Classifications

• • 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
• • 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
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
  • 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
  • C22C38/02—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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C21D2211/001—Austenite
• • 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
  • C21D2211/005—Ferrite

Definitions

• the present invention relates to a high-tensile strength hot-rolled steel sheet having excellent elongation properties and excellent stretch flangeability and a method of producing the same.
• hot-rolled steel sheets for use in automobiles, for structural members of a vehicle body, suspension members (for instance, wheels, rims, chassis and so on) and strengthening members (for instance, bumpers, door guard covers and so on), high-tensile strength hot-rolled steel sheets that have the tensile strength of 780 MPa class to 980 MPa class are used.
• suspension members for instance, wheels, rims, chassis and so on
• strengthening members for instance, bumpers, door guard covers and so on
• high-tensile strength hot-rolled steel sheets that have the tensile strength of 780 MPa class to 980 MPa class are used.
• the hot-rolled steel sheets that are used for vehicle bodies in order to attain lower fuel consumption and an improvement in collision safety of automobiles, are demanded to satisfy the high mechanical strength and high workability.
• DP steel composite structure steel having a microstructure primarily made of ferrite and martensite, and a retained austenitic steel that has a microstructure made of ferrite, bainite and retained austenite
• patent reference No.1 discloses a method of producing a hot-rolled steel sheet in which steel whose essential components are C, Si and Mn is subjected to hot finish rolling under a rolling reduction of 80% or more and at a rolling temperature from 780 to 900°C, after the rolling has come to completion, cooling is started at a cooling rate of less than 40°C/sec and finished at a predetermined temperature, subsequently cooling is applied at a cooling rate of 40°C/sec or more further followed by coiling temperature in the range of 350 to 500°C, and thereby a hot-rolled steel sheet that has a microstructure that has a space factor of polygonal ferrite of 61% or more, a ratio of the space factor of the polygonal ferrite to a grain size of 18 or more, a second phase made of bainite and retained austenite, and 5% or more of retained austenite in the second phase is obtained.
• a TS x EL value calculated from the tensile strength TS (MPa) and the elongation EL (%) can attain 20000 MPa%, that is, a hot-rolled steel sheet excellent in the elongation properties can be obtained.
• the stretch flangeability that is important characteristics demanded for automobile high-tensile strength steel sheets is not at all considered.
• the stretch flangeability is an indicator that is generally expressed by use of a hole expansion rate obtained by hole expansion test and evaluates workability of the steel sheet. There is no correlation between the stretch flangeability and the elongation properties. Accordingly, even with the technology disclosed in patent reference No.1, it is difficult to produce a high-tensile strength hot-rolled steel sheet that combines the excellent stretch flangeability and the excellent elongation properties.
• a high-tensile strength steel sheet excellent in the stretch flangeability is disclosed.
• the high-tensile strength steel sheet is characterized in that C, Si, Mn and B are contained as essential constituents, an S content is restricted to 0.02% or less, and a microstructure is made of three phases of polygonal ferrite, bainite and martensite.
• a high-tensile strength hot-rolled steel sheet excellent in the stretch flangeability is disclosed.
• the steel sheet is characterized in that it includes C, Si, Mn, Ti and Nb as essential components, the area rate of ferrite having an average grain size of 25 ⁇ m or less is 70 to 95%, and the balance is made of a microstructure that comprises martensite or retained austenite.
• the hole expansion ratio ⁇ is only 48%, the stretch flangeability is not sufficient.
• a high-tensile strength steel sheet excellent in the burring properties is disclosed.
• the steel sheet is characterized in that it contains C, Si, Mn and Ti as essential components and has a microstructure made of a primary phase (that is, ferrite) having an average grain size of 5 ⁇ m or less and a secondary phase having an average grain size of 3.5 ⁇ m or less.
• the technology intends to produce a high-tensile strength steel sheet excellent in the TS-EL balance and the TS- ⁇ balance, particularly excellent in the burring properties (that is, hole expansion workability).
• the secondary phase contains pearlite, the disclosed tensile strength is at most 740 MPa, that is, 780 MPa is not achieved.
• Patent reference No.1 JP-A-3-10049 gazette.
• Patent reference No.2 JP-A-58-167750 gazette.
• Patent reference No.3 JP-A-9-125194 gazette.
• Patent reference No.4 JP-A-2000-192191 gazette.
• a steel sheet that is a high-tensile strength hot-rolled steel sheet having the tensile strength TS of 780 MPa or more or furthermore of 980 MPa or more, and has the elongation properties capable of attaining TS ⁇ EL ⁇ 20000 MPa% and the stretch flangeability capable of attaining TS ⁇ ⁇ ⁇ 82000 MPa% is in demand. That is, in the case of, for instance, TS 780MPa, the high-tensile strength hot-rolled steel sheet capable of satisfying EL ⁇ 25.5% and ⁇ ⁇ 105% is demanded.
• TS 780MPa the high-tensile strength hot-rolled steel sheet capable of satisfying EL ⁇ 25.5% and ⁇ ⁇ 105% is demanded.
• the present invention has been carried out to overcome these problems and intends to provide a high-tensile strength hot-rolled steel sheet in which the TS is 780 MPa or more or 980 MPa or more, the elongation properties are excellent, that is, TS ⁇ EL ⁇ 20000 MPa% is satisfied, and the stretch flangeability is excellent, that is, TS ⁇ ⁇ ⁇ 82000 MPa% is satisfied, and a method of producing the same.
• the present inventors after intensively studying in order to attain the above object, have found that when, with Ti as an indispensable component, ferrite generated after the hot rolling is made finer in its grain size, and fractions of bainite and retained austenite generated from non-transformed austenite are controlled in predetermined ranges, the high-tensile strength hot-rolled steel sheet having the tensile strength of 780 MPa or more or furthermore 980 MPa or more can be remarkably improved in the elongation properties and the stretch flangeability.
• a high-tensile strength hot-rolled steel sheet comprises a composition that includes C of 0.04% by mass or more and 0.25% by mass or less, Si of 0.4% by mass or more and 2.0% by mass or less, Mn of 3.0% by mass or less, Al of 0.2% by mass or less, S of 0.007% by mass or less, Ti of 0.08% by mass or more and 0.3% by mass or less, and the balance of Fe and inevitable impurities, wherein contents of the C, the Si and the Ti satisfy the following equation (1); and a microstructure that contains ferrite, bainite and retained austenite, wherein a fraction of the ferrite in an entire microstructure is 40% or more, an average grain size of the ferrite is 5 ⁇ m or less, a fraction of the bainite is in the range of 20% to 48% with respect to an entire microstructure, and a fraction of the retained austenite is in the range of 2% to 7% with respect to an entire microstructure.
• a method of producing a high-tensile strength hot-rolled steel sheet comprises, after a steel slab having a composition that includes C of 0.04% by mass or more and 0.25% by mass or less, Si of 0.4% by mass or more and 2.0% by mass or less, Mn of 3.0% by mass or less, Al of 0.2% by mass or less, S of 0.007% by mass or less, Ti of 0.08% by mass or more and 0.3% by mass or less, and the balance of Fe and inevitable impurities, wherein contents of the C, the Si and the Ti satisfy the following equation (1), is heated to 1150°C or less, hot rolling at a finish rolling temperature of (Ar 3 transformation temperature + 20°C) or more and (Ar 3 transformation temperature + 100°C) or less; cooling the obtained hot-rolled steel sheet at a cooling rate of 30°C/sec or more followed by holding for 2 to 20 seconds in a temperature range of 600 to 750°C; subsequently cooling at a cooling rate of 15°C/sec or more followed by coil
• the hot-rolled steel sheet thus obtained has excellent elongation properties and stretch flangeability.
• Ti is necessary to be contained 0.08% by mass or more.
• Ti when Ti is contained exceeding 0.3% by mass, the austenite is very much disturbed in recrystallization, accordingly, not only the microstructure of the hot-rolled steel sheet is made coarser but also the elongation properties and stretch flangeability are deteriorated. Accordingly, Ti has to satisfy a range of 0.08% by mass or more and 0.3% by mass or less. Ti is preferably contained in the range of 0.12% by mass or more and 0.25% by mass or less.
• the C content, the Ti content and the Si content, in order to form a mixed microstructure of ferrite, bainite and retained austenite as mentioned later, have to satisfy the following equation (1). ([%C]/12 - [%Ti]/48)/([%Si]/28) ⁇ 0.4
• the bainite and retained austenite, during the cooling process after the hot rolling, are generated from the non-transformed austenite.
• C is accelerated in diffusing
• C is suppressed from diffusing.
• ferrite increases and the fractions of bainite and retained austenite decrease. That is, the diffusion behavior of C affects a great influence on the generation of ferrite, bainite and retained austenite.
• Si suppresses cementite from being generated in the hot-rolled steel sheet and promotes C to diffuse from the ferrite to the non-transformed austenite.
• the C contents in the ferrite, bainite and retained austenite reach saturation states in a very short time, accordingly, even when the cooling conditions (for instance, the cooling rate and so on) fluctuate, an influence on the generation of the ferrite, bainite and retained austenite can be suppressed. That is, Si affects a great influence upon the diffusion behavior of C.
• the diffusion behavior of C varies according to interactions of C, Si and Ti.
• the interactions of these elements can be evaluated according to an index calculated from the respective numbers of atoms. That is, when the index is in the range satisfying equation (1), the diffusion of C is promoted, and a hot-rolled steel sheet that has a mixed microstructure containing ferrite, bainite and retained austenite as described later can be stably obtained. Moreover, without being affected by the fluctuation of the cooling conditions after the hot rolling, the hot-rolled steel sheet made of ferrite, bainite and retained austenite can be obtained.
• a ferrite fraction is set at 40% or more with respect to an entire microstructure.
• the reason for this is that when the ferrite fraction is 40% or more, the elongation properties can be improved.
• the ferrite When the elongation properties are improved with the tensile strength maintained at 780 MPa class, it is preferable for the ferrite to be rendered a primary phase (that is, the ferrite fraction is made 50% or more with respect to an entire microstructure).
• an average grain size of the ferrite grains is necessary to be 5 ⁇ m or less.
• the average grain size exceeds 5 ⁇ m, the stretch flangeability deteriorates remarkably.
• the ferrite grains having an average grain size of 5 ⁇ m or less are generated, an addition amount of an alloying element can be reduced. Accordingly, without causing the deterioration of the mechanical properties such as the elongation properties and the stretch flangeability of the hot-rolled steel sheet, the tensile strength of 780 MPa class or furthermore 980 MPa class can be obtained.
• the average grain size of the ferrite grains is preferable to be 4 ⁇ m or less.
• the other phase than the ferrite phase is rendered a mixed phase that contains bainite and retained austenite.
• the bainite is softer in comparison with the retained austenite and martensite, accordingly, hardness difference with ferrite is small.
• cracks in the stretch flanging occur in an interface with hardness greatly different between phases difference (for instance, an interface between the ferrite and martensite). Accordingly, as the soft bainite is contained much, the stretch flangeability is improved.
• Such effect can. be obtained when the bainite fraction is 20% or more with respect to an entire microstructure.
• the bainite fraction exceeds 48%, the ferrite fraction decreases, resulting in deterioration of the elongation properties.
• the C content in the non-transformed austenite is largely lowered and the retained austenite decreases. This also causes the deterioration of the elongation properties.
• the bainite fraction is necessary to be from 20% to 48% with respect to an entire microstructure.
• the bainite fraction is preferable to be 40% or less, being more preferable to be in the range of 25% to 35%.
• the retained austenite owing to the generation of stress-induced martensite, exhibits uniform and high elongation properties. Such effects can be obtained when the retained austenite fraction is 2% or more in an entire microstructure.
• the retained austenite fraction is over 7%, owing to being subjected to the stretch flanging, the retained austenite becomes harder, resulting in a large hardness difference with the ferrite.
• the retained austenite fraction is necessary to be 2 to 7% with respect to an entire microstructure. It is preferable to be 4 to 6%.
• martensite In production processes of the hot-rolled steel sheet, other than the ferrite, bainite and the retained austenite, in some cases, martensite is'generated.
• the martensite is the hardest phase in the microstructure of the hot-rolled steel sheet. Accordingly, by stretch flanging, in an interface between the ferrite and the martensite, cracks tend to be generated. Accordingly, the smaller the martensite fraction is, the better, it is preferable to be 5% or less relative to an entire microstructure.
• Molten steel with the above composition is prepared, and therefrom according to a so far known method such as a continuous casting method or ingot making method, a steel slab is produced. Subsequently, the steel slab is set in a heating furnace and heated to a temperature of 1150°C or less. When the steel slab is heated to a temperature exceeding 1150°C, since TiC is dissolved, finer austenite grains cannot be obtained. As a result, the ferrite becomes coarser, resulting in deterioration of the elongation properties and stretch flangeability.
• the lower limit of the heating temperature of the steel slab in order to secure a finish rolling temperature described later, is preferable to be 1050°C or more.
• a more preferable range of the heating temperature of the steel slab is from 1050 to 1100°C.
• the finish rolling temperature of the hot rolling is set, above the Ar 3 transformation point, in a range of (Ar 3 transformation point + 20°C) or more and (Ar 3 transformation point + 100°C) or less.
• the bainite fraction can be maintained within the range of 20 to 48% in an entire microstructure.
• the finish rolling temperature is lower than (Ar 3 transformation point + 20°C)
• the bainite fraction cannot attain 20%, resulting in an increase in the ferrite fraction and the retained austenite fraction.
• the finish rolling temperature is higher than (Ar 3 transformation point + 100°C)
• the austenite grains grow and the microstructure becomes coarser, resulting in deterioration in the elongation properties and the stretch flangeability.
• the hot-rolled steel sheet obtained through the hot rolling is, according to a first step cooling, cooled at a cooling rate of 30°C/sec or more to 600 to 750°C.
• the cooling rate is set at 30°C/sec or more, the microstructure can be hindered from becoming coarser.
• a temperature where the first step cooling is stopped is outside of the range of 600 to 750°C, ferrite transformation in the second cooling described later is delayed. As a result, the ferrite, bainite and retained austenite fractions cannot be properly maintained.
• the temperature where the first cooling is stopped is preferably 650 to 700°C.
• the hot-rolled steel sheet thus obtained by stopping the first step cooling at 600 to 750°C is retained for 2 to 20 seconds in a temperature range of 600 to 750°C.
• the hot-rolled steel sheet is held at 600 to 750°C, the thickening of C into the bainite and retained austenite can be promoted.
• the retention time is less than 2 seconds, since the thickening of C into the austenite is insufficient, proper fractions of the ferrite, bainite and retained austenite cannot be maintained.
• the retention time exceeds 20 seconds the ferrite transformation excessively proceeds and pearlite is generated, resulting in deterioration of the elongation properties and stretch flangeability.
• the preferable retention time is 4 to 10 seconds. In order to hold in the above temperature range for 2 to 20 seconds, atmospheric cooling (radiational cooling) is only necessary after the first step cooling is stopped, or a heating device may be used to keep hot.
• the hot-rolled steel sheet is cooled at a cooling rate of 15°C/sec or more to 380 to 520°C according to the second cooling step, thereafter the hot-rolled steel sheet is wound.
• the cooling rate By setting the cooling rate at 15°C/sec or more, the microstructure can be inhibited from becoming coarser.
• the martensite is inhibited from being generated and thereby the bainite is generated, and at the same time owing to the bainite transformation the retained austenite can be generated.
• the stopping temperature of the second cooling step (that is, coiling temperature) is less than 380°C, because of lowering of the coiling temperature, the hot-rolled steel sheet becomes undulating. Moreover, since the martensite is excessively generated, the stretch flangeability deteriorates. On the other hand, when the stopping temperature exceeds 520°C, since the pearlite is generated, the bainite and retained austenite are suppressed from being generated, resulting in deterioration of the elongation properties and stretch flangeability.
• the preferable stopping temperature of the second cooling step (that is, coiling temperature) is preferable to be 400 to 500°C.
• Steel slabs A through D are examples that satisfy component ranges according to the invention.
• steel slab E is an example whose S content is deviated from the range of the invention
• steel slab F is an example in which the equation (1) is not satisfied and contents of Si and Ti are outside of the ranges of the invention
• steel slab G is an example whose contents of C and Mn are outside of the range of the invention
• steel slab H is an example in which contents of Si and Al are outside of the ranges of the invention
• steel slab I is an example in which the equation (1) is not satisfied and C content is deviated from the range of the invention
• steel slab J is an example in which the equation (1) is not satisfied.
• the steel slabs are hot-rolled under various conditions, and thereby hot-rolled steel sheets having thickness of 2.9 mm are produced.
• Conditions of the hot rolling are as shown in Tables 2 and 3.
• a test piece is sampled from each of thus obtained hot-rolled steel sheets, and grain size and fraction of ferrite are measured.
• the grain size measurement is performed as follows. That is, after an electron microgram is taken of a section in a rolling direction, according to an intercept method in a method for estimating ferrite grain size defined in JIS G0552, the grain size is measured. An area rate of ferrite is obtained according to an image analysis of the electron microgram, and the area rate is regarded as a fraction thereof. Results are shown in Tables 2 and 3.
• microstructures of phases other than the ferrite, the bainite fraction, the retained austenite fraction and the martensite fraction are estimated.
• the microstructure of the second phase is estimated with an electron microscope.
• the bainite fraction is estimated by applying image analysis to an electron microgram.
• the retained austenite fraction is calculated from integrated intensities of (200) and (220) planes of the austenite phase and (200) and (211) planes of the ferrite phase obtained with K alpha line of Co by use of an X-ray diffractometer.
• the martensite fraction is estimated by image analyzing the electron microgram. Results thereof are shown in Tables 2 and 3.
• JIS No. 5 test piece is sampled in a rolling width direction (that is, a direction orthogonal to a rolling direction) of the hot-rolled steel sheet and tensile test is carried out therewith. Results thereof are shown in Tables 2 and 3.
• a hot-rolled steel sheet that satisfies, in addition to the tensile strength TS of 780 MPa class or furthermore 980 MPa class, TS ⁇ EL ⁇ 20000 MPa% and TS ⁇ ⁇ ⁇ 82000 MPa%, that is, a high-tensile strength hot-rolled steel sheet excellent in the elongation properties and the stretch flangeability can be obtained.

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