JPH01157743A - Production of high corrosion resistant ferritic super stainless steel strip - Google Patents

Abstract

PURPOSE:To improve corrosion resistance of a product by directly producing a ferritic stainless steel strip and controlling staying time in the specific range of the strip temp. during cooling so as to satisfy the prescribed inequality. CONSTITUTION:In an Fe-Cr-Mo alloy steel, <=0.025% C, <=1.0% Si, <=1.0% Mn, <=0.025% N, >=22% Cr, >=1.0% Mo are contained so as to satisfy the inequality II. If necessary, <=1% Ti, Nb, Zr or <=4.0% Ni, <=0.5% Cu are added. Further, the retaining time T (min) at 700-900 deg.C of the strip temp. during production in a caster is controlled so as to satisfy the inequality I. By this method, the Cr and Mo contents in the strip can be increased while preventing increase of brittleness, and the corrosion resistance of the product is improved.

Description

[Detailed Description of the Invention] (Field of Industrial Application) High-purity ferritic stainless steel has the feature of being resistant to stress corrosion cracking in environments containing Cl ions, and is
Used for the interior and exterior of freezers, building materials, chemical plants, heat exchange tubes, etc.

Ordinary ferritic stainless steel is resistant to stress corrosion cracking, but has poor corrosion resistance, mechanical properties, and weldability, which limits its applications.

In recent years, C and N have been extremely reduced (typically C≦0.030w
t% (hereinafter simply expressed as %) (N≦0.025%), the toughness and weldability were improved.

On the other hand, the corrosion resistance improves as the Cr and Mo contents increase;
r, high Mo alloys are used in harsher environments.

However, when making a steel plate using the conventional method of molten steel → ingot → blooming → hot rolling → cold rolling, increasing the amount of Cr and Mo causes brittleness to become noticeable, especially during hot working, and in reality, blooming and hot rolling become more pronounced. becomes impossible. For this purpose, Cr is 30% and Mo is 4%.
limited to the maximum amount added.

An advantageous method for producing highly corrosion-resistant ferritic super stainless steel strip using Pe--Cr--Mo alloy steel containing such or even higher Cr or Mo content is proposed herein.

(Conventional technology) Tadao Hirano, Materials of the Futsuki Materials Society, Corrosion Prevention Division Committee,
120.1 (1984) 23 discloses typical high-purity ferritic super stainless steels, and also discloses 25Cr-4Mo-4Ni-(Ti), 26Cr
-3Mo-2,5Ni-8(Ti), 29Cr-4
Mo-2Ni.

30Cr-2Mo-0,18Ni-(Cu, Si),
19Cr-2Mo 0.48 (T%+Nb), 26C
r-1,3Mo-0,17Ni-(Cu, Si) and 26Cr-IM.

is available commercially.

(Problems to be Solved by the Invention) Increasing the addition of Cr and Mo has great advantages in that corrosion resistance improves and it can be used in harsher environments, but on the other hand, it becomes difficult to manufacture.

This is because "σ brittleness" is a typical embrittlement factor, and when heated at around 800°C, an order phase called the σ phase is formed and becomes brittle.

In the conventional process, in the process of blooming and hot rolling the ingot, the material is usually heated to over 1000℃ and processed at 600 to 1000℃ before hot working, so the generation of σ phase cannot be prevented. , Cr 30% or more, Mo4
% or more, cracks occur and break during blooming and hot rolling,
Manufacturing becomes impossible, and even if hot rolling can be carried out at temperatures above 800° C., fine racks are generated during cooling, making subsequent processes virtually impossible or reducing the yield of good products.

Therefore, by adopting a manufacturing process that is completely different from the conventional process, we have overcome the problem of brittleness and have achieved high Cr and high M.
It is an object of the present invention to propose an advantageous method for manufacturing oO highly corrosion resistant ferritic super stainless steel sheets.

(Means for Solving the Problems) The present invention provides a method for directly producing ferritic stainless steel strip by supplying Fe-Cr-Mo alloy molten steel to a strip caster, in which the strip is removed from the movable cooling body of the caster. The time T (min) during which the temperature of the strip stays at 700 to 900°C while being uncoiled, coiled and cooled is 1og T≦3-1/15 [Cr
(%)+3Mo(χ)] is a method for producing a highly corrosion-resistant ferritic super stainless steel strip, and as an embodiment, Fe
-Cr-Mo alloy molten steel has a composition of C: 0.025
wt% or less, Si: 1.0wt% or less, Mn: 1.0wt% or less, N: 0.025 wt% or less, and Cr: 22
．． 0wt% or more, Mo: 1.0wt% or more 60≧
(%)+3・Mo(χ)] ≧28, with the remainder consisting of Fe and unavoidable impurities;
-Cr-Mo alloy molten steel has the following composition: C: 0.025 wt% or less, Si: 1.0 wt% or less, Mn: 1. O'wt% or less, N: 0.025 wt% or less, and Cr: 22
．． 0wt% or more, Mo: 1.0wt% or more 60≧
(%)+3・Mo(χ)]≧28, and further contains Ti, Nb, and Zr in a total amount of 1.0 wt% or less, with the remainder consisting of Fe and unavoidable impurities. , the composition of the Fe-Cr-Mo alloy molten steel is: C: 0.025-1% or less, Si: 1.0wt% or less, Mn: 1.0wt% or less, N: 0.025 tnt% or less, and Cr : 2
2.0 wt% or more, Mo: 1. Including Oiyt% or more in the range of 60≧(%)+3・Mo(χ)]≧28,
Furthermore, it contains 4.0 wt% or less of Ni and 0.5 wt% or less of Cu, with the remainder consisting of Fe and unavoidable impurities, and the composition of the Pe-Cr-Mo alloy molten steel is C : 0.025 wt% or less, Si: 1.0 wt% or less, Mn: 1. Oiyt% or less, N: 0.025 wt% or less Cr: 22
．． 0wt% or more, Mo: 1.0wt% or more 60≧
(%)+3・Mo(χ)]≧28, and further contains Ti, Nb, and Zr in a sum of 1.0 wt% or less, 4.0 wt% or less of Ni, and 0.5 wt% or less of Cu. It is recommended that the alloy contains Fe and unavoidable impurities, with the remainder consisting of Fe and unavoidable impurities.

The bottleneck in increasing the amount of Cr and Mo is hot brittleness, so if strips are manufactured using a strip caster that eliminates blooming and hot rolling, and if necessary, they are finished into steel sheets by annealing and cold rolling. We thought that it would be possible to produce a high Cr/high Mo steel plate and conducted repeated experiments.

Strips with a thickness of 0.2 to 3.0 mm were produced using a twin roll caster as shown in FIG. 1 and a single roll caster as shown in FIG.

The strip forming ability is not particularly inferior to that of 5LIS 304 even if the amount of Cr (!: Mo) is increased. It is cooled by a refrigerant and wound into a coil by a winding machine No. 3.

However, in the case of high Cr and high Mo, cracks occur when the wound coil is unwound for the next process, and in extreme cases, even trimming is not possible.

As a result of investigation, it was found that the cause of this was due to the formation of brittle σ phase that is generated at 700 to 900°C. It has been found that a coil with good toughness can be obtained by rapidly cooling the 700-900° C. region, such as by strengthening the cooling of the strip before winding or immediately cooling the coil with water.

(Function) In order to have high corrosion resistance, especially crevice corrosion resistance and seawater resistance as the object of this invention, 22% or more Cr and 1%
The above content of Mo is a prerequisite for the composition of Fe-Cr-Mo alloy molten steel. Corrosion resistance is approximately Cr+3M.

Since it is proportional to the amount of Cr content, it is practically necessary to contain Cr content of 3Mo>28%.On the other hand, even if the content of %)+3Mo(χ) exceeds 60%, the corrosion resistance will no longer improve significantly. However, it is limited to 60% due to the cost of raw materials and the difficulty of manufacturing.

Mn is set to 1.0% or less in order to eliminate the effects of S and other substances that adversely affect toughness.

As a deoxidizing agent, Si is preferably contained in an amount up to 1.0%.

Next, in general, for ferritic stainless steel, C and N carbonate and nitride Cr at grain boundaries, worsening intergranular corrosion resistance and toughness, so it is preferable to keep them low, and each is 0.0
It shall be 25%.

The above composition is sufficient for the normal use intended for this invention, but for use in severe corrosive environments such as chemical plant containers, Ti, Nb and Zr may be added to further fix C and N. It is better to suppress Cr precipitates, and if C and N are each in the range of 0.025%, a total of 1% of Nb+Ti+Zr is sufficient.

Furthermore, Ni and Cu can be added in the present invention because they improve high-temperature strength and corrosion resistance. If the amount is too high, it becomes brittle under hot conditions, so they are contained in amounts of 4% or less and 0.5% or less, respectively.

The molten steel adjusted to the above composition is processed by a strip caster as shown in Figure 1.2 to a thickness of 0.2~
The object of the present invention can be achieved by directly casting a 3.0 mm strip, and whether the strip caster is applied with either the twin roll method of FIG. 1 or the single roll method of FIG. 2.

In addition, in the twin roll method, the different diameter and inclined twin roll methods also meet the purpose of the present invention.

The strip that leaves the casting roll typically has a thickness of 800 to 1
The material is at a temperature of 200° C., and is cooled by natural cooling or forced cooling before winding.

If the wound coil is not forcedly cooled, the internal temperature of the wound coil will appear to rise slightly (recuperation phenomenon) and then slowly drop.

It goes without saying that the thicker the strip, the slower the cooling becomes, and the essence of this invention lies in this cooling.

In other words, in stainless steel containing 20% or more of Cr and 1% or more of Mo, 700%
This is because if the strip remains in the temperature range of ~900°C for a long time, it will become brittle and cracks will occur in the next process, making the process difficult or reducing the yield, as already mentioned in this regard. This invention was completed based on this knowledge.

Cr+3M for residence time of 700-900″C.

(wt%) and stratify the quality of flexibility.
The residence time T (min) is log T≦3−−(Cr+3
It has been experimentally confirmed that the flexibility is good when the condition is Mo) (%1t%), and this condition is considered to hold regardless of whether it is a single roll, double roll, or composition. be able to.

(Example) Molten steel having the composition shown in Table 1 was directly cast into a strip having a thickness of 0.2 to 3.0 trm by twin roll and single roll methods, and wound into a coil.

From the cooling roll to the coiler, the plate temperature was measured using an optical thermometer. In addition, a PR thermocouple was inserted into the coil during winding, and the plate temperature inside the coil was measured until the coil was cooled to room temperature.

In order to vary the cooling pattern, air cooling or water cooling was applied in the cooling zone in front of the coiler, or the coil was immersed in water immediately after winding. In this way, the plate is heated to 900℃.
The residence time for cooling from to 700°C was determined.

The coil is passed on to the next process, where it is surface-obtained on a line such as pickling or grinder belt grinding, or sent through an annealing line, during which the flexibility is determined as the coil is unwound. I made a distinction between good and bad.

In other words, good means that the board has no harmful defects such as cracks.
Industrially stable threading is possible. On the other hand, if the board is inferior, cracks, surface folds, cracks, etc. are observed in the board, or they occur during unwinding, making it industrially impossible to proceed to the next process.

(Effects of the invention) There is a demand for ferritic stainless steel sheets with high corrosion resistance, but there was a limit to increasing the amount of Cr and Mo in the conventional process, but this invention uses a direct sheet manufacturing method. Moreover, by strengthening the cooling in the range of 700 to 900''C, it has become possible to increase the amounts of Cr and Mo, and it has become possible to manufacture plates with high corrosion resistance.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a twin-roll strip caster, FIG. 2 is a schematic diagram of a single-roll strip caster, and FIG. 3 is a graph showing the relationship between residence time and Cr+3Moi on flexibility.

Claims (1)

[Claims] 1. When Fe-Cr-Mo alloy molten steel is supplied to a strip caster to directly produce a ferritic stainless steel strip, the strip is separated from the movable cooling body of the caster and wound up. While the strip is being cooled, the temperature of the strip is between 700 and 700℃.
A method for producing a highly corrosion-resistant ferritic super stainless steel strip, which comprises controlling the residence time T (min) at 900°C within the range of logT≦3-1/15 [Cr (%) + 3Mo (%)]. 2. The composition of the Fe-Cr-Mo alloy molten steel is C: 0.0
25wt% or less, Si: 1.0wt% or less, Mn: 1.0wt% or less, N: 0.025wt% or less, and Cr: 22.0wt% or more, Mo: 1.0wt% or more with 60≧[Cr( %)+3.Mo(%)]≧28, with the remainder consisting of Fe and unavoidable impurities. 3. The composition of the Fe-Cr-Mo alloy molten steel is C: 0.0
25wt% or less, Si: 1.0wt% or less, Mn: 1.0wt% or less, N: 0.025wt% or less, and Cr: 22.0wt% or more, Mo: 1.0wt% or more with 60≧[Cr( %)+3・Mo(%)] in the range of ≧28, and further contains Ti, Nb, and Zr with a sum of 1.0
The method according to claim 1, wherein the content is less than or equal to wt%, with the remainder consisting of Fe and unavoidable impurities. 4. The composition of the Fe-Cr-Mo alloy molten steel is C: 0.0
25wt% or less, Si: 1.0wt% or less, Mn: 1.0wt% or less, N: 0.025wt% or less, and Cr: 22.0wt% or more, Mo: 1.0wt% or more with 60≧[Cr( %) + 3・Mo(%)] ≧ 28, and further contains 4.0 wt% or less of Ni and 0.5
2. The method according to claim 1, wherein the copper content is less than or equal to wt% of Cu, with the remainder consisting of Fe and unavoidable impurities. 5. The composition of the Fe-Cr-Mo alloy molten steel is C: 0.0
25wt% or less, Si: 1.0wt% or less, Mn: 1.0wt% or less, N: 0.025wt% or less, and Cr: 22.0wt% or more, Mo: 1.0wt% or more with 60≧[Cr( %)+3・Mo(%)]≧28, and further contains Ti, Nb, and Zr in a sum of 1.0wt%.
and below and 4.0wt% or less Ni and 0.5wt% or less C
2. The method according to claim 1, which contains u and the remainder consists of Fe and unavoidable impurities.