JPS602622A - Method for rolling continuously cast billet of ferritic stainless steel containing niobium and copper - Google Patents

Abstract

PURPOSE:To prevent cracking by forming a continuously cast billet of a ferritic stainless steel having a specified composition contg. Nb and Cu, cooling the billet to a specified temp. under specified cooling conditions, and reheating and hot rolling it. CONSTITUTION:A continuously cast billet of a ferritic stainless steel contg. Nb and Cu is formed. The steel consists of, by weight, <0.05% C, <0.04% N, 10-30% Cr, <1% Si, <1% Mn, 0.2-1% Nb [Nb% >=8X(C%+N%)], 0.2-2% Cu and the balance Fe with inevitable impurites. The billet is cooled from 700 deg.C to 200 deg.C at <=18 deg.C/hr average cooling rate, and it is cooled from 400 deg.C to 200 deg.C at <=15 deg.C/hr average cooling rate. The billet is heated again without cooling to <200 deg.C, and it is hot rolled.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for hot rolling a continuously cast slab of ferritic stainless steel containing niobium and copper (hereinafter referred to as "continuous slab").

(Prior art) Ferritic stainless steel, represented by SUS430 steel, does not contain large amounts of expensive Nl and is therefore inexpensive, so it is required to have a beautiful surface in relatively mild corrosive environments such as automobile exterior parts and architectural interior parts. It is widely used for various purposes. However, its uses have been limited because it generally has inferior corrosion resistance compared to austenitic stainless steel, typified by SUS 304 steel.

In addition, ferritic stainless steel has the disadvantage that when it is subjected to drawing or tension processing in the manufacture of parts, an uneven striped pattern called roving or roving occurs, which significantly impairs the appearance.

In response, the present applicant has invented a ferritic stainless steel in which the induction resistance is improved by bright annealing by adding Nb and Cu in combination. Invented a method for manufacturing ferritic stainless steel that is virtually zinc-free by strictly controlling the ingredients and limiting the hot rolling and rough annealing conditions (
% Application No. 57-82281). The ferritic stainless steel invented by Tora et al. not only can be used in exactly the same way as SUS 304 steel, but also has no deterioration in corrosion resistance in the heat-affected zone of welds and is not susceptible to stress corrosion cracking. This significantly expanded the scope of use of inexpensive ferritic stainless steel, allowing it to be used in fields other than steel.

However, although this Nb- and Cu-containing ferritic stainless steel has many advantages as described above, it has the disadvantage that slabs produced by continuous casting tend to crack during cooling. In other words, continuous cast slabs of ferritic stainless steel (containing Nl), (::u) cracked in the transverse direction during cooling and broke during reheating for hot rolling, forcing the rolling to be stopped. In addition, even if the cracking is slight and rolling is finished, significant ridge-like flaws and through holes are likely to remain, resulting in loss of commercial value.

On the other hand, in the past, temperatures ranging from around 800℃ to 100℃ were
There is a method of slow cooling down to around 300°C, and a method where the transition temperature of the slab is 300°C.
℃, a method of reheating without cooling to below 300° C. has been proposed (Japanese Patent Laid-Open No. 58-39732). In addition, for Nb-containing @, which does not contain Cu, thermal stress and stress are also small when the tensile strength of the slab is below 150°C. -128464), but this method is applicable to Nb, C according to the present invention.
In the case of u-containing ferritic stainless steel, even gold-Nb ferritic stainless steel has no effect.

These methods are not effective even if the slow cooling rate is lowered to about 5°C/hr, and even if they are effective by lowering the slow cooling rate to below that, they are disadvantageous in terms of cost and are not practical. The measure of not cooling the slab below 300°C is considered to be an appropriate method to prevent cracking based on the concept described below, but at temperatures above 300°C the surface quality of the slab cannot be maintained, resulting in poor surface quality. Stainless steel, which is highly valued, has serious drawbacks and is also impractical.

(Object of the Invention) An object of the present invention is to provide a practical method for rolling slabs without the above-mentioned drawbacks.

(Structure and operation of the invention) As a result of various investigations into the causes of cracks in continuously cast slabs, the present inventors found that plate-like Nb carbides were found at the grain boundaries of continuous cast slabs of Nb-containing Cu ferritic stainless steel. It was found that coarse precipitates were deposited in the form of leaves, and cracks occurred from these areas. Furthermore, it was discovered that although there are intergranular cracks near the base point, the wide cracks as a whole are mainly brittle fractures, and that there are many fine intergranular cracks in the slab in addition to dog cracks. did. From these facts, slab cracking is caused by intergranular cracking of grain boundaries with coarse weeping leaf-like precipitates due to thermal stress during cooling, stress concentrates at the cracked part, and in this state transition temperature wind (5 ) It was considered that if it were to fall downward, it would lead to large brittle cracks. Based on this idea, the countermeasure of not lowering the temperature below the transition temperature is that even if minute intergranular cracks occur, (
(Such fine cracks are crimped during rolling, so there is no problem.) Since they do not lead to large cracks, it can be said that this method is theoretically very suitable. (including Nl)
, The transition temperature of Cu ferritic stainless steel continuously cast pieces is approximately 300°C when measured in a laboratory, so 300°C
It is easy to think of countermeasures that do not lower the value below.

On the other hand, based on this consideration, slab cracking does not occur due to a simple difference in tensile strength.
b The tensile strength of ferritic stainless steel slab is 150℃
It is also well understood that cracking of slabs cannot be prevented even if measures are taken not to lower the temperature below 150°C, even though the thermal stress will be reduced below.

Based on the above considerations, the present inventors investigated various methods for lowering the transition temperature of Nb l Cu ferritic continuous stainless steel slabs, and found that there is a relationship between the cooling rate (6) of the slab and the Heading: The invention has been accomplished.

In other words, a continuously cast piece of ferritic stainless steel is generally 6
The cast pieces are cut at 00 to 800°C and then naturally air cooled for 10 to 15 hours to become so-called cold pieces with a surface temperature of 100°C or less. Of course, it is quite possible that the cooling time until it becomes a cold slab is affected by the thickness of the slab, but there is not that much difference between slabs with a thickness of 150 to 350 slabs. However, it is easy to slow down the cooling rate by immediately charging the cut slab into a heat retention furnace or the like.

Figure 1 shows the components shown in Table 1, including Nb with a thickness of 215 cm.
Cu ferritic stainless steel slab from 700℃
This is the result of measuring the transition temperature of a cold piece cooled by varying the cooling rate to 00°C. Since slow cooling was carried out in a heat retention furnace, the cooling curve almost coincides with the extended time axis of the cooling curve of natural cooling. The transition temperature is 300° C. in the case of natural cooling, but it shifts to a lower temperature side by slow cooling, and in particular, decreases to about 150° C. by performing slow cooling for 35 hours or more.

Figure 2 shows the composition of Nb r with a thickness of 215°C shown in Table 1.
50cm cut from Cu ferritic stainless steel slab
These are the results of measuring the transition temperature for the purpose of determining the temperature range for slow cooling using a sample of X 50 ttan X 100. From 700℃ to 400℃ and from 400℃ to 200℃
As shown in the figure, the cooling time from 9700°C to 400°C was measured at an average cooling rate of 18°C 7'hr or less, and from 400°C to 200°C at 1 hour.
It was found that the transition temperature can be expected to be lowered by cooling at an average cooling rate of 5° C./hr or less. If there is insufficient cooling time for either the high temperature side or the low temperature side, the effectiveness will be drastically reduced. Moreover, the cooling curve from 700° C. to 400° C. was good if it was close to the curve in the heat retention furnace. However, although we have experimentally conducted conditions close to quenching locally (possible since the test material is small), there was no significant effect, and it was possible to cool down from 700°C to 40°C.
The average cooling rate from 400°C to 200°C is 15°C/hr or less.
By setting the average cooling rate to less than hr, the transition temperature can be reduced to 20
It was below 0℃.

From the above findings, the Cu-containing ferritic stainless steel continuously cast slab was heated from 700°C to 400°C at 18°C.
/hr or less, 15℃/h from 400℃ to 200℃
We have invented a rolling method that does not cause slab cracking, in which hot rolling is performed by cooling at an average cooling rate below r p and reheating without cooling below 200°C. As a result, the surface treatment of the slab was
It became possible to carry out the process at 0 to 300°C.

Next, the reasons for limiting the starting steel components will be briefly explained.

C and N are carbonitrides of Nb, which cause surface scratches and deteriorate corrosion resistance, so they should be added at 0.05% or less and 0.0% or less, respectively.
It was set to 4 sections or less.

Cr was set in the range of 10 to 30 Cr, which is the amount of Cr normally used in ferritic stainless steel.

Sl1Mr+ can be added from the viewpoint of strength, but if it becomes too large, hot workability will deteriorate, so the upper limit for both is 1.
It spread out.

In order to fix C and N and improve corrosion resistance, Nb was set at 0.2 or more and at least 8 times the sum of C and N%. However, since the effect (10) does not change even if 160% or more is added, the upper limit was set at 1.0.

Cu improves corrosion resistance when combined with Nb, but when added in large amounts, hot workability deteriorates, so Cu is 0.2 to 1
．． It was set to 0.

TI can also be added as needed.

Adding a large amount of Nl and Mo would not only be disadvantageous in terms of cost, but also degrade hot workability, so the upper limits were set to 1 or less, 1 or less, and 3 or less, respectively.

Next, the present invention will be further explained with reference to Examples.

Nb p Cu containing the components shown in Table 2 as representative components
Continuously cast ferritic stainless steel slabs were cooled by various cooling methods, surface treated between 200 and 300°C, and then hot rolled. As a result, Table 3 shows whether or not cracks occurred in the slabs. The number of rollings is shown in parentheses. While no cracking occurred in the method of the present invention, cracking occurred in all comparative methods except for the method (A5) in which the temperature was allowed to cool naturally and the temperature was not lowered below 300°C. Further, although no cracking occurred in method A5, the surface could not be maintained at all, and a great deal of work had to be done to remove surface flaws after hot rolling.

(Effects of the Invention) As described above, according to the present invention, it is possible to hot-roll continuously cast slabs of ferritic stainless steel containing niobium and copper without causing cracks, which is an industrial advantage. There is a huge amount to do.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the transition temperature and the time required for cooling a 215 m + thick Nb continuous slab of Cu-containing ferritic stainless steel from 700 °C to 200 °C, and Figure 2 is the same (Nb # cu). It is a diagram showing the relationship between cooling rate and transition temperature of ferritic stainless steel containing ferritic stainless steel. Inventor Hide Funaki - Inside the Nippon Steel Corporation Basic Research Laboratory, 1618 Ida, Nakahara-ku, Kawasaki City Inventor Yoshio Abe Inside the Nippon Steel Corporation Basic Research Laboratory, 1618 Ida, Nakahara-ku, Kawasaki City Written amendment (spontaneous) August 29, 1980 Kazuo Wakasugi, Commissioner of the Patent Office1, Indication of the case, Patent Application No. 109881 of 1982, Title of the invention: Continuous cast slab of ferritic stainless steel containing niobium and copper Rolling method 3, relationship with the case of the person making the amendment Patent applicant 2-6-3 Otemachi, Chiyoda-ku, Tokyo (665) Company-wide representative of Nippon Steel Corporation Takeshi 1) Yutaka 4, agent 100 Tokyo 2-4-1-6 Marunouchi, Chiyoda-ku, Detailed explanation of the invention column 7 of the specification subject to amendment, Contents of the amendment (1) "Base point" on page 5, lines 12-13 and 14 of the specification
are respectively corrected to the "starting point".

Claims (1)

[Claims] C0.05% or less, NO.04% or less, cr1o~30
%, Si 1% or less, Calm 1 chi or less, Nb 0.2-1.0%
8 times or more of CS and N% * Contains Cu 0.2 to 2%, and if necessary, Ti 1% or less, Nl 1
% or less, Mo 3.
, After casting continuous cast slabs of Cu ferritic stainless steel, the average cooling rate was 18℃ from 700℃ to 400℃.
/hr or less, niobium is hot-rolled by cooling from 400°C to 200'C1 at an average cooling rate of 15°C/hr or less, and heating again without cooling to less than 200°C, A method for rolling copper-containing ferritic stainless steel continuous slabs.