JPH0649920B2 - Method for producing high-strength galvannealed steel sheet with excellent plating adhesion and workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention has remarkably excellent plating adhesion, forming workability, weldability and corrosion resistance used for mechanical structural members including automobiles and general processing. The present invention relates to a method for producing a high-strength galvannealed steel sheet.

(Prior Art) Hot-rolled steel sheet or cold-rolled steel sheet is already subjected to spot galvanizing and spot-weldable galvannealing to improve corrosion resistance of mild steel sheet. However, when it is desired to use a high-strength steel sheet as the base material for energy saving and fuel consumption reduction, problems occur in this galvannealing due to the influence of the steel components. That is, non-plating occurs, adhesion of zinc deteriorates,
Alternatively, there are problems such as insufficient alloying. Furthermore, this hot-dip galvanizing is manufactured by a continuous hot-dip galvanizing line, but in many cases the heat cycle that gives sufficient characteristics to the base material is incompatible with the reduction of the steel plate surface for plating due to the line configuration. However, in terms of imparting such characteristics, great difficulty is involved.

Several attempts have been made to produce high-strength steel sheets with good workability by restricting the hot-dip galvanizing line. For example, the technique described in Japanese Examined Patent Publication No. 56-14130 relates to P-added steel, but has a relatively low tensile strength, which is different from the high strength of 500 MPa or more targeted by the present invention. Japanese Patent Publication Sho 58-30
In the technique described in Japanese Patent No. 933, C, Si, Mn, P, etc. are appropriately added, but it cannot be said that the plating characteristics such as plating adhesion are as excellent as those of mild steel sheets, and Si is restricted. Since it was unavoidable, the properties as a high strength steel sheet were also insufficient.

Although the present invention is intended to solve these various kinds of difficulties by utilizing the clad and identifying the components of the inner layer, there are some prior arts regarding this clad steel plate. JP-A-60-145384, JP-A-60-152
The techniques described in Japanese Patent No. 684 and Japanese Patent Laid-Open No. 62-13332 are related to hot-rolled steel sheets or cold-rolled steel sheets, and impart plating characteristics and good formability when passing through a plating line. I didn't mention anything about it.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for producing a high-strength hot-dip galvanized hot-rolled steel sheet or a cold-rolled steel sheet that has excellent plating properties and good workability and has a tensile strength of 500 to 700 MPa. is there. Here, the excellent plating property means that the plating property, that is, the corrosion resistance is not deteriorated by the processing in consideration of the use for automobiles, and for that reason, not only the non-plating etc. does not occur but also the plating adhesion is excellent. In addition, it means that the galvanized layer is properly alloyed with Fe to make it weldable. In addition, good workability in the present invention means TS
It means to have a low yield ratio (YR) in order to have excellent El balance and to improve the flow of material during molding.

(Means for Solving the Problem) The present invention intends to solve such problems by using specific surface layer and inner layer components and by applying specific hot dip galvanizing line conditions. The places to be are as follows.

(1) C: 0.05% or less, Si: 0.0
3% or less, Mn: 0.05 to 0.5%, P: 0.03% or less,
S: 0.015% or less, Al: 0.1% or less, balance Fe
And unavoidable impurities, and the other inside is C:
0.05 to 0.16%, Si: 0.6 to 2.5%, Mn: 3.5%
Hereinafter, P: 0.03% or less, S: 0.015% or less, Al: 0.0.
A steel containing 1% or less and Mn + 1 / 2Si ≧ 2.9% and the balance Fe and unavoidable impurities is made into a slab, which is hot-rolled and then 750 After being heated to ~ 900 ° C, it is cooled to 400 to 460 ° C at a cooling rate of 5 ° C / s or more, and at this temperature range, 1
Hold for up to 60 s, apply hot dip galvanizing after this hold, and then raise the temperature to 530 to 600 ° C.
A method for producing a high-strength galvannealed steel sheet having remarkably excellent plating adhesion and workability, which is characterized by cooling to room temperature after alloying the surface galvanized layer for 0 seconds.

(2) C: 0.05% or less, Si: 0.0
3% or less, Mn: 0.05 to 0.5%, P: 0.03% or less,
S: 0.015% or less, Al: 0.1% or less, balance Fe
And unavoidable impurities, and the other inside is C:
0.05 to 0.16%, Si: 0.6 to 2.5%, Mn: 3.5%
Hereinafter, P: 0.03% or less, S: 0.015% or less, Al: 0.0.
1% or less, and Mn + 1 / 2Si ≧ 2.9%, Ca: 0.0010 to 0.0040%, REM: 0.00
5 to 0.04%, Zr: 0.005 to 0.04%, Cr: 0.1
After forming a steel containing at least one of 0.5% to 0.1% and the balance Fe and unavoidable impurities into a slab, hot-rolling the steel, and then hot-rolling the hot-dip galvanizing line.
After heating to 0 ° C, 400 to 46 at a cooling rate of 5 ° C / s or more
After cooling to 0 ° C., holding in this temperature range for 1 to 60 s, hot-dip galvanizing after this holding, 530 to 600 ° C.
High temperature alloyed hot-dip galvanized alloy, which is characterized by cooling to room temperature after alloying the surface zinc-plated layer by holding it for 1 to 10 s in that temperature range and cooling to room temperature. Manufacturing method of plated steel sheet.

(3) C: 0.05% or less, Si: 0.0
3% or less, Mn: 0.05 to 0.5%, P: 0.03% or less,
S: 0.015% or less, Al: 0.1% or less, balance Fe
And unavoidable impurities, and the other inside is C:
0.05 to 0.16%, Si: 0.6 to 2.5%, Mn: 3.5%
Hereinafter, P: 0.03% or less, S: 0.015% or less, Al: 0.0.
A steel containing 1% or less and Mn + 1 / 2Si ≧ 2.9% and the balance Fe and unavoidable impurities is made into a slab, which is hot-rolled and cold-rolled, and then hot-dipped galvanizing line is passed. After heating to 750-900 ° C, 5 ° C /
It is cooled to 400 to 460 ° C. at a cooling rate of s or more, is held in this temperature range for 1 to 60 s, is hot-dip galvanized after this holding, and is heated to 530 to 600 ° C. A method for producing a high-strength galvannealed steel sheet having remarkably excellent plating adhesion and workability, which is characterized by cooling to room temperature after holding for 10 seconds to alloy the surface galvanized layer.

(4) C: 0.05% or less, Si: 0.0
3% or less, Mn: 0.05 to 0.5%, P: 0.03% or less,
S: 0.015% or less, Al: 0.1% or less, balance Fe
And unavoidable impurities, and the other inside is C:
0.05 to 0.16%, Si: 0.6 to 2.5%, Mn: 3.5%
Hereinafter, P: 0.03% or less, S: 0.015% or less, Al: 0.0.
1% or less, and Mn + 1 / 2Si ≧ 2.9%, Ca: 0.0010 to 0.0040%, REM: 0.00
5 to 0.04%, Zr: 0.005 to 0.04%, Cr: 0.1
When steel containing at least one of 0.5% to 0.5% and the balance Fe and unavoidable impurities is made into a slab, hot-rolled and cold-rolled, and then passed through a hot dip galvanizing line,
After heating to 750 to 900 ° C, it is cooled to 400 to 460 ° C at a cooling rate of 5 ° C / s or more, and 1 to 60s in this temperature range.
After holding and hot-dip galvanizing after this holding, 53
High strength that is remarkably excellent in plating adhesion and workability, which is characterized by heating up to 0 to 600 ° C., alloying the surface zinc plating layer by holding for 1 to 10 seconds in that temperature range, and then cooling to room temperature. A method for manufacturing a galvannealed steel sheet.

(Operation) Next, the operation of each requirement and the reason for limiting the numerical value will be described.

First, the components of the surface layer will be described.

C: 0.05% or less. If it exceeds 0.05%, coarse carbides are generated, which deteriorates the plating characteristics of that portion and causes partial non-plating. The lower limit is not particularly specified and may be lowered to about 10 to 20 ppm by vacuum degassing or the like.

Si: The amount of Si added to the surface layer has an extremely bad influence on the hot-dip galvanizing property and its alloying characteristics, and therefore it must be reduced as much as possible. Therefore, it is set to 0.03% or less. Preferably 0.
It should be below 02%.

Mn: Mn is also insufficient for the reduction of the surface layer prior to hot dip galvanizing when the oxide layer is present. Therefore, it should be 0.5% or less. On the other hand, Mn has a role of fixing S, which is an impurity in the steel, as MnS and avoiding hot embrittlement due to S. Therefore, at least 0.05% is necessary.

P: P affects the alloying characteristics of the galvanized layer, and if it is too large, alloying will be accelerated and a Γ phase, which is unfavorable to the workability, tends to occur. Therefore, 0.03% or less.

S: S is an impurity, which acts as an inclusion and deteriorates the ductility of steel, and also causes hot brittleness, so 0.015%
Below. However, in the case of the surface layer, the adverse effect as inclusions is not so great, so it is not necessary to dare to make the extremely low S.

Al: Al is used as a deoxidizer, and as a result, it remains as solid solution Al in the steel to some extent, but if it is too much, it becomes an inclusion and hinders the ductility of the steel, so it is made 0.1% or less. Further, solid solution Al hinders reduction prior to hot dip galvanizing, so it is preferably made 0.03% or less. Alternatively, it is preferable not to use Al for Ti deoxidation or to use an extremely small amount, and in this case, the Al amount is 0.005% or less.

Next, the components of the inner layer will be described.

C: C forms a martensite phase and imparts workability and strength to the steel. For that, 0.05% is necessary. On the other hand, addition of more than 0.16% deteriorates the weldability of steel.

Si: The workability of the steel of the present invention is intended to be dramatically improved by forming a dual phase structure of ferrite and martensite, in which case Si exerts an extremely important action. In particular, Si is indispensable for manufacturing in the insufficient heat treatment line called the hot dip galvanizing line as in the present invention. That is, Si pushes C out of α into the γ phase to stabilize γ during heating in the α / γ2 phase region, and suppresses the precipitation of carbide from this γ to the target martensite to cause a two-phase structure. There is an effect that In particular, in the hot-dip galvanizing line where the cooling rate after heating is insufficient, and the retention must be entered in and before the galvanizing bath, γ easily transforms into pearlite or bainite, but with Si, By identifying Si and Mn, which will be described later, this transformation was successfully suppressed and a two-phase structure was obtained in the hot dip galvanizing line. In order to exert these effects, the Si content must be at least 0.6%. On the other hand, when the Si amount becomes about 2.5%, such an effect tends to be saturated, and the addition of excessively high concentration impairs the economical efficiency, so the upper limit of the addition is 2.
It was set to 5%.

Mn: Mn enhances hardenability and also promotes two-phase organization. However, the upper limit was set to 3.5% because it saturates at about 3.5% and also impairs economic efficiency. The lower limit is determined by the following requirements.

Mn + 1 / 2Si: Si requires a certain minimum amount, but it is compatible with Mn in the sense of enhancing the hardenability of steel.
Regulated by + 1 / 2Si. If this amount is less than 2.9%, the hardenability is insufficient and a good two-phase structure cannot be obtained.

P: In the case of a two-phase structure, P deteriorates the toughness of steel, so 0.
It should be 03% or less. Preferably, the high pure steel level should be 0.01% or less.

S: S is an impurity, which acts as an inclusion and deteriorates the ductility of steel, especially the local ductility such as stretch flangeability and hole expandability.
Therefore, it is necessary to set it to 0.015% or less. For such molding, S is preferably 0.005% or less.

Al: Al is used as a deoxidizer, and as a result, it remains as solid solution Al in the steel to some extent, but if it is too much, it becomes an inclusion and hinders the ductility of the steel, so it is made 0.1% or less.

Next, when local ductility is particularly important, C
a: 0.0010 to 0.0040%, REM: 0.005 to 0.04%, Zr:
Add one or more of 0.005-0.04%. This is to make MnS in the steel spherical and reduce the adverse effects of inclusions. If it is less than the lower limit, it has no effect, and it saturates at the upper limit. In addition, Cr: 0.
1-0.5% may be added. Below the lower limit, there is no effect, and the upper limit was set from an economic point of view.

The starting steel of the present invention is cast to have such an inner layer and an outer layer, and then hot-rolled. After hot rolling, it is pickled, and in the case of a hot rolled steel sheet original plate, it is passed through, and in the case of a cold rolled steel sheet original plate, it is passed through a hot dip galvanizing line after cold rolling. First of all, the heating temperature is important in the hot dip galvanizing line. Heating temperature is 750 ℃
If it is less than the above, a sufficient α / γ state is not obtained and a two-phase structure cannot be obtained. On the other hand, heating above 900 ° C. results in a γ single phase, and since the crystal grains become coarse, the heating temperature is set to 900 ° C. or lower. Next, the galvanizing bath is cooled, but the cooling rate is 5 ° C./s or more. If it is less than this, pearlite or the like is mixed during cooling, and sufficient characteristics cannot be obtained. The cooling end temperature is 400 to 460 ° C. This temperature is necessarily determined by the temperature of the zinc bath. However, some fluctuations can be absorbed by adjusting the plating bath temperature, and there is no effect on the material. It is held for plate temperature adjustment etc. before plating immersion,
It is natural that this time should be short because this temperature is in the bainite transformation region. Usually 1s (s is seco
Since nd (second)) is taken, the minimum value is set to 1 s. Even if the components are adjusted as in the present invention, holding for more than 60 s causes bainite transformation or the like, and a sufficient two-phase structure cannot be obtained, so the upper limit is made 60 s. After this holding, the steel strip is immersed in a zinc bath and subsequently heated for alloying treatment. The alloying treatment is usually performed at 530 to 600 ° C., 1 to
The condition of holding for 10 s is sufficient.

When the clad steel sheet of the present invention is produced by casting, for example, the two-line IN method (IN: immersion nozzle) disclosed in JP-B-44-27361 can be used. When this method is used, the tundish is divided into two chambers, and one dipping nozzle is inserted from each chamber, a total of two dipping nozzles being inserted into the mold for casting. Molten steels with different components are separately poured into each chamber in the tundish from a ladle.

In addition, a method of partitioning the inside of the mold into areas corresponding to the inner layer portion and the surface layer portion of the clad steel plate and adding an alloying element to the inner layer portion with a wire, or arranging the mold in two stages and casting the inner layer portion with the upper mold It can be manufactured by a method of casting the surface layer portion with a lower mold after the casting.

As the clad ratio, it is preferable to control the inner layer / surface layer ratio to 4 to 20. Here, the surface layer means a part where the front and back surfaces are combined. If it is less than 4, the ratio of the inner layer is small and it is difficult to obtain a predetermined material. On the other hand, if it exceeds 20, the surface layer is too thin and the components of the inner layer diffuse to affect the surface layer, degrading the plating characteristics.

After casting, it is continuously cast or slab-rolled into steel slabs, which are then hot-rolled.However, direct-rolling rolling that directly rolls without passing through a hot-rolling heating furnace, or hot-charge rolling that inserts hot strips is adopted. I don't care. When heating, the heating temperature is 1
000 to 1300 ° C. Other hot rolling conditions are not particularly limited as long as the finishing temperature is 950 ° C. or lower at the finishing transformation temperature of Ar ₃ transformation point of the component of the surface layer portion and the winding temperature is 100 to 700 ° C. However, since it is preferable to avoid coarsening of the inner layer structure, the finishing temperature is preferably 910 ° C or lower. When cold rolling, the cold rolling rate is not particularly limited. It is usually 50 to 80%.

(Example) The surface layer and the inner layer were adjusted to the chemical composition shown in Table 1, and 2
It was melted by the IN method. However, the steel with the code E'was melted up to the surface layer as this component. The clad rate was 7.5 to 9 although it varied somewhat depending on the slab site.

This slab is heated at a temperature of 1100 to 1130 ° C, a finishing temperature of 880 to 900 ° C, and a winding temperature of 600 to 630.
Hot rolling was performed under the condition of ° C. When this hot-rolled coil was used as a base plate of a hot-rolled steel plate, when it was used as a base plate of a cold-rolled steel plate, it was cold-rolled by 75% and then passed through a hot dip galvanizing line. The hot dip galvanizing line is a non-oxidizing furnace-reducing furnace system. After exiting the reduction furnace, it was rapidly cooled by gas jet cooling with a narrowed gap between the nozzle and the plate to increase the cooling capacity, or roll contact cooling. The quenching end temperature was 440 to 455 ° C.
Then, after being held, it is immersed in a galvanizing bath, and 0.1% Al was added to the galvanizing bath. Subsequently, the temperature was raised and the plating layer was alloyed. Table 2 shows the plating conditions and the properties of the obtained galvannealed steel sheet.

As is clear from Table 2 and FIG. 1, the mechanical properties of the steel sheet according to the present invention show excellent high elongation and low yield ratio in relation to the tensile strength, and also have an alloying hot-dip galvanizing property. Are better. On the other hand, the comparative steel sheets are either inferior in mechanical properties and / or do not meet the plating properties or both.

(Effects of the Invention) According to the present invention, it has been possible to impart high workability to high-strength steel sheets with high rust-prevention properties, which have been increasingly demanded. Members or underbody have high strength and high rust prevention,
The durability or strength of the car is further improved. This leads to prevention of waste of natural fuel such as reduction of fuel consumption, or improvement of collision strength to improve safety, which has great social significance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between tensile strength (TS) and elongation (El) and yield ratio (YR) of alloyed hot-dip galvanized steel sheets of Examples. In the figure, the symbol ◯ ■ represents the present invention and the symbol ＊ ×
Indicates comparison.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical indication C23C 2/02 2/06 (72) Inventor Ryuji Takahashi 1 Kimitsu, Kimitsu-shi, Chiba New Nippon Steel Stocks Incorporated by Kimitsu Works (56) References JP-A-54-126638 (JP, A) JP-A-61-217529 (JP, A) JP-B-1-44776 (JP, B2)

Claims (4)

[Claims] 1. A surface layer comprising C: 0.05% or less and S
i: 0.03% or less, Mn: 0.05 to 0.5%, P: 0.03
% Or less, S: 0.015% or less, Al: 0.1% or less,
The balance consists of Fe and unavoidable impurities, and the other internal parts are C: 0.05 to 0.16%, Si: 0.6 to 2.5%, M
Contains n: 3.5% or less, P: 0.03% or less, S: 0.015% or less, Al: 0.1% or less, and Mn + 1 / 2Si ≧ 2.
A steel containing 9% of the balance Fe and unavoidable impurities was made into a slab, which was hot-rolled and subsequently heated to 750 to 900 ° C. when passing through a hot dip galvanizing line.
It is cooled to 400 to 460 ° C. at a cooling rate of s or more, is held in this temperature range for 1 to 60 s, is hot-dip galvanized after this holding, and is heated to 530 to 600 ° C. A method for producing a high-strength galvannealed steel sheet having remarkably excellent plating adhesion and workability, which is characterized by cooling to room temperature after holding for 10 seconds to alloy the surface galvanized layer. 2. The component of the surface layer portion is C: 0.05% or less, S
i: 0.03% or less, Mn: 0.05 to 0.5%, P: 0.03
% Or less, S: 0.015% or less, Al: 0.1% or less,
The balance consists of Fe and unavoidable impurities, and the other internal parts are C: 0.05 to 0.16%, Si: 0.6 to 2.5%, M
Contains n: 3.5% or less, P: 0.03% or less, S: 0.015% or less, Al: 0.1% or less, and Mn + 1 / 2Si ≧ 2.
9%, Ca: 0.0010 to 0.0040%, RE
M: 0.005 to 0.04%, Zr: 0.005 to 0.04%,
Cr: Steel containing at least one of 0.1 to 0.5% and the balance Fe and unavoidable impurities is made into a slab, which is hot-rolled and then passed through a hot dip galvanizing line.
After heating to 750 to 900 ° C, it is cooled to 400 to 460 ° C at a cooling rate of 5 ° C / s or more, and 1 to 60s in this temperature range.
After holding and hot-dip galvanizing after this holding, 53
High strength that is remarkably excellent in plating adhesion and workability, which is characterized by heating up to 0 to 600 ° C., alloying the surface zinc plating layer by holding for 1 to 10 seconds in that temperature range, and then cooling to room temperature. A method for manufacturing a galvannealed steel sheet. 3. The surface layer component is C: 0.05% or less, S
i: 0.03% or less, Mn: 0.05 to 0.5%, P: 0.03
% Or less, S: 0.015% or less, Al: 0.1% or less,
The balance consists of Fe and unavoidable impurities, and the other internal parts are C: 0.05 to 0.16%, Si: 0.6 to 2.5%, M
Contains n: 3.5% or less, P: 0.03% or less, S: 0.015% or less, Al: 0.1% or less, and Mn + 1 / 2Si ≧ 2.
A steel containing 9% of the balance Fe and unavoidable impurities was made into a slab, which was hot-rolled and cold-rolled. Then, when hot-dip galvanizing line was passed through the plate, after heating to 750 to 900 ° C., 5 ° C./s It is cooled to 400 to 460 ° C. at the above cooling rate, is held in this temperature range for 1 to 60 s, is hot-dipped after this holding, is heated to 530 to 600 ° C., and is in this temperature range for 1 to 10 s. A method for producing a high-strength hot-dip galvanized steel sheet having remarkably excellent plating adhesion and workability, which comprises holding and alloying the surface galvanized layer, and then cooling to room temperature. 4. The component of the surface layer portion is C: 0.05% or less, S
i: 0.03% or less, Mn: 0.05 to 0.5%, P: 0.03
% Or less, S: 0.015% or less, Al: 0.1% or less,
The balance consists of Fe and unavoidable impurities, and the other internal parts are C: 0.05 to 0.16%, Si: 0.6 to 2.5%, M
Contains n: 3.5% or less, P: 0.03% or less, S: 0.015% or less, Al: 0.1% or less, and Mn + 1 / 2Si ≧ 2.
9%, Ca: 0.0010 to 0.0040%, RE
M: 0.005 to 0.04%, Zr: 0.005 to 0.04%,
Cr: A steel containing at least one of 0.1 to 0.5% and the balance Fe and unavoidable impurities is made into a slab, which is hot-rolled and cold-rolled, and then passed through a hot dip galvanizing line. After heating to 750 to 900 ° C., it is cooled to 400 to 460 ° C. at a cooling rate of 5 ° C./s or more, and at this temperature range, 1
Hold for up to 60 s, apply hot dip galvanizing after this hold, and then raise the temperature to 530 to 600 ° C.
A method for producing a high-strength galvannealed steel sheet having remarkably excellent plating adhesion and workability, which is characterized by cooling to room temperature after alloying the surface galvanized layer for 0 seconds.