JP2755677B2 - Method for producing alloyed hot-dip galvanized steel sheet for processing with excellent surface properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an alloyed hot-dip galvanized steel sheet having excellent surface properties suitable for use in bending, press forming, drawing, and deep drawing. And a method for producing the same.

<Related Art> In recent years, in the case of thin steel sheets used for automobile steel sheets and the like, in order to improve the corrosion resistance, demand for surface-treated steel sheets subjected to various surface treatments has been increasing. Among them, the galvannealed steel sheet is one of the most excellent surface-treated steel sheets in view of its excellent corrosion resistance and spot weldability.

By the way, as a method for producing an alloyed hot-dip galvanized steel sheet, conventionally, after subjecting a hot-rolled steel strip to cold rolling, a recrystallization annealing step is performed using a continuous annealing method or a box-type annealing method,
Then, hot-dip galvanized and then 450-650 ° C
Alloying treatment is performed.

However, recently, there has been an increasing demand from customers for cost reduction, and there has been a demand for an inexpensive alloyed hot-dip galvanized steel sheet having excellent workability. Therefore, in order to combine the two heat treatment steps of recrystallization annealing and alloying, which require a large amount of energy, into a single alloyed hot-dip galvanized steel sheet that is subjected to alloying that also serves as recrystallization annealing. A method has been proposed.

As such a production method, for example, JP-A-61-2769
No. 61 discloses a manufacturing method in which a cold-rolled steel sheet is subjected to a hot-dip galvanizing treatment, and then subjected to an alloying treatment also serving as a recrystallization annealing treatment in a temperature range of 700 to 850 ° C. It requires special equipment that can process at higher temperatures than processing equipment and cannot be manufactured with conventional equipment. In addition,
JP-A 61-276962 also discloses a manufacturing method in which a cold-rolled steel sheet is subjected to a hot-dip galvanizing treatment, and then subjected to an alloying treatment also serving as a recrystallization annealing treatment in a temperature range of 700 to 850 ° C. Also requires special alloying treatment equipment and cannot be manufactured with conventional equipment.

<Problems to be Solved by the Invention> An object of the present invention is to eliminate two special heat treatment steps which require a large amount of energy in a conventional equipment without requiring any special alloying treatment equipment. It is an object of the present invention to provide a method for producing an alloyed hot-dip galvanized steel sheet having excellent surface properties and properties for working only by performing an alloying treatment which also serves as a recrystallization annealing treatment.

<Means for Solving the Problems> The present inventors performed alloying processing that also serves as recrystallization annealing using conventional alloying processing equipment, so that alloyed hot-dip galvanizing that can be used for processing is performed. As a result of repeated research on steel sheets, it has been found that regulating the composition of C, Al and N to an appropriate range and performing annealing at 500-600 ° C is extremely effective in achieving the intended purpose. This has led to the present invention.

That is, in the present invention, C: 0.0015% or less, Si: 0.2% by weight%.
%, Mn: 1.0% or less, P: 0.15% or less, S: 0.0200% or less, N: 0.0030% or less, and Al: (20 ×% N) to 0.15%, and if necessary, Nb: 0.001 to 0.020% and B:
Contains 0.0002-0.0020% of one or two types, with the balance being Fe
And hot-dip galvanizing of steel consisting of unavoidable impurities and then alloying that also serves as recrystallization annealing in a temperature range of 500 ° C to 650 ° C. This is a method for producing an alloyed hot-dip galvanized steel sheet having excellent processing properties.

<Action> In the production of an alloyed hot-dip galvanized steel sheet, the present inventors used two conventional heat treatment processes, a recrystallization annealing treatment and an alloying heat treatment, mainly for the purpose of energy saving, using conventional alloying treatment equipment. Of alloying treatment which also serves as recrystallization annealing treatment 1
In order to make it into a process, we have been diligently studying a steel sheet having a low recrystallization annealing temperature and excellent workability.

First, the experimental results that led to the present invention will be described.

It consists of various compositions in the range shown in Table 1 and has a weight ratio of Al
A cold-rolled steel sheet having a thickness of 0.8 mm adjusted so that / N became 10 or 30 was subjected to an annealing treatment at a temperature range of 500 to 900 ° C for a soaking time of 40 s, and the material properties were investigated.

FIG. 1 shows the results of investigating the effect of steel components on the annealing temperature that gives a constant Rankford value (r value = 1.5) after annealing.

As is clear from FIG. 1, the Al / N ratio is 30 and C is 0.0015.
r value when it is within the range of wt% (hereinafter simply referred to as%)
It was found that the annealing temperature satisfying 1.5 was significantly reduced.

It is considered that the reason why excellent workability can be obtained at a low annealing temperature under the above conditions is as follows. That is, an increase in Al / N and a decrease in C are advantageous in workability.
1｝ Infiltration type solid solution elements (C,
N), the segregation state of the grain boundary changes with the decrease of the solid solution C and N, and the effect of reducing the infiltration type solid solution C and N is added. It is considered that workability was obtained.

In addition, the reason why the surface properties are excellent is that, compared to the conventional steel sheet annealed at a relatively high temperature, there is less surface enrichment of components that have an adverse effect on the plating properties, so that there are fewer surface defects due to plating. it is conceivable that.

Next, the reasons for limiting the components in the present invention will be described below.

The range of C: C is particularly important in the present invention, and in order to obtain a steel sheet having good workability at a conventional alloying treatment temperature, that is, good elongation and r value, as shown in FIG. However, the carbon material must be extremely low carbon, which is lower than that of conventional low carbon materials. Therefore, C is set to 0.0015% or less.

Si: If Si is present in excess of 0.2%, the adhesion of the plating will be significantly degraded.

Mn: If Mn exceeds 1.0%, the elongation and drawability of the steel sheet are deteriorated.

P: If P is present in excess of 0.15%, the amount of segregation to grain boundaries increases and the steel sheet becomes embrittled.

S: If S exceeds 0.0200%, the corrosion resistance is remarkably deteriorated. Therefore, S is set to 0.0200% or less. However, if it is too low, the descaling property during hot rolling is deteriorated and the surface properties are deteriorated. Therefore, 0.0035% or more is desirable.

N: When N is increased, not only does the material deteriorate, but also the necessary amount of Al becomes excessive as described later, deteriorating the surface properties. A particularly desirable range is 0.
0020% or less.

The range of Al: Al is also particularly important in the present invention. The effect is greater as Al / N is larger, so that Al / N is set to 20 or more. A particularly desirable range is that Al / N is 30 or more. However, the presence of more than 0.15% degrades the surface properties, so the content is set to 0.15% or less.

In addition to the above, in the present invention, for the purpose of improving workability and in-plane anisotropy, Nb: 0.001 to 0.020
% And B: 0.0002-0.0020%.

 Further, the reasons for limiting the process conditions will be described below.

The steelmaking and hot rolling steps may be performed according to a conventional method, and the present invention does not particularly need to limit those conditions. In the cold rolling step, the conditions are not particularly limited, but it is desirable that the rolling reduction is 70% or more for the purpose of improving recrystallization.

In the present invention, the following steps are important. In the conventional method, annealing is performed before hot-dip galvanizing, but in the present invention, hot-dip galvanizing is performed without annealing. The temperature of the steel sheet when inserted into the hot-dip galvanizing bath is not particularly limited, but is approximately the same as that of the hot-dip galvanizing bath due to plating adhesion, that is, 40
It is desirable to heat to 0 to 550 ° C. After a while
The alloying process is performed.
450-650 ° C, from the recrystallization of the component steel of the present invention
500-650 ° C.

After the plating, temper rolling may be performed within a normal range for the purpose of correcting the shape of the sheet.

<Examples> Hot-dip galvanizing (amount of coating per side: about 40 g / side) was carried out by immersing a 0.8 mm thick cold-rolled steel sheet composed of various components shown in Table 2 in a hot-dip galvanizing bath at a bath temperature of 450 ° C. m ² )
Subsequently, alloying treatment was performed at various alloying temperatures shown in Table 3 and then temper rolling of 0.8% was performed to obtain a steel sheet.

Tensile properties of each steel material: Yield point (YP), tensile strength (TS),
Table 3 also shows the elongation (El), the r value and the surface defect rate due to plating. Here, the surface defects caused by plating are mainly white streak defects, poor plating, and poor alloying.

As can be seen from Table 3, all of the test pieces steel-made with the steel components of the present invention have good workability (r value ≧ 1.5, El ≧ 47).
%) And the surface defect rate, but a good material could not be obtained in Comparative Examples where the conditions were not met.

The tensile properties and r values were determined using JIS No. 5 tensile test pieces.

<Effect of the Invention> According to the present invention, in particular, the composition of C, Al, and N is regulated to an appropriate range, and alloying treatment also serving as recrystallization annealing treatment at 500 to 650 ° C is performed using conventional equipment. As a result, a galvannealed steel sheet for processing having excellent surface properties and good quality can be manufactured, so that ordinary recrystallization annealing treatment can be omitted and the accompanying cost reduction can be achieved.

[Brief description of the drawings]

FIG. 1 is a graph showing the influence of the Al / N ratio and the carbon concentration on the annealing temperature required to obtain an r value of 1.5.

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corporation Research and Development Headquarters (72) Inventor Hideo Abe 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corp. (56) References JP-A-2-163356 (JP, A) JP-A-61-60860 (JP, A) JP-A-61-56245 (JP, A) JP-A-58-25436 (JP) JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] (1) In weight%, C: 0.0015% or less, Si: 0.2% or less, Mn: 1.0% or less, P: 0.15% or less, S: 0.0200% or less, N:
A steel containing 0.0030% or less and Al: (20 ×% N) to 0.15%, the balance being Fe and unavoidable impurities is subjected to hot-dip galvanizing after cold rolling, and then to 500 ° C to 650 ° C.
A method for producing an alloyed hot-dip galvanized steel sheet having excellent surface properties, comprising performing an alloying treatment also serving as a recrystallization annealing treatment in a temperature range of ° C. 2. In% by weight, C: 0.0015% or less, Si: 0.2% or less, Mn: 1.0% or less, P: 0.15% or less, S: 0.0200% or less, N:
0.0030% or less and Al: (20 ×% N) to 0.15%, and further Nb: 0.001 to 0.020% and B: 0.0002 to 0.0020
%, One or two types, the balance consisting of Fe and unavoidable impurities, hot-dip galvanized after cold rolling, and also served as recrystallization annealing in a temperature range of 500 ° C to 650 ° C A method for producing an alloyed hot-dip galvanized steel sheet for processing having excellent surface properties, comprising performing an alloying treatment.