JPH05345913A - Production of hot-dip aluminum coated austenitic stainless steel sheet reduced in sensitivity to sensitization phenomenon - Google Patents

Abstract

PURPOSE:To prevent the occurrence of sensitization phenomenon at the time of applying hot-dip metal coating to an austenitic stainless steel. CONSTITUTION:At the time of preannealing a starting sheet for plating, the cooling velocity (T deg.C/sec) at annealing is controlled so that, in the relationship with the amount of [C ]] contained in the starting sheet, it satisfies T=([C]-0.005)X(500 when 0.010<[C%]<=0.020, T=([C]-0.005)X750 when 0.020<[C%]<=0.040, and T=([C]-0.005)X(1000 when 0.040<[C%]<=0.070, respectively. Further, holding time at the maximum heating temp. in a hot dipping stage is controlled so that it is <=60sec when the maximum heating temp. is >=800 deg.C, <=120sec when 700-<800 deg.C, <=180sec when 600-<700 deg.C, and <=240sec when 500-<600 deg.C, respectively.

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 austenitic stainless steel hot-dip aluminum-plated steel sheet having a low susceptibility to sensitization, which is obtained by applying a hot-dip aluminum-based plating such as aluminum or aluminum-zinc. ..

[0002]

2. Description of the Related Art Conventionally, stainless steel sheets have been utilized for their excellent properties such as corrosion resistance, high temperature oxidation resistance and high temperature strength, for example, plant members for chemical industry, construction members such as roofs and walls, kitchens. It is used in many fields such as appliances and household appliances such as bathtubs. Recently, it has started to be widely used in exhaust mufflers for automobiles and motorcycles, exhaust pipes for dust incinerators, etc., and when higher workability is required, austenitic stainless steel is used, and corrosion progresses at high temperatures. If further durability is required in a severely corrosive environment, the austenitic stainless steel whose surface is coated with molten metal such as aluminum or aluminum-zinc is already used in some fields. It has been put to practical use in Japan, and the amount is increasing recently.

A plating base plate of an austenitic stainless steel hot-dip galvanized steel sheet having such excellent properties is disclosed in, for example, Japanese Patent Application Laid-Open No. 3-100124, "Cooling rate during solidification is 100 ° C./Sec or more and thickness is 10 mm or less. A thin strip cast slab is cooled from a high temperature to a temperature of 650 ° C or lower at a rate of 20 ° C / Sec or higher, wound, and rolled at a rolling reduction ratio of 4
It is manufactured by applying many conditions to the rolling step and the annealing step, as in "a method for manufacturing an austenitic stainless steel thin plate in which cold rolling is performed at 0% or more, annealing is performed, and then temper rolling is performed." However, the annealing temperature and the holding time of austenitic stainless steel differ depending on the steel type and cross-sectional thickness.
Generally, it is said that the annealing temperature is 925 to 1125 ° C. and the holding time is 5 to 120 minutes. Further, cooling from the annealing temperature to room temperature must be rapidly cooled due to sensitization and formation of carbide. Sensitization is generally 4
Chromium carbide (carbide) precipitates at the grain boundaries in the temperature range of 50 to 800 ° C and loss of chromium in the vicinity occurs, and when this steel is placed in an environment that is easily corroded, the passivation film is destroyed and each grain The so-called intergranular corrosion, in which the parts are separated into pieces, and only this part is corroded earlier than other parts.

There are two ways to prevent this sensitization.
One of them is to heat the chromium carbide generated at the grain boundaries to a high temperature to redissolve it, decompose it, and then quench it. Another method is to reduce the carbon content to the range where chromium carbide precipitation does not occur. However, the latter is generally adopted because the latter requires a special process. The austenitic stainless steel plated original plate produced in this manner has a temperature of 690 after the surface is cleaned and dried as described in JP-A-55-97460.
There is a method for producing an aluminum-coated austenitic stainless steel sheet which is preheated to 750 ° C. and then passed through a molten aluminum bath at about 700 ° C., which has excellent corrosion resistance and good workability, or a steel containing Cr; 3 to 20%. Manufacturing method, for example, "Temperature 620 to 690" as disclosed in JP-A-63-47356.
It is manufactured under the same or similar thermal conditions as in "Manufacturing method of plating with Al-Si-based plating bath at ℃". The austenitic stainless steel hot-dip aluminum-plated steel sheet produced in this way exhibits excellent properties such as corrosion resistance, high-temperature oxidation resistance, and high-temperature strength, but some of them are highly susceptible to sensitization and have deteriorated corrosion resistance. There was a problem that was manufactured.

[0005]

In order to stably produce austenitic stainless steel hot-dip aluminum-plated steel sheet which is less susceptible to the sensitization phenomenon, the inventors of the present invention have many plating methods. A method for manufacturing a steel sheet was examined. As a result, they have found that the target plated steel sheet can be obtained by controlling the heating temperature of the austenitic stainless steel of the plating original plate before hot dipping and the cooling rate suitable for the C content in the steel. The present invention is configured on the basis of such findings, the gist thereof is, after cold rolling in a normal thin steel plate manufacturing process, a final austenitic stainless steel plated original plate that is annealed once or twice or more. Annealing treatment is performed according to the following heating temperature and heating time: 60 seconds or less in the case of 800 ° C. or higher, 120 seconds or less under 800 ° C. to 700 ° C. or lower, 180 seconds or lower in 700 ° C. to 600 ° C. or higher 600 240 seconds or less at less than ℃ ~ 500 ℃

Subsequently, the cooling temperature range from the heating temperature to 600 ° C. is cooled at the following rate (T ° C./Sec) determined by the amount of [C] contained in the original plate, and [C%] ≦ 0. 0.020 T = ([C] −0.005) × 500 0.020 <[C%] ≦ 0.040 T = ([C] −0.005) × 750 0.040 <[C%] ≦ 0 0.070 T = ([C] −0.005) × 1000 Then, a method for producing an austenitic stainless steel hot-dip aluminum-plated steel sheet having a small sensitivity to the sensitization phenomenon of subsequent hot-dip aluminum plating.

The present invention will be described in detail below. Usually, hot or cold rolling is performed after the molten steel produced by using a melting furnace such as a converter or an electric furnace or its auxiliary processing equipment is made into a slab by the continuous casting method or the ingot agglomeration method. The original plating plate of the austenitic stainless steel cold-rolled steel plate manufactured through the manufacturing process of 1. is annealed once or twice or more. This annealing is required to heat the final annealing of the plated original plate to a temperature of 500 ° C. or higher to remove the internal strain remaining after cold working during cold rolling to soften and provide workability. Depending on the degree of properties, the annealing is repeated once or twice or more. The upper limit of the annealing temperature is not particularly limited, but excessive annealing temperature must be avoided from the problem of increasing the sensitivity of the sensitization phenomenon and coarsening the crystal grains,
It is preferably 950 ° C or lower. Further, the heating time at that temperature is related to the annealing temperature, and is controlled within the following range in which the sensitivity of the sensitization phenomenon is low and the workability is imparted to the plated steel sheet.

In the case of 800 ° C. or higher, 60 seconds or less Less than 800 ° C. to 700 ° C. or less, 120 seconds or less Less than 700 ° C. to 600 ° C. or less, 180 seconds or less 600 ° C. to 500 ° C. or more, 240 seconds or less The steel sheet that has undergone such annealing and has a high temperature heat has a temperature of 600 ° C. that promotes the reaction between the original plating plate and the plating bath without cooling and solidifying the plating bath and preventing the precipitation of grain boundaries of CrC. Control cooling. In addition, it is necessary to perform controlled cooling during that time. Table 1 shows the experimental results of observing the relationship between the annealing temperature and the cooling rate by observing the state of grain boundaries in the oxalic acid etch test method of JIS G-0571.

That is, the cooling rate (T ° C./Se of the present invention
c) shows the experimental results of Table 1 for steel (C: 0.008 to 0.0
7%, Ni: 8.0 to 9.5%, Cr: 17.2-1
8.5% C content in austenitic stainless steel sheet) is arranged in the following formula, and the growth of CrC precipitated at the grain boundaries and the intergranular corrosion in the heating process of the steel sheet cooled at the speed in this range To prevent. [C%] ≦ 0.020 T = ([C] −0.005) × 500 0.020 <[C%] ≦ 0.040 T = ([C] −0.005) × 750 0.040 < [C%] ≦ 0.070 T = ([C] −0.005) × 1000

However, the thus-cooled original plating plate which retains heat at a temperature of 600 ° C. is a molten aluminum-based plating bath (Al, Al-) which is kept at a normal plating bath temperature (for example, 570 to 700 ° C.). Si, Al-Zn, Al-
After passing through a molten aluminum-based plating bath to which a small amount of corrosion-resistant metal such as Mn or Mg, Cr, Cu, Ti, and Ni is selectively added and wiped to a predetermined coating weight, if necessary, overaging treatment may be performed. It is given to the product. According to the above-mentioned manufacturing method, an austenitic stainless steel hot-dip aluminum-plated steel sheet with low sensitivity to sensitization phenomenon is manufactured, and excellent properties such as corrosion resistance, high temperature oxidation resistance and high temperature strength are also obtained.

[0011]

[Table 1]

[0012]

EXAMPLES Next, examples of the present invention will be described. Table 2
Is a plate of an austenitic stainless steel of each composition that has been annealed and heated at each temperature in a continuous plating line and held there, and then controlled and cooled to a temperature of 600 ° C., followed by A
L-Si plating bath (plating temperature 660 ° C) or AL-
It was passed through a Zn plating bath (plating temperature 590 ° C.). The degree of sensitization at that time is shown in the same table.

[0013]

[Table 2]

[0014]

As is apparent from the test results described above, the present invention is capable of producing a hot-dip aluminium-plated steel sheet of austenitic stainless steel in which sensitization is prevented as compared with the comparative material.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C21D 9/52 102 C23C 2/12

Claims (1)

[Claims] 1. A final annealing treatment of an austenitic stainless steel plated original plate which is annealed once or twice or more after cold rolling in a thin steel sheet manufacturing process, after the following relationship between heating temperature and heating time. In the case of 800 ° C. or higher, 60 seconds or less Less than 800 ° C. to 700 ° C. or less, 120 seconds or less Less than 700 ° C. to 600 ° C. or less, 180 seconds or less 600 ° C. to 500 ° C. or more, 240 seconds or less Cooling temperature range from heating temperature to 600 ℃,
The plate is cooled at the following rate (T ° C / Sec) determined by the amount of [C] contained in the original plate, and [C%] ≦ 0.020 T = ([C] −0.005) × 500 0.020 <[C%] ≦ 0.040 T = ([C] −0.005) × 750 0.040 <[C%] ≦ 0.070 T = ([C] −0.005) × 1000 Then melted A method for producing an austenitic stainless steel hot-dip aluminum-plated steel sheet which is less susceptible to the sensitization phenomenon characterized by aluminum-based plating.