JPH1112708A - Production of galvannealed dipped steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable alloying at high speed by specifying a dew point at reducing atmosphere from a specific temp. at the time of recrystallize-annealing to a specific temp. in a cooling stage after reaching the max. temp. and a dew point just before dipping into a galvanizing bath, and executing galvanizing and alloying. SOLUTION: The dew point from 600 deg.C at the time of recrystallize-annealing to 700 deg.C in the cooling stage after reaching the max. temp., is regulated to 0 deg.C-20 deg.C, and the dew point just before dipping into the galvanizing bath is regulated to <=-20 deg.C. It is desirable to use a steel containing >=0.01 wt.% P. The suitable atmosphere at the time of executing anneal-reduction, is an N2 atmosphere containing <=2% H2 , and the dew point of <=700 deg.C at the temp. of anneal-heating is ordinally the one between the dew point at 700 deg.C and the dew point just before galvanizing, because the dew point just before galvanizing is regulated to <-20 deg.C. However, any dew point is not surely regulated. The productivity can be improved by quickening a line speed and the deterioration of the galvanized quality can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet for an automobile body having excellent alloying and plating properties.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanizing is widely used as a steel sheet for automobiles because it is excellent in corrosion resistance and weldability and can be manufactured at low cost. However, in mass production of steel sheets, the capacity of the line's alloying furnace becomes a bottleneck, making it impossible to increase the line speed, and even if the line speed is increased, there are unalloyed materials that do not complete alloying sufficiently. There is a problem. In particular, P in steel for high strength
In a steel sheet to which is added, alloying delay is remarkable (for example, “iron and steel”: vol 68, 1400).

[0003] As a countermeasure, there are a method of reducing the line speed and lengthening the alloying time, a method of increasing the furnace temperature, a method of reducing the Al concentration in the zinc bath, a method of changing the zinc bath temperature, and a method of changing the temperature of the steel. There is known a method of adding Ti to steel to promote alloying. However, a decrease in line speed degrades productivity, and the method based on an increase in furnace temperature makes it difficult to control alloying, and conversely, tends to overalloy. When the alloy is over-alloyed, an adverse effect such as a decrease in the adhesion of the plating becomes a problem. In addition, changing the Al and bath temperature in the bath is time-consuming and inefficient in a continuous hot-dip galvanizing line for plating various steel types. It is better to keep the state of the zinc bath as constant as possible, since solids are easily generated and adhere to the steel sheet to cause defects. In addition, adding a large amount of Ti to the steel causes problems in that it is not known whether the intended strength can be obtained, and that the secondary work brittleness resistance is deteriorated. In addition, a small amount of Ti has little effect of promoting alloying. .

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention was conducted by changing various conditions and conducting experiments.
It was found that the conditions during annealing before hot-dip plating greatly affected the alloying speed. In particular, it was found that high-speed alloying was possible by increasing the dew point of the reducing atmosphere during recrystallization annealing.

An object of the present invention is to provide such a technique.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to achieve high-speed alloying, and its technical means is to provide a continuous hot-dip galvanizing line with a 600 ° C recrystallization annealing.
The dew point of the reducing atmosphere from 0 ° C to + 20 ° C after reaching the maximum temperature from 700 ° C to the cooling stage 700 ° C,
The dew point of the atmosphere immediately before immersion in the hot dip galvanizing bath is -2.
This is a method for producing an alloyed hot-dip galvanized steel sheet, wherein hot-dip galvanizing is performed at a temperature of 0 ° C. or lower and further alloying is performed. In this case, if steel containing 0.01% by weight or more of P is used, the increase in line speed is remarkable, which is preferable.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have found that by changing the dew point during annealing, the surface enrichment behavior of Mn, Si and P is different, and alloying is promoted particularly in a region where P is easily enriched. Was found. The term “surface concentration” as used herein means that components in the steel move to the surface of the steel sheet and combine with oxygen on the surface of the steel sheet to form an oxide.

FIG. 1 shows the alloying speed of a steel sheet having a dew point changed from 600 ° C. to 700 ° C. through 800 ° C. FIG. 1 plots the amount of Fe in the plating layer while keeping the alloying temperature and time constant. It can be said that the alloying speed increases as the amount of iron in the plating layer increases. -10
It can be seen that the temperature is rapidly increased above ℃. FIG. 2 shows the relationship between the dew point and the surface concentration. As the amount of surface enrichment, the integrated value of the surface layer intensity by GDS (glow discharge emission spectroscopy) was used for Mn, Si, and P. Mn and Si have a large surface concentration at a dew point of −30 ° C. to −40 ° C., and the surface concentration decreases at lower and higher temperatures. On the other hand, in P, the surface concentration is hardly observed at -10 ° C or lower, and the surface concentration increases at the high temperature side of 0 ° C or higher. FIG. 3 shows the results of measurement of the surface condensate of P by ESCA (X-ray photoelectron spectroscopy). A peak that was not observed on the low temperature side was observed at + 10 ° C., and it is considered that phosphoric acid was found from the peak position.

The relationship between the surface concentration and the alloying rate is thought to be that the surface concentrate of Mn or Si inhibits the diffusion of iron, that is, slows down the alloying. The rapid change in the alloying rate cannot be explained only by the phenomenon of the surface concentration of Mn or Si. Therefore, the steel sheet was subjected to brittle fracture in AES (Auger electron spectroscopy), and the P content at the extremely surface layer grain boundary was measured. Table 1 shows the results. It can be seen that on the high temperature side where the dew point is 0 ° C. or higher, the amount of the grain boundary P in the surface layer decreases. This is presumably because P was oxidized and concentrated on the surface at the high dew point, so that the amount of P dissolved in the surface layer decreased.

From the above results, the effect of the P oxide concentrated on the surface to the alloying rate is small, and conversely, the amount of solid solution P in the surface layer is reduced by the surface concentration, so that the diffusion of iron is fast. It is estimated that the alloying speed was increased. The upper limit of the dew point is set to + 20 ° C. due to the saturation of the alloying rate shown in FIG. 1 and the deterioration of plating adhesion due to the formation of iron oxide on the surface.

[0011] Next, the dew point immediately before plating is as follows. As a result of various tests, if the dew point is high, an oxide film on the zinc bath surface is likely to be generated, and surface defects such as scratches are likely to occur. It is good to be below ° C. Here, the term “immediately before plating” refers to a time when the steel sheet passes through a portion usually called a snout, and is a portion where the steel sheet after the cooling zone is guided to the zinc bath.

The present invention is not particularly effective only in steel sheets to which P is added, and a sufficient effect is obtained even in steel sheets in which P is not intentionally added and the P content is about 0.01% by weight. It has been seen. By increasing the dew point to 0.01% by weight of P steel, it is possible to increase the line speed by about 20 to 30%.
An alloying promoting effect of about 60% has been observed.

The atmosphere during the reduction by annealing in the present invention is 2%
The above N ₂ atmosphere containing H ₂ is suitable, and the dew point of 600 ° C. or less during annealing heating is not particularly specified.
Further, the dew point of 700 ° C. or less upon cooling after annealing is usually between the dew point at 700 ° C. and the dew point immediately before plating in order to keep the atmosphere immediately before plating at −20 ° C. or less. However, it is not specified.

[0014]

EXAMPLES After smelting the test materials shown in Table 2 in a converter, slabs were produced by continuous casting. This slab is heated to a slab heating temperature (SRT) of 1250 ° C., the final finishing temperature of the hot rolling step is set to 900 ° C., and the slab is wound at 550 ° C. 3.2
A hot-rolled sheet coil having a thickness of mm was prepared. After the black scale was removed by pickling, cold rolling was performed to obtain a cold-rolled steel sheet having a thickness of 0.8 mm.
The annealing conditions of this cold-rolled steel sheet were changed in a continuous hot-dip galvanizing line as shown in Table 3 to prepare an alloyed hot-dip galvanized steel sheet. For the evaluation of the alloying speed, the maximum line speed for completing the alloying was used. Here, the completion of alloying refers to when the Fe% in the plating layer becomes 8.6% or more and 10.7% or less.

The atmosphere during annealing was 3-6% H ₂ -N ₂ . The plating bath conditions were as follows: the Al concentration in the bath was 0.140% by weight, the bath temperature was 460 ° C to 470 ° C, and the intrusion plate temperature was 460 ° C to 4 ° C.
80 ° C. and the alloying temperature were kept constant at 530 ° C. Further, the zinc adhesion amount was 45 to 55 g / m ² . As a quality test, the adhesion of the plating was confirmed by a 90-degree bending back test.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

[Table 3]

[0019]

[Table 4]

[0020]

According to the present invention, in the method for producing an alloyed hot-dip galvanized steel sheet, the line speed can be increased to improve the productivity, and the deterioration of the plating quality can be prevented.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the dew point during annealing and the amount of Fe in a plating layer.

FIG. 2 is a graph showing the relationship between the dew point at the time of annealing and the surface concentration of Mn, Si, and P in a plating layer.

FIG. 3 is an ESCA analysis chart showing a relationship between binding energy and surface concentration of P.

Claims (2)

[Claims] In a continuous hot-dip galvanizing line, the dew point of the reducing atmosphere in the recrystallization annealing after reaching the maximum temperature from 600 ° C. to a maximum cooling temperature of 700 ° C. is set to 0 ° C. or more and + 20 ° C. or less, A method for producing an alloyed hot-dip galvanized steel sheet, wherein the dew point of the atmosphere immediately before immersion is set to −20 ° C. or less, hot-dip galvanizing is performed, and further alloying is performed. 2. The method for producing a galvannealed steel sheet according to claim 1, wherein a steel containing 0.01% by weight or more of P is used.