JPH05311244A - Manufacture of galvannealed steel sheet excellent in stretch flanging property using high strength hot rolled original steel sheet - Google Patents

Abstract

PURPOSE:To obtain the subjective high strength galvanized steel sheet having excellent workability superior to that of a hot rolled original steel sheet, particularly, excellent stretch flanging properties without executing cold rolling by using a high strength hot rolled steel sheet having >=45kg tensile strength as an original sheet. CONSTITUTION:Steel having compn. contg., by weight, 0.02 to 0.30% C, <=1.50% Si, 0.60 to 3.0% Mn, <=0.20% P, <=0.05% S and 0.01 to 0.10% Al, and the balance Fe with inevitable impurities is subjected to hot rolling at the Ac3 point or above. This hot rolled steel sheet is picked, and after that, is held under heating to the Ac1 point or above without executing cold rolling, in a continuous annealing and galvanizing line, and subsequently, is quenched to the Ms point or below during it reaches a hot dip zinc bath to produce partial or complete martensite in the steel sheet. Next the steel sheet is heated to the hot dip zinc bath temp. and galvannealing temp., i.e., the Ms point or above to produce partial or complete tempered martensite in the steel sheet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention uses a high-strength hot-rolled steel sheet having a tensile strength of 45 kg or more as an original sheet to strengthen the transformation structure by heat history in a continuous annealing hot-dip galvanizing line without cold rolling. The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet having excellent workability equivalent to or higher than that of a hot-rolled raw sheet, particularly excellent stretch flangeability.

[0002]

2. Description of the Related Art In recent years,
There is a strong need for hot-rolled galvanized steel sheets with strong working and high strength from the viewpoint of reducing the weight of reinforcing members and underbody members of automobiles and improving rust prevention performance.

However, hot-dip galvanized steel sheets are mainly cold-rolled steel sheets (JP-A-63-179024, JP-A-6-196024).
0-204827), hot-dip galvanizing of high-strength hot-rolled steel sheets has not been put into practical use yet, but recently, there is a tendency to be galvanized.
No. 44423 proposes a method for producing a hot-dip galvanized steel sheet having excellent workability.

[0004] However, this proposal specifies only the soaking temperature before plating, and under such condition specifications, sufficient workability cannot always be obtained with high component steel having a TS of 50 kg or more. ..

On the other hand, as for the high-strength hot-rolled steel sheet excellent in stretch flangeability, a transformation-structure strengthened ferrite-bainite-structured steel has already been developed as it is (Japanese Patent Application No. 59-252242). , Japanese Patent Application No. 60-298
542).

[0006] However, when hot-dip galvanizing the transformation structure reinforcing material is subject to heat history due to plating, the local ductility is markedly reduced, and there is concern about workability, and the above-mentioned plating cannot be applied. ..

[0007] However, the applicant of the present invention has previously found that the continuous annealing hot-dip galvanizing line has a high λ with excellent stretch flangeability.
Manufacturing conditions for obtaining a hot-rolled raw sheet include: lowering the soaking temperature (T) (≦ 700 ° C.), and lowering the secondary cooling rate (CR ₂ ) (≦ 10 ° C./s). Although it was proposed (Japanese Patent Application No. 2-416280), the appropriate range of T and CR ₂ becomes narrower as the tensile strength becomes higher, and the stretch flangeability (λ
Value) is not easy to obtain.

On the other hand, in order to widen the range of manufacturing conditions in the plating line, a method of manufacturing a high stretch-flangeability (λ) steel plate by only heat treatment of the plating line can be considered regardless of the characteristics of the hot-rolled base plate. In the conventional hot-dip galvanizing process, a high-λ hot-rolled hot-dip galvanized steel sheet having excellent plating adhesion has not been obtained.

The reason is that the tempered martensitic steel has the most excellent mechanical property value and λ value, but it has not been heated before entering the plating tank (pot). Producing martensite by cooling control after soaking and heating in the plating line, even after tempering, it is difficult to galvanize because of the low temperature of the steel sheet before the plating tank, and hot-dip galvanized steel sheet with excellent workability is obtained. Because I could not get it.

The present invention solves the above-mentioned problems of the prior art, and uses a high-strength hot-rolled steel sheet having a tensile strength of 45 kg or more as a base sheet, in a continuous annealing hot-dip galvanizing line without cold rolling. It is an object of the present invention to provide a method for producing a high-strength hot-dip galvanized steel sheet having excellent workability equal to or higher than that of a hot-rolled raw sheet, particularly excellent stretch flangeability.

[0011]

In order to solve these problems, the present inventor is keenly aware of a method for producing a high-strength hot-dip galvanized steel sheet having excellent stretch-flange formability using a hot-rolled steel sheet as a base sheet. As a result of repeated research, by using the temperature control zone in front of the plating tank in the continuous annealing hot-dip galvanizing line, ・ Ac soaking heating of ₁ point or more, ・ cooling after soaking heating, ・ after cooling, plating bath It was discovered that by controlling the structure, carbide shape, and solid solution C under a wide range of manufacturing conditions such as heating to a temperature higher than the bath temperature before entering, it is possible to stably manufacture high-strength hot-dip galvanized steel sheet with excellent local ductility. Then, the present invention is completed here.

That is, according to the present invention, C: 0.02 to 0.3.
0%, Si: 1.50% or less, Mn: 0.60 to 3.0%,
P: 0.20% or less, S: 0.05% or less, Al: 0.01
~ 0.10%, if necessary, further Mo: 0.0
1-1.0%, Cr: 0.10-1.5%, Nb: 0.01-
0.05%, Ti: 0.01 to 0.5%, B: 5 to 30 pp
m, Ca: For a steel containing at least one of 0.01% or less and the balance being Fe and inevitable impurities, pickling hot-rolled steel sheet hot-rolled at Ac ₃ points or more After that, in a continuous annealing hot-dip galvanizing line without cold rolling, after heating and holding at Ac ₁ point or higher, it is rapidly cooled to Ms point or lower until reaching the hot dip galvanizing bath, and partially or in the steel sheet. After forming all the partial martensite, it is heated by the molten zinc bath temperature and the alloying furnace temperature above the Ms point to form the partially or fully tempered martensite, which is excellent in stretch flangeability. The gist is a method of manufacturing a galvannealed steel sheet.

The present invention will be described in more detail below.

[0014]

[Action]

First, the reasons for limiting the chemical composition of steel in the present invention will be shown.

C: C component has the effect of improving the strength of the steel, but if the content is less than 0.02%, the desired strength cannot be secured, while 0.30% is required. If it exceeds, the weldability is deteriorated, so the C content is from 0.02 to
The range is 0.30%.

Si: The Si component has a function of effectively increasing the strength and the ductility of steel even if added in a small amount through solid solution hardening. However, if the content exceeds 1.50%, the weldability is deteriorated, so the Si content is 1.5.
It is 0% or less. It should be noted that, particularly when the scale property of the steel sheet surface is strictly demanded, it is preferably made 0.07% or less.

Mn: Mn components include solid solution strengthening, transformation strengthening,
Fine grain strengthening has the effect of improving both strength and toughness of steel. However, if its content is less than 0.60%, the desired effect cannot be obtained, while if it exceeds 3.0%, the weldability deteriorates, so the Mn content is 0.60-3.
The range is 0%.

P: The P component is an element unfavorable for the ductility of steel, and it is preferable that the content is as small as possible. In the present invention, the upper limit is set to 0.20% in consideration of the fact that the adverse effect of P can be reduced in the low temperature winding of hot rolling and the economy. It is preferably 0.02% or less.

S: The S component is combined with Mn to form an A-type inclusion, which lowers the ductility and deteriorates the vertical cracking resistance. Therefore, the S content is preferably as small as possible. Considering economy, the upper limit is 0.0
5%. It is preferably 0.005% or less.

Al: The Al component is an element necessary for deoxidizing steel, and 0.01 is necessary for ensuring a sufficient deoxidizing effect.
% Or more must be contained. However, if it exceeds 0.10%, the effect is saturated, and the number of cluster-like inclusions increases, and the workability deteriorates. Therefore, the upper limit is 0.10.
%.

The above elements are essential components, but if necessary, at least one of the following Mo, Cr, Nb, Ti, B and Ca can be contained in an appropriate amount.

Mo, Cr: The addition of Mo and Cr is effective in stabilizing the austenite phase and facilitating the formation of a low temperature transformation product in the cooling process. In order to exert this effect effectively, it is necessary to add Mo in an amount of 0.01% or more and Cr in an amount of 0.10% or more. Since alloying elements are effective, the upper limits are set to 1.0% for Mo and 1.5% for Cr.

Nb: The Nb component is a preferred element that imparts a large increase in strength and excellent low temperature toughness when added in a trace amount. Further, it facilitates the formation of a low temperature generation phase such as bainite, which is effective in improving the local ductility. The lower limit is 0.01%, which is the amount at which such effects can be expected. However, if the content exceeds 0.05%, the weldability deteriorates, so the upper limit is 0.0.
5%.

Ti: The Ti component, like Nb, is added to stabilize the quality inside the coil and strengthen the steel. To obtain the effect, 0.01% or more is required. But,
If it exceeds 0.5%, the effect is saturated and it is economically disadvantageous, so the upper limit is made 0.5%.

B: The addition of the B component is effective for enhancing the hardenability of the steel and obtaining the required structure, and at least 5 ppm is necessary for the effective expression of such effect. .. However, even if added excessively, not only the effect of improving the hardenability is saturated, but also the ductility is deteriorated, so the upper limit of the addition amount is set to 30 ppm.

The Ca: Ca component is preferable for improving low temperature toughness and ductility by controlling the morphology of inclusions, but if it is too large, the amount of inclusions in the steel increases and ductility deteriorates, so the upper limit is 0.01. %.

Next, the manufacturing conditions in the present invention will be described.

The steel is subjected to hot rolling, but without cold rolling, to a continuous annealed galvanizing line. The hot rolling conditions are not particularly limited, but hot rolling is preferably performed at an Ac ₃ point or higher.

However, the thermal history conditions of the continuous annealing hot dip galvanizing line must be the following conditions.

Soaking and heating temperature (T): In order to obtain a uniform tempered martensite by uniformly or partially converting the whole or part of the hot-rolled base plate into martensite, the hot-rolled base plate is made of Ac ₁ It is necessary to perform uniform heating above the point (two-phase region or γ region).

Cooling rate (CR): quenching in a short striping time,
Since tempering is required, it is necessary to control the cooling rate and generate martensite at a cooling stop temperature as close to the bath temperature as possible. For this purpose, it is necessary to rapidly cool it below the Ms point before reaching the molten zinc bath. The cooling rate (C
R) is preferably a cooling rate that satisfies the following equation in relation to the components. LnCR ≧ −1.18 Mneq + 3.37 where Mneq = Mn + 1.10Cr + 0.10Si + 2.1
P + 1.52

Heating temperature in plating bath and alloying furnace: Ms
Martensite produced by quenching below the melting point is tempered at a molten zinc bath temperature and alloying temperature above the Ms point in the plating tank and alloying furnace during the subsequent plating treatment to produce partially or fully tempered martensite. To do. As shown in FIG. 1, it is necessary to install a heater in the continuous annealing hot-dip galvanizing line and raise the temperature in front of the plating tank. Thereby, the molten zinc is well adapted to the surface of the steel sheet, and the effect of improving the plating adhesion of the zinc layer can be obtained.

Next, examples of the present invention will be described.

[0035]

【Example】

[Table 1] For the sample steel with the chemical composition shown in Fig. 1, a hot-rolled raw sheet with a ferrite + pearlite structure coiled at 580 ° C was prepared.

[Table 2] It processed by the continuous annealing hot-dip galvanizing line shown in. The plating bath temperature is 450 ° C and the alloying temperature is 560 ° C.
Further, the cooling stop temperature in Table 2 is a stop temperature when the soaking temperature is rapidly cooled, and the pre-plating bath temperature is a temperature immediately before heating after entering the plating bath after the rapid cooling.

Table 2 shows the mechanical properties and surface quality of the obtained steel sheet. From the table, all of the examples of the present invention have excellent workability.
It can be seen that the surface quality is shown as (λ value> 80%).

[0037]

As described in detail above, according to the present invention,
High strength hot rolled steel sheet with tensile strength of 45 kg or more
High-strength hot-dip galvanized steel with excellent workability equivalent to or higher than that of hot-rolled sheet, especially stretch flangeability, by strengthening transformation structure by heat history in continuous annealing hot-dip galvanizing line without performing cold rolling. A plated steel sheet can be manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a heat history of a continuous annealing hot-dip galvanizing line.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C23C 2/40 // C22C 38/00 301 T S 38/06 38/32 (72) Inventor Roadside Masayoshi 1 Kanazawa-cho, Kakogawa-shi, Hyogo Kobe Steel Works Kakogawa Works

Claims (2)

[Claims] 1. In% by weight (hereinafter the same), C: 0.02 to
0.30%, Si: 1.50% or less, Mn: 0.60 to 3.0
%, P: 0.20% or less, S: 0.05% or less, Al: 0.0.
For a steel containing 0.1 to 0.10% and the balance being Fe and unavoidable impurities, hot-rolled steel sheet hot-rolled at Ac ₃ points or more was pickled and continuously without cold rolling. In an annealing hot-dip galvanizing line, after heating and holding at Ac ₁ point or more, and then rapidly cooling to Ms point or less until reaching the hot dip galvanizing bath to form partial or whole martensite in the steel sheet, A method for producing an alloyed hot-dip galvanized steel sheet having excellent stretch-flange formability, which comprises heating at a hot-dip zinc bath temperature and an alloying furnace temperature at or above the Ms point to produce partially or fully tempered martensite. 2. The steel further comprises Mo: 0.01 to 1.0.
%, Cr: 0.10 to 1.5%, Nb: 0.01 to 0.05
%, Ti: 0.01 to 0.5%, B: 5 to 30 ppm, Ca:
The method according to claim 1, which contains at least one or more of 0.01% or less.