JP2011117069A - Method for manufacturing high-strength hot-dip galvanized steel sheet and high-strength hot-dip galvannealed steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet which use a high-strength steel sheet containing Si as its base metal and has no unplated part. <P>SOLUTION: A base steel sheet includes, by mass%, 0.05-0.30% C, 1.5-3.0% Si, 0.5-3.0% Mn, 0.01-3.0% Al, 0.001-0.01% S and 0.001-0.1% P. The manufacturing method includes: heating the base steel sheet in an atmosphere containing 0.01-20 vol.% O<SB>2</SB>and 1-50 vol.% H<SB>2</SB>O so as to keep a temperature in the range of 873-1,123 K; subsequently heating the steel sheet in an atmosphere containing 1-50 vol.% H<SB>2</SB>, and in such conditions that partial pressure of water vapor PH<SB>2</SB>O, partial pressure of carbon dioxide PCO<SB>2</SB>, the highest arrival temperature of the steel sheet T(K) and a Si content of the steel sheet [Si%] satisfy the relations of 0<PH<SB>2</SB>O/PCO<SB>2</SB><323.6-15.2logT<SP>2</SP>-71.5[Si%], 0<log(PH<SB>2</SB>O+15PCO<SB>2</SB>)<2.3 and 1,023≤T≤1,173; and then subjecting the steel sheet to hot-dip galvanizing treatment. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet manufacturing method using a Si-containing high-strength steel sheet as a base material, and in particular, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel having a beautiful surface appearance without unplating. The present invention relates to a method for producing a plated steel sheet.

  In recent years, in the fields of automobiles, home appliances, building materials, etc., surface-treated steel sheets imparted with rust resistance to raw steel sheets, particularly hot-dip galvanized steel sheets and galvannealed steel sheets excellent in rust resistance have been used.

  Generally, a hot dip galvanized steel sheet is manufactured by the following method. First, using a thin steel plate obtained by hot-rolling, cold-rolling or heat-treating a slab, the base steel plate surface is degreased and / or pickled and cleaned in the pretreatment step, or the pretreatment step is omitted. After the oil on the surface of the base steel plate is burned and removed, recrystallization annealing is performed by heating in a non-oxidizing atmosphere or a reducing atmosphere. Thereafter, the steel sheet is cooled to a temperature suitable for plating in a non-oxidizing atmosphere or a reducing atmosphere, and in a molten zinc bath to which a small amount of Al (about 0.1 to 0.2 mass%) is added without being exposed to the air. Immerse in.

  In addition, the alloyed hot-dip galvanized steel sheet is manufactured by subsequently heat-treating the steel sheet in an alloying furnace after hot-dip galvanizing.

  By the way, in recent years, weight reduction has been promoted along with higher performance of raw steel plates, and higher strength of raw steel plates has been demanded, and the amount of high-strength hot-dip galvanized steel plates having rust prevention properties is increasing.

  Addition of solid solution strengthening elements such as Si, Mn, P, and Al is performed to increase the strength of the steel sheet. Among these, Si and Al have an advantage that the strength can be increased without impairing the ductility of the steel, and the Si-containing steel plate is promising as a high-strength steel plate. However, when producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that use a high-strength steel sheet containing a large amount of Si as a base material, there are 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 873 to 1173K in a non-oxidizing atmosphere or a reducing atmosphere. However, Si in steel is an easily oxidizable element, and is selectively oxidized in a generally used non-oxidizing atmosphere or reducing atmosphere to concentrate on the surface to form an oxide. Since this oxide reduces the wettability with molten zinc during the plating process and causes non-plating, the wettability rapidly decreases as the Si concentration in the steel increases and non-plating occurs frequently. In addition, even when non-plating does not occur, there is a problem that the plating adhesion is poor.

  Further, when Si in the steel is selectively oxidized and concentrated on the surface, a significant alloying delay occurs in the alloying process after hot dip galvanizing. As a result, productivity is significantly inhibited. If an alloying treatment is attempted at an excessively high temperature in order to ensure productivity, there is a problem that the powdering resistance is deteriorated, and it is difficult to achieve both high productivity and good powdering resistance.

  In order to solve such a problem, a method has been proposed in which surface oxides are removed by performing pickling after annealing a steel sheet, and then annealing and hot-dip galvanizing are performed again (for example, Patent Document 1).

  In addition, it has been proposed to improve wettability with molten zinc by heating steel sheets in an oxidizing atmosphere in advance to form iron oxide on the surface and then performing reduction annealing (for example, Patent Document 2). .

  The technique described in Patent Document 1 performs annealing twice and suppresses the formation of surface concentrate during the second annealing by pickling and removing the surface concentrate of Si generated on the surface after the first annealing. It is what. However, when the Si concentration is high, pickling cannot completely remove the surface concentrate, so that the problem of the performance of the plating layer cannot be solved as described above. Furthermore, there is another problem that the pickling equipment for removing the surface concentrate of Si is newly required, which is costly.

  Furthermore, the technique described in Patent Document 2 attempts to suppress Si surface concentration during reduction annealing by heating in an oxidizing atmosphere in advance to form iron oxide on the steel sheet surface. However, as is generally known, since the oxidation rate on the steel sheet surface greatly decreases with an increase in the Si concentration in the steel, it is necessary to suppress the surface concentration of Si only by the description in Patent Document 2. It is difficult to obtain an adequate amount of iron oxide.

JP 2000-290730 A JP-A-4-202630

  The present invention provides a method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance without unplating using a Si-containing high-strength steel sheet as a base material, and alloying and melting having a beautiful surface appearance without unplating. It is an object to provide a method for producing a galvanized steel sheet.

  As described above, in the case of a steel sheet having a high Si concentration in steel, it is difficult to completely suppress non-plating by either removing the Si surface enrichment or suppressing the surface enrichment by oxidation.

  Here, for steel sheets with a high Si concentration in steel, it is considered that the Si surface enrichment suppression technology by oxidation treatment is more effective than the conventional technology, but the oxidation becomes difficult as the Si concentration in steel increases. I know that. Therefore, the inventors thought that the Si surface concentration could be suppressed with a small amount of oxidation, and investigated the steel sheet surface after annealing in detail. As a result, it was found that the reduced iron covered the entire steel sheet surface in the steel sheet having a low Si concentration in the steel, whereas the steel sheet having a high Si concentration in the steel had a region not covered by the reduced iron. Furthermore, this phenomenon becomes more prominent as the Si concentration in the steel increases. When the Si concentration in the steel exceeds 2%, reduced iron is present in the form of islands.

As a result of further investigation, when performing the first heating process for oxidizing the steel sheet surface and the second heating process for reducing iron oxide, the H ₂ O partial pressure in the reducing atmosphere of the second heating process is reduced. It turned out that the hot-dip galvanized steel plate without unplating is obtained by making the coverage of reduced iron high and making a coverage 60% or more. This is probably because the reduction reaction of iron oxide is promoted. The reduction reaction of iron oxide with H ₂ is represented by the following formula.

FeO _x + xH ₂ = Fe + xH ₂ O
Here, when the H ₂ O partial pressure is lowered, the above reaction proceeds to the right. Therefore, when the H ₂ O partial pressure in the atmosphere is low, the reduction reaction of iron oxide is promoted, and it is presumed that the reduction rate increases and the coverage of reduced iron is improved. It was also found that the upper limit value of the H ₂ O partial pressure at which the reduced iron coverage was 60% or more changed according to the Si concentration in the steel, and the higher the Si concentration, the lower the upper limit value. Moreover, although iron oxide is a small amount, it is also reduced by C in the steel to generate CO ₂ . In order to increase the reduction rate, it is necessary to reduce the CO ₂ partial pressure, but the reduction reaction by C involves decarburization, which is not desirable in terms of material. Therefore, when an experiment was performed by changing the H ₂ O partial pressure and the CO ₂ partial pressure, the decarburization was suppressed by controlling the partial pressure ratio and the partial pressure sum of H ₂ O and CO ₂ to appropriate values. However, it was found that the reduction rate of iron oxide can be increased.

FIG. 1 shows the relationship between the H ₂ O and CO ₂ partial pressure ratio (PH ₂ O / PCO ₂ ) in the second heating step, the Si concentration in the steel, and the reduced iron coverage on the steel sheet surface after the second heating step. It is an example shown. When the reduced iron coverage was 60% or more, the plating appearance was good, and when the reduced iron coverage was less than 60%, the plating appearance was poor.

Based on these findings, for steel sheets with different Si concentrations, the relationship between the heating conditions in the second heating step, the reduced iron coverage on the steel sheet surface after the second heating step, and the surface appearance after plating were investigated. H ₂ : 1-50 vol%, the balance being one or more of N ₂ , H ₂ O, O ₂ , CO, CO ₂ and unavoidable impurities, and water vapor content in the atmosphere Pressure: PH ₂ O (Pa), carbon dioxide partial pressure: PCO ₂ (Pa), maximum reached temperature of steel plate: T (K), Si content of steel plate: [Si%] (mass%) ) To (3), it was found that 60% or more of the steel sheet surface can be coated with reduced iron after the second heating step, and a good surface appearance can be obtained after the hot dip galvanizing treatment.
0 <PH ₂ O / PCO ₂ <323.6-15.2 logT ² -71.5 [Si%] (1)
0 <log (PH ₂ O + 15PCO ₂ ) <2.3 (2)
1023 ≦ T ≦ 1173 (3)
Further, the first heating step is divided into a first stage and a second stage, the first stage is a step of oxidizing the steel sheet, and the second stage is a step of reducing a part of the oxide generated in the first stage heating step. It was found that the effect of preventing the pickup by the in-furnace roll is more excellent.

  The present invention is based on the above findings. The gist of the present invention is as follows.

[1] As chemical components, in mass%, C: 0.05 to 0.30%, Si: 1.5 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to In performing hot-dip galvanizing on a steel sheet containing 3.0%, S: 0.001 to 0.01%, P: 0.001 to 0.1%, and the balance Fe and inevitable impurities, O ₂ : _0.01~20vol%, H 2 O: containing 1~50vol%, the balance being _N 2, CO, one or more, and the steel sheet in an atmosphere consisting of unavoidable impurities of CO ₂ 873~1123K first heating step of heating so that a temperature in the range, _then, H 2: containing 1～50Vol%, the balance being _{N 2,} H ₂ O, O 2, CO, 1 or 2 or of the CO ₂ above and in an atmosphere consisting of unavoidable impurities, water vapor partial pressure in the atmosphere: PH 2 O _(P ), Partial pressure of carbon _dioxide: PCO 2 (Pa), the maximum temperature of the steel sheet: T (K), Si content of steel: satisfying [Si%] (mass%) satisfies the following expressions (1) to (3) A surface appearance characterized by performing a second heating step that heats under conditions (however, the steel plate temperature in the second heating step is higher than the steel plate temperature in the first heating step), and then performing hot dip galvanizing treatment. A method for producing an excellent high-strength hot-dip galvanized steel sheet.
0 <PH ₂ O / PCO ₂ <323.6-15.2 logT ² -71.5 [Si%] (1)
0 <log (PH ₂ O + 15PCO ₂ ) <2.3 (2)
1023 ≦ T ≦ 1173 (3)
[2] In the first heating step, the first stage is such that the steel sheet is brought to a temperature in the range of 873 to 1023 K in an atmosphere containing O ₂ : 0.1 to ₂₀ vol% and H ₂ O: 1 to 50 vol%. In the subsequent stage, the steel sheet is heated to a temperature in the range of 973 to 1123K in an atmosphere containing O ₂ : 0.01 to less than 0.1 vol% and H ₂ O: 1 to 20 vol% or less. (However, the temperature of the subsequent steel sheet is higher than the temperature of the preceding steel sheet.) The method for producing a high-strength hot-dip galvanized steel sheet having excellent surface appearance according to the above [1].

  [3] In the first heating step, the first stage is heated by a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF) under the condition of an air ratio of 1 to 1.35, and the latter stage is a direct-fired furnace or a non-oxidizing furnace. The method for producing a high-strength hot-dip galvanized steel sheet having excellent surface appearance according to the above [2], wherein the heating is performed under the condition that the air ratio is less than 1.

  [4] The steel sheet further includes, as a chemical component, mass%, Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Ti: 0.01 to 0.1%, One or more selected from Nb: 0.01 to 0.1% and B: 0.0005 to 0.0050% are contained in any one of the above [1] to [3] The manufacturing method of the high intensity | strength hot-dip galvanized steel plate which is excellent in the surface appearance of description.

  [5] A high-strength alloyed and melted material with excellent surface appearance, wherein a high-strength hot-dip galvanized steel sheet is produced by the method described in any one of [1] to [4] above, and further alloyed. Manufacturing method of galvanized steel sheet.

  According to the production method of the present invention, even when a Si-containing high-strength steel sheet is used as a base material, a hot-dip galvanized steel sheet having a beautiful surface appearance without unplating and a beautiful surface appearance without unplating are provided. An alloyed hot-dip galvanized steel sheet is obtained.

An example showing the relationship between the H ₂ O and CO ₂ partial pressure ratio (PH ₂ O / PCO ₂ ) in the second heating step, the Si concentration in the steel, and the reduced iron coverage on the steel sheet surface after the second heating step. is there.

  Hereinafter, the present invention will be specifically described.

  First, the reason why the component composition of the steel sheet is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, the "%" display regarding a component shall mean mass%.

C: 0.05-0.30%
C is an element that stabilizes the austenite phase, and is an element necessary for increasing the strength of the steel sheet. If the C content is less than 0.05%, it is difficult to ensure the strength, and if the C content exceeds 0.30%, the weldability decreases. Therefore, the C content is in the range of 0.05 to 0.30%.

Si: 1.5-3.0%
Si has the effect of improving the formability of the steel sheet by concentrating the solid solution C in the ferrite phase in the austenite phase and increasing the temper softening resistance of the steel. The problem that reduced iron is distributed in islands after reduction annealing and lowers the plating quality becomes apparent when the Si content is 1.5% or more. On the other hand, Si has an effect of suppressing oxidation of the steel sheet, and if the content exceeds 3.0%, the amount of iron oxide cannot be secured. Therefore, even if the production process of the present invention described later is applied, the coverage of reduced iron Is less than 60%, and the surface appearance is not sufficiently improved. Therefore, the Si amount is set in the range of 1.5 to 3.0%.

Mn: 0.5 to 3.0%
Mn is an element useful for increasing the hardenability and increasing the strength of the steel sheet. The effect cannot be obtained at less than 0.5%. On the other hand, if the content exceeds 3.0%, segregation of Mn occurs and the workability decreases. Therefore, the amount of Mn is in the range of 0.5 to 3.0%.

Al: 0.01 to 3.0%
Al is an element added in a complementary manner to Si, and is preferably contained in an amount of 0.01% or more. However, if the Al content exceeds 3.0%, the weldability and strength ductility balance are adversely affected. Therefore, the Al content is preferably in the range of 0.01 to 3.0%.

S: 0.001 to 0.01%
S is an element inevitably contained in steel, and lowers formability by producing plate-like inclusions MnS after cold rolling. MnS is not produced until the S content is 0.01%, but excessive reduction is accompanied by an increase in desulfurization cost in the steelmaking process. Therefore, the S content is within the range of 0.001 to 0.01%.

P: 0.001 to 0.1%
P is an element inevitably contained in steel, and is an element contributing to strength improvement. On the other hand, it is also an element that deteriorates weldability, and when the amount of P exceeds 0.1%, the influence appears remarkably. On the other hand, excessive P reduction is accompanied by an increase in manufacturing cost in the steelmaking process. Therefore, the P content is within the range of 0.001 to 0.1%.

  In the present invention, the above component composition is an essential component, and the balance is iron and unavoidable impurities. However, if necessary, one or more of the following components can be appropriately contained.

Cr: 0.1 to 1.0%
Cr is an effective element for improving the hardenability of steel. To obtain this effect, addition of 0.1% or more is required. Cr strengthens the ferrite phase in a solid solution, reduces the hardness difference between the martensite phase and the ferrite phase, and effectively contributes to improving the formability. However, if the Cr content exceeds 1.0%, this effect is saturated, but rather the surface quality is significantly degraded. Therefore, the Cr content is within the range of 0.1 to 1.0%.

Mo: 0.1 to 1.0%
Mo is an element effective for improving the hardenability of steel and is an element that develops tempering secondary hardening. In order to obtain this effect, addition of 0.1% or more is required. However, if the amount of Mo exceeds 1.0%, this effect is saturated and causes an increase in cost. Therefore, the Mo amount is set within a range of 0.1 to 1.0%.

Ti: 0.01 to 0.1%
Ti effectively acts to impart fine grain refinement and precipitation strengthening after annealing by forming fine carbides and fine nitrides with C or N in steel. In order to obtain this effect, addition of 0.01% or more is necessary. However, this effect is saturated when the Ti content exceeds 0.1%. Therefore, the Ti amount is within the range of 0.01 to 0.1%.

Nb: 0.01 to 0.1%
Nb is an element that contributes to improvement in strength by solid solution strengthening or precipitation strengthening. In order to obtain this effect, addition of 0.01% or more is required. However, if the content exceeds 0.1%, the ductility of ferrite is lowered, and the workability is lowered. Therefore, the Nb content is in the range of 0.01 to 0.1%.

B: 0.0005 to 0.0050%
B is necessary for improving the hardenability, suppressing the formation of ferrite during annealing cooling, and obtaining a desired amount of martensite. In order to acquire this effect, it is necessary to add B amount 0.0005% or more, but if it exceeds 0.0050%, this effect will be saturated. Therefore, the B amount is within the range of 0.0005 to 0.0050%.

Next, the manufacturing method of a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet will be described. Unless otherwise specified, “%” display and “ppm” display relating to the atmosphere mean vol% and vol ppm, respectively, and units of water vapor partial pressure PH ₂ O and carbon dioxide partial pressure PCO _{2 in} the atmosphere are not particularly specified. As long as it is “Pa”.

  The steel slab having the above composition is heated in the hot rolling process, then subjected to rough rolling and finish rolling, and then the hot-rolled sheet surface scale is removed in the pickling process, followed by cold rolling. The hot rolling process to the cold rolling process is not particularly limited, and may be a conventional method.

  The cold-rolled steel sheet is subjected to a heat treatment consisting of the following two steps of a first heating step and a second heating step, followed by plating. This heat treatment is an important requirement in the present invention, and in particular, the second heating step is the most important requirement. After the first heating step, by performing the second heating step under the following conditions, 60% or more of the steel plate surface is covered with reduced iron at the end of the second heating step, thereby suppressing Si concentration on the steel plate surface. In addition, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that are excellent in surface appearance even with a steel sheet containing a large amount of Si can be produced.

First heating step: atmosphere containing O ₂ : 0.01 to 20%, H ₂ O: 1 to 50%, the balance being one or more of N ₂ , CO, CO ₂ and unavoidable impurities The steel sheet is heated so that the maximum temperature of the steel sheet is within the range of 873 to 1123K.

Second heating step: H ₂ : 1-50 vol%, the balance is N ₂ , H ₂ O, O ₂ , CO, CO _{2 in} an atmosphere composed of one or more and unavoidable impurities, Water vapor partial pressure in the atmosphere: PH ₂ O (Pa), carbon dioxide partial pressure: PCO ₂ (Pa), maximum reached temperature of steel plate: T (K), Si content of steel plate: [Si%] (mass%) Is heated under conditions that satisfy the following formulas (1) to (3).
0 <PH ₂ O / PCO ₂ <323.6-15.2 logT ² -71.5 [Si%] (1)
0 <log (PH ₂ O + 15PCO ₂ ) <2.3 (2)
1023 ≦ T ≦ 1173 (3)
The reasons for limiting the first heating step and the second heating step will be described.

A 1st heating process is performed in order to form iron oxide on the steel plate surface. Therefore, the heating atmosphere _{O 2: 0.01~20%, H 2} O: containing 1% to 50%, with the balance being _N 2, CO, 1 or 2 or more and unavoidable impurities CO ₂ The atmosphere.

When the O ₂ concentration in the heating atmosphere is less than 0.01%, Fe is not oxidized, and when it exceeds 20%, the cost is increased. Further, H ₂ O is made 1% or more in order to promote oxidation. Considering the humidification cost, 50% or less is preferable.

Further, the CO ₂ concentration is desirably 10% or less, and the CO concentration is desirably 5% or less.

  When the steel plate temperature is less than 873K, the steel plate is not oxidized. When the steel plate temperature exceeds 1123K, the steel plate is excessively oxidized, and push scoops are generated by picking up with the in-furnace roll in the second heating step, so the steel plate temperature is 873K or more and 1123K. The steel sheet is heated to a temperature within the range.

Further, in order to more effectively suppress the occurrence of push rods due to pick-up by the roll in the furnace in the second heating step, the first heating step is divided into two stages, the former stage is a step of oxidizing the steel sheet, and the latter stage is the preceding stage. It is preferable to set it as the process of reducing a part of oxide produced | generated by this. Specifically, the former stage is heated in an atmosphere containing O ₂ : 0.1 to ₂₀ vol% and H ₂ O: 1 to 50 vol% so that the steel sheet temperature is in the range of 873 to 1023K, and the latter stage In an atmosphere containing O ₂ : 0.01 to less than 0.1 vol% and H ₂ O: 1 to 20 vol% or less, the steel sheet is heated so that the steel sheet temperature is within the range of 973 to 1123 K ( However, the latter steel plate temperature is preferably higher than the former steel plate temperature.)

The heating in the previous stage is performed to oxidize the steel sheet, and O ₂ needs a sufficient amount to oxidize and is 0.1% or more. For economic reasons, it is preferably 20% or less of the atmospheric level. H ₂ O is made 1% or more in order to promote oxidation. Further, considering the humidification cost, 50% or less is preferable. It is difficult to oxidize when the steel plate temperature after heating in the former stage is less than 873K, and it is excessively oxidized if it exceeds 1023K, and when this oxide is reduced in the latter stage, the oxide cannot be sufficiently reduced, and the second heating step The effect of suppressing roll pickup is insufficient. Therefore, the former stage is heated so that the steel sheet temperature becomes 873-1023K.

The latter stage heating is performed to reduce the surface of the steel plate once oxidized and suppress the pressing. Therefore, it is possible to reduce the surface of the steel sheet by heating in the latter stage, and heating is performed under conditions where pick-up does not occur, that is, low-temperature reducing heating conditions in a low O ₂ concentration atmosphere. In the next second heating step, reduction treatment is performed to such an extent that it reacts with the in-furnace roll and pick-up does not occur. When O ₂ becomes 0.1% or more, the reduction becomes insufficient, and the effect of preventing pickup of the in-furnace roll in the second heating step becomes insufficient, so O ₂ is made less than 0.1%. If H ₂ O is contained in a large amount, the steel sheet is oxidized, so the content is set to 20% or less (however, 1 vol% or more). If the steel plate temperature is less than 973K, it is difficult to reduce, and if it exceeds 1123K, heating costs are required. Therefore, the latter is heated so that the steel plate temperature is within the range of 973 to 1123K (however, the steel plate temperature in the second heating step) Is higher than the steel plate temperature in the first heating step).

  When the pre-stage heating is performed by a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF), it is preferable that the combustion gas is C gas generated in a coke oven and the air ratio is 1 to 1.35. This is because when the air ratio is less than 1, the steel sheet is not oxidized, and when it exceeds 1.35, pickup occurs due to overoxidation. In addition, when post-stage heating is performed in a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF), it is preferable that the combustion gas is C gas generated in a coke oven and the air ratio is 0.6 to less than 1. . This is because if the air ratio exceeds 1, iron oxide on the surface of the steel sheet cannot be reduced, and if the air ratio is less than 0.6, the combustion efficiency deteriorates. Here, the air ratio is the ratio of the amount of introduced air to the amount of air required for complete combustion.

The second heating step is performed subsequent to the first heating step, and a reduction treatment of the oxide (iron oxide) formed on the steel plate surface in the first heating step and recrystallization annealing of the steel plate are performed. For this reason, the heating atmosphere contains H ₂ : 1 to 50 vol%, and the balance is an atmosphere composed of one or more of N ₂ , H ₂ O, O ₂ , CO, CO ₂ and inevitable impurities. If H ₂ is less than 1%, iron oxide is difficult to reduce, and if it exceeds 50%, the cost increases.

Furthermore, the partial pressure of water vapor in the heating atmosphere: PH ₂ O (Pa), the partial pressure of carbon dioxide: PCO ₂ (Pa), the maximum temperature reached by the steel plate: T (K), the Si content of the steel plate: [Si%] (mass) %) Must satisfy the following formulas (1) to (3).
0 <PH ₂ O / PCO ₂ <323.6-15.2 logT ² -71.5 [Si%] (1)
0 <log (PH ₂ O + 15PCO ₂ ) <2.3 (2)
1023 ≦ T ≦ 1173 (3)
When the ratio of the H ₂ O partial pressure and the CO ₂ partial pressure, PH ₂ O / PCO ₂ is 323.6-15.2 logT ² -71.5 [Si%] or more, the reduction rate of iron oxide decreases, so that the reduction occurs. The iron coverage is less than 60%.

When the sum of the H ₂ O partial pressure and the CO ₂ partial pressure, the logarithm of PH ₂ O + 15 PCO ₂ is 2.3 or more, decarburization is facilitated. Further, this is because the cost increases when the sum of the H ₂ O partial pressure and the CO ₂ partial pressure, or the logarithm of PH ₂ O + 15 PCO ₂ , is 0 or less.

Further, it is desirable that the O ₂ concentration is less than 5 ppm and the CO concentration is less than 10 ppm.

  If the maximum temperature of the steel sheet is less than 1023 K, the strain introduced during cold rolling remains and the workability deteriorates. Moreover, since the reduction | restoration rate of iron oxide falls, the coverage of reduced iron falls. Moreover, when it exceeds 1173K, it will become disadvantageous at the point of heating cost.

  After the heating in the above two steps, it is cooled and immersed in a hot dip galvanizing bath to perform hot dip galvanizing. When the above two steps are performed, 60% or more of the steel sheet surface is covered with reduced iron at the end of the second heating step. Since such steel sheets have suppressed surface concentration of Si, hot dip galvanization suppresses the occurrence of non-plating due to surface concentration of Si, prevents deterioration of plating adhesion, and forms an alloy. Since the occurrence of alloying delay is prevented when the treatment is performed, both high productivity and good powdering resistance can be achieved.

  For the production of hot dip galvanized steel sheet, a bath temperature of 713 to 823K and a zinc plating bath with an Al concentration in the bath of 0.14 to 0.24% are used. For the production of an alloyed hot dip galvanized steel sheet, the bath temperature is 713 to 823K. It is preferable to use a galvanizing bath having an Al concentration in the bath of 0.10 to 0.20%.

  If the bath temperature is less than 713K, the place where the temperature variation in the bath is large is not suitable because there is a possibility that the solidification of Zn occurs, and if it exceeds 823K, the problem of heating costs and the vaporized Zn vaporized into the furnace will enter the furnace. There are operational problems due to adhesion. Furthermore, since alloying proceeds during plating, it tends to be overalloyed.

  If the Al concentration in the bath is less than 0.14% when producing a hot dip galvanized steel sheet, Fe—Zn alloying progresses and plating adhesion deteriorates, and if it exceeds 0.24%, defects due to Al oxide occur. When the alloyed hot-dip galvanized steel sheet is produced, if the Al concentration in the bath is less than 0.10%, a large amount of ζ phase is generated and powdering properties deteriorate, and if it exceeds 0.20%, Fe-Zn alloying progresses. Absent.

The alloying process is optimally performed at a temperature exceeding 773K and less than 843K. Below 773K, the alloying progresses slowly, and above 843K, a hard and brittle Zn-Fe alloy layer formed at the base iron interface due to overalloy is formed too much, not only the plating adhesion deteriorates, but also the residual austenite phase decomposes. Therefore, the strength ductility balance is also deteriorated. The plating adhesion amount is not particularly defined, but is preferably 10 g / m ² or more (adhesion amount per one surface) for corrosion resistance and plating adhesion amount control. Moreover, since adhesion will fall when there is much adhesion amount, 120 g / m < ² > or less (attachment amount per one side) is desirable.

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

A slab having a steel composition shown in Table 1 was heated in a heating furnace at 1533K for 60 minutes, subsequently hot-rolled to a thickness of 2.8 mm, and wound at 813K. Next, the black scale was removed by pickling and cold rolled to a thickness of 1.6 mm. Thereafter, the first heating step and the second heating step were performed under the heat treatment conditions shown in Table 2 or Table 3 using a furnace capable of adjusting the atmosphere and keeping the gas flow rate constant during the heat treatment. At this time, the first heating step was performed in a DFF type heating furnace or a NOF type heating furnace. C gas generated in a coke oven was used as the fuel gas. The second heating step was performed in an RTF type heating furnace, and H ₂ —N ₂ gas was supplied as an atmospheric gas.

Subsequently, hot dip galvanizing treatment was performed in a 733K Al-containing Zn bath to obtain a hot dip galvanized steel sheet. The Al concentration in the bath was 0.18% during the production of the hot dip galvanized steel sheet, 0.14% during the production of the alloyed hot dip galvanized steel sheet, and the adhesion amount was adjusted to 40 g / m ² per side by gas wiping. Moreover, after performing hot dip galvanization, the alloying hot dip galvanized steel sheet was obtained by performing an alloying process at 773K. Moreover, the steel plate after the 2nd heating process was obtained by letting a plating bath pass. Elemental mapping was performed on the surface of this steel plate with EPMA in the range of 500 μm × 500 μm, and the reduced iron coverage was determined.

  The surface appearance was investigated by the method shown below with respect to the hot dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) obtained as described above.

<Surface appearance>
Judging by visual inspection for appearance defects such as non-plating and pushing rods, good (○) when there is no appearance defect, good (△) when there is a slight but poor appearance. When there was an appearance defect, it was determined as a defect (x).

The water vapor partial pressure PH ₂ O and the carbon dioxide partial pressure PCO ₂ in the atmosphere were calculated from the following formulas based on the H ₂ O concentration and the CO ₂ concentration in the atmosphere, respectively.
Water vapor partial pressure PH ₂ O (Pa) = H ₂ O concentration (ppm) × 10 ⁻⁶ × 101325
Carbon dioxide partial pressure PCO ₂ (Pa) = CO ₂ concentration (ppm) × 10 ⁻⁶ × 101325
The obtained results are shown in Table 2 (Invention Example) and Table 3 (Comparative Example) together with the conditions.

  As can be seen from Tables 2 and 3, the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet of the examples of the present invention have a beautiful surface appearance with no unplating or pressing, despite containing Si. On the other hand, the hot-dip galvanized steel sheet and the galvannealed steel sheet of the comparative example are inferior in surface appearance.

  In the inventive examples, there was no problem of a decrease in plating adhesion and a delay in alloying.

Similarly to Example 1, a slab having the steel composition shown in Table 1 was heated in a heating furnace at 1533 K for 60 minutes, subsequently hot-rolled to 2.8 mm, and wound at 813 K. Next, the black scale was removed by pickling and cold rolled to 1.6 mm. Thereafter, using a furnace in which the atmosphere can be adjusted and the gas flow rate can be kept constant during the heat treatment, the first heating step, the first heating step, and the second heating step under the heat treatment conditions shown in Table 4 or 5 Went. At this time, the first heating step was performed in a DFF type heating furnace or a NOF type heating furnace. C gas generated in a coke oven was used as the fuel gas. The second heating step was performed in an RTF type heating furnace, and H ₂ —N ₂ gas was supplied as an atmospheric gas.

Subsequently, hot dip galvanizing treatment was performed in a 733K Al-containing Zn bath to obtain a hot dip galvanized steel sheet. The Al concentration in the bath was 0.18% during the production of the hot dip galvanized steel sheet, 0.14% during the production of the alloyed hot dip galvanized steel sheet, and the adhesion amount was adjusted to 40 g / m ² per side by gas wiping. Moreover, after performing hot dip galvanization, the alloying hot dip galvanized steel sheet was obtained by performing an alloying process at 773K. Moreover, the steel plate after the 2nd heating process was obtained by letting a plating bath pass. Elemental mapping was performed on the surface of this steel plate with EPMA in the range of 500 μm × 500 μm, and the reduced iron coverage was determined.
The surface appearance was investigated by the method shown below with respect to the hot dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) obtained as described above. The surface appearance criteria are the same as those in the first embodiment. The atmospheric water vapor partial pressure PH ₂ O and the carbon dioxide partial pressure PCO ₂ were calculated in the same manner as in Example 1.

  The obtained results are shown in Table 4 (Invention Examples) and Table 5 (Comparative Examples) together with the conditions.

  As can be seen from Tables 4 and 5, the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet of the examples of the present invention have a beautiful surface appearance without any unplating or pressing, despite containing Si. Among the examples of the present invention, those satisfying the invention scope of claim 3 (Nos. 1 to 16) have a more beautiful surface appearance than those outside the invention scope of claim 3 (Nos. 17 and 18). Have. On the other hand, the hot-dip galvanized steel sheet and the galvannealed steel sheet of the comparative example are inferior in surface appearance.

  In the inventive examples, there was no problem of a decrease in plating adhesion and a delay in alloying.

  The high-strength hot-dip galvanized steel sheet and high-strength galvannealed steel sheet produced by the method of the present invention have a beautiful surface appearance, and are expected to be used in a wide range of applications, especially in the fields of automobiles, home appliances, and building materials. .

Claims (5)

As chemical components, in mass%, C: 0.05 to 0.30%, Si: 1.5 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to 3.0 %, S: 0.001 to 0.01%, P: 0.001 to 0.1%, and when applying hot dip galvanizing to a steel plate composed of the remaining Fe and inevitable impurities, O ₂ : 0.01 _~20vol%, H 2 O: containing 1~50vol%, the balance being _N 2, CO, one or more, and the steel sheet in an atmosphere consisting of unavoidable impurities in CO ₂ in the range of 873~1123K first heating step of heating so as to temperature, _then, H 2: containing 1～50Vol%, the balance being _{N 2,} H ₂ O, O 2, CO, 1 or 2 or more of CO ₂ and unavoidable in an atmosphere consisting of impurities, water vapor partial pressure in the _atmosphere: PH 2 O (Pa), Carbon monoxide partial _pressure: PCO 2 (Pa), the maximum temperature of the steel sheet: T (K), Si content of the steel sheet: [Si%] (mass% ) is in a condition satisfying the following formula (1) to (3) The second heating step for heating is performed (however, the steel plate temperature in the second heating step is higher than the steel plate temperature in the first heating step), followed by hot dip galvanizing treatment. A manufacturing method of high strength hot-dip galvanized steel sheet.
0 <PH ₂ O / PCO ₂ <323.6-15.2 logT ² -71.5 [Si%] (1)
0 <log (PH ₂ O + 15PCO ₂ ) <2.3 (2)
1023 ≦ T ≦ 1173 (3) In the first heating step, the former stage is such that the steel sheet is heated to a temperature in the range of 873 to 1023 K in an atmosphere containing O ₂ : 0.1 to ₂₀ vol% and H ₂ O: 1 to 50 vol%. In the latter stage, the steel sheet is heated to a temperature in the range of 973 to 1123K in an atmosphere containing O ₂ : 0.01 to less than 0.1 vol% and H ₂ O: 1 to 20 vol% (provided that The method for producing a high-strength hot-dip galvanized steel sheet having excellent surface appearance according to claim 1, wherein the steel plate temperature at the latter stage is higher than the steel plate temperature at the former stage. In the first heating step, the first stage is heated by a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF) at an air ratio of 1 or more and 1.35 or less, and the latter stage is heated by a direct-fired furnace or a non-oxidizing furnace. The method for producing a high-strength hot-dip galvanized steel sheet having excellent surface appearance according to claim 2, wherein heating is performed under a condition where the ratio is less than 1. The steel sheet has, as chemical components, mass%, Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Ti: 0.01 to 0.1%, Nb: 0 The surface appearance according to any one of claims 1 to 3, comprising one or more selected from 0.01 to 0.1% and B: 0.0005 to 0.0050%. For producing high-strength hot-dip galvanized steel sheets with excellent resistance. A high-strength galvannealed steel sheet having excellent surface appearance, characterized by further alloying after producing a high-strength galvanized steel sheet by the method according to any one of claims 1 to 4. Production method.

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