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
  • C21D8/0263—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
  • 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
  • C22C38/001—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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • 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
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C21D2211/009—Pearlite

Definitions

• the present invention relates to a high-strength steel sheet suitable for use as a strength member for automobile parts, which requires excellent workability (stretch flangeability), and a method for producing the same.
• DP steel sheet duplex steel sheet having a two-phase structure composed of a ferrite phase and a martensite phase
• a steel sheet having a composite structure including a ferrite phase, a martensite phase, and a bainite phase Composite steel sheets have been proposed.
• Patent Document 1 C: 0.08 to 0.30%, Si: 0.1 to 2.5%, Mn: 0.5 to 2.5%, P: 0.01 to 0.15 the cold-rolled steel sheet of percent composition containing, recrystallization annealing at a c1 point or higher, then, after forced cooling to a temperature range in the range of a r1 point to 600 ° C., the cooling rate of more than 100 ° C.
• the quenching start temperature is increased to increase the volume ratio of the low-temperature transformation generation phase, and then an overaging treatment is performed at 350 to 600 ° C. to precipitate C in the ferrite, and the low temperature
• the transformation generation phase is softened to reduce Hv (L) / Hv ( ⁇ ) and improve local elongation.
• Patent Document 2 C: 0.02 to 0.25%, Si: 2.0% or less, Mn: 1.6 to 3.5%, P: 0.03 to 0.20%, S : 0.02% or less, Cu: 0.05 to 2.0%, sol.
• a steel slab containing Al: 0.005 to 0.100% and N: 0.008% or less is hot-rolled to form a hot-rolled coil. After pickling, the hot-rolled coil is 720 to 950 ° C. in a continuous annealing line. Describes a method for producing a low-yield ratio, high-tensile hot-rolled steel sheet excellent in corrosion resistance that is annealed at a temperature of 5 ° C. According to the technique described in Patent Document 2, a high-tensile hot-rolled steel sheet having a composite structure that maintains a low yield ratio, high ductility, and good hole expandability, and has excellent corrosion resistance can be manufactured.
• Patent Document 3 discloses that C: 0.03 to 0.17%, Si: 1.0% or less, Mn: 0.3 to 2.0%, P: 0.010% or less, S: 0.0.
• a high-strength cold-rolled steel sheet having a structure and satisfying (second-phase Vickers hardness) / (ferrite-phase Vickers hardness) less than 1.6 and having an excellent strength-stretch flangeability balance is described.
• the high-strength cold-rolled steel sheet described in Patent Document 3 is obtained by hot rolling a steel (slab) having the above composition, winding it at a temperature of 650 ° C. or lower, pickling it, and then cold rolling, , a 1 point or more, (a 3 point + 50 ° C.) soaking at a temperature, then slowly cooled below 20 ° C. / s until temperature T 1 of between the range of 750 ⁇ 650 ° C., then, from T 1 It is said that it can be obtained by performing an annealing treatment of cooling to 500 ° C. at a rate of 20 ° C./s or more, and subsequently overaging at a temperature of 500 to 250 ° C.
• JP-A-63-293121 JP 05-111282 A Japanese Patent Laid-Open No. 10-60593
• Patent Document 1 requires a continuous annealing facility capable of rapid cooling (quenching) after recrystallization annealing and suppresses a rapid strength decrease due to an overaging treatment at a high temperature.
• a large amount of alloying element is required.
• Patent Document 2 it is essential to add a large amount of P and Cu in combination.
• P has a strong tendency to segregate in the steel, and this segregated P has a problem of causing embrittlement of the welded part in addition to lowering the stretch flangeability of the steel sheet.
• the high-strength cold-rolled steel sheet described in Patent Document 3 is excellent in stretch flangeability, but in the case of a high strength of 540 MPa or more, the elongation is less than 26%, and the desired excellent workability can be maintained. There is a problem that sufficient growth cannot be secured.
• An object of the present invention is to solve such problems of the prior art and to provide a high-strength steel sheet having a thin plate thickness of about 1.0 to 1.8 mm and excellent workability, and a method for producing the same.
• “high strength” refers to a case where the tensile strength TS is 540 MPa or more, preferably 590 MPa or more, and “excellent workability” means elongation El: 30% or more. (When a JIS No. 5 test piece is used), the hole expansion rate ⁇ in a hole expansion test in accordance with Japan Iron and Steel Federation standard JFST 1001-1996 is 80% or more.
• the present inventors conducted extensive research on the influence of the composition and the microstructure on the strength and workability.
• the hot-rolled sheet with the alloy element amount adjusted to an appropriate range is subjected to an annealing process and an appropriate cooling process, which are heated to an appropriate two-phase temperature range, without performing cold rolling.
• the main phase and the second phase can be made mainly of fine pearlite, which can ensure the desired high strength, greatly improve the workability, and achieve the desired elongation and desired hole expansion. It was found that a high-strength steel sheet excellent in workability that combines the efficiency can be obtained.
• the present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows. (1) In mass%, C: 0.08 to 0.15%, Si: 0.5 to 1.5%, Mn: 0.5 to 1.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, the composition composed of the balance Fe and inevitable impurities, the ferrite phase as the main phase, and at least pearlite The ferrite phase is 75 to 90%, the pearlite is 10 to 25%, and the average particle size of the pearlite is 5 ⁇ m or less. Furthermore, the high-strength steel sheet excellent in workability, wherein the pearlite is 70% or more in terms of the area ratio with respect to the total area of the second phase.
• a method for producing a high-strength steel sheet having excellent workability characterized by performing a continuous annealing step for performing a cooling treatment with a residence time of 30 to 400 s.
• the method further comprises mass: B: 0.0003 to 0.0050%. .
• a high-strength steel sheet excellent in workability which has a high strength of tensile strength TS: 540 MPa or more, an elongation of El: 30% or more, and an elongation flangeability of ⁇ : 80% or more. It can be manufactured easily and inexpensively, and has a remarkable industrial effect.
• the present invention has an effect that cold rolling can be omitted, and the manufacturing cost can be reduced and productivity can be greatly improved.
• the steel sheet according to the present invention is applied particularly to automobile body parts, it can greatly contribute to weight reduction of the automobile body.
• C 0.08 to 0.15%
• C is an element that contributes to an increase in the strength of the steel sheet and effectively acts on the formation of a composite structure composed of a ferrite phase and a second phase other than the ferrite phase.
• the desired tensile strength In order to ensure a high strength of 540 MPa or more, it is necessary to contain 0.08% or more. On the other hand, if the content exceeds 0.15%, spot weldability is lowered, and workability such as ductility is further lowered. Therefore, C is limited to the range of 0.08 to 0.15%. Note that the content is preferably 0.10 to 0.15%.
• Si 0.5 to 1.5% Si is an element that dissolves in steel and effectively acts to strengthen the ferrite, and also contributes to the improvement of ductility.
• the content of Si is 0.8. It needs to contain 5% or more.
• an excessive content exceeding 1.5% promotes the generation of red scale and the like, lowers the surface properties of the steel sheet, and lowers the chemical conversion treatment property.
• excessive inclusion of Si is accompanied by an increase in electrical resistance during resistance welding, and impedes resistance weldability. For this reason, Si was limited to the range of 0.5 to 1.5%. In addition, Preferably it is 0.7 to 1.2%.
• Mn 0.5 to 1.5%
• Mn is an element that contributes to an increase in the strength of the steel sheet and that effectively acts in the formation of a composite structure.
• the Mn content needs to be 0.5% or more.
• a content exceeding 1.5% tends to form a martensite phase in the cooling process during annealing, and causes deterioration in workability, particularly stretch flangeability.
• Mn was limited to the range of 0.5 to 1.5%. In addition, Preferably it is 0.7 to 1.5%.
• P 0.1% or less
• P is an element that has the effect of increasing the strength of the steel sheet by solid solution in steel, but has a strong tendency to segregate to the grain boundary, lowering the bonding strength of the grain boundary, and processing
• it concentrates on the surface of the steel sheet to reduce chemical conversion properties, corrosion resistance, and the like.
• Such an adverse effect of P becomes remarkable when the content exceeds 0.1%.
• P was limited to 0.1% or less.
• P is preferably 0.1% or less and preferably reduced as much as possible.
• excessive reduction leads to an increase in manufacturing cost, so it should be about 0.001% or more. Is preferred.
• S 0.01% or less S forms sulfides (inclusions) such as MnS mainly in steel and lowers the workability of the steel sheet, particularly the local elongation. In addition, the presence of sulfide (inclusions) also reduces weldability. Such an adverse effect of S becomes remarkable when the content exceeds 0.01%. For this reason, S was limited to 0.01% or less. In order to avoid such an adverse effect of S, S is preferably 0.01% or less, and is preferably reduced as much as possible. However, excessive reduction leads to an increase in manufacturing cost, so it should be about 0.0001% or more. Is preferred.
• Al acts as a deoxidizer and is an essential element for improving the cleanliness of the steel sheet, and also effectively acts for improving the yield of carbide forming elements.
• 0.01% or more of content is required. If the content is less than 0.01%, the removal of Si-based inclusions that are the starting point of delayed fracture becomes insufficient, and the risk of delayed fracture increases. On the other hand, even if the content exceeds 0.1%, the above-described effect is saturated, an effect commensurate with the content cannot be expected, and it becomes economically disadvantageous, workability is reduced, and surface defects tend to occur. Increase. For this reason, Al was limited to the range of 0.01 to 0.1%.
• the content is preferably 0.01 to 0.05%.
• N 0.005% or less N is an element that is essentially harmful in the present invention, and it is desirable to reduce it as much as possible, but up to 0.005% is acceptable. For this reason, N was limited to 0.005% or less. In addition, since excessive reduction of N causes a rise in manufacturing cost, it is preferable to make it about 0.0001% or more.
• the above components are basic components.
• Cr 0.05 to 0.5%
• V 0.005 to 0.2%
• Mo 0.005 to One or more selected from 0.2% and / or one selected from Ti: 0.01 to 0.1% and Nb: 0.01 to 0.1% Or 2 and / or B: 0.0003 to 0.0050% and / or Ni: 0.05 to 0.5%
• Cu 0.05 to 0.5% 1 type or 2 types and / or 1 or 2 types selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% are selected and contained.
• Can do
• Cr 0.05 to 0.5%, V: 0.005 to 0.2%, Mo: 0.005 to 0.2% Cr, V, and Mo are Any of them is an element that increases the strength of the steel sheet and contributes to the formation of the composite structure, and can be selected as necessary and contained in one or more kinds.
• excessive content exceeding Cr: 0.5%, V: 0.2%, Mo: 0.2%, respectively makes it difficult to produce a desired amount of pearlite during the cooling treatment after the annealing treatment, A desired composite structure cannot be secured, stretch flangeability is lowered, and workability is lowered.
• it may be limited to the ranges of Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, and Mo: 0.005 to 0.2%, respectively. preferable.
• Ti and Nb are both elements that increase the steel sheet strength by precipitation strengthening. These can be selected as necessary and can be contained alone or in combination. In order to obtain such an effect, it is desirable to contain Ti: 0.01% or more and Nb: 0.01% or more, respectively, but Ti: 0.1% and Nb: more than 0.1%, respectively. Containment reduces processability and shape freezing property. For this reason, when it contains, it is preferable to limit in the range of Ti: 0.01-0.1% and Nb: 0.01-0.1%, respectively.
• B 0.0003 to 0.0050%
• B is an element that segregates at the austenite grain boundary and has an action of suppressing the formation and growth of ferrite from the grain boundary, and can be contained as necessary. In order to acquire such an effect, it is desirable to contain 0.0003% or more, but inclusion exceeding 0.0050% reduces workability. For this reason, when contained, B is preferably limited to a range of 0.0003 to 0.0050%. In addition, in order to acquire the effect of B as mentioned above, it is necessary to suppress the production
• Ni and Cu both have an action of increasing the strength of the steel sheet, It is an element that also has an effect of promoting oxidation and improving plating adhesion, and can be selected and contained as necessary. In order to obtain such an effect, it is desirable to contain Ni: 0.05% or more and Cu: 0.05% or more, respectively, but Ni: 0.5% and Cu: 0.5% are exceeded. Containment makes it difficult to produce a desired amount of pearlite during the cooling treatment after the annealing treatment, making it impossible to secure a desired composite structure, lowering stretch flangeability, and lowering workability. For this reason, when it contains, it is preferable to limit to Ni: 0.05-0.5% and Cu: 0.05-0.5%.
• REM One or two selected from 0.001 to 0.005%
• Ca and REM are elements that contribute to sulfide morphology control. It has the effect of suppressing the adverse effect on the workability of sulfides, particularly the stretch flangeability, by making the shape of sulfides spherical.
• Excessive inclusion causes an increase in inclusions, resulting in frequent occurrence of surface defects and internal defects. For this reason, when it contains, it is preferable to limit to Ca: 0.001-0.005% and REM: 0.001-0.005%.
• the balance other than the components described above consists of Fe and inevitable impurities.
• the steel sheet of the present invention has the above-described composition and a structure composed of a ferrite phase as a main phase and a second phase containing at least pearlite.
• the area ratio of the ferrite phase that is the main phase is 75 to 90% in terms of the area ratio with respect to the entire structure. If the area ratio of the ferrite phase is less than 75%, the desired elongation and the desired hole expansion rate cannot be ensured, and the workability deteriorates. On the other hand, if the area ratio of the ferrite phase exceeds 90%, the area ratio of the second phase decreases, and a desired high strength cannot be ensured. For this reason, the area ratio of the ferrite phase as the main phase is limited to a range of 75 to 90%. A preferable area ratio of the ferrite phase is 80 to 90%.
• the second phase contains at least pearlite.
• the area ratio of pearlite is the area ratio with respect to the whole structure, and is 10 to 25%. If the area ratio of pearlite is less than 10%, a desired hole expansion rate cannot be ensured, the stretch flangeability is lowered, and the workability is lowered. On the other hand, when the area ratio of pearlite exceeds 25%, the interface between the ferrite phase and pearlite increases, voids are likely to be generated during processing, stretch flangeability decreases, and workability decreases.
• pearlite is a fine particle having an average particle size of 5 ⁇ m or less.
• the average grain size of pearlite exceeds 5 ⁇ m and becomes coarse, stress concentrates on the pearlite grains (interface) during the processing of the steel sheet, and microvoids are generated, so that stretch flangeability is lowered and workability is lowered.
• the average particle size of pearlite was limited to 5 ⁇ m or less.
• Preferably it is 4.0 micrometers or less.
• the second phase in the structure of the steel sheet of the present invention is a phase mainly composed of pearlite that contains at least pearlite, and the pearlite has an area ratio of 70% or more with respect to the total area of the second phase. If the pearlite is less than 70% in area ratio with respect to the total area of the second phase, the hard martensite phase, bainite phase, or residual ⁇ increases too much, and the workability tends to be lowered. For this reason, pearlite was limited to 70% or more in the area ratio with respect to the total area of a 2nd phase. Preferably, it is 75 to 100%.
• the second phase may contain bainite, martensite, retained austenite (residual ⁇ ), etc., but in particular, bainite and martensite are hard phases, and residual ⁇ is transformed during processing. It transforms into martensite, which degrades workability. For this reason, it is desirable that these bainite, martensite and retained austenite be as small as possible, and the total area ratio with respect to the entire structure is preferably 5% or less. More preferably, the total content is 3% or less.
• a steel material having the above composition is used as a starting material.
• the method for producing the steel material is not particularly limited, but the molten steel having the above composition is melted by a conventional melting method such as a converter or an electric furnace, and a slab or the like is obtained by a conventional casting method such as a continuous casting method. It is preferable to use a steel material from the viewpoint of productivity. It is also possible to apply an ingot-bundling rolling method, a thin slab casting method, or the like.
• the steel material having the above composition is subjected to a hot rolling process to obtain a hot rolled sheet.
• the steel material is heated to a temperature in the range of 1100 to 1280 ° C., and then hot rolled to a hot rolling finish temperature of 870 to 950 ° C. to form a hot rolled sheet,
• the hot-rolled sheet is preferably a step of winding at a winding temperature of 350 to 720 ° C. If the heating temperature of the steel material is less than 1100 ° C., the deformation resistance becomes too high, the rolling load becomes excessive, and hot rolling may be difficult.
• the heating temperature for hot rolling is preferably set to a temperature in the range of 1100 to 1280 ° C. More preferably, it is less than 1280 degreeC.
• the hot rolling finish temperature is less than 870 ° C.
• ferrite ( ⁇ ) and austenite ( ⁇ ) are generated during rolling, and a band-like structure is easily generated on the steel sheet.
• This band-like structure remains even after annealing, and may cause anisotropy in the obtained steel sheet characteristics or cause a decrease in workability.
• the hot rolling end temperature is preferably 870 to 950 ° C.
• the winding temperature after the hot rolling is less than 350 ° C.
• bainitic ferrite, bainite, martensite, etc. are generated, and it is easy to form a hard and non-sized hot rolled structure, and in the subsequent annealing treatment In some cases, it inherits the hot-rolled structure, tends to become a non-sized structure, and cannot secure the desired workability.
• the winding temperature is preferably set to a temperature in the range of 350 to 720 ° C. More preferably, the temperature is 500 to 680 ° C.
• the hot-rolled sheet obtained through the hot-rolling process is subjected to pickling according to a conventional method, and then cold-rolling the hot-rolled sheet.
• the continuous annealing process which performs an annealing process and a subsequent cooling process in a continuous annealing line directly is performed.
• the annealing process is a process of holding for 5 to 400 s in the first temperature range from the A c1 transformation point to the A c3 transformation point.
• the temperature (heating temperature) in the first temperature range of the annealing treatment is less than the Ac1 transformation point, or the holding time (annealing time) in the first temperature range is less than 5 s, hot rolling Since the carbide in the plate does not dissolve sufficiently, or the ⁇ ⁇ ⁇ transformation does not occur or is insufficient, the desired composite structure cannot be secured by the subsequent cooling treatment, so the desired elongation and hole expansion rate are satisfied. A steel sheet having ductility and stretch flangeability cannot be obtained.
• the heating temperature of the annealing process becomes higher than the Ac3 transformation point, coarsening of the austenite grains becomes remarkable, the structure generated by the subsequent cooling process becomes coarse, and workability may be lowered.
• the annealing treatment is limited to a treatment for holding for 5 to 400 s in the first temperature range from the A c1 transformation point to the A c3 transformation point.
• the A c1 transformation point of each steel plate is the following equation (1), and the A c3 transformation point is the value calculated by the following equation (2).
• the element is calculated as zero.
• a c1 transformation point (° C.) 723 + 29.1Si-10.7Mn-16.9Ni + 16.9Cr + 6.38W + 290As (1)
• Ac 3 transformation point (° C.) 910 ⁇ 203 ⁇ C + 44.7Si-30Mn + 700P + 400Al-15.2Ni-11Cr-20Cu + 31.5Mo + 104V + 400Ti + 13.1W + 120As (2)
• C, Si, Mn, Ni, Cr, W, As, C, P, Al, Cu, Mo, V, Ti Content of each element (mass%)
• the cooling treatment after the annealing treatment is performed by cooling from the first temperature range to 700 ° C. at an average cooling rate of 5 ° C./s or more, and further in the second temperature range of 700
• the cooling rate from the first temperature range to 700 ° C. is less than 5 ° C./s, the amount of ferrite produced increases too much, the desired composite structure cannot be obtained, the workability decreases, and the desired tensile strength is further reduced.
• the strength (540 MPa or more) may not be ensured.
• the cooling rate from the first temperature range to 700 ° C. was limited to 5 ° C./s or more on average.
• the temperature is preferably 20 ° C./s or less, more preferably 5 to 15 ° C./s.
• the residence time in the second temperature range of 700 ° C. to 400 ° C. is an important factor for the formation of pearlite contained in the second phase.
• the “residence time” means the time of staying in the above-mentioned second temperature range, and when holding at the specific temperature of the second temperature range, The case where it cools with a cooling rate and the case where it cools with the pattern which mixed them are included. If the residence time in the second temperature range is less than 30 s, pearlite transformation does not occur or the amount of pearlite produced is insufficient, so a desired composite structure cannot be ensured. On the other hand, when the residence time in the second temperature range is longer than 400 s, productivity is lowered.
• the residence time in the second temperature range is limited to the range of 30 to 400 s. In addition, Preferably it is 150 s or less.
• the cooling time in the temperature range of 700 to 550 ° C. is 10 s or more, that is, the cooling rate in the temperature range of 700 to 550 ° C. is 15 ° C./s or less on average. It is preferable for securing a desired amount of pearlite. If the cooling time in the temperature range of 700 to 550 ° C. is less than 10 s, the formation of pearlite becomes insufficient, the desired composite structure cannot be obtained, and the desired workability may not be ensured.
• Molten steel having the composition shown in Table 1 was melted and used as a steel material by a conventional method. These steel materials are hot-rolled at the heating temperature and hot rolling end temperature shown in Table 2 to form a 1.6 mm thick hot rolled sheet, and after hot rolling, coiled at the winding temperature shown in Table 2 Rolled up. Thereafter, pickling was performed. Some hot-rolled sheets (thickness: 3.2 mm) were pickled and then cold-rolled at a reduction ratio of 50% to obtain 1.6 mm-thick cold-rolled sheets as comparative examples. .
• the obtained hot-rolled sheet or cold-rolled sheet is further heated to the temperature in the first temperature range under the conditions shown in Table 2, and the annealing treatment to be held, from the temperature in the first temperature range to 700 ° C, Cooling is performed at the average cooling rate shown in Table 2, and 700 to 550 ° C. of the second temperature range is further cooled at the cooling rate (cooling time) shown in Table 2, and then the second temperature of 700 to 400 ° C.
• the region residence time is shown in Table 2 and the residence time is used. Cooling treatment is performed, and a continuous annealing step is performed to obtain an annealing plate.
• the transformation point of each steel plate shown in Table 2 is a value calculated using the above-described equations (1) and (2).
• Specimens were collected from the obtained annealed plate and subjected to a structure observation, a tensile test, and a hole expansion test.
• the test method was as follows.
• the average crystal grain size of pearlite is determined by measuring the area of each pearlite grain, calculating the equivalent circle diameter from the area, arithmetically averaging the equivalent circle diameter of each obtained grain, It was.
• the measured number of pearlite particles was 20 or more.
• the area ratio with respect to the total area of the second phase of pearlite was also calculated.
• All of the examples of the present invention have high tensile strength TS: 540 MPa or more, elongation El: high ductility of 30% or more, and excellent stretch flangeability of hole expansion ratio ⁇ : 80% or more. It is a high-strength steel sheet with excellent properties.
• the desired high strength is not obtained, the desired elongation is not obtained, or the desired hole expansion ratio ⁇ is not obtained. , Workability is degraded.

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