WO2019189841A1 - 高強度亜鉛めっき鋼板、高強度部材およびそれらの製造方法 - Google Patents
高強度亜鉛めっき鋼板、高強度部材およびそれらの製造方法 Download PDFInfo
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- WO2019189841A1 WO2019189841A1 PCT/JP2019/014220 JP2019014220W WO2019189841A1 WO 2019189841 A1 WO2019189841 A1 WO 2019189841A1 JP 2019014220 W JP2019014220 W JP 2019014220W WO 2019189841 A1 WO2019189841 A1 WO 2019189841A1
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- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
Definitions
- the present invention relates to a high-strength galvanized steel sheet, a high-strength member, and a method for producing them, which are excellent in plating properties and bendability and are suitable for building materials and automobile collision-resistant parts.
- the steel sheet strength increases, its processing performance decreases.
- the plated steel sheet tends to have poor processing performance.
- Non-Patent Documents 1 and 2 while the steel structure of the steel sheet is made ferrite and martensite, the steel structure is once made ferrite and martensite, and then tempered to soften the martensite and improve bendability.
- Patent Document 1 the maximum tensile stress of 900 MPa or more with good ductility and bendability with a structure homogeneity index of 0.4 or less, which is an index indicating the homogeneity of the steel sheet given by the standard deviation of the Rockwell hardness of the steel sheet surface
- a high-strength steel sheet having the following and its manufacturing method are disclosed.
- As a factor affecting the bendability a steel sheet that has been obtained as a result of improving the inhomogeneity of the solidified structure at the time of casting has been proposed.
- Patent Document 1 in order to ensure good plating properties at this time, in an annealing furnace of a continuous hot dip galvanizing line, a hydrogen concentration of 1 to 60 vol%, the balance N 2 , H 2 O, O 2 and unavoidable
- the atmosphere is made of an impurity, and the logarithm log (P H2O / P H2 ) of the moisture pressure and the hydrogen partial pressure in the atmosphere is defined as ⁇ 3 ⁇ log (P H2O / P H2 ) ⁇ ⁇ 0.5.
- the ratio of the hardness Hvs of the steel sheet surface layer to the 1/4 thickness Hvb of the steel sheet is 0.35 to 0.90 in a composite structure steel sheet containing 50% or more of bainite and 3 to 30% of retained austenite. It stipulates that there is.
- the plating property is secured in a high alloy system.
- the bendability is ensured by defining a decarburized ferrite layer, and as a method for producing a plated steel sheet, it consists of 2 to 20 vol% of hydrogen, the balance containing nitrogen and impurities, and the dew point is A method for adjusting to an atmosphere of more than ⁇ 30 ° C. and not more than 20 ° C. is disclosed.
- the steel structure has been mainly optimized to improve the bendability of the steel sheet.
- this is a certain level of improvement, and further improvement is required.
- hydrogen in the atmosphere in the plating process is considered to be hydrogen in steel remaining in the steel plate product. This hydrogen in steel is considered to hinder improvement in bendability.
- it is necessary to improve both bendability and plating properties.
- An object of the present invention is to improve the bendability of a plated steel sheet from a new viewpoint, and to provide a high-strength galvanized steel sheet, a high-strength member and a method for producing them, which are excellent in plateability and bendability.
- high strength means that the tensile strength (TS) is 1100 MP or more.
- the present inventors have intensively studied to solve the above problems.
- the steel sheet has a specific component composition, and the inclusions present in the range from the surface to the position of the plate thickness 1/3 are adjusted so as to have a steel structure in a predetermined existence state.
- strength galvanized steel plate which has favorable bendability and plating property was obtained by adjusting the amount of hydrogen remaining in steel appropriately.
- the high-strength galvanized steel sheet of this invention can be manufactured by adjusting suitably the conditions of each manufacturing process, such as the conditions of the furnace atmosphere at the time of annealing.
- the present invention provides the following.
- Steel composition is mass%, C: 0.08% or more and 0.20% or less, Si: less than 2.0%, Mn: 1.5% to 3.5%, P: 0.02% or less, S: 0.002% or less, A component composition containing Al: 0.10% or less and N: 0.006% or less, the balance being Fe and inevitable impurities;
- the average particle diameter of inclusions containing at least one of Al, Si, Mg, and Ca existing in the range from the surface to the plate thickness 1/3 position is 50 ⁇ m or less, and the average closest distance of the inclusions is 20 ⁇ m or more.
- the component composition is further in mass%, Sb: 0.001% or more and 0.1% or less, and Sn: The high-strength galvanized steel sheet according to any one of [1] to [3], which contains at least one of 0.001% to 0.1%.
- the steel structure has an area ratio of 30% to 85% martensite, 60% or less (including 0%) ferrite, 15% or less (including 0%) bainite, and less than 5%. Having residual austenite (including 0%), The high-strength galvanized steel sheet according to any one of [1] to [5], wherein the average grain size of the ferrite is 15 ⁇ m or less.
- a steel material is produced by casting the steel having the composition described in any one of [1] to [5] under a condition that the molten steel flow velocity at the solidification interface near the mold meniscus is 16 cm / second or more. Casting process; A hot rolling step of hot rolling the steel material after the casting step; Pickling step of pickling the steel plate after the hot rolling step; A cold rolling step of cold rolling the steel plate after the pickling step at a rolling reduction of 20% to 80%; The steel sheet after the cold rolling process is heated to a pretreatment heating temperature of 720 ° C. or more and 880 ° C. or less, and then the pretreatment heating temperature to 500 ° C. is an average cooling rate of 2 ° C./second or more and 499 ° C.
- annealing temperature 740 ° C. or higher (Ac3 + 20) ° C. or lower in a continuous annealing line, with the hydrogen concentration in the furnace atmosphere in the temperature range of 500 ° C. or higher being over 0 vol% and 12 vol% or lower. Then, an annealing step for cooling the annealing temperature to 600 ° C.
- a steel plate after the annealing step is plated, and after the plating treatment, a plating step of cooling a temperature range from 450 ° C. to 250 ° C. at an average cooling rate of 3 ° C./second or more is produced.
- the dew point of the atmosphere at 600 ° C or more and the pretreatment heating temperature or less is Y ° C or more
- the annealing step when heating at the annealing temperature, the dew point in the temperature range of 700 ° C or higher in the continuous annealing line is Z ° C or lower, and the pretreatment heating temperature is X ° C
- Said X, Y and Z are the manufacturing methods of the high intensity
- the method further includes a post-treatment step of heating in a temperature range of 50 to 400 ° C. for 30 seconds or more in an atmosphere having a hydrogen concentration of 5 vol% or less and a dew point of 50 ° C. or less.
- the present invention it is possible to provide a high-strength galvanized steel sheet, a high-strength member, and methods for producing them, which are excellent in plating properties and bendability.
- the high-strength galvanized steel sheet according to the present invention is applied to a skeleton member of an automobile body, it can greatly contribute to improvement of collision safety and weight reduction.
- the high-strength galvanized steel sheet of the present invention has a steel sheet and a galvanized layer formed on the steel sheet surface.
- component composition steel composition
- % which is a unit of component content, means “mass%”.
- C 0.08% or more and 0.20% or less C is an element effective for increasing the strength of a steel sheet, and contributes to increasing the strength by forming martensite, which is one of the hard phases of the steel structure. Further, depending on the manufacturing method, the formation of carbide forming elements such as Nb, Ti, V, and Zr and fine alloy compounds or alloy carbonitrides also contributes to high strength. In order to obtain these effects, the C content needs to be 0.08% or more. On the other hand, when the C content exceeds 0.20%, the martensite becomes excessively hard, and the bending workability tends not to be improved even if the inclusions and the hydrogen content in the steel are controlled. Therefore, the C content is 0.08% or more and 0.20% or less. From the viewpoint of stabilizing TS to 1100 MPa or more, the C content is preferably 0.09% or more.
- Si Less than 2.0% Si is an element that contributes to high strength mainly by solid solution strengthening. The decrease in ductility is relatively small as the strength increases, and it improves the balance between strength and ductility as well as strength. Contribute. Increased ductility leads to improved bendability.
- Si tends to form Si-based oxides on the surface of the steel sheet, which may cause unplating, and if it is excessively contained, a remarkable scale is formed during hot rolling, and scale marks are attached to the surface of the steel sheet. The surface properties may be deteriorated and the pickling property may be deteriorated. Therefore, it is sufficient to add only the amount necessary for ensuring the strength, but from the viewpoint of plating properties, the Si content is set to less than 2.0%.
- the Si content is preferably 0.5% or less, more preferably 0.3% or less. Further, the lower limit of the Si content is not particularly specified, but if it is less than 0.001%, it tends to be difficult to control the production. Therefore, the Si content is preferably 0.001% or more.
- Mn 1.5% or more and 3.5% or less Mn is effective as an element contributing to high strength by solid solution strengthening and martensite formation. To obtain this effect, it is necessary to contain 1.5% or more. is there.
- the Mn content is preferably 1.9% or more.
- Mn is an oxide or composite oxide on the steel sheet surface. It tends to oxidize externally and may cause non-plating. Therefore, the Mn content is 3.5% or less.
- P 0.02% or less
- P is an effective element that contributes to increasing the strength of the steel sheet by solid solution strengthening, but it also affects the plateability. In particular, the wettability with the steel sheet is deteriorated and the alloying rate of the plating layer is delayed, and the influence is large particularly in a high alloy system that obtains a high-strength steel sheet. Therefore, the P content is set to 0.02% or less.
- the P content is preferably 0.01% or less.
- the lower limit of the P content is not particularly specified, but if it is less than 0.0001%, the production efficiency is lowered and the dephosphorization cost is increased in the production process. Therefore, the P content is preferably 0.0001% or more.
- S 0.002% or less S tends to form sulfide inclusions in steel.
- MnS inclusions are easily formed.
- S causes hot brittleness and adversely affects the manufacturing process. Therefore, it is preferable to reduce as much as possible.
- up to 0.002% is acceptable.
- the lower limit of the S content is not particularly specified, but if it is less than 0.0001%, the production efficiency is lowered and the cost is increased in the production process. Therefore, the S content is preferably 0.0001% or more.
- Al 0.10% or less Al is added as a deoxidizer.
- Al 0.001% or more of Al is preferably contained.
- the Al content is 0.10% or less, preferably 0.08% or less as sol.Al in the steel.
- the N content is 0.006% or less, preferably 0.005% or less.
- the content is preferably as low as possible from the viewpoint of improving ductility by cleaning, but the N content is set to 0.0001% or more in order to reduce the production efficiency and increase the cost in the manufacturing process. It is preferable.
- the mass ratio of Si content to the Mn content in the steel (Si / Mn) is less than 0.1.
- the component composition of the steel sheet may contain Si and Mn in the above range, but it suppresses the occurrence of non-plating defects, improves the plating peel resistance (adhesion), and makes it less likely to cause uneven appearance.
- the mass ratio (Si / Mn) of the Si content to the Mn content in the steel is less than 0.1.
- the mass ratio of the Si content to the Mn content in the steel (Si / Mn) is more preferably less than 0.10, and even more preferably less than 0.08. Yes, particularly preferably less than 0.06.
- the steel of the present invention basically contains the above component composition, and the balance is iron and inevitable impurities.
- the above component composition may further contain the following components as optional components as long as the effects of the present invention are not impaired.
- the optional components shall be included as inevitable impurities.
- the component composition may contain Mg, La, Ce, Bi, W, and Pb as inevitable impurities up to a total of 0.002%.
- the above component composition may further contain at least one of the following (1) to (3) as an optional component in mass%.
- Ti, Nb, V and Zr form carbides and nitrides (also carbonitrides) with C and N. .
- these elements have the effect of refining the structure of the hot-rolled coil, and by refining the steel structure after the subsequent cold rolling / annealing, it contributes to improving workability such as strength increase and bendability.
- Ti, Nb, V and Zr is 0.1% or less in total.
- the elements Mo, Cr, Cu, and Ni are elements that contribute to high strength in order to increase the hardenability and facilitate the formation of martensite.
- the lower limit of 0.01% is defined as a preferable lower limit.
- Mo, Cr, Cu and Ni excessive addition leads to saturation of the effect and cost increase, and Cu induces cracks during hot rolling and causes surface defects. Therefore, it is preferable that at least one of Mo, Cr, Cu, and Ni be 0.5% or less in total.
- the B content is preferably 0.0003% or more and 0.005% or less.
- the above component composition may further contain the following components as optional components.
- Sb 0.001% or more and 0.1% or less
- Sn 0.001% or more and 0.1% or less
- Sb and Sn suppress decarburization, denitrification, deboronation, etc.
- the content is preferably 0.001% or more because it is an effective element for suppressing the strength reduction.
- the upper limit is preferably made 0.1%.
- Ca 0.0005% or less
- the Ca content is preferably 0.0005% or less.
- the minimum of Ca content is not prescribed
- the average particle diameter of inclusions containing at least one of Al, Si, Mg, and Ca existing in the range from the surface to the plate thickness 1/3 position is 50 ⁇ m or less, and the average closest distance of inclusions is 20 ⁇ m or more.
- the bendability can be improved by adjusting the average particle diameter and average closest distance of the inclusions to the above ranges and further setting the amount of diffusible hydrogen in the steel to a specific range.
- the closest distance of inclusions only inclusions containing at least one of Al, Si, Mg, and Ca are not counted.
- the average particle size of the inclusions is 50 ⁇ m or less, preferably 30 ⁇ m or less, more preferably 20 ⁇ m or less. Since the average particle size of inclusions is preferably small, the lower limit is not particularly defined, but is often 1 ⁇ m or more.
- the average closest distance of inclusions is 20 ⁇ m or more, preferably 30 ⁇ m or more, and more preferably 50 ⁇ m or more.
- the upper limit of the average closest distance of inclusions is not particularly limited, but is often 500 ⁇ m or less.
- the average particle diameter of inclusions and the average closest distance of inclusions are measured by the method described in the examples.
- the steel structure of the steel sheet is, in terms of area ratio, 30% to 85% martensite, 60% or less (including 0%) ferrite, 15% or less (including 0%) bainite, Further, it is preferable that the retained austenite is less than 5% (including 0%), and the average particle diameter of the ferrite is 15 ⁇ m or less.
- Martensite 30% or more and 85% or less Martensite is hard and is effective and essential for increasing the strength of the steel sheet.
- the area ratio is preferably 30% or more. From the viewpoint of ensuring the stability of TS, it is preferably 45% or more.
- the martensite said here contains the tempered martensite self-tempered during manufacture and the tempered martensite tempered by the subsequent heat processing depending on the case. Further, from the viewpoint of the balance between bendability and strength, martensite is preferably 85% or less.
- Ferrite 60% or less (including 0%) When heat treatment and plating are applied in an atmosphere where hydrogen is present, hydrogen penetrates and remains in the steel. As one technique for reducing hydrogen in the steel of the final product as much as possible, ferrite and bainite having a BCC structure appear in the steel structure before plating. This makes use of the fact that ferrite or bainite having a BCC structure has a lower hydrogen solubility than austenite having an FCC structure. Soft ferrite improves the ductility of the steel sheet and improves bendability. However, if the ferrite content exceeds 60%, the strength cannot be secured, so the preferable upper limit is set to 60%. In many cases, ferrite is 2% or more.
- the average particle diameter of ferrite is preferably 15 ⁇ m or less. As the ferrite particle size is smaller, the generation and connection of voids on the bending surface can be suppressed, and the bendability can be improved.
- the average particle diameter of the ferrite is more preferably 10 ⁇ m or less, and further preferably 4 ⁇ m or less.
- Bainite 15% or less (including 0%) Bainite may be included because it contributes to improvement in bendability, but if it is included excessively, the desired strength cannot be obtained, so it is preferably made 15% or less. In many cases, bainite is 2% or more.
- Residual austenite is less than 5% (including 0%) Austenite is an fcc phase and has a high hydrogen storage capacity and is slow to diffuse in steel compared to ferrite (bcc phase), and therefore tends to remain in steel. Furthermore, when this retained austenite undergoes work-induced transformation to martensite, there is a concern of increasing the diffusible hydrogen in the steel. Therefore, in the present invention, the retained austenite is preferably less than 5%.
- the steel structure may include precipitates such as pearlite and carbide in the balance as a structure other than the above-described structure (phase), and these are 10% or less in terms of the total area ratio at the position of the sheet thickness 1/4 from the steel sheet surface. Acceptable if any. Preferably, it is 5% or less (including 0%).
- the galvanized layer has a plating adhesion amount of 20 to 120 g / m 2 on one side. If the adhesion amount is less than 20 g / m 2, it is difficult to ensure corrosion resistance. Therefore, the adhesion amount is 20 g / m 2 or more, preferably 25 g / m 2 or more, more preferably 30 g / m 2 or more. On the other hand, when it exceeds 120 g / m 2 , the plating peel resistance deteriorates. Therefore, the adhesion amount is 120 g / m 2 or less, preferably 100 g / m 2 or less, more preferably 80 g / m 2 or less.
- the composition of the galvanized layer is not particularly limited and may be a general one.
- a hot-dip galvanized layer or an alloyed hot-dip galvanized layer generally, Fe: 20% by mass or less, Al: 0.001% by mass to 1.0% by mass, and further, Pb, One or two or more selected from Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM in total 0 to 3.5% by mass It is contained below, and the balance is composed of Zn and inevitable impurities.
- a hot dip galvanized layer having a plating adhesion amount of 20 to 120 g / m 2 on one side, and an alloyed hot dip galvanized layer obtained by alloying it.
- the Fe content in the plated layer is less than 7% by mass.
- the Fe content in the plated layer is 7 to 20% by mass. % Is preferred.
- the amount of diffusible hydrogen in the steel obtained by measurement by the method described in Examples is less than 0.25 ppm by mass. Diffusible hydrogen in steel degrades bendability. When the amount of diffusible hydrogen in the steel is 0.25 mass ppm or more, the bendability deteriorates even if the inclusions and the steel structure are appropriately formed.
- the amount of diffusible hydrogen in steel is less than 0.25 mass ppm.
- it is 0.20 mass ppm or less, More preferably, it is 0.15 mass ppm or less.
- a minimum is not specifically limited, Since it is so preferable that it is small, a minimum is 0 mass ppm.
- the diffusible hydrogen in the steel needs to be less than 0.25 ppm by mass before the steel sheet is formed or welded.
- the high-strength galvanized steel sheet of the present invention has a high tensile strength (TS). Specifically, the tensile strength (TS) measured by the method described in the examples is 1100 MPa or more. Moreover, the plate
- the production method of the present invention includes a casting process, a hot rolling process, a pickling process, a cold rolling process, a pretreatment process, an annealing process, and a plating process.
- a casting process a hot rolling process
- a pickling process a cold rolling process
- a pretreatment process a pretreatment process
- an annealing process a plating process.
- each step will be described.
- the temperature at the time of heating or cooling a slab (steel material), a steel plate, etc. shown below means the surface temperature of a slab (steel material), a steel plate, etc. unless otherwise specified.
- the casting process is a process in which steel having the above composition is cast into a steel material under the condition that the molten steel flow velocity at the solidification interface near the mold meniscus is 16 cm / second or more.
- the steel used in the production method of the present invention is generally produced by a continuous casting method called slab, which prevents macro segregation of alloy components. This is the purpose, and it may be manufactured by an ingot-making method or a thin slab casting method.
- the molten steel flow velocity (hereinafter also simply referred to as molten steel flow velocity) at the solidification interface near the mold meniscus is 16 cm / second or more.
- the molten steel flow rate is preferably 17 cm / second or more. Since the steel plate according to the present invention can be easily obtained by increasing the molten steel flow rate, the upper limit is not particularly specified, but is preferably 50 cm / second or less from the viewpoint of operational stability.
- Near mold meniscus means an interface between powder and molten steel used during continuous casting in a mold. In the case of agglomeration, it is preferable that the inclusions are sufficiently levitated during solidification and the floated and gathered portions are cut off and used for the next step.
- the hot rolling process is a process of hot rolling the steel material after the casting process.
- the method of hot rolling is not particularly limited, but it is preferably performed under the following conditions.
- the heating temperature of the steel slab is preferably in the range of 1100 ° C to 1350 ° C. This is because the precipitates present in the steel slab are likely to be coarsened, which is disadvantageous when, for example, securing strength by precipitation strengthening. Alternatively, coarse precipitates may be used as nuclei, which may adversely affect the structure formation in the subsequent annealing process. In addition, it is beneficial as product quality to reduce cracks and irregularities on the surface of the steel sheet by scaling off bubbles and defects on the surface of the slab by heating to achieve a smooth steel sheet surface.
- the slab heating temperature is defined from such a viewpoint. In order to acquire such an effect, it is preferable to set it as 1100 degreeC or more. On the other hand, when the temperature exceeds 1350 ° C., the austenite grains are coarsened, and the steel structure of the final product is also coarsened to cause a decrease in strength and bendability of the steel sheet.
- the steel slab is generally made into a sheet bar by rough rolling and hot rolled into a coil by finishing rolling. There is no problem if it becomes the size of.
- the finish rolling temperature is preferably in the range of 800 ° C. or higher and 950 ° C. or lower.
- the temperature is preferably in the range of 800 ° C. or higher and 950 ° C. or lower.
- the hot-rolled coil (hot-rolled sheet) that has been subjected to the above hot rolling starts cooling within 3 seconds after finishing rolling in order to refine and homogenize the structure. It is preferable that the temperature range of ⁇ 100] ° C. is cooled at an average cooling rate of 10 to 250 ° C./s and wound around the coil at a temperature range of 450 to 700 ° C.
- the pickling process is a process of pickling the steel sheet after the hot rolling process. Remove scale by pickling. What is necessary is just to set pickling conditions suitably.
- the cold rolling process is a process of cold rolling the steel sheet after the pickling process at a rolling reduction of 20% to 80%.
- the rolling reduction is 20% or more is to obtain a uniform and fine steel structure in the subsequent annealing step. If it is less than 20%, it tends to become coarse or non-uniform in structure during annealing, and there is a concern that the strength and workability of the final product plate may deteriorate as described above.
- the high reduction ratio is set to 80% because it may cause a shape defect in addition to a decrease in productivity due to a rolling load. In addition, you may pickling after cold rolling.
- the pretreatment step refers to heating to a pretreatment heating temperature of 720 ° C. or more and 880 ° C. or less after the cold rolling step. This is a step of cooling to 0 ° C. at an average cooling rate of 3 ° C./second or more, and performing pickling after the cooling.
- the pretreatment heating temperature is set to 720 ° C. or more and 880 ° C. or less.
- the pretreatment heating temperature is preferably 720 ° C. or higher and Ac 3 ° C. or lower.
- the average cooling rate from the pretreatment heating temperature to 500 ° C. is set to 2 ° C./second or more.
- the average cooling rate from the pretreatment heating temperature to 500 ° C. is preferably 3 ° C./second or more.
- the upper limit of the average cooling rate from the pretreatment heating temperature to 500 ° C. is not particularly specified, but is preferably 200 ° C./s or less from the viewpoint of energy saving of the cooling facility.
- the average cooling rate from 499 ° C. to 200 ° C. is preferably 4 ° C./second or more.
- the upper limit of the average cooling rate from 499 degreeC to 200 degreeC is not prescribed
- the annealing process means that the steel sheet after the pretreatment process is subjected to an annealing temperature of 740 ° C. or more (Ac3 + 20) ° C. in a continuous annealing line, with the hydrogen concentration in the furnace atmosphere in the temperature range of 500 ° C. or more exceeding 0 vol% and 12 vol% or less.
- it is a step of cooling from the annealing temperature to 600 ° C. at an average cooling rate of 3 ° C./second or more.
- the cooling stop temperature for cooling is not particularly limited.
- the element symbol in the above formula means the content (% by mass) of each element, and the component not contained is 0.
- Hydrogen concentration shall be more than 0 vol% and 12 vol% or less.
- the hydrogen concentration is preferably 10 vol% or less. Further, from the viewpoint of improving the plating property, the hydrogen concentration is preferably 1 vol% or more, and more preferably 3 vol% or more.
- the annealing temperature is too high, there is a problem that the amount of diffusible hydrogen in the steel specified by the present invention exceeds the upper limit, and if it is too low, there is a problem that the tensile strength specified by the present invention cannot be obtained. Is 740 ° C. or higher and (Ac3 + 20) ° C. or lower.
- the average cooling rate from the annealing temperature to 600 ° C. is set to 3 ° C./second or more.
- the average cooling rate from the annealing temperature to 600 ° C. is preferably 4 ° C./second or more.
- the reason for focusing on the temperature range from the annealing temperature to 600 ° C. is that this temperature range is a temperature range that affects the amount of austenite that becomes martensite.
- the upper limit of the average cooling rate from annealing temperature to 600 degreeC is not prescribed
- the plating step is a step of plating the steel sheet after the annealing step and cooling the temperature range from 450 ° C. to 250 ° C. at an average cooling rate of 3 ° C./second or more after the plating treatment.
- the average cooling rate from 450 ° C. to 250 ° C. after the plating treatment is preferably 5 ° C./second or more.
- the reason for paying attention to the temperature range of 450 to 250 ° C. is that the martensitic transformation start temperature (Ms point) is conscious of the plating and / or plating alloying temperature.
- the upper limit of the average cooling rate from 450 ° C. to 250 ° C. after the plating treatment is not particularly defined, but is preferably 2000 ° C./s or less from the viewpoint of energy saving of the cooling facility.
- Zinc plating is performed by immersing in a hot dip galvanizing bath, for example.
- the hot dip galvanizing treatment may be performed by a conventional method, and the plating adhesion amount per one side is adjusted to be in the above range.
- an alloying treatment of galvanizing can be performed as necessary. In that case, it may be held in the temperature range of 480 to 580 ° C. for about 1 to 60 seconds.
- oxides such as Si and Mn may be externally oxidized on the surface layer of the steel sheet to impair plating properties, so it is necessary to undergo pickling before annealing.
- oxides such as Si and Mn may be externally oxidized on the surface layer of the steel sheet to impair plating properties, so it is necessary to undergo pickling before annealing.
- the amount of external oxidation during pretreatment heating increases, external oxidation in the continuous annealing line can be suppressed, so that a good plating appearance can be obtained. For this reason, conventionally, it has been considered that a better plating appearance is more easily obtained as the dew point of the pre-heat treatment temperature and the highest reached temperature are higher and the dew point in the continuous annealing line in the annealing process is lower.
- the dew point of the atmosphere of 600 ° C. or more and the pretreatment heating temperature or less is Y ° C. or more
- the annealing step when heating at the annealing temperature, when the dew point in the temperature range of 700 ° C. or higher in the continuous annealing line is Z ° C. or lower and the pretreatment heating temperature is X ° C.
- the X, Y and Z are expressed by the following relational expressions (i), ( It is preferable to satisfy ii) or (iii).
- the dew point Y ° C. is preferably ⁇ 50 ° C. or higher and ⁇ 25 ° C. or lower.
- the dew point Y ° C. is less than ⁇ 50 ° C., the amount of external oxidation such as Si and Mn during the pre-heat treatment becomes insufficient, leading to poor plating appearance and plating adhesion.
- the dew point Y ° C. is less than ⁇ 50 ° C.
- the dew point Z ° C. of the continuous annealing line is not particularly set, but if the dew point Z ° C. is less than ⁇ 55 ° C., it is difficult to ensure the airtightness of the furnace body, which tends to increase the cost.
- the dew point Z ° C exceeds -30 ° C, external oxidation after continuous annealing tends to be excessive, which may lead to poor plating appearance and plating adhesion. Therefore, the dew point Z of the continuous annealing line is preferably ⁇ 55 ° C. or higher and ⁇ 30 ° C. or lower.
- the annealing step or after the plating step After the annealing step or after the plating step, it has a post-treatment step of heating in a temperature range of 50 to 400 ° C. for 30 seconds or more in an atmosphere having a hydrogen concentration of 5 vol% or less and a dew point of 50 ° C. or less. It is preferable from the viewpoint of reduction. In addition, it is preferable that a post-processing process is implemented as a next process of an annealing process or a plating process.
- the atmosphere has a dew point of 50 ° C. or lower.
- the heating time in the temperature range of 50 to 400 ° C is short, the effect of reducing the amount of diffusible hydrogen in the steel is small, and this process is merely an increase in the number of steps, so the heating time in the temperature range of 50 to 400 ° C is reduced. It is preferable to make it 30 seconds or more. Note that the reason for focusing on the temperature range of 50 to 400 ° C. is that the dehydrogenation reaction proceeds more than the hydrogen intrusion in this temperature range, and there is a concern that the properties of the material and the plating layer may deteriorate at this temperature or higher. .
- a width trim step for performing a width trim may be further included.
- the plate width direction end of the steel plate is sheared. In addition to adjusting the product width, this has the effect of reducing the amount of diffusible hydrogen in the steel by removing diffusible hydrogen from the shear end face.
- the production of the high-strength galvanized steel sheet of the present invention may be performed in a continuous annealing line or may be performed off-line.
- the high-strength member of the present invention is obtained by subjecting the high-strength galvanized steel sheet of the present invention to at least one of forming and welding.
- the manufacturing method of the high strength member of this invention has the process of performing at least one of a shaping
- the high-strength member of the present invention is excellent in bendability, cracks after bending are suppressed, and the structural reliability of the member is high.
- the high-strength member is excellent in plating properties, particularly plating peeling resistance. Therefore, for example, when a steel plate is press-molded to form a member, it is possible to suppress adhesion of zinc powder or the like to a press mold due to galvanization peeling, and to suppress occurrence of surface defects on the steel plate caused by the adhesion. Therefore, it has the effect that the productivity at the time of press molding is high.
- a general processing method such as pressing can be used without limitation.
- general welding such as spot welding and arc welding can be used without limitation.
- the high-strength member of the present invention can be suitably used for, for example, automobile parts.
- Example 1 In order to confirm the influence of the amount of hydrogen in the steel, the examination shown in Example 1 was performed.
- Molten steel having the composition shown in Table 1 was melted in a converter, and a slab was formed at an average molten steel flow velocity of 18 cm / sec at the solidification interface near the mold meniscus and an average casting speed of 1.8 m / min.
- This slab was heated to 1200 ° C. to obtain a hot rolled coil at a finish rolling temperature of 840 ° C. and a winding temperature of 550 ° C.
- a cold-rolled steel sheet having a cold reduction ratio of 50% and a thickness of 1.4 mm was obtained.
- This cold-rolled steel sheet was heated to 790 ° C. (Ac3 point + within 20 ° C.
- Hydrogen content in steel (diffusible hydrogen content) The amount of hydrogen in the steel was measured by the following method. First, a test piece of about 5 ⁇ 30 mm was cut out from the plated steel sheet, and the plating on the surface of the test piece was removed using a router (precision grinder) and placed in a quartz tube. Next, after replacing the inside of the quartz tube with Ar, the temperature was raised at 200 ° C./hr, and hydrogen generated up to 400 ° C. was measured by a gas chromatograph. In this way, the amount of released hydrogen was measured by a temperature rising analysis method. The cumulative amount of hydrogen detected in the temperature range from room temperature (25 ° C.) to less than 210 ° C. was defined as the amount of diffusible hydrogen in the steel.
- Bendability A strip test piece of 25 ⁇ 100 mm was cut out from the manufactured plated steel sheet so that the direction parallel to the rolling direction was the short side. Next, a 90 ° V bending test was performed so that the rolling direction was a ridge line when bent. The stroke speed was set to 50 mm / min, and pressing was performed for 5 seconds with a load of 10 tons. The test was performed by changing the tip R of the V-shaped punch in various steps in 0.5 steps, and the vicinity of the ridge line of the test piece was observed with a 20 ⁇ lens to confirm the presence or absence of cracks.
- Example 2 In Example 2, the following galvanized steel sheet was manufactured and evaluated.
- the molten steel having the composition shown in Table 2 was melted in a converter, and the slab cast under the conditions shown in Table 3 was reheated to 1200 ° C, hot-rolled at a finishing temperature of 800 to 830 ° C, and coiled A hot rolled coil was manufactured at a temperature of 560 ° C. at the time of taking.
- After pickling the hot-rolled steel sheet obtained from the hot-rolled coil by carrying out the steps of cold rolling, pretreatment, annealing, plating treatment, width trimming, and post-treatment under the conditions shown in Table 3.
- a 1.4 mm thick galvanized steel sheet was produced.
- galvanizing alloying treatment was performed at 500 ° C. for 20 seconds.
- the width trimming and post-processing steps were performed only under some manufacturing conditions.
- a sample is taken from the obtained plated steel sheet, and the structure observation and tensile test are performed by the following method to determine the tensile strength (TS), the amount of hydrogen in the steel (diffusible hydrogen amount), the bendability and the steel structure. The fraction was evaluated and measured. Moreover, the plating property was evaluated.
- the evaluation method is as follows.
- Tensile test A JIS No. 5 tensile test piece (JISZ2201) was taken from a plated steel sheet in a direction perpendicular to the rolling direction, and a tensile test was performed at a constant tensile speed (crosshead speed) of 10 mm / min.
- the tensile strength was a value obtained by dividing the maximum load in the tensile test by the initial cross-sectional area of the parallel part of the specimen.
- the plate thickness in calculating the cross-sectional area of the parallel portion the plate thickness value including the plating thickness was used.
- F represents ferrite
- M represents martensite (including tempered martensite)
- B represents bainite
- ⁇ represents retained austenite.
- the average particle diameter of the ferrite was obtained by observing 10 particles with an SEM, obtaining the area ratio of each, calculating the equivalent circle diameter, and averaging the obtained.
- Non-plating defects are in the order of several ⁇ m to several mm and mean areas where there is no plating and the steel sheet is exposed.
- the peel resistance (adhesion) of the manufactured hot dip galvanized steel sheet was examined.
- a cellophane tape was pressed against a processed part obtained by bending a hot-dip galvanized steel sheet by 90 ° to transfer the peeled material to the cellophane tape, and the amount of the peeled material on the cellophane tape was determined by the fluorescent X-ray method as the Zn count number.
- the measurement conditions were as follows: mask diameter 30 mm, fluorescent X-ray acceleration voltage 50 kV, acceleration current 50 mA, and measurement time 20 seconds.
- the plating property was evaluated according to the following criteria. The results are shown in Table 4. In the present invention, the following rank A, B, or C having no unplating defect was regarded as acceptable.
- D Non-plating defect occurs.
- the galvanized steel sheet of the present invention obtained with the components and production conditions within the scope of the present invention has high strength at TS ⁇ 1100 MPa, excellent bendability at R / t ⁇ 3.5, and excellent plating properties. It was. [Example 3] In Example 3, the following galvanized steel sheet was manufactured and evaluated. In addition, in the manufacturing method of Example 3, the preferable manufacturing method of the galvanized steel sheet of this invention was examined in detail by controlling a dew point with a pre-heat treatment and a continuous annealing line.
- a slab made of molten steel having the composition shown in Table 5 in a converter and cast under the conditions shown in Table 6 is reheated to 1200 ° C, hot-rolled at a finishing temperature of 800 to 830 ° C, and coiled.
- a hot rolled coil was manufactured at a temperature of 560 ° C. at the time of taking. After pickling the hot-rolled steel sheet obtained from the hot-rolled coil, by carrying out the steps of cold rolling, pretreatment, annealing, plating treatment, width trimming, and post-treatment under the conditions shown in Table 6.
- a 1.4 mm thick galvanized steel sheet was produced.
- galvanizing alloying treatment was performed at 500 ° C. for 20 seconds.
- the width trimming and post-processing steps were performed only under some manufacturing conditions.
- Example 2 As described above, a sample is taken from the obtained plated steel sheet and subjected to structure observation and tensile test in the same manner as in Example 2 to obtain tensile strength (TS), hydrogen amount in steel (diffusible hydrogen amount), bendability. And the fraction of steel structure was evaluated and measured. In addition, the plating property was evaluated in the same manner as in Example 2.
- a galvanized steel sheet was also manufactured under the same manufacturing conditions as manufacturing conditions 3-7 in Table 6 except that the galvanizing alloying treatment was not performed. Then, the surface properties (appearance) of the galvanized steel sheet are visually observed, and on the order of several ⁇ m to several mm, the presence / absence of an area where the steel sheet is exposed without plating (existence of non-plating defects) is examined. It was. As a result of examination, it was confirmed that this galvanized steel sheet had no non-plating defects and had good plating properties.
- the galvanized steel sheet of the present invention obtained with the components and production conditions within the scope of the present invention has high strength at TS ⁇ 1100 MPa, excellent bendability at R / t ⁇ 3.5, and excellent plating properties. It was. Moreover, it turned out that control of the dew point in a pre-processing process and an annealing process is important for plating property improvement.
- Example 4 No. in Table 6 of Example 3.
- the galvanized steel sheet of 3-2 (invention example) was press-molded to produce a member of the invention example. Furthermore, No. 6 in Table 6 of Example 3 was used. No. 3-2 (invention example) galvanized steel sheet and No.
- a member of the present invention was manufactured by joining the galvanized steel sheet of 3-3 (example of the present invention) by spot welding. Since these members of the present invention were excellent in bendability and plating properties, it was confirmed that they could be suitably used for automobile parts and the like.
- the high-strength galvanized steel sheet of the present invention has not only high tensile strength but also good bendability and plating properties. Therefore, when the high-strength galvanized steel sheet of the present invention is applied mainly to the framework parts of automobile bodies, especially around the cabin that affects collision safety, the safety performance is improved and the weight of the vehicle body is reduced by the effect of thinning high strength. Can contribute to environmental aspects such as CO 2 emissions. In addition, since it has good surface properties and plating quality, it can be actively applied to places where there is concern about corrosion due to rain and snow, such as undercarriage, and the performance of rust prevention and corrosion resistance of the car body is also improved. Can be expected. Such a characteristic is an effective material not only for automobile parts but also in the fields of civil engineering / architecture and home appliances.
Abstract
Description
C:0.08%以上0.20%以下、
Si:2.0%未満、
Mn:1.5%以上3.5%以下、
P:0.02%以下、
S:0.002%以下、
Al:0.10%以下、及び
N:0.006%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成と、
表面から板厚1/3位置までの範囲に存在するAl、Si、Mg及びCaの少なくとも1種を含有する介在物の平均粒径が50μm以下、前記介在物の平均最近接距離が20μm以上である鋼組織と、を有する鋼板と、
前記鋼板の表面に、片面あたりのめっき付着量が20g/m2以上120g/m2以下の亜鉛めっき層と、を備え、
鋼中に含まれる拡散性水素量が0.25質量ppm未満であり、
引張強さが1100MPa以上である高強度亜鉛めっき鋼板。
[2]鋼中のMn含有量に対するSi含有量の質量比(Si/Mn)が0.1未満である、[1]に記載の高強度亜鉛めっき鋼板。
[3]前記成分組成は、さらに、質量%で、下記(1)~(3)のうち少なくとも1つを含有する[1]又は[2]に記載の高強度亜鉛めっき鋼板。
(1)Ti、Nb、V及びZrのうち1種以上を合計で0.005%以上0.1%以下
(2)Mo、Cr、Cu及びNiのうち1種以上を合計で0.01%以上0.5%以下
(3)B:0.0003%以上0.005%以下
[4]前記成分組成は、さらに、質量%で、Sb:0.001%以上0.1%以下及びSn:0.001%以上0.1%以下のうち少なくとも1種を含有する[1]~[3]のいずれか一つに記載の高強度亜鉛めっき鋼板。
[5]前記成分組成は、さらに、質量%で、Ca:0.0005%以下を含有する[1]~[4]のいずれか一つに記載の高強度亜鉛めっき鋼板。
[6]前記鋼組織は、面積率で、30%以上85%以下のマルテンサイト、60%以下(0%を含む)のフェライト、15%以下(0%を含む)のベイナイト、かつ5%未満(0%を含む)の残留オーステナイトを有し、
フェライトの平均粒径が15μm以下である[1]~[5]のいずれか一つに記載の高強度亜鉛めっき鋼板。
[7][1]~[5]のいずれか一つに記載の成分組成を有する鋼を、鋳型メニスカス近傍の凝固界面の溶鋼流速が16cm/秒以上となる条件で鋳造して鋼素材とする鋳造工程と、
前記鋳造工程後の鋼素材を熱間圧延する熱延工程と、
前記熱延工程後の鋼板を酸洗する酸洗工程と、
前記酸洗工程後の鋼板を、圧下率20%以上80%以下で冷間圧延する冷延工程と、
前記冷延工程後の鋼板を、前処理加熱温度720℃以上880℃以下に加熱し、その後、前記前処理加熱温度から500℃までを平均冷却速度2℃/秒以上、499℃から200℃までを平均冷却速度3℃/秒以上で冷却し、該冷却後、酸洗を行う前処理工程と、
前記前処理工程後の鋼板を、連続焼鈍ラインにて、500℃以上の温度域の炉内雰囲気の水素濃度を0vol%超12vol%以下として、焼鈍温度740℃以上(Ac3+20)℃以下で加熱した後、前記焼鈍温度から600℃までを平均冷却速度3℃/秒以上で冷却する焼鈍工程と、
前記焼鈍工程後の鋼板をめっき処理し、該めっき処理後、450℃から250℃までの温度域を平均冷却速度3℃/秒以上で冷却するめっき工程と、を有する高強度亜鉛めっき鋼板の製造方法。
[8]前記前処理工程において、前記前処理加熱温度で加熱する際に、600℃以上前記前処理加熱温度以下の雰囲気の露点がY℃以上であり、
前記焼鈍工程において、前記焼鈍温度で加熱する際に、前記連続焼鈍ラインにおける700℃以上の温度域の露点がZ℃以下であり、かつ
前記前処理加熱温度をX℃としたとき、
前記X、Y及びZは、以下の関係式(i)、(ii)、又は(iii)を満たす、[7]に記載の高強度亜鉛めっき鋼板の製造方法。
(i) 720≦X≦800、かつY-Z≧-5
(ii) 800<X≦840、かつY-Z≧0
(iii) 840<X≦880、かつY-Z≧5
[9]前記めっき工程後、幅トリムを行う幅トリム工程を、さらに有する[7]又は[8]に記載の高強度亜鉛めっき鋼板の製造方法。
[10]前記焼鈍工程後又は前記めっき工程後、水素濃度が5vol%以下、露点が50℃以下の雰囲気で、50~400℃の温度域で30秒以上加熱する後処理工程をさらに有する[7]~[9]のいずれか一つに記載の高強度亜鉛めっき鋼板の製造方法。
[11]前記めっき工程において、前記めっき処理直後に合金化処理を行う[7]~[10]のいずれか一つに記載の高強度亜鉛めっき鋼板の製造方法。
[12][1]~[6]のいずれか一つに記載の高強度亜鉛めっき鋼板が、成形加工及び溶接の少なくとも一方がされてなる高強度部材。
[13][7]~[11]のいずれか一つに記載の高強度亜鉛めっき鋼板の製造方法によって製造された高強度亜鉛めっき鋼板を、成形加工及び溶接の少なくとも一方を行う工程を有する、高強度部材の製造方法。
Cは鋼板の高強度化に有効な元素であり、鋼組織の硬質相の一つであるマルテンサイトを形成することで高強度化に寄与する。また、製造方法によってはNbやTi、V及びZrといった炭化物形成元素と微細な合金化合物或いは合金炭窒化物を形成することでも高強度化に寄与する。これらの効果を得るためには、C含有量は0.08%以上とすることが必要である。一方、C含有量が0.20%を超えると、マルテンサイトが過剰に硬質化し、介在物や鋼中水素量を制御しても曲げ加工性が改善しなくなる傾向にある。したがってC含有量は0.08%以上0.20%以下とする。TSを安定して1100MPa以上にする観点からは、C含有量は0.09%以上が好ましい。
Siは主に固溶強化により高強度化に寄与する元素であり、強度上昇に対して延性の低下が比較的少なく、強度のみならず強度と延性のバランス向上にも寄与する。延性の向上は曲げ性の改善につながる。一方で、Siは鋼板表面にSi系酸化物を形成しやすく、不めっきの原因となる場合があると共に、過剰に含有すると熱間圧延時に著しいスケールが形成されて鋼板表面にスケール跡疵が付き、表面性状が悪くなることがあり、また酸洗性が低下することがある。したがって、強度確保に必要な分だけ添加すれば良いが、めっき性の観点から、Si含有量は2.0%未満とする。また、より優れためっき性を得る観点から、Si含有量は好ましくは0.5%以下であり、より好ましくは0.3%以下である。また、Si含有量の下限は特に規定されないが、0.001%未満では製造での制御が難しくなる傾向があるので、Si含有量は0.001%以上とすることが好ましい。
Mnは固溶強化およびマルテンサイト形成により高強度化に寄与する元素として有効であり、この効果を得るために1.5%以上の含有が必要である。Mn含有量は好ましくは1.9%以上である。一方、Mn含有量が3.5%を超えるとMnの偏析などに起因して鋼組織にムラを生じやすくなり、加工性の低下を招くほか、Mnは鋼板表面に酸化物あるいは複合酸化物として外部酸化しやすく、不めっきの原因となる場合がある。そのため、Mn含有量は3.5%以下とする。
Pは、固溶強化により鋼板の高強度化に寄与する有効な元素であるが、一方でめっき性に影響を与える。特に鋼板との濡れ性の劣化、めっき層の合金化速度の遅延を招き、特に高強度鋼板を得るような高合金系では影響が大きい。そのため、P含有量は0.02%以下とした。P含有量は、好ましくは0.01%以下である。P含有量の下限は特に規定しないが0.0001%未満では製造過程において生産能率低下と脱燐コスト増を招くため、P含有量は0.0001%以上とすることが好ましい。
Sは鋼中で硫化物系の介在物を形成しやすい。特に高強度化のためMnを多量に添加する場合はMnS系の介在物を形成しやすくなる。これは曲げ性を損なう原因にもなるほか、Sは熱間脆性を起こす原因となって製造工程に悪影響を及ぼすため、極力低減することが好ましい。本発明では0.002%までは許容できる。S含有量の下限は特に規定しないが0.0001%未満では製造過程において生産能率低下とコスト増を招くため、S含有量は0.0001%以上とすることが好ましい。
Alは脱酸剤として添加される。脱酸剤としてAlを添加する場合、その効果を得るために、Alを0.001%以上含有することが好ましい。一方、Al含有量が0.10%を超えると製造工程中で介在物を形成しやすくなり、曲げ性を劣化させる。そのため、Al含有量は0.10%以下とし、好ましくは鋼中のsol.Alとして0.08%以下である。
N含有量が0.006%を超えると鋼中に過剰な窒化物が生成して加工性を低下させるほか、鋼板の表面性状の悪化を招くことがある。このためN含有量は0.006%以下、好ましくは0.005%以下とする。フェライトが存在する場合、その清浄化による延性向上の観点からは含有量は極力少ない方が好ましいが、製造過程における生産能率低下とコスト増を招くため、N含有量は0.0001%以上とすることが好ましい。
より優れためっき性を得るためには鋼中で酸化しやすい元素の制御することが好ましい。鋼板の成分組成には、SiとMnは上記範囲で含有すればよいが、不めっき欠陥の発生を抑制し、耐めっき剥離性(密着性)を向上させ、かつ外観ムラを発生しにくくするという効果を有効に得る観点からは、鋼中のMn含有量に対するSi含有量の質量比(Si/Mn)を0.1未満とすることが好ましい。これらの効果をより有効に得るためには、鋼中のMn含有量に対するSi含有量の質量比(Si/Mn)は、より好ましくは0.10未満であり、さらに好ましくは0.08未満であり、特に好ましくは0.06未満である。
(1)Ti、Nb、V及びZrのうち1種以上を合計で0.005%以上0.1%以下
(2)Mo、Cr、Cu及びNiのうち1種以上を合計で0.01%以上0.5%以下
(3)B:0.0003%以上0.005%以下
Ti、Nb、V及びZrは、CやNと炭化物や窒化物(炭窒化物の場合もある)を形成する。微細析出物とすることで鋼板の高強度化に寄与する。特に軟質なフェライトに析出させることでその強度を高め、マルテンサイトとの強度差を低減する効果は曲げ性のほか伸びフランジ性の改善にも寄与する。さらにこれらの元素は熱延コイルの組織を微細化する作用があり、その後の冷延・焼鈍後の鋼組織を微細化することでも強度上昇や曲げ性などの加工性向上に寄与する。この効果を得る観点から、Ti、Nb、V及びZrのうち1種以上を合計で0.005%以上含有することが好ましい。しかしながら過剰な添加は、冷間圧延時の変形抵抗を高めて生産性を阻害するほか、過剰な或いは粗大な析出物の存在はフェライトの延性を低下させ、鋼板の延性や曲げ性を低下させる傾向がある。このため、Ti、Nb、V及びZrのうち1種以上を合計で0.1%以下とすることが好ましい。
SbやSnは脱炭や脱窒、脱硼などを抑制して、鋼板の強度低下抑制に有効な元素であるため0.001%以上の含有が好ましい。しかしながら過剰な添加は表面性状を低下させるためその上限を0.1%とすることが好ましい。
Caを少量添加すると、硫化物の形状を球状化させ、鋼板の曲げ性を向上させる効果が得られる。一方、過剰に添加すると、Caが鋼中で硫化物や酸化物を過剰に形成し、鋼板の加工性、特に曲げ性を低下させるため、Ca含有量を0.0005%以下とすることが好ましい。また、Ca含有量の下限は特に規定されないが、Caを含有する場合は、Ca含有量は0.0001%以上であることが多い。
マルテンサイトは硬質であり、鋼板強度を高めるために有効かつ必須である。引張強さ(TS)1100MPa以上を確保するために面積率で30%以上とすることが好ましい。TSの安定確保の観点からは45%以上とすることが好ましい。また、ここで言うマルテンサイトは、製造中に自己焼戻しされたオートテンパードマルテンサイトや場合によってはその後の熱処理で焼き戻された焼戻しマルテンサイトを含む。また、曲げ性と強度とのバランスの観点からマルテンサイトは85%以下とすることが好ましい。
水素が存在する雰囲気中で熱処理およびめっきを付与する工程を施した場合、鋼中に水素が侵入および残存する。最終製品の鋼中水素を極力低減する1つの手法として、めっきを付与する前の鋼組織にBCC構造であるフェライトやベイナイトを出現させる。これは、FCC構造であるオーステナイトよりもBCC構造であるフェライトやベイナイトの方が水素の固溶度が小さいことを利用するものである。また軟質なフェライトは鋼板の延性を向上させ、曲げ性を向上させる。しかし、フェライトが60%を超えると強度を確保することができなくなるため好ましい上限を60%とする。なお、フェライトは2%以上となることが多い。
ベイナイトは曲げ性向上に寄与するため、含んでもよいが、過剰に含むと所望の強度が得られないため、15%以下とすることが好ましい。なお、ベイナイトは2%以上となることが多い。
オーステナイトはfcc相であり、フェライト(bcc相)に比べ、水素の吸蔵能が高くかつ鋼中での拡散が遅いため鋼中に残存しやすい。さらにこの残留オーステナイトがマルテンサイトに加工誘起変態した場合には、鋼中の拡散性水素を増加させる懸念がある。そのため、本発明では、残留オーステナイトは5%未満が好ましい。
本発明の製造方法で使用する鋼は、一般的にスラブとよばれる連続鋳造方法で製造されたものを用いるが、これは合金成分のマクロ偏析を防止する目的であり、造塊法や薄スラブ鋳造法などで製造してもよい。
「鋳型メニスカス近傍」とは鋳型内の連続鋳造時に使用するパウダーと溶鋼の界面を意味する。造塊の場合、凝固中介在物を十分浮上させかつその浮上集合した箇所を切り捨てて次工程へ用いることが好ましい。
Ac3(℃)=910-203(C)1/2+44.7Si-30Mn-11P+700S+400Al+400Tiとする。
(ii) 800<X≦840、かつY-Z≧0
(iii) 840<X≦880、かつY-Z≧5
上記露点Y℃は、-50℃以上-25℃以下が好ましい。露点Y℃が-50℃未満の場合、前熱処理時のSiやMnなどの外部酸化量が不十分となり、めっき外観及びめっき密着性の不良につながる。露点Y℃が-25℃超えの場合、SiやMn酸化物によるピックアップが起こりやすくなり、操業が困難となる。連続焼鈍ラインの露点Z℃については、露点の下限は特に設定しないが、露点Z℃が-55℃未満では、炉体の気密性確保が困難になりやすく、コストアップにつながりやすい。一方、露点Z℃が-30℃超えの場合、連続焼鈍後の外部酸化が過剰となりやすく、めっき外観及びめっき密着性の不良につながる可能性がある。そのため、連続焼鈍ラインの露点Zは、-55℃以上-30℃以下が好ましい。
<高強度部材およびその製造方法>
本発明の高強度部材は、本発明の高強度亜鉛めっき鋼板が、成形加工及び溶接の少なくとも一方がされてなるものである。また、本発明の高強度部材の製造方法は、本発明の高強度亜鉛めっき鋼板の製造方法によって製造された高強度亜鉛めっき鋼板を、成形加工及び溶接の少なくとも一方を行う工程を有する。
鋼中の水素量の影響を確認するために、実施例1に示す検討を行った。
鋼中の水素量は以下の方法で測定した。先ずめっき鋼板から、5×30mm程度の試験片を切り出し、ルータ(精密グラインダ)を使って試験片表面のめっきを除去して石英管中に入れた。次いで、石英管中をArで置換した後、200℃/hrで昇温し、400℃までに発生した水素をガスクロマトグラフにより測定した。このように、昇温分析法にて放出水素量を測定した。室温(25℃)から210℃未満の温度域で検出された水素量の累積値を鋼中の拡散性水素量とした。
製造しためっき鋼板から、圧延方向に平行方向が短辺となるように、25×100mmの短冊試験片を切出した。次いで圧延方向が曲げたときの稜線になるように90°V曲げ試験を行った。ストローク速度は50mm/minとし、荷重10トンで5秒間ダイスに押し付ける決め押しをした。V型パンチの先端Rを0.5ステップで種々変化させて試験を行い、試験片稜線近傍を20倍のレンズで観察して亀裂(割れ)の有無を確認した。亀裂が発生しなかった最小のRと、試験片の板厚(tmm、千分の一の位で四捨五入した百分の一の位までの値を使用)から、R/tを算出し、これを曲げ性の指標とした。R/tの値が小さいほど曲げ性は良好である。
実施例2では、以下に示す亜鉛めっき鋼板を製造して評価した。
めっき鋼板から圧延方向に対して直角方向にJIS5号引張試験片(JISZ2201)を採取し、引張速度(クロスヘッドスピード)10mm/min一定で引張試験を行った。引張強さは引張試験における最大荷重を初期の試験片平行部断面積で除した値とした。平行部の断面積算出における板厚はめっき厚込みの板厚値を用いた。
実施例1と同様の方法で行った。
実施例1と同様の方法で行った。なお、本評価では、R/t≦3.5を曲げ性に優れると評価した。
製造した溶融亜鉛鍍金鋼板から組織観察用試験片を採取し、L断面(圧延方向に平行な板厚断面)を研磨後、ナイタール液で腐食しSEMで1500倍の倍率で3視野以上を観察して撮影した画像を解析した(観察視野ごとに面積率を測定し、平均値を算出した)。観察位置は、板厚表面から板厚1/4付近の位置とした。ただし、残留オーステナイトの体積率(体積率を面積率とみなす)についてはX線回折強度により定量したため、各組織の合計が100%を超える結果になる場合がある。表4のFはフェライト、Mはマルテンサイト(焼戻しマルテンサイトを含む)、Bはベイナイト、γは残留オーステナイトを意味する。フェライトの平均粒径はSEMで10個の粒子を観察し、各々の面積率を求め、円相当径を算出し、これを平均することで求めた。
90°V曲げ試験をおこなった試験片の稜線部を強制的に破断させて、鋼板の断面をSEMで観察した。試験片表層、すなわち曲げ外側表面から板厚1/3位置までに存在した介在物に関し、EDXによる定性分析で組成を確認し、Al、Si、Mg及びCaの少なくとも1種類以上を含有する酸化物を同定したのち、画像中の介在物の最長径(粒子幅の最も長い部分の寸法)を測定して、その最長径を粒径とみなし、その平均粒径を求めた。また、該視野内で、表面から板厚1/3位置までの範囲に存在する任意の介在物に対し、最近接に位置する介在物までの距離(最近接距離)を求め、全ての介在物についてこの距離を平均して平均最近接距離を求めた。
製造した溶融亜鉛鍍金鋼板の表面性状(外観)を目視観察し、不めっき欠陥の有無を調べた。不めっき欠陥とは数μm~数mm程度のオーダーで、めっきが存在せず鋼板が露出している領域を意味する。
A:不めっき欠陥が全くなく、かつZnカウント数が7000未満である。
B:不めっき欠陥が全くなく、かつZnカウント数が7000以上8000未満である。
C:不めっき欠陥が全くなく、かつZnカウント数が8000以上である。
D:不めっき欠陥が発生する。
[実施例3]
実施例3では、以下に示す亜鉛めっき鋼板を製造して評価した。なお、実施例3の製造方法では、前熱処理及び連続焼鈍ラインで露点を制御することによって、本発明の亜鉛めっき鋼板の好ましい製造方法をより詳細に検討した。
[実施例4]
実施例3の表6のNo.3-2(本発明例)の亜鉛めっき鋼板を、プレス成形をして、本発明例の部材を製造した。さらに、実施例3の表6のNo.3-2(本発明例)の亜鉛めっき鋼板と、表6のNo.3-3(本発明例)の亜鉛めっき鋼板とをスポット溶接により接合し、本発明例の部材を製造した。これらの本発明例の部材は、曲げ性及びめっき性に優れているため、自動車部品等に好適に用いることができることを確認できた。
Claims (13)
- 鋼組成が質量%で、
C:0.08%以上0.20%以下、
Si:2.0%未満、
Mn:1.5%以上3.5%以下、
P:0.02%以下、
S:0.002%以下、
Al:0.10%以下、及び
N:0.006%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成と、
表面から板厚1/3位置までの範囲に存在するAl、Si、Mg及びCaの少なくとも1種を含有する介在物の平均粒径が50μm以下、前記介在物の平均最近接距離が20μm以上である鋼組織と、を有する鋼板と、
前記鋼板の表面に、片面あたりのめっき付着量が20g/m2以上120g/m2以下の亜鉛めっき層と、を備え、
鋼中に含まれる拡散性水素量が0.25質量ppm未満であり、
引張強さが1100MPa以上である高強度亜鉛めっき鋼板。 - 鋼中のMn含有量に対するSi含有量の質量比(Si/Mn)が0.1未満である、請求項1に記載の高強度亜鉛めっき鋼板。
- 前記成分組成は、さらに、質量%で、下記(1)~(3)のうち少なくとも1つを含有する請求項1又は2に記載の高強度亜鉛めっき鋼板。
(1)Ti、Nb、V及びZrのうち1種以上を合計で0.005%以上0.1%以下
(2)Mo、Cr、Cu及びNiのうち1種以上を合計で0.01%以上0.5%以下
(3)B:0.0003%以上0.005%以下 - 前記成分組成は、さらに、質量%で、Sb:0.001%以上0.1%以下及びSn:0.001%以上0.1%以下のうち少なくとも1種を含有する請求項1~3のいずれか一項に記載の高強度亜鉛めっき鋼板。
- 前記成分組成は、さらに、質量%で、Ca:0.0005%以下を含有する請求項1~4のいずれか一項に記載の高強度亜鉛めっき鋼板。
- 前記鋼組織は、面積率で、30%以上85%以下のマルテンサイト、60%以下(0%を含む)のフェライト、15%以下(0%を含む)のベイナイト、かつ5%未満(0%を含む)の残留オーステナイトを有し、
フェライトの平均粒径が15μm以下である請求項1~5のいずれか一項に記載の高強度亜鉛めっき鋼板。 - 請求項1~5のいずれか一項に記載の成分組成を有する鋼を、鋳型メニスカス近傍の凝固界面の溶鋼流速が16cm/秒以上となる条件で鋳造して鋼素材とする鋳造工程と、
前記鋳造工程後の鋼素材を熱間圧延する熱延工程と、
前記熱延工程後の鋼板を酸洗する酸洗工程と、
前記酸洗工程後の鋼板を、圧下率20%以上80%以下で冷間圧延する冷延工程と、
前記冷延工程後の鋼板を、前処理加熱温度720℃以上880℃以下に加熱し、その後、前記前処理加熱温度から500℃までを平均冷却速度2℃/秒以上、499℃から200℃までを平均冷却速度3℃/秒以上で冷却し、該冷却後、酸洗を行う前処理工程と、
前記前処理工程後の鋼板を、連続焼鈍ラインにて、500℃以上の温度域の炉内雰囲気の水素濃度を0vol%超12vol%以下として、焼鈍温度740℃以上(Ac3+20)℃以下で加熱した後、前記焼鈍温度から600℃までを平均冷却速度3℃/秒以上で冷却する焼鈍工程と、
前記焼鈍工程後の鋼板をめっき処理し、該めっき処理後、450℃から250℃までの温度域を平均冷却速度3℃/秒以上で冷却するめっき工程と、を有する高強度亜鉛めっき鋼板の製造方法。 - 前記前処理工程において、前記前処理加熱温度で加熱する際に、600℃以上前記前処理加熱温度以下の雰囲気の露点がY℃以上であり、
前記焼鈍工程において、前記焼鈍温度で加熱する際に、前記連続焼鈍ラインにおける700℃以上の温度域の露点がZ℃以下であり、かつ
前記前処理加熱温度をX℃としたとき、
前記X、Y及びZは、以下の関係式(i)、(ii)、又は(iii)を満たす、請求項7に記載の高強度亜鉛めっき鋼板の製造方法。
(i) 720≦X≦800、かつY-Z≧-5
(ii) 800<X≦840、かつY-Z≧0
(iii) 840<X≦880、かつY-Z≧5 - 前記めっき工程後、幅トリムを行う幅トリム工程を、さらに有する請求項7又は8に記載の高強度亜鉛めっき鋼板の製造方法。
- 前記焼鈍工程後又は前記めっき工程後、水素濃度が5vol%以下、露点が50℃以下の雰囲気で、50~400℃の温度域で30秒以上加熱する後処理工程をさらに有する請求項7~9のいずれか一項に記載の高強度亜鉛めっき鋼板の製造方法。
- 前記めっき工程において、前記めっき処理直後に合金化処理を行う請求項7~10のいずれか一項に記載の高強度亜鉛めっき鋼板の製造方法。
- 請求項1~6のいずれか一項に記載の高強度亜鉛めっき鋼板が、成形加工及び溶接の少なくとも一方がされてなる高強度部材。
- 請求項7~11のいずれか一項に記載の高強度亜鉛めっき鋼板の製造方法によって製造された高強度亜鉛めっき鋼板を、成形加工及び溶接の少なくとも一方を行う工程を有する、高強度部材の製造方法。
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EP3748029B1 (en) | 2022-05-04 |
JP6673534B2 (ja) | 2020-03-25 |
CN111936648A (zh) | 2020-11-13 |
JPWO2019189841A1 (ja) | 2020-04-30 |
KR20200123242A (ko) | 2020-10-28 |
US20210130920A1 (en) | 2021-05-06 |
US11473165B2 (en) | 2022-10-18 |
EP3748029A4 (en) | 2020-12-09 |
EP3748029A1 (en) | 2020-12-09 |
MX2020010281A (es) | 2020-10-28 |
CN111936648B (zh) | 2021-11-02 |
KR102469709B1 (ko) | 2022-11-22 |
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