WO2022249919A1 - 高強度合金化溶融亜鉛めっき鋼板およびその製造方法 - Google Patents
高強度合金化溶融亜鉛めっき鋼板およびその製造方法 Download PDFInfo
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- WO2022249919A1 WO2022249919A1 PCT/JP2022/020394 JP2022020394W WO2022249919A1 WO 2022249919 A1 WO2022249919 A1 WO 2022249919A1 JP 2022020394 W JP2022020394 W JP 2022020394W WO 2022249919 A1 WO2022249919 A1 WO 2022249919A1
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- steel sheet
- less
- hot
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- dip galvanized
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- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 5
- 239000000463 material Substances 0.000 description 5
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- 229910001209 Low-carbon steel Inorganic materials 0.000 description 2
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- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
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- 240000007594 Oryza sativa Species 0.000 description 1
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- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
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- 230000000977 initiatory effect Effects 0.000 description 1
- 229910052749 magnesium Inorganic materials 0.000 description 1
- 239000000155 melt Substances 0.000 description 1
- 238000005272 metallurgy Methods 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
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- 229910052720 vanadium Inorganic materials 0.000 description 1
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- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
-
- 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 a high-strength cold-rolled steel sheet having a galvannealed layer on its surface, and a method for producing the same.
- the tensile strength TS of the steel sheet is simply referred to as “tensile strength”
- the 0.2% proof stress ⁇ 0.2 and upper yield point UYP are also collectively referred to as "yield strength”.
- steel sheets used for automotive body structural members are required to have high tensile strength and high yield strength.
- the composition of steel sheets is being increased, that is, the content of additive elements in steel sheets is being increased.
- the increase in the content of such additive elements is progressing also in the base steel sheet used for the galvanized steel sheet.
- LME cracks liquid metal embrittlement
- steel sheets that are currently applied to automotive body structural members are required to have high tensile strength and high yield strength, as well as excellent LME cracking resistance.
- Patent Document 1 has an area ratio of autotempered martensite as a steel structure of 80% or more, ferrite of less than 5%, and bainite of 10% or less. Retained austenite satisfies 5% or less, the average number of precipitated iron-based carbides of 5 nm or more and 0.5 ⁇ m or less in the autotempered martensite is 5 ⁇ 10 4 or more per 1 mm 2 , and the tensile strength is 1400 MPa.
- a high-strength steel sheet characterized by the above is disclosed.
- Patent Document 3 discloses that three welding pulses are used in one spot welding plan as a resistance spot welding method capable of suppressing the occurrence of LME cracking, and a first welding pulse and a second welding pulse are used.
- a welding pulse is used to suppress the formation of nuggets and the initiation of liquid metal embrittlement cracking, said first welding pulse having a diameter of 3.75T 1/2 to 4.25T 1/2 , where T is the thickness of the steel sheet), the second welding pulse causes the nugget to grow slowly, and the third welding pulse, which is a tempering pulse, is used to improve the plasticity of the weld spot.
- a resistance spot welding method has been proposed.
- Patent Document 4 proposes a technique of obtaining excellent LME resistance by setting the thickness of the softened surface layer of the steel sheet to 5 ⁇ m or more.
- Patent Documents 1 and 2 in addition to the tensile strength of the steel sheet, only the workability or yield strength is examined, and the LME cracking resistance is not considered.
- the spot welding method proposed in Patent Document 3 does not take into consideration the case of welding various plate sets including three or more steel plates, or the case of adding disturbance conditions during welding. In some cases, the occurrence of LME cracks may not be suppressed. With the technique disclosed in Patent Document 4, the tensile strength of the obtained steel plate is only less than 1150 MPa, and Patent Document 4 does not consider countermeasures for LME crack resistance of high-strength materials of 1150 MPa or more.
- the present invention has been made in view of such problems, and has a yield strength of 970 MPa or more, a tensile strength of 1470 MPa or more, and a high-strength alloyed hot-dip galvanized steel sheet having excellent LME crack resistance, and
- An object of the present invention is to provide a method for manufacturing such a high-strength galvannealed steel sheet.
- a high-strength galvannealed steel sheet is a high-strength galvannealed steel sheet having a coating layer on the surface of a base steel sheet
- the base steel plate is mass %, C: 0.19 to 0.30%, Si: more than 0%, 0.70% or less, Mn: 1.8-3.0%, P: more than 0%, 0.020% or less, S: more than 0%, 0.05% or less, Al: 0.015-0.060%, Cr: 0.05-0.8%, Ti: 0.015 to 0.080%, B: 0.0010 to 0.0150%, Mo: more than 0%, 0.40% or less, N: 0.0100% or less, and O: 0.0030% or less, containing, the balance consisting of iron and inevitable impurities,
- the Cr content [Cr] (mass%) and the Si content [Si] (mass%) of the base steel sheet satisfy 2 ⁇ [Cr] - [Si] ⁇ 0.1, In the metal structure
- a method for producing a high-strength galvannealed steel sheet according to another aspect of the present invention includes hot-rolling a slab having the above composition, Winding the hot-rolled steel sheet obtained by hot rolling at 620 ° C. or higher, Unwinding and cold-rolling the wound hot-rolled steel sheet, A cold-rolled steel sheet obtained by cold rolling is heated, held for 11 seconds or longer in a temperature range of Ac3 or higher, and the heated and held cold-rolled steel sheet is cooled to a temperature range of 540 to 580 ° C. at an average cooling rate of 3 ° C./s. above, and further cooled to a temperature range of 410 to 480 ° C.
- Hot-dip galvanizing is applied to the base steel sheet obtained by cooling the cold-rolled steel sheet, A hot-dip galvanized steel sheet obtained by hot-dip galvanizing is heated to a temperature range of 550 ° C. or less to alloy the hot-dip galvanized, Cooling the alloyed hot-dip galvanized steel sheet obtained by subjecting the hot-dip galvanized steel sheet to a temperature range of 230 to 340° C. at an average cooling rate of 5° C./s or more.
- FIG. 1 is a schematic diagram of a heat pattern in an annealing process according to an embodiment of the present invention.
- FIG. 2 is an example of a scanning electron micrograph of a cross section of a test piece.
- FIG. 3 is a schematic diagram showing how the number of laths is measured by the cutting method.
- FIG. 4 is a schematic front view of a sample for evaluation of LME crack resistance.
- FIG. 5 is an example of an optical microscope photograph of a sample for observing LME cracks.
- a high-strength alloyed hot-dip galvanized steel sheet according to an embodiment of the present invention will be described below.
- the high-strength galvannealed steel sheet according to the present embodiment is a high-strength galvannealed steel sheet having a coating layer on the surface of the base steel sheet.
- Each component of the high-strength galvannealed steel sheet will be described below.
- a high-strength galvannealed steel sheet having the following composition has a yield strength of 970 MPa or more and a tensile strength of 1470 MPa or more, and has excellent LME crack resistance.
- the base steel sheet of the high-strength galvannealed steel sheet according to the present embodiment has the following chemical composition. "%” in the following description of the chemical composition means “% by mass”.
- the base steel sheet and the high-strength alloyed hot-dip galvanized steel sheet are also collectively referred to simply as “steel sheet”.
- the tensile strength TS of the steel sheet is simply referred to as “tensile strength”
- the 0.2% proof stress ⁇ 0.2 and upper yield point UYP are collectively referred to as “yield strength”.
- “Tensile strength” and “yield strength” are collectively referred to simply as "strength”.
- (C: 0.19-0.30%) C is an element necessary for ensuring the strength of the steel sheet.
- the amount of C is set to 0.19% or more.
- the lower limit of the C content is preferably 0.20% or more, more preferably 0.21% or more.
- the amount of C is set to 0.30% or less.
- the upper limit of the C content is preferably 0.290% or less, more preferably 0.280% or less, still more preferably 0.270% or less, and even more preferably 0.260% or less.
- Si is one of the important elements in the present invention.
- Si is known as a solid-solution strengthening element, and is an element that effectively acts to improve the tensile strength while suppressing the deterioration of the ductility of the steel sheet.
- Si is also an element that has the effect of increasing the temper softening resistance of the martensitic structure.
- the amount of Si is set to an amount exceeding 0%.
- the lower limit of the amount of Si is preferably 0.06% or more.
- the lower limit of the amount of Si is more preferably 0.07% or more, still more preferably 0.08% or more.
- the amount of Si is set to 0.70% or less.
- the upper limit of the amount of Si is preferably 0.60% or less, more preferably 0.50% or less.
- Mn is an element that contributes to increasing the strength of the steel sheet. In order to effectively exhibit such effects, the amount of Mn is set to 1.8% or more.
- the lower limit of the Mn content is preferably 1.9% or more, more preferably 2.0% or more.
- an excessive amount of Mn may cause slab breakage, an increase in cold rolling load, and the like. Therefore, the Mn content is set to 3.0% or less.
- the upper limit of the Mn amount is preferably 2.9% or less, more preferably 2.8% or less.
- P is an element that is inevitably contained in steel, and is an element that segregates at grain boundaries of steel to promote intergranular embrittlement. It is preferable to reduce the amount of P as much as possible in order to avoid breakage or the like during processing of the steel sheet. Therefore, the amount of P is set to 0.020% or less.
- the upper limit of the amount of P is preferably 0.015% or less, more preferably 0.012% or less.
- P is an impurity that is inevitably mixed in steel, and it is impossible to reduce its amount to 0% in terms of industrial production.
- S is an element that is unavoidably contained in steel. S forms inclusions together with other elements in steel. The inclusions may cause breakage or the like during processing of the steel sheet. In order to avoid such breakage of the steel sheet, it is preferable to reduce the amount of S as much as possible. Therefore, the amount of S is set to 0.05% or less.
- the upper limit of the S amount is preferably 0.04% or less, more preferably 0.03% or less. As with P, it is impossible for industrial production to reduce the amount of S to 0%.
- Al 0.015-0.060%
- Al is an element that acts as a deoxidizing agent in steel.
- the amount of Al is set to 0.015% or more.
- the lower limit of the Al content is preferably 0.025% or more, more preferably 0.030% or more.
- the Al content is set to 0.060% or less.
- the upper limit of the Al content is preferably 0.055% or less, more preferably 0.050% or less.
- Cr 0.05-0.8%) Cr is one of the important elements in the present invention.
- Cr is an element that contributes to increasing the strength of the steel sheet.
- Cr is an element that improves the hardenability of the steel sheet, reduces bainite generated during hardening, increases the number of martensite laths, and effectively increases the strength of the steel sheet.
- Cr is an element that hardly deteriorates the LME cracking resistance even when the content is increased.
- the amount of Cr is set to 0.05% or more.
- the lower limit of Cr content is preferably 0.1% or more.
- the Cr content is set to 0.8% or less.
- the upper limit of the Cr content is preferably 0.7% or less, more preferably 0.6% or less.
- Ti is an element that forms carbides and nitrides to improve the strength of the steel sheet.
- Ti is an effective element for effectively exhibiting the hardenability-improving effect of B, which will be described later. That is, Ti reduces N in steel by forming nitrides. As a result, the formation of B nitrides is suppressed, B enters a solid solution state, and the hardenability-enhancing effect of B can be effectively exhibited. In this way, Ti improves the hardenability of the steel sheet due to B, thereby contributing to increasing the strength of the steel sheet. In order to effectively exhibit such effects, the amount of Ti should be 0.015% or more.
- the lower limit of the Ti amount is preferably 0.018% or more, more preferably 0.020% or more. However, if the amount of Ti becomes excessive, Ti carbides and Ti nitrides become excessive, which may cause cracks during working of the steel sheet. Therefore, the amount of Ti should be 0.080% or less.
- the upper limit of the Ti amount is preferably 0.070% or less, more preferably 0.060% or less.
- B is an element that improves hardenability and contributes to increasing the strength of the steel sheet.
- the amount of B is set to 0.0010% or more.
- the lower limit of the B content is preferably 0.0012% or more, more preferably 0.0014% or more.
- the amount of B is set to 0.0150% or less.
- the upper limit of the amount of B is preferably 0.0140% or less.
- Mo more than 0%, 0.40% or less
- Mo is an element that contributes to increasing the strength of the steel sheet. The effect increases as the amount of Mo increases. In order to effectively exhibit the effect, the amount of Mo should be more than 0%.
- the lower limit of Mo content is preferably 0.10% or more, more preferably 0.20% or more. However, if the amount of Mo becomes excessive, the effect will be saturated and the cost will increase. Therefore, the amount of Mo is set to 0.40%.
- the upper limit of Mo content is preferably 0.30% or less.
- the Cr content and the Si content of the base steel sheet of the high-strength galvannealed steel sheet according to the present embodiment satisfy the following formula (1).
- the formula (1) the hardenability of the high-strength galvannealed steel sheet can be improved.
- the balance between the Si content and the Cr content becomes appropriate, and it becomes possible to reduce the occurrence of LME cracks in the region around the weld zone. That is, it is possible to achieve both high strength of the high-strength alloyed hot-dip galvanized steel sheet and excellent resistance to LME cracking.
- [element symbol] is the content (% by mass) of the element in the base steel sheet.
- the basic components of the base steel sheet of the high-strength galvannealed steel sheet according to the present embodiment are as described above, and the balance is substantially iron.
- the balance of the basic components of the base steel sheet consists of iron and unavoidable impurities.
- unavoidable impurities include, for example, N, O, etc., in addition to P and S described above. N and O preferably fall within the following ranges.
- N is an impurity element that is unavoidably contained in steel. An excessive amount of N may cause cracks during processing of the steel sheet. Therefore, the amount of N is made 0.0100% or less.
- the upper limit of the amount of N is preferably 0.0060% or less. It is difficult in terms of industrial production to make the amount of N 0%.
- O is an impurity element inevitably contained in steel. An excessive amount of O may cause cracks during processing of the steel sheet. Therefore, the amount of O is made 0.0030% or less.
- the upper limit of the amount of O is preferably 0.0020% or less. It is difficult in terms of industrial production to reduce the O content to 0%.
- the base steel sheet of the high-strength galvannealed steel sheet according to the present embodiment may contain Ca within the following range.
- the base steel sheet contains one or more elements selected from the group consisting of Nb, V, Cu, Ni, Mg, and rare earth elements (REM) with or without Ca. You may make it contain in the range shown below. By containing these elements singly or in combination, the properties of the base steel sheet are improved depending on the contained elements.
- Ca is an element effective in spheroidizing sulfides in steel and improving the bendability of the steel sheet.
- the effect is exhibited when the amount of Ca exceeds 0%, and increases as the amount of Ca increases.
- the lower limit of the amount of Ca is preferably 0.0005% or more, more preferably 0.0010% or more.
- the upper limit of the amount of Ca is preferably 0.0040% or less, more preferably 0.0030% or less.
- Nb is an element that contributes to increasing the strength of the steel sheet. The effect is exhibited when the Nb content exceeds 0%, and increases as the Nb content increases.
- the lower limit of the Nb content is preferably 0.003% or more, more preferably 0.005% or more.
- the upper limit of the Nb content is preferably 0.020% or less, more preferably 0.018% or less, and still more preferably 0.015% or less.
- V is an element that contributes to increasing the strength of the steel sheet. The effect is exhibited when the amount of V exceeds 0%, and increases as the amount of V increases.
- the lower limit of the V content is preferably 0.005% or more, more preferably 0.010% or more.
- the upper limit of the V content is preferably 0.30% or less, more preferably 0.25% or less, and even more preferably 0.20% or less.
- Cu more than 0%, 0.30% or less
- Cu is an element effective in improving the corrosion resistance of steel sheets. The effect is exhibited when the amount of Cu exceeds 0%, and increases as the amount of Cu increases.
- the lower limit of the amount of Cu is preferably 0.01% or more, more preferably 0.05% or more.
- the upper limit of the amount of Cu is preferably 0.30% or less, more preferably 0.20% or less, and still more preferably 0.15% or less.
- Ni is an element effective in improving the corrosion resistance of steel sheets. The effect is exhibited when the amount of Ni exceeds 0%, and increases as the amount of Ni increases.
- the lower limit of the Ni amount is preferably 0.03% or more, more preferably 0.05% or more.
- the upper limit of the Ni content is preferably 0.30% or less, more preferably 0.20% or less, and even more preferably 0.15% or less.
- Mg is an element that contributes to the improvement of the formability of the steel sheet. The effect is exhibited if the amount of Mg exceeds 0%. However, if the amount of Mg becomes excessive, the pickling property, weldability, hot workability, and economic efficiency of the steel sheet deteriorate. Therefore, the upper limit of the Mg content is preferably 0.0100% or less, more preferably 0.0040% or less.
- REM more than 0%, 0.010% or less
- REM is an element that contributes to improving the formability of the steel sheet. The effect is exhibited if the amount of REM exceeds 0%. However, if the amount of REM becomes excessive, the pickling property, weldability, hot workability, and economic efficiency of the steel sheet deteriorate. Therefore, the upper limit of the REM amount is preferably 0.010% or less, more preferably 0.0040% or less.
- the metal structure of the base steel sheet of the high-strength galvannealed steel sheet according to the present embodiment is martensite (including tempered martensite and self-tempered martensite) of 82% by volume or more, bainite of 13% by volume or less, and retained austenite. is 5% by volume or less.
- the high-strength galvannealed steel sheet according to the present embodiment can have specified tensile strength and yield strength.
- the method for measuring the ratio of each metal structure can be the method described in the examples below.
- Martensite in the metal structure is the base structure of the base steel sheet according to this embodiment.
- the volume fraction of martensite is more preferably 83% by volume or more. There is no upper limit to the volume fraction of martensite, and it may be 100%.
- the martensite of this embodiment includes not only as-quenched martensite, but also tempered martensite and self-tempered martensite (autotempered martensite).
- Ferrite, pearlite and bainite 13% by volume or less in total
- Ferrite, pearlite, and bainite are softer than martensite, which is the base structure of the base steel sheet.
- the total content of ferrite, pearlite and bainite is set to 13% by volume or less with respect to the entire metal structure of the base steel sheet.
- There is no lower limit to the volume percentages of ferrite, pearlite and bainite and they may be 0% by volume.
- ferrite, pearlite, and bainite are also collectively referred to as “bainite, etc.”.
- the retained austenite in the metal structure should be 5% by volume or less with respect to the entire metal structure of the base steel sheet.
- retained austenite a small amount of film-like retained austenite existing at the boundaries of laths of martensite has the effect of increasing tensile strength and yield strength by suppressing the movement of dislocations when stress is applied to the steel sheet. have.
- the retained austenite itself is softer than the martensite structure. Therefore, even in the form of a film, when excessive retained austenite exists, both the yield strength and tensile strength of the steel sheet are lowered. From this point of view, the retained austenite should be 5% by volume or less. Retained austenite may be 0% by volume.
- the percentage of retained austenite can be determined by polishing the ground surface of a test piece cut out from the base steel plate by chemical polishing or electrolytic polishing, and applying the X-ray diffraction method to the ground surface after polishing.
- can be measured by Electrolytic polishing is preferable to chemical polishing as a method for polishing the ground surface from the viewpoint of reducing the burden on the environment.
- the base steel sheet of the high-strength galvannealed steel sheet according to the present embodiment has high tensile strength and high yield strength. Therefore, in an image obtained by observing the metal structure of the base steel sheet with a scanning electron microscope, the number of laths in a total length of 300 ⁇ m measured by a cutting method (hereinafter also simply referred to as “the number of laths in a total length of 300 ⁇ m”) is 200 or more. do. When the number of laths in a total length of 300 ⁇ m is less than 200, at least one of yield strength and tensile strength is lowered.
- the number of laths in a total length of 300 ⁇ m is preferably 210 or more, more preferably 220 or more.
- the “lath” here is the substructure of martensite, which is a crystal elongated in one direction.
- the structure of martensite is multi-layered as explained below. Martensite is formed by transformation of quenched austenite.
- a plurality of packets which are groups of grains having the same habit plane, exist in one prior austenite grain.
- blocks which are parallel strips.
- each block has a set of laths containing dislocations at a high density with almost the same crystal orientation.
- the "number of laths in a total length of 300 ⁇ m measured by the cutting method" defined in the present invention is measured at 1/4 part of the plate thickness of the cross section parallel to the rolling direction of the steel plate. Specifically, the cross section of the polished steel plate is corroded using nital, and the cross section is photographed at a magnification of 3000 times using an FE-SEM (Field Emission Scanning Electron Microscope). do. A cut method is applied to this photograph to determine the number of laths.
- the cutting method in this embodiment is a method of drawing a line (a test line of a straight line or an arc) with a total length of 300 ⁇ m on a photographed FE-SEM image and obtaining the number of laths intersecting with the test line.
- the method for measuring the number of laths by cutting will be described in more detail in the examples below.
- the plating layer is obtained by subjecting hot-dip galvanization formed on the surface of the base steel sheet to alloying treatment, which will be described later.
- Hot-dip galvanizing is not particularly limited and any commonly used one can be applied.
- the high-strength galvannealed steel sheet according to the present embodiment which satisfies the above requirements, is obtained by hot-rolling a slab having the composition of the base steel sheet described above, winding it at a predetermined temperature, and unrolling the wound hot-rolled steel sheet.
- the cold-rolled steel sheet obtained by cold rolling is cold-rolled, the cold-rolled steel sheet obtained by the cold rolling is annealed, and the cold-rolled steel sheet obtained by cooling the annealed cold-rolled steel sheet is hot-dip galvanized and hot-dip galvanized. It is obtained by performing an alloying treatment on a hot-dip galvanized steel sheet and then cooling it. Each step will be described below.
- the conditions for hot rolling are, for example, as follows.
- a slab having the composition of the steel sheet described above is hot rolled.
- the heating temperature of the slab before hot rolling is preferably 1100° C. or higher.
- the heating temperature of the slab before hot rolling is more preferably 1200° C. or higher.
- the upper limit of the heating temperature before hot rolling is preferably 1350° C. or lower, more preferably 1300° C. or lower.
- the finish rolling temperature of hot rolling is low, the deformation resistance of the slab during rolling increases, which may make operation difficult. Therefore, the finish rolling temperature is preferably 850°C or higher, more preferably 870°C or higher. However, if the finish rolling temperature becomes too high, the strength of the hot-rolled steel sheet may become excessively high. Therefore, the finish rolling temperature is preferably 980°C or lower, more preferably 950°C or lower.
- the average cooling rate of the hot-rolled steel sheet obtained by hot rolling from finish rolling to coiling is preferably 10° C./s or more, more preferably 20° C./s or more, in consideration of productivity. be. On the other hand, if the average cooling rate is too fast, the equipment cost will increase. Therefore, the average cooling rate is preferably 100° C./s or less, more preferably 50° C./s or less.
- Winding process of hot-rolled steel sheet A hot-rolled steel sheet obtained by hot rolling is coiled at 620° C. or higher. If the coiling temperature of the hot-rolled steel sheet is less than 620°C, the strength of the hot-rolled steel sheet becomes too high, making cold rolling difficult.
- the coiling temperature of the hot-rolled steel sheet is preferably 630°C or higher, more preferably 640°C or higher. On the other hand, if the coiling temperature of the hot-rolled steel sheet becomes too high, the pickling property for scale removal deteriorates. Therefore, the winding temperature is preferably 800° C. or lower, more preferably 750° C. or lower.
- the lower limit of the rolling reduction during cold rolling is preferably 10% or more.
- the rolling reduction in the present embodiment is synonymous with the "rolling reduction". Specifically, when the thickness of the steel sheet before rolling is h1 and the thickness of the steel sheet after rolling is h2, the rolling rate (%) is "(h1 ⁇ h2)/h1 ⁇ 100". If the rolling reduction during cold rolling is less than 10%, the thickness of the hot-rolled steel sheet must be reduced in the hot-rolling process in order to obtain a steel sheet with a predetermined thickness. When the hot-rolled steel sheet is thinned, the length of the hot-rolled steel sheet becomes long, so pickling takes time and productivity decreases.
- the lower limit of the rolling reduction during cold rolling is more preferably 25% or more.
- the upper limit of the rolling reduction during cold rolling is preferably 70% or less, more preferably 65% or less.
- FIG. 1 is a schematic diagram of a heat pattern of a steel sheet after cold rolling according to this embodiment.
- the heat pattern shown in FIG. 1 includes (a) a soaking step, (b) a first cooling step, (c) a second cooling step, (d) an alloying step, (e) a third cooling step, and (f) a fourth cooling step.
- the conditions of each step (a) to (e) are appropriately adjusted. It is important to.
- the soaking step the cold-rolled steel sheet is heated and held for 11 seconds or longer in a temperature range of Ac3 or higher. If the heating temperature is less than the Ac3 point, there is a possibility that soft ferrite that lowers the yield strength and tensile strength of the steel sheet will remain. Therefore, the lower limit of the holding temperature in the soaking step is Ac3.
- the lower limit of the holding temperature in the soaking step is preferably (3 points of Ac+5)°C.
- the holding temperature in the soaking step may be equal to or lower than the solidus temperature of the cold-rolled steel sheet, and there is no particular upper limit.
- the upper limit of the holding temperature in the soaking process is preferably 980 ° C. or less. be.
- the temperature of the cold-rolled steel sheet may be kept constant, or the temperature of the cold-rolled steel sheet may fluctuate within the holding temperature range.
- the holding time in the temperature range of Ac3 or higher is set to 11 seconds or longer.
- the lower limit of the holding time in the temperature range of Ac3 or higher is preferably 12 s or longer, more preferably 15 s.
- the retention time is preferably less than 600 s, because productivity deteriorates if the retention time is too long.
- the Ac3 point can be calculated by the following formula (2) (William C. Leslie, "Leslie Iron and Steel Materials Science", Maruzen Co., Ltd., p.273).
- [Element symbol] in the formula (2) represents the content (% by mass) of the element.
- Ac3 (°C) 910 - 203 x [C] 1/2 - 15.2 x [Ni] + 44.7 x [Si] + 104 x [V] + 31.5 x [Mo] + 13.1 x [W] - ⁇ 30 ⁇ [Mn]+11 ⁇ [Cr]+20 ⁇ [Cu] ⁇ 700 ⁇ [P] ⁇ 400 ⁇ [Al] ⁇ 120 ⁇ [As] ⁇ 400 ⁇ [Ti] ⁇ (2)
- the cold-rolled steel sheet heated and held in the temperature range of Ac 3 or more in the soaking step is averaged to a temperature range of 540 to 580 ° C. (first temperature range). Cool at a cooling rate of 3°C/s or more. Specifically, after starting cooling in the soaking step, cooling is performed from the Ac3 point to the first temperature range at an average cooling rate of 3° C./s or more. If this average cooling rate is less than 3° C./s, the possibility of ferrite formation increases, making it difficult to ensure the yield strength and tensile strength specified in the present invention.
- the average cooling rate should be 3° C./s or higher, preferably 4° C./s or higher, and more preferably 5° C./s or higher.
- the upper limit of the average cooling rate is 50° C./s or less, preferably 40° C./s or less.
- Second cooling step In the second cooling step, after the first cooling step, the cold-rolled steel sheet is cooled to a temperature range of 410 to 480 ° C. (second temperature range) within 90 seconds to cool the base steel sheet. get More specifically, the cold-rolled steel sheet is cooled from the first temperature range to the second temperature range in a time of 90 seconds or less while ensuring a temperature equal to or higher than the Ms point. If the time for cooling from the first temperature range to the second temperature range exceeds 90 seconds, there is concern that bainite will increase, so the time for cooling from the first temperature range to the second temperature range is set to 90 seconds or less.
- the upper limit of the time for cooling from the first temperature range to the second temperature range is preferably 70 seconds or less.
- the upper limit of the second temperature range is preferably 470°C or lower, more preferably 460°C or lower.
- the second cooling step it is preferable to keep the cold-rolled steel sheet at a temperature equal to or higher than the Ms point.
- the cold-rolled steel sheet falls below the Ms point in the second cooling step, martensite is formed before the subsequent alloying step, and the lath spacing of the final structure after the heat treatment is reduced, resulting in a decrease in tensile strength. It is from.
- the “Ms point” is the temperature at which austenite starts transforming to martensite, and can be easily obtained from the chemical composition of the steel sheet based on the following formula (3) (“Lecture on Modern Metallurgy Materials Ed. Vol. 4 Iron and Steel Materials", The Japan Institute of Metals, June 1985, p.45). [Element symbol] in the formula (3) represents the content (% by mass) of the relevant element in the steel sheet, and the element not contained in the steel sheet is assumed to be 0 in the calculation.
- Ms point (°C) 550 - 361 x [C] - 39 x [Mn] - 35 x [V] - 20 x [Cr] - 17 x [Ni] - 10 x [Cu] - 5 x ([Mo] + [W]) + 15 x [Co] + 30 x [Al] (3)
- the base steel sheet is hot-dip galvanized.
- the base steel sheet that has been cooled to the second temperature range enters a plating pot containing a hot-dip galvanizing bath and undergoes immersion treatment in the plating bath. By this immersion treatment, the base steel sheet is hot-dip galvanized, and a hot-dip galvanized steel sheet is obtained.
- the alloying step the obtained hot-dip galvanized steel sheet is alloyed.
- the hot-dip galvanized steel sheet is heated to a temperature range of 550°C or less to perform the hot-dip galvanizing alloying process. Specifically, by heating the hot-dip galvanized steel sheet, zinc contained in the hot-dip galvanized steel sheet is alloyed with iron contained in the base steel sheet. If the heating temperature in this alloying treatment exceeds 550° C., the possibility of forming ferrite in the base steel sheet increases, and the tensile strength of the alloyed hot-dip galvanized steel sheet may decrease. In addition to this, the diffusion of iron from the base steel sheet to the hot-dip galvanized zinc is excessive, and the coating layer is more likely to peel off during press forming or the like.
- the upper limit of the heating temperature in the alloying step is preferably 540°C or lower, more preferably 530°C or lower.
- the heating temperature in the alloying step may be higher than the temperature of the hot-dip galvanized steel sheet immediately after immersion in the plating bath, preferably 420° C. or higher, more preferably 430° C. or higher.
- the alloyed hot-dip galvanized steel sheet obtained in the alloying step is cooled down to a temperature range of 230 to 340°C (third temperature range) at an average cooling rate of 5.5. Cool at 0°C/s or higher. More specifically, cooling is performed at an average cooling rate of 5.0° C./s or more from immediately after alloying to the cooling stop temperature in the third temperature range. Thereby, the high-strength alloyed hot-dip galvanized steel sheet according to the present embodiment described above can be obtained. If the average cooling rate in the third cooling step is less than 5.0°C/s, there is concern that bainite will increase.
- the average cooling rate in the third cooling step is set to 5.0° C./s or higher.
- the average cooling rate in the third cooling step is preferably 8.0°C/s or higher, more preferably 10°C/s or higher.
- the upper limit of the average speed is not particularly specified, increasing the cooling capacity of the cooling equipment causes a large load on the cooling equipment. Therefore, the upper limit of the average speed is preferably 50° C./s or less, more preferably 40° C./s or less.
- the fourth cooling step is performed.
- the high-strength galvannealed steel sheet is preferably cooled from the third temperature range to a cooling stop temperature of 50°C or less at an average cooling rate of 5.0°C/s or less. If the average cooling rate in the fourth cooling step exceeds 5.0° C./s, self-tempering of martensite in the base steel sheet may not progress, resulting in a brittle structure.
- the average cooling rate in the fourth cooling step is more preferably 4.0°C or less.
- the average cooling rate in the fourth cooling step is preferably 0.05 ° C./s or more, more preferably 0.10 ° C./s. s or more.
- cooling stop temperature in the third cooling step is higher than 230°C, the average cooling rate from the cooling stop temperature in the third cooling step to 230°C does not matter. Further, in the fourth cooling step, cooling to room temperature may be performed at any cooling rate as long as no heating is performed after the third cooling step.
- the high-strength alloyed hot-dip galvanized steel sheet may be processed and tempered as necessary.
- the high-strength galvannealed steel sheet cooled in the fourth cooling process has sufficiently high tensile strength and yield strength without temper rolling.
- Such an improvement in yield strength is due to a decrease in mobile dislocations of martensite in the base steel sheet due to working of the plated steel sheet.
- Mobile dislocations of martensite in the base steel sheet are preferably small because they reduce the yield strength of the plated steel sheet.
- the upper limit of the processing amount of the plated steel sheet is not specified. However, the plated steel sheet suffers from shape deterioration and strength anisotropy due to processing.
- the upper limit of the amount of processing in temper rolling is preferably 5% or less, more preferably 4% or less in terms of elongation in the rolling direction of the high-strength galvannealed steel sheet.
- processing using a leveler may be performed.
- the preferred processing amount for processing using a leveler is the same as for temper rolling.
- the tempering temperature is not specified, if it exceeds approximately 500°C, excessive tempering occurs, the number of martensite laths in the base steel sheet decreases, and the tensile strength of the high-strength galvannealed steel sheet decreases. , leading to a decrease in yield strength. Therefore, the upper limit of the tempering temperature is preferably 500°C or less.
- the high-strength alloyed hot-dip galvanized steel sheet of the present invention is not limited to those obtained by the above manufacturing method.
- the high-strength alloyed hot-dip galvanized steel sheet of the present invention may be obtained by other manufacturing methods as long as it satisfies the requirements specified in the present invention.
- the high-strength galvannealed steel sheet according to one aspect of the present invention is a high-strength galvannealed steel sheet having a coating layer on the surface of the base steel sheet
- the base steel plate is mass %, C: 0.19 to 0.30%, Si: more than 0%, 0.70% or less, Mn: 1.8-3.0%, P: more than 0%, 0.020% or less, S: more than 0%, 0.05% or less, Al: 0.015-0.060%, Cr: 0.05-0.8%, Ti: 0.015 to 0.080%, B: 0.0010 to 0.0150%, Mo: more than 0%, 0.40% or less, N: 0.0100% or less, and O: 0.0030% or less, containing, the balance consisting of iron and inevitable impurities,
- the Cr content [Cr] (mass%) and the Si content [Si] (mass%) of the base steel sheet satisfy 2 ⁇ [Cr] - [Si] ⁇ 0.1, In
- the base steel sheet may further contain, in mass%, Ca: more than 0% and 0.0040% or less.
- the base steel sheet further contains, in mass%, Nb: more than 0%, 0.020% or less, V: more than 0%, 0.30% or less, Cu: more than 0%, 0.30% or less, Ni: more than 0%, 0.30% or less, Mg: more than 0%, 0.0100% or less, and REM: more than 0%, 0.010% or less, It may contain one or more selected from the group consisting of.
- a method for producing a high-strength galvannealed steel sheet according to another aspect of the present invention includes hot-rolling a slab having the above composition, Winding the hot-rolled steel sheet obtained by hot rolling at 620 ° C. or higher, Unwinding and cold-rolling the wound hot-rolled steel sheet, A cold-rolled steel sheet obtained by cold rolling is heated, held for 11 seconds or longer in a temperature range of Ac3 or higher, and the heated and held cold-rolled steel sheet is cooled to a temperature range of 540 to 580 ° C. at an average cooling rate of 3 ° C./s. above, and further cooled to a temperature range of 410 to 480 ° C.
- Hot-dip galvanizing is applied to the base steel sheet obtained by cooling the cold-rolled steel sheet, A hot-dip galvanized steel sheet obtained by hot-dip galvanizing is heated to a temperature range of 550 ° C. or less to alloy the hot-dip galvanized, Cooling the alloyed hot-dip galvanized steel sheet obtained by subjecting the hot-dip galvanized steel sheet to a temperature range of 230 to 340° C. at an average cooling rate of 5° C./s or more.
- the cooled alloyed hot-dip galvanized steel sheet may be processed at an elongation rate of 5% or less.
- Table 1 also shows the Ac3 point and Ms point of each steel type.
- the Ac3 points were calculated using the above formula (2), and the Ms points were calculated using the above formula (3).
- the contents of the elements that were not added and the elements that were below the measurement limit were set to 0.
- Table 1 also shows the value of “2 ⁇ [Cr] ⁇ [Si]” of each steel type (described as “CS value” in Table 1).
- [Cr] and [Si] are contents (% by mass) of the relevant elements in the slab.
- the CS value is 0.1 or more, the following formula (1) is satisfied. 2 ⁇ [Cr] ⁇ [Si] ⁇ 0.1 (1)
- Steel grades A to F and H to J slabs are heated to 1250°C, and steel grade G slabs are heated to a temperature range of 1265 to 1275°C.
- Rolled steel plate The temperature of the hot-rolled steel sheet at the completion of finish rolling was 900° C. for steel types A to F and H to J, and 920° C. for steel type G.
- the average cooling rate of the hot-rolled steel sheet from the completion of finish rolling of hot rolling to the start of coiling of the hot-rolled steel sheet was set to 10 to 30° C./s.
- the coiling start temperature of the hot-rolled steel sheet was set to 650° C. for steel types A to F and 680° C. for steel types G to J, and coiling of the hot-rolled steel sheet and processing equivalent to coil winding were performed.
- the cold rolling rate (rolling rate at the time of cold rolling) of any steel grade was within the range of 10 to 60%.
- the heat treatment Nos. shown in Table 2 were applied to the obtained cold-rolled steel sheets of steel grades A to J. 1 to 14 heat treatments were performed, and Experiment No. 1 to 18 steel sheets (steel sheets or galvannealed steel sheets) were produced. Heat treatment no. 1 to 7, 13 and 14, a lab simulator was used as the heat treatment furnace. In 8 to 12, actual equipment was used. Experiment no.
- the "time from 3 points of Ac to the highest temperature” in the soaking step (step (a) shown in FIG. 1) is 10 seconds, and during this time, "cooling from the highest temperature to 3 points of Ac
- Heat treatment No. For the steel sheets subjected to 1 to 7, 13, and 14, the structure was observed and the mechanical properties were evaluated after heat treatment simulating the hot-dip galvanizing treatment and alloying treatment (step (d) shown in FIG. 1). .
- Heat treatment no. For the steel sheets subjected to 8 to 12, the structure was observed and the mechanical properties were evaluated after plating or specified work hardening treatment (pass rolling).
- volume fraction of retained austenite The volume fraction of retained austenite was measured as follows. A test piece was cut into a size of 20 mm ⁇ 20 mm from a steel plate (thickness: 1.4 to 1.6 mm) after heat treatment. This test piece was ground from the surface to 1 ⁇ 4 of the plate thickness, and the ground surface was polished by chemical polishing (Experiment No. 8) or electrolytic polishing (Experiment Nos. 16 to 18). The volume fraction of retained austenite on the ground surface after polishing was measured by the X-ray diffraction method (ISIJ Int. Vol. 33. (1993), No. 7, P. 776). For the measurement, a two-dimensional micro X-ray diffractometer (RINT-RAPID II, manufactured by Rigaku Corporation) was used. Co was used as the target.
- the volume fraction of retained austenite is 5% or less from the C content or Si content, or the heat treatment conditions under the third cooling condition or the fourth cooling condition. rice field. Therefore, the volume fraction of retained austenite was not measured for such test pieces by the X-ray diffraction method.
- the maximum volume fraction of retained austenite of 5% was used as an assumed value.
- the volume fractions of bainite and martensite were measured and calculated by the following procedure.
- a test piece was cut into a size of 20 mm ⁇ 20 mm from a steel plate (thickness: 1.4 to 1.6 mm) after heat treatment.
- a section parallel to the rolling direction of this test piece was polished, and the polished surface was subjected to nital corrosion.
- a photograph (magnification: 3000 times) of the structure of 1/4 part of the plate thickness of the polished surface subjected to nital corrosion was taken using an FE-SEM.
- the structure was classified into bainite or martensite based on the color of grains in the structure photograph, and the area ratios of bainite and martensite were measured by the point counting method.
- orthogonal grids with 3 ⁇ m intervals (9 mm intervals on the photograph) were provided on the photographed FE-SEM image, and the structure at points where the lattices intersected at right angles (lattice points) was classified into bainite or martensite.
- the division of the structure was performed for 100 grid points, and the results were used to calculate the area ratio of bainite and the area ratio of martensite. Measurement was performed for one field of view (one photograph) for each test piece.
- Fig. 2 is an example of a scanning electron micrograph at a magnification of 3000 times of a structure of a quarter part of the plate thickness of the polished surface subjected to nital corrosion in a cross section parallel to the rolling direction of the test piece.
- the structure that looks black is bainite and the rest is martensite.
- the volume ratio of retained austenite and the area ratio of bainite and martensite are measured by different methods, so the total ratio of each structure is not necessarily 100%. Not necessarily.
- the number of laths in a total length of 300 ⁇ m was measured by a cutting method.
- the cutting method is usually a technique for measuring particle size (JIS G 0551:2013).
- the cutting method is applied as a method for measuring the number of laths.
- FIG. 3 is a schematic diagram showing how the number of laths is measured by the cutting method.
- a line (test line) with a total length of 300 ⁇ m was drawn on the captured FE-SEM image, and the number of laths that the line passed through (the number of laths intersecting the test line) was measured as shown in FIG.
- the number of laths intersecting the test line with a total length of 300 ⁇ m is referred to as “the number of laths in a total length of 300 ⁇ m”.
- Table 3 shows the measured number of laths in a total length of 300 ⁇ m.
- Table 3 shows the measured tensile strength TS and 0.2% yield strength ⁇ 0.2 .
- the mechanical properties were regarded as acceptable.
- LME cracking resistance is greatly affected by the chemical composition of the steel sheet, and the effect of heat treatment is small compared to the chemical composition. Therefore, LME cracking resistance can be evaluated by chemical composition.
- the LME cracking resistance of each steel plate was evaluated by the following method.
- As-cold-rolled steel sheets of steel grades A to F were electroplated so that the amount of zinc coating deposited was 50 g/m 2 .
- the obtained galvanized steel sheet was heated to 350° C. to carry out galvanizing alloying treatment.
- Each of the obtained galvannealed steel sheets was cut, and two samples of 140 mm ⁇ 35 mm were taken.
- FIG. 4 is a schematic front view of a sample for evaluation of LME crack resistance.
- a plate set was formed by sandwiching a mild steel plate 2 between two samples 1 taken, and both ends of the three plate set were fixed with clamps.
- the two samples 1 are hereinafter also referred to as an upper plate and a lower plate, respectively.
- Resistance spot welding was performed on the center of the fixed three-plate set to prepare a sample for LME crack resistance evaluation.
- As the mild steel plate 2 a GA steel plate (alloyed hot-dip galvanized steel plate) having a tensile strength of 270 MPa, a coating weight on one side of 55 g/m 2 and dimensions of 0.75 mm ⁇ 140 mm ⁇ 35 mm was used.
- One sample for LME cracking resistance evaluation was prepared for each of the steel types A to J. Welding conditions were as follows.
- Welder AC inverter type resistance welder Electrodes: Upper and lower DR type Cu-Cr (made by Dome Radius) Electrode striking angle: 5° Electrode diameter: tip diameter 8 mm Cooling water flow rate: Up and down about 2L/min Pressure: 350kgf Initialization pressure time: 60 cycles/60 Hz Upslobe: 1 cycle/60Hz First stage of energization Current value: 7.2 kA Time (cycles/60Hz): 8 2nd energization stage Current value: 9kA Time (cycles/60Hz): 17 Downslobe (cycles/60Hz): 30 Hold time (cycle/60Hz): 5
- FIG. 5 is an example of an optical microscope photograph of a sample for observing LME cracks. As shown in FIG. 5, cracks with a length of 50 ⁇ m or more were judged to be LME cracks in the weld zone. As a result of observation of the LME crack resistance evaluation samples of each steel type, the evaluation results are shown in Table 4, with those with cracks indicated as ⁇ (improper) and those without cracks indicated as ⁇ (good). The evaluation result of steel grade G is an estimate.
- Experiment No. 1, 5, 6, and 11 are comparative examples that do not satisfy any of the requirements defined in the present invention. These comparative steel sheets did not meet the acceptance criteria for mechanical properties.
- Experiment No. 5 and 11 are examples in which the number of laths in the total length of 300 ⁇ m specified in the present invention was not satisfied.
- the steel sheet of No. 5 had a low temperature of 150°C in the first cooling step (b), and although the metal structure satisfied the provisions of the present invention, it satisfied the number of laths in the total length of 300 ⁇ m specified in the present invention. didn't. Therefore, the tensile strength TS was low, and the mechanical properties did not satisfy the acceptance criteria.
- Experiment no. Steel sheet No. 11 has a holding time of less than 11 seconds in a temperature range of Ac 3 or more in the soaking step (a), the number of laths in a total length of 300 ⁇ m is less than the specification of the present invention, and the volume of bainite etc. The rate was higher than specified in the present invention. Therefore, the tensile strength TS and 0.2% proof stress ⁇ 0.2 were low, and the mechanical properties did not satisfy the acceptance criteria.
- Experiment No. 1 and 6 are examples using steel grades (steel grades A and D in Table 1) that do not satisfy the chemical composition specified in the present invention.
- Experiment no. Steel plate No. 1 had a Cr content of steel type A less than the specified content of the present invention, a CS value of less than 0.1, and did not satisfy formula (1). As a result, the hardenability was insufficient, and the volume fraction of bainite and the like was higher than specified in the present invention. Therefore, the tensile strength TS and 0.2% proof stress ⁇ 0.2 were low, and the mechanical properties did not satisfy the acceptance criteria.
- the steel plate No. 6 had a Si content larger than that specified in the present invention, a CS value of less than 0.1, and did not satisfy the formula (1). As a result, the hardenability was insufficient, and the volume fraction of bainite and the like was higher than specified in the present invention. Therefore, the tensile strength TS and 0.2% proof stress ⁇ 0.2 were low, and the mechanical properties did not satisfy the acceptance criteria.
- Experiment No. Steel plate No. 7 is a comparative example using a steel type (steel type E in Table 1) that does not satisfy the chemical composition specified in the present invention, but it satisfied the acceptance criteria for mechanical properties.
- the CS value is less than 0.1
- the steel type A that does not satisfy the formula (1), the Si content is greater than the specified amount of the present invention
- the steel type D that does not satisfy the formula (1), the Si content is the present invention.
- Each steel plate of steel type E, which has a larger amount than the specified amount was inferior in LME crack resistance. Therefore, experiment no. Steel sheets Nos. 1, 6 and 7 are considered to be inferior in resistance to LME cracking.
- the present invention has wide industrial applicability in the technical field of high-strength alloyed hot-dip galvanized steel sheets and manufacturing methods thereof.
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Abstract
Description
前記素地鋼板が質量%で、
C :0.19~0.30%、
Si:0%超、0.70%以下、
Mn:1.8~3.0%、
P :0%超、0.020%以下、
S :0%超、0.05%以下、
Al:0.015~0.060%、
Cr:0.05~0.8%、
Ti:0.015~0.080%、
B :0.0010~0.0150%、
Mo:0%超、0.40%以下、
N :0.0100%以下、および
O :0.0030%以下、
を含有し、残部が鉄および不可避不純物からなり、
前記素地鋼板のCr量[Cr](質量%)およびSi量[Si](質量%)は、2×[Cr]-[Si]≧0.1を満足し、
前記素地鋼板の金属組織において、マルテンサイト(焼戻しマルテンサイトおよび自己焼戻しマルテンサイトを含む)が82体積%以上、フェライト、パーライトおよびベイナイトが13体積%以下、残留オーステナイトが5体積%以下であり、
前記素地鋼板の金属組織を走査型電子顕微鏡で観察した像において、切断法で測定した総長300μmにおけるラスの個数が200個以上であり、降伏強度が970MPa以上かつ引張強さが1470MPa以上である。
熱間圧延により得られた熱延鋼板を620℃以上で巻き取り、
巻き取った前記熱延鋼板を繰り出して冷間圧延し、
冷間圧延により得られた冷延鋼板を加熱し、Ac3点以上の温度域で11s以上保持し、加熱、保持した前記冷延鋼板を540~580℃の温度域まで平均冷却速度3℃/s以上で冷却し、さらに90s以内に410~480℃の温度域まで冷却し、
前記冷延鋼板を冷却して得られた素地鋼板に溶融亜鉛めっきを施し、
溶融亜鉛めっきを施して得られた溶融亜鉛めっき鋼板を550℃以下の温度域に加熱して前記溶融亜鉛めっきの合金化処理を行い、
前記溶融亜鉛めっき鋼板に合金化処理を行って得られた合金化溶融亜鉛めっき鋼板を230~340℃の温度域まで平均冷却速度5℃/s以上で冷却することを含む。
本実施形態に係る高強度合金化溶融亜鉛めっき鋼板の素地鋼板は、以下の化学成分組成を有する。以下の化学成分組成の説明における「%」は「質量%」を意味する。以下では、素地鋼板および高強度合金化溶融亜鉛めっき鋼板を総称して、単に「鋼板」ともいう。また、鋼板の引張強さTSを単に「引張強さ」、0.2%耐力σ0.2および上降伏点UYPを総称して「降伏強度」ともいう。「引張強さ」および「降伏強度」を総称して単に「強度」ともいう。
Cは、鋼板の強度を確保するために必要な元素である。C量が不足すると、鋼板の引張強さが低下する。十分な鋼板の強度を確保するため、C量は0.19%以上とする。C量の下限は、好ましくは0.20%以上であり、より好ましくは0.21%以上である。しかしながら、C量が過剰になると、残留オーステナイトの体積率が過大となり、鋼板の降伏強度の低下を招くおそれがある。そこで、C量は0.30%以下とする。C量の上限は、好ましくは0.290%以下であり、より好ましくは0.280%以下であり、さらに好ましくは0.270%以下であり、さらにより好ましくは0.260%以下である。
Siは本発明における重要な元素の一つである。Siは固溶強化元素として知られており、鋼板の延性の低下を抑えつつ、引張強さを向上させることに有効に作用する元素である。また、Siはマルテンサイト組織の焼戻し軟化抵抗を高める効果を有する元素でもある。これらの効果を有効に発揮させるため、Si量は0%を超えた量とする。Si量の下限は、好ましくは0.06%以上である。Si量の下限は、さらに好ましくは、0.07%以上であり、さらにより好ましくは、0.08%以上である。しかしながら、Si量が過剰になると、残留オーステナイトの体積率が過大となり、鋼板の降伏強度の低下を招くおそれがある。またSi量が過剰になると、鋼板のLME割れ耐性を悪化させるおそれがある。そのためSi量は、0.70%以下とする。Si量の上限は、好ましくは0.60%以下であり、より好ましくは0.50%以下である。
Mnは鋼板の高強度化に寄与する元素である。このような効果を有効に発揮させるため、Mn量は1.8%以上とする。Mn量の下限は、好ましくは1.9%以上であり、より好ましくは2.0%以上である。しかしながら、Mn量が過剰になると、スラブ折損、冷間圧延荷重の増大等を招くおそれがある。そのためMn量は、3.0%以下とする。Mn量の上限は、好ましくは2.9%以下であり、より好ましくは2.8%以下である。
Pは鋼に不可避的に含まれる元素であり、鋼の結晶粒界に偏析して粒界脆化を助長する元素である。鋼板の加工時の破断等を回避するため、P量はできるだけ低減することが好ましい。そのためP量は0.020%以下とする。P量の上限は、好ましくは0.015%以下であり、より好ましくは0.012%以下である。なお、Pは、上述のように鋼中に不可避的に混入する不純物であり、その量を0%にすることは工業生産上不可能である。
SもPと同様に鋼に不可避的に含まれる元素である。Sは鋼中の他の元素とともに介在物を生成する。当該介在物に起因して鋼板の加工時に破断等が生じるおそれがある。このような鋼板の破断等を回避するため、S量はできるだけ低減することが好ましい。そのためS量は、0.05%以下とする。S量の上限は、好ましくは0.04%以下であり、より好ましくは0.03%以下である。なお、SもPと同様に、その量を0%にすることは工業生産上不可能である。
Alは鋼において脱酸剤として作用する元素である。こうした作用を有効に発揮させるため、Al量は0.015%以上とする。Al量の下限は、好ましくは0.025%以上であり、より好ましくは0.030%以上である。しかしながら、Al量が過剰になると、鋼板中にアルミナなどの介在物が多く生成し、鋼板の加工時に破断を生じるおそれがある。そのため、Al量は0.060%以下とする。Al量の上限は、好ましくは0.055%以下であり、より好ましくは0.050%以下である。
Crは本発明における重要な元素の一つである。Crは鋼板の高強度化に寄与する元素である。具体的には、Crは鋼板の焼入れ性を向上させる元素であり、焼入れ時に生成するベイナイトを低減させ、マルテンサイトのラスの数を増加させ、鋼板の高強度化に有効に作用する。さらに、Crは含有量を増加してもLME割れ耐性を悪化させにくい元素である。これらの効果を有効に発揮させるため、Cr量は0.05%以上とする。Cr量の下限は、好ましくは0.1%以上である。しかしながら、Cr量が過剰になると、溶融亜鉛めっき鋼板や合金化溶融亜鉛めっき鋼板において、不めっきを発生させることがある。そのため、Cr量は0.8%以下とする。Cr量の上限は、好ましくは0.7%以下であり、より好ましくは0.6%以下である。
Tiは、炭化物や窒化物を形成して鋼板の強度を向上させる元素である。また、Tiは、後述するBによる焼入れ性向上効果を有効に発揮させる上でも有効な元素である。すなわち、Tiは、窒化物を形成することによって鋼中のNを低減する。その結果、B窒化物の形成が抑制され、Bが固溶状態となって、Bによる焼入れ性向上効果が有効に発揮できる。このように、TiはBによる鋼板の焼入れ性を向上させることにより、鋼板の高強度化に寄与する。このような効果を有効に発揮させるため、Ti量は0.015%以上とする。Ti量の下限は、好ましくは0.018%以上であり、より好ましくは0.020%以上である。しかしながら、Ti量が過剰になると、Ti炭化物やTi窒化物が過剰となり、鋼板の加工時に割れを引き起こすおそれがある。そのため、Ti量は0.080%以下とする。Ti量の上限は、好ましくは0.070%以下であり、より好ましくは0.060%以下である。
Bは、焼入れ性を向上させて鋼板の高強度化に寄与する元素である。このような効果を有効に発揮させるため、B量は0.0010%以上とする。B量の下限は、好ましくは0.0012%以上であり、より好ましくは0.0014%以上である。しかしながら、B量が過剰になると、その効果が飽和し、コストが増加するだけである。そのため、B量は0.0150%以下とする。B量の上限は、好ましくは0.0140%以下である。
Moは、鋼板の高強度化に寄与する元素である。その効果はMo量が増加するにつれて増大する。その効果を有効に発揮させるため、Mo量は0%超とする。Mo量の下限は、好ましくは0.10%以上であり、より好ましくは0.20%以上である。しかしながら、Mo量が過剰になると、その効果が飽和するとともに、コストが増加する。そのため、Mo量は0.40%とする。Mo量の上限は、好ましくは0.30%以下である。
本実施形態に係る高強度合金化溶融亜鉛めっき鋼板の素地鋼板のCr量およびSi量は、下記式(1)を満たす。式(1)を満たすことにより、高強度合金化溶融亜鉛めっき鋼板の焼入れ性を向上させることができる。焼入れ性を向上させることにより、焼入れ時におけるフェライトおよびベイナイトの生成量を制限することができる。また、式(1)を満たすことでSi量とCr量のバランスが適切となり、溶接部周辺の領域におけるLME割れの発生を低減することが可能となる。すなわち、高強度合金化溶融亜鉛めっき鋼板の高強度化と優れたLME割れ耐性とを両立させることができる。
2×[Cr]-[Si]≧0.1 …(1)
式(1)において、[元素記号]は、素地鋼板の当該元素の含有量(質量%)である。
本実施形態に係る高強度合金化溶融亜鉛めっき鋼板の素地鋼板の基本成分は上記の通りであり、残部は実質的に鉄である。ただし、原材料、資材、製造設備等の状況によって不可避的に持ち込まれる不純物が鋼中に含まれることは当然に許容される。すなわち、素地鋼板の基本成分の残部は、鉄および不可避不純物からなる。こうした不可避不純物としては、上述したP、Sの他、例えば、N、O等が含まれる。N、Oは、それぞれ以下の範囲であることが好ましい。
Nは鋼に不可避的に含まれる不純物元素である。N量が過剰であると、鋼板の加工時に割れを引き起こすおそれがある。そのため、N量は0.0100%以下とする。N量の上限は、好ましくは0.0060%以下である。N量を0%にすることは工業生産上困難である。
Oは鋼に不可避的に含まれる不純物元素である。O量が過剰であると、鋼板の加工時に割れを引き起こすおそれがある。そのため、O量は0.0030%以下とする。O量の上限は、好ましくは0.0020%以下である。O量を0%にすることは工業生産上困難である。
本実施形態に係る高強度合金化溶融亜鉛めっき鋼板の素地鋼板には、必要に応じて、Caを以下に示す範囲で含有させてもよい。また、当該素地鋼板には、Caとともに、またはCaを含有させずに、Nb、V、Cu、Ni、Mg、および希土類元素(REM)からなる群から選ばれる1種または2種以上の元素を以下に示す範囲で含有させてもよい。これらの元素を単独または適宜組み合わせて含有させることにより、含有される元素に応じて素地鋼板の特性がより良好となる。
Caは、鋼中の硫化物を球状化し、鋼板の曲げ性を高めるのに有効な元素である。その効果は、Ca量が0%を超えた量であれば発揮され、Ca量が増加するにつれて増大する。その効果をより有効に発揮させるため、Ca量の下限は、好ましくは0.0005%以上であり、より好ましくは0.0010%以上である。しかしながら、Ca量が過剰になると、その効果が飽和するとともに、コストが増加する。そのため、Ca量の上限は、好ましくは0.0040%以下であり、より好ましくは0.0030%以下である。
Nbは、鋼板の高強度化に寄与する元素である。その効果は、Nb量が0%を超えた量であれば発揮され、Nb量が増加するにつれて増大する。その効果を有効に発揮させるため、Nb量の下限は、好ましくは0.003%以上であり、より好ましくは0.005%以上である。しかしながら、Nb量が過剰になると、鋼板の焼入れ性を劣化させる。そのため、Nb量の上限は、好ましくは0.020%以下であり、より好ましくは0.018%以下であり、さらに好ましくは0.015%以下である。
Vは、鋼板の高強度化に寄与する元素である。その効果は、V量が0%を超えた量であれば発揮され、V量が増加するにつれて増大する。その効果を有効に発揮させるため、V量の下限は、好ましくは0.005%以上であり、より好ましくは0.010%以上である。しかしながら、V量が過剰になると、その効果が飽和するとともに、コストが増加する。そのため、V量の上限は、好ましくは0.30%以下であり、より好ましくは0.25%以下であり、さらに好ましくは0.20%以下である。
Cuは、鋼板の耐食性向上に有効な元素である。その効果は、Cu量が0%を超えた量であれば発揮され、Cu量が増加するにつれて増大する。その効果を有効に発揮させるため、Cu量の下限は、好ましくは0.01%以上であり、より好ましくは0.05%以上である。しかしながら、Cu量が過剰になると、その効果が飽和するとともに、コストが増加する。そのため、Cu量の上限は、好ましくは0.30%以下であり、より好ましくは0.20%以下であり、さらに好ましくは0.15%以下である。
Niは、鋼板の耐食性向上に有効な元素である。その効果は、Ni量が0%を超えた量であれば発揮され、Ni量が増加するにつれて増大する。その効果を有効に発揮させるため、Ni量の下限は、好ましくは0.03%以上であり、より好ましくは0.05%以上である。しかしながら、Ni量が過剰になると、その効果が飽和するとともに、コストが増加する。そのため、Ni量の上限は、好ましくは0.30%以下であり、より好ましくは0.20%以下であり、さらに好ましくは0.15%以下である。
Mgは鋼板の成形性の向上に寄与する元素である。その効果は、Mg量が0%を超えた量であれば発揮される。しかしながら、Mg量が過剰になると、鋼板の酸洗性、溶接性、熱間加工性、経済性が悪化する。そのため、Mg量の上限は、好ましくは0.0100%以下であり、より好ましくは0.0040%以下である。
REMは、鋼板の成形性の向上に寄与する元素である。その効果は、REM量が0%を超えた量であれば発揮される。しかしながら、REM量が過剰になると、鋼板の酸洗性、溶接性、熱間加工性、経済性が悪化する。そのため、REM量の上限は、好ましくは0.010%以下であり、より好ましくは0.0040%以下である。
本実施形態に係る高強度合金化溶融亜鉛めっき鋼板の素地鋼板の金属組織は、マルテンサイト(焼戻しマルテンサイトおよび自己焼戻しマルテンサイトを含む)が82体積%以上、ベイナイトが13体積%以下、残留オーステナイトが5体積%以下である。これにより、本実施形態に係る高強度合金化溶融亜鉛めっき鋼板を規定の引張強さおよび降伏強度とすることができる。また、各金属組織の割合の測定方法は、後述の実施例において説明する方法とすることができる。
金属組織中のマルテンサイトは、本実施形態に係る素地鋼板の基地組織である。マルテンサイトを、素地鋼板の金属組織全体に対して82体積%以上とすることで、鋼板の降伏強度および引張強さを上昇させる。マルテンサイトが82体積%未満となると、他の軟質な組織が低応力で塑性変形を開始してしまい、鋼板の降伏強度および引張強さが低下する。マルテンサイトの体積率は、より好ましくは83体積%以上である。マルテンサイトの体積率に上限はなく、100%でもよい。本実施形態のマルテンサイトは、焼入れままマルテンサイトだけでなく、焼戻しマルテンサイトおよび自己焼戻しマルテンサイト(オートテンパードマルテンサイト)を含む。
フェライト、パーライトおよびベイナイトは、素地鋼板の基地組織であるマルテンサイトに比べて軟質である。鋼板においてこれらの組織が増加すると、低応力でこれらの組織が塑性変形を開始してしまい、鋼板の降伏強度および引張強さが低下する。こうした観点から、フェライト、パーライトおよびベイナイトは、素地鋼板の金属組織全体に対して合計で13体積%以下とする。フェライト、パーライトおよびベイナイトの体積率に下限はなく、0体積%でもよい。以下では、フェライト、パーライトおよびベイナイトを総称して「ベイナイト等」ともいう。
金属組織中の残留オーステナイトは、素地鋼板の金属組織全体に対して、5体積%以下とする。残留オーステナイトのうち、マルテンサイトのラスの境界に存在する少量のフィルム状の残留オーステナイトは、鋼板に応力が付加された際に転位の移動を抑制することで、引張強さや降伏強度を高める効果を有する。しかし、残留オーステナイトそのものはマルテンサイト組織に比べて軟質である。そのため、フィルム状であっても残留オーステナイトが過剰に存在すると、鋼板の降伏強度および引張強さとも低下する。こうした観点から、残留オーステナイトは5体積%以下とする。残留オーステナイトは0体積%であってもよい。
本実施形態に係る高強度合金化溶融亜鉛めっき鋼板の素地鋼板は、高い引張強さ、高い降伏強度を満足する。そのため、素地鋼板の金属組織を走査型電子顕微鏡で観察した像において、切断法で測定した総長300μmにおけるラスの個数(以下、単に「総長300μmにおけるラスの個数」ともいう。)を200個以上とする。総長300μmにおけるラスの個数が200個未満となると、降伏強度および引張強さの少なくとも一方が低下する。総長300μmにおけるラスの個数は、好ましくは210個以上であり、より好ましくは220個以上である。
めっき層は、素地鋼板の表面に形成された溶融亜鉛めっきに後述の合金化処理が施されたものである。溶融亜鉛めっきは、一般に使用されているものを適用でき、特に制限されない。
本実施形態に係る高強度合金化溶融亜鉛めっき鋼板の製造方法について説明する。
熱間圧延の条件は、例えば以下のとおりである。熱間圧延工程では、上述の鋼板の組成を有するスラブを熱間圧延する。熱間圧延前のスラブの加熱温度が低いと、TiC等の炭化物がオーステナイト中に固溶し難くなるおそれがある。そのため、熱間圧延前のスラブの加熱温度は、好ましくは1100℃以上である。この熱間圧延前のスラブの加熱温度は、さらに好ましくは1200℃以上である。しかしながら、熱間圧延前の加熱温度が高くなり過ぎるとコストアップとなる。そのため、熱間圧延前の加熱温度の上限は、好ましくは1350℃以下であり、さらに好ましくは1300℃以下である。
熱間圧延により得られた熱延鋼板は、620℃以上で巻き取る。熱延鋼板の巻取り温度が、620℃未満になると、熱延鋼板の強度が高くなり過ぎて、冷間圧延が困難となる。熱延鋼板の巻取り温度は、好ましくは630℃以上であり、より好ましくは640℃以上である。一方、熱延鋼板の巻取り温度が高くなり過ぎると、スケール除去のための酸洗性が劣化する。そのため、巻取り温度は、好ましくは800℃以下であり、より好ましくは750℃以下である。
巻き取られた熱延鋼板は、繰り出された後、冷間圧延に供される。繰り出された熱延鋼板は、必要に応じてスケール除去のために酸洗が施される。
均熱工程では、冷延鋼板を加熱し、Ac3点以上の温度域で11s以上保持する。加熱温度がAc3点未満の場合、鋼板の降伏強度や引張強さを低下させる軟質なフェライトが残存する可能性がある。そのため、均熱工程での保持温度の下限はAc3点とする。均熱工程での保持温度の下限は、好ましくは(Ac3点+5)℃である。また、均熱工程での保持温度は、冷延鋼板の固相線温度以下であればよく、上限については特に設けない。しかし、均熱工程での保持温度を上げすぎると生産性の悪化、または炉の燃費の増大による経済性の悪化が生じるため、均熱工程での保持温度の上限は、好ましくは980℃以下である。均熱工程では、冷延鋼板の温度を一定に保ってもよく、また、上記保持温度の範囲であれば冷延鋼板の温度が変動してもよい。
Ac3(℃)=910-203×[C]1/2-15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W]-{30×[Mn]+11×[Cr]+20×[Cu]-700×[P]-400×[Al]-120×[As]-400×[Ti]} …(2)
第1の冷却工程では、均熱工程でAc3点以上の温度域に加熱、保持した冷延鋼板を540~580℃の温度域(第1の温度域)まで平均冷却速度3℃/s以上で冷却する。具体的には、均熱工程で冷却を開始した後、Ac3点から第1の温度域まで平均冷却速度3℃/s以上で冷却する。この平均冷却速度が、3℃/s未満になると、フェライトが生成する可能性が高くなり、本発明で規定する降伏強度および引張強さの確保が難くなる。そのため、上記平均冷却速度は3℃/s以上とする必要があり、好ましくは4℃/s以上であり、より好ましくは5℃/s以上である。一方、上限は特に設けないものの、上記平均冷却速度が50℃/sを超えると、鋼板温度を制御し難くなり、設備コストが増加する。そのため、上記平均冷却速度の上限は50℃/s以下、好ましくは40℃/s以下である。
第2の冷却工程では、第1の冷却工程の後、冷延鋼板を90s以内に410~480℃の温度域(第2の温度域)まで冷却して素地鋼板を得る。より具体的には、冷延鋼板を、第1の温度域から90s以下の時間でMs点以上の温度を確保しつつ第2の温度域まで冷却する。第1の温度域から第2の温度域まで冷却する時間が90sを超えるとベイナイトの増加が懸念されるため、第1の温度域から第2の温度域まで冷却する時間は90s以内とする。第1の温度域から第2の温度域まで冷却する時間の上限は、好ましくは70s以下である。第2の温度域の上限は、好ましくは470℃以下であり、より好ましくは460℃以下である。
Ms点(℃)=550-361×[C]-39×[Mn]-35×[V]-20×[Cr]-17×[Ni]-10×[Cu]-5×([Mo]+[W])+15×[Co]+30×[Al] …(3)
第2の冷却工程の後、合金化工程に先立って、素地鋼板に溶融亜鉛めっきを施す。第2の温度域に冷却された素地鋼板は、溶融亜鉛めっき浴を収容するめっきポットに侵入し、めっき浴への浸漬処理が施される。この浸漬処理により、素地鋼板に溶融亜鉛めっきが施され、溶融亜鉛めっき鋼板が得られる。合金化工程では、得られた溶融亜鉛めっき鋼板に合金化処理が施される。
第3の冷却工程では、合金化工程で得られた合金化溶融亜鉛めっき鋼板を、230~340℃の温度域(第3の温度域)まで平均冷却速度5.0℃/s以上で冷却する。より具体的には、合金化直後から第3の温度域の冷却停止温度まで平均冷却速度5.0℃/s以上で冷却する。これにより、上述の本実施形態に係る高強度合金化溶融亜鉛めっき鋼板を得ることができる。第3の冷却工程での平均冷却速度が5.0℃/s未満になると、ベイナイトの増加が懸念される。また、ベイナイトの生成を抑制しても、Ms点通過後に生成するマルテンサイトから未変態オーステナイトへの炭素の分配が進むことでオーステナイトが安定化する。オーステナイトが安定化すると、マルテンサイトに変態するオーステナイト量が低下する。その結果、5体積%を超える残留オーステナイトを含みやすくなる。そのため、第3の冷却工程での平均冷却速度は、5.0℃/s以上とする。第3の冷却工程での平均冷却速度は、好ましくは8.0℃/s以上であり、より好ましくは10℃/s以上である。当該平均速度の上限は、特に規定しないものの、冷却設備の冷却能力を増大すると、冷却設備に大きな負荷が生じる。そのため、当該平均速度の上限は、好ましくは50℃/s以下であり、より好ましくは40℃/s以下である。
第3の冷却工程後に引き続き第4の冷却工程を行う。第4の冷却工程では、高強度合金化溶融亜鉛めっき鋼板を、第3の温度域から50℃以下の冷却停止温度まで平均冷却速度5.0℃/s以下で冷却することが好ましい。第4の冷却工程での平均冷却速度が5.0℃/sを超えると、素地鋼板のマルテンサイトの自己焼き戻しが進まず、脆性的な組織となるおそれがある。第4の冷却工程での平均冷却速度は、より好ましくは4.0℃以下である。また、高強度合金化溶融亜鉛めっき鋼板の生産性が悪化するため、第4の冷却工程での平均冷却速度は、好ましくは0.05℃/s以上であり、より好ましくは0.10℃/s以上である。
第4の冷却工程の後に、高強度合金化溶融亜鉛めっき鋼板に、必要に応じて加工を行ってもよく、焼き戻しを行ってもよい。
前記素地鋼板が質量%で、
C :0.19~0.30%、
Si:0%超、0.70%以下、
Mn:1.8~3.0%、
P :0%超、0.020%以下、
S :0%超、0.05%以下、
Al:0.015~0.060%、
Cr:0.05~0.8%、
Ti:0.015~0.080%、
B :0.0010~0.0150%、
Mo:0%超、0.40%以下、
N :0.0100%以下、および
O :0.0030%以下、
を含有し、残部が鉄および不可避不純物からなり、
前記素地鋼板のCr量[Cr](質量%)およびSi量[Si](質量%)は、2×[Cr]-[Si]≧0.1を満足し、
前記素地鋼板の金属組織において、マルテンサイト(焼戻しマルテンサイトおよび自己焼戻しマルテンサイトを含む)が82体積%以上、フェライト、パーライトおよびベイナイトが13体積%以下、残留オーステナイトが5体積%以下であり、
前記素地鋼板の金属組織を走査型電子顕微鏡で観察した像において、切断法で測定した総長300μmにおけるラスの個数が200個以上であり、降伏強度が970MPa以上かつ引張強さが1470MPa以上である。
Nb:0%超、0.020%以下、
V :0%超、0.30%以下、
Cu:0%超、0.30%以下、
Ni:0%超、0.30%以下、
Mg:0%超、0.0100%以下、および
REM:0%超、0.010%以下、
からなる群から選ばれる1種または2種以上を含有してもよい。
熱間圧延により得られた熱延鋼板を620℃以上で巻き取り、
巻き取った前記熱延鋼板を繰り出して冷間圧延し、
冷間圧延により得られた冷延鋼板を加熱し、Ac3点以上の温度域で11s以上保持し、加熱、保持した前記冷延鋼板を540~580℃の温度域まで平均冷却速度3℃/s以上で冷却し、さらに90s以内に410~480℃の温度域まで冷却し、
前記冷延鋼板を冷却して得られた素地鋼板に溶融亜鉛めっきを施し、
溶融亜鉛めっきを施して得られた溶融亜鉛めっき鋼板を550℃以下の温度域に加熱して前記溶融亜鉛めっきの合金化処理を行い、
前記溶融亜鉛めっき鋼板に合金化処理を行って得られた合金化溶融亜鉛めっき鋼板を230~340℃の温度域まで平均冷却速度5℃/s以上で冷却することを含む。
溶鋼を鋳造して表1に示す化学成分組成(鋼種:A、B、C、D、E、F、G、H、I、J)のスラブを製造した。表1中、「<」を付した数値は測定限界未満であったことを、それぞれ意味する。P、S、N、Oは、上述の通り不可避不純物であり、P、S、N、Oの欄に示した値は不可避的に含まれた量を意味する。表1に示す化学成分組成の残部は、鉄、およびP、S、N、O以外の不可避不純物である。
2×[Cr]-[Si]≧0.1 …(1)
このようにして得られた実験No.1~18の各鋼板について、マルテンサイト、ベイナイトおよび残留オーステナイトの各組織の体積率、切断法で測定した総長300μmにおけるラスの個数、ならびに機械的特性(0.2%耐力σ0.2および引張強さTS)を下記の手順に従って測定した。
本実施例の製造方法によれば、各鋼板においてマルテンサイト、ベイナイトおよび残留オーステナイト以外の組織(例えば、フェライトやパーライト)が存在する可能性は極めて低い。そのため、マルテンサイト、ベイナイトおよび残留オーステナイト以外の組織は測定しなかった。以下では、各組織の体積率の測定方法について、残留オーステナイト、ベイナイトおよびマルテンサイトの順に説明する。
残留オーステナイトの体積率は、次のように測定した。試験片は、熱処理後の鋼板(板厚1.4~1.6mm)から20mm×20mmの大きさに切り出した。この試験片を、表面から板厚の1/4部まで研削し、研削面を化学研磨(実験No.8)または電解研磨(実験No.16~18)により研磨した。研磨後の研削面について、X線回折法により残留オーステナイトの体積率を測定した(ISIJ Int.Vol.33.(1993),No.7,P.776)。測定には、2次元微小部X線回折装置(株式会社リガク製、RINT-RAPID II)を使用した。ターゲットはCoを使用した。
ベイナイトおよびマルテンサイトの体積率は、次の手順で測定、算出した。試験片は、熱処理後の鋼板(板厚1.4~1.6mm)から20mm×20mmの大きさに切り出した。この試験片の圧延方向と平行な断面を研磨し、研磨面にナイタール腐食を施した。ナイタール腐食を施した研磨面の板厚の1/4部の組織の写真(倍率3000倍)を、FE-SEMを用いて撮影した。組織写真の粒の色などに基づいて組織をベイナイトまたはマルテンサイトに区分し、ベイナイトおよびマルテンサイトの面積率を点算法で測定した。具体的には、撮影したFE-SEM像上に3μm間隔(写真上では9mm間隔)の直交格子を設け、格子が直角に交わる点(格子点)における組織をベイナイトまたはマルテンサイトに区分した。組織の区分は、格子点100点について行い、その結果を用いてベイナイトの面積率およびマルテンサイトの面積率を算出した。測定は、各試験片とも1視野(写真1枚)について行った。
総長300μmにおけるラスの個数は、切断法で測定した。切断法は、通常粒径を測定する手法である(JIS G 0551:2013)。本実施例では、切断法をラスの個数を測定する手法として応用した。
熱処理を実施した鋼板の機械的特性として、引張強さTSおよび0.2%耐力σ0.2を測定した。引張強さTSおよび0.2%耐力σ0.2は、冷間圧延の圧延方向と直角な方向が試験片の長手となるように採取したJIS5号試験片(板状試験片)を用いて測定した。実験No.11~15の測定条件は、JIS Z 2241:2011に基づく。実験No.1~10、16~18の測定条件は、クロスヘッド変位速度を10mm/minで一定とした以外は、JIS Z 2241:2011に基づく。測定した引張強さTSおよび0.2%耐力σ0.2を表3に示した。引張強さTSが1470MPa以上、かつ0.2%耐力σ0.2が970MPa以上の場合、機械的特性について合格とした。
LME割れ耐性は、鋼板の化学成分組成が大きく影響し、熱処理の影響は化学成分組成に比べて小さい。そのため、LME割れ耐性は、化学成分組成による評価が可能である。本実施例では、以下の方法で各鋼板のLME割れ耐性を評価した。
電極:上下DR型Cu-Cr(ドームラジアス製)
電極打角:5°
電極径:先端直径8mm
冷却水流量:上下約2L/分
加圧力:350kgf
初期化圧力時間:60サイクル/60Hz
アップスローブ:1サイクル/60Hz
通電1段目
電流値:7.2kA
時間(サイクル/60Hz):8
通電2段目
電流値:9kA
時間(サイクル/60Hz):17
ダウンスローブ(サイクル/60Hz):30
ホールド時間(サイクル/60Hz):5
このようにして作製したLME割れ耐性評価用試料から、LME割れ観察用試料を準備した。LME割れ観察用試料は、観察面が溶接ナゲットの直径を通る断面となるように作製した。光学顕微鏡を用いて25~100倍で、LME割れ観察用試料の上板および下板の表層部の観察を行い、割れの有無を調査した。図5は、LME割れ観察用試料の光学顕微鏡写真の一例である。図5に示すように、長さ50μm以上の割れを溶接部のLME割れと判断した。各鋼種のLME割れ耐性評価用試料の観察の結果、割れがあったものを×(不可)、割れがなかったものを〇(良好)として、表4に評価結果を示した。鋼種Gの評価結果は推定である。
(機械的特性)
表2に示した結果から、以下のように考察できる。実験No.2~4、8~10、12~18は、本発明で規定する化学成分組成を満足する鋼種(鋼種B、C、F~J)を用い、適切な熱処理条件で鋼板を製造した実施例である。これらの実施例の鋼板では、金属組織中の各組織の割合、および総長300μmにおけるラスの個数が適切に調整され、引張強さTSが1470MPa以上、かつ0.2%耐力σ0.2が970MPa以上であり、機械的特性について合格基準を満足していた。
表4に示したように、本発明で規定する化学成分組成を満足する鋼種(鋼種B、C、F~J)の鋼板は、優れたLME割れ耐性を有していた。そのため、鋼種B、C、F~Jを使用した実験No.2~4、8~10、12~18の鋼板もLME割れ耐性に優れていたと考えられる。
Claims (5)
- 素地鋼板の表面にめっき層を有する高強度合金化溶融亜鉛めっき鋼板であって、
前記素地鋼板が質量%で、
C :0.19~0.30%、
Si:0%超、0.70%以下、
Mn:1.8~3.0%、
P :0%超、0.020%以下、
S :0%超、0.05%以下、
Al:0.015~0.060%、
Cr:0.05~0.8%、
Ti:0.015~0.080%、
B :0.0010~0.0150%、
Mo:0%超、0.40%以下、
N :0.0100%以下、および
O :0.0030%以下、
を含有し、残部が鉄および不可避不純物からなり、
前記素地鋼板のCr量[Cr](質量%)およびSi量[Si](質量%)は、2×[Cr]-[Si]≧0.1を満足し、
前記素地鋼板の金属組織において、マルテンサイト(焼戻しマルテンサイトおよび自己焼戻しマルテンサイトを含む)が82体積%以上、フェライト、パーライトおよびベイナイトが13体積%以下、残留オーステナイトが5体積%以下であり、
前記素地鋼板の金属組織を走査型電子顕微鏡で観察した像において、切断法で測定した総長300μmにおけるラスの個数が200個以上であり、降伏強度が970MPa以上かつ引張強さが1470MPa以上である、高強度合金化溶融亜鉛めっき鋼板。 - 前記素地鋼板が、さらに、質量%で、
Ca:0%超、0.0040%以下、
を含有する、請求項1に記載の高強度合金化溶融亜鉛めっき鋼板。 - 前記素地鋼板が、さらに、質量%で、
Nb:0%超、0.020%以下、
V :0%超、0.30%以下、
Cu:0%超、0.30%以下、
Ni:0%超、0.30%以下、
Mg:0%超、0.0100%以下、および
REM:0%超、0.010%以下、
からなる群から選ばれる1種または2種以上を含有する、請求項1または請求項2に記載の高強度合金化溶融亜鉛めっき鋼板。 - 高強度合金化溶融亜鉛めっき鋼板の製造方法であって、
前記請求項1で規定する組成を有するスラブを熱間圧延し、
熱間圧延により得られた熱延鋼板を620℃以上で巻き取り、
巻き取った前記熱延鋼板を繰り出して冷間圧延し、
冷間圧延により得られた冷延鋼板を加熱し、Ac3点以上の温度域で11s以上保持し、加熱、保持した前記冷延鋼板を540~580℃の温度域まで平均冷却速度3℃/s以上で冷却し、さらに90s以内に410~480℃の温度域まで冷却し、
前記冷延鋼板を冷却して得られた素地鋼板に溶融亜鉛めっきを施し、
溶融亜鉛めっきを施して得られた溶融亜鉛めっき鋼板を550℃以下の温度域に加熱して前記溶融亜鉛めっきの合金化処理を行い、
前記溶融亜鉛めっき鋼板に合金化処理を行って得られた合金化溶融亜鉛めっき鋼板を230~340℃の温度域まで平均冷却速度5℃/s以上で冷却することを含む、高強度合金化溶融亜鉛めっき鋼板の製造方法。 - 冷却した前記合金化溶融亜鉛めっき鋼板に、伸び率5%以下で加工を行う請求項4に記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。
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