US20100319819A1 - High-strength hot-rolled steel sheet and method for manufacturing same - Google Patents

High-strength hot-rolled steel sheet and method for manufacturing same Download PDF

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US20100319819A1
US20100319819A1 US12/866,382 US86638209A US2010319819A1 US 20100319819 A1 US20100319819 A1 US 20100319819A1 US 86638209 A US86638209 A US 86638209A US 2010319819 A1 US2010319819 A1 US 2010319819A1
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steel sheet
strength
cooling
hot
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Shinjiro Kaneko
Kaneharu Okuda
Tetsuo Shimizu
Noriaki Moriyasu
Masahide Watabe
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: MORIYASU, NORIAKI, OKUDA, KANEHARU, SHIMIZU, TETSUO, WATABE, MASAHIDE, KANEKO, SHINJIRO
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    • 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
    • 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
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/14—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
    • C21D2211/004—Dispersions; Precipitations
    • 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

  • This disclosure relates to a high-intensity hot-rolled steel sheet having a tensile strength (TS) of 540 to 780 MPa, only small variations in strength, and excellent uniformity in strength between coils and within a coil, and being useful as a steel sheet for automobiles and so forth, and to a method for manufacturing the same.
  • TS tensile strength
  • Japanese Unexamined Patent Application Publication No. 4-289125 discloses the following method: In the case of hot-rolling Nb-containing low-manganese steel (Mn: 0.5% or less), a rough-rolled sheet bar is temporarily wound into a coil. Next, the sheet bar is joined to the preceding sheet bar while being unwound, and then continuously finish-rolled to achieve uniformity in the strength of the high-strength hot-rolled steel sheet in a coil.
  • Japanese Unexamined Patent Application Publication No. 2002-322541 discloses a high-strength hot-rolled steel sheet with excellent uniformity in strength, i.e., only small variations in strength, produced by the addition of both Ti and Mo to form very fine precipitates uniformly dispersed therein.
  • JP 4-289125 has a problem in which when the sheet is wound into a coil, the sheet is divided. Furthermore, the addition of Nb causes an increase in cost, which is economically disadvantageous. In the steel sheet described in JP 2002-322541, which is a Ti system, it is necessary to add Mo, which is expensive, thus causing an increase in cost. Moreover, in both of those publications, two-dimensional uniformity in strength in the in-plane directions including both of the width direction and the longitudinal direction of the coil is not taken into consideration. Disadvantageously, even if the coiling temperature is uniformly controlled, the variations in the in-plane strength of the coil are inevitably caused by different cooling histories for each position in the wound coil.
  • FIG. 1 shows the investigation results of the relationship between the volume fraction of polygonal ferrite (%) and the tensile strength TS (MPa).
  • FIG. 2 shows the investigation results of the relationship between the proportion of the amount of Ti contained in a precipitate having a size of less than 20 nm with respect to Ti* and the tensile strength TS (Mpa).
  • An example of a target steel sheet is a coiled steel sheet having a weight of five tons or more and a steel sheet width of 500 mm or more.
  • the innermost turn including the front end in the longitudinal direction, the outermost turn including the rear end in the longitudinal direction, and regions extending from both sides to 10 mm from both sides in the width direction are not evaluated.
  • Variations in the strength of the steel sheet are evaluated on the basis of tensile-strength distribution obtained from two-dimensional measurement at least 10 points in the longitudinal direction and at least 5 points in the width direction.
  • TS tensile strength
  • the units of the content of each component in the steel composition are “percent by mass” and are simply indicated by “%” unless otherwise specified.
  • C is an important element as well as Ti described below in our steel sheets.
  • C forms a carbide with Ti and is effective in increasing the strength of a steel sheet by precipitation strengthening.
  • the C content is preferably 0.05% or more and more preferably 0.06% or more from the viewpoint of precipitation strengthening.
  • a C content exceeding 0.12% liable to adversely affect satisfactory elongation and flangeability.
  • the upper limit of the C content is set to 0.12% and preferably 0.10% or less.
  • the Si is effective in enhancing solid-solution strengthening and improving ductility.
  • the Si content is effectively 0.01% or more.
  • a Si content exceeding 0.5% is liable to cause the occurrence of a surface defect called red scale during hot rolling, which can reduce the quality of surface appearance when a steel sheet is produced.
  • the Si content is preferably 0.5% or less and more preferably 0.3% or less.
  • Mn is effective in achieving higher strength and has the effect of reducing the transformation point and the ferrite grain size.
  • the Mn content needs to be 0.8% or more. More preferably, the Mn content is set to 1.0% or more. A Mn content exceeding 1.8% causes the formation of a low-temperature transformation phase after hot rolling to reduce the ductility and is liable to make TiC precipitation unstable. Thus, the upper limit of the Mn content is set to 1.8%.
  • P is an element effective for solid-solution strengthening. P also has the effect of reducing scale defects due to Si. An excessive P content more than 0.030%, however, is liable to cause the segregation of P into grain boundaries and reduce toughness and weldability. Thus, the upper limit of the P content is set to 0.030%.
  • S is an impurity and causes hot tearing. Furthermore, S is present in the form of an inclusion in steel, deteriorating the various characteristics of a steel sheet. Thus, the S content needs to be minimized. Specifically, the S content is set to 0.01% because the S content is allowable to 0.01%.
  • Al is useful as a deoxidizing element for steel.
  • Al also has the effect of fixing dissolved N present as an impurity, thereby improving resistance to room-temperature aging.
  • the Al content needs to be 0.005% or more.
  • An Al content exceeding 0.5% leads to an increase in alloy cost and is liable to cause surface defects.
  • the upper limit of the Al content is set to 0.1%.
  • N is an element which degrades the resistance to room-temperature aging and in which the N content is preferably minimized.
  • a higher N content causes a reduction in resistance to room-temperature aging.
  • To fix dissolved N it is necessary to perform the addition of large amounts of Al and Ti.
  • the upper limit of the N content is set to 0.01%.
  • Ti is an important element to strengthen steel by precipitation strengthening. Ti contributes to precipitation strengthening by forming a carbide with C.
  • fine precipitates each having a size of less than 20 nm. Furthermore, it is important to increase the proportion of the fine precipitates (each having a size of less than 20 nm).
  • precipitates having a size of 20 nm or more are less likely to provide the effect of suppressing dislocation migration and fail to sufficiently harden polygonal ferrite, which can reduce strength. It is thus preferred that the precipitates have a size of less than 20 nm.
  • the fine precipitates containing Ti and each having a size of less than 20 nm are formed by the addition of Ti and C within the above ranges.
  • the precipitates containing Ti and C are generically referred to as a “Ti-containing carbide.”
  • the Ti-containing carbide include TiC and Ti 4 C 2 S 2 .
  • the carbide may further contain N as a component and may be precipitated in combination with, for example, MnS.
  • the Ti-containing carbide is mainly precipitated in polygonal ferrite. This is probably because supersaturated C is easily precipitated as a carbide in polygonal ferrite because of a low solid-solubility limit of C in polygonal ferrite. The precipitates allow soft polygonal ferrite to harden, thereby achieving a tensile strength (TS) of 540 MPa to 780 MPa. Furthermore, Ti is readily bonded to dissolved N and thus an element suitable for fixation of dissolved N. From that standpoint, the Ti content is set to 0.030% or more.
  • composition of the balance other than the components described above be substantially iron and incidental impurities.
  • the steel sheet has a microstructure whose volume fraction of polygonal ferrite is 70% or more, and the amount of Ti in a precipitate having a size of less than 20 nm is 50% or more of the value of Ti* calculated using formula (I).
  • the strength of the high-strength hot-rolled steel sheet is determined by the superposition of the amounts of strengthening based on three strengthening mechanisms, i.e., solid-solution strengthening, microstructural strengthening, and precipitation strengthening, on the base strength of the steel itself.
  • the base strength is inherent strength of iron.
  • the amount of solid-solution strengthening is almost uniquely determined by a chemical composition. Thus, these two strengthening mechanisms are negligibly involved in the variations in strength in a coil.
  • the strengthening mechanism that is the most closely related to the variations in strength is precipitation strengthening, followed by microstructural strengthening.
  • the amount of strengthening by precipitation strengthening is determined by the size and dispersion of precipitates (specifically, precipitate spacing).
  • the dispersion of precipitates can be expressed by the amount and size of the precipitates. Thus, if the size and amount of the precipitates are determined, the amount of strengthening by precipitation strengthening will be determined.
  • Microstructural strengthening is determined by the type of steel microstructure. The type of steel microstructure is determined by a transformation-temperature range from austenite. If a chemical composition and a steel microstructure are determined, the amount of strengthening will be determined.
  • each sheet bar was subjected to natural cooling for 10 seconds.
  • Each sheet bar was inserted into an electric furnace having a temperature of 500° C. to 700° C. and wound.
  • the holding time in the furnace was changed between 1 and 300 minutes.
  • water cooling was performed at a cooling rate of 25° C./s in such a manner that the sheet bar had a temperature 30° C. higher than the furnace temperature.
  • Hot-rolled steel sheets having different precipitation states of Ti and different steel microstructures were manufactured by the method described above. The hot-rolled steel strips were subjected to pickling and then temper rolling at an elongation of 0.5%. Test pieces for a tensile test and analytical samples of precipitates were taken.
  • FIG. 1 shows the investigation results of the relationship between the volume fraction of polygonal ferrite (%) and the tensile strength TS (MPa). As shown in FIG. 1 , the tensile strength TS tends to decrease as the volume fraction of polygonal ferrite increases. At a volume fraction of polygonal ferrite of 70% or more, a change in TS is small, and TS is stabilized.
  • the volume fraction of polygonal ferrite can be determined as follows. A portion of an L section (a section parallel to a rolling direction) of a steel sheet, the portion excluding surface layers each having a thickness equal to 10% of the thickness of the sheet, is etched with 5% nital. The microstructures of the etched portion are photographed with a scanning electron microscope (SEM) at a magnification of 1000 ⁇ . Smooth ferrite crystal grains in which grain boundaries have a small step height of less than 0.1 ⁇ m and corrosion marks are not left in the grains are defined as polygonal ferrite. Polygonal ferrite is distinguished from other ferrite phases and different transformation phases such as pearlite and bainite. These are color-coded with image-analysis software. The area ratio is defined as the volume fraction of polygonal ferrite.
  • FIG. 2 shows the investigation results of the relationship between the proportion of the amount of Ti contained in a precipitate having a size of less than 20 nm with respect to Ti* expressed as formula (1) described below and the tensile strength TS (MPa).
  • the precipitates each having a size of less than 20 nm and contributing to precipitation strengthening are composed of added Ti.
  • TS tensile strength
  • TS tends to increase as the amount of Ti contained in the precipitate having a size of less than 20 nm increases.
  • the amount of Ti contained in the precipitate is 50% or more of Ti*, a change in TS is small, and TS is stabilized.
  • [Ti] and [N] represent a Ti content (percent by mass) and a N content (percent by mass), respectively, of the steel sheet.
  • a steel sheet has microstructures whose volume fraction of polygonal ferrite is 70% or more and that the amount of Ti contained in a precipitate having a size of less than 20 nm is 50% or more of Ti* expressed as formula (1) described above are met, at any position of a steel sheet, even if the cooling histories of a coil are different for each position, substantially the same amount of strengthening is obtained at any position of the steel sheet.
  • the steel sheet has only small variation in strength and excellent uniformity in strength.
  • the amount of Ti contained in a precipitate having a size of less than 20 nm can be measured by a method described below.
  • the test piece After a sample is electrolyzed in an electrolytic solution by a predetermined amount, the test piece is taken out of the electrolytic solution and immersed in a solution having dispersibility. Then precipitates contained in this solution are filtered with a filter having the pore size of 20 nm. Precipitates passing through the filter having a pore size of 20 nm together with the filtrate each have a size of less than 20 nm. After the filtration, the filtrate is appropriately analyzed by inductively-coupled-plasma (ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry, or the like to determine the amount of Ti in the precipitates having a size of less than 20 nm.
  • ICP inductively-coupled-plasma
  • the composition of a steel slab used in the manufacturing method is the same as the composition of the steel sheet described above. Furthermore, the reason for the limitation of the composition is the same as above.
  • the high-strength hot-rolled steel sheet is manufactured through a hot-rolling step of subjecting a raw material to rough hot rolling to form a hot-rolled steel sheet, the raw material being the steel slab having a composition within the range described above.
  • the hot-rolled steel sheet is preferably heated to 1150° C. or higher such that an undissolved Ti-containing carbide, such as TiC, may not be present in a heating stage.
  • an undissolved Ti-containing carbide such as TiC
  • the upper limit of the heating temperature of the slab is preferably set to 1300° C.
  • the steel slab heated under the foregoing conditions is subjected to hot rolling in which rough rolling and finish rolling are performed.
  • the steel slab is formed into a sheet bar by the rough rolling.
  • the conditions of the rough rolling need not be particularly specified.
  • the rough rolling may be performed according to a common method. It is preferred to use what is called a “sheet-bar heater” from the viewpoint of reducing the heating temperature of the slab and preventing problems during the hot rolling.
  • the sheet bar is subjected to finish rolling to form a hot-rolled steel sheet.
  • a high finishing temperature results in coarse grains to reduce formability and is liable to cause scale defects.
  • the finishing temperature is set to 950° C. or lower.
  • a finishing temperature of less than 800° C. results in an increase in rolling force to increase the rolling load and an increase in rolling reduction to develop an abnormal texture in austenite non-recrystallization, which is not preferred from the viewpoint of achieving uniform strength.
  • the finishing temperature is set in the range of 800° C. to 950° C. and preferably 840° C. to 920° C.
  • Lubrication rolling is effective from the viewpoint of improving uniformity in the shape of a steel sheet and uniformity in strength.
  • the coefficient of friction during the lubrication rolling is preferably in the range of 0.10 to 0.25.
  • a continuous rolling process is preferred in which a preceding sheet bar and a succeeding sheet bar are joined to each other and then the joined sheet bars are continuously finish-rolled. The use of the continuous rolling process is desirable from the viewpoint of achieving the stable operation of the hot rolling.
  • Cooling primary cooling at a cooling rate of 20° C./s or more within 2 seconds after finish hot rolling
  • the natural cooling temperature is set in the range of 650° C. to 750° C.
  • a natural cooling time of less than 2 seconds results in an insufficient amount of the Ti-containing carbide precipitating. It is thus difficult to ensure the amount of strengthening required.
  • a natural cooling time exceeding 15 seconds causes a reduction in strengthening ability because coarse Ti-containing carbide grains are sparsely distributed. Therefore, the natural cooling time is set in the range of 2 seconds to 15 seconds.
  • Cooling (secondary cooling) at a cooling rate of 100° C./s
  • the cooling rate subsequent to the natural cooling treatment is 100° C. or more, the controllability of a coiling temperature is reduced, causing difficulty in achieving stable strength.
  • the cooling rate is set to less than 100° C./s.
  • the lower limit of the cooling rate is not particularly limited but is 5° C./s or more from the viewpoint of inhibiting the coarsening of precipitates.
  • the coiling temperature is set in the range of 550° C. to 650° C.
  • the precipitation of the Ti-containing carbide such as TiC proceeds mainly in a cooling stage after completion of the winding.
  • the front end and the rear end of the coil are rapidly cooled so that precipitation of the Ti-containing carbide does not sufficiently proceed in some cases.
  • the temperatures of the front and rear ends of the coil are increased with respect to the temperature of the inner portion of the coil other than the front and rear ends, thereby further improving the variations in strength.
  • Molten steels having compositions shown in Table 1 were made with a converter and formed into slabs by a continuous casting process. These steel slabs were heated to 1250° C. and rough-rolled into sheet bars. Then the resulting sheet bars were subjected to a hot-rolling step in which finish rolling was performed under conditions shown in Table 2, thereby forming hot-rolled steel sheets.
  • test pieces for a tensile test were taken in a direction (L direction) parallel to a rolling direction and processed into JIS No. 5 test pieces.
  • the tensile test was performed according to the regulation of JIS Z 2241 at a crosshead speed of 10 mm/min to determine tensile strength (TS).
  • Table 2 shows the investigation results of tensile properties of the resulting hot-rolled steel sheets.
  • microstructures With respect to microstructures, a portion of an L section (a section parallel to a rolling direction) of each of the steel sheets, the portion excluding surface layers each having a thickness equal to 10% of the thickness of the sheet, was etched with nital. The microstructures of the etched portion were identified with a scanning electron microscope (SEM) at a magnification of 1000 ⁇ . The volume fraction of polygonal ferrite was measured by the method described above with image processing software.
  • the quantification of Ti in a precipitate having a size of less than 20 nm was performed by a quantitative procedure described below.
  • test pieces Each of the test pieces was subjected to constant-current electrolysis in a 10% AA-containing electrolytic solution (10 vol % acetylacetone-1 mass % tetramethylammonium chloride-methanol) at a current density of 20 mA/cm 2 so as to be reduced in weight by about 0.2 g.
  • 10% AA-containing electrolytic solution (10 vol % acetylacetone-1 mass % tetramethylammonium chloride-methanol) at a current density of 20 mA/cm 2 so as to be reduced in weight by about 0.2 g.
  • each of the test pieces having surfaces to which precipitates adhered was taken from the electrolytic solution and immersed in an aqueous solution of sodium hexametaphosphate (500 mg/l) (hereinafter, referred to as an “SHMP aqueous solution”). Ultrasonic vibration was applied thereto to separate the precipitates from the test piece. The separated precipitates were collected in the SHMP aqueous solution.
  • the SHMP aqueous solution containing the precipitates was filtered with a filter having a pore size of 20 nm. After the filtration, the resulting filtrate was analyzed with an ICP emission spectrometer to measure the absolute quantity of Ti in the filtrate.
  • the absolute quantity of Ti was divided by an electrolysis weight to obtain the amount of Ti (percent by mass) contained in the precipitates each having a size of less than 20 nm.
  • the electrolysis weight was determined by measuring the weight of the test piece after the separation of the precipitates and subtracting the resulting weight from the weight of the test piece before electrolysis.
  • the resulting amount of Ti (percent by mass) contained in the precipitates each having a size of less than 20 nm was divided by Ti* calculated by substituting the Ti content and the N content shown in Table 1 in formula (1), thereby determining the proportion (%) of the amount of Ti contained in the precipitates each having a size of less than 20 nm.
  • values of the proportion of the volume fraction of polygonal ferrite, the amount of Ti contained in precipitates each having a size of less than 20 nm with respect to Ti* expressed as formula (1), and the tensile strength TS are defined as representative values at a middle portion in the longitudinal and transverse directions.
  • the proportion of compliant steel microstructures is defined as the proportion of points where both requirements of the volume fraction of polygonal ferrite and the proportion of the amount of Ti in the precipitates each having a size of less than 20 nm is satisfied to 189 measurement points.
  • the proportion of compliant TS is defined as the proportion of points where TS is 540 MPa or more to 189 measurement points.
  • ⁇ TS is a value obtained by determining the standard deviation ⁇ of TS values at 189 measurement points and multiplying the standard deviation ⁇ by 4.
  • TS tensile strength
  • automotive parts reduces variations in the amount of springback after formation using the high-tensile steel sheet and variations in collision characteristics, thus making it possible to design automobile bodies with higher accuracy and to contribute sufficiently to the collision safety and weight reduction of automobile bodies.

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JP2008028455A JP5194858B2 (ja) 2008-02-08 2008-02-08 高強度熱延鋼板およびその製造方法
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PCT/JP2009/052244 WO2009099237A1 (ja) 2008-02-08 2009-02-04 高強度熱延鋼板およびその製造方法

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