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
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0268—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment between cold rolling steps
• • 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
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C21D2211/002—Bainite
• • 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 hot-rolled flat steel product produced from a complex phase steel and a method for producing such a product.
• EP 2 028 282 A1 discloses a cold-rolled flat steel product which is produced from a dual phase steel and which, in addition to a tensile strength of at least 950 MPa and good deformability, also has a surface quality which, using a simple production method, enables the flat product produced from this steel, in the uncoated state or a state provided with a covering which protects against corrosion, to be formed into a complex shaped component, such as a component of a vehicle bodywork.
• the steel according to the invention comprises from 20 to 70% of martensite, up to 8% of residual austenite and as the remainder ferrite and/or bainite and contains (in % by weight): C: 0.10-0.20%, Si: 0.10-0.60%, Mn: 1.50-2.50%, Cr: 0.20%-0.80%, Ti: 0.02-0.08%, B: ⁇ 0.0020%, Mo: ⁇ 0.25%, Al: ⁇ 0.10%, P: ⁇ 0.2%, S: 0.01%, N: 0.012% with the remainder being iron and inevitable impurities.
• the flat steel products produced from such a steel in practice achieve tensile strengths of up to 1050 MPa.
• the hot-rolled strip is subsequently cooled with a cooling speed over the runout rolling operation of at least 30° C./s, so that the conversion of the steel is carried out to the greatest possible extent in the bainite stage and a conversion of the steel into perlite is prevented. Proportions of martensite in the structure of the hot-rolled strip can further increase the tensile strengths. Furthermore, the comparatively rapid cooling contributes to the precipitation of extremely fine particles, by means of which the strength is further increased.
• the cooling operation is intended to be ended at a temperature below 600° C. by the strip being wound on a coiler and subsequently being further cooled in the coil.
• the hot-rolled strip obtained in this manner regularly reaches tensile strengths of up to 1150 MPa.
• an object of the invention was to provide a flat steel product in which further increased tensile strengths with good elongation properties and consequently inherently good deformation properties are combined.
• a method for producing such a flat steel product is also intended to be set out.
• the complex phase steel used for the production of a hot-rolled flat steel product according to the invention contains in addition to iron and inevitable impurities (in % by weight), C: 0.13-0.2%, Mn: 1.8-2.5%, Si: 0.70-1.3%, Al: up to 0.1%, P: up to 0.1%, S: up to 0.01%, Cr: 0.25% -0.70%, optionally Mo, the total of the Cr and Mo contents being 0.25-0.7%, Ti: 0.08-0.2% and B: 0.0005-0.005%.
• a flat steel product hot-rolled from the steel according to the invention has a high strength, with good elasticity at the same time.
• the structure of a flat steel product according to the invention owing to the alloy thereof selected within narrow limits, is characterised in that the structure thereof comprises at the most 10% by volume of residual austenite, 10-60% by volume of martensite, at the most 30% by volume of ferrite and bainite as the remainder, the proportion being intended to be at least 10% by volume.
• Perlite is in any case present in a flat steel product according to the invention in ineffective traces, the perlite proportion being minimised where possible.
• Flat steel products according to the invention thus reach in the hot-rolled state a tensile strength Rm, which is greater than 1100 MPa, in particular regularly at least 1150 MPa and more, and a yield point Re of also regularly at least 720 MPa.
• Rm tensile strength
• Rm yield point of also regularly at least 720 MPa.
• Carbon is added to the complex phase steel used according to the invention in order to harden the structure and to form extremely fine precipitations. Owing to the presence of C in the contents predetermined according to the invention of 0.13% to 0.2% by weight, a martensite and bainite proportion which is sufficiently high for the desired hardness is thus achieved in the structure. At contents of more than 0.20% by weight, carbon prevents the occurrence of the desirably high bainitic structure proportion. Relatively high C contents also have a negative effect on the weldability, which is particularly significant for the use of the material according to the invention, for example, in the field of automotive construction. The advantageous effect of carbon in a steel which is used to produce a flat steel product according to the invention can be used in a particularly reliable manner when the C content is 0.15-0.18% by weight, in particular a maximum of 0.17% by weight.
• Manganese at a content of at least 1.8% by weight delays the transformation and brings about the formation of hard, strength-increasing transformation products. The occurrence of martensite is thus promoted by the presence of Mn.
• the content is limited according to the invention to a maximum of 2.5% by weight, the advantageous influences of Mn then appearing in a particularly reliable manner when the Mn content of the steel according to the invention is limited to 2.05 to 2.2% by weight.
• Si also serves to increase the strength, on the one hand by promoting the solid solution hardening of the ferrite or bainite and, on the other hand, by stabilising the residual austenite.
• the residual austenite proportion contributes to increasing the elasticity and strength (TRIP effect).
• steel according to the invention has 0.70-1.3% by weight of Si, in particular at least 0.75% by weight of Si.
• the strength and elasticity-increasing effect occurs in particular when the Si content of a steel according to the invention is at least 0.75% by weight, in particular at least 0.85% by weight.
• the upper limit of the Si content has been determined to 1.3% by weight at the same time.
• the danger of grain boundary oxidation is also minimised.
• An unfavourable influence of Si on the properties of the steel used according to the invention can be prevented with even greater reliability by the Si content of the steel according to the invention being limited to 1.1% by weight, in particular 0.95% by weight.
• the steel which the flat steel product according to the invention consists of is Al-stabilised. Aluminium is used in the melting of a steel according to the invention for deoxidation and for binding nitrogen which may be contained in the steel. To this end, Al in contents of less than 0.1% by weight may be added to the steel according to the invention, if necessary, the desired effect of Al then occurring in a particularly reliable manner when the contents thereof are in the range from 0.01-0.06% by weight, in particular 0.020-0.050% by weight.
• Phosphorus can be used to further increase the solid solution hardening, but should not exceed a content of 0.1% by weight for reasons of weldability, owing to the otherwise increasing risk of the formation of segregations.
• chromium prevents the formation of ferrite and perlite. Accordingly, it promotes the formation of a hardening structure and consequently the strength of the steel used for the flat steel product according to the invention.
• the content thereof should be limited to a maximum of 0.7% by weight.
• the Or content of a steel according to the invention being limited to 0.7% by weight, the risk of the occurrence of grain boundary oxidation is reduced and the good elongation properties of the steel according to the invention are ensured.
• this upper limit is complied with, a surface of the flat steel product produced from the steel is also achieved which can readily be provided with a metallic coating.
• the optionally provided contents of molybdenum contribute in the same manner as Cr to increasing the strength of a steel according to the invention, by promoting the formation of extremely fine precipitations and martensite in the structure of the steel.
• the presence of Mo does not have a negative effect on the ability to coat the flat product with a metal coating and the elasticity thereof. Practical tests have shown that the positive effects of Mo can be used in a particularly effective manner up to contents of 0.25% by weight, in particular 0.22% by weight, including from a cost point of view. Thus, Mo contents of 0.05% by weight already have a positive effect on the properties of the steel according to the invention.
• the total of the Cr and Mo contents in a steel used according to the invention is limited to from 0.25% to 0.7% by weight.
• titanium in contents of from at least 0.08% to a maximum of 0.2% by weight, in particular 0.09% -0.15% by weight, in steel according to the invention the formation of extremely fine precipitations in the form of TiC or Ti(C, N) with a hardening effect can be promoted and grain refinement can be brought about.
• Another positive effect of Ti involves the binding of any nitrogen which may be present so that the formation of boron nitrides in the steel according to the invention is prevented. Owing to the presence of Ti consequently, in the case of an addition of boron to increase strength, it is also ensured that the boron can fully develop its effect in the dissolved state.
• the positive effect of Ti can be used in a particularly reliable manner in a steel according to the invention when the Ti content thereof is 0.11-0.13% by weight.
• boron improves the hardenability when B is present in contents of from 0.0005-0.005% by weight.
• austenite boron segregates at the grain boundaries and prevents the formation of ferrite and perlite. In this instance, boron brings about a significant increase of the strength with only a slight reduction of the deformability.
• the favourable influences of B on the alloy according to the invention are produced in a particularly reliable manner when the B content of the steel according to the invention is determined to 0.001% -0.002% by weight.
• Flat steel products which are produced in a manner according to the invention are distinguished by a particularly high level of grain fineness, a high yield point and increased strength.
• the proportions of martensite, bainite and extremely fine precipitations contained in the structure thereof contribute to the high degree of strength.
• the residual austenite and ferrite portions of the structure ensure the good elongation properties thereof.
• the hot-rolled strips may be provided with a metallic protective coating before or after they are shaped to form a component. This can be carried out by means of hot-dip galvanising, or electrolytic coating.
• a steel melt having a composition which falls under the alloy of the steel used according to the invention is first cast to form a preliminary product which is typically a strand which is cut into slabs or thin slabs.
• the preliminary product is heated to a temperature of 1150-1350° C. in order to ensure for the subsequent hot rolling a completely austenitic structure of the steel and to bring the precipitations into a solution.
• the preliminary product is then hot-rolled to form a hot-rolled strip, the end temperature of the hot rolling being 800-950° C.
• the rolling end temperature should be in the range of the homogeneous austenite and consequently not be below 800° C. in order to keep deformation-induced precipitations low and to enable the development of the desired structure composition.
• the hot-rolled strip which is obtained is cooled to the coiling temperature selected in each case at a cooling speed of at least 30° C./sec.
• the cooling conditions are intended to be selected in such a manner that a transformation to perlite is prevented and the transformation is carried out to the greatest possible extent in such a manner that the high bainite proportions and the proportions predetermined according to the invention of martensite and residual austenite are obtained.
• the cooling operation is ended when the range of the coiling temperature of 400-570° C. predetermined according to the invention is achieved, in which the bainite stage of the steel according to the invention is achieved.
• the hot-rolled strip which is cooled accordingly is then wound to form a coil and is further cooled in the coil. Further transformations into bainite and martensite and the formation of precipitations occur.
• steel according to the invention is particularly suitable for producing profile-members which are highly loaded in practical use and for crash- and strength-relevant components for vehicle bodyworks.
• the blocks were subsequently heated to 1270° C. and, starting from this temperature, hot-rolled to form hot-rolled strip having a thickness of 2.5 mm.
• the hot rolling end temperature was 900° C.
• the hot-rolled strip obtained was slowly cooled in an oven after the hot rolling operation at a cooling speed of 80° C./sec. and at a temperature of 490° C. in order to simulate the cooling in the coil.
• the hot-rolled strip obtained had transversely relative to the rolling direction a tensile strength Rm of 1192 MPa and an elongation A80 of 10.5%.
• the structure obtained comprises 35-40% by volume of martensite, approximately 5% by volume of ferrite, 6% by volume of residual austenite and the balance comprises bainite.
• the hot-rolled strips produced in the manner explained above, after the hot rolling, were first cooled to a temperature of 75° C. and subsequently slowly further cooled to ambient temperature in the oven in order to also simulate the cooling in the coil in this instance.
• the hot-rolled strips obtained in this manner had a tensile strength Rm of 1550 MPa and a comparatively low elongation A80 of 5.9%. They were primarily martensitic.
• the above-explained hot-rolled strips after the hot rolling operation, were first cooled to a temperature of 600° C. corresponding to the “coiling temperature” and subsequently slowly cooled again to ambient temperature in order to simulate the cooling in the coil.
• the hot-rolled strips obtained in this manner had a tensile strength Rm of 955 MPa and an expansion A80 of 15.5%.
• the structure consisted of ferrite having a perlite proportion of 25-30% by volume.
• the blocks were subsequently heated to 1270° C. and, starting from this temperature, were hot-rolled to form hot-rolled strip having a thickness of 2.5 mm.
• the hot rolling end temperature was 900° C.
• the hot-rolled strip obtained had a tensile strength Rm of 1180 MPa and an elongation A80 of 11%.
• the structure thereof had a martensite proportion of 35-40% by volume, a residual austenite content of 7.5% by volume, a ferrite content of 10% by volume and, as the remainder, bainite.
• a steel having the alloy set out in Table 3 according to the invention was melted and cast to form a strand.
• the slabs separated from the strand were subsequently reheated to a temperature of approximately 1260° C., subsequently hot-rolled at a hot rolling end temperature WET to form hot-rolled strips having a thickness D and finally cooled at a cooling rate V T to a coiling temperature HT, at which they were wound to form a coil.
• the parameters adjusted in each case and the mechanical properties of the hot-rolled strips obtained are set out in Table 4.

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• Chemical & Material Sciences (AREA)
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• Physics & Mathematics (AREA)
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• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

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• 2011
  • 2011-02-18 EP EP11154973.9A patent/EP2489748B1/de active Active
  • 2011-12-29 US US13/985,420 patent/US20140041767A1/en not_active Abandoned
  • 2011-12-29 CN CN201180067938.XA patent/CN103380217B/zh active Active
  • 2011-12-29 KR KR1020137024831A patent/KR20140005293A/ko not_active Application Discontinuation
  • 2011-12-29 JP JP2013553818A patent/JP5864619B2/ja not_active Expired - Fee Related
  • 2011-12-29 WO PCT/EP2011/074251 patent/WO2012110165A1/de active Application Filing
  • 2011-12-29 CA CA2825240A patent/CA2825240A1/en not_active Abandoned

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