WO2011118459A1 - Ultra high strength cold rolled steel sheet and method for producing same - Google Patents
Ultra high strength cold rolled steel sheet and method for producing same Download PDFInfo
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- WO2011118459A1 WO2011118459A1 PCT/JP2011/056128 JP2011056128W WO2011118459A1 WO 2011118459 A1 WO2011118459 A1 WO 2011118459A1 JP 2011056128 W JP2011056128 W JP 2011056128W WO 2011118459 A1 WO2011118459 A1 WO 2011118459A1
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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
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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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to an ultra-high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more and used for a vehicle body structural member such as an automobile center pillar or door impact beam manufactured mainly by press working or roll forming, and a method for manufacturing the same. Is.
- the martensitic single-phase steel sheet is an upper critical cooling in which a second phase typified by a ferrite phase and a pearlite phase is not formed in a steel sheet having an austenite phase single-phase structure by soaking at a temperature equal to or higher than the Ac 3 transformation point. It is generally understood that it is manufactured by cooling to the Ms point or less at a cooling rate that is higher than or equal to the rate (hereinafter, this cooling is also referred to as “quenching”).
- the deterioration of the shape of the steel sheet due to the above quenching not only affects the operability in the continuous annealing process and the manufacturability in the subsequent process, but also when the steel sheet is processed into a vehicle structural member by press forming or roll forming. However, it causes problems such as operation troubles in the molding line and adverse effects on the dimensional accuracy of the product. Therefore, in order to stably use the martensitic single-phase steel sheet as a material for the structural member of an automobile body, it is also important that the steel sheet has excellent flatness in addition to high strength.
- the warp height in the width direction of the product steel plate as shown in FIG. 1 is desired to be 10 mm or less.
- Patent Document 1 discloses the metal structure of a steel sheet based on the results of investigating the relationship between the warpage height of a steel sheet after continuous annealing with a tensile strength of 1470 MPa to 1960 MPa and the martensite volume ratio in the metal structure.
- a technique for obtaining predetermined mechanical characteristics and an excellent steel sheet shape by forming a two-phase structure composed of a martensite phase having a volume ratio of 80 to 97% and the balance of a ferrite phase is disclosed.
- Patent Document 2 after continuous annealing to obtain a martensitic single-phase steel sheet having a tensile strength of 1049 to 1240 MPa, temper rolling is performed so that the average roughness Ra of the steel sheet surface is 1.4 ⁇ m or more.
- the technique of obtaining a favorable steel plate shape by applying is disclosed.
- the method of correcting the steel sheet shape by temper rolling as in the technique of Patent Document 2 is not a technique for suppressing the deterioration of the steel sheet shape that occurs during quenching, it is necessary to improve the operability in the continuous annealing process. I can not connect it.
- shape correction by temper rolling requires, for example, a very high rolling load for a high-strength steel sheet having a tensile strength of 1320 MPa or more, and a sufficient shape correction effect cannot be obtained with existing rolling equipment.
- the increase in the surface roughness of the steel sheet is unsuitable for applications that require surface aesthetics, and there is a problem that the fatigue characteristics may be deteriorated due to the increase in the surface roughness.
- the present invention has been made in view of the above problems, and its purpose is to suppress the deterioration of the shape of the steel sheet during quenching in continuous annealing itself, thereby achieving ultrahigh strength cold rolling having high flatness. It is to provide a steel sheet and to propose an advantageous manufacturing method thereof.
- the inventors have intensively studied to solve the above problems of the conventional technology.
- the deterioration of the shape of the martensitic single-phase steel sheet caused by volume shrinkage due to high-speed cooling during quenching and volume expansion due to martensitic transformation is caused by cooling from the soaking temperature to the Ms point during quenching during continuous annealing. It is divided into primary cooling that cools to the vicinity immediately above and secondary cooling that cools to the temperature below 100 ° C. from the vicinity immediately above the Ms point, and during that time, the steel sheet is held at the temperature near the Ms point for a predetermined time to equalize the steel sheet temperature. Was found to be effective, and the present invention was completed.
- the present invention includes C: 0.05 to 0.40 mass%, Si: 2.0 mass% or less, Mn: 1.0 to 3.0 mass%, P: 0.05 mass% or less, S: 0.02 mass%.
- Al 0.01 to 0.05 mass%
- N less than 0.005 mass%
- the balance is composed of Fe and inevitable impurities
- the metal structure is a single martensite phase
- the ultra-high-strength cold-rolled steel sheet of the present invention is characterized in that the metal structure is a tempered martensite single phase.
- the ultra-high strength cold-rolled steel sheet of the present invention is characterized in that the tensile strength is 1320 MPa or more.
- the ultra-high strength cold-rolled steel sheet of the present invention further includes Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, B: 0.0005 to 0.0030 mass%, and Cu: It contains one or two or more selected from 0.20 mass% or less.
- C 0.05 to 0.40 mass%, Si: 2.0 mass% or less, P: 0.05 mass% or less, S: 0.02 mass% or less, Al: 0.01 to 0.05 mass %, N: less than 0.005 mass%, Mn: 1.0 to 3.0 mass%, and the steel sheet after cold rolling having a component composition with the balance consisting of Fe and inevitable impurities is subjected to continuous annealing and tensile strength
- the temperature ranges from the soaking temperature not lower than the Ac 3 transformation point to the temperature range of Ms point to Ms point + 200 ° C. determined by the following formula (1).
- the method for producing an ultra-high strength cold-rolled steel sheet according to the present invention is characterized in that after secondary cooling, it is reheated and tempered at 100 to 250 ° C. for 120 to 1800 seconds.
- the manufacturing method of the ultra-high strength cold-rolled steel sheet of the present invention is characterized in that primary cooling and secondary cooling are performed by water cooling.
- the steel sheet after cold rolling in the production method of the present invention further includes Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, B: 0.0005 to 0.0030 mass%.
- Cu It contains 1 type, or 2 or more types chosen from 0.20 mass% or less, It is characterized by the above-mentioned.
- the present invention it is possible to suppress the deterioration of the shape itself that occurs at the time of quenching the steel sheet in the continuous annealing process, so that not only the improvement in manufacturability in the continuous annealing process etc., but also the shape correction cost by temper rolling etc. It also contributes greatly to reduction.
- the technology of the present invention can also be applied to ultra-high-strength steel sheets having a tensile strength of 1320 MPa or more, which is considered difficult to correct the shape in temper rolling, etc., so that the use of ultra-high-strength martensitic single-phase steel sheets is expanded. Also contributes.
- an ultra-high-strength cold-rolled steel sheet having sufficient flatness can be stably obtained, so that the productivity when manufacturing a structural member for automobiles by press molding or roll molding is improved. And can greatly contribute to quality improvement such as dimensional accuracy.
- the shape deterioration that occurs in a martensitic single-phase steel sheet during quenching in the continuous annealing process is due to the occurrence of non-uniform stress inside the steel sheet due to volume shrinkage associated with high-speed cooling and volume expansion associated with martensitic transformation. .
- the volume shrinkage accompanying high-speed cooling and the stress generated thereby increase in proportion to the temperature difference between the temperature at which cooling starts and the temperature at which cooling ends.
- the volume expansion accompanying the martensitic transformation is uniform when the metal structure after the final cooling is a martensite single phase structure.
- the influence on the steel sheet shape due to quenching can be considered to be only a uniform volume expansion accompanying martensitic transformation, and the temperature range below the Ms point.
- the effect of the cooling rate on the steel plate shape is considered to be small.
- the difference between the cooling start temperature and the cooling end temperature may be reduced in order to reduce the stress generated in the steel sheet due to the volume shrinkage during quenching. Therefore, in the present invention, after the primary cooling in which the quenching of the steel sheet in the continuous annealing process is cooled from the soaking temperature not lower than the Ac 3 transformation point to the temperature immediately above the Ms point, the steel sheet temperature is maintained at the temperature near the Ms point for a predetermined time. Then, the temperature distribution in the steel sheet was made uniform, and then secondary cooling was performed from the temperature immediately above the Ms point to 100 ° C. or less to cause martensitic transformation. In this way, the steel sheet of the present invention can be manufactured by minimizing the stress generated with volume shrinkage during quenching.
- C 0.05-0.40 mass%
- C is an element that stabilizes the austenite phase and is an element necessary for ensuring the strength of the steel sheet.
- C is less than 0.05 mass%, it is difficult to obtain a martensitic single-phase steel sheet having a desired tensile strength (980 MPa or more).
- the amount of C exceeds 0.40 mass%, rolling before the continuous annealing process becomes difficult, and the transformation strain and transformation stress accompanying martensitic transformation may remarkably increase to cause firing cracks. This is not preferable in production. Therefore, in the present invention, C is set in the range of 0.05 to 0.40 mass%. The range is preferably from 0.15 to 0 and 30 mass%.
- Si 2.0 mass% or less
- Si is a substitutional solid solution strengthening element that is effective for increasing the strength without impairing the workability of the steel sheet.
- Si is also an element that shifts the Ac 3 transformation point to the high temperature side
- excessive addition of Si is not preferable because it causes an increase in annealing temperature and an increase in annealing cost.
- Si is set to 2.0 mass% or less. Preferably it is 1.5 mass% or less.
- Mn 1.0 to 3.0 mass%
- Mn is an element that stabilizes the austenite phase and makes it easier to obtain a martensite structure.
- Mn is less than 1.0 mass%, the hardenability of the steel is not sufficient, and the ferrite phase, the pearlite phase, and the bainite phase start to form early during cooling from the soaking temperature during annealing, and the present invention
- it is difficult to stably obtain the intended martensite single-phase structure.
- Mn is in the range of 1.0 to 3.0 mass%.
- it is in the range of 1.5 to 2.5 mass%.
- P 0.05 mass% or less
- P is also an element that segregates at the grain boundary and promotes grain boundary destruction. Therefore, P is set to 0.05 mass% or less. Preferably it is 0.02 mass% or less, More preferably, it is 0.01 mass% or less. In addition, from a viewpoint of improving weldability, 0.008 mass% or less is desirable.
- S 0.02 mass% or less Since S becomes a sulfide-based inclusion such as MnS and induces a decrease in impact resistance and delayed fracture resistance, it is desirable that S be as low as possible. Therefore, the upper limit of S is 0.02 mass%. Preferably it is 0.002 mass% or less.
- Al 0.01 to 0.05 mass%
- Al is an element added for deoxidation in the steelmaking process, and it is necessary to add 0.01 mass% or more in order to obtain a sufficient deoxidation effect. On the other hand, when it adds excessively, the inclusion in a steel plate will increase and the ductility will fall. Therefore, Al is set in the range of 0.01 to 0.05 mass%.
- N Less than 0.005 mass% N is an element that forms a nitride. In particular, when the content is 0.005 mass% or more, the decrease in ductility at high and low temperatures due to the formation of nitride increases. Therefore, N is limited to less than 0.005 mass%.
- Nb, Ti, B and Cu can be added to the ultra-high strength cold-rolled steel sheet of the present invention within the following ranges depending on the purpose.
- Nb 0.1 mass% or less
- Ti 0.1 mass% or less
- Nb and Ti are effective elements for refining crystal grains and increasing the strength of a steel sheet.
- Nb and Ti are added in amounts exceeding 0.1 mass%, the effect is saturated, which is not economically preferable. Therefore, when adding Nb and Ti, respectively, it shall be 0.1 mass% or less.
- B 0.0005 to 0.0030 mass%
- B is an element effective for enhancing the hardenability and increasing the steel sheet strength. However, if B is less than 0.0005 mass%, the above-mentioned strength increasing effect cannot be expected. On the other hand, if B exceeds 0.0030 mass%, the hot workability deteriorates, which is not preferable for production. Therefore, when adding B, it is set as the range of 0.0005-0.0030 mass%.
- Cu 0.20 mass% or less Cu stabilizes the austenite phase, makes it easy to obtain a martensite single-phase structure, and forms a concentrated layer on the surface of the steel sheet in a corrosive environment. It is an element that has the effect of suppressing penetration and improving delayed fracture resistance. However, when the addition amount exceeds 0.20 mass%, these effects are saturated. Therefore, it is preferable to add Cu with an upper limit of 0.20 mass%.
- the balance other than the above elements is Fe and inevitable impurities. However, addition of other elements is not rejected as long as the effects of the present invention are not impaired.
- the ultra-high-strength cold-rolled steel sheet of the present invention requires that the metal structure is a martensite single phase.
- the martensite phase may not be generated in the range of 10 ⁇ m in the thickness direction from the steel sheet surface due to the influence of decarburization or the like in the manufacturing process, it is necessary to exclude this range.
- an austenite phase may remain in the matrix structure of the steel sheet, which is called a retained austenite phase. If the residual austenite phase is less than 0.5% in volume ratio, it can be regarded as a martensite single phase structure.
- carbides, nitrides, and inclusions are inevitably present in the steel sheet structure, but these are not included in the evaluation when determining whether or not these are martensite single phase structures.
- the ultra-high-strength cold-rolled steel sheet of the present invention has a martensitic single phase as-quenched metal structure, but when subjected to a tempering treatment described later after secondary cooling, it becomes a tempered martensite single-phase structure. .
- the residual austenite phase needs to be less than 0.5% in volume ratio.
- the manufacturing method of the ultra high strength cold-rolled steel sheet of the present invention is characterized by the continuous annealing process described below.
- a conventionally known manufacturing method is adopted. can do.
- the reason for limitation of the continuous annealing process which is the feature of the present invention will be described.
- the steel sheet structure before quenching needs to be an austenite single phase, so the soaking temperature in the continuous annealing is set to the Ac 3 transformation point or higher.
- the Ac 3 transformation point was described in “Metal Heat Treatment Technology Handbook 3rd Edition” (Metal Heat Treatment Technology Handbook Editorial Committee: Nikkan Kogyo Shimbun, (1966), p. 137) from the chemical composition of the steel sheet.
- the following formula (2); Ac 3 (° C.) 910 ⁇ 203 ⁇ C 1/2 + 44.7 ⁇ Si ⁇ 30 ⁇ Mn ⁇ 20 ⁇ Cu + 700 ⁇ P + 400 ⁇ Al + 400 ⁇ Ti (2)
- the element symbol in the above formula represents the content (mass%) of each element. Can be used to calculate.
- the soaking time above the Ac 3 transformation point is preferably 30 to 1200 seconds, and more preferably 300 to 900 seconds from the viewpoint of suppressing the annealing cost.
- the cooling stop temperature in the quenching process be as low as possible.
- the primary cooling stop temperature is less than the Ms point, stress due to volume shrinkage due to rapid cooling and volume expansion unevenness due to martensitic transformation is generated inside the steel sheet, causing shape deterioration. Therefore, in the present invention, in order to reduce stress generated due to volume shrinkage accompanying cooling, a primary cooling step in which the quenching step is cooled from the soaking temperature to a temperature immediately above the Ms point, and from the vicinity immediately above the Ms point to 100. It was decided to control separately from the secondary cooling step of cooling to below ° C.
- the cooling stop temperature in the primary cooling needs to be in the temperature range from the Ms point to the Ms point + 200 ° C. in the vicinity immediately above the Ms point.
- the martensitic transformation proceeds, and stress due to volume expansion due to the martensitic transformation is generated, so that the effect of suppressing shape deterioration cannot be obtained.
- a second phase such as a ferrite phase or a pearlite phase may be generated in the subsequent holding step, and if the secondary cooling start temperature increases, The accompanying volume shrinkage becomes large, leading to deterioration of the shape.
- the element symbol in the above formula represents the content (mass%) of each element. Can be used to calculate.
- the average cooling rate in the primary cooling needs to be 20 ° C./second or more. This is because, at an average cooling rate of less than 20 ° C./second, a second phase such as a ferrite phase and a pearlite phase is generated before the primary cooling stop temperature is reached, and a martensite single phase structure cannot be obtained.
- the steel sheet after the primary cooling described above needs to be held in the temperature range of Ms point to Ms point + 200 ° C., which is the primary cooling stop temperature, for 0.1 to 60 seconds in order to make the temperature in the steel plate uniform.
- the holding time in this holding process is shorter than 0.1 seconds, temperature unevenness due to the difference in cooling rate in the plate thickness direction or width direction of the steel plate is not sufficiently eliminated, so it is sufficient for reducing the stress in the steel plate. The effect is not obtained.
- the holding time in the holding step is set in the range of 0.1 to 60 seconds.
- the range is preferably 2 to 30 seconds.
- Secondary cooling step After completion of the holding step, secondary cooling is performed from the primary cooling stop temperature (Ms point to Ms point + 200 ° C.) to 100 ° C. or lower at an average cooling rate of 100 ° C./second or higher in order to obtain a martensite single phase structure. Cooling needs to be done. When the average cooling rate is less than 100 ° C./second, a second phase such as a ferrite phase, a pearlite phase, or a bainite phase is generated during cooling, and a martensite single phase structure cannot be obtained.
- the stress generated by the volume shrinkage accompanying cooling that occurs in this step and the volume expansion accompanying martensite transformation reduces the temperature difference from the martensite transformation point by the primary cooling, and the volume shrinkage generated in this step Can be suppressed to a minimum by making the temperature in the steel plate uniform in the holding step and reducing the generation of uneven stress in the steel plate width direction.
- the steel plate subjected to the above-described quenching treatment has a predetermined strength and sufficient flatness, so that it can be made as it is, but in order to improve toughness and workability, 100 A tempering treatment for 120 to 1800 seconds may be performed at a temperature of ⁇ 250 ° C. If the tempering temperature is lower than 100 ° C. or the tempering time is shorter than 120 seconds, the tempering effect cannot be sufficiently obtained. On the other hand, if the tempering temperature is higher than 250 ° C. or the tempering time is longer than 1800 seconds, the softening of the martensite phase proceeds excessively, resulting in a significant decrease in strength and an increase in manufacturing cost. More preferable tempering conditions are in the range of 130 to 220 ° C. ⁇ 300 to 1200 seconds.
- the cooling after the tempering treatment is not particularly limited and may be either air cooling or water cooling. In addition, it is preferable to perform this tempering process using the overaging zone of a continuous annealing line.
- the steel sheet after continuous annealing does not need to be subjected to temper rolling for the purpose of shape correction, but from the viewpoint of surface roughness adjustment and material adjustment of the steel sheet, temper rolling is appropriately performed. Also good.
- Thickness of steel of steel symbols A to S having the composition shown in Table 1 is made into a slab, the slab is heated to 1250 ° C., and then finished by hot rolling with a finish rolling finish temperature of 900 ° C.
- An 8 mm hot-rolled steel sheet was taken up at a winding temperature of 650 ° C. Thereafter, the hot-rolled steel sheet was pickled to remove the surface scale, and then cold-rolled to obtain a cold-rolled steel sheet having a plate thickness of 1.0 mm and a plate width of 800 to 1400 mm.
- the cold-rolled steel sheet is soaked under the conditions described in Table 2 and then subjected to continuous annealing that is quenched through primary cooling, holding, and secondary cooling, or further subjected to tempering treatment.
- a rolled steel sheet was obtained.
- Table 1 the Ms point and Ac 3 transformation point obtained from the above-described formulas (1) and (2) from the chemical components of each steel type are also shown.
- the maximum warp height in the width direction was measured by the method shown in FIG. Specifically, the steel plate was placed on a surface plate, and the distance from the surface plate to the lower surface of the steel plate at the position where the height of the steel plate was the highest was measured. Moreover, the test piece was extract
- each of the steel sheets of Examples 1 to 13 has a martensite single-phase structure (tempered martensite single-phase structure), and the maximum warp height of the warp generated in the steel sheet is 6 mm or less, which is highly flat.
- the martensite single-phase structure intended by the present invention is obtained, but the maximum warp height of warpage is as large as 23 mm, and sufficient flatness is not obtained.
- the steel sheet of the present invention has a hole expansion ratio ⁇ , which is an index of tensile characteristics and stretch flange characteristics, having a value equivalent to that of a martensite single phase steel sheet (No.
- the cooling rate of the primary cooling step is lower than the range of the present invention.
- No. 15 since all austenite phases were transformed into ferrite phases or pearlite phases during primary cooling, no martensite single phase structure was obtained.
- No. 1 in which the primary cooling stop temperature is higher than the range of the present invention.
- No. 16 no pearlite phase was generated, but most of the austenite phase was transformed into a ferrite phase, and a predetermined metal structure was not obtained.
- the holding time in the holding step is longer than the range of the present invention.
- No. 17 since a large amount of ferrite phase and pearlite phase are generated during the holding step, a predetermined metal structure is not obtained.
- the cooling rate in the secondary cooling step was less than the cooling rate of the present invention.
- No. 18 since a ferrite phase and a pearlite phase were generated during cooling from the primary cooling stop temperature to the Ms point, a martensite single phase structure was not obtained. From the above results, the martensitic single-phase steel sheet according to the present invention can achieve excellent flatness while having the same strength and processing characteristics as the martensitic single-phase steel sheet manufactured by the conventional method. It was confirmed that it was possible.
- the ultra-high-strength martensitic single-phase steel sheet obtained by the present invention produces automotive structural members such as automobile door impact beams and center pillars formed by press molding or roll molding with high productivity and dimensional accuracy. It can make a big contribution.
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Abstract
Description
また、特許文献2には、連続焼鈍して引張強さが1049~1240MPaのマルテンサイト単相組織鋼板とした後、鋼板表面の平均粗さRaが1.4μm以上となるように調質圧延を施すことにより、良好な鋼板形状を得る技術が開示されている。 Several improvement techniques have been proposed for the problem of deterioration of the steel plate shape. For example, Patent Document 1 discloses the metal structure of a steel sheet based on the results of investigating the relationship between the warpage height of a steel sheet after continuous annealing with a tensile strength of 1470 MPa to 1960 MPa and the martensite volume ratio in the metal structure. A technique for obtaining predetermined mechanical characteristics and an excellent steel sheet shape by forming a two-phase structure composed of a martensite phase having a volume ratio of 80 to 97% and the balance of a ferrite phase is disclosed.
In Patent Document 2, after continuous annealing to obtain a martensitic single-phase steel sheet having a tensile strength of 1049 to 1240 MPa, temper rolling is performed so that the average roughness Ra of the steel sheet surface is 1.4 μm or more. The technique of obtaining a favorable steel plate shape by applying is disclosed.
Ms(℃)=550−361×C−39×Mn−35×V−20×Cr−17×Ni−10×Cu−5×(Mo+W)+15×Co+30×Al・・・(1)
ここで、上記式中の元素記号は、それぞれの元素の含有量(mass%)を表す。 In the present invention, C: 0.05 to 0.40 mass%, Si: 2.0 mass% or less, P: 0.05 mass% or less, S: 0.02 mass% or less, Al: 0.01 to 0.05 mass %, N: less than 0.005 mass%, Mn: 1.0 to 3.0 mass%, and the steel sheet after cold rolling having a component composition with the balance consisting of Fe and inevitable impurities is subjected to continuous annealing and tensile strength In the method for producing an ultra-high strength cold-rolled steel sheet of 980 MPa or more, in the above-mentioned continuous annealing, the temperature ranges from the soaking temperature not lower than the Ac 3 transformation point to the temperature range of Ms point to Ms point + 200 ° C. determined by the following formula (1). Primary cooling is performed at an average cooling rate of at least ° C./second, and is maintained in the above temperature range for 0.1 to 60 seconds, followed by secondary cooling to 100 ° C. or less at an average cooling rate of at least 100 ° C./second. Ultra high strength cold rolled steel To propose a method of manufacture.
Ms (° C.) = 550-361 × C-39 × Mn-35 × V-20 × Cr-17 × Ni-10 × Cu-5 × (Mo + W) + 15 × Co + 30 × Al (1)
Here, the element symbol in the above formula represents the content (mass%) of each element.
連続焼鈍工程での焼入れ時にマルテンサイト単相組織鋼板に発生する形状悪化は、高速冷却に伴う体積収縮とマルテンサイト変態に伴う体積膨張によって、鋼板内部に不均一な応力が発生することに起因する。一般に、高速冷却に伴う体積収縮およびそれによって発生する応力は、冷却を開始する温度と冷却終了温度との温度差に比例して大きくなると考えられる。一方、マルテンサイト変態に伴う体積膨張は、最終冷却後の金属組織がマルテンサイト単相組織である場合には均一である。したがって、冷却に伴う体積収縮とそれに伴い発生する応力が小さい場合、焼入れによる鋼板形状への影響は、ほぼマルテンサイト変態に伴う一様な体積膨張のみと考えることができ、Ms点以下の温度域における冷却速度が鋼板形状に及ぼす影響は小さいものと考えられる。 First, the basic technical idea of the present invention will be described.
The shape deterioration that occurs in a martensitic single-phase steel sheet during quenching in the continuous annealing process is due to the occurrence of non-uniform stress inside the steel sheet due to volume shrinkage associated with high-speed cooling and volume expansion associated with martensitic transformation. . In general, it is considered that the volume shrinkage accompanying high-speed cooling and the stress generated thereby increase in proportion to the temperature difference between the temperature at which cooling starts and the temperature at which cooling ends. On the other hand, the volume expansion accompanying the martensitic transformation is uniform when the metal structure after the final cooling is a martensite single phase structure. Therefore, when the volume shrinkage accompanying cooling and the stress generated thereby are small, the influence on the steel sheet shape due to quenching can be considered to be only a uniform volume expansion accompanying martensitic transformation, and the temperature range below the Ms point. The effect of the cooling rate on the steel plate shape is considered to be small.
C:0.05~0.40mass%
Cは、オーステナイト相を安定化させる元素であるとともに、鋼板強度を確保するのに必要な元素である。Cが0.05mass%未満では、所望の引張強さ(980MPa以上)のマルテンサイト単相組織鋼板を得ることは困難である。一方、C量が0.40mass%を超えると、連続焼鈍工程前の圧延が困難となったり、マルテンサイト変態に伴う変態歪および変態応力が著しく増大し、焼き割れを起こしたりするおそれがあるため、製造上好ましくない。よって、本発明では、Cを0.05~0.40mass%の範囲とする。好ましくは0.15~0、30mass%の範囲である。 Next, the reason for limiting the component composition of the ultra high strength cold rolled steel sheet of the present invention will be described.
C: 0.05-0.40 mass%
C is an element that stabilizes the austenite phase and is an element necessary for ensuring the strength of the steel sheet. When C is less than 0.05 mass%, it is difficult to obtain a martensitic single-phase steel sheet having a desired tensile strength (980 MPa or more). On the other hand, if the amount of C exceeds 0.40 mass%, rolling before the continuous annealing process becomes difficult, and the transformation strain and transformation stress accompanying martensitic transformation may remarkably increase to cause firing cracks. This is not preferable in production. Therefore, in the present invention, C is set in the range of 0.05 to 0.40 mass%. The range is preferably from 0.15 to 0 and 30 mass%.
Siは、鋼板の加工性を害することなく高強度化するのに有効な置換型固溶強化元素である。しかし、SiはAc3変態点を高温側に移行させる元素でもあるため、過度なSi添加は、焼鈍温度の上昇、ひいては焼鈍コストの上昇を招くため好ましくない。またSiを過剰に添加すると、熱間圧延でのスケール生成が顕著になり、最終製品の表面欠陥が増加し、品質上も好ましくない。よって、Siは2.0mass%以下とする。好ましくは1.5mass%以下である。 Si: 2.0 mass% or less Si is a substitutional solid solution strengthening element that is effective for increasing the strength without impairing the workability of the steel sheet. However, since Si is also an element that shifts the Ac 3 transformation point to the high temperature side, excessive addition of Si is not preferable because it causes an increase in annealing temperature and an increase in annealing cost. Moreover, when Si is added excessively, the scale production | generation by hot rolling will become remarkable, the surface defect of a final product will increase, and it is unpreferable also on quality. Therefore, Si is set to 2.0 mass% or less. Preferably it is 1.5 mass% or less.
Mnは、オーステナイト相を安定化させて、マルテンサイト組織を得やすくする元素である。しかし、Mnが1.0mass%未満では、鋼の焼入れ性が十分ではなく、焼鈍時の均熱温度からの冷却中に、フェライト相やパーライト相、ベイナイト相が早期に生成を開始し、本発明が意図するマルテンサイト単相組織を安定して得ることが困難となる。一方、3.0mass%を超えて添加すると、偏析が顕著となったり、加工性が低下したりするおそれがある。また、耐遅れ破壊特性も低下する。よって、Mnは1.0~3.0mass%の範囲とする。好ましくは1.5~2.5mass%の範囲である。 Mn: 1.0 to 3.0 mass%
Mn is an element that stabilizes the austenite phase and makes it easier to obtain a martensite structure. However, if Mn is less than 1.0 mass%, the hardenability of the steel is not sufficient, and the ferrite phase, the pearlite phase, and the bainite phase start to form early during cooling from the soaking temperature during annealing, and the present invention However, it is difficult to stably obtain the intended martensite single-phase structure. On the other hand, if it is added in an amount exceeding 3.0 mass%, segregation may become remarkable or processability may be deteriorated. In addition, the delayed fracture resistance also decreases. Therefore, Mn is in the range of 1.0 to 3.0 mass%. Preferably, it is in the range of 1.5 to 2.5 mass%.
Pは、粒界に偏析して粒界破壊を助長する元素でもあるので、低いほど望ましい。よって、Pは0.05mass%以下とする。好ましくは0.02mass%以下、より好ましくは0.01mass%以下である。なお、溶接性を向上する観点からは、0.008mass%以下が望ましい。 P: 0.05 mass% or less P is also an element that segregates at the grain boundary and promotes grain boundary destruction. Therefore, P is set to 0.05 mass% or less. Preferably it is 0.02 mass% or less, More preferably, it is 0.01 mass% or less. In addition, from a viewpoint of improving weldability, 0.008 mass% or less is desirable.
Sは、MnSなどの硫化物系介在物となって、耐衝撃特性や耐遅れ破壊特性の低下を誘引するため、極力低い方が望ましい。よって、Sの上限は0.02mass%とする。好ましくは0.002mass%以下である。 S: 0.02 mass% or less Since S becomes a sulfide-based inclusion such as MnS and induces a decrease in impact resistance and delayed fracture resistance, it is desirable that S be as low as possible. Therefore, the upper limit of S is 0.02 mass%. Preferably it is 0.002 mass% or less.
Alは、製鋼工程において脱酸のために添加される元素であり、十分な脱酸効果を得るためには0.01mass%以上添加する必要がある。一方、過剰に添加すると、鋼板中の介在物が増加し、延性の低下を招く。よって、Alは0.01~0.05mass%の範囲とする。 Al: 0.01 to 0.05 mass%
Al is an element added for deoxidation in the steelmaking process, and it is necessary to add 0.01 mass% or more in order to obtain a sufficient deoxidation effect. On the other hand, when it adds excessively, the inclusion in a steel plate will increase and the ductility will fall. Therefore, Al is set in the range of 0.01 to 0.05 mass%.
Nは、窒化物を形成する元素である。特に含有量が0.005mass%以上になると、窒化物の形成による高温および低温での延性の低下が大きくなる。よって、Nは0.005mass%未満に制限する。 N: Less than 0.005 mass% N is an element that forms a nitride. In particular, when the content is 0.005 mass% or more, the decrease in ductility at high and low temperatures due to the formation of nitride increases. Therefore, N is limited to less than 0.005 mass%.
Nb:0.1mass%以下、Ti:0.1mass%以下
NbおよびTiは、結晶粒を微細化させ、鋼板の強度を上昇させるのに有効な元素である。しかし、Nb,Tiは、それぞれ0.1mass%を超えて添加しても、その効果は飽和するため、経済的に好ましくない。よって、NbおよびTiを添加する場合には、それぞれ0.1mass%以下とする。 In addition to the above essential elements, Nb, Ti, B and Cu can be added to the ultra-high strength cold-rolled steel sheet of the present invention within the following ranges depending on the purpose.
Nb: 0.1 mass% or less, Ti: 0.1 mass% or less Nb and Ti are effective elements for refining crystal grains and increasing the strength of a steel sheet. However, even if Nb and Ti are added in amounts exceeding 0.1 mass%, the effect is saturated, which is not economically preferable. Therefore, when adding Nb and Ti, respectively, it shall be 0.1 mass% or less.
Bは、焼入れ性を高めて、鋼板強度を上昇させるのに有効な元素である。しかし、Bが0.0005mass%未満では、上記強度上昇効果が期待できない。一方、Bが0.0030mass%を超えると、熱間加工性が低下するため、製造上好ましくない。よって、Bを添加する場合には、0.0005~0.0030mass%の範囲とする。 B: 0.0005 to 0.0030 mass%
B is an element effective for enhancing the hardenability and increasing the steel sheet strength. However, if B is less than 0.0005 mass%, the above-mentioned strength increasing effect cannot be expected. On the other hand, if B exceeds 0.0030 mass%, the hot workability deteriorates, which is not preferable for production. Therefore, when adding B, it is set as the range of 0.0005-0.0030 mass%.
Cuは、オーステナイト相を安定化させ、マルテンサイト単相組織を得やすくするとともに、腐食環境下で鋼板表層に濃化層を形成することによって、鋼中への水素の侵入を抑制し、耐遅れ破壊特性を向上する効果がある元素である。しかし、添加量が0.20mass%を超えると、これらの効果が飽和するので、Cuは、0.20mass%を上限として添加するのが好ましい。 Cu: 0.20 mass% or less Cu stabilizes the austenite phase, makes it easy to obtain a martensite single-phase structure, and forms a concentrated layer on the surface of the steel sheet in a corrosive environment. It is an element that has the effect of suppressing penetration and improving delayed fracture resistance. However, when the addition amount exceeds 0.20 mass%, these effects are saturated. Therefore, it is preferable to add Cu with an upper limit of 0.20 mass%.
本発明の超高強度冷延鋼板は、その金属組織が、マルテンサイト単相であることが必要である。ただし、鋼板表面から板厚方向に10μmの範囲は、製造過程での脱炭等の影響により、マルテンサイト相が生成しない場合があるので、この範囲は除外する必要がある。なお、鋼板母相組織中には、オーステナイト相が残存することがあり、残留オーステナイト相と呼ばれる。この残留オーステナイト相が体積率にして0.5%未満であれば、マルテンサイト単相組織と見做すことができる。また、鋼板組織中には不可避的に炭化物、窒化物、介在物も存在するが、これらはマルテンサイト単相組織であるか否かを判定する上では評価の対象には含めない。 Next, the metal structure of the ultra high strength cold rolled steel sheet of the present invention will be described.
The ultra-high-strength cold-rolled steel sheet of the present invention requires that the metal structure is a martensite single phase. However, since the martensite phase may not be generated in the range of 10 μm in the thickness direction from the steel sheet surface due to the influence of decarburization or the like in the manufacturing process, it is necessary to exclude this range. Note that an austenite phase may remain in the matrix structure of the steel sheet, which is called a retained austenite phase. If the residual austenite phase is less than 0.5% in volume ratio, it can be regarded as a martensite single phase structure. In addition, carbides, nitrides, and inclusions are inevitably present in the steel sheet structure, but these are not included in the evaluation when determining whether or not these are martensite single phase structures.
本発明の超高強度冷延鋼板の製造方法は、以下に述べる連続焼鈍工程に特徴があり、それ以前の工程、すなわち、製鋼工程から冷間圧延工程までについては、従来公知の製造方法を採用することができる。以下、本発明の特徴である連続焼鈍工程の限定理由について説明する。 Next, the manufacturing method of the ultra high strength cold-rolled steel sheet of the present invention will be described.
The manufacturing method of the ultra-high strength cold-rolled steel sheet of the present invention is characterized by the continuous annealing process described below. For the processes before that, that is, from the steel making process to the cold rolling process, a conventionally known manufacturing method is adopted. can do. Hereinafter, the reason for limitation of the continuous annealing process which is the feature of the present invention will be described.
本発明が意図するマルテンサイト単相組織を得るためには、焼入れ前の鋼板組織をオーステナイト単相とする必要があることから、連続焼鈍における均熱温度はAc3変態点以上とする必要がある。ここで、Ac3変態点は、鋼板の化学成分から、「金属熱処理技術便覧 第3版」(金属熱処理技術便覧編集委員会:日刊工業新聞社、(1966)、p.137)に記載された下記(2)式;
Ac3(℃)=910−203×C1/2+44.7×Si−30×Mn−20×Cu+700×P+400×Al+400×Ti ・・・(2)
ここで、上記式中の元素記号は、各元素の含有量(mass%)を表す。
を用いて計算することができる。
なお、Ac3変態点以上に均熱する時間は30~1200秒が好ましく、焼鈍コストを抑制する観点からは300~900秒の範囲がより好ましい。 Soaking process In order to obtain the martensite single phase structure intended by the present invention, the steel sheet structure before quenching needs to be an austenite single phase, so the soaking temperature in the continuous annealing is set to the Ac 3 transformation point or higher. There is a need. Here, the Ac 3 transformation point was described in “Metal Heat Treatment Technology Handbook 3rd Edition” (Metal Heat Treatment Technology Handbook Editorial Committee: Nikkan Kogyo Shimbun, (1966), p. 137) from the chemical composition of the steel sheet. The following formula (2);
Ac 3 (° C.) = 910−203 × C 1/2 + 44.7 × Si−30 × Mn−20 × Cu + 700 × P + 400 × Al + 400 × Ti (2)
Here, the element symbol in the above formula represents the content (mass%) of each element.
Can be used to calculate.
The soaking time above the Ac 3 transformation point is preferably 30 to 1200 seconds, and more preferably 300 to 900 seconds from the viewpoint of suppressing the annealing cost.
一般に、焼入れ工程における冷却停止温度は可能な限り低温であることが望ましい。しかし、一次冷却停止温度をMs点未満とした場合、急速冷却による体積収縮とマルテンサイト変態による体積膨張のムラに起因した応力が鋼板内部に発生し、形状悪化を引き起こす。そこで、本発明は、冷却に伴う体積収縮に起因して発生する応力を低減するため、焼入れ工程を均熱温度からMs点直上近傍温度まで冷却する一次冷却工程と、上記Ms点直上近傍から100℃以下まで冷却する二次冷却工程とに分けて制御することとした。 Primary cooling process Generally, it is desirable that the cooling stop temperature in the quenching process be as low as possible. However, when the primary cooling stop temperature is less than the Ms point, stress due to volume shrinkage due to rapid cooling and volume expansion unevenness due to martensitic transformation is generated inside the steel sheet, causing shape deterioration. Therefore, in the present invention, in order to reduce stress generated due to volume shrinkage accompanying cooling, a primary cooling step in which the quenching step is cooled from the soaking temperature to a temperature immediately above the Ms point, and from the vicinity immediately above the Ms point to 100. It was decided to control separately from the secondary cooling step of cooling to below ° C.
Ms(℃)=550−361×C−39×Mn−35×V−20×Cr−17×Ni−10×Cu−5×(Mo+W)+15×Co+30×Al・・・(1)
ここで、上記式中の元素記号は、それぞれの元素の含有量(mass%)を表す。
を用いて計算することができる。 In addition, Ms point (Martensite transformation start point) is the following (1) formula from the chemical component of a steel plate;
Ms (° C.) = 550-361 × C-39 × Mn-35 × V-20 × Cr-17 × Ni-10 × Cu-5 × (Mo + W) + 15 × Co + 30 × Al (1)
Here, the element symbol in the above formula represents the content (mass%) of each element.
Can be used to calculate.
上記の一次冷却後の鋼板は、鋼板内の温度を均一化するため、一次冷却停止温度であるMs点~Ms点+200℃の温度範囲に0.1~60秒間保持する必要がある。この保持工程における保持時間が0.1秒よりも短い場合、鋼板の板厚方向あるいは幅方向での冷却速度の違いに起因する温度ムラが十分には解消されないため、鋼板内の応力低減に十分な効果が得られない。一方、保持時間が60秒よりも長くなると、保持中にフェライト相やパーライト相、ベイナイト相が生成し、マルテンサイト単相組織が得られなくなる。よって、保持工程における保持時間は0.1~60秒の範囲とする。好ましくは2~30秒の範囲である。 Holding Step The steel sheet after the primary cooling described above needs to be held in the temperature range of Ms point to Ms point + 200 ° C., which is the primary cooling stop temperature, for 0.1 to 60 seconds in order to make the temperature in the steel plate uniform. When the holding time in this holding process is shorter than 0.1 seconds, temperature unevenness due to the difference in cooling rate in the plate thickness direction or width direction of the steel plate is not sufficiently eliminated, so it is sufficient for reducing the stress in the steel plate. The effect is not obtained. On the other hand, if the holding time is longer than 60 seconds, a ferrite phase, a pearlite phase, and a bainite phase are generated during holding, and a martensite single phase structure cannot be obtained. Therefore, the holding time in the holding step is set in the range of 0.1 to 60 seconds. The range is preferably 2 to 30 seconds.
保持工程終了後は、マルテンサイト単相組織を得るために、一次冷却停止温度(Ms点~Ms点+200℃)から100℃以下までを平均冷却速度100℃/秒以上で二次冷却を行う必要がある。平均冷却速度が100℃/秒未満の場合、冷却中にフェライト相やパーライト相、ベイナイト相等の第二相が生成し、マルテンサイト単相組織が得られない。なお、この工程で起こる冷却に伴う体積収縮とマルテンサイト変態に伴う体積膨張とによって発生する応力は、上記一次冷却によりマルテンサイト変態点との温度差を低減し、本工程で発生する体積収縮量を低減していること、および、上記保持工程において鋼板内の温度を均一化し、鋼板幅方向の不均一な応力発生を低減していることにより、最小限に抑えることができる。 Secondary cooling step After completion of the holding step, secondary cooling is performed from the primary cooling stop temperature (Ms point to Ms point + 200 ° C.) to 100 ° C. or lower at an average cooling rate of 100 ° C./second or higher in order to obtain a martensite single phase structure. Cooling needs to be done. When the average cooling rate is less than 100 ° C./second, a second phase such as a ferrite phase, a pearlite phase, or a bainite phase is generated during cooling, and a martensite single phase structure cannot be obtained. The stress generated by the volume shrinkage accompanying cooling that occurs in this step and the volume expansion accompanying martensite transformation reduces the temperature difference from the martensite transformation point by the primary cooling, and the volume shrinkage generated in this step Can be suppressed to a minimum by making the temperature in the steel plate uniform in the holding step and reducing the generation of uneven stress in the steel plate width direction.
また、本発明では、連続焼鈍後の鋼板には、形状矯正を目的とする調質圧延を施す必要はないが、鋼板の表面粗度調整や材質調整の観点から、調質圧延を適宜施してもよい。 As a cooling method in continuous annealing, it is desirable to use water cooling in order to achieve uniform cooling and a high cooling rate, but roll cooling, gas cooling, mist cooling (air-water cooling), or the like may be used. In addition, as a method of maintaining the steel sheet temperature within the temperature range of Ms point to Ms point + 200 ° C., it may be a method of immersing in a salt bath or a metal bath in which the temperature is adjusted to the primary cooling stop temperature range in combination with the primary cooling. Or you may use the method of reheating to a primary cooling stop temperature range using an induction heating apparatus after a primary cooling stop.
In the present invention, the steel sheet after continuous annealing does not need to be subjected to temper rolling for the purpose of shape correction, but from the viewpoint of surface roughness adjustment and material adjustment of the steel sheet, temper rolling is appropriately performed. Also good.
また、当該鋼板から試験片を採取して、金属組織、引張特性および伸びフランジ特性の評価を下記のようにして行った。
(1)金属組織の観察
上記の各冷延鋼板から試験片を採取し、圧延方向に平行な断面を鏡面研磨し、ナイタールエッチングをして金属組織を現出させ、光学顕微鏡または走査型電子顕微鏡を用いて微細な金属組織を観察し、マルテンサイト相、焼戻しマルテンサイト相、フェライト相などの構成相の種類を同定するとともに、撮影した組織写真を画像解析装置で2値化することにより、マルテンサイト相と第二相の体積率を求めた。なお、上記冷延鋼板には、残留オーステナイト相が存在する可能性もあるため、発明例の鋼板についてはX線(Mo−Kα線)測定により残留オーステナイト相の体積率の測定を試みたが、その存在量はいずれも0.5%未満であり、マルテンサイト単相組織あるいは焼戻しマルテンサイト単相組織と見做せることができた。
(2)引張試験
上記の各冷延鋼板から圧延方向に直角な方向にJIS5号引張試験片を採取し、JIS Z2241に準拠して引張試験を行い、0.2%耐力(PS)、引張強さ(TS)、破断伸び(El)を測定した。
(3)伸びフランジ特性
伸びフランジ特性は、日本鉄鋼連盟規格JFST1001の規定に準拠して穴拡げ試験を行い評価した。すなわち、上記の各冷延鋼板から採取した試験片に10mmφのポンチ穴を開け、バリが外側になるようにして、頂角60°の円錐ポンチを用いて、板厚を貫通する割れが発生するまで穴拡げ加工を行い、下記式を用いて穴拡げ率λを求めた。
λ(%)={(d−d0)/d0}×100
ここで、d0:初期穴内径(mm)、d:割れ発生時の穴内径(mm) About the various cold-rolled steel plates obtained as described above, the maximum warp height in the width direction was measured by the method shown in FIG. Specifically, the steel plate was placed on a surface plate, and the distance from the surface plate to the lower surface of the steel plate at the position where the height of the steel plate was the highest was measured.
Moreover, the test piece was extract | collected from the said steel plate, and metal structure, a tensile characteristic, and the stretch flange characteristic were evaluated as follows.
(1) Observation of metal structure A test piece is taken from each of the above-mentioned cold-rolled steel sheets, a cross section parallel to the rolling direction is mirror-polished, and a metal structure is revealed by performing a nital etching, and an optical microscope or scanning electron By observing a fine metal structure using a microscope, identifying the types of constituent phases such as martensite phase, tempered martensite phase, and ferrite phase, and binarizing the photographed structure photograph with an image analyzer, The volume ratio of the martensite phase and the second phase was determined. In addition, since there is a possibility that the retained austenite phase exists in the cold-rolled steel sheet, for the steel sheet of the inventive example, an attempt was made to measure the volume ratio of the retained austenite phase by X-ray (Mo-Kα ray) measurement. The abundance was less than 0.5%, and could be regarded as a martensite single phase structure or a tempered martensite single phase structure.
(2) Tensile test JIS No. 5 tensile test specimens were taken from each of the above cold-rolled steel sheets in a direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with JIS Z2241, 0.2% proof stress (PS), tensile strength. (TS) and elongation at break (El) were measured.
(3) Stretch flange characteristics Stretch flange characteristics were evaluated by performing a hole expansion test in accordance with the provisions of the Japan Iron and Steel Federation Standard JFST1001. That is, a 10 mmφ punch hole is made in the test piece taken from each cold-rolled steel sheet, and a crack penetrating the plate thickness is generated using a conical punch with an apex angle of 60 ° so that the burr is on the outside. Hole expansion was performed until the hole expansion rate λ was obtained using the following formula.
λ (%) = {(d−d 0 ) / d 0 } × 100
Where d 0 : initial hole inner diameter (mm), d: hole inner diameter (mm) when cracking occurs
一方、一次冷却工程の冷却速度が本発明範囲より低いNo.15では、一次冷却中に全てのオーステナイト相がフェライト相あるいはパーライト相に変態したため、マルテンサイト単相組織が得られていない。同様に、一次冷却停止温度を本発明の範囲よりも高温としたNo.16では、パーライト相は生成していないものの、オーステナイト相の大部分がフェライト相に変態しており、所定の金属組織が得られていない。また、保持工程における保持時間が本発明の範囲より長いNo.17では、保持工程中に多量のフェライト相およびパーライト相が生成するために、所定の金属組織が得られていない。また、二次冷却工程における冷却速度を本発明の冷却速度未満としたNo.18では、一次冷却停止温度からMs点までの冷却中にフェライト相およびパーライト相が生成したため、マルテンサイト単相組織は得られていない。
以上の結果から、本発明のマルテンサイト単相組織鋼板は、従来法で製造したマルテンサイト単相組織鋼板と同等の強度特性および加工特性を有しながらも、優れた平坦度を実現することができることが確認された。 The results are shown in Table 3. No. suitable for the present invention. Each of the steel sheets of Examples 1 to 13 has a martensite single-phase structure (tempered martensite single-phase structure), and the maximum warp height of the warp generated in the steel sheet is 6 mm or less, which is highly flat. In contrast to the No. 1 in which the conventional quenching method was carried out. In Comparative Example 14, the martensite single-phase structure intended by the present invention is obtained, but the maximum warp height of warpage is as large as 23 mm, and sufficient flatness is not obtained. Furthermore, the steel sheet of the present invention has a hole expansion ratio λ, which is an index of tensile characteristics and stretch flange characteristics, having a value equivalent to that of a martensite single phase steel sheet (No. 14) manufactured by a conventional method. Yes.
On the other hand, the cooling rate of the primary cooling step is lower than the range of the present invention. In No. 15, since all austenite phases were transformed into ferrite phases or pearlite phases during primary cooling, no martensite single phase structure was obtained. Similarly, No. 1 in which the primary cooling stop temperature is higher than the range of the present invention. In No. 16, no pearlite phase was generated, but most of the austenite phase was transformed into a ferrite phase, and a predetermined metal structure was not obtained. Moreover, the holding time in the holding step is longer than the range of the present invention. In No. 17, since a large amount of ferrite phase and pearlite phase are generated during the holding step, a predetermined metal structure is not obtained. Moreover, the cooling rate in the secondary cooling step was less than the cooling rate of the present invention. In No. 18, since a ferrite phase and a pearlite phase were generated during cooling from the primary cooling stop temperature to the Ms point, a martensite single phase structure was not obtained.
From the above results, the martensitic single-phase steel sheet according to the present invention can achieve excellent flatness while having the same strength and processing characteristics as the martensitic single-phase steel sheet manufactured by the conventional method. It was confirmed that it was possible.
Claims (8)
- C:0.05~0.40mass%、Si:2.0mass%以下、Mn:1.0~3.0mass%、P:0.05mass%以下、S:0.02mass%以下、Al:0.01~0.05mass%、N:0.005mass%未満を含有し、残部がFeおよび不可避不純物からなる成分組成を有し、金属組織がマルテンサイト単相で、引張強さが980MPa以上、鋼板の平坦度が10mm以下である超高強度冷延鋼板。 C: 0.05 to 0.40 mass%, Si: 2.0 mass% or less, Mn: 1.0 to 3.0 mass%, P: 0.05 mass% or less, S: 0.02 mass% or less, Al: 0. 01 to 0.05 mass%, N: less than 0.005 mass%, the balance is a component composition composed of Fe and inevitable impurities, the metal structure is a martensite single phase, the tensile strength is 980 MPa or more, An ultra-high strength cold-rolled steel sheet having a flatness of 10 mm or less.
- 金属組織が焼戻しマルテンサイト単相であることを特徴とする請求項1に記載の超高強度冷延鋼板。 The ultrahigh-strength cold-rolled steel sheet according to claim 1, wherein the metal structure is a tempered martensite single phase.
- 引張強さが1320MPa以上であることを特徴とする請求項1または2に記載の超高強度冷延鋼板。 The ultra-high-strength cold-rolled steel sheet according to claim 1 or 2, wherein the tensile strength is 1320 MPa or more.
- 上記成分組成に加えてさらに、Ti:0.1mass%以下、Nb:0.1mass%以下、B:0.0005~0.0030mass%およびCu:0.20mass%以下のうちから選ばれる1種または2種以上を含有することを特徴とする請求項1~3のいずれか1項に記載の超高強度冷延鋼板。 In addition to the above component composition, Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, B: 0.0005 to 0.0030 mass%, and Cu: 0.20 mass% or less The ultra-high-strength cold-rolled steel sheet according to any one of claims 1 to 3, comprising two or more kinds.
- C:0.05~0.40mass%、Si:2.0mass%以下、P:0.05mass%以下、S:0.02mass%以下、Al:0.01~0.05mass%、N:0.005mass%未満、Mn:1.0~3.0mass%を含有し、残部がFeおよび不可避不純物からなる成分組成を有する冷間圧延後の鋼板を連続焼鈍して引張強さ980MPa以上の超高強度冷延鋼板を製造する方法において、上記連続焼鈍では、Ac3変態点以上の均熱温度から下記(1)式で求められるMs点~Ms点+200℃の温度範囲まで20℃/秒以上の平均冷却速度で一次冷却し、上記温度範囲に0.1~60秒間保持した後、100℃/秒以上の平均冷却速度で100℃以下まで二次冷却することを特徴とする超高強度冷延鋼板の製造方法。
Ms(℃)=550−361×C−39×Mn−35×V−20×Cr−17×Ni−10×Cu−5×(Mo+W)+15×Co+30×Al・・・(1)
ここで、上記式中の元素記号は、それぞれの元素の含有量(mass%)を表す。 C: 0.05 to 0.40 mass%, Si: 2.0 mass% or less, P: 0.05 mass% or less, S: 0.02 mass% or less, Al: 0.01 to 0.05 mass%, N: 0.00. Ultra high strength with a tensile strength of 980 MPa or more by continuously annealing a cold-rolled steel sheet having a component composition of less than 005 mass%, Mn: 1.0 to 3.0 mass%, the balance being composed of Fe and inevitable impurities In the method for producing a cold-rolled steel sheet, in the above-described continuous annealing, an average of 20 ° C./second or more is obtained from a soaking temperature not lower than the Ac 3 transformation point to a temperature range of Ms point to Ms point + 200 ° C. obtained by the following equation (1). An ultra-high strength cold-rolled steel sheet that is primarily cooled at a cooling rate, held in the above temperature range for 0.1 to 60 seconds, and then secondarily cooled to 100 ° C. or less at an average cooling rate of 100 ° C./second or more. Manufacturing method.
Ms (° C.) = 550-361 × C-39 × Mn-35 × V-20 × Cr-17 × Ni-10 × Cu-5 × (Mo + W) + 15 × Co + 30 × Al (1)
Here, the element symbol in the above formula represents the content (mass%) of each element. - 二次冷却後、再加熱し、100~250℃×120~1800秒の焼戻し処理を施すことを特徴とする請求項5に記載の超高強度冷延鋼板の製造方法。 6. The method for producing an ultra-high strength cold-rolled steel sheet according to claim 5, wherein after the secondary cooling, reheating is performed and a tempering treatment is performed at 100 to 250 ° C. × 120 to 1800 seconds.
- 一次冷却および二次冷却を水冷却で行うことを特徴とする請求項5または6に記載の超高強度冷延鋼板の製造方法。 The method for producing an ultra-high-strength cold-rolled steel sheet according to claim 5 or 6, wherein primary cooling and secondary cooling are performed by water cooling.
- 上記冷間圧延後の鋼板は、上記成分組成に加えてさらに、Ti:0.1mass%以下、Nb:0.1mass%以下、B:0.0005~0.0030mass%およびCu:0.20mass%以下のうちから選ばれる1種または2種以上を含有することを特徴とする請求項5~7のいずれか1項に記載の超高強度冷延鋼板の製造方法。 In addition to the above component composition, the steel sheet after the cold rolling further includes Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, B: 0.0005 to 0.0030 mass%, and Cu: 0.20 mass%. The method for producing an ultra-high strength cold-rolled steel sheet according to any one of claims 5 to 7, comprising one or more selected from the following.
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