range of geometrical dimensions.
• the aspect indicated above has been achieved by a method for manufacturing flat steel products, wherein according to the invention a steel that forms a multi-phase microstructure, which (in wt. %) contains 0.08-0.12% C, 1.70-2.00% Mn, up to 0.030% P, up to 0.004% S, up to 0.20% Si, 0.01-0.06% Al, up to 0.0060% N, 0.20-0.50% Cr, 0.010-0.050% Ti, 0.0010-0.0045% B and remainder iron and unavoidable impurities, is cast into a cast strip having a thickness of 1-4 mm, wherein the cast strip is hot-rolled in-line into a hot-rolled strip having a thickness of 0.5 to 3.2 mm in a continuous process at a final hot-rolling temperature ranging from 800 to 1100° C., the deformation degree being greater than 20%, and wherein the hot-rolled strip is coiled at a coiling temperature ranging from 250 to 570° C., so as to obtain a hot-rolled
• the invention provides a method of casting to convert a particularly high-tensile, possibly peritectically solidifying multi-phase steel into a hot-rolled strip. Since the cast strip itself in this case already possesses a narrow thickness, only relatively low deformation degrees must be maintained in the course of hot-rolling this strip, in order to manufacture flat products with narrow thicknesses, as they are needed particularly in the field of automotive construction. Thus it is possible with the method according to the invention, by specifying a corresponding initial thickness of the cast strip, to produce without any problems hot-rolled strip, which with an optimal characteristic distribution has a maximum thickness of 1.5 mm and from which components for the support structure of a vehicle for example can be manufactured.
• the rolling forces necessary for this are low, so that hot-rolled strip of large width, which lies substantially above the width of hot-rolled strip of the same strength and thickness cast in the conventional way, can be produced without any problems with the method according to the invention.
• the invention permits high-tensile hot-rolled strip, consisting of a martensitic steel with the composition indicated and processed according to the invention, the width of which is greater than 1,200 mm, in particular greater than 1,600 mm, to be reliably produced.
• the application according to the invention of the strip casting process for converting high-tensile steels of the type composed according to the invention apart from the advantages mentioned above, due to their characteristics and process variables specific to the method (hot-rolling final temperature, cooling, coiling temperature for example) offers the possibility, also in respect to their solidification behavior, of reliably casting critical steel compositions of the type processed according to the invention.
• very rapid solidification of the cast strip, characteristic of strip casting leads to a substantially reduced risk of the emergence of center liquations, compared to conventional production, with the consequence that the hot-rolled strip produced according to the invention has a particularly uniform characteristic distribution and microstructure over its cross section and its length.
• a further special advantage of the method according to the invention is that the hot-rolled strip produced according to the invention has a high strength of at least 800 MPa, without in addition a special cooling cycle of the hot-rolled strip having to be maintained between the end of hot-rolling and coiling, which is prescribed for example in EP 1 072 689 B I as the result of the need for a cooling interruption.
• it In carrying out the method according to the invention, it must only be ensured that hot-rolling is terminated in a relatively closely confined temperature window and also that coiling is carried out in a precisely defined temperature range. Single-step cooling takes place in the interim.
• a further advantage of the method according to the invention is that an extension in the range of mechanical properties of the strip produced according to the invention can be achieved, based on a single steel analysis, by varying the cooling and rolling conditions.
• Hot-rolled strip produced according to the invention is particularly suitable for subsequent conversion into cold-rolled strip. Accordingly, one practical embodiment of the invention makes provision for the hot-rolled strip to be cold-rolled into cold-rolled strip having a thickness of 0.5-1.4 mm, in particular 0.7 mm up to 1.3 mm, as is needed for constructing automotive bodies.
• the cold-rolled strip can be annealed at an annealing temperature of 750-850° C.
• a minimum tensile strength of 800 MPa can be reliably ensured.
• a 50 of the cold-rolled strip is 10%.
• the cold-rolled strip is provided in the way known per se with a metallic coating, in which, for example, this can be a zinc coating.
• the strength and elongation values of hot-rolled strip produced according to the invention can be adjusted over a large range by corresponding coordination of the final hot-rolling and coiling temperatures. If for example hot-rolled strip, which has a minimum breaking elongation A 80 of the obtained hot-rolled strip of 10% and a minimum tensile strength R m of 800 MPa, is to be manufactured, this can be achieved due to the final hot-rolling temperature being 900-1000° C. and the coiling temperature being 420-510° C.
• a hot-rolled strip with a higher tensile strength R m of at least 1000 MPa at a minimum breaking elongation A 80 of 5% is to be manufactured, in order to do this a final hot-rolling temperature ranging from 900 to 1100° C. and a coiling temperature ranging from 450 to 570° C. are selected.
• the strips cast out of the steels A and B have been hot-rolled in-line directly after the strip was cast into a hot-rolled strip, having a thickness of 1.25 mm, at a final hot-rolling temperature WET. Subsequently, the obtained hot-rolled strip in each case was directly cooled in a cooling step to a coiling temperature HT and coiled. After coiling the hot-rolled strips produced from the steels A and B in each case had a tensile strength R m and a breaking elongation A 80 , which are indicated in Table 2 as the final hot-rolling temperature WET and coiling temperature HT maintained in each case during their production.
• the tensile strength R m of the cold-rolled strip obtained in this way was 835 MPa.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Crystallography & Structural Chemistry (AREA)
• Thermal Sciences (AREA)
• Physics & Mathematics (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Heat Treatment Of Steel (AREA)
• Metal Rolling (AREA)
• Continuous Casting (AREA)

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• 2006
  • 2006-10-30 AT AT06123139T patent/ATE432376T1/de active
  • 2006-10-30 PL PL06123139T patent/PL1918406T3/pl unknown
  • 2006-10-30 EP EP06123139A patent/EP1918406B1/de not_active Not-in-force
  • 2006-10-30 ES ES06123139T patent/ES2325962T3/es active Active
  • 2006-10-30 DE DE502006003835T patent/DE502006003835D1/de active Active
• 2007
  • 2007-10-24 KR KR1020097007485A patent/KR101461583B1/ko active IP Right Grant
  • 2007-10-24 US US12/447,621 patent/US20100043513A1/en not_active Abandoned
  • 2007-10-24 JP JP2009533822A patent/JP5350253B2/ja not_active Expired - Fee Related
  • 2007-10-24 WO PCT/EP2007/061390 patent/WO2008052919A1/de active Application Filing
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