WO2017169329A1 - 高強度鋼板およびその製造方法 - Google Patents
高強度鋼板およびその製造方法 Download PDFInfo
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- WO2017169329A1 WO2017169329A1 PCT/JP2017/006608 JP2017006608W WO2017169329A1 WO 2017169329 A1 WO2017169329 A1 WO 2017169329A1 JP 2017006608 W JP2017006608 W JP 2017006608W WO 2017169329 A1 WO2017169329 A1 WO 2017169329A1
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- 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
- C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
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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
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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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
- C22C38/18—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
- This disclosure relates to a high-strength steel sheet that can be used for various applications including automobile parts and a method for manufacturing the same.
- Patent Document 1 discloses a steel sheet having a tensile strength of 980 to 1180 MPa and exhibiting good deep drawability.
- the tensile strength is required to be 780 MPa or more.
- a high yield strength (YS) in addition to a high tensile strength (TS).
- the product TS ⁇ EL of TS and total elongation EL is required to be 22000 MPa% or more. Furthermore, in order to ensure the moldability at the time of component molding, the LDR (Limiting Drawing Ratio), which is an index indicating deep drawability and is required by the method described in Examples below, is 2.05 or more, and the hole It is also required that the hole expansion ratio ⁇ indicating the expandability is 20% or more.
- LDR Liting Drawing Ratio
- Patent Document 1 the high-strength steel sheet disclosed in Patent Document 1 is difficult to satisfy all these requirements, and a high-strength steel sheet that can satisfy all these requirements has been demanded.
- Embodiments of the present invention have been made to meet such demands, and include tensile strength TS, yield ratio YR, TS ⁇ EL of total elongation EL, LDR indicating deep drawability, and holes.
- An object of the present invention is to provide a high-strength steel sheet having a high level of the hole expansion ratio ⁇ indicating the expandability and a method for manufacturing the same.
- the high-strength steel plate according to the embodiment of the present invention that has solved the above problems is Ingredient composition is mass%, C: 0.15 to 0.35%, Total of Si and Al: 0.5 to 2.5% Mn: 1.0 to 4.0%, P: more than 0% and 0.05% or less, and S: more than 0% and 0.01% or less, with the balance consisting of Fe and inevitable impurities,
- the steel structure is the ratio of the entire structure.
- Ferrite more than 5 area% and 50 area% or less
- a total of tempered martensite and bainite 30 area% or more
- retained austenite 10 vol% or more, further having MA
- Average circle equivalent diameter of MA 1.0 ⁇ m or less
- the average equivalent circle diameter of retained austenite 1.0 ⁇ m or less and the retained austenite having an equivalent circle diameter of 1.5 ⁇ m or more satisfies a volume ratio of 5% or more of the total retained austenite.
- the amount of C in the component composition is preferably 0.30% or less. Further, the Al content in the component composition may be less than 0.10%.
- the high-strength steel plate is further mass%, (A) A total of one or more selected from the group consisting of Cu, Ni, Mo, Cr, and B is more than 0% and 1.0% or less, (B) a total of one or more selected from the group consisting of V, Nb, Ti, Zr, and Hf, more than 0% and 0.2% or less, (C) a total of at least one selected from the group consisting of Ca, Mg, and REM (Rare Earth Metal) is more than 0% and not more than 0.01%, May be included.
- the manufacturing method of the high-strength steel plate according to the embodiment of the present invention is a method for manufacturing the high-strength steel plate, and uses an original plate that satisfies the component composition, (Ac 1 point ⁇ 0.8 + Ac 3 point ⁇ 0.2) Step A of heating to a temperature T1 that is equal to or higher than Ac 3 point, After the heating, a step B of rapidly cooling from a rapid cooling start temperature T2 of 650 ° C. or higher to a cooling stop temperature T3a of 300 to 500 ° C.
- step C of performing slow cooling at an average cooling rate of 0 ° C./second or more and 10 ° C./second or less in a temperature range of 300 to 500 ° C. for 10 seconds or more and less than 300 seconds after the rapid cooling
- step D of cooling from a slow cooling end temperature T3b of 300 ° C. or higher to a cooling stop temperature T4 of 100 to 300 ° C. at an average cooling rate exceeding 10 ° C./second, and reheating at 300 to 500 ° C.
- Process E for reheating to temperature T5 Are included in this order.
- the step C may include holding at a constant temperature in a temperature range of 300 to 500 ° C.
- the tensile strength TS, the yield ratio YR, the product TS ⁇ EL of the TS and the total elongation EL, the high-strength steel plate having a high level of hole expandability and deep drawability, and its manufacture A method can be provided.
- FIG. 1 is a diagram schematically illustrating the heat treatment according to the manufacturing method of the embodiment of the present invention.
- the inventors have determined that the tensile strength TS, the yield ratio YR, the product TS ⁇ EL of the TS and the total elongation EL, the hole expandability and the deep drawability can be obtained by setting the steel composition to a predetermined component.
- the steel structure will be described below.
- the ferrite content is more than 5 area% in order to improve the ductility and obtain an excellent strength-ductility balance.
- the amount of ferrite is preferably 7 area% or more, more preferably 10 area% or more.
- the ferrite content is set to 50 area% or less in order to ensure an excellent hole expansion ratio ⁇ and a high yield ratio.
- the amount of ferrite is preferably 45 area% or less, more preferably 40 area% or less.
- total of tempered martensite and bainite 30 area% or more
- the bainite in the embodiment of the present invention refers to tempered bainite, untempered bainite, and bainitic ferrite.
- the total of the tempered martensite and bainite is preferably 35 area% or more.
- the upper limit of the total of tempered martensite and bainite is about 85% by area in consideration of the fractions of ferrite and residual austenite, which are other essential structures.
- Residual austenite 10% by volume or more
- Residual austenite is a structure in which a TRIP phenomenon that transforms into martensite by processing-induced transformation occurs during processing such as press processing, and a large elongation can be obtained. Further, the formed martensite has a high hardness. Therefore, an excellent strength-ductility balance can be obtained. Furthermore, deep drawability can also be improved by controlling the particle size distribution of retained austenite as described later. In particular, the amount of retained austenite is 10% by volume or more in order to obtain an excellent strength-ductility balance with TS ⁇ EL of 22000 MPa% or more and to fully exhibit the effect of improving deep drawability by controlling the particle size distribution of retained austenite described later. To do.
- the amount of retained austenite is preferably 12% by volume or more, more preferably 13% by volume or more, still more preferably 14% by volume or more, and still more preferably 15% by volume or more.
- the upper limit of the amount of retained austenite is approximately 40% by volume.
- MA is an abbreviation for martensite-austenite constituent and is a composite structure of martensite and austenite.
- [Average circle equivalent diameter of MA: 1.0 ⁇ m or less] MA is a hard phase, and the vicinity of the interface between the mother phase and the hard phase acts as a void formation site during deformation. As the MA size becomes larger, strain concentration at the interface between the mother phase and the hard phase occurs, and breakage based on voids formed in the vicinity of the interface between the mother phase and the hard phase tends to occur. As a result, the hole expandability is lowered. For this reason, the average circle equivalent diameter of MA is suppressed to 1.0 ⁇ m or less.
- the average equivalent circle diameter of the MA is preferably 0.8 ⁇ m or less, more preferably 0.7 ⁇ m or less. The smaller the average equivalent circle diameter of MA is, the smaller the hole expansion property is, and the better. However, in consideration of the component composition and manufacturing conditions defined in the embodiment of the present invention, the lower limit of the average equivalent circle diameter of MA is about 0.2 ⁇ m. Become.
- the area ratio of MA containing retained austenite is not particularly limited, and the area ratio of retained austenite may be within the above range as described above. As described above, in the embodiment of the present invention, since the amount of retained austenite is 10% by volume or more, MA in which most of the retained austenite is present may also be 10% by area or more. In the embodiment of the present invention, the area ratio of desired ferrite, tempered martensite and bainite needs to be ensured, so the upper limit of the area ratio of MA is less than 65 area%.
- the drawing process proceeds easily and good deep drawability is obtained.
- the deformation behavior of the flange portion is deformed in a state in which the compressive stress is applied because the compressive stress in the circumferential direction of the board surface is strongly applied.
- martensitic transformation is accompanied by volume expansion, martensitic transformation is unlikely to occur under compressive stress.
- the work-induced martensite transformation of retained austenite is suppressed, so that work hardening is small, and as a result, deep drawability is improved.
- the volume expansion amount during martensitic transformation increases, in other words, because martensitic transformation is further suppressed under compressive stress, and deep drawability is further improved. Conceivable.
- the inventors of the present invention should preferably cause a work-induced transformation over a wide stress range. It has been noted that unstable residual austenite that undergoes work-induced transformation and stable retained austenite that does not cause work-induced transformation unless it is under high stress may be mixed.
- the flange portion contributes to suppression of martensite transformation
- the vertical wall portion is relatively coarse residual austenite as unstable residual austenite that easily undergoes processing-induced transformation under relatively low stress.
- a steel structure containing a predetermined amount of fine retained austenite was studied.
- the inventors set the average equivalent circle diameter of the retained austenite to 1.0 ⁇ m or less, and the retained austenite having a circle equivalent diameter of 1.5 ⁇ m or more accounts for 5% or more of the total retained austenite. It was found that a high work hardening rate can be maintained during deformation, and excellent deep drawability is exhibited.
- the “residual austenite having an equivalent circle diameter of 1.5 ⁇ m or more” may be referred to as “relatively coarse retained austenite”.
- the average equivalent circle diameter of the retained austenite is preferably 0.95 ⁇ m or less, more preferably 0.90 ⁇ m or less. Considering the component composition and production conditions defined in the embodiment of the present invention, the lower limit of the average equivalent-circle diameter of retained austenite is approximately 0.40 ⁇ m.
- the volume ratio of the residual austenite having an equivalent circle diameter of 1.5 ⁇ m or more to the total residual austenite is preferably 10% or more, more preferably 15% or more. Note that if the proportion of the relatively coarse retained austenite is excessive, the size of the MA is likely to be coarse, and the hole expandability is liable to be lowered. From this viewpoint, the volume ratio of the retained austenite having an equivalent circle diameter of 1.5 ⁇ m or more is preferably 50% or less, and more preferably 40% or less.
- the martensite structure formed by the process-induced transformation is hard and acts as a starting point for fracture. Larger martensite structures are more likely to be the origin of destruction.
- the average equivalent circular diameter of retained austenite is 1.0 ⁇ m or less as described above, the size of martensite formed by the processing-induced transformation can be reduced, and breakage can be suppressed.
- the average equivalent circle diameter of the retained austenite and the ratio of the retained austenite amount with an equivalent circle diameter of 1.5 ⁇ m or more to the total retained austenite amount are crystals using SEM (Scanning Electron Microscope) as shown in the examples described later. This can be obtained by creating a Phase map using an analysis method EBSD (Electron Back Scatter Diffraction Patterns) method.
- SEM Sccanning Electron Microscope
- the metallographic structure of the steel sheet according to the embodiment of the present invention includes ferrite, tempered martensite, bainite, retained austenite, and MA, and may be composed of only these, but the effect of the present invention is achieved. A remaining structure such as pearlite may be present as long as it is not impaired.
- composition of the high-strength steel sheet according to the embodiment of the present invention will be described below.
- unit% display of a component composition means the mass% altogether.
- C is an essential element for obtaining a desired structure such as retained austenite and ensuring high characteristics such as TS ⁇ EL.
- the C content is set to 0.15% or more.
- the amount of C is preferably 0.17% or more, more preferably 0.20% or more.
- the amount of C exceeds 0.35%, MA and retained austenite become coarse, and as a result, both deep drawability and hole expansibility deteriorate. Further, if the amount of C is excessive, the weldability is also inferior. Therefore, the C amount is set to 0.35% or less.
- the amount of C is preferably 0.33% or less, more preferably 0.30% or less.
- Total of Si and Al each have a function of suppressing the precipitation of cementite and promoting the formation of retained austenite.
- Si and Al are contained in a total of 0.5% or more.
- the total of Si and Al is preferably 0.7% or more, more preferably 1.0% or more.
- the total of Si and Al exceeds 2.5%, tempered martensite and bainite cannot be secured, and MA and retained austenite become coarse. Therefore, the total of Si and Al is 2.5% or less, preferably 2.0% or less.
- Al may be added in an amount that functions as a deoxidizing element, that is, less than 0.10%.
- Al may be contained as much as 0.7% or more, for example.
- Mn is an element necessary for securing tempered martensite / bainite and the like other than ferrite by suppressing the formation of ferrite.
- the amount of Mn is set to 1.0% or more.
- the amount of Mn is preferably 1.5% or more, more preferably 2.0% or more.
- the Mn content is 4.0% or less, preferably 3.7% or less, more preferably 3.5% or less.
- P more than 0% and 0.05% or less
- P is unavoidably present as an impurity element.
- the amount of P exceeds 0.05%, the total elongation EL and the hole expandability deteriorate. Therefore, the P content is 0.05% or less, preferably 0.03% or less.
- S more than 0% and 0.01% or less
- S inevitably exists as an impurity element.
- the amount of S exceeds 0.01%, sulfide inclusions such as MnS are formed, and this becomes a starting point of cracking and the hole expanding property is lowered.
- the amount of S is 0.01% or less, preferably 0.005% or less.
- the balance of the component composition of the high-strength steel sheet is Fe and inevitable impurities.
- the inevitable impurities for example, mixing of trace elements such as As, Sb, and Sn, which are brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like, is allowed.
- the content for example, like P and S, it is usually preferable that the content is small. Therefore, although it is an unavoidable impurity, there is an element that separately defines the composition range as described above. For this reason, in this specification, the term “inevitable impurities” constituting the balance is a concept that excludes elements whose composition ranges are separately defined.
- the high-strength steel plate according to the embodiment of the present invention may optionally contain other elements as long as the characteristics can be maintained.
- Other elements considered to be able to be selectively contained without impairing the effects of the present invention based on experience so far are exemplified below.
- Cu, Ni, Mo, Cr, and B are elements that act as elements that increase the tensile strength of the steel sheet, and that act effectively to stabilize retained austenite and ensure a predetermined amount.
- the above elements are preferably contained in a total amount of 0.001% or more, more preferably 0.01% or more. However, even if the above elements are contained excessively, the effect is saturated and economically wasteful, so the total amount of the above elements is preferably 1.0% or less, more preferably 0.5% or less.
- the said element can be made to contain 2 or more types chosen independently or arbitrarily.
- V, Nb, Ti, Zr, and Hf are elements that exert the effects of precipitation strengthening and microstructure refinement, and are useful for increasing the strength of the steel sheet.
- the above elements are preferably contained in a total amount of 0.01% or more, more preferably 0.02% or more.
- the said element is 0.2% or less in total amount, More preferably, it is 0.1% or less.
- the said element can be made to contain 2 or more types chosen independently or arbitrarily.
- Ca, Mg, and REM are elements that control the form of sulfide in steel and effectively act on workability.
- the above elements are preferably contained in a total amount of 0.001% or more, more preferably 0.002% or more.
- the total amount of the element is preferably 0.01% or less, more preferably 0.005% or less.
- the said element can be made to contain 2 or more types chosen independently or arbitrarily. Examples of the REM include Sc, Y, and lanthanoids.
- the high-strength steel plate according to the embodiment of the present invention has high levels of TS, YR, TS ⁇ EL, ⁇ , and LDR. These characteristics of the high-strength steel sheet according to the embodiment of the present invention will be described in detail below.
- Tensile strength TS The high strength steel plate according to the embodiment of the present invention has a tensile strength of 780 MPa or more.
- the tensile strength is preferably 880 MPa or more, more preferably 980 MPa or more.
- the higher the strength, the better, but the upper limit of the tensile strength is approximately 1300 MPa in consideration of the composition of the steel sheet according to the embodiment of the present invention, the production conditions, and the like.
- the high strength steel plate according to the embodiment of the present invention has a yield ratio of 0.60 or more. Thereby, high yield strength can be realized in combination with the above-described high tensile strength. As a result, the final product obtained by processing such as deep drawing can be used without deformation even under high load. Preferably, it has a yield ratio of 0.65 or higher. The higher the yield ratio, the better. However, the upper limit of the yield ratio is approximately 0.75 in consideration of the composition of the steel sheet according to the embodiment of the present invention, production conditions, and the like.
- TS ⁇ EL of tensile strength TS and total elongation EL
- TS ⁇ EL is 22000 MPa% or more.
- TS ⁇ EL is preferably 23000 MPa% or more.
- TS ⁇ EL is preferably as high as possible, but the upper limit of TS ⁇ EL is approximately 35000 MPa% in consideration of the composition of the steel sheet according to the embodiment of the present invention, production conditions, and the like.
- the high-strength steel sheet according to the embodiment of the present invention has a hole expansion ratio ⁇ of 20% or more. Thereby, excellent workability such as press formability can be obtained.
- the hole expansion ratio ⁇ is preferably 30% or more. The higher the hole expansion ratio ⁇ , the better. However, considering the composition of the steel sheet according to the embodiment of the present invention, the manufacturing conditions, and the like, the upper limit of the hole expansion ratio ⁇ is approximately 60%.
- the deep drawability of the steel sheet is evaluated by LDR as shown in the examples described later.
- the high-strength steel sheet according to the embodiment of the present invention has an LDR of 2.05 or more, preferably 2.10 or more, and has excellent deep drawability.
- the upper limit of the LDR is approximately 2.25 in consideration of the composition of the steel sheet according to the embodiment of the present invention, production conditions, and the like.
- the high-strength steel sheet according to the embodiment of the present invention is excellent in TS, YR, TS ⁇ EL, ⁇ , and LDR, so that it can be suitably used for steel molded parts for various uses including automobile parts. it can.
- Examples of the original sheet to be heat-treated include a hot-rolled steel sheet obtained by hot rolling and a cold-rolled steel sheet obtained by further cold rolling, but the conditions for the hot rolling and cold rolling are not particularly limited.
- Step A Heating to a temperature T1 of (Ac 1 point ⁇ 0.8 + Ac 3 points ⁇ 0.2) or more and less than Ac 3 points]
- Step A corresponds to [2] in FIG.
- heating is performed to a temperature T1 of (Ac 1 point ⁇ 0.8 + Ac 3 points ⁇ 0.2) or more and less than Ac 3 points.
- the heating temperature T1 is lower than (Ac 1 point ⁇ 0.8 + Ac 3 point ⁇ 0.2)
- the reverse transformation to austenite is insufficient and the processed structure remains, and a certain amount of ferrite can be secured. Therefore, the strength-ductility balance is lowered.
- the heating temperature T1 is (Ac 1 point ⁇ 0.8 + Ac 3 points ⁇ 0.2) or more, preferably (Ac 1 point ⁇ 0.8 + Ac 3 points ⁇ 0.2) + 5 ° C. or more, more preferably (Ac 1 point ⁇ 0.8 + Ac 3 points ⁇ 0.2) + 10 ° C. or more.
- the heating temperature T1 is set to less than Ac 3 points.
- the heating temperature T1 is preferably Ac 3 points ⁇ 10 ° C. or lower, more preferably Ac 3 points ⁇ 20 ° C. or lower.
- the holding time t1 at the heating temperature T1 is preferably set to 1 to 1800 seconds in consideration of securing the effect by heating and productivity.
- Ac 1 point and said Ac 3 point are respectively shown in "Leslie Steel Materials” (Maruzen Co., Ltd., issued May 31, 1985, page 273), the following formula (1) and the following formula: Calculated from (2).
- each element in the formula is the content in mass% of each element in the steel sheet, and elements not included are calculated as zero.
- Ac 1 point 723-10.7Mn + 29.1Si + 16.9Cr
- Ac 3 points 910-203C 0.5 + 44.7Si + 104V + 31.5Mo-30Mn-11Cr + 700P + 400Al + 400Ti (2)
- the temperature may be raised at an arbitrary heating rate in the temperature raising stage shown in [1] of FIG.
- the temperature is raised from room temperature to the heating temperature T1 at an average heating rate HR1 of 1 ° C./second or more and 20 ° C./second or less.
- Step B After the heating, a step of cooling from a rapid cooling start temperature T2 of 650 ° C. or higher to a cooling stop temperature T3a of 300 to 500 ° C. at an average cooling rate of 30 ° C./second or more and less than 200 ° C./second]
- Step B corresponds to [4] in FIG.
- rapid cooling with an average cooling rate CR2 of 30 ° C./second or higher is performed.
- excessive ferrite formation can be suppressed.
- the average cooling rate CR2 is preferably 35 ° C./second or more, more preferably 40 ° C./second or more. However, if the average cooling rate CR2 is too high, excessive thermal distortion is likely to occur due to rapid cooling, so the average cooling rate CR2 is less than 200 ° C./second, preferably 100 ° C./second or less.
- the rapid cooling with the average cooling rate CR2 of 30 ° C./second or more starts from the rapid cooling start temperature T2 of 650 ° C. or more. If the rapid cooling start temperature T2 is lower than 650 ° C., there is a problem that excessive growth of ferrite occurs before the rapid cooling starts, and a desired amount of ferrite cannot be secured.
- the rapid cooling start temperature T2 is preferably 680 ° C. or higher, more preferably 700 ° C. or higher. Note that the upper limit of the rapid cooling start temperature T2 is not particularly limited, and may be the heating temperature T1 or less.
- the cooling stop temperature T3a of 300 to 500 ° C. is the rapid cooling end temperature of the rapid cooling and is also the starting temperature of slow cooling in the following step C.
- the temperature T3a is preferably 480 ° C. or lower, more preferably 460 ° C. or lower.
- the temperature T3a is lower than 300 ° C., that is, when quenching is performed to a low temperature range, the carbon concentration region becomes small, and as a result, relatively coarse residual austenite having an equivalent circle diameter of 1.5 ⁇ m or more can be secured. However, deep drawability is reduced.
- the temperature T3a is preferably 320 ° C. or higher, more preferably 340 ° C. or higher.
- the average cooling rate CR1 from the heating temperature T1 to 650 ° C. shown in [3] of FIG. 1 is not particularly limited.
- Examples of the average cooling rate CR1 include cooling at a relatively slow average cooling rate of 0.1 ° C./second or more and 10 ° C./second or less.
- Step C Step of performing slow cooling at an average cooling rate of 0 ° C./second to 10 ° C./second in a temperature range of 300 to 500 ° C. after the rapid cooling for 10 seconds to less than 300 seconds]
- Step C corresponds to [5] in FIG.
- this slow cooling start temperature rapid cooling end temperature T3a to the slow cooling end temperature T3b shown in FIG. 1 are within a temperature range of 300 to 500 ° C., and the above T3a to T3b
- the average cooling rate is 0 ° C./second or more and 10 ° C./second or less, and the time from T3a to T3b, that is, the slow cooling time t3 is 10 seconds or more and less than 300 seconds.
- bainite is partially formed.
- This bainite has a lower solid solubility limit of carbon than austenite, and therefore expels carbon beyond the solid solubility limit.
- a carbon-enriched austenite region is formed around the bainite. This region becomes a relatively coarse retained austenite after cooling in step D and reheating in step E, which will be described later. The presence of this relatively coarse retained austenite can improve the deep drawability as described above.
- the reason why the temperature range of the slow cooling is set to 300 to 500 ° C. is as follows. That is, when the slow cooling start temperature T3a is higher than 500 ° C., as described above, the carbon concentration region becomes too large, and not only the retained austenite but also the MA becomes coarse, so the hole expansion rate decreases.
- the temperature T3a is preferably 480 ° C. or lower, more preferably 460 ° C. or lower.
- the slow cooling end temperature T3b is lower than 300 ° C., the carbon-concentrated region becomes fine due to the progress of the bainite transformation at a low temperature, and a relatively coarse residual austenite cannot be secured sufficiently. Sex is reduced.
- the temperature T3b is preferably 320 ° C. or higher, more preferably 340 ° C. or higher.
- the slow cooling time t3 is 10 seconds or longer, preferably 30 seconds or longer, more preferably 50 seconds or longer.
- the slow cooling time t3 is less than 300 seconds, preferably 200 seconds or less, more preferably 100 seconds or less, still more preferably 80 seconds or less, and still more preferably 60 seconds or less.
- the average cooling rate of the slow cooling is larger than 10 ° C./second, sufficient bainite transformation does not occur, and as a result, a sufficient carbon enriched region is not formed, and the amount of relatively coarse retained austenite is insufficient.
- the average cooling rate is preferably 8 ° C./second or less, more preferably 3 ° C./second or less.
- the average cooling rate may be kept at 0 ° C./second, that is, at a constant temperature. Further, within the above temperature range, the cooling rate may be changed within the above range, or slow cooling and holding at a constant temperature may be combined.
- Preferred conditions for slow cooling include slow cooling for 30 to 80 seconds at an average cooling rate of 0 ° C./second to 8 ° C./second within a temperature range of 320 to 480 ° C. More preferably, slow cooling is performed for 50 to 60 seconds at an average cooling rate of 0 ° C./second to 3 ° C./second within a temperature range of 340 to 460 ° C.
- Patent Document 1 seems to perform reheating after supercooling without performing the above slow cooling.
- the relatively coarse amount of retained austenite that is, the ratio of retained austenite having an equivalent circle diameter of 1.5 ⁇ m or more is low, and the deep drawability is inferior to that of the present invention.
- Step D Step of cooling from the slow cooling end temperature T3b of 300 ° C. or higher to the cooling stop temperature T4 of 100 to 300 ° C. at an average cooling rate exceeding 10 ° C./second after the slow cooling
- Step D corresponds to [6] in FIG.
- a part of the above-described carbon enriched region is martensitic transformed.
- the remaining carbon-enriched region that has not undergone martensitic transformation can be refined, and as a result, fine MA can be obtained.
- the average cooling rate CR3 is 10 ° C./second or less, the carbon concentration region spreads more than necessary during cooling, and the MA becomes coarse, so the hole expansion rate decreases.
- the average cooling rate CR3 is preferably 15 ° C./second or more, more preferably 20 ° C./second or more.
- the slow cooling end temperature T3b is 500 ° C. or lower as described in the step C.
- the cooling stop temperature T4 is controlled at the average cooling rate CR3 within a temperature range of 100 to 300 ° C., the amount of austenite remaining without transforming into martensite is adjusted, and the final retained austenite Control the amount.
- this cooling stop temperature T4 is less than 100 ° C., the amount of retained austenite becomes insufficient. As a result, although TS increases, EL decreases and TS ⁇ EL balance is insufficient.
- the cooling stop temperature T4 is preferably 120 ° C. or higher, more preferably 140 ° C. or higher.
- the cooling stop temperature T4 is set to 300 ° C. or lower.
- the cooling stop temperature T4 is preferably 280 ° C. or lower, more preferably 260 ° C. or lower.
- One of the preferred embodiments is to set the slow cooling end temperature T3b as the cooling start temperature as shown in [6] in FIG.
- the holding time t4 is preferably 1 to 600 seconds. This is because even if the holding time t4 is long, the characteristics are hardly affected, but if it exceeds 600 seconds, the productivity is lowered.
- Step E Step of reheating to 300 to 500 ° C. reheating temperature T5
- Step E corresponds to [9] in FIG.
- a reheating temperature T5 of 300 to 500 ° C. and holding at the reheating temperature T5 carbon in martensite is discharged and carbon concentration in the surrounding austenite is increased. Is promoted, and austenite can be stabilized. Thereby, the amount of retained austenite finally obtained can be increased.
- the reheating temperature T5 is set to 300 ° C. or higher, preferably 320 ° C. or higher, more preferably 340 ° C. or higher.
- the reheating temperature T5 is higher than 500 ° C., carbon precipitates as cementite, and a sufficient amount of retained austenite cannot be obtained.
- the reheating temperature T5 is set to 500 ° C. or lower, preferably 480 ° C. or lower, more preferably 460 ° C. or lower.
- the reheating time t5 held at the reheating temperature T5 is not particularly limited, and may be, for example, 50 seconds or longer, further 70 seconds or longer, and further 90 seconds or longer.
- the upper limit of the reheating time t5 is, for example, 1200 seconds or less, further 900 seconds or less, and further 600 seconds or less.
- the average heating rate HR2 in the heating step from the above-described cooling stop temperature T4 to the reheating temperature T5 shown in [8] in FIG. 1 is not particularly limited. Further, the cooling after the reheating shown in [10] in FIG. 1 may be performed at an average cooling rate CR4 of 200 ° C. or lower, for example, to room temperature, preferably 2 to 20 ° C./second.
- the hot-rolled steel sheet was pickled to remove the surface scale, and then cold-rolled to obtain a 1.4 mm cold-rolled steel sheet as an original sheet.
- the original plate was heat-treated under the conditions shown in Table 2-1 and Table 2-2 to obtain a sample.
- the average heating rate HR2 in the temperature raising process from the cooling stop temperature T4 to the reheating temperature T5 was set to 30 ° C./second except for No. 17.
- the steel structure was evaluated and the characteristics were evaluated as follows.
- Total area ratio of tempered martensite and bainite The total area ratio of tempered martensite and bainite is obtained by subtracting the above-described ferrite fraction from the entire steel structure; and the MA fraction determined by the following method, that is, the sum of residual austenite and as-quenched martensite. was determined by
- the amount of retained austenite is calculated by obtaining the diffraction intensity ratio of ferrite, bainite, tempered martensite and martensite which are body-centered cubic lattices or body-centered tetragonal lattices; and austenite which is face-centered cubic lattices by X-ray diffraction. Can be obtained. In detail, it measured as follows. In other words, after polishing using a sandpaper of # 1000 to # 1500 to a thickness of 1/4, the surface is further electropolished to a depth of about 10 to 20 ⁇ m, and then an X-ray diffraction apparatus manufactured by Rigaku Corporation.
- Measurement was performed using a two-dimensional microscopic X-ray diffractometer (RINT-RAPIDII). Specifically, a Co target was used, a Co—K ⁇ ray was used as the X-ray source, an output of about 40 kV-200 mA was output, and a range of 40 ° to 130 ° was measured at 2 ⁇ . From the obtained diffraction peaks (110), (200), (211) of bcc ( ⁇ ), and diffraction peaks (111), (200), (220), (311) of fcc ( ⁇ ), Quantitative measurement was performed.
- RINT-RAPIDII two-dimensional microscopic X-ray diffractometer
- the average equivalent circle diameter of retained austenite and the ratio of the retained austenite amount with an equivalent circle diameter of 1.5 ⁇ m or more to the total retained austenite amount are obtained by using the EBSD method, which is a crystal analysis method using SEM, as described above. Sought by creating. Specifically, the surface of the sample is electropolished, and at a quarter of the plate thickness, a total of 3 regions (the size of each region is 100 ⁇ m ⁇ 100 ⁇ m), one step: 180,000 points of EBSD measurement at 0.25 ⁇ m ( Temse Laboratories OIM system).
- the residual austenite having an equivalent circle diameter of 1.5 ⁇ m or more occupies the total retained austenite. Got a proportion.
- the ratio of the retained austenite having an equivalent circle diameter of 1.5 ⁇ m or more to the total retained austenite thus obtained is an area ratio, but is equivalent to a volume ratio.
- LDR Deep drawability was evaluated by LDR.
- LDR is obtained as follows. That is, in the cylindrical drawing, the diameter of the obtained cylinder is d, and the maximum diameter of a disk-shaped steel plate (blank) that can obtain a cylinder without being broken by one deep drawing is D. LDR is obtained from d / D.
- a disk-shaped sample having a plate thickness of 1.4 mm and various diameters was subjected to cylindrical deep drawing with a die having a punch diameter of 50 mm, a punch angle radius of 6 mm, a die diameter of 55.2 mm, and a die angle radius of 8 mm.
- the sample diameter (maximum diameter D) that was drawn without breaking was determined, and d / D was determined to be LDR. And it evaluated that the case where LDR was 2.05 or more was excellent in deep drawability.
- No. 1, 2, 7, 11-16, 20, 22-27, and 34-40 all have the component composition defined in the embodiment of the present invention, and are manufactured under the defined conditions and have a desired steel structure.
- the component composition defined in the embodiment of the present invention are manufactured under the defined conditions and have a desired steel structure.
- high tensile strength and excellent deep drawability but also excellent strength-ductility balance, high yield ratio and excellent hole expandability, that is, high hole expansion ratio are exhibited.
- examples other than the above do not satisfy the prescribed component composition or do not satisfy the prescribed production conditions, so that a desired steel structure cannot be obtained, and as a result, the result is inferior to at least one of the above characteristics. Details are shown below.
- No. No. 17 is an example in which the steps [6] to [9] in FIG. 1 are not performed, and after the rapid cooling of [4] in FIG.
- this example since cooling and reheating were not performed at an average cooling rate CR3, martensitic transformation did not occur, and MA and residual austenite became coarse. As a result, the hole expandability decreased.
- this No. 17 Since No. 17 is significantly different from the structure defined in the embodiment of the present invention, the average equivalent circle diameter of retained austenite is coarse, but the deep drawability is ensured.
- No. No. 18 is an example in which there is no residence step of [5] in FIG. In this case, retained austenite having an equivalent circle diameter of 1.5 ⁇ m or more could not be secured, and the deep drawability deteriorated.
- No. No. 32 is an example in which the amount of Mn is insufficient, so the ferrite becomes excessive, and tempered martensite and bainite cannot be obtained sufficiently. As a result, TS and YR decreased.
- Aspect 1 Ingredient composition is mass%, C: 0.15 to 0.35%, Total of Si and Al: 0.5 to 2.5% Mn: 1.0 to 4.0%, P: more than 0% and 0.05% or less, and S: more than 0% and 0.01% or less, with the balance consisting of Fe and inevitable impurities,
- the steel structure is the ratio of the entire structure.
- Aspect 2 The high strength steel plate according to aspect 1, wherein the C content in the component composition is 0.30% or less.
- Aspect 3 The high-strength steel sheet according to aspect 1 or 2, wherein the Al content in the component composition is less than 0.10%.
- Aspect 4 The high-strength steel sheet according to any one of aspects 1 to 3, further comprising, in mass%, one or more selected from the group consisting of Cu, Ni, Mo, Cr, and B in total exceeding 0% and 1.0% or less.
- Aspect 5 The high-strength steel sheet according to any one of aspects 1 to 4, further comprising, in mass%, one or more selected from the group consisting of V, Nb, Ti, Zr, and Hf in total exceeding 0% and 0.2% or less.
- Aspect 6 The high-strength steel sheet according to any one of aspects 1 to 5, further comprising, in mass%, one or more selected from the group consisting of Ca, Mg, and REM in total exceeding 0% and 0.01% or less.
- step C of performing slow cooling at an average cooling rate of 0 ° C./second or more and 10 ° C./second or less in a temperature range of 300 to 500 ° C. for 10 seconds or more and less than 300 seconds after the rapid cooling
- step D of cooling from a slow cooling end temperature T3b of 300 ° C. or higher to a cooling stop temperature T4 of 100 to 300 ° C. at an average cooling rate exceeding 10 ° C./second, and reheating at 300 to 500 ° C.
- Process E for reheating to temperature T5
- Aspect 8 The production method according to aspect 7, wherein the step C includes holding at a constant temperature in a temperature range of 300 to 500 ° C.
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Abstract
Description
成分組成が、質量%で、
C:0.15~0.35%、
SiとAlの合計:0.5~2.5%、
Mn:1.0~4.0%、
P:0%超0.05%以下、および
S:0%超0.01%以下
を含み、残部がFeおよび不可避不純物からなり、
鋼組織が、全組織に占める割合で、
フェライト:5面積%超50面積%以下、
焼戻しマルテンサイトとベイナイトの合計:30面積%以上、および
残留オーステナイト:10体積%以上
を満たし、更にMAを有するものであり、かつ、
MAの平均円相当直径:1.0μm以下、
残留オーステナイトの平均円相当直径:1.0μm以下、および
円相当直径が1.5μm以上の残留オーステナイトが、全残留オーステナイトに占める体積割合:5%以上
を満たすところに特徴を有する。
(a)Cu、Ni、Mo、Cr、およびBよりなる群から選ばれる1種以上を合計で0%超1.0%以下、
(b)V、Nb、Ti、Zr、およびHfよりなる群から選ばれる1種以上を合計で0%超0.2%以下、
(c)Ca、Mg、およびREM(Rare Earth Metal:希土類元素)よりなる群から選ばれる1種以上を合計で0%超0.01%以下、
を含んでもよい。
(Ac1点×0.8+Ac3点×0.2)以上Ac3点未満の温度T1に加熱する工程A、
前記加熱後、650℃以上の急冷開始温度T2から、300~500℃の冷却停止温度T3aまでを、平均冷却速度30℃/秒以上200℃/秒未満で急冷する工程B、
前記急冷後、300~500℃の温度域で、平均冷却速度0℃/秒以上10℃/秒以下の徐冷却を10秒以上300秒未満行う工程C、
前記徐冷却後、300℃以上の徐冷却終了温度T3bから、100~300℃の冷却停止温度T4までを、平均冷却速度10℃/秒超で冷却する工程D、および
300~500℃の再加熱温度T5まで再加熱を行う工程E
をこの順に含むところに特徴を有する。
以下に本発明の実施形態に係る高強度鋼板の鋼組織の詳細を説明する。尚、以下の鋼組織の説明では、そのような組織を有することにより各種の特性を向上できるメカニズムについて説明している場合がある。これらは本発明者らが現時点で得られている知見により考えたメカニズムであるが、本発明の技術的範囲を限定するものではないことに留意されたい。
フェライトは、一般的に延性に優れるものの、強度が低いという問題を有する。本発明の実施形態では、延性を向上させて優れた強度-延性バランスを得るため、フェライト量を5面積%超とした。フェライト量は、好ましくは7面積%以上、より好ましくは10面積%以上である。一方、フェライトが多いと穴広げ性および/または降伏比が低下する。よって優れた穴広げ率λおよび高い降伏比を確保すべく、フェライト量を50面積%以下とする。フェライト量は、好ましくは45面積%以下、より好ましくは40面積%以下である。
全組織に占める焼戻しマルテンサイトとベイナイトの合計を30面積%以上とすることで、高強度と高い穴広げ性を両立できる。なお、本発明の実施形態におけるベイナイトとは、焼戻しベイナイト、未焼戻しベイナイト、およびベイニティックフェライトをさす。前記焼戻しマルテンサイトとベイナイトの合計は好ましくは35面積%以上である。尚、前記焼戻しマルテンサイトとベイナイトの合計の上限は、その他の必須組織であるフェライトおよび残留オーステナイトの分率を考慮すると、85面積%程度となる。
残留オーステナイトは、プレス加工等の加工中に、加工誘起変態によりマルテンサイトに変態するTRIP現象を生じ、大きな伸びを得ることのできる組織である。また、形成されるマルテンサイトは高い硬度を有する。このため、優れた強度-延性バランスを得ることができる。更には、後記の通り残留オーステナイトの粒径分布を制御することで、深絞り性も高めることができる。特にTS×ELが22000MPa%以上の優れた強度-延性バランスを得ると共に、後記の残留オーステナイトの粒径分布制御による深絞り性向上効果を十分に発揮させるべく、残留オーステナイト量を10体積%以上とする。残留オーステナイト量は好ましくは12体積%以上、より好ましくは13体積%以上、更に好ましくは14体積%以上、より更に好ましくは15体積%以上である。尚、本発明の実施形態で規定する成分組成および製造条件等を考慮すると、残留オーステナイト量の上限はおおよそ40体積%となる。
MAは硬質相であり、変形時に母相/硬質相の界面近傍がボイド形成サイトとして働く。MAサイズが粗大になるほど、母相/硬質相の界面への歪集中が起こり、母相/硬質相の界面近傍に形成されたボイドを基点とした破壊が生じ易くなる。その結果、穴広げ性の低下をまねく。このため、MAの平均円相当直径を1.0μm以下に抑える。該MAの平均円相当直径は、好ましくは0.8μm以下、より好ましくは0.7μm以下である。MAの平均円相当直径は、小さいほど穴広げ性が高まるため好ましいが、本発明の実施形態で規定する成分組成および製造条件を考慮すると、MAの平均円相当直径の下限はおおよそ0.2μmとなる。
[円相当直径が1.5μm以上の残留オーステナイトが、全残留オーステナイトに占める割合:5%以上]
本発明の実施形態では、上述の通り、残留オーステナイトの平均粒径を一定以下に抑えつつ、比較的大きな残留オーステナイトの割合を一定以上確保することによって、特に優れた深絞り性を達成できることを見出した。以下、この様に規定した理由について詳述する。
以下に本発明の実施形態に係る高強度鋼板の組成について説明する。なお、成分組成について単位の%表示は、すべて質量%を意味する。
Cは残留オーステナイト等の所望の組織を得て、TS×EL等の高い特性確保のために必須の元素である。このような作用を有効に発揮させるためC量を0.15%以上とする。C量は、好ましくは0.17%以上、より好ましくは0.20%以上である。一方、C量が0.35%超だと、MAおよび残留オーステナイトが粗大となり、その結果、深絞り性と穴広げ性のどちらも低下する。また、C量が過剰であると溶接性にも劣る。よってC量は0.35%以下とする。C量は、好ましくは0.33%以下、より好ましくは0.30%以下である。
SiとAlは、それぞれ、セメンタイトの析出を抑制し、残留オーステナイトの形成を促進する働きを有する。このような作用を有効に発揮させるため、SiとAlを合計で0.5%以上含有させる。SiとAlの合計は、好ましくは0.7%以上、より好ましくは1.0%以上である。一方、SiとAlの合計が2.5%を超えると、焼戻しマルテンサイトおよびベイナイトを確保できなくなるとともに、MAおよび残留オーステナイトが粗大となる。よって、SiとAlの合計は、2.5%以下、好ましくは2.0%以下とする。
Mnは、フェライトの形成を抑制して、フェライト以外の焼戻しマルテンサイト/ベイナイト等の確保に必要な元素である。このような作用を有効に発揮させるため、Mn量を1.0%以上とする。Mn量は、好ましくは1.5%以上、より好ましくは2.0%以上である。ただしMn量が4.0%を超えると、製造過程でベイナイト変態が抑制されて、上述した比較的粗大な残留オーステナイトが十分に得られず、その結果、優れた深絞り性を確保できない。よって、Mn量は4.0%以下、好ましくは3.7%以下、より好ましくは3.5%以下とする。
Pは不純物元素として不可避的に存在する。P量が0.05%を超えると、全伸びELおよび穴広げ性が劣化する。よって、P量は0.05%以下、好ましくは0.03%以下とする。
Sは不純物元素として不可避的に存在する。S量が0.01%を超えると、MnS等の硫化物系介在物が形成され、これが割れの起点となって穴広げ性が低下する。このため、S量は0.01%以下、好ましくは、0.005%以下である。
Cu、Ni、Mo、Cr、およびBは、鋼板の引張強度を高める元素として作用すると共に、残留オーステナイトを安定化し、所定量確保するために有効に作用する元素である。こうした効果を有効に発揮させるには、上記元素は合計量で0.001%以上含有することが好ましく、より好ましくは0.01%以上である。しかし、上記元素を過剰に含有させてもその効果は飽和し、経済的に無駄であるので、上記元素は合計量で1.0%以下が好ましく、より好ましくは0.5%以下である。なお、上記元素は、単独で、または任意に選ばれる2種以上を含有させることができる。
V、Nb、Ti、Zr、およびHfは、析出強化および組織微細化の効果を発揮させる元素であり、鋼板の高強度化に有用に作用する。こうした効果を有効に発揮させるには、上記元素は合計量で0.01%以上含有することが好ましく、より好ましくは0.02%以上である。しかし、上記元素を過剰に含有させてもその効果は飽和し、経済的に無駄であるので、上記元素は合計量で0.2%以下が好ましく、より好ましくは0.1%以下である。なお、上記元素は、単独で、または任意に選ばれる2種以上を含有させることができる。
Ca、Mg、およびREMは、鋼中硫化物の形態を制御し、加工性向上に有効に作用する元素である。こうした効果を有効に発揮させるには、上記元素は合計量で0.001%以上含有することが好ましく、より好ましくは0.002%以上である。しかし、上記元素を過剰に含有させてもその効果は飽和し、経済的に無駄であるので、上記元素は合計量で0.01%以下、さらには0.005%以下とするのが好ましい。なお、上記元素は、単独で、または任意に選ばれる2種以上を含有させることができる。上記REMは、Sc、Yおよびランタノイド等が挙げられる。
上述のように本発明の実施形態に係る高強度鋼板は、TS、YR、TS×EL、λおよびLDRが何れも高いレベルにある。本発明の実施形態に係る高強度鋼板のこれらの特性について以下に詳述する。
本発明の実施形態に係る高強度鋼板は、引張強度が780MPa以上である。該引張強度は、好ましくは880MPa以上、より好ましくは980MPa以上である。強度が高いほど好ましいが本発明の実施形態に係る鋼板の成分組成および製造条件等を考慮すると、引張強度の上限はおおよそ1300MPaである。
本発明の実施形態に係る高強度鋼板は、降伏比が0.60以上である。これにより上述の高い引張強度と相まって高い降伏強度を実現できる。その結果、深絞り加工等の加工により得た最終製品は、高い負荷がかかった状態でも変形せずに使用できる。好ましくは、0.65以上の降伏比を有する。降伏比は高いほど好ましいが本発明の実施形態に係る鋼板の成分組成および製造条件等を考慮すると、降伏比の上限はおおよそ0.75である。
本発明の実施形態に係る高強度鋼板は、TS×ELが22000MPa%以上である。22000MPa%以上のTS×ELを有することで、高い強度と高い延性とを同時に有する、高いレベルの強度-延性バランスを得ることができる。TS×ELは、好ましくは23000MPa%以上である。TS×ELは高いほど好ましいが本発明の実施形態に係る鋼板の成分組成および製造条件等を考慮すると、TS×ELの上限はおおよそ35000MPa%である。
本発明の実施形態に係る高強度鋼板は、穴広げ率λが20%以上である。これによりプレス成形性等の優れた加工性を得ることができる。穴広げ率λは好ましくは30%以上である。穴広げ率λは高いほど好ましいが本発明の実施形態に係る鋼板の成分組成および製造条件等を考慮すると、穴広げ率λの上限はおおよそ60%である。
本発明の実施形態では、鋼板の深絞り性を後記の実施例に示す通りLDRで評価する。本発明の実施形態に係る高強度鋼板は、LDRが2.05以上であり、好ましくは2.10以上であり、優れた深絞り性を有している。LDRは高いほど好ましいが本発明の実施形態に係る鋼板の成分組成および製造条件等を考慮すると、LDRの上限はおおよそ2.25である。
次に本発明の実施形態に係る高強度鋼板の製造方法について説明する。本発明者らは、所定の組成を有する原板に対し、下記に詳述する工程A~工程Eを含む熱処理、特にはマルチステップのオーステンパー処理を行うことにより、上述の所望の鋼組織を有し、その結果、上述の所望の特性を有する高強度鋼板が得られることを見いだした。以下にその詳細を説明する。
工程Aは、図1では[2]に相当する。この図1の[2]に示すように、(Ac1点×0.8+Ac3点×0.2)以上Ac3点未満の温度T1に加熱する。この加熱温度T1が(Ac1点×0.8+Ac3点×0.2)を下回ると、オーステナイトへの逆変態が不充分となって加工組織が残存したままとなり、一定量のフェライトを確保できず、強度-延性バランスが低下する。従って、上記加熱温度T1は(Ac1点×0.8+Ac3点×0.2)以上、好ましくは(Ac1点×0.8+Ac3点×0.2)+5℃以上、より好ましくは(Ac1点×0.8+Ac3点×0.2)+10℃以上とする。しかし、加熱温度T1がAc3点以上になると、所望のフェライト量を確保できず、延性が劣化する。よって、加熱温度T1はAc3点未満とする。加熱温度T1は、好ましくはAc3点-10℃以下、より好ましくはAc3点-20℃以下である。
Ac1点=723-10.7Mn+29.1Si+16.9Cr・・・(1)
Ac3点=910-203C0.5+44.7Si+104V+31.5Mo-30Mn-11Cr+700P+400Al+400Ti・・・(2)
工程Bは、図1では[4]に相当する。図1の[4]に示す通り、上記650℃以上の急冷開始温度T2から、300~500℃の冷却停止温度T3aまでの間を、平均冷却速度CR2が30℃/秒以上の急冷とすることで、過剰なフェライトの形成を抑制できる。前記平均冷却速度CR2は、好ましくは35℃/秒以上、より好ましくは40℃/秒以上である。しかし上記平均冷却速度CR2が速すぎると、急激な冷却による過大な熱歪みが生じやすくなるため、上記平均冷却速度CR2は200℃/秒未満、好ましくは100℃/秒以下とする。
工程Cは、図1では[5]に相当する。図1の[5]に示す通り、この徐冷却開始温度=急冷終了温度T3aから、図1に示す徐冷却終了温度T3bまでが、300~500℃の温度域内であって、かつ上記T3aからT3bまでの平均冷却速度を0℃/秒以上10℃/秒以下、上記T3aからT3bまでの時間、即ち徐冷却時間t3を10秒以上300秒未満とする。これによって、部分的にベイナイトを形成させる。このベイナイトはオーステナイトよりも炭素の固溶限が低いため、固溶限を超えた炭素をはき出す。その結果、ベイナイト周囲に炭素の濃化したオーステナイトの領域が形成される。この領域が、後述する工程Dの冷却、および工程Eの再加熱を経て、比較的粗大な残留オーステナイトとなる。この比較的粗大な残留オーステナイトを存在させることによって、上述の通り、深絞り性を高めることができる。
工程Dは、図1では[6]に相当する。図1の[6]に示すように、300℃以上の徐冷却終了温度T3bから、100~300℃の冷却停止温度T4までを、10℃/秒超の平均冷却速度CR3で冷却することによって、上述の炭素濃化領域の一部をマルテンサイト変態させる。これにより、マルテンサイト変態を生じなかった残りの炭素濃化領域を微細化することができ、その結果、微細なMAが得られる。前記平均冷却速度CR3が10℃/秒以下であると、冷却中に炭素濃化領域が必要以上に広がり、MAが粗大になるため穴広げ率が低下する。前記平均冷却速度CR3は、好ましくは15℃/秒以上であり、より好ましくは20℃/秒以上である。尚、徐冷却終了温度T3bは前記工程Cで述べた通り500℃以下である。
工程Eは、図1では[9]に相当する。図1の[9]に示す通り、300~500℃の再加熱温度T5まで加熱し、該再加熱温度T5で保持することにより、マルテンサイト中の炭素が排出されて周囲のオーステナイトへの炭素濃化が促進され、オーステナイトを安定化させることができる。これにより、最終的に得られる残留オーステナイト量を増大させることができる。
表1に記載した成分組成(残部は鉄および不可避不純物)を有する鋳造材を真空溶製で製造した後、この鋳造材を熱間鍛造で板厚30mmとし、次いで、熱間圧延を施した。なお、表1には成分組成から計算したAc1点およびAc3点、並びに「0.8×Ac1点+0.2×Ac3点」の値も併せて示す。前記熱間圧延は、1200℃に加熱した後、多段圧延で板厚2.5mmとした。この時、熱間圧延の終了温度は880℃とした。その後、600℃まで30℃/秒で冷却し、冷却を停止し、600℃に加熱した炉に挿入後、30分保持し、その後、炉冷して熱延鋼板を得た。この熱延鋼板に対し、酸洗を施して表面のスケールを除去した後、冷間圧延を施して1.4mmの冷延鋼板を原板として得た。この原板に対し、表2-1および表2-2に示す条件の熱処理を行ってサンプルを得た。尚、本実施例では、冷却停止温度T4から再加熱温度T5までの昇温工程における平均加熱速度HR2は、No.17を除き30℃/秒とした。
得られたサンプルの板厚t/4位置の鋼組織を以下の通り測定した。
サンプルを電解研磨した後、レペラーで腐食し、光学顕微鏡(1000倍)で3視野(100μm×100μmサイズ/視野)観察し、格子間隔5μmおよび格子点数20×20の点算法にてフェライトの面積率を測定し、平均値を算出した。この面積比で求めた値はそのまま体積比(体積%)の値として用いることができる。
焼戻しマルテンサイトおよびベイナイトの合計面積率は、鋼組織全体から上述のフェライト分率と;下記の方法で求めたMAの分率、すなわち、残留オーステナイトと焼入れたままのマルテンサイトの合計と;を引くことにより求めた。
サンプルを電解研磨した後、ナイタール腐食を行い、SEM(5000倍)で3視野(20μm×20μmサイズ/視野)観察し、写真中の任意の位置に10μmの直線を20本引き、その直線とMAが交わる切片長を測定し、それら切片長の平均値を算出することでMAの平均円相当直径を求めた。MAの面積率は、MAの平均円相当直径と視野内に観察されるMA数に基づいて算出し、3視野の平均を求めた。
残留オーステナイト量は、X線回折により体心立方格子または体心正方格子であるフェライト、ベイナイト、焼戻しマルテンサイトおよびマルテンサイトと;面心立方格子であるオーステナイトと;の回折強度比を求めて算出することにより得ることができる。詳細には、次の通り測定した。即ち、板厚1/4位置まで#1000~#1500のサンドペーパーを使用して研磨した後、更に表面を深さ10~20μm程度まで電解研磨してから、X線回折装置として株式会社リガク製2次元微小部X線回折装置(RINT-RAPIDII)を用いて測定した。具体的にはCoターゲットを使用し、X線源としてはCo-Kα線を用い、40kV-200mA程度出力して2θで40°~130°の範囲を測定した。そして得られたbcc(α)の回折ピーク(110)、(200)、(211)、及びfcc(γ)の回折ピーク(111)、(200)、(220)、(311)から残留γの定量測定を行った。
残留オーステナイトの平均円相当直径と、円相当直径が1.5μm以上の残留オーステナイト量の全残留オーステナイト量に占める比率は、上述の通りSEMを用いた結晶解析手法であるEBSD法を用い、Phaseマップを作成することで求めた。詳細には、サンプル表面を電解研磨し、板厚の1/4位置にて、合計3領域(1領域のサイズは100μm×100μm)について、1ステップ:0.25μmで18万点のEBSD測定(テクセムラボラトリーズ社製OIMシステム)を実施した。この測定で得られたPhaseマップから、個々のオーステナイト相(残留オーステナイト)の面積を求め、その面積から個々のオーステナイト相の円相当直径を求め、その平均値を、残留オーステナイトの平均円相当直径とした。
得られたサンプルについて、下記に示す通り、引張試験を行って降伏強度YS、引張強度TS、全伸びELを測定し、降伏比YRおよびTS×ELを算出した。また、下記に示す方法で穴広げ率λを測定すると共に、深絞り性を評価した。
上記サンプルから、圧延方向に対して垂直な方向が長手方向となるようにJIS Z2201で規定される5号試験片を採取した。そして該試験片を用い、引張試験機にてJIS Z2241の条件で引張試験を行い、YS、TS、YR、EL、およびTS×ELを求めた。そして、前記TSが780MPa以上、前記YRが0.60以上、前記TS×ELが22000MPa%以上の場合を、高強度、高い降伏比、かつ強度-延性バランスに優れていると評価した。
穴広げ率λは、日本鉄鋼連盟規格 JFS T1001に従って求めた。詳細には、試験片に直径d0(d0=10mm)の打ち抜き穴を開け、先端角度が60°のポンチをこの打ち抜き穴に押し込み、発生した亀裂が試験片の板厚を貫通した時点の打ち抜き穴の直径dを測定し、下記の式より求めた。そして、該穴広げ率λが20%以上の場合を、プレス成形性等の加工性に優れていると評価した。
λ(%)={(d-d0)/d0}×100
深絞り性はLDRで評価した。LDRは次の様にして求められる。即ち円筒絞り成形において、得られる円筒の直径をdとし、1回の深絞り加工で破断を生じずに円筒を得ることができる円盤状の鋼板(ブランク)の最大直径をDとする。そしてLDRは、d/Dから求められる。本実施例では、板厚1.4mmで各種径を有する円盤状の試料を、パンチ径50mm、パンチ角半径6mm、ダイ径55.2mmおよびダイ角半径8mmの金型で円筒深絞りを行い、破断することなく絞り抜けた試料径(最大直径D)を求め、d/Dを求めてLDRとした。そしてLDRが2.05以上の場合を深絞り性に優れていると評価した。
態様1:
成分組成が、質量%で、
C:0.15~0.35%、
SiとAlの合計:0.5~2.5%、
Mn:1.0~4.0%、
P:0%超0.05%以下、および
S:0%超0.01%以下
を含み、残部がFeおよび不可避不純物からなり、
鋼組織が、全組織に占める割合で、
フェライト:5面積%超50面積%以下、
焼戻しマルテンサイトとベイナイトの合計:30面積%以上、および
残留オーステナイト:10体積%以上
を満たし、更にMAを有するものであり、かつ、
MAの平均円相当直径:1.0μm以下、
残留オーステナイトの平均円相当直径:1.0μm以下、および
円相当直径が1.5μm以上の残留オーステナイトが、全残留オーステナイトに占める体積割合:5%以上
を満たすことを特徴とする高強度鋼板。
態様2:
前記成分組成におけるC量が0.30%以下である態様1に記載の高強度鋼板。
態様3:
前記成分組成におけるAl量が0.10%未満である態様1または2に記載の高強度鋼板。
態様4:
更に、質量%で、Cu、Ni、Mo、Cr、およびBよりなる群から選ばれる1種以上を合計で0%超1.0%以下含む態様1~3のいずれかに記載の高強度鋼板。
態様5:
更に、質量%で、V、Nb、Ti、Zr、およびHfよりなる群から選ばれる1種以上を合計で0%超0.2%以下含む態様1~4のいずれかに記載の高強度鋼板。
態様6:
更に、質量%で、Ca、Mg、およびREMよりなる群から選ばれる1種以上を合計で0%超0.01%以下含む態様1~5のいずれかに記載の高強度鋼板。
態様7:
態様1~6のいずれかに記載の高強度鋼板を製造する方法であって、
態様1~6のいずれかに記載の成分組成を満たす原板を用い、
(Ac1点×0.8+Ac3点×0.2)以上Ac3点未満の温度T1に加熱する工程A、
前記加熱後、650℃以上の急冷開始温度T2から、300~500℃の冷却停止温度T3aまでを、平均冷却速度30℃/秒以上200℃/秒未満で急冷する工程B、
前記急冷後、300~500℃の温度域で、平均冷却速度0℃/秒以上10℃/秒以下の徐冷却を10秒以上300秒未満行う工程C、
前記徐冷却後、300℃以上の徐冷却終了温度T3bから、100~300℃の冷却停止温度T4までを、平均冷却速度10℃/秒超で冷却する工程D、および
300~500℃の再加熱温度T5まで再加熱を行う工程E
をこの順に含むことを特徴とする高強度鋼板の製造方法。
態様8:
前記工程Cでは、300~500℃の温度域にて、一定温度で保持することを含む態様7に記載の製造方法。
Claims (6)
- 成分組成が、質量%で、
C:0.15~0.35%、
SiとAlの合計:0.5~2.5%、
Mn:1.0~4.0%、
P:0%超0.05%以下、および
S:0%超0.01%以下
を含み、残部がFeおよび不可避不純物からなり、
鋼組織が、全組織に占める割合で、
フェライト:5面積%超50面積%以下、
焼戻しマルテンサイトとベイナイトの合計:30面積%以上、および
残留オーステナイト:10体積%以上
を満たし、更にMAを有するものであり、かつ、
MAの平均円相当直径:1.0μm以下、
残留オーステナイトの平均円相当直径:1.0μm以下、および
円相当直径が1.5μm以上の残留オーステナイトが、全残留オーステナイトに占める体積割合:5%以上
を満たすことを特徴とする高強度鋼板。 - 前記成分組成におけるC量が0.30%以下である請求項1に記載の高強度鋼板。
- 前記成分組成におけるAl量が0.10%未満である請求項1に記載の高強度鋼板。
- 更に、質量%で、以下の(a)~(c)のいずれか1つ以上を含有する請求項1に記載の高強度鋼板。
(a)Cu、Ni、Mo、Cr、およびBよりなる群から選ばれる1種以上を合計で0%超1.0%以下
(b)V、Nb、Ti、Zr、およびHfよりなる群から選ばれる1種以上を合計で0%超0.2%以下
(c)Ca、Mg、およびREMよりなる群から選ばれる1種以上を合計で0%超0.01%以下 - 請求項1~4のいずれかに記載の高強度鋼板を製造する方法であって、
請求項1~4のいずれかに記載の成分組成を満たす原板を用い、
(Ac1点×0.8+Ac3点×0.2)以上Ac3点未満の温度T1に加熱する工程A、
前記加熱後、650℃以上の急冷開始温度T2から、300~500℃の冷却停止温度T3aまでを、平均冷却速度30℃/秒以上200℃/秒未満で急冷する工程B、
前記急冷後、300~500℃の温度域で、平均冷却速度0℃/秒以上10℃/秒以下の徐冷却を10秒以上300秒未満行う工程C、
前記徐冷却後、300℃以上の徐冷却終了温度T3bから、100~300℃の冷却停止温度T4までを、平均冷却速度10℃/秒超で冷却する工程D、および
300~500℃の再加熱温度T5まで再加熱を行う工程E
をこの順に含むことを特徴とする高強度鋼板の製造方法。 - 前記工程Cでは、300~500℃の温度域にて、一定温度で保持することを含む請求項5に記載の製造方法。
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JP6569842B1 (ja) * | 2018-12-11 | 2019-09-04 | 日本製鉄株式会社 | 成形性、靱性、及び、溶接性に優れた高強度鋼板、及び、その製造方法 |
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EP3896185A4 (en) * | 2018-12-11 | 2022-04-13 | Nippon Steel Corporation | HIGH-STRENGTH STEEL PLATE EXCELLENT IN FORMABILITY, TENACITY AND WELDABILITY, AND PRODUCING METHOD THEREOF |
CN113166865B (zh) * | 2018-12-11 | 2022-07-12 | 日本制铁株式会社 | 成形性、韧性及焊接性优异的高强度钢板及其制造方法 |
KR102536689B1 (ko) | 2018-12-11 | 2023-05-30 | 닛폰세이테츠 가부시키가이샤 | 성형성, 인성 및 용접성이 우수한 고강도 강판, 및 그 제조 방법 |
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