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
  • 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
  • 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
• • 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
  • C22C38/001—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
  • 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
• • 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

• a hot rolled thick steel sheet herein is a hot rolled steel sheet having a sheet thickness of 6 mm or more and 12 mm or less, which is a relatively thick hot rolled steel sheet.
• Such a hot rolled thick steel sheet is suitably used as a material for manufacturing structural components of, for example, automobiles and construction equipment (hereinafter also, referred to as “construction machines”).
• die quench in which parts are quenched while being pressed, has been put to practical use as a means for strengthening small thin-walled parts.
• die quench is applied to large thick-walled parts, there are various problems in that huge equipment needs to be prepared, desired strength cannot be achieved because parts are not quenched to their center due to their thick wall, and the brittle failure unique to thick-walled parts is caused when the parts are as quenched.
• die quench is unsuitable for large thick-walled parts.
• Japanese Unexamined Patent Application Publication No. 2002-309344 discloses a method for manufacturing a thin steel sheet including a step of hot-rolling a steel material at a coiling temperature of 720° C. or less, the steel material containing C: 0.10 to 0.37% and proper amounts of Si, Mn, P, S, and Al and containing B and N so as to satisfy 14B/10.8N: 0.50 or more, wherein BN that is an intrasteel precipitate has an average grain size of 0.1 ⁇ m or more, and prior austenite after quenching has a grain size of 2 to 25 ⁇ m.
• a thin steel sheet having excellent hardenability at a low temperature for a short time after processing, excellent toughness after quenching, and little variation of characteristics according to quenching conditions can be manufactured.
• Japanese Unexamined Patent Application Publication No. 2002-309345 discloses a method for manufacturing a thin steel sheet that is excellent in toughness for impact after quenching, the method including a step of hot-rolling a steel material at a coiling temperature of 720° C. or less, the steel material containing C: 0.10 to 0.37% and proper amounts of Si, Mn, P, S, Al, and Ti and containing B and N so as to satisfy effective B amount: 0.0005% or more, wherein TiN that is an intrasteel precipitate has an average grain size of 0.06 to 0.30 ⁇ m, and prior austenite after quenching has a grain size of 2 to 25 ⁇ m.
• a thin steel sheet having excellent hardenability at a low temperature for a short time after processing, excellent toughness for impact after quenching, and little variation of characteristics according to quenching conditions can be manufactured.
• a hot rolled thick steel sheet used for large thick-walled parts such as structural components of automobiles, construction machines, and the like needs to be quenched after hot rolling.
• quenching after hot rolling causes the cooling rate at an outer layer of the steel sheet (particularly around the edges in a sheet width direction) to become too high, which causes martensitic transformation.
• the outer layer of the steel sheet is hardened, and a hot rolled steel sheet partially having large deviation of hardness along thickness is obtained.
• a hot rolled thick steel sheet that is “excellent in strength and toughness after heat treatment” herein is a hot rolled steel sheet having high strength and high ductility, specifically a tensile strength of 980 MPa or more and an elongation of 15% or more (GL: 50 mm) in a typical water quenching and tempering treatment (about 930° C. heating water quenching-about 200° C. tempering); and having a high toughness, specifically a ductile-brittle fracture transition temperature vTrs of ⁇ 60° C. or less in a Charpy impact test.
• the heat treatment conditions applied to components composed of the steel sheet are not limited to the above-described typical water quenching and tempering treatment (about 930° C. heating water quenching-about 200° C. tempering).
• desired heat treatment conditions such as about 930° C. heating water quenching-about 400° C. tempering can be used.
• the microstructure can form a bainitic ferrite single phase that is uniform across the entire thickness, whereby the deviation of hardness along thickness comes within 10% from the average.
• the hot rolled steel sheet is mainly used for large structural components of automobiles, construction machines, and the like, the sheet thickness is limited to 6 mm or more and 12 mm or less.
• C is an element that forms a carbide in a steel and effectively contributes to an increase in the strength of a steel sheet.
• C is an element that facilitates martensitic transformation and effectively contributes to strengthening of a microstructure caused by a martensitic phase.
• a C content of 0.10% or more is necessary. When the C content is less than 0.10%, it is difficult to achieve desired sheet strength (tensile strength: 440 MPa or more) and desired strength after heat treatment (tensile strength: 980 MPa or more). On the other hand, when the C content is more than 0.20%, the sheet strength and the strength after heat treatment become too high, which reduces formability and toughness, thereby decreasing weldability. Thus, the C content is limited to 0.10 to 0.20%. Si: 0.01 to 1.0%
• Si is an element that effectively contributes to an increase in the strength of steel through solution hardening.
• a Si content of 0.01% or more is necessary to produce such an effect.
• the Si content is more than 1.0%, unevenness called a red scale is formed on a surface and surface properties are degraded. This decreases elongation and fatigue strength.
• the Si content is limited to 0.01 to 1.0%.
• the Si content is 0.35% or less.
• Mn is an element that effectively contributes to an increase in the strength of steel through solution hardening and an increase in the strength of steel through the improvement in hardenability.
• a Mn content of 0.5% or more is necessary to produce such an effect.
• the Mn content is more than 2.0%, segregation appears markedly and it is difficult to form a bainitic ferrite single phase across the entire thickness. Consequently, the characteristics of a steel sheet and the quality of a material after heat treatment are degraded.
• the Mn content is limited to 0.5 to 2.0%.
• the Mn content is 1.0 to 2.0%.
• the P content is preferably reduced as much as possible, but excess reduction increases material costs.
• the P content is more than 0.03%, segregation appears markedly.
• the P content is limited to 0.03% or less.
• the P content is 0.02% or less.
• the S content is present as a sulfide in steel and decreases ductility, thereby reducing bending workability and the like. Therefore, the S content is preferably reduced as much as possible, but excess reduction increases material costs. When the S content is more than 0.01%, toughness after heat treatment is significantly reduced. Thus, the S content is limited to 0.01% or less. Preferably, the S content is 0.005% or less.
• Al is an element that functions as a deoxidizer. Such an effect markedly appears when an Al content is 0.01% or more. However, an Al content of more than 0.1% decreases formability and hardenability. Thus, the Al content is limited to 0.01 to 0.1%. Preferably, the Al content is 0.05% or less.
• N decreases formability by forming nitrides such as TiN and AlN in steel. N also reduces the amount of B solid solution that is effective for improving hardenability by forming BN during quenching. Such an adverse effect of N is permissible when the N content is 0.005% or less. Thus, the N content is limited to 0.005% or less.
• Ti is an element that effectively contributes to allowing a microstructure after hot rolling to be constituted by bainitic ferrite and that contributes to producing an effect of improving hardenability through a B solid solution because Ti forms a nitride prior to B. Such effects are produced when a Ti content is 0.01% or more. However, a Ti content of more than 0.15% increases deformation resistance during hot rolling and excessively increases rolling load, thereby decreasing toughness after heat treatment. Thus, the Ti content is limited to 0.01 to 0.15%. Preferably, the Ti content is 0.03 to 0.10%.
• B is an element that suppresses the formation of polygonal ferrite and pearlite during cooling performed after hot rolling and that effectively contributes to improving hardenability and toughness during heat treatment.
• a B content is 0.0010% or more.
• a B content of more than 0.0050% increases deformation resistance during hot rolling and excessively increases rolling load.
• such a B content forms bainite and martensite after hot rolling and poses a problem such as sheet cracking.
• the B content is limited to 0.0010 to 0.0050%.
• the B content is 0.0015 to 0.0040%.
• the balance other than the components described above is Fe and incidental impurities.
• Fe incidental impurities
• Cu 0.3% or less
• Cr 0.3% or less are permissible as incidental impurities.
• the hot rolled thick steel sheet has the above-described composition and a bainitic ferrite single phase across the entire thickness.
• a single phase herein is constituted by a bainitic ferrite phase having an area ratio of 95% or more.
• a bainitic ferrite phase includes needle-shaped ferrite and acicular ferrite. Note that 5% or less of a polygonal ferrite phase, a pearlite phase, a cementite phase, a bainite phase, a martensite phase, and the like on an area ratio basis are permissible as a microstructure other than the bainitic ferrite phase.
• a hot rolled thick steel sheet By forming a bainitic ferrite single phase across the entire thickness, a hot rolled thick steel sheet can be provided that has desired high strength and high ductility, specifically a tensile strength of 440 MPa or more and 640 MPa or less and an elongation of 20% or more (GL: 50 mm), that is excellent in formability such as a flexural property, and that can be processed into large thick-walled parts such as structural components of automobiles, construction machines, and the like.
• the area ratio of the bainitic ferrite phase is less than 95%, both the desired high strength and high ductility cannot be achieved.
• the phase fraction of the bainitic ferrite phase is decreased to less than 95%, the uniformity of the microstructure is reduced.
• the area ratios of a bainitic ferrite phase are obtained at a depth of 0.1 mm from the surface, at a position of a quarter the way through the sheet thickness, and at a position of a half the way through the sheet thickness. When the area ratios are 95% or more at all of the three positions, it is judged that a bainitic ferrite single phase is formed across the entire thickness.
• a molten steel having the above-described composition is preferably smelted by a typical smelting method using a converter, a vacuum melting furnace, or the like to make a steel material such as a slab through a typical casting method such as continuous casting or an ingot making-blooming method.
• a typical smelting method using a converter, a vacuum melting furnace, or the like to make a steel material such as a slab through a typical casting method such as continuous casting or an ingot making-blooming method.
• the method for making a steel material is not limited to this example, and any typical method for making a steel material can be suitably applied.
• a steel material having the above-described composition is hot-rolled to obtain a hot rolled thick steel sheet having a sheet thickness of 6 mm or more and 12 mm or less.
• the sheet thickness is more than 12 mm, a sufficient reduction ratio is not achieved in hot rolling and the microstructure is coarsened after the hot rolling, which tends to produce martensite during cooling.
• the sheet thickness is preferably 12 mm or less.
• the heating temperature for hot rolling is not particularly limited, and a finisher delivery temperature in hot rolling described below needs only to be ensured.
• the heating temperature is preferably 1000 to 1300° C., which is a typical heating temperature. When the heating temperature is more than 1300° C., crystal grains are coarsened and hot formability is easily decreased.
• the heating temperature is less than 1000° C.
• deformation resistance is excessively increased and a burden on rolling equipment is increased, which easily poses a problem such as a difficulty in rolling.
• the heating temperature is less than 1000° C.
• TiC that is present in a steel material is insufficiently melted, which easily causes a difficulty in achieving a desired microstructure and desired strength after hot rolling.
• the finisher delivery temperature of finish rolling is 820 to 880° C.
• the finisher delivery temperature of finish rolling is 820° C. or more
• ferrite transformation is suppressed in the following cooling step.
• a bainitic ferrite phase (bainitic ferrite single phase) having an area ratio of 95% or more can be formed.
• the finisher delivery temperature of finish rolling is less than 820° C.
• ferrite transformation is facilitated in the following cooling step.
• a bainitic ferrite single phase is not easily formed.
• the finisher delivery temperature of finish rolling is more than 880° C., not only ferrite transformation but also bainitic ferrite transformation is suppressed.
• a bainitic ferrite single phase is not easily formed and a bainite phase and a martensite phase are easily formed.
• the formation of a bainite phase and a martensite phase may excessively increase the strength of a steel sheet and cause cracking on a steel sheet in coiling or rewinding, of a coil.
• the finisher delivery temperature of finish rolling is limited to 820 to 880° C.
• the hot rolled steel sheet is cooled at a cooling rate of 15 to 50° C./s on a sheet surface temperature basis until a surface temperature reaches a temperature range of 550 to 650° C.
• a cooling rate is adjusted so as to be 15° C./s or more on a sheet surface temperature basis in the cooling performed after the completion of rolling.
• the cooling rate is less than 15° C./s on a surface temperature basis, a polygonal ferrite phase is easily precipitated, for example, in the center in a sheet thickness direction, which makes it difficult to form a uniform bainitic ferrite single phase in a sheet thickness direction.
• the cooling rate is more than 50° C./s on a surface temperature basis, martensite is produced on an outer layer and a uniform bainitic ferrite single phase cannot be formed in a sheet thickness direction.
• the deviation of hardness along thickness becomes significant and it is difficult to adjust the deviation of hardness along thickness to be within 10% from the arithmetic mean hardness (average) in a sheet thickness direction.
• water cooling is adopted.
• the cooling rate is preferably adjusted by changing the amount and time of water injection. For this reason, in the cooling performed after the completion of rolling, the cooling rate is adjusted to 15 to 50° C./s on a sheet surface temperature basis.
• the above-described cooling rate on a surface temperature basis is an average value of actually measured surface temperatures between the finisher delivery temperature of finish rolling and the cooling stop temperature.
• the above-described cooling stop temperature is in a temperature range in which the surface temperature of a steel sheet is 550 to 650° C.
• the cooling stop temperature is less than 550° C. on a surface temperature basis, a bainite phase and a martensite phase are produced and a bainitic ferrite single phase cannot be formed.
• cracking is caused on a hot rolled steel sheet during coiling and the formability of a steel sheet is decreased due to too high strength.
• the cooling stop temperature is more than 650° C., a polygonal ferrite phase and a pearlite phase are produced and a bainitic ferrite single phase cannot be formed.
• the strength of a steel sheet may fall short of desired strength.
• the cooling stop temperature after the completion of rolling is limited to a temperature range of 550 to 650° C.
• the hot rolled steel sheet is coiled in the temperature range.
• the coiling temperature is less than 550° C.
• a bainite phase and a martensite phase are produced and a bainitic ferrite single phase cannot be formed.
• the coiling temperature is more than 650° C.
• a polygonal ferrite phase and a pearlite phase are produced and a bainitic ferrite single phase cannot be formed. Consequently, the desired strength of a steel sheet cannot be achieved and the uniformity in a sheet thickness direction is decreased.
• the coiling temperature is limited to a temperature range of 550 to 650° C. on a sheet surface temperature basis.
• the obtained hot rolled steel sheet was evaluated for strength, ductility, the uniformity of hardness in a sheet thickness direction, and formability (bending workability) by performing a microstructure observation, a tensile test, a hardness test, and a bending test. Furthermore, after a test panel was prepared from the obtained hot rolled steel sheet and then pickled to remove scales on the steel sheet surface, heat treatment (quenching-tempering treatment) was performed. The test panel was evaluated for strength, ductility, and toughness after heat treatment by performing a microstructure observation, a tensile test, and an impact test. The heat treatment was constituted by quenching and tempering. In the quenching treatment, the test panel was heated to 930° C.
• test panel was heated to 200° C. and held for 60 minutes, and then cooled in the air. After the cooling, a test piece was prepared from the test panel to perform the tests.
• the test methods are as follows.
• a JIS No. 5 test piece (GL: 50 mm) was prepared from the obtained hot rolled steel sheet (or the test panel) such that the pulling direction was perpendicular to the rolling direction.
• a tensile test was performed in conformity to JIS Z 2241.
• Tensile characteristics (yield strength YS, tensile strength TS, and elongation El) were obtained to evaluate strength and ductility.
• a test piece for hardness measurement was prepared from the obtained hot rolled steel sheet, and sheet sections that were parallel to the rolling direction of the test piece were then polished.
• the hardness measurement was started at a position of 0.2 mm from a surface. When a point to be measured next reached a position within 0.2 mm from another surface, the point was not measured and the hardness measurement was finished.
• the average hardness (average value) HV mean of the hot rolled steel sheet was calculated by averaging the obtained hardness values in the sheet thickness direction using an arithmetic mean.
• the difference ⁇ HV between the maximum hardness and the minimum hardness was calculated to obtain [ ⁇ HV/HV mean ] ⁇ 100(%).
• the uniformity in the sheet thickness direction was evaluated.
• a test piece for a bending test (size: sheet thickness t ⁇ 100 ⁇ 200 mm) was prepared from the obtained hot rolled steel sheet such that a direction perpendicular to the rolling direction was a longitudinal direction of the test piece.
• 180 degree bending was performed at various bend radii such as bend radii of 0.5 times, 1.0 time, 1.5 times, and 2.0 times the sheet thickness such that the longitudinal direction of the test piece was a circumferential direction.
• the minimum bend radius was expressed as a ratio to the sheet thickness of the test piece.
• a V-notch test piece was prepared from the obtained test panel in conformity to JIS Z 2242 such that the longitudinal direction of the test piece was perpendicular to the rolling direction.
• a Charpy impact test was performed to obtain a ductile-brittle fracture transition temperature vTrs (° C.), which is a temperature at which percent ductile fracture is 50%. Thus, the toughness after heat treatment was evaluated.
• a bainitic ferrite phase having an area ratio of 95% or more (bainitic ferrite single phase) is uniformly formed in a sheet thickness direction, whereby there is provided a high strength hot rolled thick steel sheet with excellent formability that has a tensile strength of 440 MPa or more and an elongation of 20% or more; that is excellent in uniformity because the deviation of hardness ⁇ HV along thickness is within 10% from the average hardness value (average) HV mean ; and that is excellent in bending workability with a minimum bend radius of 0.5t or less.
• high strength with a tensile strength of 980 MPa or more, high ductility with an elongation of 15% or more, and high toughness with a vTrs of ⁇ 60° C. or less can be achieved by performing quenching and tempering treatment.
• a uniform bainitic ferrite phase is not formed and “strength or ductility” or “strength and ductility” do not reach the above-described desired values.
• the deviation of hardness ⁇ HV along thickness becomes large and the uniformity in the sheet thickness direction is decreased.
• one or more of strength, ductility, and toughness after quenching and tempering treatment do not reach the above-described desired values, which provides a hot rolled steel sheet that lacks any of strength, ductility, and toughness after quenching and tempering treatment.

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• 2007
  • 2007-06-29 JP JP2007171898A patent/JP5040475B2/ja active Active
• 2008
  • 2008-06-06 WO PCT/JP2008/060805 patent/WO2009004909A1/fr active Application Filing
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