JP2003105513A - High strength galvanized steel sheet having excellent appearance and workability, and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength galvanized steel sheet which has suppressed occurrence of unplating, and has excellent appearance and workability, and to provide a production method therefor. SOLUTION: The high strength galvanized steel sheet having excellent appearance and workability is obtained by providing the surface of a steel sheet containing, by mass, 0.0001 to 0.3% C, 0.001 to <0.1% Si, 0.01 to 3% Mn, 0.001 to 4% Al, 0.001 to 1% Mo, 0.0001 to 0.3% P and 0.0001 to 0.1% S, and the balance Fe with inevitable impurities with a plated layer having a composition containing 0.001 to 3% Mn, 0.001 to 4% Al, 0.0001 to 1% Mo and 5 to 20% Fe, and the balance Zn with inevitable impurities. Provided that the content of Mn in the steel is defined as X (mass%), the content of Si in the steel as Y (mass%), and the content of Si in the plating as Z (mass%), X, Y and Z satisfy the following inequality: 0.6-(X/18+Y+Z)>=0 (1).

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to building materials, home electric appliances,
The present invention relates to a high-strength hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet that are suitable for automobiles and have excellent workability and a method for manufacturing the same.

[0002]

2. Description of the Related Art Hot-dip galvanizing is applied for the purpose of preventing corrosion of steel sheets and is widely used in building materials, home appliances, automobiles and the like. As its manufacturing method, in a continuous line,
After degreasing and cleaning, it is heated in a non-oxidizing atmosphere, annealed in a reducing atmosphere containing H ₂ and N ₂ , cooled to near the plating bath temperature, immersed in a molten zinc bath, and then cooled or reheated.
There is a Sendzimer method in which an e-Zn alloy phase is generated and then cooled, which is often used for treating steel sheets.

Regarding annealing before plating, after degreasing and cleaning,
In some cases, a total reduction furnace system is also used in which annealing is immediately performed in a reducing atmosphere containing H ₂ and N ₂ without heating in a non-oxidizing atmosphere. Further, a flux method is also performed in which after degreasing and pickling a steel sheet, a flux treatment is performed using ammonium chloride or the like, and the steel sheet is immersed in a plating bath and then cooled.

A small amount of Al is added to the plating bath used in these plating treatments for deoxidizing molten zinc. In the Zenzimer method, the Zn plating bath contains about 0.1% by mass of Al. Since Al in this bath has a stronger affinity with Fe than Fe-Zn, when the steel is immersed in the plating bath, an Fe-Al alloy phase, that is, a concentrated layer of Al, is formed on the surface of the steel, and Fe-Zn It is known to suppress the reaction. Due to the presence of a concentrated layer of Al, the Al content in the obtained plating layer is usually A in the plating bath.
It becomes higher than 1 content rate.

In recent years, particularly from the viewpoint of weight reduction of automobile bodies for the purpose of improving fuel economy, demand for high-strength steel sheets having high ductility is increasing. As an inexpensive strengthening method, Si is added to steel, and particularly in high ductility and high strength steel plates, it may be contained in an amount of 1 mass% or more.

On the other hand, from the viewpoint of plating, S in steel
When the content of i exceeds 0.3% by mass, the normal A
In the Zenzimer method using a plating bath containing 1 l, the plating wettability is significantly reduced and non-plating occurs, resulting in poor appearance quality. The reason for this is that S is formed on the surface of the steel sheet during reduction annealing.
It is said that this is because the i oxide is concentrated and the wettability of Si oxide with respect to molten zinc is poor.

As means for solving this problem, Japanese Patent Laid-Open No. Hei 3
No. 28359, Japanese Patent Laid-Open No. 3-64437, etc., the plating property is improved by applying a specific plating. However, in this method, it is newly added to the front stage of the hot dip line annealing furnace. There is a problem that a plating facility must be provided, or a plating process must be performed in advance in an electroplating line, resulting in a significant cost increase.

Further, Si scale scratches generated during hot rolling also cause damage to the plating appearance later. To suppress this, it is essential to reduce the amount of Si in steel,
In a retained austenitic steel and a multi-phase steel sheet, which are typical high-strength high-strength steel sheets, Si is an extremely effective additive element in terms of enhancing the strength-ductility balance.

[0009]

DISCLOSURE OF THE INVENTION The present invention solves the above problems and provides a high-strength galvannealed steel sheet and a hot-dip galvanized steel sheet which suppress non-plating and have excellent appearance and workability, and a method for producing the same. The purpose is to provide.

[0010]

Means for Solving the Problems As a result of various investigations, the inventors have found that the plating layer contains a specific element in an appropriate concentration, and it is combined with a component of a steel sheet.
The wettability of hot-dip galvanizing high-strength steel sheets and the promotion of alloying in alloying plating were found. This effect is mainly due to
It can be made to appear by controlling the Al concentration in the plating phase and the Mn in steel.

Mn content in steel is X (mass%), Si in steel is
When the content rate is Y (mass%) and the Al content rate during plating is Z (mass%), X, Y, and Z satisfy the formula (1),
It has been found that extremely good plating can be obtained.

0.6- (X / 18 + Y + Z) ≧ 0 (1) The present invention has been completed based on the above findings, and the gist thereof is as follows. [1]% by mass, C: 0.0001 to 0.3%, S
i: 0.001 to less than 0.1%, Mn: 0.01 to 3
%, Al: 0.001 to 4% Mo: 0.001 to 1%
P: 0.0001 to 0.3% S: 0.0001 to 0.1
%, On the surface of the steel sheet consisting of balance Fe and unavoidable impurities, in mass%, Mn: 0.001 to 3% Al: 0.001 to 4% Mo: 0.0001 to 1% Fe: 5 to 20 %, With the balance being Zn and unavoidable impurities in the hot-dip galvanized steel sheet, wherein the Mn content in the steel is X (mass%), the Si content in the steel is Y (mass%), and the plating is A high-strength hot-dip galvanized steel sheet excellent in appearance and workability, characterized in that X, Y, and Z satisfy the following equation, where the medium Al content is Z (mass%).

0.6- (X / 18 + Y + Z) ≧ 0 (1) [2]% by mass, C: 0.0001 to 0.3%, S
i: 0.001 to less than 0.1%, Mn: 0.01 to 3
%, Al: 0.001 to 4% Mo: 0.001 to 1% P: 0.0001 to 0.3% S: 0.0001 to 0.1%, and a balance Fe and unavoidable impurities %, Mn: 0.001 to 3% Al: 0.001 to 4% Mo: 0.0001 to 1% Fe: less than 5% plating, the balance Zn and unavoidable impurities A hot dip galvanized steel sheet having a layer, wherein Mn content in steel is X (mass%), Si content in steel is Y (mass%), and Al content in plating is Z (mass%), X, A high-strength hot-dip galvanized steel sheet excellent in appearance and workability, characterized in that Y and Z satisfy the following expressions. 0.6− (X / 18 + Y + Z) ≧ 0 (1) [3] The plating layer is further mass%, Si: 0.001 to 0.1% W: 0.001 to 0.1% Zr : 0.001 to 0.1% Cs: 0.001 to 0.1%, Rb: 0.001 to 0.
1%, K: 0.001-0.1%, Ag: 0.001-
5%, Na: 0.001 to 0.05%, Cd: 0.00
1 to 3%, Cu: 0.001 to 3%, Ni: 0.001
~ 0.5%, Co: 0.001-1%, La: 0.00
1 to 0.1%, Tl: 0.001 to 8%, Nd: 0.0
01-0.1%, Y: 0.001-0.1%, In:
0.001-5%, Be: 0.001-0.1%, C
r: 0.001 to 0.05%, Pb: 0.001 to 1
%, Hf: 0.001-0.1%, Tc: 0.001-
0.1%, Ti: 0.001 to 0.1%, Ge: 0.0
01-5%, Ta: 0.001-0.1%, V: 0.0
01-0.2%, B: 0.001-0.1%, 1 type or 2 types or more are contained, and it is excellent in the appearance and workability described in [1] or [2]. High strength galvanized steel sheet. [4] Steel is further mass%, Cr: 0.001-2
5%, Ni: 0.001-10%, Cu: 0.001-
5%, Co: 0.001 to 5% W: 0.001 to 5% One or more types are contained, and the appearance according to any one of [1] to [3]. And high strength galvanized steel sheet with excellent workability. [5] Steel further contains Nb, Ti, V, and Z in mass%.
One or two or more of r, Hf and Ta in total of 0.00
A high-strength hot-dip galvanized steel sheet having excellent appearance and workability according to any one of [1] to [4], which is characterized by containing 1 to 1%. [6] Steel further contains B: 0.0001% by mass.
A high-strength hot-dip galvanized steel sheet excellent in appearance and workability according to any one of [1] to [5], which is characterized by containing 0.1%. [7] Steel further contains Y, Rem, Ca, M in mass%.
One or two or more of g and Ce in total of 0.0001 to 1
%, A high-strength hot-dip galvanized steel sheet excellent in appearance and workability according to any one of [1] to [6]. [8] The microstructure of steel has a total volume fraction of 50 to 97.
% Of the ferrite phase or the ferrite phase and the bainite phase as the main phase, and the balance is a composite structure composed of one or both of the martensite phase and the retained austenite phase in a volume fraction of 3 to 50%. Do [1] ~
[7] A high-strength hot-dip galvanized steel sheet excellent in appearance and workability according to any one of [7].

[9] The microstructure of the steel sheet has a volume fraction of 70 to 97%
Ferrite as the main phase and its average grain size is 20 μm or less, and the second phase is austenite and / or martensite with a volume fraction of 3 to 30%, and the second phase has an average grain size of 10 μm or less. [1]-
A high-strength hot-dip galvanized steel sheet having excellent appearance and workability according to any one of [8]. [10] The second phase of the steel sheet is austenite, the carbon content in the steel: C (mass%), the Mn content in the steel: Mn (mass%), the austenite volume ratio: Vγ (%), ferrite and bainite. Volume ratio of: Vα (%) and (2)
[1] which is characterized by satisfying the formula

Any one of [9]
High-strength hot-dip galvanized steel sheet having excellent appearance and workability as described in the item. (Vγ + Vα) / Vγ × C + Mn / 8 ≧ 2.000 (2) [11] The microstructure of the steel sheet has a volume fraction of 50 to 95.
% Ferrite as the main phase and its average particle size is 20 μm or less, and the second phase contains austenite and / or martensite in a volume fraction of 3 to 30%, and their average particle size is 10 μm or less. , And 2 in volume fraction
Characterized by 47% bainite [1]
A high-strength hot-dip galvanized steel sheet having excellent appearance and workability according to any one of [10]. [12] A method for producing the high-strength hot-dip galvanized steel sheet according to any one of [1] to [11],
A casting slab made of the steel sheet component according to any one of [1] and [4] to [7] is heated to 1180 to 1250 ° C. again as it is cast or once cooled, and then heated to 880 to 1100.
After the hot rolling is finished at ℃, the rolled hot rolled steel sheet is pickled, cold rolled, and then 0.1 × (Ac ₃ −Ac ₁ ) + Ac ₁
10 seconds or more in a temperature range of (° C.) or more and Ac ₃ +50 (° C.) or less
After annealing for 30 minutes, it is cooled to a temperature range of 650 to 700 ° C. at a cooling rate of 0.1 to 10 ° C./second, and then 0.1 to 10 ° C.
After cooling to a plating bath temperature of −50 ° C. to a plating bath temperature of +50 (° C.) at a cooling rate of 100 ° C./sec, the plate is immersed in a plating bath, and the plating bath temperature −50 ° C. to the plating bath temperature of +50 including the immersion time. A high-strength molten zinc excellent in workability, which is characterized by holding in a temperature range of (° C) for 2 to 200 seconds, and then, if necessary, alloying in a temperature range of 400 to 550 ° C and cooling to room temperature. Manufacturing method of plated steel sheet.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.

The inventors have found that, in mass%, C: 0.000.
1 to 0.3%, Si: 0.001 to less than 0.1%, M
n: 0.01 to 3%, Al: 0.001 to 4% Mo: 0.001 to 1% P: 0.0001 to 0.3% S: 0.0001 to 0.1%, and the balance A steel plate composed of Fe and unavoidable impurities is annealed, immersed in a Zn plating bath at a temperature of 450 to 470 ° C. for 3 seconds, and 500 to 530 ° C. for some samples.
It heated for 10 to 60 seconds. After that, the appearance was evaluated on a scale of 5 based on the rate of occurrence of defects on the surface of the plated steel sheet. In addition, the tensile properties were also evaluated in terms of mechanical properties. As a result, assuming that the Mn content in steel is X (mass%), the Si content in steel is Y (mass%), and the Al content in plating is Z (mass%), X, Y and Z are expressed by the formula (1). It has been found that a composition satisfying the above conditions gives a rating of 5 in which appearance defects hardly occur. 0.6- (X / 18 + Y + Z) ≧ 0 (1) For each of the ratings 1 to 5, the appearance of plating was visually evaluated for the state of non-plating and the state of defects such as scratches and patterns. The evaluation index is as follows. Rating 5: No plating, almost no scratches or patterns (1% or less in area ratio) Rating 4: No plating, minute scratches or patterns (more than 1% in area ratio 1
0% or less) Rating 3: Non-plating, scratches and patterns are small (area ratio over 10% 5
(Less than 0%) Rating 2: Non-plating, many scratches and patterns (over 50% in area ratio) Rating 1: No wetting of plating and the reason why non-plating and other defects are suppressed are unknown. , Al added in the plating bath
It is considered that non-plating occurs due to the poor wettability with the SiO ₂ formed on the steel plate surface. That is, it is possible to suppress the occurrence of non-plating by adding an element that removes the adverse effect of Al added to the Zn bath. As a result of diligent studies by the present inventors, it was found that the notational purpose can be achieved by adding Mn in an appropriate concentration range. Further, it is considered that the reduction of the amount of Si in the steel suppresses the generation of scratches due to Si scale that occurs during hot rolling, which is also effective for improving the appearance. In addition, regarding the deterioration of the material due to the reduction of Si, it was also found that the ductility can be secured by the manufacturing conditions and the addition of other components: Al and Mo, and that the reduction of Si and the addition of Al are effective in promoting alloying. It was

Further, W, Zr, Cs, R is contained in the plating layer.
b, K, Ag, Na, Cd, Cu, Ni, Co, La,
Tl, Nd, Y, In, Be, Cr, Pb, Hf, T
It has been found that the inclusion of one or more of c, Ti, Ge, Ta, V, and B suppresses non-plating and promotes alloying.

The coating weight is not particularly limited, but from the viewpoint of corrosion resistance, the coating weight on one side is preferably 5 g / m ² or more. Applying an upper layer plating on the hot-dip Zn-coated steel sheet of the present invention for the purpose of improving paintability and weldability, and various treatments such as chromate treatment, phosphate treatment, lubricity improving treatment, weldability improving treatment, etc. Even if you give
It does not depart from the invention.

The Mn content in the plating layer is 0.001 to 3% by mass.
The reason why it is set within the range is that no plating occurs in this range and plating with a good appearance can be obtained. Mn
When the amount exceeds the upper limit of 3% by mass, Mn-Z is generated in the plating bath.
The n compound precipitates and is taken into the plating layer, which significantly deteriorates the appearance.

The amount of Al in the plating layer is 0.001 to 4% by mass.
When the content is less than 0.001 mass%, dross generation is remarkable and a good appearance cannot be obtained, and when Al is added in excess of 4 mass%, the alloying reaction is significantly suppressed, This is because it becomes difficult to form a chemical-dip galvanized layer.

The reason for setting the amount of Mo in the plating layer within the range of 0.0001 to 1 mass% is that non-plating is suppressed and plating with a good appearance is obtained in this range. M
If the amount of o exceeds the upper limit of 1% by mass, the dross containing Mo is generated, and the plating appearance is significantly deteriorated.

Fe is incorporated into the plating layer by the alloying treatment, and a high-strength galvanized steel sheet having excellent coatability and spot weldability can be obtained. In the invention [1], if the Fe content of the plating layer is less than 5 mass%, the spot weldability becomes insufficient. On the other hand, when the amount of Fe exceeds 20% by mass, the adhesion of the plating layer itself is impaired, and the plating layer is broken / falls off during processing and adheres to the mold, causing defects during molding. Therefore, the range of the amount of Fe in the plated layer when alloying is performed is 5 to 20 mass%.

When no alloying treatment is performed, the Fe content in the plating layer may be less than 5% by mass. That is, in the invention according to the above [2], even if the Fe content of the plating layer is less than 5% by mass,
Other than the effect of the alloying of the present invention according to [1], the effects such as appearance, workability and corrosion resistance are good.

The amount of Si in the plating layer is 0.0001 to 0.1.
% By mass, 0.001 to 0.1% by mass of W, 0.001 to 0.1% by mass of Zr, and 0.001 to 0.
1% by mass, Rb amount from 0.001 to 0.1% by mass, K amount from 0.001 to 0.1% by mass, Ag amount from 0.001 to 5% by mass, Na amount from 0.001 to 0. 05% by mass, Cd amount of 0.001 to 3% by mass, Cu amount of 0.001 to 3% by mass, Ni amount of 0.001 to 0.5% by mass, and Co amount of 0.
001 to 1 mass%, La content of 0.001 to 0.1 mass%, Tl content of 0.001 to 8 mass%, and Nd content of 0.00.
1 to 0.1% by mass, Y amount of 0.001 to 0.1% by mass,
In content is 0.001 to 5 mass%, Be content is 0.001 to
0.1 mass%, Cr content 0.001-0.05 mass%,
Pb amount is 0.001-1 mass%, Hf amount is 0.001-
0.1% by mass, Tc amount 0.001 to 0.1% by mass, T
i amount is 0.001 to 0.1% by mass, Ge amount is 0.001
.About.5 mass%, Ta content of 0.001 to 0.1 mass%, V content of 0.001 to 0.2 mass%, B content of 0.001 to 0.
The reason why the content is within the range of 1% by mass is that non-plating is suppressed in each of these ranges and plating with a good appearance is obtained. If the amount of each element exceeds the upper limit, the appearance of plating is significantly deteriorated due to the formation of dross containing each element.

Next, the reasons for limiting the components of the steel sheet in the present invention will be described.

C: To secure the strength, the lower limit of the amount of C was set to 0.0001% by mass. Further, it is an indispensable additional element necessary to secure a sufficient amount and stability of retained austenite. On the other hand, the upper limit for maintaining weldability was 0.3% by mass.

Si: 0.001% or more in order to secure manufacturability and material strength, and Si is set to less than 0.1% to reduce scale scratches. Addition in excess of this will cause frequent scale scratches, leading to deterioration of the plating appearance and lowering of the yield of steel sheets.

The range of Mn: 0.01 to 3% by mass is that the strengthening effect appears at 0.01% by mass or more, and the upper limit is 3% by mass. This is because

Al: The range of 0.001 to 4 mass% is 0.001 mass% or more for the purpose of deoxidizing because of low Si. It also has the effect of improving the strength-ductility balance and promoting the alloying behavior of plating. On the other hand, excessive addition adversely affects weldability, plating wettability, and manufacturability, so 4% was made the upper limit.

Mo: A strengthening element. Since the content of Si is low, 0.001 mass% or more is added to suppress the precipitation of pearlite or carbide that adversely affects the strength-ductility balance instead of Si. On the other hand, excessive addition causes the generation and stabilization of retained austenite and hardening of ferrite, and therefore ductility is lowered, so the upper limit was made 1%.

The amount of P is set in the range of 0.0001 to 0.3% by mass because the strengthening effect appears at 0.0001% by mass or more and the extremely low temperature is economically disadvantageous. The lower limit was set. The upper limit of 0.3% by mass is that addition of an amount exceeding this adversely affects weldability and manufacturability during casting and hot rolling.

The amount of S is set to the range of 0.0001 to 0.1% by mass because the extremely low temperature is economically disadvantageous.
The lower limit of 0.0001 mass% and the upper limit of 0.1 mass% are that addition of an amount exceeding this adversely affects weldability and manufacturability during casting and hot rolling.

Further, in the steel targeted by the present invention, Cr, Ni, Cu, Co and W are used for the purpose of further improving the strength.
1 type or 2 types or more of these can be contained.

The amount of Cr is set in the range of 0.001 to 25% by mass because the strengthening effect appears at 0.001% by mass or more, and the upper limit of 25% by mass is that the amount of addition exceeds this amount. This is because the workability is adversely affected.

The amount of Ni is set in the range of 0.001 to 10% by mass because the strengthening effect appears at 0.001% or more.
The reason why the upper limit is 10% by mass is that addition of an amount exceeding this has an adverse effect on workability.

The amount of Cu is set in the range of 0.001 to 5% by mass because the strengthening effect appears at 0.001% by mass or more, and the upper limit of 25% by mass is that when the amount of addition exceeds this amount. This is because the workability is adversely affected.

The Co content in the range of 0.001 to 5% by mass means that the strengthening effect appears at 0.001% by mass or more, and the upper limit of 5% by mass is that the amount of addition exceeds this range. This is because the workability is adversely affected.

The amount of W is set to the range of 0.001 to 5% by mass because the strengthening effect appears at 0.001% by mass or more.
The reason why the upper limit is 5% by mass is that the workability is adversely affected if the amount of addition exceeds this range.

Further, the steel targeted by the present invention is N which is a strong carbide forming element for the purpose of further improving the strength.
One or more of b, Ti, V, Zr, Hf and Ta may be contained.

These elements form fine carbides, nitrides or carbonitrides, and are extremely effective in strengthening the steel sheet. Therefore, if necessary, one kind or two or more kinds in total of 0.
The amount added was 001% by mass or more. On the other hand, since it inhibits the deterioration of ductility and the concentration of C in the retained austenite,
The upper limit of the total addition amount of one or more kinds was set to 1% by mass.

B can also be added if necessary. B
Is effective in strengthening the grain boundaries and strengthening the strength of steel by adding 0.0001 mass% or more, but if the addition amount exceeds 0.1 mass%, not only the effect will be saturated, but more than necessary. Since the steel plate strength is increased and the workability is deteriorated, the upper limit is set to 0.1% by mass.

Y, Rem, Ca, Mg, Ce ,: Added for the purpose of suppressing generation of Si-based internal grain boundary oxidized phase which deteriorates the wettability of the plating. Rather than forming a grain boundary oxide like Si-based oxides, a relatively fine oxide can be dispersed and formed. 1 out of these element groups
One or two or more elements in total was added at 0.0001% or more. On the other hand, excessive addition reduces manufacturability such as castability and hot workability and the ductility of steel sheet products, so the upper limit was 1% by mass.

Next, a preferable microstructure of the base steel sheet will be described. In order to ensure sufficient workability, it is desirable that the main structure is a ferrite phase having a volume fraction of 50% or more, preferably 70% or more, but a bainite phase may be included in consideration of high strength. On the other hand, an increase in the volume fraction of ferrite increases ductility but leads to a decrease in strength. Therefore, the upper limit is 97% by volume when the bainite phase is not contained.
%, When containing a bainite phase, the volume fraction is 95%. In order to achieve both high strength and high ductility, a composite structure containing a retained austenite phase and / or a martensite phase is used. Due to the high strength and high ductility, the total volume of the retained austenite phase and the martensite phase is 3% or more. When the volume ratio exceeds 50% in total, embrittlement tends to occur, so 50% or less is desirable, and 30% or less is desirable.

In order to secure high ductility of the steel sheet itself, the average grain size of ferrite is set to 20 μm or less, and the average grain size of the second phase austenite and / or martensite is set to 10 μm.
Specified as less than μm. Further, here, it is desirable that the second phase is austenite and / or martensite, and a ratio of 0.7 or less is ensured with respect to the average grain size of ferrite as the main phase. On the other hand, it is difficult to set the average particle size of the second phase, austenite and / or martensite, to be less than 0.01 times the average particle size of ferrite. preferable.

Further, in order to achieve a good balance between plating adhesion and high strength ductility / ductility, when the second phase of the steel sheet is austenite, the carbon content in the steel: C (mass%), Mn amount: Mn (mass%), volume ratio of austenite: Vγ (%), volume ratio of ferrite and bainite: Vα (%). (Vγ + Vα) / Vγ × C + Mn / 8 ≧ 2.000 (2) By satisfying this formula, a steel sheet having particularly excellent balance of strength and ductility and good plating adhesion can be obtained.

The volume fraction and the like in the case of including bainite will be described below. When the bainite phase is contained in a volume fraction of 2% or more, it contributes to high strength, and when it coexists with the austenite phase, it contributes to the stabilization of austenite, resulting in a high n-value. Also,
This phase is basically fine and also contributes to plating adhesion during high processing. Especially when the second phase is austenite, setting the volume fraction of bainite to 2% or more further improves the balance between plating adhesion and ductility. On the other hand, if it is excessively generated, ductility is deteriorated, so that the bainite phase is limited to 47% or less in terms of volume fraction.

In addition to the above, as the remaining structure of the microstructure,
The case where one or more kinds of carbides, nitrides, sulfides and oxides are contained is also within the scope of the steel sheet of the present invention.
It is preferable that the species or two or more species have a volume fraction of 1% or less. The ferrite, bainite, austenite, martensite and the remaining structure of the above microstructure are identified, the existing positions are observed, and the average grain size (average circle equivalent diameter) and space factor are measured by using Nital reagent and JP-A-59-59. -2
The reagent disclosed in Japanese Patent No. 19473 corrodes a steel plate rolling direction cross section or a cross section perpendicular to the rolling direction to 500 times or more.
Quantification is possible by observing with a 1000 times optical microscope. Here, measuring the particle size of martensite may be difficult when using an optical microscope. In this case, the block boundary of martensite, the packet boundary, or a set thereof is observed and particle size is measured using a scanning electron microscope to determine the average equivalent circle diameter.

The average particle size is defined as a value determined by JIS based on the results obtained by observing 20 fields of view by the above method.

A method for manufacturing a high strength galvanized steel sheet having such a structure will be described below.

When the steel sheet of the present invention is manufactured by hot rolling and cold rolling / annealing, a slab adjusted to have predetermined components is cast or once cooled and then reheated to 1180 ° C. or higher to obtain a uniform scale. Is formed on the surface of the billet to enhance the descaling property. On the other hand, heating above 1250 ° C. promotes local abnormal oxidation, so this was made the upper limit of the heating temperature. Further, hot rolling is terminated at 880 ° C. or higher for the purpose of suppressing excessive internal oxidation formation, followed by pickling, cold rolling and annealing to obtain a final product. At this time, the hot rolling completion temperature is generally higher than the Ar ₃ transformation temperature determined by the chemical composition of the steel, but the characteristics of the final steel sheet are not deteriorated if the temperature is from Ar ₃ to a low temperature of about 10 ° C. On the other hand, the hot rolling completion temperature is set to 1100 ° C. or lower in order to avoid the production of a large amount of oxide scale.

Further, by setting the coiling temperature after cooling to be the bainite transformation start temperature determined by the chemical composition of steel or more, it is possible to avoid increasing the load during cold rolling more than necessary, but the total reduction ratio of cold rolling is Not only this when it is small,
Even when wound below the bainite transformation temperature of steel, the properties of the final steel sheet are not deteriorated. Also, the total reduction ratio of cold rolling is
It is set based on the relationship between the final plate thickness and the cold rolling load, but if it is 40% or more, the properties of the final steel plate are not deteriorated.

When annealing after cold rolling, the annealing temperature is determined by the temperature of the chemical components Ac ₁ and Ac ₃ (for example, "Steel Material Science": W. C. Leslie, translated by Shigeyasu Koda, Maruzen P273). , Expressed by 0.1 × (Ac
_In the case of less than ( _3- Ac ₁ ) + Ac ₁ (° C.), the amount of austenite obtained at the annealing temperature is small, so that the retained austenite phase or martensite phase cannot be left in the final steel sheet, and therefore this is annealed. The lower limit of temperature was set. Moreover, even if the annealing temperature exceeds Ac ₃ +50 (° C.), the upper limit of the annealing temperature is set to Ac ₃ +50 (° C.) so that the characteristics of the steel sheet cannot be improved and the manufacturing cost is increased. The annealing time at this temperature needs to be 10 seconds or more in order to make the temperature of the steel plate uniform and to secure austenite. However, if it exceeds 30 minutes, not only the effect will be saturated, but also the cost will increase, so this was made the upper limit.

The subsequent primary cooling is important for promoting the transformation from the austenite phase to the ferrite phase and concentrating C in the untransformed austenite phase to stabilize the austenite. If the cooling rate is less than 0.1 ° C./sec, the required production line length is lengthened and the production rate is extremely slowed down. Therefore, the lower limit of the cooling rate is set to 0. It was set to 1 ° C./second.
On the other hand, when the cooling rate is higher than 10 ° C./sec, ferrite transformation does not sufficiently occur, it becomes difficult to secure the retained austenite phase in the final steel sheet, and the amount of hard phase such as martensite phase increases. Therefore, this is the upper limit.

When this primary cooling is performed to a temperature lower than 650 ° C., pearlite is not formed or sufficient ferrite is not formed during cooling, so this is the lower limit. However, if the cooling is completed at a temperature higher than 700 ° C., the progress of ferrite transformation is not sufficient, so this was made the upper limit.

The rapid cooling of the secondary cooling to be performed subsequently is
A cooling rate of at least 0.1 ° C./second or more is required so that pearlite transformation or precipitation of iron carbide does not occur during cooling. However, it is difficult to increase the cooling rate to more than 100 ° C / sec because of the facility capacity.
Seconds were set as the cooling rate range.

When the cooling stop temperature of the secondary cooling is lower than the plating bath temperature of -50 ° C, it causes a serious problem in operation, and when it exceeds the plating bath temperature of +50 (° C), carbide precipitation occurs in a short time, and thus the retained austenite obtained. The amount of martensite and martensite cannot be secured. For this reason, the stop temperature of the secondary cooling is set to the plating bath temperature −50 ° C. or higher and the plating bath temperature +50.
(° C). In order to stabilize the austenite phase remaining in the steel sheet at room temperature, it is essential to further increase the carbon concentration in the austenite by transforming part of it into the bainite phase. In order to advance the bainite transformation in a short time together with the alloying treatment, 2 to 3 including the immersion time in the temperature range of the plating temperature −50 ° C. to the plating temperature + 50 ° C.
It was decided to hold for 200 seconds.

If the plating temperature is lower than -50 ° C, bainite transformation is unlikely to occur, and if the plating temperature is higher than + 50 ° C, carbides are generated and it becomes difficult to leave a sufficient retained austenite phase.

Unlike the case of the retained austenite phase, the formation of the martensite phase does not require the bainite transformation to occur. On the other hand, it is necessary to suppress the formation of carbides and pearlite phases as in the case of the retained austenite phase. Therefore, it is necessary to perform sufficient alloying treatment after secondary cooling.
The alloying treatment is performed in the temperature range of 00 to 550 ° C.

[0058]

The present invention will be described in more detail with reference to the following examples.

A steel plate having a composition as shown in Table 1 was made into 1180
To 1250 ° C., hot rolling is completed at 880 to 1100 ° C., and after cooling, a steel strip wound at a bainite transformation start temperature or higher determined by the chemical composition of each steel is pickled and then cold rolled to 1.0
The thickness was mm.

[0060]

[Table 1]

Then, the Ac ₁ and Ac ₃ transformation temperatures were calculated from the components (mass%) of each steel according to the following formulas.

Ac ₁ = 723-10.7 × Mn% + 2
_{9.1 × Si%, Ac 3 =} 910-203 × (C%) 1/2 + 44.7 × S
i% + 31.5 × Mo% −30 × Mn% −11 × Cr%
+ 400 × Al%, after heating and holding at an annealing temperature calculated from these Ac ₁ and Ac ₃ transformation temperatures in a 10% H ₂ —N ₂ atmosphere, and then at a cooling rate of 0.1 to 10 ° C./sec. 650-700 ° C
By cooling to a temperature range, subsequently cooling to a plating bath temperature at a cooling rate of 0.1 to 20 ° C./sec, and immersing in a zinc plating bath at 460 to 470 ° C. with various bath compositions for 3 seconds. Plated.

For some steel sheets, Fe--Zn
As an alloying treatment, the plated steel plate is 400 to 550 ° C.
The temperature was maintained for 15 seconds to 20 minutes, and the Fe content in the plating layer was adjusted to be 5 to 20% by mass. The appearance of the plating was evaluated by visually observing the appearance of dross in the appearance of the plating surface and measuring the area of the unplated portion. The prepared plating was dissolved in a 5% hydrochloric acid solution containing an inhibitor and subjected to chemical analysis to determine the composition, which is shown in Table 2.

From Tables 2 and 3, the steels of the present invention all have appearance ratings of 5 and are excellent in strength / elongation balance.
On the other hand, all the comparative examples that do not satisfy the scope of the present invention have low appearance scores and poor balance of strength and elongation. Further, the steel sheet produced within the scope of the claims of the present invention has the microstructure described above, and is excellent in appearance and strength / elongation balance.

[0065]

[Table 2]

[0066]

[Table 3]

[0067]

[Table 4]

[0068]

EFFECTS OF THE INVENTION The high-strength hot-dip galvanized steel sheet of the present invention has an extremely excellent plating appearance and good workability, and is extremely effective for building materials, home appliances, automobile body applications and the like.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/00 302 C22C 38/00 302Z 38/12 38/12 38/58 38/58 C23C 2/02 C23C 2/02 2/40 2/40 (72) Inventor Manabu Takahashi 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technical Development Division (72) Inventor Yasuhide Morimoto 20-1 Shintomi, Futtsu City Nippon Steel Technology Development Headquarters (72) Inventor Masao Kurosaki 20-1 Shintomi, Futtsu City Nippon Steel Corporation (72) Inventor Akihiro Miyasaka 20-1 Shintomi, Futtsu City Nippon Steel Corporation F-Term within Company Technology Development Headquarters (reference) 4K027 AA02 AA05 AA23 AB02 AB14 AB28 AB42 AB44 AC12 AC73 AE12 AE22 AE27 4K037 EA01 EA02 EA05 EA06 EA09 EA10 EA11 EA12 EA32 EA21 EA32 EA21 EA32 EA21 EA23 EA21 EA21 EA21 EA21 EA21 EA21 EA21 EA21 EA21 EA20 33 EA35 EA36 EB07 EB08 EB11 EB12 EC02 FA02 FA03 FB00 FC04 FC05 FG00 FH00 FJ05 FJ06 FJ07 FK01 FK02 FK03 FK08 GA05 HA05

Claims (13)

[Claims] 1. In mass%, C: 0.0001 to 0.3% Si: 0.001 to less than 0.1% Mn: 0.01 to 3% Al: 0.001 to 4% Mo: 0. 001 to 1% P: 0.0001 to 0.3% S: 0.0001 to 0.1% on the surface of the steel sheet containing the balance Fe and unavoidable impurities, in mass%, Mn: 0.001 to A hot dip galvanized steel sheet containing 3% Al: 0.001 to 4% Mo: 0.0001 to 1% Fe: 5 to 20%, and a balance of Zn and unavoidable impurities in the steel. Assuming that the Mn content is X (mass%), the Si content in steel is Y (mass%), and the Al content in plating is Z (mass%), X, Y and Z satisfy the formula (1). High-strength hot-dip galvanized steel sheet with excellent appearance and workability. 0.6- (X / 18 + Y + Z) ≧ 0 (1) 2. In mass%, C: 0.0001 to 0.3% Si: 0.001 to less than 0.1% Mn: 0.01 to 3% Al: 0.001 to 4% Mo: 0. 001 to 1% P: 0.0001 to 0.3% S: 0.0001 to 0.1% on the surface of the steel sheet containing the balance Fe and unavoidable impurities, in mass%, Mn: 0.001 to A hot dip galvanized steel sheet containing 3% Al: 0.001 to 4% Mo: 0.0001 to 1% Fe: less than 5%, and a balance of Zn and an unavoidable impurity. When the content is X (mass%), the Si content in steel is Y (mass%), and the Al content in plating is Z) (mass%), X, Y and Z satisfy the formula (1). High-strength hot-dip galvanized steel sheet with excellent appearance and workability. 0.6- (X / 18 + Y + Z) ≧ 0 (1) 3. The plating layer is further mass%, Si: 0.0001-0.1%, W: 0.001-0.1%, Zr: 0.001-0.1%, Cs: 0. 0.001-0.1%, Rb: 0.001-0.1%, K: 0.001-0.1%, Ag: 0.001-5%, Na: 0.001-0.05%, Cd: 0.001 to 3%, Cu: 0.001 to 3%, Ni: 0.001 to 0.5%, Co: 0.001 to 1%, La: 0.001 to 0.1%, Tl : 0.001-8%, Nd: 0.001-0.1%, Y: 0.001-0.1%, In: 0.001-5%, Be: 0.001-0.1%, Cr: 0.001 to 0.05%, Pb: 0.001 to 1%, Hf: 0.001 to 0.1%, Tc: 0.001 to 0.1%, Ti: 0.001 to 0. %, Ge: 0.001 to 5%, Ta: 0.001 to 0.1%, V: 0.001 to 0.2%, B: 0.001 to 0.1%, one kind or two kinds. The high-strength hot-dip galvanized steel sheet having excellent appearance and workability according to claim 1 or 2, containing the above. 4. The steel further contains, by mass%, Cr: 0.001 to 25%, Ni: 0.001 to 10%, Cu: 0.001 to 5%, Co: 0.001 to 5%, W : 0.001 to 5% of 1 type, or 2 or more types, The high-strength hot-dip galvanized steel sheet excellent in the external appearance and workability of any one of Claims 1-3 characterized by the above-mentioned. 5. The steel further comprises Nb, Ti, and
A total of 0.001 to 1% of one or more of V, Zr, Hf, and Ta is contained.
4. A high-strength hot-dip galvanized steel sheet excellent in appearance and workability according to any one of 4 above. 6. The steel further contains B: 0.000 in mass%.
1 to 0.1% is contained, The 1 to 5 characterized by the above-mentioned.
1. A high-strength hot-dip galvanized steel sheet excellent in appearance and workability according to any one of 1. 7. The steel further comprises Y, Rem, C in mass%.
a, Mg, Ce, or a combination of 0.001 to 1
% Content of any one of claims 1 to 6
High-strength hot-dip galvanized steel sheet having excellent appearance and workability as described in the item. 8. The microstructure of steel has a volume fraction of 50-9.
7% of ferrite phase or ferrite phase and bainite phase as main phase, balance of martensite phase, retained austenite phase or both, total 3-50% in volume fraction
A high-strength hot-dip galvanized steel sheet excellent in appearance and workability according to any one of claims 1 to 7, characterized in that it has a composite structure of. 9. The microstructure of the steel sheet has a volume fraction of 70 to 70.
Main phase is 97% ferrite and its average grain size is 20μ
m or less, consisting of austenite and / or martensite with a volume fraction of 3 to 30% as the second phase, and the average particle size of the second phase is 10 μm or less. A high-strength hot-dip galvanized steel sheet excellent in appearance and workability according to any one of items. 10. The second phase of the steel sheet is austenite, the carbon content in the steel: C (mass%), and the Mn content in the steel: Mn.
(Mass%), volume ratio of austenite: Vγ (%), volume ratio of ferrite and bainite: Vα (%) is (2)
A high-strength galvanized steel sheet having excellent appearance and workability according to any one of claims 1 to 9, which satisfies the formula. (Vγ + Vα) / Vγ × C + Mn / 8 ≧ 2.000 (2) 11. The microstructure of a steel sheet has a volume fraction of 50.
~ 95% ferrite as the main phase, the average grain size is 20
μm or less, containing 3 to 30% by volume of austenite and / or martensite as the second phase,
The average particle size thereof is 10 μm or less, and further, it is made of bainite having a volume fraction of 2 to 47%, and is highly excellent in appearance and workability according to any one of claims 1 to 10. High strength galvanized steel sheet. 12. A method for producing the high-strength galvanized steel sheet according to any one of claims 1 to 11, wherein the components of the steel sheet according to any one of claims 1 and 4 to 7. Cast slab consisting of 11 as cast or after cooling 11
After heating again at 80 to 1250 ° C. and ending hot rolling at 880 to 1100 ° C., the rolled hot rolled steel sheet is pickled and cold rolled, and then 0.1 × (Ac ₃ −Ac ₁ ) + Ac ₁ (℃) or more A
After annealing for 10 seconds to 30 minutes in a temperature range of c ₃ +50 (° C.) or less, 650 to 70 at a cooling rate of 0.1 to 10 ° C./second.
It is cooled to a temperature range of 0 ° C, and subsequently 0.1 to 100 ° C /
Plating bath temperature −50 ° C ~ Plating bath temperature +5 at cooling rate of 2 seconds
After being cooled to 0 (° C), it is immersed in a plating bath and, including the immersion time, the plating bath temperature is -50 ° C to the plating bath temperature +50.
A method for producing a high-strength hot-dip galvanized steel sheet excellent in appearance and workability, which is characterized by holding in a temperature range of (° C) for 2 to 200 seconds and then cooling to room temperature. 13. The high strength excellent in appearance and workability according to claim 12, wherein alloying treatment is performed in a temperature range of 400 to 550 ° C. after immersion and holding in a plating bath and cooling to room temperature. Manufacturing method of hot-dip galvanized steel sheet.