JPH0617142A - Production of steel sheet for high strength galvannealing excellent in deep drawability and plating adhesion and production of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To produce a high strength galvannealed steel sheet excellent in plating adhesion as well as in deep drawability and also to produce a steel sheet for plating suitable for the steel sheet. CONSTITUTION:A steel having a composition containing, by weight, <=0.005% C, <=2% Mn, 0.05-0.15% P, <=0.005% N, <=0.02% S, and Ti by the amount in the range between {4[C]+3.43[N]+1.5[S]} and 0.1% (where [C], [N], and [S] represent respective weight percentages of C, N, and S) is soaked at >=1150 deg.C for >=30min and hot-rolled, and finish rolling is completed at a temp. in the range between the Ar3 transformation point and (Ar3 transformation point +100 deg.C). The resulting steel plate is coiled at 630-550 deg.C, pickled, and cold-rolled. Further, the resulting steel sheet for plating is used and this steel sheet is annealed at a temp. not lower than the recrystallization temp. and subjected to hot dipping by the use of a hot-dip galvanizing bath, in which Al content is controlled so that prescribed requirements are satisfied, and then to alloying treatment.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is for producing a high-strength hot-dip galvanized steel sheet which is excellent in workability, particularly deep drawability, and also has excellent plating adhesion and does not cause powdering during press forming. The present invention relates to a method, a method for producing a plated steel sheet for obtaining such a plated steel sheet, and the like.

[0002]

2. Description of the Related Art A base plate of an alloyed hot-dip galvanized steel sheet for use in difficult-to-form parts, such as inner and outer sheets of automobiles, is conventionally a steel sheet obtained by decarburizing and annealing low carbon Al killed steel, or Ti or N.
An ultra-low carbon steel plate (so-called IF steel plate) or the like to which a carbonitride forming element such as b was added was used. However, as the demand for weight reduction of the vehicle body for the purpose of improving fuel efficiency and reducing the amount of exhaust gas originated from the recent global environmental problems, or the demand for improving the production efficiency is gradually increasing, the difficult-to-form parts as described above are formed. On the other hand, in order to reduce the thickness of a steel sheet by increasing the strength, a high-strength galvannealed steel sheet having higher workability has been required.

As a method for obtaining a high-strength galvannealed steel sheet having good workability, Si, M is added to the ultra low carbon IF steel.
A method is already known in which a steel sheet to which a strengthening element such as n or P is added is used as a base material and hot dip galvannealing is applied to the base material.
For example, in JP-A-61-60860, P-added ultra-low carbon IF
A method of producing a galvanized steel sheet for deep drawing having good plating adhesion by disposing hot-dip galvanizing on a steel as a base and controlling the plating bath temperature and the Al concentration in the bath is disclosed. Further, in Japanese Patent Laid-Open No. 1-191748, ultra-low carbon Ti-added steel and Ti-Nb composite-added steel are added to Si, Mn,
A steel sheet to which P or the like is added is disclosed.

[0004]

However, when the inventors of the present invention have examined the techniques up to now from various angles, they have found the following problems. Ti
In the case of ultra-low carbon steel containing a large amount of P, if it is a mild steel sheet with a small amount of P added, it is deep-drawn after annealing by hot rolling at 650 ° C or higher after hot rolling and sufficient precipitation of carbides. It improves the sex. That is, when C is present in a solid solution state at the stage of hot-rolled sheet, during cold rolling and annealing, formation and development of a recrystallization texture preferable for deep drawing are inhibited, so that C
Is deposited as a carbide to make the adverse effect of solid solution C harmless. However, in a high-strength steel sheet containing a large amount of P, precipitation of FeTiP compound (hereinafter, simply referred to as P compound) occurs in the hot-rolled sheet due to high temperature winding, which rather deteriorates deep drawability after annealing.

FIG. 1 shows 0.0028% C-0.01% Si-0.25%
In Mn-0.071% P-0.053% Ti-0.0019% N steel, it is a graph which shows the result of having investigated how winding temperature affects deep drawability (r value) and P content in P compound. . As is clear from FIG. 1, the deep drawability after annealing is the best when the coiling temperature is around 600 ° C., and at higher temperatures, the deep drawability decreases conversely. In addition, when the precipitates in the hot-rolled steel sheet were investigated, the P compound was precipitated in the form of coarse feathers at the grain boundaries in the winding temperature range higher than 600 ° C. The influence of the taking temperature was obvious. Furthermore, as a result of a detailed study by the present inventors, even if the P compound is wound at a low temperature at which it is expected that the P compound will not precipitate, if the slab heating temperature is low, the P compound is precipitated in the austenite by heating for a long time. It was found that the deep drawability after annealing deteriorated as a result.

By the way, when a steel sheet containing Ti or P is used, these elements have a great influence on the alloying rate of Fe and Zn, and when proper alloying conditions cannot be obtained, in the alloyed plating layer In addition, insufficient alloying in which Zn remains in an unalloyed state, or excessive alloying in which the fragile Γ phase (Fe ₃ Zn ₁₀ ) grows thick due to excessive alloying within the surface of the alloyed plating layer is likely to appear. , Weldability, plating adhesion, or corrosion resistance after coating is significantly reduced. For example, it is said that Ti promotes the above alloying, and P delays the alloying conversely. However, when these elements are added in a complex manner, the actions of both are complicatedly entangled, and thus alloying is insufficient or alloying is performed. It is extremely difficult to quickly and accurately grasp the optimum operating conditions that do not cause excessive alloying, and a large reduction in yield due to excess or deficiency of alloying cannot be avoided.

In the above-mentioned Japanese Patent Laid-Open No. 61-60860, in order to obtain a plating layer having excellent quality characteristics, the plating bath temperature is 430-50.
Although the Al concentration in the bath is specified to be 0.05% or more at 0 ° C, T
The same conditions are applied to all steels in the range of i: 0.03% or less and P: 0.02 to 0.1%, and it cannot be said that the optimum conditions are necessarily obtained according to the steel composition. More Ti and P
In the steel to which is added, since the effects of these elements are complicatedly entangled as described above, it is more difficult to obtain a plating layer having a good degree of alloying.

On the other hand, Japanese Patent Publication No. 60-55589 discloses an alloyed zinc plating having excellent plating adhesion by adjusting the P content in steel and the effective Al concentration in the plating bath to values specified by a relational expression. A method of making a steel sheet is disclosed. However, the steel sheet applied by this method is an ordinary low carbon steel having a C content of 0.02 to 0.12% and containing no Ti, and is not an extremely low carbon steel to which Ti is added, but has a great influence on the alloying behavior. No consideration is given to Ti which influences.

The present invention has been made in view of the above circumstances, and an object thereof is a method for producing a high-strength galvannealed steel sheet which is excellent in both deep drawability and plating adhesion, And a method for producing a steel sheet for plating to obtain such a plated steel sheet.

[0010]

The method for producing a steel sheet for high-strength galvannealing according to the present invention, which has been able to solve the above-mentioned problems, is C: 0.005% by weight or less, Mn: 2% by weight or less, P : 0.05 to 0.15 wt%, N: 0.005 wt% or less, S: 0.02 wt% or less, Ti: {4 [C] +3.43 [N] +
1.5 [S]} to 0.1% by weight (however, [C], [N], and [S] are
Steels containing C, N, and S respectively) are soaked at a temperature of 1150 ° C. or higher for 30 minutes or more, and then hot-rolled, and at the Ar ₃ transformation point to (Ar ₃ transformation point + 100 ° C.) The point is to finish rolling at the above temperature, wind at 630 to 550 ° C., and then pickle and cold roll.
Further, after using the steel sheet for plating obtained by the above method and annealing it for recrystallization or more, hot dip galvanizing is performed using a hot dip galvanizing bath whose Al content is adjusted so as to satisfy the requirements of the following formula [I]. And then by alloying treatment,
The desired plated steel sheet can be obtained.

[0011]

[Equation 2]

[0012]

The present invention is constructed as described above, but in short,
From the viewpoint of improving the deep drawability, a steel sheet for plating having desired characteristics was obtained by controlling the precipitation behavior of the P compound in the hot rolling stage in the ultra-low carbon steel to which Ti and P were added in combination. In addition, by using the steel plate for plating obtained in this way and by strictly defining the optimum plating conditions that accurately correspond to the steel composition from the viewpoint of improving the quality of the plating layer, it is possible to achieve deep drawability and plating adhesion. In each case, excellent high-strength galvannealing was realized. Each constituent element of the present invention will be described below. First, the reasons for limiting the chemical composition of the steel sheet used in the present invention are as follows.

C: 0.005% by weight or less C is preferably as small as possible in improving press workability, particularly ductility. If the C content exceeds 0.005% by weight, sufficient ductility cannot be obtained, and the amount of Ti added to precipitate and fix the solid solution C in steel increases, so the upper limit is 0.005% by weight.

Mn: 2 wt% or less Mn is a safe element in that it does not impair plating adhesion even if it is contained in a large amount, and although it cannot be expected so much, it is effective for the time being. It is positioned as a strengthening element. Therefore, Mn is also added to the steel of the present invention, but if the Mn content exceeds 2% by weight, the deep drawability deteriorates and adversely affects the workability, so the upper limit is 2% by weight. .

P: 0.05 to 0.15% by weight P is less likely to deteriorate the deep drawability, while strengthening the steel. Therefore, in the present invention, the strength of the steel is increased mainly by adding P, and for that purpose, at least 0.
It is necessary to contain 05% by weight. However, if the content of P becomes excessive, not only will the weldability of the steel plate deteriorate, but
Since it also promotes secondary work brittleness, it should be 0.15% by weight or less.

N: 0.005 wt% or less If N is too large, the amount of addition of Ti necessary for depositing and fixing the N increases, which not only increases cost but also deteriorates workability, so 0.005 wt% is the upper limit. . S: 0.02 wt% or less S combines with Ti to form a sulfide. If it is too large, the ductility of the steel sheet deteriorates, so 0.02% by weight is the upper limit.

Ti: {4 [C] +3.43 [N] +1.5 [S]} to 0.1
% By weight (however, [C], [N], and [S] represent the weight% of C, N, and S, respectively) Ti is used to precipitate and fix C, N, and S in steel as precipitates. , Add more than each equal amount. Therefore Ti is
Addition of 4 [C] +3.43 [N] +1.5 [S] or more is required, and if insufficient, it causes deterioration of deep drawability and aging. On the other hand, not only is the effect saturated when added in excess of 0.1% by weight,
This is the upper limit because it will increase the cost.

The steel sheet used in the present invention contains each of the above elements as a basic component, but it is possible to further add 0.004% by weight or more of Nb in order to improve the anisotropy.
However, since Nb remarkably raises the recrystallization temperature, when Nb is added, its addition amount needs to be suppressed to 0.05% by weight or less. Further, in order to improve the secondary processing brittleness resistance, B can be further added in an amount of 0.0003% by weight or more. However, even if a large amount of B is added, the effect is saturated, so B
When adding, the upper limit of the addition amount is preferably 0.0025% by weight. Further, the steel of the present invention may contain Si or Al, but the range of these components should be specified as follows.

Si: 0.3 wt% or less Si can be an effective strengthening element, but if it is added in a large amount, the plating adhesion will be significantly deteriorated, so Si should be 0.3 wt% or less. If it is 0.3% by weight or less, the effect on the alloying behavior is very small and can be ignored.

Al: 0.01 to 0.1 wt% Al is preferably added in an amount of 0.01 wt% or more for deoxidation. However, if added excessively, not only the cost will be increased but also the surface properties will be deteriorated, so the amount should be 0.1% by weight or less.

The reasons for limiting each manufacturing condition in the present invention are as follows. First, the soaking condition of the slab is
At a temperature of 1150 ° C or higher for 30 minutes or longer. As a result, the FeTiP compound that precipitates during slow cooling before the slab is charged into the heating furnace is redissolved. If the temperature is lower than 1150 ° C, a considerable amount of the P compound remains as precipitated and deteriorates the deep drawability after annealing. In addition, in order to completely heat the slab to 1150 ° C or above, it is necessary to load it in the furnace for at least 30 minutes and soak it. The slab may be a cold piece slab cooled to room temperature or a high temperature slab of HCR as long as it is heated to 1150 ° C or higher.

Next, the hot rolling finishing temperature needs to be set to the Ar ₃ transformation point or higher. Below the Ar ₃ transformation point, a texture unfavorable for deep drawability after annealing develops, which is not preferable, and a hot rolled steel sheet with a random texture is obtained. Therefore, hot finishing may be performed at the Ar ₃ transformation point or higher. is necessary. However, if it is finished at a too high temperature, the grain growth of austenite will be remarkable and the ferrite grain size after transformation will become coarse, which will have an unfavorable effect on the deep drawability after annealing, so it is set to about (Ar ₃ transformation point + 100 ° C). There is a need.
Immediately after finish rolling, strong cooling of 50 to 100 ° C / sec is started to suppress precipitation of P compounds and to suppress ferrite grain growth in favor of deep drawability after annealing. It is preferable that the crystal grain is refined.

On the other hand, if the coiling temperature exceeds 630 ° C., precipitation of P compound occurs, and if it is less than 550 ° C., precipitation of TiC is insufficient and solute C remains in the steel, adversely affecting deep drawability after annealing. Therefore, the winding temperature must be 630 to 550 ° C.

After hot rolling, pickling, cold rolling and annealing are carried out in the usual manner. Cold rolling is a way to improve deep drawability.
It is desirable to carry out at 70 to 90%. From the viewpoint of workability, it is better to anneal at a temperature higher than the recrystallization temperature. However, if it is heated to a temperature above the transformation point, the workability deteriorates, which is not preferable.

After the recrystallization annealing, it is cooled, and subsequently hot-dip galvanized and alloyed. At this time, the alloying of Fe-Zn is started by destroying the Fe-Al alloy layer generated by the reaction between Al in the plating bath and the base Fe by mutual diffusion of the base Fe and Zn. The properties of the Fe-Al alloy layer are strongly affected by Ti and P in steel and Al in the bath, Ti promotes alloying, and P and Al in the bath delay alloying. Here, Al in the bath means Al dissolved in the bath.
And Al that forms dross by combining with Fe in the bath at a weight ratio of 1: 1. However, the influence on the properties of the Fe-Al alloy layer is dissolution in the bath, which is said to be effective Al. Is Al.

In the present invention, this effective Al (A in the bath A
(1 concentration-Fe concentration in the bath), the interaction between promotion of alloying by Ti and delay of alloying by P in the steel sheet is adjusted by the effective Al concentration to precisely control the alloying rate. . That is, the present inventors used various steel sheets having different Ti contents and P contents, performed a galvanizing treatment using hot dip galvanizing baths having different effective Al concentrations, and then immediately performed an alloying heat treatment. The conversion rate was investigated. As a result, it has been clarified that the Fe-Zn alloying rate becomes almost constant when the relationship of the following equation is established, and that an extremely accurate alloying operation with no excess or deficiency of alloying can be performed. Effective Al concentration in bath (%) = 0.100-1 / 12 x [P] +1/5 x [Ti] (where [P] and [Ti] are the amount of P and Ti in steel [wt%]
Represents)

That is, by adjusting the effective Al concentration in the plating bath to the value obtained from the term on the right side of the above formula according to the steel composition, the alloying treatment could be performed accurately and quickly. is there. Although it is optimal to operate under the conditions that satisfy the above equation, in practice, effective Al in the bath is
If the concentration is within the range of ± 5% of the optimum concentration, alloying will not be insufficient or over-alloying.
The relationship between the amount of P and the Al concentration in the bath was defined as in the above formula [I]. Therefore, if the above formula is less than 0.95 or more than 1.05, corrosion resistance and weldability after coating due to insufficient alloying or deterioration of plating adhesion due to excessive alloying will result.

As described above, in the present invention, the effective Al concentration in the hot dip galvanizing bath is adjusted according to the Ti and P contents in the raw steel sheet to control the alloying rate by the mutual diffusion of the raw materials Fe and Zn. The hot dip galvanizing bath at 430 to 480 ° C (this hot dip galvanizing bath is not particularly limited, although the hot dipping conditions and alloying heat treatment conditions are not particularly limited. Inside, a suitable amount of Al satisfying the above requirements is added) is a method of immersing and running the steel sheet to be treated, and the alloying heat treatment, the plated steel sheet 480 immediately after the plating treatment.
This is a method of heating at about 700 ° C for about 3 to 120 seconds. Needless to say, temper rolling may be performed after the galvannealing treatment.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the type of steel sheet, plating pretreatment method, hot dip plating conditions, alloys, etc. It is within the technical scope of the present invention to carry out by appropriately changing the chemical heat treatment conditions and the like as necessary.

[0030]

EXAMPLE Steels having the chemical compositions shown in Table 1 were melted and continuously cast into slabs. This slab has a slab heating temperature of 1150
1 to 3 hours at ~ 1200 ℃, 3.6mm at finishing temperature 900 ~ 940 ℃
It was hot rolled to a thickness and wound at the winding temperatures shown in Table 2, respectively. After pickling, cold rolling to a thickness of 0.8 mm, recrystallization annealing under the soaking conditions shown in Table 2 and subsequent effective A in the bath shown in Table 2
An alloying treatment was performed by plating in a 1-concentration plating bath. After temper rolling with an elongation of 0.5%, mechanical properties and plating adhesion, corrosion resistance after coating, secondary work embrittlement resistance, etc. were investigated. The results are also shown in Table 2.

The methods for evaluating the plating adhesion, the corrosion resistance after coating, and the secondary processing brittleness are as follows. <Plating adhesion> For each test steel plate that has undergone alloying heat treatment,
V-shaped bending was performed at an angle of 60 °, and the amount of plating removal when the compression side was tape-peeled was evaluated in three levels. ○: excellent, △: good, ×: bad

<Corrosion resistance after coating> After each plated steel sheet is treated with phosphate, it is coated with 3 coats for automobiles (specifically, electrodeposition paint + intermediate coat paint + top coat paint 3 layers, baking at 150 ° C for 20 minutes) ). A cross cut reaching the surface of the plating layer was put into this coated steel sheet, a salt water spray test was conducted for 1000 hours, and then a three-stage evaluation was made based on the degree of swelling of the coating film from the cross cut portion. ○: excellent, △: good, × bad

<Secondary work embrittlement resistance> Each test steel sheet has a drawing ratio of 2.
Deep-drawing was performed into a φ40 mm cylindrical cup at 3 and the cup opening was expanded with a truncated cone punch at 0 ° C., and the presence or absence of vertical cracking was evaluated. ○: No crack, ×: Crack occurred

[0034]

[Table 1]

[0035]

[Table 2]

As is clear from Table 2, the invention examples (No.
1, 3, 6, 8 to 10) have deep drawability with an r value of 2.0 or more, and show good characteristics of passing level in both corrosion resistance after coating and plating adhesion, whereas Comparative Examples ( No.5
7, 11 to 13), the r value is low because either the hot rolling coiling temperature or the chemical components are different from the conditions of the present invention, and Comparative Example (No. 2, 4) is a bath suitable for the component system. Since the medium effective Al concentration is not obtained, the corrosion resistance after coating and the plating adhesion are inferior to those of the examples of the present invention. Further, the comparative example (No. 14) has the same high r-value as the steel of the present invention, but the P content is high and vertical cracking occurred in the squeeze cup expansion test.

[0037]

As described above, according to the present invention, it is possible to obtain a high-strength galvannealed steel sheet having good plating adhesion and excellent press formability.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of the coiling temperature on the deep drawability of a P-added ultra-low carbon Ti steel and the amount of compound P in the hot-rolled sheet.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Internal reference number FI technical display location // C22C 38/00 301 ST 38/14 (72) Inventor Tomohiro Kase 1 Kanazawa-cho, Kakogawa-shi Kobe Steel Co., Ltd. Kakogawa Steel Works (72) Inventor Masaaki Urai No. 1 Kanazawa-machi, Kakogawa City Kobe Steel Co., Ltd. Kakogawa Steel Works

Claims (4)

[Claims] 1. C: 0.005 wt% or less, Mn: 2 wt%
Below, P: 0.05 to 0.15% by weight, N: 0.005% by weight or less,
S: 0.02 wt% or less, Ti: {4 [C] +3.43 [N] +1.5
Steel containing [S]} to 0.1 wt% (however, [C], [N], and [S] represent the wt% of C, N, and S, respectively) for 30 minutes or longer at a temperature of 1150 ° C or higher. After soaking and hot rolling, A
High strength alloyed molten zinc characterized by finishing rolling at a temperature of r ₃ transformation point to (Ar ₃ transformation point + 100 ° C.), winding at 630 to 550 ° C., pickling and cold rolling. Manufacturing method of steel plate for plating. 2. The manufacturing method according to claim 1, further comprising using steel containing Nb: 0.004 to 0.05% by weight. 3. The production method according to claim 1, further comprising a steel containing B: 0.0003 to 0.0025% by weight. 4. A steel sheet for plating obtained by the method according to claim 1, after being annealed at a temperature equal to or higher than a recrystallization temperature, Al is formed so as to satisfy the requirement of the following formula [I]. A method for producing a high-strength galvannealed steel sheet with excellent deep drawability and plating adhesion, which comprises hot-dip galvanizing using a hot-dip galvanizing bath whose content is adjusted, and then performing an alloying treatment. [Equation 1]