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
  • 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
  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous 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
  • C21D8/0221—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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold 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
  • 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
  • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/06—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • 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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the present invention relates to a cold-rolled steel sheet having a thickness of 0.4 mm or more and 3.0 mm or less suitable for use in automobiles, electric machines, etc., and in particular, a cold-rolled steel sheet excellent in spot weldability having a tensile strength of 980 MPa or more and its It relates to a manufacturing method.
• the weld metal becomes a coarse columnar solidified martensite single phase structure by heating the steel sheet to the melting point and then rapidly cooling it.
• a welding heat-affected zone heated to a temperature range of Ac 3 or higher (hereinafter also referred to as a weld heat-affected zone of Ac 3 or higher) also has a relatively coarse martensitic structure. For this reason, the weld metal and the weld heat-affected zone at Ac 3 or higher have a higher hardness than the base metal and are easily embrittled.
• the base material is high strength As the value becomes, the degree of softening of the base material tends to increase.
• the welded portion has a discontinuous shape, so that stress tends to concentrate, and generation of residual stress due to welding heat history is inevitable.
• discontinuity in the strength of the region extending from the weld metal, the weld heat-affected zone, and the base metal becomes prominent, and the fracture strength of the spot weld is likely to decrease compared to the base metal.
• JP 2012-167338 A Japanese Patent No. 4530606 Japanese Patent No. 4883216 Japanese Patent No. 514068 Japanese Patent No. 5323552
• the high strength steel sheets proposed in Patent Documents 1 to 5 and the like have high tensile strength of 980 MPa or more and sufficient improvement of spot weldability under sufficient economy and productivity.
• the current situation is that it has not been achieved.
• the present invention was developed in view of the above-mentioned present situation, and without causing an increase in manufacturing cost and a decrease in productivity, a cold-rolled steel sheet excellent in spot weldability having a tensile strength of 980 MPa or more, It is intended to provide with its advantageous manufacturing method.
• excellent spot weldability means that, in a cross tension test in accordance with JIS Z 3137 (1999), the cross tension force is 10 kN / spot or more, and the rupture form is plug rupture. In the cross-sectional test of the spot welded part according to 3139 (2009), it means that the difference ⁇ HV between the maximum value and the minimum value of Vickers hardness in the region from the weld metal part to the base metal part is less than 120.
• Tensile strength In order to achieve 980 MPa or more, it is important to strictly adjust the chemical composition of the steel sheet and to properly control the mass% ratio (Ti / N) of Ti and N. This is because, by properly controlling Ti / N, grain refinement strengthening and precipitation strengthening due to the formation of TiN appear. In addition, through the suppression of Nb nitride formation, it becomes possible to secure Nb in a solid solution state during the annealing process, which has the effect of delaying the recrystallization progress during heating, which increases the strength of the steel sheet. It is because it contributes to.
• the gist configuration of the present invention is as follows. 1. % By mass C: 0.05 to 0.13%, Si: 0.05 to 2.0%, Mn: 1.5 to 4.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.01 to 0.10%, Cr: 0.05 to 1.0%, Nb: 0.010 to 0.070%, Ti: 0.005 to 0.040% and N: 0.0005 to 0.0065%
• the mass ratio of Ti and N: Ti / N is 2.5 or more and 7.5 or less, and the balance has a steel composition consisting of Fe and inevitable impurities, While 70 mass% or more of Ti in steel exists as precipitates, 15 mass% or more of Nb in steel exists as solute Nb, A cold-rolled steel sheet excellent in spot weldability having a tensile strength of 980 MPa or more.
• the steel composition is further mass%, Mo: 0.01 to 1.0%, Cu: 1.0% or less,
• the steel material having the steel composition described in 1 or 2 above is heated to a temperature range of (Ts ⁇ 50) ° C. or more and (Ts + 200) ° C. when Ts is set to a temperature represented by the following formula (1).
• Rolling end temperature after performing hot rolling at 850 ° C. or higher, and then winding at a temperature of 650 ° C. or lower to form a hot rolled steel sheet, Cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet;
• the cold-rolled steel sheet is heated to a temperature range of 700 ° C. to 900 ° C., and in the subsequent cooling process, the average cooling rate: 12 ° C./s to 100 ° C./s to 200 ° C. to 450 ° C.
• a method for producing a cold-rolled steel sheet having excellent spot weldability comprising: a step of cooling and holding for 30 seconds to 600 seconds in the temperature range and performing a continuous annealing.
• Ts (° C.) 6770 / [2.26 ⁇ log 10 ⁇ [% Nb] ⁇ ([% C] +0.86 [% N]) ⁇ ]-273 (1)
• [% Nb], [% C] and [% N] indicate the contents (mass%) of Nb, C and N in the steel, respectively.
• a cold-rolled steel sheet excellent in spot weldability with a tensile strength of 980 MPa or more can be obtained without causing an increase in production cost or a decrease in productivity.
• the cold-rolled steel sheet of the present invention it is possible to improve the production efficiency when manufacturing a steel structure such as an automobile and the safety for passengers of the automobile, and further contribute greatly to the reduction of the environmental load accompanying the improvement of fuel consumption. Can do.
• C 0.05 to 0.13% C is the most important element for strengthening steel and has a high solid solution strengthening ability. In order to acquire such an effect, 0.05% or more of C is required.
• the amount of C exceeds 0.13%, the martensite phase in the base material is increased and markedly hardened, and the hole expandability deteriorates.
• the C content is limited to a range of 0.05 to 0.13%. Preferably it is 0.06 to 0.12% of range.
• Si 0.05-2.0%
• Si is a necessary element in steelmaking that acts as a deoxidizing material. Moreover, Si has the effect of increasing the strength of a steel sheet by solid solution in steel and solid solution strengthening. In order to obtain such an effect, it is necessary to contain 0.05% or more of Si.
• the amount of Si exceeds 2.0%, the toughness of the weld metal and the weld heat affected zone is significantly deteriorated, and the fracture strength of the weld is lowered. For this reason, the amount of Si is limited to the range of 0.05 to 2.0%. Preferably it is 0.10 to 1.60% of range.
• Mn 1.5 to 4.0% Mn has the effect of increasing the hardenability of steel at a relatively low cost, and in order to ensure a base material strength of tensile strength: 980 MPa or more, it is necessary to make the amount of Mn 1.5% or more. is there.
• the amount of Mn exceeds 4.0%, the fracture strength of the welded portion decreases and the microsegregation of the base material increases, which promotes the occurrence of delayed fracture starting from the base material segregated portion.
• the amount of Mn is limited to the range of 1.5 to 4.0%. Preferably it is in the range of 1.7 to 3.8%.
• P 0.05% or less P is an element having a large solid solution strengthening ability, but promotes microsegregation together with Mn. For this reason, if the amount of P exceeds 0.05%, not only the base material becomes brittle, but also the grain boundary segregation part tends to be the starting point of delayed fracture. Therefore, it is desirable to reduce P as much as possible with 0.05% as the upper limit. However, excessive P reduction raises the refining cost and is economically disadvantageous, so the lower limit of P is preferably about 0.005%.
• S 0.005% or less S is segregated at the grain boundary to lower the ductility during hot rolling, so it is desirable to reduce 0.005% as much as possible.
• Al acts as a deoxidizer and is the most widely used element in the molten steel deoxidation process for steel sheets. Moreover, it has the effect which suppresses the embrittlement by solid solution N by fixing solid solution N in steel and forming AlN. In order to obtain such an effect, it is necessary to contain 0.01% or more of Al. On the other hand, if the Al content exceeds 0.10%, surface cracks during slab production are promoted. For this reason, the Al content is limited to a range of 0.01 to 0.10%. Preferably it is 0.02 to 0.07% of range.
• Cr 0.05 to 1.0% Cr has the effect of increasing the hardenability of steel relatively inexpensively, delays the bainite transformation of the intermediate hardness phase in the annealing process, generates martensite of the high hardness phase, and contributes to the improvement of the strength of the steel. It is an element. In order to obtain such an effect, it is necessary to contain 0.05% or more of Cr. On the other hand, if the amount of Cr exceeds 1.0%, not only does it promote embrittlement due to an excessive increase in strength, but it also becomes economically disadvantageous. For this reason, the Cr content is limited to a range of 0.05 to 1.0%. Preferably, it is in the range of 0.07 to 0.8%.
• Nb 0.010 to 0.070% Nb is present as a solid solution Nb in the annealing heating after cold rolling, thereby producing a solution drag effect, and delaying the recrystallization of the processed structure generated by cold rolling, thereby allowing the steel plate after annealing to It is an important element for increasing strength.
• generated by a hot rolling and annealing process refines
• the Nb content exceeds 0.070%, coarse carbonitride precipitates, which may promote surface cracking during slab production and may be a starting point for fracture.
• the Nb content is limited to the range of 0.010 to 0.070%. Preferably it is 0.015 to 0.060% of range.
• Ti 0.005 to 0.040%
• Ti is an important alloying element in the present invention, and has the effect of suppressing the coarsening of crystal grains in the base metal, the weld metal and the weld heat affected zone by fixing solid solution N to form TiN. , It has the effect of suppressing embrittlement by reducing the solid solution N.
• the formation of TiN effectively contributes to securing a predetermined amount of solid solution Nb through suppressing the formation of Nb nitride in the hot rolling and annealing processes, and increasing the strength of the steel sheet after annealing. In order to obtain such an effect, it is necessary to contain 0.005% or more of Ti.
• the amount of Ti exceeds 0.040%, very hard and brittle TiC precipitates and promotes embrittlement. For this reason, the amount of Ti is limited to the range of 0.005 to 0.040%. Preferably, the content is 0.010 to 0.035%.
• N 0.0005 to 0.0065% N is contained in steel as an unavoidable impurity, but by adding an appropriate amount of Ti, TiN is formed, and the effect of suppressing the coarsening of crystal grains in the weld metal and weld heat affected zone during welding is expressed. To do. In order to obtain such an effect, the N content needs to be 0.0005% or more. On the other hand, when the N content exceeds 0.0065%, the aging resistance is remarkably lowered due to an increase in the solid solution N. Therefore, the N content is limited to a range of 0.0005 to 0.0065%. Preferably, the content is 0.0010 to 0.0060%.
• Ti / N 2.5 or more and 7.5 or less
• mass ratio of Ti and N Ti / N appropriately with the above-described component composition.
• Ti / N 2.5 or more and 7.5 or less
• Ti / N is less than 2.5, the solid solution N in the steel sheet increases and promotes embrittlement.
• Ti / N exceeds 7.5, very hard and brittle TiC is generated in the steel sheet, and the ductility is lowered and the embrittlement becomes remarkable.
• Ti / N is limited to the range of 2.5 to 7.5. Preferably, it is in the range of 3.0 to 7.0.
• the 1 type (s) or 2 or more types selected from Mo, Cu, Ni, and V can be contained as needed.
• Mo: 0.01 to 1.0% Mo is an element that contributes to improving the strength of steel. In order to obtain such an effect, it is necessary to add 0.01% or more of Mo. On the other hand, if the amount of Mo exceeds 1.0%, not only does it promote embrittlement due to an excessive increase in strength, but it becomes economically disadvantageous. For this reason, when Mo is contained, the amount of Mo is in the range of 0.01 to 1.0%. Preferably it is 0.03 to 0.8% of range.
• Cu 1.0% or less
• Cu is an element that contributes to improving the strength of steel.
• the amount of Cu exceeds 1.0%, hot brittleness is caused and the surface properties of the steel sheet are deteriorated. For this reason, when Cu is contained, the Cu content is 1.0% or less.
• Ni 1.0% or less
• Ni is an element that contributes to improving the strength of steel.
• the amount of Ni exceeds 1.0%, the effect is saturated and economically disadvantageous. For this reason, when Ni is contained, the Ni content is 1.0% or less.
• V 0.1% or less V is an element that contributes to improving the strength of steel. However, if the amount of V exceeds 0.1%, the base metal ductility is deteriorated. For this reason, when V is contained, the V amount is 0.1% or less.
• the components other than the above are Fe and inevitable impurities.
• Ratio of Ti present as precipitates in steel 70% by mass or more
• the structure is refined by Ti precipitates, and the hole expandability of the finally obtained cold-rolled steel sheet is improved.
• Ti exists as a precipitate in the cold-rolled steel sheet after annealing, coarsening of the crystal grains in the weld heat affected zone due to the welding heat history during welding is suppressed, and the fracture strength of the weld zone is improved.
• 70 mass% or more needs to exist as a precipitate among Ti in steel.
• the upper limit of the ratio of Ti existing as precipitates in the steel is not particularly specified, but when it becomes 100% by mass, the toughness is greatly deteriorated due to the remaining solid solution N. For this reason, it is preferable to make the ratio of Ti which exists as a precipitate in steel into less than 100 mass%, and it is more preferable to set it as less than 98 mass%.
• the form of the precipitate is mainly TiN alone or a composite precipitate of TiN and other precipitates, but Ti oxide or Ti carbide is less than 10% of the total number of Ti-based precipitates. If there is, the effect is negligible even if mixed. Moreover, the presence form of Ti in steel other than a precipitate is solute Ti.
• Ratio of Nb present as solid solution Nb in steel 15% by mass or more
• Nb When Nb is present in a solid solution state, in the annealing process, it effectively contributes to increasing the strength of steel due to the effect of suppressing recrystallization during heating. At the same time, it has the effect of suppressing the softening of the weld heat affected zone of less than Ac 3 point.
• 15% by mass or more of Nb in the steel needs to be present as solute Nb.
• the upper limit of the ratio of Nb which exists as solid solution Nb in steel is not prescribed
• the ratio of Nb which exists as solid solution Nb in steel shall be 70 mass% or less.
• the presence form of Nb in steel other than solute Nb is an Nb precipitate, and examples of such an Nb precipitate include Nb carbide such as NbC, Nb carbonitride, and the like.
• the temperature of the steel plate in manufacturing conditions shall mean the surface temperature of a steel plate.
• Molten steel having the above component composition is melted by a known method such as a converter or an electric furnace, and a steel material such as a slab having a predetermined size is obtained by a known method such as a continuous casting method or an ingot-bundling rolling method. .
• treatments such as ladle refining and vacuum degassing may be added to the molten steel.
• the obtained steel material was immediately or once cooled, heated to a temperature range of (Ts-50) ° C. or higher and (Ts + 200) ° C. or lower, and hot rolled at a finish rolling finish temperature of 850 ° C. or higher.
• Ts is defined by the following equation (1).
• Ts (° C.) 6770 / [2.26 ⁇ log 10 ⁇ [% Nb] ⁇ ([% C] +0.86 [% N]) ⁇ ]-273 (1)
• [% Nb], [% C] and [% N] indicate the contents (mass%) of Nb, C and N in the steel, respectively.
• Heating temperature (Ts ⁇ 50) ° C. or more and (Ts + 200) ° C. or less
• the carbonitride containing coarse Nb crystallized during the melting of the steel material does not contribute to increasing the strength of the steel sheet.
• the coarse Nb-based crystallized product is once dissolved in steel in the heating stage before hot rolling, and then again in the process of rolling, cooling, annealing, etc., again with fine Nb carbides and It is important to deposit as carbonitride.
• the heating temperature is less than (Ts-50) ° C., the heating is not sufficient, so that the Nb-based crystallized substance does not sufficiently dissolve in the steel, and the strength after annealing is insufficient.
• the heating temperature is set to (Ts ⁇ 50) ° C. or more and (Ts + 200) ° C. or less. It is preferably (Ts ⁇ 20) ° C. or higher and (Ts + 170) ° C. or lower.
• Finish rolling end temperature 850 ° C. or more
• finish rolling end temperature 850 ° C. or more
• Winding temperature 650 ° C. or less
• NbC precipitated during winding is excessively coarsened, so that it easily becomes brittle and tends to be a starting point of fracture.
• the coiling temperature of a hot-rolled steel sheet needs to be 650 degrees C or less.
• it is 620 degrees C or less. Note that the lower limit of the coiling temperature of the hot-rolled steel sheet does not need to be specified in particular.
• the obtained hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
• the conditions for cold rolling need not be specified, but in order to ensure a desired strength after annealing, the total rolling reduction is preferably 30% or more. On the other hand, in order to avoid an excessive load on the rolling mill, the total rolling reduction is preferably 80% or less.
• the cold-rolled steel sheet obtained as described above is subjected to continuous annealing under the following conditions.
• Heating temperature in continuous annealing 700 ° C. or more and 900 ° C. or less
• the heating temperature in continuous annealing is less than 700 ° C.
• the reverse transformation of austenite becomes insufficient, and the amount of hard martensite or bainite generated during subsequent cooling is insignificant. It becomes sufficient and the desired strength cannot be obtained.
• the heating temperature in continuous annealing shall be 700 degreeC or more and 900 degrees C or less.
• they are 720 degreeC or more and 880 degrees C or less.
• the holding time after heating need not be specified, but it is preferable to hold for 15 seconds or more in order to ensure a uniform temperature distribution and a stable microstructure.
• holding for a long time not only lowers the production efficiency but also causes coarsening of austenite grains, so the holding time is preferably 600 s or less.
• Average cooling rate 12 ° C./s or more and 100 ° C./s or less If the average cooling rate in the cooling process after heating is less than 12 ° C./s, a soft ferrite phase is excessively generated during cooling and the desired strength is obtained. It becomes difficult to secure. Moreover, since Nb reprecipitates excessively in the middle of cooling, it becomes difficult to secure a desired amount of solid solution Nb. In addition, a coarse ferrite phase or pearlite phase is generated during cooling, and the strength decreases. On the other hand, when the average cooling rate after annealing exceeds 100 ° C./s, it becomes difficult to ensure the shape of the steel sheet. For this reason, the average cooling rate after annealing treatment shall be 12 degrees C / s or more and 100 degrees C / s or less. Preferably they are 14 degreeC / s or more and 70 degrees C / s or less.
• Cooling stop temperature 200 ° C. or higher and 450 ° C. or lower If the cooling stop temperature is lower than 200 ° C., the conveyance speed of the steel sheet is extremely reduced, which is not preferable in terms of production efficiency. On the other hand, when the cooling is stopped at a temperature exceeding 450 ° C., a relatively soft bainite phase is excessively generated after the cooling is stopped, and it becomes difficult to secure a desired strength. Moreover, since Nb reprecipitates excessively after cooling is stopped, it is difficult to secure a desired amount of solid solution Nb. Furthermore, a soft structure such as ferrite is excessively generated and the strength is insufficient. For this reason, cooling stop temperature shall be 200 degreeC or more and 450 degrees C or less. Preferably they are 230 degreeC or more and 420 degrees C or less.
• Holding time in the cooling stop temperature region 30 s or more and 600 s or less
• the holding time in the cooling stop temperature region is less than 30 s, the temperature and the uniformity of the material in the steel sheet are lowered.
• the holding time in the cooling stop temperature region exceeds 600 s, the manufacturing efficiency is lowered. For this reason, the holding time in the cooling stop temperature region is set to 30 s or more and 600 s or less.
• the steel having the composition shown in Table 1 was melted in a converter and then smelted in a ladle and made into a steel slab by continuous casting. Subsequently, the steel slab was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. Thereafter, these hot-rolled steel sheets were subjected to cold rolling and continuous annealing under the conditions shown in Table 2 to obtain cold-rolled steel sheets to be product plates.
• the cold-rolled steel sheet thus obtained was subjected to (1) extraction residue analysis of precipitates, (2) tensile test, and (3) spot welding test in the following manner.
• the cross-sectional test was implemented based on JISZ3139 (2009). That is, two cold-rolled steel plates of the same steel type were spot-welded under the same conditions as the above-mentioned cross-shaped tensile test piece production conditions. Next, the welded section cut out perpendicular to the steel sheet surface was polished and then subjected to nital corrosion to obtain a test piece for hardness measurement.
• the tensile strength is 980 MPa or more
• the cross tensile force is 10 kN / spot or more
• the breaking mode is plug breaking
• the maximum and minimum values of Vickers hardness are shown.
• Excellent spot weldability with a value difference ⁇ HV of less than 120 was obtained.
• the total elongation was 13% or more.
• at least one of the tensile strength and total elongation of the base material, the cross tensile force and the fracture mode in the spot welding test, and the difference between the maximum value and the minimum value ( ⁇ HV) of Vickers hardness is sufficient. I could't say that.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

Family

ID=55263465

Family Applications (1)

Country Status (6)

Cited By (1)

Families Citing this family (3)

Citations (3)

Family Cites Families (10)

• 2014
  • 2014-08-08 JP JP2014162834A patent/JP5935843B2/ja active Active
• 2015
  • 2015-07-31 EP EP15829208.6A patent/EP3178954B1/en not_active Not-in-force
  • 2015-07-31 US US15/329,026 patent/US20170204492A1/en not_active Abandoned
  • 2015-07-31 CN CN201580042291.3A patent/CN106661693B/zh active Active
  • 2015-07-31 MX MX2017001687A patent/MX2017001687A/es active IP Right Grant
  • 2015-07-31 WO PCT/JP2015/003881 patent/WO2016021169A1/ja active Application Filing

Patent Citations (3)

Also Published As

Similar Documents

Legal Events