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
  • C21D8/0221—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
  • 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
  • C21D2211/005—Ferrite
• • 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 producing hot strip with a two-phase structure from a slab previously produced by block casting or continuous casting, which contains iron, carbon, manganese, silicon and chromium as essential constituents, by heating the slab to the rolling temperature Hot and finish rolling at a temperature> A, 3 , by accelerated cooling from the rolling heat and by coiling at a relatively low temperature.
• Such a process for the production of hot strip with a two-phase structure made of fine-grained ferrite (> 70%) and grains of martensite dispersed therein, in which the slab essentially containing carbon, manganese, silicon and chromium is first heated to hot rolling temperature, then warm above Ar3 - and finish-rolled, then accelerated and finally coiled at low temperature, is known from EP-B-19193 and from EP-A-72 867.
• the slab which is essentially 0.05-0.20% C, 0.5-1.5% Mn and 0.5-2.0% Si and possibly Cr, V, Mo, Ti and Nb, the rest contains iron, hot-rolled in the austenitic state, then cooled to a temperature in the range of approx. 800 ⁇ 650 ° C, coiled and held at this temperature for at least one minute. Subsequently, the strip is unwound in a further process step, cooled to a temperature ⁇ 450 ° C. at a speed> 10 ° C./s and finally rewound at this temperature.
• the slab heated to the rolling temperature which is essentially 0.02-0.20% C, 0.5-2.0% Mn, 0.05-2.0% Si and 0.3-1.5% Cr as well as ⁇ 0.15% P and ⁇ 0.1% Al, the rest contains iron, hot rolled with a finish rolling temperature> 780 ° C.
• the hot strip is cooled at a speed> 40 ° C / s to an intermediate temperature T N in the order of approximately 750-650 ° C and kept at this temperature for at least 5 seconds. This is followed by a further accelerated cooling at a rate> 50 ° C / s to a temperature in the range of 500-200 ° C before the strip is finally coiled at this temperature.
• both cooling processes mean complex cooling sections or, in the case of the process known from EP-B-19 193, a second unwinding and winding device for the finished rolled hot strip.
• EP-A-68 598 discloses a method for producing hot strip with a two-phase structure, low yield ratio and good formability, in which, in contrast to the two aforementioned methods, the hot strip after finishing rolling to a low coiling temperature without additional effort is cooled.
• This is essentially achieved in that the slab in addition to 0.03-0.15% C, 0.6 ⁇ 1.8% Mn, ⁇ 0.10% Al, ⁇ 0.008% S and possibly 0.2-2.0 % Si alone or together with Cr contains an increased phosphorus content in the range of 0.04-0.20%, balance iron.
• the slab must be heated to a certain temperature in the specified range of 1,100-1,250 ° C before it is subsequently hot-rolled and finish-rolled at a temperature in the range of 900-780 ° C and after finish-rolling at a speed in the range of 10-200 ° C / s is cooled and finally can be coiled at a temperature ⁇ 450 ° C.
• This previously known method has the advantage that the hot strip can be produced on conventional rolling mills with the associated cooling section without additional equipment.
• the increase in the phosphorus content means a deterioration in the weldability of the hot strip.
• the tendency of the hot strip to embrittlement increases with increasing or increased phosphorus content.
• This temper embrittlement is particularly noticeable when the sheet made from the hot strip with the increased phosphorus content has to be welded during further processing, for example.
• the temperature of the furnace for heating and heating the slab must be accurate to the rolling temperature can be set, this temperature from the predetermined temperature range is also below the usual temperatures.
• the object of the invention is to provide a method for producing hot strip with a two-phase structure which, with at least the same property profile, namely low yield ratio (below approx. 0.70), isotropic cold formability, good weldability, with simple means, especially on conventional rolling mills with associated cooling section, ie can be produced without additional equipment.
• the manganese content of the steel from which the slab is first produced and then the hot strip is reduced and to a low value in the range from 0.20 to 0.40%. is set.
• the carbon content of the steel is preferably set to a value in the range from 0.05 to 0.12%.
• the slab produced from a steel of the claimed composition can then be heated to the usual rolling temperature and heated through using the method according to the invention. No special measures are required in this regard.
• the hot and finished rolling of the heated slab into the hot strip takes place at a temperature above and as close as possible to A, 3 .
• this deformation during hot rolling and in particular during finish rolling in the last two stands of the finishing train no further special measures are necessary when using the method according to the invention.
• the deformation in the last two stands of the prefabricated structure is a maximum of 25%, preferably about 15%.
• the hot strip is, according to the invention, cooled immediately after finish rolling with a final rolling temperature above A r3 at an average speed in the range from 30 to 70 ° C./s and without interruptions and then coiled at a temperature in the range from 350 to 190 ° C. .
• the slab additionally contains 0.01 to 0.04% titanium in a stoichiometric ratio to nitrogen in order to achieve an improvement in the cold formability at the low coiling temperature according to the invention.
• nitrogen aging of the finished rolled hot strip or of the sheets produced therefrom is avoided.
• the main advantage of the method according to the invention is that hot strip with a two-phase structure made of fine-grained, globular ferrite (> 80%) and grains of martensite dispersed therein can be produced on conventional hot strip mills with the associated downstream cooling section. Furthermore, the method according to the invention enables known measures to accelerate ferrite formation and its negative effects, such as to dispense with a high final deformation during hot rolling and finish rolling in the two-phase area.
• a high final deformation means undesirable high rolling forces and a deterioration in strip flatness and strip geometry and finish rolling in the two-phase area also mean high rolling forces, a deterioration in cold formability and anisotropic mechanical properties of the hot rolled strip.
• an Mn content of at least 0.6% is required to set the two-phase structure
• this also has a reduced Mn content according to the invention in the range of 0.2-0.4% is possible.
• Another advantage of the method according to the invention is that the manufacturing costs are reduced due to the reduced Mn content.
• the low Mn content according to the invention advantageously has the effect that practically no stretched sulfides (MnS) form, which usually cause a deterioration in the cold formability, particularly in the transverse direction, of high-strength steels.
• the process according to the invention makes it possible to dispense with the increased addition of phosphorus, which leads to embrittlement.
• the preferred phosphorus content is limited to - ⁇ 0.015%.
• hot strip with a two-phase structure made of> 80% fine-grained, globular ferrite and martensite and with a yield ratio ⁇ 0.07 can be produced with the aid of the method according to the invention, which can be welded without problems and in both the longitudinal and transverse directions has good, uniform cold mobility.
• Another advantage of the method according to the invention is that, in the hot strip produced by the method, after deformation and subsequent tempering treatment, for example, by baking an applied layer of lacquer, an additional increase in the yield strength is achieved. Furthermore, the low alloy content enables the production of hot strip with a two-phase structure with tensile strengths of 500 to 600 N / mm 2 , which are particularly suitable for the production of parts that require high cold formability.
• Table 2 shows that the reel temperature HT should preferably be set to a temperature above 200 ° C, because at a lower reel temperature, s.
• Samples A3 and B3 the yield strength ratio increased again and the elongation at break A 5 decreased to even worse values. Both have an unfavorable effect on the cold formability of the hot strip or sheet.
• an artificial aging treatment at 100 ° C and for about an hour showed no change in the yield strength.
• an increase in the yield strength in the range of 4080 N / mm 2 was determined after a tempering treatment at approx. 170 ° C. and 20 minutes after the strips or sheets had previously been subjected to a three percent pre-ver f styling aid were subjected.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Physics & Mathematics (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Heat Treatment Of Strip Materials And Filament Materials (AREA)
• Heat Treatment Of Steel (AREA)
• Networks Using Active Elements (AREA)
• Inductance-Capacitance Distribution Constants And Capacitance-Resistance Oscillators (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (3)

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Family Applications (1)

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• 1984
  • 1984-11-08 DE DE3440752A patent/DE3440752A1/de active Granted
• 1985
  • 1985-11-02 DE DE8585113944T patent/DE3581591D1/de not_active Expired - Fee Related
  • 1985-11-02 AT AT85113944T patent/ATE60624T1/de not_active IP Right Cessation
  • 1985-11-02 EP EP85113944A patent/EP0181583B1/de not_active Expired - Lifetime
  • 1985-11-07 CA CA000494751A patent/CA1269256A/en not_active Expired - Fee Related
  • 1985-11-08 JP JP60249086A patent/JPH0676616B2/ja not_active Expired - Fee Related
• 1987
  • 1987-05-19 US US07/051,892 patent/US4790889A/en not_active Expired - Lifetime

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