JP2010065314A - High-strength hot-dip-galvanized steel sheet and production method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high-strength hot-dip-galvanized steel sheet which is excellent in plating property without going through a step of forming iron oxide on a surface of the steel sheet. <P>SOLUTION: The high-strength hot-dip-galvanized steel sheet has a plating layer (including those subjected to alloying treatment after hot-dip plating) formed on the surface of a steel sheet having a composition comprising 0.01-2.0 mass% C, 0.2-3.0 mass% Mn, 0.10-1.0 mass% Cr, 0.01-5.0 mass% Al, &le;0.2 mass% P, &le;0.02 mass% S and the balance being Fe and unavoidable impurities, formed by conducting hot-dip-plating in a hot-dip-galvanizing bath containing Al concentration of &ge;0.001 mass%. The hot-dip-galvanized steel sheet has parts in which Al, Mn and Cr concentrations are at least three times larger than those in the ground steel sheet, respectively, in a region in the plating layer within 1 &mu;m from the interface between the plating and the ground steel sheet and/or in a region in the ground steel sheet within 1 &mu;m from the interface between the plating and the ground steel sheet. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a hot-dip galvanized steel sheet using a high-strength steel sheet containing Mn and Cr as a base material and a method for producing the same.

  In recent years, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that can be manufactured at low cost and have excellent anti-rust properties have been used as surface-treated steel sheets that have been imparted with rust prevention properties in the fields of automobiles, home appliances, and building materials ing.

  In general, hot-dip galvanized steel sheets use thin steel sheets obtained by hot-rolling, cold-rolling or heat-treating slabs as the base material, and after cleaning the surface of the base steel sheet by degreasing and / or pickling in the pretreatment step Alternatively, the pretreatment step is omitted and the oil component on the surface of the base steel plate is burned and removed in the preheating furnace, and then annealed by heating at a temperature of about 700 to 900 ° C. in a reducing atmosphere, and the reduction treatment is performed. . Thereafter, in a molten zinc bath in which a small amount of Al (about 0.1 to 0.2% by mass) is added without cooling to a temperature suitable for plating in a non-oxidizing atmosphere or a reducing atmosphere without touching the air. It is manufactured by dipping in The alloyed hot-dip galvanized steel sheet is manufactured by subsequently heat-treating the hot-dip galvanized steel sheet in an alloying furnace and forming a plated layer into an Fe—Zn alloy.

  By the way, in recent years, steel sheets used in the fields of automobiles, home appliances, building materials, etc. have been promoted to improve performance and light weight, and there has been a demand for higher strength of steel sheets. The amount of steel sheets and high-strength galvannealed steel sheets is increasing. Addition of solid solution strengthening elements such as Mn, Si, P, and Al is performed to increase the strength of the steel sheet. However, high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets that use Mn-containing high-strength steel sheets as base materials have the following problems.

  As described above, the hot-dip galvanized steel sheet is subjected to hot-dip galvanization after being heat-annealed at a temperature of about 700 to 900 ° C. in a reducing atmosphere. However, Mn in steel is an easily oxidizable element, and is selectively surface oxidized in a generally used reducing atmosphere to be concentrated on the surface to form an oxide. Since the oxide of Mn reduces the wettability with molten zinc at the time of the plating process and causes non-plating, the wettability rapidly decreases with the increase of the Mn concentration in the steel, and non-plating frequently occurs. Moreover, even if it does not lead to non-plating, there exists a problem that it is inferior to plating adhesion.

  Further, when Mn in steel is selectively surface oxidized and concentrated on the surface, a significant alloying delay occurs in the alloying treatment step after hot dip galvanizing. As a result, there is a problem that significantly impedes productivity, and there is a problem that causes deterioration of powdering resistance when alloying at an excessively high temperature in order to ensure productivity. It is difficult to achieve both productivity and good powdering resistance.

  For such problems, it is known that wettability with molten zinc is improved by performing reduction annealing by heating the steel sheet in an oxidizing atmosphere in advance to form iron oxide on the surface and then heating. (See Patent Documents 1 and 2).

Further, in Patent Document 3, sulfur or a sulfur compound is deposited as an S amount in an amount of 0.1 to 1000 mg / m ² prior to hot dipping, and then a preheating step is performed in a weakly oxidizing atmosphere, followed by non-oxidation containing hydrogen. A method of improving plating adhesion and powdering resistance by annealing in a neutral atmosphere is disclosed.

  However, in the case of a steel sheet having a high Mn concentration in steel, the methods described in Patent Documents 1 and 2 tend to generate a Γ phase in the plating layer, resulting in deterioration of powdering resistance. Moreover, when the amount of oxidation increases, the oxide of the steel sheet adheres to the roll in the annealing furnace, which causes defects such as pressing irons, and the productivity decreases due to the removal of the oxide attached to the roll. appear. Furthermore, since it is difficult to control the amount of oxidation, unevenness of oxidation occurs, and the appearance after plating is impaired.

In the method described in Patent Document 3, the amount of oxidation is excessively large, and it is difficult to control the amount of oxidation, so that uneven oxidation occurs and the appearance after plating is often impaired.
Japanese Patent No. 2587724 JP 2002-220737 A Japanese Patent Laid-Open No. 11-50223

  The present invention has been made in view of such circumstances, and uses a high Mn content steel sheet as a base material, and without undergoing a step of forming iron oxide on the steel sheet surface as described in Patent Documents 1 to 3, hot dip plating. Plated steel sheet without post-alloying treatment has a beautiful surface appearance without unplating and excellent plating adhesion, and plated steel sheet that has been alloyed after hot dipping has a beautiful surface appearance without unplating and excellent resistance to plating. An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having powdering properties and excellent plating properties, and a method for producing the same.

The present inventors have found the following matters.
Diffusion to the steel sheet surface accompanying the selective oxidation of Mn in the steel that occurs during annealing heating is inhibited by adding Cr to the steel. In steel sheets containing appropriate amounts of Mn and Cr in steel, many oxides containing Mn and Cr formed by surface concentration during annealing are formed. An oxide containing Mn and Cr (a composite oxide of Mn and Cr) is more easily reduced by Al in the bath than an oxide containing only Mn (Mn _W O _V ), and its particle size is small. The included oxide is easily reduced, so that the reactivity between the steel sheet and Al in the bath is improved. As a result, a steel sheet containing appropriate amounts of Mn and Cr in the steel has improved wettability between the steel sheet surface and molten zinc, and a good plating appearance can be obtained. Further, when alloying is performed after hot dip galvanizing, the alloying reaction proceeds easily, and the powdering resistance can be improved.

  In addition, from the investigation results on the state of oxide in the plating layer and the surface layer of the underlying steel plate, one of the region from the plating-underlying steel plate interface of the plating layer to 1 μm and the region of the underlying steel plate from the plating-underlying steel plate interface to 1 μm Or, in both regions, if there are portions where the respective concentrations of Al, Mn and Cr are 3 times or more of the respective concentrations of Al, Mn and Cr in the underlying steel plate, good plating appearance is obtained when hot dip galvanizing is performed. Plating adhesion can be obtained, and a good plating appearance and powdering resistance can be obtained when alloying after hot dip galvanization.

  The present invention was made by further studying this finding. Means of the present invention for solving the above-mentioned problems are as follows.

  (1) C: 0.01-2.0 mass%, Mn: 0.2-3.0 mass%, Cr: 0.10-1.0 mass%, Al: 0.01-5.0 mass% , P: 0.2 mass% or less, S: 0.02 mass% or less, and the Al concentration in the bath is 0.001 mass% or more on the surface of the steel sheet having a composition composed of the remaining Fe and inevitable impurities. A high-strength hot-dip galvanized steel sheet having a plated layer formed by hot-dip galvanizing in a hot-dip galvanizing bath or a plated layer that has been alloyed after hot dip plating, and a region from the plating-underlying steel plate interface to 1 μm In each or both of the regions from the plating of the base steel plate to the base steel plate interface to 1 μm, the concentrations of Al, Mn, and Cr are three times the concentrations of Al, Mn, and Cr in the base steel plate, respectively. It is characterized by the existence of the above part High-strength hot-dip galvanized steel sheet with excellent plating properties.

(2) After the annealing step, the steel sheet having the composition described in (1) is subjected to hot dip galvanization in a hot dip galvanizing bath having an Al concentration in the bath of 0.001% by mass or more, or further subjected to alloying treatment to obtain hot dip zinc. When manufacturing a galvanized steel sheet, the annealing step is performed in a H ₂ —N ₂ gas atmosphere, which is a reducing atmosphere in which the dew point of the furnace atmosphere is −60 ° C. or more and 0 ° C. or less and H ₂ : 1.0 vol% or more. Then, the relative flow rate with respect to the steel plate in the atmosphere is set to 300 mpm or more and 1000 mpm or less, and the steel plate is heated so that the heating time is 1 second or more and 100 seconds or less in the temperature range of 600 ° C. or more and 700 ° C. or less. A method for producing a high-strength hot-dip galvanized steel sheet having excellent plating properties, characterized in that the heating time is 5 seconds or longer in a temperature range of 900 ° C. or lower.

  According to the present invention, a high-Mn steel plate is used as a base material, and a plated steel plate that is not alloyed after hot dip plating has a beautiful surface appearance with no plating and excellent plating adhesion, and is an alloy after hot dip plating. The high-strength galvanized plating with excellent surface appearance, excellent powdering resistance, and excellent strength-elongation balance while ensuring high productivity, while the plated steel sheet treated with heat treatment A steel plate can be obtained.

  In addition, since no oxidation treatment is required to form iron oxide on the steel sheet surface before reduction annealing, iron oxide formed on the steel sheet surface adheres to the roll in the annealing furnace and generates defects such as pushing iron. There is no problem of reducing productivity or productivity.

  The present invention will be described in detail below. The unit of the chemical composition of the steel sheet and the content of each element of the plating layer is “mass%”, but is simply “%” unless otherwise specified. Further, “%” of the atmospheric gas component in the furnace is “vol%”, but is simply “%” unless otherwise specified.

  First, chemical components of the steel plate will be described.

C: 0.01 to 2.0%
C is an element effective for increasing the strength of the steel sheet, and further effective for generating retained austenite and a low-temperature transformation phase, and is an element effective for ensuring improvement in TS × El. However, if the C content is less than 0.01%, it is difficult to obtain desired mechanical properties (TS × El). On the other hand, if it exceeds 2.0%, the weldability is deteriorated. From the above, C is limited to a range of 0.01% to 2.0%.

Mn: 0.2 to 3.0%
Mn strengthens the steel by solid solution strengthening, improves the hardenability of the steel, further promotes the formation of retained austenite and low-temperature transformation phase, and is an effective element for obtaining a good material. is there. Such an effect is recognized when the Mn content is 0.2% or more. On the other hand, if the content exceeds 3.0%, the effect is saturated, and an effect commensurate with the content cannot be expected, leading to an increase in cost. From the above, Mn is limited to the range of 0.2% to 3.0%.

Cr: 0.10 to 1.0%
When Cr is not added, the oxide of Mn has poor wettability with molten zinc, so that non-plating defects occur. However, by adding Cr, an oxide containing both Mn and Cr can be formed on the surface of the steel sheet after annealing, and wettability with molten zinc can be improved. Such an effect is recognized when the Cr content is 0.10% or more. On the other hand, when the content exceeds 1.0%, the amount of concentration due to selective oxidation of Cr increases, so that wettability with molten zinc is deteriorated and plating properties are inhibited. From the above, Cr is limited to the range of 0.10% to 1.0%. If the Cr content is 0.4% or more, a composite oxide of Mn and Cr can be formed more easily, so that the effect of improving the plating property and preventing the occurrence of defects such as non-plating is improved. However, if it exceeds 0.7%, this effect is saturated and the cost is increased. Therefore, the Cr content is more preferably in the range of 0.4% to 0.7%.

It is preferable that Mn and Cr in the steel satisfy the following formula (1).
0.5 ≦ Mn / Cr ≦ 0.5 + 1.8 / Cr (1)
However, Mn is Mn content (%), Cr is Cr content (%).
This is because it is more desirable to form spinel type MnCr ₂ O ₄ as a complex oxide of Mn and Cr. This is because Al can also take the form of a spinel oxide (MnAl ₂ O ₄ ), so that the reduction effect by Al is greater and the wettability improvement effect with molten zinc is greater. When Mn / Cr is less than 0.5, the amount of spinel formation decreases and Cr ₂ O ₃ increases. On the other hand, when it exceeds 0.5 + 1.8 / Cr, the amount of spinel formation similarly decreases and MnO increases.

Al: 0.01-5.0%
Al, like Si, suppresses the formation of carbides and promotes the formation of residual austenite phase, and is an effective element for obtaining a good material. Such an effect is recognized when the content is 0.01% or more. On the other hand, the content exceeding 5.0% increases the amount of inclusions in the steel and lowers the ductility. From the above, Al is limited to the range of 0.01% to 5.0%.

  Other additive elements do not impede the effect of the present invention and are not particularly limited. However, as necessary, Si, Ti, Nb, Mo, B, N, V, Zr, Cu, Ni, W By adding an appropriate amount of one or more of these elements, mechanical characteristics and plating characteristics can be improved.

Si: 0.01-3.0%
Si strengthens steel by solid solution strengthening, suppresses the formation of carbides, stabilizes austenite, promotes the formation of residual austenite phase, and is effective for expressing the effect of improving elongation. It is an element. Such an effect is recognized when the Si content is 0.01% or more. On the other hand, if the content exceeds 3.0%, Si is selectively oxidized on the surface of the steel sheet during the pre-plating annealing, so that the plating property is significantly deteriorated. From the above, Si is preferably in the range of 0.01% to 3.0%.

One or two or more of V: 0.001 to 0.1%, Nb: 0.001 to 0.1%, Ti: 0.001 to 0.1%, N: 0.00001 to 0.1% It is an element that forms carbonitride and has the effect of increasing the strength of steel by the precipitation effect, and can be added as necessary. Such an effect is recognized by containing V, Nb, and Ti of 0.001% or more and N of 0.1 ppm or more. On the other hand, when V, Nb and Ti are contained in excess of 0.1% and N is contained in excess of 1000 ppm, the strength is excessively increased and ductility is deteriorated. From the above, when contained, V is 0.001% to 0.1%, Nb is 0.001% to 0.1%, Ti is 0.001% to 0.1%, and N is 0.00. It is preferably 00001% or more and 0.1% or less.

Mo: 0.01 to 1.0%, B: 0.001 to 0.01%, one or more elements An element that has the effect of improving the hardenability of the steel and promoting the formation of a low-temperature transformation phase. is there. Such an effect is recognized by containing Mo: 0.01% or more and B: 0.001% or more. On the other hand, even if the content exceeds Mo: 1.0% and B: 0.01%, the effect is saturated, an effect commensurate with the content cannot be expected, and this is economically disadvantageous. From the above, when contained, Mo is preferably 0.01% or more and 1.0% or less, and B is preferably 0.001% or more and 0.01% or less.

One or more of Cu: 0.01 to 2.0%, Ni: 0.01 to 2.0%, W: 0.001 to 0.1%, Zr: 0.001 to 0.1 Si By adding together with Mn, there is an effect of suppressing the formation of the Γ phase and improving the plating adhesion. Such an action is recognized by containing Cu 0.01% or more, Ni 0.01% or more, and W, Zr 0.001% or more. On the other hand, even if Cu is contained in 2.0%, Ni is contained in 2.0%, and W and Zr are contained in excess of 0.1%, the effect is saturated, and an effect corresponding to the content cannot be expected, which is economically disadvantageous. . Accordingly, when contained, Cu is preferably 0.01% to 2.0%, Ni is 0.01% to 2.0%, and W and Zr are preferably 0.001% to 0.1%.

  In addition, the steel plate used for this invention consists of remainder Fe and an unavoidable impurity except the above-mentioned chemical component. As an unavoidable impurity, P is an element that can increase the strength without degrading the deep drawability. However, excessive addition delays alloying or degrades secondary work brittleness. Limited to 0.2% or less. S is an inclusion such as MnS, which causes deterioration of impact resistance and cracks along the metal flow of the weld, so it is better to be as low as possible. To do.

  In the present invention, each concentration of Al, Mn, and Cr is in one or both of the region from the plating-underlying steel plate interface of the plating layer to 1 μm and the region of the undercoating steel plating-underlying steel plate interface to 1 μm. In addition, it is defined that there are portions that are at least three times the respective concentrations of Al, Mn, and Cr in the base steel plate.

  Since the oxide containing Mn and Cr formed on the steel sheet surface during annealing reacts with Al in the bath during hot dip galvanization, the steel sheet surface is wetted with molten zinc compared to the case where the steel sheet surface is an oxide of Mn alone. Improves. Oxide present on the steel sheet surface during annealing reacts with Al during immersion in hot dip galvanizing and diffuses in the molten galvanized layer or locally reacts with Zn and diffuses in the underlying steel sheet in the molten state. There is a possibility, but the diffusion distance is at most 1 μm. Concentrated Al is often present as an oxide when Al in the bath reduces the Mn-Cr composite oxide, but Al in the steel concentrates to form a composite oxide with Mn and Cr. Some of them are due to this. Then, after hot dip galvanization or hot dip galvanization and alloying treatment, one or both of the region of the plating layer from the plating-underlying steel plate interface to 1 μm and the region of the undercoating steel plate from the underlaying steel plate interface to 1 μm In the region, the plating property is improved when there is a portion where each concentration of Al, Mn and Cr is 3 times or more of the concentration of Al, Mn and Cr in the underlying steel plate, and otherwise, the plating property Turned out not to improve.

Region from base steel interface to 1 [mu] m, and the plating of the substrate steel sheet - - plating the plating layer in each region from the substrate steel sheet interface to 1 [mu] m, Al, the concentration of Mn and Cr, the beam diameter of 0.01 [mu] m ² Measure with an analytical instrument (EDX). In consideration of segregation of the concentrated portion, 20 points or more are measured. To that the beam diameter and 0.01 [mu] m ^2, when the beam diameter is 0.01 [mu] m ^2, more than accuracy of analysis the influence of compounds present in the periphery is lowered. And when one or more places which are 3 times or more the density | concentration of Al, Mn, and Cr in a base steel plate exist in any area | region, 3 of each density | concentration of Al, Mn, and Cr in a base steel plate It is determined that there is a part that is twice or more.

  Next, a manufacturing method will be described.

  The steel sheet having the above composition may be either a hot-rolled steel sheet that has been descaled in the pickling process after hot rolling or a cold-rolled steel sheet that has been cold-rolled. The manufacturing method of this steel plate is not limited and may be a conventional method.

  In the present invention, the steel sheet is annealed after normal (known) pretreatment such as degreasing and pickling. Although annealing may be performed after performing an oxidation treatment for forming iron oxide on the surface of the steel plate after the treatment, this oxidation treatment is not particularly required in the present invention.

Relative flow rate for steel plate in annealing furnace atmosphere: 300 mpm or more and 1000 mpm or less Increasing the relative flow rate for steel plate in annealing furnace increases the supply amount of atmospheric gas to the steel plate surface, and the oxygen potential at the steel plate-atmosphere interface is compared. Therefore, the diffusion of Mn to the steel sheet surface is suppressed, the outward diffusion of Cr is promoted, and the selective oxidation on the Cr surface is promoted. As a result, a complex oxide of Mn and Cr is easily formed on the steel sheet surface. Since the composite oxide of Mn and Cr is more easily reduced by Al in the bath than the oxide of Mn alone (MnwOv), Mn and Cr are more concentrated than the oxide of the Mn alone (MnwOv). The oxide containing both of these improves the reactivity with Al in the bath. As a result, when an oxide containing both Mn and Cr is formed, the wettability with molten zinc becomes better than when an oxide of Mn alone (MnwOv) is formed. When the relative flow rate exceeds 1000 mpm, this effect is saturated and disadvantageous economically. When the relative flow rate is less than 300 mpm, a complex oxide of Mn and Cr is formed on the surface of the steel sheet, but a large amount of a single oxide of Mn is also formed, so that the plating properties cannot be improved. From the above, the relative flow velocity is defined as 300 mpm or more and 1000 mpm or less.

  The relative flow velocity can be easily adjusted by blowing atmospheric gas in the direction opposite to the steel strip traveling direction or increasing the line speed. However, the method for realizing the relative flow velocity is not limited to the above method.

Furnace atmosphere: the dew point is H ₂ -N ₂ gas dew point is a reducing atmosphere containing -60 ° C. or higher 0 ℃ less and H ₂ 1.0% or more becomes 0 ℃ greater than would be steel is oxidized, reduced It is unsuitable for annealing treatment. If it is less than -60 degreeC, the effect of the reduction annealing of a steel plate will be saturated and it will be economically disadvantageous. The atmosphere is preferably an H ₂ —N ₂ gas system, and the H ₂ concentration is preferably 1.0 to 90%. If it is less than 1.0%, the reduction is insufficient even if the dew point is lowered, and if it exceeds 90%, it is economically disadvantageous.

The heating time in the temperature range where the steel sheet temperature is 600 ° C. or more and 700 ° C. or less is 1 second or more and 100 seconds or less. In 600 ° C. to 700 ° C., the recrystallization of the steel sheet structure is an insufficient temperature range. Diffusion of Mn into the steel sheet surface occurs. When the recrystallization of the steel sheet structure is insufficient, the crystal grain size is remarkably small and the number of grain boundaries serving as diffusion paths is very large, so that the surface concentration of Cr and Mn increases. As a result, the particle size (average of the major axis and the minor axis) of the oxide containing Cr or / and Mn present on the surface of the steel sheet after annealing becomes very large, and the plating properties deteriorate. Accordingly, the heating time in this temperature range is preferably as short as possible. If it exceeds 100 seconds, the plating property deteriorates. On the other hand, if it is less than 1 second, the concentration of Cr or Mn on the surface is small, and this effect is saturated. Therefore, it is necessary to set the heating time in the temperature range of 600 ° C. to 700 ° C. to 1 second to 100 seconds.

When the heating temperature for the reduction of the steel sheet surface and the recrystallization of the steel sheet structure is less than 700 ° C., the reduction of the steel sheet surface and the recrystallization of the steel sheet structure is performed at a temperature range of 700 ° C. to 900 ° C. In addition, the effect is saturated at 900 ° C. or higher, which is economically disadvantageous. Furthermore, if the heating time is less than 5 seconds, the reduction is insufficient, so that it is 5 seconds or more.

  Of the oxide containing granular Mn and / or Cr present on the surface of the steel sheet after annealing before hot dip galvanization, the ratio (number ratio) of the oxide having a particle size of 0.1 μm or less is 70% or more. It is preferable. This is because the effect of improving the wettability between the steel sheet surface and molten zinc is more excellent. The particle diameter is the average of the major axis and the minor axis.

By making the surface oxide spinel type MnCr ₂ O ₄ , the ratio of the oxide having a particle size of 0.1 μm or less can be made 70% or more. Spinel type MnCr ₂ O ₄ tends to have a very small particle size compared to MnO and Cr ₂ O ₃ . Specifically, Cr and Mn in steel can be satisfied by satisfying 0.4 ≦ Cr ≦ 0.7 and 0.5Cr ≦ Mn ≦ 0.5Cr + 1.8. When the amount of either Cr or Mn is small or large, the proportion of the spinel type in the formed surface oxide becomes small, so the proportion of oxide having a particle size of 0.1 μm or less is made 70% or more. I can't do that. Specifically, when Cr is very small, the proportion of MnO in the surface oxide increases, and when Mn is very small, the proportion of Cr ₂ O ₃ increases. From the above, it is desirable that Cr and Mn in steel satisfy the above.

The reason for this is not clear, but is presumed as follows. In general, when the particle diameter of an oxide containing Mn or / and Cr is increased, Al in the bath becomes difficult to come into contact with the exposed Fe exposed surface, the reactivity is lowered, and the wettability is deteriorated. By adding an appropriate amount of Cr, the ratio of the Mn-containing steel sheet surface after Mn-containing steel sheet after the pre-plating reduction annealing whose Mn and / or Cr-containing oxide particle size is 0.1 μm or less increases, and also contains Mn and Cr. The oxide is more easily reduced by Al in the bath than the oxide containing only Mn, and the reactivity with Al in the bath is improved. On the other hand, when Cr is not added and when Cr is added in excess of 1.0%, the particle size of the oxide on the surface may exceed 0.1 μm, and at this time, wettability with molten zinc deteriorates. Of the oxide containing Mn and / or Cr particulate observed in the region of 1 [mu] m ^2, if the proportion of the oxide particle size is 0.1μm or less is less than 70%, is deteriorated platability .

Ratio of particle size oxide is 0.1μm or less, the steel sheet surface after annealing, to observe the region of 1 [mu] m ² by electron microscopy, the composition of the oxide, to identify particle size, Mn and / or Cr The ratio of the oxide whose particle size is 0.1 μm or less among the oxides containing can be determined. In addition, a ratio is observed and measured about 10 places, and is the average value.

  After the reductive annealing treatment, it is cooled to a temperature suitable for plating in a non-oxidizing or reducing atmosphere, and immersed in a plating bath for plating.

  The hot dip galvanizing process may be performed according to a conventionally performed method. For example, the plating bath temperature is about 440 to 520 ° C., and the plating bath immersion temperature of the steel sheet is substantially equal to the plating bath temperature.

Al concentration in the bath: 0.001% or more The Al concentration in the galvanizing bath is preferably 0.001% or more. This is because the Al in the bath reacts with the composite oxide of Mn and Cr formed during annealing, so that the wettability between the composite oxide and molten zinc is improved. This effect is not manifested when the Al concentration is lower than 0.001%. Further, the Al concentration in the bath is generally 0.1 to 0.2%, but the upper limit is not particularly limited. Or, depending on the use of the plated steel sheet, the plating conditions such as the plating temperature and the plating bath composition may be changed. However, the difference in the plating conditions does not contribute to the effect of the present invention and is not particularly limited. . For example, even if elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn other than Al are mixed in the plating bath, the effect of the present invention is not changed.

The method of adjusting the plating thickness after plating is not particularly limited, but generally, gas wiping is used, and is adjusted by the gas pressure of gas wiping, the distance between the wiping nozzle / steel plate and the like. At this time, the thickness of the plating layer is not particularly limited, but is preferably ²⁰ to 150 g / m ² per side. If it is less than 20 g / m ² , sufficient rust prevention properties cannot be obtained. On the other hand, in the 150 g / ^{m 2} exceeds corrosion resistance is saturated, while the workability, so impair the economical efficiency is preferably at 150 g / ^{m 2} or less. However, the difference in the thickness of the plating layer does not hinder the effect of the present invention and is not particularly limited.

  In the present invention, an alloying treatment is performed as necessary after the hot dip galvanizing. As described above, according to the present invention, since an oxidation treatment for forming iron oxide on the steel sheet surface is not required, the Γ phase is not excessively formed, and as a result, an alloy having excellent powdering resistance. The hot-dip galvanized steel sheet can be manufactured without impairing productivity. The alloying treatment method may be any heating method conventionally used, such as gas heating, induction heating, and current heating, and is not particularly limited.

  The alloying treatment conditions are not particularly limited. For example, the alloying treatment plate temperature is generally about 460 to 600 ° C., and the alloying holding time is about 5 to 60 seconds. The difference in conditions does not hinder the effects of the present invention.

  According to the present invention, a plated steel sheet which uses a high Mn content steel sheet as a base material and is not alloyed after hot dipping has a beautiful surface appearance without unplating and excellent plating adhesion, and is alloyed after hot dipping. Treated plated steel sheet provides high strength alloyed hot dip galvanized steel sheet that has a beautiful surface appearance without plating and excellent powdering resistance while ensuring high productivity. be able to.

  In addition, since no oxidation treatment is required to form iron oxide on the steel sheet surface before reduction annealing, iron oxide formed on the steel sheet surface adheres to the roll in the annealing furnace and generates defects such as pushing iron. There is no problem of reducing productivity or productivity.

  In addition, the hot dip galvanized steel sheet targeted by the present invention is a plated steel sheet (so-called hot dip galvanized steel sheet) that has been hot dip galvanized in a Zn bath containing about 0.1 to 0.2% Al in the plating bath, A typical example is a plated steel sheet (so-called alloyed hot-dip galvanized steel sheet) that has been further alloyed after hot dip galvanizing, but not only this plated steel sheet but also a Zn bath having a lower or higher Al concentration in the bath. And a galvannealed steel sheet that has been subjected to an alloying treatment after the galvanizing.

  Hereinafter, the present invention will be specifically described based on examples.

(1) Preparation of test-plated steel sheet A steel slab having the steel composition shown in Table 1 and comprising the remainder Fe and inevitable impurities was heated in a heating furnace at 1260 ° C. for 60 minutes, and then hot-rolled to 2.8 mm. And wound up at 540 ° C. Thereafter, the black scale was removed by pickling and cold rolled to 1.6 mm.

  The cold-rolled steel sheet was annealed and then hot-dip galvanized using a CGL equipped with a pre-oxidation (DFF) type annealing furnace or a CGL equipped with an RTF type annealing furnace, and further subjected to alloying treatment.

The pre-oxidation (DFF) type annealing furnace includes a direct fired furnace (Direct Fired Furnace) and an RTF type reduction annealing furnace, and the direct fired heating furnace (Direct Fired Furnace) is composed of O ₂ , CO, and CO ₂ . It is possible to control the concentration in the furnace and the amount of Fe oxidation of the steel sheet.

The RTF type annealing furnace includes an RTF (Radiant Tube Furnace) type heating annealing furnace, and the atmosphere is N ₂ —H ₂ (H ₂ is generally 3 to 8%). is there.

In CGL equipped with a pre-oxidation (DFF) type annealing furnace, DFF heats up to 550-650 ° C. at an air ratio: 0.9, oxidizes the steel sheet surface, and heats at 800 ° C. in a reduction annealing furnace (RTF). After reduction annealing (H ₂ concentration in N ₂ —H ₂ atmosphere is 5%), hot dip galvanization was subsequently performed in an Al-containing Zn bath at 460 ° C. (Al concentration 0.14%). Some of them were hot dip galvanized by changing the Al concentration in the Zn bath. The plating adhesion amount was adjusted to ²⁰ to 150 g / m ² per side by gas wiping. The alloying treatment temperature was 540 ° C., and the treatment time was adjusted so that the Fe concentration was 10%.

In CGL equipped with an RTF type annealing furnace, reduction annealing was performed by changing the heating temperature of the RTF in various ways. After the reduction annealing, the conditions were the same as those in CGL equipped with the above-described DFF type annealing furnace. The RTF was an N ₂ —H ₂ atmosphere, and the H ₂ concentration of the atmosphere, the dew point, and the relative flow rate with respect to the steel plate of the atmosphere were changed. The dew point was adjusted by increasing or decreasing the high humidity N ₂ —H ₂ flow rate. The relative flow velocity was adjusted by increasing / decreasing the total gas flow rate into the furnace and increasing / decreasing the line speed. The relative flow rate was obtained by calculating the relative speed from the gas flow rate and the line speed.

(2) After annealing, the molten zinc in the plating pot was emptied, the plating pot was evacuated, the alloying furnace was also evacuated, and the steel sheet after annealing before hot dip galvanizing was collected. The steel plate surface is observed with an electron microscope, and the particle size (average of major axis and minor axis) is 0.1 μm or less among the granular oxides containing Mn and / or Cr observed in the region of 1 μm ^2. It is investigated whether the ratio of the oxide is 70% or more, and among the oxides containing granular Mn or / and Cr observed in the region of 1 μm ² , the particle diameter (average of major axis and minor axis) ) Is 0.1 μm or less, the case where the ratio of the oxide is 70% or more is indicated by “◯”, and the case where it is less than 70% is indicated by “X”. The ratio is an average value observed and measured at 10 locations.

(3) By observation with a cross-sectional electron microscope of the alloyed plated steel sheet (measuring instrument (EDX) with a beam diameter of 0.01 μm ² ), plating of the plating layer—plating of the region from the base steel sheet interface to 1 μm and the base steel sheet— Observe 20 areas of 0.01 μm ² in the area from the base steel plate interface to 1 μm, and each concentration of Al, Mn and Cr is more than 3 times the concentration of Al, Mn and Cr in the base steel plate, respectively. The case where one or more areas concentrated in the density of 1 was observed was marked as ◯, and the case where it was not observed was marked as x. The concentrations shown in Table 1 were used for the concentrations of Al, Mn, and Cr in the base steel plate.

(4) Evaluation of plating appearance The surface appearance of the plated steel sheet as hot dip galvanized was visually observed to evaluate the presence or absence of non-plating. Good if there is no unplating (◎), good in case of minor defects that are somewhat unplated but do not impair the surface quality (○), bad if there is unplating and the surface quality is inferior ( X). ○ and ◎ are acceptable. The surface appearance of the plated steel sheet after the alloying treatment was visually observed, and the presence or absence of appearance unevenness due to the alloying delay was evaluated. Those with no unevenness were judged good, and those with unevenness were judged bad.
Further, when both evaluations were good, it was judged that ◯: good appearance, and at least one of them was bad, x: judged as poor appearance.

(5) Evaluation of plating adhesion of a galvanized steel sheet as hot-dip galvanized A ball impact test was conducted to evaluate the plating peeling state when the tape was peeled off. The test conditions were as follows: a 2.8 kg weight was dropped from a height of 1 m onto a hot dip galvanized steel sheet placed on a hemispherical projection having a diameter of 1/2 inch, and then the tape was peeled off from the convex side, followed by plating. The following evaluation was made according to the degree of peeling. ○ and ◎ are acceptable.
A: No peeling of plating (good)
○: Slight peeling of plating (generally good)
×: Plating peeling (defect)

(6) Evaluation of powdering resistance of galvanized steel sheet treated with alloying Powdering resistance is applied by applying adhesive tape to the plated steel sheet and bending back 90 ° at a bending radius of 1.6 mm with the tape application surface inside. The amount of peeling per unit length was measured with a fluorescent X-ray, and the Zn count was measured with a fluorescent X-ray. ) 5 were evaluated as bad (x). ○ and ◎ are acceptable.
X-ray fluorescence count: Rank 0 to less than 500: 1 (good)
Less than 500-1000: 2
Less than 1000-3000: 3
Less than 3000-5000: 4
5000 or more: 5 (poor)

(7) Evaluation of corrosion resistance The alloyed hot-dip galvanized steel sheet with dimensions of 70 mm x 150 mm was subjected to a salt spray test based on JIS Z 2371 (2000) for 3 days, and the corrosion product was treated with chromic acid (concentration 200 g / L, 80 ° C). ) For 1 minute, and measured per weight (g / m ² · day) of plating corrosion weight loss before and after the test by the weight method, and evaluated according to the following criteria.
○ (Good): Less than 20 g / m ² · day x (Bad): 20 g / m ² · day or more

(8) Evaluation of mechanical properties Mechanical properties were evaluated by taking a JIS No. 5 tensile specimen from an alloyed hot-dip galvanized steel sheet and performing a tensile test to measure the tensile strength TS (MPa) and elongation El ( %), When the value of TS × El is 20000 MPa ·% or more, the mechanical strength is good (◯), indicating a good balance of strength and ductility, and the value of TS × El is less than 20000 MPa ·%. Was defined as a mechanical property defect (×). This is because if the value of TS × El is 20000 MPa ·% or more, it is considered that the strength / elongation balance is sufficient for automotive applications.

  Tables 2 to 7 show the composition, production conditions, and evaluation results of the test materials.

  As is apparent from Tables 2 to 7, the plated steel sheets of the examples produced by the method of the present invention were either hot-dip galvanized or alloyed after being alloyed despite containing high concentrations of Mn. Also, the plating appearance is good, and the plating adhesion and powdering resistance are good. In addition, the strength and ductility balance is excellent, and there is no problem that productivity is lowered.

  The high-strength hot-dip galvanized steel sheet of the present invention, when not alloyed after hot-dip plating, has a beautiful surface appearance without unplating and excellent plating adhesion, and was alloyed after hot-dip plating. In this case, while ensuring high productivity, it has a beautiful surface appearance without plating, excellent powdering resistance, and an excellent balance of strength and elongation. It can be suitably used as a steel plate used in fields such as automobiles, home appliances, and building materials.

  The method for producing a high-strength hot-dip galvanized steel sheet according to the present invention can be used as a method for producing the aforementioned high-strength hot-dip galvanized steel sheet while ensuring high productivity.

Claims (2)

C: 0.01-2.0 mass%, Mn: 0.2-3.0 mass%, Cr: 0.10-1.0 mass%, Al: 0.01-5.0 mass%, P: A molten zinc system containing 0.2% by mass or less and S: 0.02% by mass or less and having a composition comprising the balance Fe and unavoidable impurities and having an Al concentration in the bath of 0.001% by mass or more. A high-strength hot-dip galvanized steel sheet having a plated layer formed by hot-dip plating in a plating bath or a plated layer that has been alloyed after hot-plating, a region from the plating-underlying steel plate interface of the plated layer to 1 μm; and A portion in which each concentration of Al, Mn and Cr is 3 times or more of each concentration of Al, Mn and Cr in the base steel plate in one or both of the regions from the plating of the base steel plate to the base steel plate to 1 μm Characterized by the presence of A high-strength hot-dip galvanized steel sheet with excellent clarity. The steel sheet having the composition of claim 1 is subjected to a hot dip galvanizing bath having an Al concentration in the bath of 0.001% by mass or more after the annealing step, or further subjected to an alloying treatment to obtain a hot dip galvanized steel sheet. in making the annealing process, and the dew point of the furnace atmosphere is -60 ° C. or higher 0 ℃ less _H 2: in _H 2 -N ₂ gas atmosphere is a reducing atmosphere containing at least 1.0 vol%, the atmosphere The steel plate is heated so that the heating time is 1 second or more and 100 seconds or less in a temperature range of 600 ° C. or more and 700 ° C. or less, and 700 ° C. or more and 900 ° C. or less. A method for producing a high-strength hot-dip galvanized steel sheet having excellent plating properties, characterized in that the heating time is 5 seconds or more.