WO2019189067A1 - 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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Definitions
- the present invention is a high-strength galvannealed steel sheet having a low amount of diffusible hydrogen and excellent delayed fracture resistance, preferably a high-strength galvannealed steel sheet further excellent in ductility and hole expansibility, and their It relates to a manufacturing method.
- delayed fracture is a state in which a high-strength steel material is subjected to static load stress (load stress below the tensile strength), and when a certain amount of time has passed, with no apparent plastic deformation, This is a phenomenon where sudden brittle fracture occurs.
- Baking treatment is known as a treatment for releasing (leaving) hydrogen that has entered the steel material from the steel material (for example, Patent Document 1).
- the steel material into which hydrogen has entered is heated at a predetermined temperature (for example, around 200 ° C.), thereby diffusing and releasing (detaching) hydrogen from the steel material surface.
- Patent Document 2 discloses a method for baking a hot-dip galvanized steel sheet in a steam atmosphere.
- hot dip plating has a thicker plating layer than electroplating, it is difficult to efficiently release hydrogen from the surface of the steel plate simply by baking (heating) the hot dip galvanized steel plate. For this reason, the improvement of the delayed fracture resistance tends to be insufficient, and problems such as the occurrence of hydrogen blistering and a prolonged baking process also occur.
- An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having a low amount of diffusible hydrogen and excellent delayed fracture resistance, and a method for producing the same, by solving the above-described problems of the prior art.
- Another object of the present invention is to provide a high-strength hot-dip galvanized steel sheet that is also excellent in ductility and hole expansibility and a method for producing the same.
- the present inventors have intensively studied to find a method capable of appropriately removing diffusible hydrogen contained in a hot dip galvanized steel sheet.
- the Fe-Zn intermetallic compound constituting the plating layer of the GA steel sheet is a brittle material
- an external force is applied to the Fe-Zn intermetallic compound (plating layer), which is the brittle material.
- the idea was to secure a hydrogen desorption path by introducing a crack, and then perform baking treatment to release diffusible hydrogen contained in the steel sheet through the desorption path.
- a fine crack is formed in the plated layer by rolling it (which may be rolled under relatively light pressure).
- diffusible hydrogen can be appropriately removed from the steel sheet, and the amount of diffusible hydrogen in the steel sheet can be reduced to a predetermined level. It was found that it can be reduced. That is, a method capable of effectively removing diffusible hydrogen in a steel sheet by utilizing the properties of a coating layer of a GA steel sheet different from an EG steel sheet (electroplated steel sheet) or a GI steel sheet (hot dip galvanized steel sheet). It is what I found.
- the diffusible hydrogen contained in the GA steel sheet penetrates mainly in the annealing process of CGL and the detachment of the diffusible hydrogen is inhibited by the hot dip galvanizing applied thereafter.
- the inventors of the present invention have significantly lower ductility (total elongation) and hole expansibility (limit hole expansion ratio) of GA steel sheets based on high-Mn-added steel sheets aimed at high strength and high ductility than cold-rolled steel sheets. This was presumed to be caused by diffusible hydrogen in the steel sheet.
- the present inventors use a high Mn-added steel sheet as a base material, and perform a baking process after introducing a fine crack into the plating layer by rolling the GA steel sheet having a predetermined Fe concentration in the plating layer. As a result, it was found that the ductility and hole expansibility can be greatly improved.
- the baking method can be performed at a relatively low temperature and the atmosphere is not particularly required to be controlled by the above method.
- the baking process can be performed at a relatively low temperature, and the atmosphere control is not particularly required.
- a method for producing an galvannealed steel sheet using a high-strength steel sheet as a base material, A rolling step (x) for rolling an alloyed hot-dip galvanized steel plate having a plating layer with an Fe concentration of 8 to 17% by mass, and a plated steel plate that has undergone the rolling step (x) are represented by the following formulas (1) and (2)
- fill are represented by the following formulas (1) and (2)
- the above-mentioned steel sheet is further mass%, B: 0.001 to 0.005%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.080%, Cr: 0 0.001 to 1.000%, Mo: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, Sb: 0.001 to 0.200 %.
- the steel sheet temperature (° C.) is set to [Ac 1 + (Ac 3 ⁇ Ac 1 ) / 6] to 950 ° C.
- the high-strength alloyed molten zinc according to any one of [2] to [5], wherein the holding time at temperature is 60 to 600 seconds, and in the alloying treatment step (c), the alloying treatment temperature is 460 to 650 ° C.
- Manufacturing method of plated steel sheet [7]
- the region where the steel sheet temperature is 600 to 900 ° C. is set to an atmosphere having an H 2 concentration of 3 to 20 vol% and a dew point of ⁇ 60 ° C. to ⁇ 30 ° C.
- the above-described steel sheet is further mass%, B: 0.001 to 0.005%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.080%, Cr: 0 0.001 to 1.000%, Mo: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, Sb: 0.001 to 0.200
- the average value (L) per unit area of fine cracks entering the plating layer on the steel sheet surface is 0.010 ⁇ m / ⁇ m 2 or more and 0.070 ⁇ m / [mu] m 2 or less, high strength according to any one of these, the proportion of cracks extending substantially perpendicular direction is 60% or less of the total length of the crack with respect to the rolling direction [8] to [10] Alloyed hot
- the present invention it is possible to stably provide a high-strength galvannealed steel sheet having a small amount of diffusible hydrogen and having excellent delayed fracture resistance.
- a base steel plate having a predetermined component composition with high Mn addition it is possible to stably provide a high-strength, high-ductility galvannealed steel plate with excellent ductility and hole expandability. can do.
- FIG. 1 is a graph showing the relationship between the heating temperature T satisfying the formula (1) and the holding time at the heating temperature T in the heat treatment step (y) according to the present invention.
- FIG. FIG. 15 is a view showing an example of a steel sheet surface according to the present invention in FIG.
- the method for producing a high-strength galvannealed steel sheet according to the present invention comprises a rolling step of rolling an galvannealed steel sheet having a high-strength steel sheet as a base material and a plating layer having an Fe concentration of 8 to 17% by mass (x ) And a heat treatment step (y) for heating the plated steel sheet that has undergone the rolling step (x) under predetermined heating conditions.
- the strength of the high-strength steel sheet that is the base material of the GA steel sheet there is no particular limitation on the strength of the high-strength steel sheet that is the base material of the GA steel sheet, but it is generally preferable to target a steel sheet having a tensile strength of 590 MPa or more.
- the problem of diffusible hydrogen is likely to occur particularly when a steel sheet having a tensile strength of 980 MPa or more is used as a base material. Therefore, the present invention is particularly applicable to a GA steel sheet using a steel sheet having a tensile strength of 980 MPa or more as a base material. It can be said that it is more useful. Even more useful is a GA steel plate whose base material is a steel material having a tensile strength of 1180 MPa or more.
- the manufacturing method of the present invention can further include an annealing step, a plating treatment step, and an alloying treatment step performed by CGL or the like. That is, this manufacturing method includes a steel sheet annealing step (a), a plating treatment step (b) in which hot dip galvanizing is applied to the steel plate that has undergone the annealing step (a), and a plating layer obtained in the plating treatment step (b).
- the manufacturing method of the present invention utilizes the brittleness of the Fe—Zn intermetallic compound constituting the plating layer of the GA steel sheet, and rolls the GA steel sheet in the rolling step (x), thereby providing a hydrogen separation path in the plating layer.
- a fine crack is introduced and a baking process is performed thereon.
- the rolling step (x) may be rolling at a relatively low reduction rate (light reduction), and cracks are generated by crushing the plating layer by this rolling.
- the Fe concentration of the plating layer is important in order to introduce a fine crack that becomes a hydrogen separation path into the plating layer by rolling in the rolling step (x). Since Zn is a metal, it has ductility. Even if processing such as rolling is applied, cracks will not occur in the plating layer unless the degree of processing is extremely large. On the other hand, as the alloying of Zn and Fe (base material) in the plating layer proceeds, the ratio of the Zn phase having ductility decreases (that is, the ratio of the Fe—Zn intermetallic compound increases), and the plating layer becomes brittle. Therefore, it becomes easy to crack.
- the Fe concentration of the plating layer is preferably 8% by mass or more.
- the Fe concentration in the plating layer is preferably 17% by mass or less. From the above, in the present invention, the Fe concentration of the plated layer of the GA steel sheet subjected to the rolling step (x) is 8 to 17% by mass. The Fe concentration in the plating layer is more preferably 9% by mass or more.
- the Fe concentration in the plating layer is more preferably 15% by mass or less. This is because when the Fe concentration in the plating layer exceeds 15% by mass, a fragile ⁇ phase may be partially formed at the steel plate-plating interface, and cracks concentrate at the corresponding portions, and hydrogen at the portions where cracks are difficult to enter. This is because the withdrawal speed may decrease.
- the rolling means used in the rolling step (x) may be a general rolling mill or a rolling roll.
- the rolling reduction is more preferably 0.2% or more.
- the rolling reduction is more preferably 1.0% or less, and still more preferably 0.5% or less for the purpose of introducing cracks described later.
- the crack introduction direction is often perpendicular to the rolling direction.
- peeling of the plating may increase when subjected to press processing as an automobile part, resulting in poor powdering.
- the powdering resistance deteriorates as compared with the case where the crack introduction direction is not constant.
- the ratio of the length of cracks extending in a direction substantially perpendicular to the rolling direction is 60% or less of the total crack length.
- the length of the crack extending in a direction substantially perpendicular to the rolling direction is more preferably 55% or less, and even more preferably 50% or less of the entire crack length.
- the “rolling direction” is the direction in which the steel sheet to be rolled passes, and the “substantially perpendicular direction to the rolling direction” refers to rolling as described in the examples described later.
- the direction is in the range of 80 to 100 ° with respect to the passing direction of the steel plate to be formed.
- the average value (L) per unit area of fine cracks entering the plating layer is 0.010 ⁇ m / It is preferable that they are not less than ⁇ m 2 and not more than 0.070 ⁇ m / ⁇ m 2 .
- the average value (L) is more preferably 0.020 ⁇ m / ⁇ m 2 or more, and even more preferably 0.030 ⁇ m / ⁇ m 2 or more.
- the average value (L) is more preferably 0.075 ⁇ m / ⁇ m 2 or less, and even more preferably 0.060 ⁇ m / ⁇ m 2 or less.
- the rolling reduction is preferably 0.10 to 0.5%, and the work roll diameter when rolling (light rolling) is preferably 600 mm or less.
- the rolling reduction is less than 0.1%, the introduction of fine cracks is insufficient.
- the rolling reduction exceeds 0.5%, the average length per unit area of the fine cracks (L) Is more than 0.07 ⁇ m / ⁇ m 2 , so that the powdering resistance is deteriorated.
- the rolling reduction is more preferably 0.2% or more.
- the rolling reduction is more preferably 0.4% or less.
- the work roll diameter exceeds 600 mm, the contact area between the steel sheet and the roll increases during rolling, thereby increasing the time for receiving the force in the shearing direction (rolling direction) from the roll, and cracks in the rolling direction. This is because it becomes easier to enter the right angle direction.
- the work roll diameter is more preferably 500 mm or less.
- the roughness of the work roll surface used for rolling (light rolling) is preferably 1.5 ⁇ m or less.
- the roughness of the work roll surface used for rolling (light rolling) is preferably 1.0 ⁇ m or more.
- the GA steel sheet that has undergone the rolling step (x) is subjected to a heat treatment (baking treatment) for the purpose of removing diffusible hydrogen in the heat treatment step (y).
- the heat treatment step (y) when the heating temperature is relatively high, the temperature in the coil is not uniform, and there is a risk of variations in mechanical properties in the coil. In order to discharge, it is necessary to lengthen the heating time (holding time) as the heating temperature is lower. From these viewpoints, in the present invention, the plated steel sheet is heated under conditions that satisfy the following expressions (1) and (2). Moreover, it is more desirable to heat the plated steel sheet under conditions that satisfy the following expressions (1) and (3).
- FIG. 1 shows the relationship between the heating temperature T satisfying the expression (1) and the holding time t at the heating temperature T.
- T Heating temperature of plated steel sheet (° C)
- T Holding time at heating temperature T (hr)
- the holding time may be about 1 to 500 hours.
- the heating time is more preferably 5 hours or more, and even more preferably 8 hours or more.
- the heating time is more preferably 300 hours or less, and even more preferably 100 hours or less.
- the GA steel sheet exhibits variations in mechanical properties. Without causing it, the amount of diffusible hydrogen can be reduced to a sufficiently low desired level.
- the heat treatment step (y) can be carried out in an air atmosphere without special control of the atmosphere. Moreover, there is no restriction
- the component composition of the high-strength steel sheet used as the base material of the GA steel sheet is not particularly limited. -0.35%, Si: 0.01-2.00%, Mn: 2.0-10.0%, Al: 0.001-1.000%, P: 0.10% or less, S: 0 0.01% or less is preferably contained, and if necessary, B: 0.001 to 0.005%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.080%, Cr: 0.001 to 1.000%, Mo: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, Sb: 0.001 to One or more selected from 0.200% can be contained.
- B 0.001 to 0.005%
- Nb 0.005 to 0.050%
- Ti: 0.005 to 0.080% Cr: 0.001 to 1.000%
- Mo 0.05 to 1.00%
- Sb 0.001 to One or more selected from 0.
- ⁇ C: 0.03-0.35% C is an element having an effect of increasing the strength of the steel sheet, and therefore, the C content is preferably 0.03% or more.
- the C content is preferably set to 0.35% or less.
- C is more preferably 0.05% or more, and still more preferably 0.08% or more.
- C is more preferably 0.30% or less, and still more preferably 0.28% or less.
- ⁇ Si: 0.01-2.00% Si is an element effective for strengthening steel and improving ductility. For this reason, the Si content is preferably 0.01% or more.
- the Si content is preferably 2.00% or less.
- Si is more preferably 0.02% or more, and even more preferably 0.05% or more.
- Si is more preferably 1.80% or less, still more preferably 1.70% or less.
- ⁇ Mn 2.0 to 10.0% Mn is an element that stabilizes the austenite phase and greatly improves ductility, and is an important element in high-strength and high-ductility GA steel sheets. In order to obtain such an effect, the Mn content is preferably 0.1% or more, and more preferably 2.0% or more.
- the Mn content exceeds 10.0%, the slab castability and weldability deteriorate, so the Mn content is preferably 10.0% or less.
- Mn is more preferably 2.50% or more, and still more preferably 3.00% or more.
- Mn is more preferably 8.50% or less, still more preferably 8.00% or less.
- ⁇ Al: 0.001-1.000% Al is added for the purpose of deoxidizing molten steel, but if the Al content is less than 0.001%, the purpose is not achieved.
- the Al content exceeds 1.000%, Al forms an oxide on the surface of the steel sheet and the plating appearance (surface appearance) deteriorates. Therefore, the Al content is preferably 0.001 to 1.000%.
- Al is more preferably 0.005% or more, and still more preferably 0.010% or more. Al is more preferably 0.800% or less, and still more preferably 0.500% or less.
- -P 0.10% or less P is one of the elements inevitably contained, and the slab productivity deteriorates as P increases. Furthermore, the inclusion of P suppresses the alloying reaction and causes uneven plating. For this reason, the P content is preferably 0.10% or less, and more preferably 0.05% or less. On the other hand, in order to reduce the P content to less than 0.005%, there is a concern about an increase in cost, so the P content is preferably 0.005% or more. P is more preferably 0.05% or less, and still more preferably 0.01% or less.
- S 0.01% or less S is an element inevitably contained in the steelmaking process, but if contained in a large amount, weldability deteriorates, so the S content is preferably 0.01% or less. S is more preferably 0.08% or less, and still more preferably 0.006% or less. S is more preferably 0.001% or more, and still more preferably 0.002% or more.
- ⁇ B 0.001 to 0.005% When B is 0.001% or more, a quenching promoting effect is obtained. On the other hand, when it exceeds 0.005%, chemical conversion property deteriorates. For this reason, when B is contained, the content is preferably 0.001 to 0.005%.
- Nb 0.005 to 0.050%
- the content is more preferably 0.01% or more, and even more preferably 0.02% or more.
- the content is more preferably 0.045% or less, and even more preferably 0.040% or less.
- Ti 0.005 to 0.080%
- strength adjustment stress improvement
- chemical conversion processability is deteriorated. Therefore, when Ti is contained, its content is preferably 0.005 to 0.080%.
- the content is more preferably 0.010% or more, and still more preferably 0.015% or more.
- Ti is contained, its content is more preferably 0.070% or less, and even more preferably 0.060% or less.
- ⁇ Cr 0.001-1.000% Cr has a hardenability effect of 0.001% or more.
- the content is more preferably 0.08% or more.
- Mo When Mo is contained, its content is more preferably 0.80% or less.
- Cu 0.05 to 1.00% When Cu is 0.05% or more, the effect of promoting the formation of residual ⁇ phase is obtained. On the other hand, if it exceeds 1.00%, cost increases. Therefore, when Cu is contained, its content is preferably 0.05 to 1.00%. When Cu is contained, its content is more preferably 0.08% or more, and even more preferably 0.10% or more. When it contains Cu, its content is more preferably 0.80% or less, and even more preferably 0.60% or less.
- ⁇ Ni 0.05-1.00% When Ni is 0.05% or more, the effect of promoting the formation of residual ⁇ phase is obtained.
- Ni when Ni is contained, its content is preferably 0.05 to 1.00%. When Ni is contained, its content is more preferably 0.10% or more, and still more preferably 0.12% or more. When Ni is contained, its content is more preferably 0.80% or less, and even more preferably 0.50%.
- ⁇ Sb 0.001 to 0.200% Sb can be contained from the viewpoint of suppressing decarburization in the tens of microns region of the steel sheet surface caused by nitriding, oxidation, or oxidation of the steel sheet surface.
- the content is preferably 0.001 to 0.200%. In the case of containing Sb, the content is more preferably 0.003% or more, and even more preferably 0.005% or more. When it contains Sb, its content is more preferably 0.100% or less, and even more preferably 0.080% or less.
- the balance other than the basic components and optional additives described above is Fe and inevitable impurities.
- the steel plate (base material steel plate) has a tensile strength of 980 MPa or more, and the product of the tensile strength (TS) and total elongation (EL) (TS ⁇ EL) is 16000 MPa ⁇ %. The above is preferable.
- tensile strength (TS) and total elongation (EL) are measured by a tensile test.
- TS tensile strength
- EL total elongation
- Annealing process (a) Although there is no special restriction on the annealing conditions in the annealing step (a), in order to ensure the optimal strength / ductility balance, in particular, the strength / ductility balance of the GA steel plate based on the high Mn-added steel plate having the component composition described above. Further, the steel plate temperature (° C.) corresponding to Ac 1 point and Ac 3 point of the steel plate is set to [Ac 1 + (Ac 3 ⁇ Ac 1 ) / 6] to 950 ° C., and the holding time at the temperature is 60 to 600. Preferably it is seconds.
- the steel plate temperature (° C.) is more preferably [Ac 1 + (Ac 3 ⁇ Ac 1 ) / 6] to 900 ° C.
- the steel plate temperature (° C.) is more preferably 870 ° C. or less.
- the steel sheet temperature (° C.) is more preferably 650 ° C. or higher, and even more preferably 670 ° C. or higher.
- Ac 1 point of the steel plate (°C) and Ac 3 point (°C) can be obtained by the following formulas.
- the main purpose of annealing in CGL and the like is to improve workability by recrystallization of the work structure of a steel sheet and to form a structure before cooling.
- the steel sheet temperature (° C.) By setting the steel sheet temperature (° C.) to [Ac 1 + (Ac 3 ⁇ Ac 1 ) / 6] or higher, the amount of austenite phase during annealing can be set to 20 vol% or higher, and then cooled to martensite.
- tempered martensite, bainite and retained austenite structure are formed, and martensite and tempered martensite are responsible for strength, and retained austenite is responsible for elongation.
- the steel sheet temperature (° C.) is preferably set to [Ac 1 + (Ac 3 ⁇ Ac 1 ) / 6] to 950 ° C.
- the steel plate temperature (° C.) is more preferably 900 ° C. or less, and even more preferably 870 ° C. or less.
- the steel sheet temperature (° C.) is more preferably 650 ° C. or higher, and even more preferably 670 ° C. or higher.
- the holding time at the steel plate temperature (° C.) is preferably 60 to 600 seconds.
- the holding time at the steel sheet temperature (° C.) is more preferably 500 seconds or less.
- the holding time at the steel sheet temperature (° C.) is more preferably 30 seconds or more.
- the region where the steel sheet temperature is 600 to 900 ° C. is an atmosphere where the H 2 concentration is 3 to 20 vol% and the dew point is ⁇ 60 ° C. to ⁇ 30 ° C.
- the H 2 concentration is more preferably 5 to 15 vol%. Even more preferably, the H 2 concentration is 12 vol% or less. Even more preferably, the dew point is -15 ° C or lower. It is even more preferable that the dew point is ⁇ 20 ° C. or higher.
- annealing in CGL or the like it is possible to prevent surface oxidation by heating the steel sheet in a reducing atmosphere, and to suppress a decrease in wettability with respect to molten zinc.
- Annealing in such a reducing atmosphere is sufficiently effective if the steel sheet temperature is set in the range of 600 to 900 ° C. where the reaction rate is high.
- the H 2 concentration in the annealing atmosphere is preferably 3 vol% or more.
- the H 2 concentration exceeds 20 vol%, the amount of hydrogen entering the steel plate increases, and the amount of diffusible hydrogen in the steel plate is sufficiently reduced even when the rolling step (x) and the heat treatment step (y) are performed. There is a risk that it will not be possible.
- the internal oxidation of the steel sheet can be controlled by managing the dew point of the annealing atmosphere with the steel sheet temperature in the range of 600 to 900 ° C. where the reaction rate is high.
- the reaction in which internal oxidation is caused by water vapor is expressed as follows, where M is the alloy element to be oxidized.
- the steel sheet temperature (° C.) is more preferably 870 ° C. or less, and even more preferably 860 ° C. or less.
- the steel plate temperature (° C.) is more preferably 620 ° C. or higher, and even more preferably 640 ° C. or higher.
- M + XH 2 O MO X + XH 2 Hydrogen generated by this reaction tends to remain in the steel. If the dew point in the annealing atmosphere is larger than -30 ° C, the amount of hydrogen generated by internal oxidation increases, and the amount of diffusible hydrogen in the steel sheet is sufficient even when the rolling process (x) and the heat treatment process (y) are performed. There is a risk that it cannot be reduced. On the other hand, even if the dew point is less than ⁇ 60 ° C., the effect of controlling the dew point is saturated, and on the contrary, the economy is impaired.
- the H 2 concentration is 3 to 20 vol% and the dew point is ⁇ 60 ° C. to ⁇ 30 ° C.
- the H 2 concentration is more preferably 5 vol% or more. More preferably, the H 2 concentration is 15 vol% or less.
- the dew point is more preferably ⁇ 55 ° C. or higher, and still more preferably ⁇ 50 ° C. or higher.
- the dew point is more preferably ⁇ 35 ° C. or lower.
- the atmosphere in other regions is arbitrary and may be a non-oxidizing atmosphere.
- the plating treatment step (b) In the plating treatment step (b), after annealing in the annealing step (a), the steel sheet cooled to a predetermined temperature is immersed in a hot dip galvanizing bath to perform hot dip galvanizing treatment.
- the plating basis weight is usually adjusted by gas wiping or the like with respect to the plated steel sheet from the hot dip galvanizing bath.
- the plating adhesion amount is preferably 20 g / m 2 or more from the viewpoint of corrosion resistance and plating adhesion amount control, and also from the viewpoint of adhesion To 120 g / m 2 or less.
- the plating adhesion amount is more preferably 25 g / m 2 or more, and even more preferably 30 g / m 2 or more.
- the plating adhesion amount is more preferably 100 g / m 2 or less, and still more preferably 70 g / m 2 or less.
- the composition of the hot dip galvanizing bath an appropriate amount of, for example, one or more of Al, Mg, Si, etc. can be contained as the plating component other than Zn (the balance is Zn and inevitable impurities) as in the conventional one.
- the Al concentration in the bath is desirably about 0.001 to 0.2% by mass.
- the Al concentration in the bath is more preferably 0.01% or more, and even more preferably 0.05% or more.
- the Al concentration in the bath is more preferably 0.17% or less, and still more preferably 0.15% or less.
- the effect of the present invention can be obtained. does not change.
- -Alloying process (c) In the alloying treatment step (c), the steel sheet that has undergone the plating treatment step (b) is heated to alloy the hot dip galvanized layer.
- the alloying treatment temperature (the maximum temperature reached by the steel sheet) is preferably 460 to 650 ° C, more preferably 480 to 570 ° C.
- the alloying treatment temperature is less than 460 ° C., the rate of the alloying reaction is slow, and there is a risk that the desired Fe concentration of the plating layer may not be obtained.
- a brittle Zn—Fe alloy layer may be formed thick and the plating adhesion may be deteriorated, and the strength and ductility balance may be lowered due to decomposition of the retained austenite phase.
- the alloying treatment temperature maximum steel plate temperature
- the alloying treatment temperature (steel plate maximum temperature) be 490 ° C. or higher.
- the GA steel sheet obtained through the above annealing step (a), plating treatment step (b), alloying treatment step (c) is subjected to the rolling step (x) and the heat treatment step (y under the conditions described above. ) Thereby, the amount of diffusion hydrogen is reduced to a sufficiently low level, and a high-strength GA steel sheet having excellent delayed fracture resistance is obtained. Further, as described above, a high strength / high ductility GA steel sheet having excellent ductility and hole expansibility can be obtained by using a base steel sheet having a predetermined component composition with high Mn addition.
- the high-strength GA steel sheet of the present invention is a GA steel sheet obtained by the above-described production method of the present invention and having a high-strength steel sheet as a base material.
- the composition is such that the Fe concentration in the plating layer is 8 to 17% by mass, and among the hydrogen present in the steel sheet, the amount of hydrogen released when the steel sheet is heated to 200 ° C. is 0.35 mass ppm. It is the following.
- the reason for the limitation that the Fe concentration of the plating layer is 8 to 17% by mass is as described above. Further, the preferable tensile strength (TS) of the steel plate and the reason thereof are also as described above.
- the amount of hydrogen released when the steel plate is heated to 200 ° C. is 0.35.
- Mass ppm or less means that the amount of diffusible hydrogen is sufficiently reduced, and thus has excellent delayed fracture resistance.
- the amount of hydrogen released is preferably 0.20 ppm by mass or less. Even more preferably, the amount of hydrogen released is 0.10 mass ppm or less.
- the amount of hydrogen released is preferably 0 as much as possible, but long-time heat treatment causes an increase in production cost. Therefore, a residual hydrogen amount of 0.02 mass ppm or less that does not significantly affect the material is observed.
- the amount of hydrogen released when the steel sheet is heated to 200 ° C.” can be measured as follows. First, the plating layers on the front and back sides of the GA steel sheet are removed. As a method for removal, the plating layer may be physically removed using a router or the like, or the plating layer may be chemically removed using an alkali. However, when physically shaving, the grinding amount of the steel plate is 5% or less of the plate thickness. After removal of the plating layer, the amount of hydrogen in the test piece is measured by a temperature rising analysis by gas chromatography.
- the rate of temperature increase is not particularly limited, but if it is too large, accurate measurement may not be possible, so 500 ° C./hr or less is preferable, and about 200 ° C./hr is particularly preferable. It is even more preferable that the rate of temperature rise be about 100 ° C./hr.
- the value obtained by dividing the amount of hydrogen measured in this way by the mass of the steel sheet is expressed as “amount of hydrogen released when the steel sheet is heated to 200 ° C. (mass ppm) among the hydrogen present in the steel sheet”. To do.
- the temperature increase is usually started from room temperature. A specific value for room temperature is, for example, 20 ° C.
- the high-Mn-added high-strength and high-ductility GA steel plate as described above is not limited to the above-described configuration, and the steel plate is in mass% and C: 0.00.
- Si 0.01 to 2.00%, Mn: 2.0 to 10.0%, Al: 0.001 to 1.000%, P: 0.10% or less, S: 0.01% or less, and if necessary, B: 0.001 to 0.005%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.080%, Cr: 0.001 to 1.000%, Mo: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, Sb: 0.001 to 0.00.
- the balance contains at least one selected from 200%, the balance has a component composition consisting of Fe and inevitable impurities, and a tensile strength of 980 Pa or more, the product of the tensile strength (TS) and total elongation (EL) (TS ⁇ EL) is at 16,000 MPa ⁇ % or more, and a coating weight of the plating layer is per side 20 ⁇ 120g / m 2.
- the component composition of the base material, the mechanical characteristic value, and the reason for limiting the amount of coating are as described above.
- the plating layer has fine cracks.
- the plating layer since the GA steel sheet of the present invention has undergone the rolling process (x), the plating layer has a slightly crushed structure, and thus has fine cracks.
- the high-Mn-added high-strength and high-ductility GA steel sheets having the specific composition described above are excellent in hole expansibility.
- excellent hole expandability means that, depending on the tensile strength TS, the limit hole expansion rate ⁇ (the measurement method of this limit hole expansion rate ⁇ is described in the examples described later) is as follows: It is that.
- the plating layer (alloyed hot-dip galvanized layer) of the GA steel sheet of the present invention has an Fe concentration of 8 to 16% by mass by alloying treatment.
- a plating component other than Zn for example, An appropriate amount of one or more of Al, Mg, Si, etc. can be contained (the balance being Zn and inevitable impurities).
- Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, Sn, and the like may be contained.
- the GA steel sheet of the present invention is suitable for automobile use as a surface-treated steel sheet capable of reducing the weight and increasing the strength of the vehicle body.
- As a steel plate it can be applied to a wide range of uses including home appliances and building materials.
- a slab having a steel composition shown in Table 1 was heated at 1260 ° C. for 60 minutes in a heating furnace, then hot-rolled to a plate thickness of 2.8 mm, and wound at 540 ° C.
- the hot-rolled steel sheet was pickled to remove the black scale, and then cold-rolled to a thickness of 1.6 mm to obtain a cold-rolled steel sheet.
- the continuous hot-dip galvanizing equipment equipped with a reduction furnace (radiant tube type heating furnace), cooling zone, hot-dip zinc pot, alloying IH furnace, and light rolling machine in order from the entry side, the conditions shown in Table 2 and Table 4
- the cold-rolled steel sheet is subjected to annealing (annealing step (a)), plating treatment (plating treatment step (b)), alloying treatment (alloying treatment step (c)) and light rolling (rolling step (x)). After applying sequentially, it was wound up.
- the GA steel sheet (coil) was subjected to heat treatment (heat treatment step (y)) under the conditions described in Table 2 and Table 4. This heat treatment was performed in an air atmosphere without any control other than the temperature of the atmosphere.
- the roll diameter of the work roll used for the light rolling was 530 mm, and the surface roughness of the work roll was 1.3 ⁇ m.
- a H 2 —N 2 mixed gas was used as the atmosphere gas of the reducing furnace, and the dew point of the atmosphere was controlled by introducing a humidified gas into the reducing furnace. Further, the hot dip galvanizing bath held in the hot dip zinc pot was adjusted so that the bath temperature was 500 ° C. and the bath composition was 0.1% by mass of Al and the balance was Zn and inevitable impurities. After the steel sheet was immersed in a hot dip galvanizing bath, the amount of plating was controlled by gas wiping. The alloying treatment after hot dip galvanization was performed by heating the steel sheet with an IH heater.
- Limit hole expansion rate (%) ⁇ (D f ⁇ D 0 ) / D 0 ⁇ ⁇ 100
- D f Hole diameter at the time of crack occurrence (mm)
- D 0 Initial hole diameter (mm)
- ⁇ critical hole expansion ratio
- the amount of Zn and Fe dissolved in hydrochloric acid was measured using ICP emission spectroscopy, and ⁇ Fe dissolved amount / (Fe dissolved amount + Zn dissolved amount) ⁇ ⁇ 100 was expressed as Fe concentration (mass%) of the plating layer. It was.
- ⁇ Measurement of “the amount of hydrogen released when the steel plate is heated to 200 ° C. among the hydrogen present in the steel plate” Physically scraping the plating layers on the front and back of the test piece of GA steel plate using a router Removed. The grinding amount of the steel plate at this time was 5% or less of the plate thickness. After removal of the plating layer, the amount of hydrogen in the test piece was measured by temperature analysis using gas chromatography.
- the temperature reached when the test piece was heated was 200 ° C.
- the rate of temperature increase was 200 ° C./hr.
- the value obtained by dividing the amount of hydrogen measured in this way by the mass of the steel sheet is expressed as “amount of hydrogen released when the steel sheet is heated to 200 ° C. (mass ppm) among the hydrogen present in the steel sheet”. did. -Evaluation of plating appearance
- the plating appearance of the GA steel sheet was evaluated as follows.
- the appearance of the plating surface of the GA steel sheet was observed, and the plating appearance was evaluated by the presence or absence of non-plating and the presence or absence of a pattern recognized as a color tone difference on the plating surface. That is, for the GA steel sheet, a range of 1 m 2 was randomly selected, and the presence or absence of non-plating and the presence or absence of a pattern recognized as a color tone difference were examined visually, and the plating appearance was evaluated as follows.
- a crack whose direction was in the range of 80 to 100 ° with respect to the rolling direction was regarded as a crack that propagated perpendicular to the rolling direction, and its length was measured, and the ratio to the total crack was calculated. Those with this ratio exceeding 60% were judged as bad (x), and those with 60% or less were judged as good ( ⁇ ). For the case where L is less than 0.010 ⁇ m / ⁇ m 2 or 0.070 ⁇ m / ⁇ m 2 or more, the crack ratio was not calculated.
- -Measurement of powdering resistance The powdering resistance of GA steel sheet was measured as follows.
- Cellotape (registered trademark) is applied to the GA steel sheet, the tape surface is bent 90 degrees, bent back, and the tape is peeled off.
- the amount of plating peeled off from the steel sheet attached to the peeled tape was measured as the Zn count number by fluorescent X-rays. (Triangle
- the delayed fracture resistance of GA steel sheet was evaluated as follows.
- the test piece obtained by the preliminary processing was ground to obtain a secondary test piece of 30 mm ⁇ 100 mm. This secondary test piece was bent 180 ° with a curvature radius of 10 mmR, and the space between the plates was narrowed by 12 mm to obtain a test piece for delayed fracture evaluation.
- test pieces for delayed fracture evaluation were immersed in hydrochloric acid aqueous solutions of pH 1 and pH 3, respectively, and the presence or absence of cracking after 96 hours was investigated. This test was carried out for three specimens of each steel plate, and cracking was considered to occur when even one specimen was cracked. The test results were evaluated as follows.
- the high-strength GA steel sheets of the examples of the present invention have excellent delayed fracture resistance because the amount of diffusible hydrogen is kept low, and further, ductility, hole expansibility, Excellent plating appearance.
- the high strength GA steel sheet of the comparative example is inferior in delayed fracture resistance due to a large amount of diffusible hydrogen, and inferior in one or more of ductility, hole expansibility, and plating appearance.
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Abstract
Description
[1] 高強度鋼板を母材とする合金化溶融亜鉛めっき鋼板の製造方法であって、
Fe濃度が8~17質量%のめっき層を有する合金化溶融亜鉛めっき鋼板を圧延する圧延工程(x)と、該圧延工程(x)を経ためっき鋼板を下記(1)式および(2)式を満たす条件で加熱する加熱処理工程(y)を有する高強度合金化溶融亜鉛めっき鋼板の製造方法。
40≦T≦160 ・・・(2)
但し、T:めっき鋼板の加熱温度(℃)
t:加熱温度Tでの保持時間(hr)
[2]前述の圧延工程(x)の前に、鋼板の焼鈍工程(a)と、該焼鈍工程(a)を経た鋼板に溶融亜鉛めっきを施すめっき処理工程(b)と、該めっき処理工程(b)で得られためっき層に合金化処理を施し、前述のFe濃度が8~17質量%のめっき層とする合金化処理工程(c)とを有する[1]に記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。
[3]前述の圧延工程(x)では、めっき鋼板を圧下率0.10~1%で軽圧下圧延する[1]または[2]に記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。
[4]前述の鋼板が、質量%で、C:0.03~0.35%、Si:0.01~2.00%、Mn:2.0~10.0%、Al:0.001~1.000%、P:0.10%以下、S:0.01%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を有するとともに、引張強度が980MPa以上、引張強度(TS)と全伸び(EL)の積(TS×EL)が16000MPa・%以上であり、めっき層のめっき付着量が片面当たり20~120g/m2である[1]~[3]のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。
[5]前述の鋼板が、さらに、質量%で、B:0.001~0.005%、Nb:0.005~0.050%、Ti:0.005~0.080%、Cr:0.001~1.000%、Mo:0.05~1.00%、Cu:0.05~1.00%、Ni:0.05~1.00%、Sb:0.001~0.200%の中から選ばれる1種以上を含有する[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。
[6]前述の焼鈍工程(a)では、鋼板のAc1点およびAc3点に応じて鋼板温度(℃)を[Ac1+(Ac3-Ac1)/6]~950℃とし、当該温度における保持時間を60~600秒とし、合金化処理工程(c)では、合金化処理温度を460~650℃とする[2]~[5]のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。
[7]前述の焼鈍工程(a)では、鋼板温度が600~900℃の領域をH2濃度が3~20vol%、露点が-60℃~-30℃の雰囲気とする[2]~[6]のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。
[8]高強度鋼板を母材とする合金化溶融亜鉛めっき鋼板であって、めっき層のFe濃度が8~17質量%であり、鋼板中に存在する水素のうち、鋼板を200℃まで昇温した際に放出される水素量が0.35質量ppm以下である高強度合金化溶融亜鉛めっき鋼板。
[9]前述の鋼板が、質量%で、C:0.03~0.35%、Si:0.01~2.00%、Mn:2.0~10.0%、Al:0.001~1.000%、P:0.10%以下、S:0.01%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を有するとともに、引張強度が980MPa以上、引張強度(TS)と全伸び(EL)の積(TS×EL)が16000MPa・%以上であり、めっき層のめっき付着量が片面当たり20~120g/m2である[8]に記載の高強度合金化溶融亜鉛めっき鋼板。
[10]前述の鋼板が、さらに、質量%で、B:0.001~0.005%、Nb:0.005~0.050%、Ti:0.005~0.080%、Cr:0.001~1.000%、Mo:0.05~1.00%、Cu:0.05~1.00%、Ni:0.05~1.00%、Sb:0.001~0.200%の中から選ばれる1種以上を含有する[9]に記載の高強度合金化溶融亜鉛めっき鋼板。
[11]前述の高強度合金化溶融亜鉛めっき鋼板において、鋼板表面のめっき層に入った微細な亀裂の単位面積当たりの長さの平均値(L)が0.010μm/μm2以上0.070μm/μm2以下であり、このうち、圧延方向に対して略直角方向に延びる亀裂の割合が亀裂の長さ全体の60%以下である[8]~[10]のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板。
40≦T≦160 ・・・(2)
60≦T≦120 ・・・(3)
但し、T:めっき鋼板の加熱温度(℃)
t:加熱温度Tでの保持時間(hr)
本発明では、加熱処理工程(y)での加熱条件は上記(1)式および(2)式に従うことが望ましいが、より広い加熱条件で加熱処理してもよく、例えば、加熱温度に関わりなく保持時間を1~500時間程度としてもよい。加熱時間は、より好ましくは5時間以上であり、更により好ましくは8時間以上である。加熱時間は、より好ましくは300時間以下であり、更により好ましくは100時間以下である。
まず、GA鋼板の母材となる高強度鋼板について説明する。なお、以下の説明において、各元素の含有量の単位は「質量%」であるが、便宜上「%」で示す。
・C:0.03~0.35%
Cは鋼板の強度を高める効果を有する元素であり、このためC含有量は0.03%以上とすることが好ましい。一方、C含有量が0.35%を超えると自動車や家電の素材として用いる場合に必要な溶接性が劣化するので、C含有量は0.35%以下とすることが好ましい。Cは、より好ましくは0.05%以上、更により好ましくは0.08%以上である。Cは、より好ましくは0.30%以下、更により好ましくは0.28%以下である。
・Si:0.01~2.00%
Siは鋼を強化し、延性を向上させるのに有効な元素であり、このためSi含有量は0.01%以上とすることが好ましい。一方、Si含有量が2.00%を超えると、Siが鋼板表面に酸化物を形成し、めっき外観が劣化するので、Si含有量は2.00%以下とすることが好ましい。Siは、より好ましくは0.02%以上、更により好ましくは0.05%以上である。Siは、より好ましくは1.80%以下、更により好ましくは1.70%以下である。
・Mn:2.0~10.0%
Mnはオーステナイト相を安定化させ、延性を大きく向上させる元素であり、高強度・高延性GA鋼板において重要な元素である。そのような効果を得るために、Mn含有量は0.1%以上、望ましくは2.0%以上とすることが好ましい。一方、Mn含有量が10.0%を超えるとスラブ鋳造性や溶接性が劣化するので、Mn含有量は10.0%以下とすることが好ましい。Mnは、より好ましくは2.50%以上、更により好ましくは3.00%以上である。Mnは、より好ましくは8.50%以下、更により好ましくは8.00%以下である。
・Al:0.001~1.000%
Alは溶鋼の脱酸を目的に添加されるが、Al含有量が0.001%未満では、その目的が達成されない。一方、Al含有量が1.000%を超えると、Alが鋼板表面に酸化物を形成し、めっき外観(表面外観)が劣化する。このためAl含有量は0.001~1.000%とすることが好ましい。Alは、より好ましくは0.005%以上、更により好ましくは0.010%以上である。Alは、より好ましくは0.800%以下、更により好ましくは0.500%以下である。
・P:0.10%以下
Pは不可避的に含有される元素のひとつであり、Pの増加に伴いスラブ製造性が劣化する。さらに、Pの含有は合金化反応を抑制し、めっきムラを引き起こす。このためP含有量は0.10%以下とすることが好ましく、0.05%以下とすることがより好ましい。一方、P含有量を0.005%未満にするには、コストの増大が懸念されるため、P含有量は0.005%以上が望ましい。Pは、より好ましくは0.05%以下、更により好ましくは0.01%以下である。Pは、より好ましくは0.007%以上、更により好ましくは0.008%以上である。
・S:0.01%以下
Sは製鋼過程で不可避的に含有される元素であるが、多量に含有すると溶接性が劣化するので、S含有量は0.01%以下とすることが好ましい。Sは、より好ましくは0.08%以下、更により好ましくは0.006%以下である。Sは、より好ましくは0.001%以上、更により好ましくは0.002%以上である。
・B:0.001~0.005%
Bは0.001%以上で焼き入れ促進効果が得られる。一方、0.005%を超えると化成処理性が劣化する。このためBを含有する場合には、その含有量は0.001~0.005%とすることが好ましい。Bを含有する場合には、その含有量は0.002%以上がより好ましい。Bを含有する場合には、その含有量は0.004%以下がより好ましい。
・Nb:0.005~0.050%
Nbは0.005%以上で強度調整(強度向上)の効果が得られる。一方、0.050%を超えるとコストアップを招く。このためNbを含有する場合には、その含有量は0.005~0.050%とすることが好ましい。Nbを含有する場合には、その含有量は0.01%以上がより好ましく、0.02%以上が更により好ましい。Nbを含有する場合には、その含有量は0.045%以下がより好ましく、0.040%以下が更により好ましい。
・Ti:0.005~0.080%
Tiは0.005%以上で強度調整(強度向上)の効果が得られる。一方、0.080%を超えると化成処理性の劣化を招く。このためTiを含有する場合には、その含有量は0.005~0.080%とすることが好ましい。Tiを含有する場合には、その含有量は0.010%以上がより好ましく、0.015%以上が更により好ましい。Tiを含有する場合には、その含有量は0.070%以下がより好ましく、0.060%以下が更により好ましい。
・Cr:0.001~1.000%
Crは0.001%以上で焼き入れ性効果が得られる。一方、1.000%を超えるとCrが鋼板表面に濃化するため、溶接性が劣化する。このためCrを含有する場合には、その含有量は0.001~1.000%とすることが好ましい。Crを含有する場合には、その含有量は0.005%以上がより好ましく、0.100%以上が更により好ましい。Crを含有する場合には、その含有量は0.950%以下がより好ましく、0.900%以下が更により好ましい。
・Mo:0.05~1.00%
Moは0.05%以上で強度調整(強度向上)の効果が得られる。一方、1.00%を超えるとコストアップを招く。このためMoを含有する場合には、その含有量は0.05~1.00%とすることが好ましい。Moを含有する場合には、その含有量は0.08%以上がより好ましい。Moを含有する場合には、その含有量は0.80%以下がより好ましい。
・Cu:0.05~1.00%
Cuは0.05%以上で残留γ相形成促進効果が得られる。一方、1.00%を超えるとコストアップを招く。このためCuを含有する場合には、その含有量は0.05~1.00%とすることが好ましい。Cuを含有する場合には、その含有量は0.08%以上がより好ましく、0.10%以上が更により好ましい。Cuを含有する場合には、その含有量は0.80%以下がより好ましく、0.60%以下が更により好ましい。
・Ni:0.05~1.00%
Niは0.05%以上で残留γ相形成促進効果が得られる。一方、1.00%を超えるとコストアップを招く。このためNiを含有する場合には、その含有量は0.05~1.00%とすることが好ましい。Niを含有する場合には、その含有量は0.10%以上がより好ましく、0.12%以上が更により好ましい。Niを含有する場合には、その含有量は0.80%以下がより好ましく、0.50%が更により好ましい。
・Sb:0.001~0.200%
Sbは鋼板表面の窒化、酸化、或いは酸化により生じる鋼板表面の数十ミクロン領域の脱炭を抑制する観点から含有させることができる。窒化や酸化を抑制することで鋼板表面においてマルテンサイトの生成量が減少するのを防止し、疲労特性や表面品質が改善する。このような効果は、0.001%以上で得られる。一方、0.200%を超えると靭性が劣化する。このためSbを含有する場合には、その含有量は0.001~0.200%とすることが好ましい。Sbを含有する場合には、その含有量は0.003%以上がより好ましく、0.005%以上が更により好ましい。Sbを含有する場合には、その含有量は0.100%以下がより好ましく、0.080%以下が更により好ましい。
・焼鈍工程(a)
焼鈍工程(a)の焼鈍条件に特別な制限はないが、最適な強度・延性バランス、特に上述した成分組成を有する高Mn添加鋼板を母材とするGA鋼板の強度・延性バランスを確保するために、鋼板のAc1点とAc3点に応じた鋼板温度(℃)を[Ac1+(Ac3-Ac1)/6]~950℃とするとともに、当該温度における保持時間を60~600秒とすることが好ましい。また、鋼板温度(℃)は[Ac1+(Ac3-Ac1)/6]~900℃とすることがより好ましい。鋼板温度(℃)は870℃以下とすることがさらにより好ましい。鋼板温度(℃)は650℃以上とすることが、より好ましく、670℃以上とすることがさらにより好ましい。
Ac1点(℃)=750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4Nb-169.4Al-894.7B
ここで、上記式中のC、Si、Mn、Cu、Ni、Cr、Mo、V、Ti、Nb、Al、Bは、鋼板中でのそれぞれの元素の含有量(質量%)である。
この反応により発生する水素は鋼中に残存しやすい。焼鈍雰囲気の露点が-30℃よりも大きいと、内部酸化により発生する水素量が多くなり、圧延工程(x)と加熱処理工程(y)を実施しても鋼板中の拡散性水素量を十分低減することができなくなるおそれがある。一方、露点を-60℃未満にしても、露点を制御することによる効果は飽和するので、却って経済性を損なう。
なお、その他の領域での雰囲気は任意であり、非酸化性の雰囲気であればよい。
・めっき処理工程(b)
めっき処理工程(b)では、焼鈍工程(a)で焼鈍後、所定温度まで冷却された鋼板を溶融亜鉛めっき浴に浸漬して溶融亜鉛めっき処理を施す。溶融亜鉛めっき浴を出ためっき鋼板に対して、通常、ガスワイピングなどによりめっき目付量の調整がなされる。めっき処理条件に特別な制限はないが、めっき付着量(片面当たりの付着量)は、耐食性およびめっき付着量制御上の観点から20g/m2以上とすることが好ましく、また、密着性の観点から120g/m2以下とすることが好ましい。めっき付着量は25g/m2以上とすることがより好ましく、30g/m2以上とすることがさらにより好ましい。めっき付着量は100g/m2以下とすることがより好ましく、70g/m2以下とすることがさらにより好ましい。
・合金化処理工程(c)
合金化処理工程(c)では、めっき処理工程(b)を経た鋼板を加熱し、溶融亜鉛めっき層を合金化処理する。合金化処理条件に特別な制限はないが、合金化処理温度(鋼板最高到達温度)は460~650℃が望ましく、480~570℃がより好ましい。合金化処理温度が460℃未満では合金化反応の速度が遅くなり、めっき層の所望のFe濃度が得られなくなるおそれがあり、一方、650℃を超えると、過合金により地鉄界面に硬くて脆いZn-Fe合金層が厚く生成してめっき密着性が劣化するおそれがあるとともに、残留オーステナイト相が分解することにより強度・延性バランスも低下してしまうおそれがある。合金化処理温度(鋼板最高到達温度)は550℃以下とすることがさらにより好ましい。合金化処理温度(鋼板最高到達温度)は490℃以上とすることがさらにより好ましい。
1180≦TS<1470の場合、λ≧20%
1470≦TSの場合、λ≧15%
本発明のGA鋼板が有するめっき層(合金化溶融亜鉛めっき層)は、合金化処理によるFe濃度が8~16質量%であるが、従来のGA鋼板と同じく、Zn以外のめっき成分として、例えば、Al、Mg、Siなどの1種以上を適量含有する(残部はZn及び不可避不純物)ことができる。さらに、Pb、Sb、Fe、Mg、Mn、Ni、Ca、Ti、V、Cr、Co、Sn等の1種以上が含有される場合がある。
・引張強度(TS)および全伸び(EL)の測定
引張強度(TS)、全伸び(EL)は引張試験により測定した。この引張試験は、引張方向が鋼板の圧延方向と直角方向となるようにサンプルを採取したJIS5号試験片を用いて、JIS Z2241(2011)に準拠して行い、引張強度(TS)、全伸び(EL)を測定した。ここで、高強度・高延性GA鋼板としては、TS≧980MPa以上かつ引張強度(TS)×全伸び(EL)が16000MPa・%以上が“好ましい特性”であるといえる。
・限界穴拡げ率(λ)の測定
限界穴拡げ率(λ)は、穴拡げ試験により測定した。この穴拡げ試験は、JIS Z2256(2010)に準拠して行った。GA鋼板を100mm×100mmのサイズに切断して供試体とし、この供試体にクリアランス12%±1%で直径10mmの穴を打ち抜いた後、内径75mmのダイスを用いてしわ押さえ力9ton(88.26kN)で抑えた状態で、60°円錐のポンチを穴に押し込んで亀裂発生限界における穴直径を測定した。ポンチの押し込み速度は10mm/minとした。下記の式から限界穴拡げ率を求め、この限界穴広げ率から穴拡げ性を評価した。
但し、Df:亀裂発生時の穴径(mm)
D0:初期穴径(mm)
ここで、高強度・高延性GA鋼板としては、限界穴拡げ率(λ)が以下の場合が“好ましい特性”であるといえる。
1180≦TS<1470の場合、λ≧20%
・めっき付着量およびめっき層のFe濃度の測定
鉄に対する腐食抑制剤(朝日化学工業(株)製「イビット」(登録商標))を添加した10質量%塩酸中に供試体(GA鋼板)を浸漬し、めっき層を溶解させた。溶解に伴う供試体の質量減少量を測定し、その値を鋼板の表面積で規格化した値をめっき付着量(g/m2)とした。また、ICP発光分光分析法を使用して塩酸に溶解したZn、Feの量を測定し、{Fe溶解量/(Fe溶解量+Zn溶解量)}×100をめっき層のFe濃度(質量%)とした。
・「鋼板中に存在する水素のうち、鋼板を200℃まで昇温した際に放出される水素量」の測定
GA鋼板の試験片の表裏のめっき層を、リューターを用いて物理的に削って除去した。この際の鋼板の研削量は板厚の5%以下とした。めっき層の除去後、試験片中の水素量をガスクロマトグラフィーによる昇温分析により測定した。この分析における試験片の昇温時到達温度を200℃とし、昇温速度は200℃/hrとした。このようにして測定された水素量を鋼板の質量で除した値を、「鋼板中に存在する水素のうち、鋼板を200℃まで昇温した際に放出される水素量(質量ppm)」とした。
・めっき外観の評価
GA鋼板のめっき外観を以下のように評価した。
△:5箇所すべてにおいて不めっきが認められないが、1箇所以上で模様が認められる(良好)
×:1箇所以上で不めっきが認められる(不良)
・GA鋼板の亀裂の確認
GA鋼板の亀裂の確認は以下のように行った。走査型電子顕微鏡(SEM)でGA表面を観察し、領域内に存在する亀裂の長さを測定して、観察領域の面積で割った数値を計算した。これを任意の領域10か所で行い、その平均値をLとした。さらに、亀裂の方向が圧延方向に対して80~100°の範囲にあるものを、圧延方向に対して直角に進展した亀裂として、その長さを測定し全体の亀裂に対する割合を計算した。この割合が60%超のものを不良(×)、60%以下のものを良好(○)とした。Lが0.010μm/μm2未満または0.070μm/μm2以上のものについては、亀裂割合の計算は行わなかった。
・耐パウダリング性の測定
GA鋼板の耐パウダリング性は以下のように測定した。GA鋼板にセロテープ(登録商標)を貼り、テープ面に90度曲げ、曲げ戻しを施し、テープを剥がす。剥がしたテープに付着した鋼板から剥離しためっきの量を、蛍光X線によるZnカウント数として測定し、下記基準に照らしてランク2以下のものを特に良好(○)、ランク3のものを良好(△)、4以上のものを不良(×)と評価し、ランク3以下を合格とした。またFe濃度が8質量%未満の鋼板については、耐パウダリング試験は行わなかった。
蛍光X線カウント数 ランク
0以上2000未満 :1 (良)
2000以上5000未満 :
5000以上8000未満 :
8000以上12000未満:
12000以上 :5 (劣)
・耐遅れ破壊性の評価
GA鋼板の耐遅れ破壊性を以下のようにして評価した。予備加工で得られた試験片に研削加工を施して30mm×100mmの二次試験片を得た。この二次試験片を曲率半径10mmRで180°曲げ加工し、板間を12mm絞め込み、遅れ破壊評価用試験片とした。この遅れ破壊評価用試験片を、pH1とpH3の塩酸水溶液中にそれぞれ浸漬し、96時間後の割れの発生の有無を調査した。本試験は、各鋼板3検体ずつ実施し、1検体でも割れが発生した場合は、割れ発生とした。この試験結果を以下のように評価した。
○:pH1の塩酸水溶液による試験では割れ発生。pH3の塩酸水溶液による試験では割れ発生無し(良好)
×:pH1の塩酸水溶液による試験とpH3の塩酸水溶液による試験のいずれでも割れ発生(不良)
以上の測定・評価結果を製造条件とともに表2~表5に示す。
Claims (11)
- 高強度鋼板を母材とする合金化溶融亜鉛めっき鋼板の製造方法であって、
Fe濃度が8~17質量%のめっき層を有する合金化溶融亜鉛めっき鋼板を圧延する圧延工程(x)と、
該圧延工程(x)を経ためっき鋼板を下記(1)式および(2)式を満たす条件で加熱する加熱処理工程(y)を有する高強度合金化溶融亜鉛めっき鋼板の製造方法。
(273+T)×(20+2× log10(t))≧8000 ・・・(1)
40≦T≦160 ・・・(2)
但し、T:めっき鋼板の加熱温度(℃)
t:加熱温度Tでの保持時間(hr) - 前記圧延工程(x)の前に、
鋼板の焼鈍工程(a)と、
該焼鈍工程(a)を経た鋼板に溶融亜鉛めっきを施すめっき処理工程(b)と、
該めっき処理工程(b)で得られためっき層に合金化処理を施し、前記Fe濃度が8~17質量%のめっき層とする合金化処理工程(c)とを有する請求項1に記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。 - 前記圧延工程(x)では、めっき鋼板を圧下率0.10~1%で軽圧下圧延する請求項1または2に記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。
- 前記鋼板が、質量%で、
C:0.03~0.35%、
Si:0.01~2.00%、
Mn:2.0~10.0%、
Al:0.001~1.000%、
P:0.10%以下、
S:0.01%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を有するとともに、
引張強度が980MPa以上、引張強度(TS)と全伸び(EL)の積(TS×EL)が16000MPa・%以上であり、めっき層のめっき付着量が片面当たり20~120g/m2である請求項1~3のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。 - 前記鋼板が、さらに、質量%で、
B:0.001~0.005%、
Nb:0.005~0.050%、
Ti:0.005~0.080%、
Cr:0.001~1.000%、
Mo:0.05~1.00%、
Cu:0.05~1.00%、
Ni:0.05~1.00%、
Sb:0.001~0.200%の中から選ばれる1種以上を含有する請求項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。 - 前記焼鈍工程(a)では、鋼板のAc1点およびAc3点に応じて鋼板温度(℃)を[Ac1+(Ac3-Ac1)/6]~950℃とし、当該温度における保持時間を60~600秒とし、
合金化処理工程(c)では、合金化処理温度を460~650℃とする請求項2~5のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。 - 前記焼鈍工程(a)では、鋼板温度が600~900℃の領域をH2濃度が3~20vol%、露点が-60℃~-30℃の雰囲気とする請求項2~6のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板の製造方法。
- 高強度鋼板を母材とする合金化溶融亜鉛めっき鋼板であって、
めっき層のFe濃度が8~17質量%であり、鋼板中に存在する水素のうち、鋼板を200℃まで昇温した際に放出される水素量が0.35質量ppm以下である高強度合金化溶融亜鉛めっき鋼板。 - 前記鋼板が、質量%で、
C:0.03~0.35%、
Si:0.01~2.00%、
Mn:2.0~10.0%、
Al:0.001~1.000%、
P:0.10%以下、
S:0.01%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を有するとともに、
引張強度が980MPa以上、引張強度(TS)と全伸び(EL)の積(TS×EL)が16000MPa・%以上であり、めっき層のめっき付着量が片面当たり20~120g/m2である請求項8に記載の高強度合金化溶融亜鉛めっき鋼板。 - 前記鋼板が、さらに、質量%で、
B:0.001~0.005%、
Nb:0.005~0.050%、
Ti:0.005~0.080%、
Cr:0.001~1.000%、
Mo:0.05~1.00%、
Cu:0.05~1.00%、
Ni:0.05~1.00%、
Sb:0.001~0.200%の中から選ばれる1種以上を含有する請求項9に記載の高強度合金化溶融亜鉛めっき鋼板。 - 前記高強度合金化溶融亜鉛めっき鋼板において、鋼板表面のめっき層に入った微細な亀裂の単位面積当たりの長さの平均値(L)が0.010μm/μm2以上0.070μm/μm2以下であり、このうち、圧延方向に対して略直角方向に延びる亀裂の長さの割合が亀裂の長さ全体の60%以下である請求項8~10のいずれかに記載の高強度合金化溶融亜鉛めっき鋼板。
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JP7131719B1 (ja) * | 2020-10-27 | 2022-09-06 | Jfeスチール株式会社 | 熱間プレス部材および熱間プレス用鋼板ならびにそれらの製造方法 |
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KR102490152B1 (ko) | 2023-01-18 |
EP3778980A4 (en) | 2021-02-17 |
US11643702B2 (en) | 2023-05-09 |
US11597983B2 (en) | 2023-03-07 |
JPWO2019189067A1 (ja) | 2020-08-20 |
EP3778980A1 (en) | 2021-02-17 |
JP6962452B2 (ja) | 2021-11-05 |
US20210010100A1 (en) | 2021-01-14 |
US20220017986A1 (en) | 2022-01-20 |
CN111936659A (zh) | 2020-11-13 |
KR20200127216A (ko) | 2020-11-10 |
CN111936659B (zh) | 2022-12-27 |
MX2020010068A (es) | 2020-10-28 |
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