JPH0333777B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to austenitic stainless steels, and particularly to austenitic stainless steels that contain 10 to 40% delta ferrite in the hot rolling stage and 15% or less delta ferrite in the product stage. This invention relates to technology for preventing the occurrence of edge cracks and surface flaws (surface microcracks). Among various austenitic stainless steels, the amount of delta ferrite is 10 to 10% during hot rolling.
About 40% of them are based on literature (e.g. “Deformation
"under hot working conditions" 1966, The
As shown in Iron and Steel Institute (Published by p. 142), the hot deformability is low and therefore the hot workability is difficult, such as duplex stainless steel containing more than 50% delta ferrite during hot rolling. It is known that edge cracks and surface microcracks are much more likely to occur during hot rolling compared to the conventional method. That is, for example, 17-7PH stainless steel contains about 15 to 40% delta ferrite during the slab heating stage, and electrode materials for overlay welding of austenitic stainless steel require dilution of the components during overlay welding. The composition is designed to have a delta ferrite content of about 10 to 30% in order to prevent cracking during welding and to prevent cracking during welding. Cracks are likely to occur at the boundary between the high-strength austenite phase and the low-strength delta-ferrite phase, and therefore edge cracks and surface microcracks are likely to occur during hot rolling. On the other hand, in the case of so-called duplex stainless steel with a delta ferrite content of 50% or more in the hot rolling stage, the high strength austenite phase is contained in the low strength delta ferrite phase, resulting in edge cracking. The occurrence of surface microcracks is lower than when the amount of delta ferrite is about 40% or less. Therefore, the hot crack prevention measures that are usually applied, namely, the means of lowering the S content (however, in that case, low S content has an S content of 0.001%
Usually 0.002 to 0.003%
degree), it was possible to suppress the occurrence of edge cracks and surface microcracks to an extent that did not cause any practical problems. However, in the case of austenitic stainless steels, which are the subject of this invention, 10 to 40
In steels containing % delta ferrite,
It has been difficult to prevent cracking during hot rolling with the level of S reduction measures that have been conventionally applied to duplex stainless steels. The occurrence of edge cracks during hot rolling not only causes a significant decrease in yield, but also increases the number of steps and causes various troubles. For example, edge cracks can cause breakage of the steel strip during hot rolling, or fragments of edge cracks can fly into the center of the steel strip during hot rolling, creating indentation flaws, or even causing holes. This may lead to a situation where you open the door. Additionally, if surface microcracks occur during hot rolling, it becomes necessary to grind the surface with a coil grinder etc. when cold rolling the hot rolled sheet, and if the cracks are severe, the product must be scrapped without being able to completely remove the flaws. Sometimes there isn't. Therefore, in order to obtain products on a mass-produced scale at low cost, it is extremely important to prevent edge cracks and microcracks from occurring during the hot rolling process. Conventionally, materials with poor hot workability, such as the steel that is the object of this invention, have been cast into steel ingots and then processed into slabs by forging, which is extremely inefficient, instead of blooming. Either measures should be taken to improve workability during hot rolling, or the edge cracks from hot rolling should be cut off before production, or surface microcracks should be removed using a coil grinder, etc. The reality is that the only countermeasure that could be taken was to grind it with a grinder. Therefore, these facts indicate that the amount of delta ferrite in the hot rolling stage targeted by this invention is 10~
This hindered the mass production of 40% austenitic stainless steel using hot strip mills, and became a bottleneck for the development of this type of material. Therefore, the inventors of the present invention have conducted various research on such difficult-to-hot-work materials, and have already filed a patent application in 1982-
As proposed in No. 25437, hot workability is improved by adjusting the component balance of steel, or as proposed in Japanese Patent Application No. 145501/1984, hot workability is applied to continuously cast slabs. There is a method for preventing edge cracking by applying reduction and subsequent soaking at high temperature, and as proposed in Japanese Patent Application No. 1058-64589, a method for preventing edge cracking is proposed in Japanese Patent Application No. 58-64589. How to roll while avoiding the temperature range of 900℃,
etc. are being developed. However, with these methods, although the degree of edge cracking can be considerably reduced, it has not yet been completely eliminated, and there are also problems such as long-term heating and the inability to easily control the rolling temperature. The reality is that there was still room for improvement. This invention was made against the background of the above circumstances, and the amount of delta ferrite was reduced to 10 in the hot rolling stage, which was generally considered to have extremely poor hot workability.
By improving the hot workability of ~40% austenitic stainless steel, it is possible to prevent edge cracking and surface cracking during hot rolling even in normal slab heating and hot rolling, without using the methods proposed above. The object is to provide a material that can almost completely prevent the occurrence of microcracks. In other words, the present inventors conducted detailed experiments and studies on the hot workability of austenitic stainless steel containing 10 to 40% delta ferrite during this type of hot rolling stage, and found that impurities in the steel By reducing the S content to an extremely low level that far exceeds conventional wisdom, even slabs obtained by continuous casting will be free from edge cracks and surface cracks during hot rolling without any special operations. It was discovered that microcracks can be almost completely prevented, leading to the creation of this invention. In that case, at the same time the S content is reduced to an extremely low level, Ca or
It has been found that adding REM is also effective. To explain in more detail the new findings of the present inventors, the present inventors have detailed the influence of the amount of delta ferrite and impurity elements in steel (S, P, O, etc.) on hot workability. As a result of the study, it was found that only the amount of S in the steel plays an important role in hot workability, and the decrease in the amount of delta ferrite due to the temperature drop during hot rolling has an adverse effect on the hot workability of S. It became clear that it was amplifying the In other words, if the negative effects of S in steel are completely eliminated, 10 to 40
It has been found that even if % of delta ferrite is present, the hot workability is good and the occurrence of edge cracks and surface microcracks during the hot rolling process can be prevented. The degree of S reduction required for this purpose is the conventional, common sense low S reduction for improving hot workability.
It has been found that it is necessary to reduce the S content to 0.0010% by weight or less. In addition to extremely low S, it also has the effect of fixing S.
Adding Ca or REM is effective in improving hot workability, and in this case, the allowable range of S is slightly expanded compared to when these elements are not added, and the S content increases.
It has been found that if the content is 0.0015% by weight or less, no edge cracks or surface microcracks will occur even in steel with a delta ferrite content of 10 to 40% at the start of hot rolling. If the amount of delta ferrite in the product exceeds 15%, the amount of delta ferrite exceeds 40% in the hot rolling stage, which improves the deformability during hot rolling. It was found that the occurrence of edge cracks and surface microcracks during hot rolling can be prevented by reducing S to a certain degree. Therefore, the first invention of the present application has C0.1% by weight.
less than 1%, Si 1% or less, Mn 3% or less, Ni 5~16%,
Cr15~30%, Mo3% or less, Al1.5% or less, Nb1.5
% or less, and contains delta ferrite in the range of 10% or more but less than 40% at the stage of starting hot rolling, and the amount of delta ferrite is 15% or less at the product stage. In austenitic stainless steel, the S content is
It is characterized in that it is 0.0010% by weight or less, and the balance with respect to each of the above-mentioned elements consists of Fe and inevitable impurities. Further, the second invention of the present application, like the first invention, provides C,
Contains Si, Mn, Ni, Cr, Mo, Al, and Nb, and the amount of delta ferrite at the start of hot rolling is 10% or more and less than 40%, and the amount of delta ferrite at the product stage is similar to the first invention. In an austenitic stainless steel containing delta ferrite in an amount of 15% or less, the S content is 0.0015% by weight or less, and Ca is more than twice the S content and 0.01% by weight or less,
It is characterized in that it contains one or more of REM (rare earth elements) in an amount not less than 4 times the S content and not more than 0.08% by weight, and the balance of each of the above elements is Fe and unavoidable impurities. This invention will be explained in more detail below. First, the reasons for limiting the basic components of the steel of this invention are as follows. C: C inhibits the inherent corrosion resistance of stainless steel and inhibits cold formability, but up to about 0.1%
The original characteristics can be exhibited by the balance of Si and Cr. If C exceeds 0.1%, the amount of Si and Cr required to maintain properties becomes too large, leading to σ embrittlement during the manufacturing process, so it is limited to less than 0.1%. Preferably it is 0.08% or less. Si: Si is an effective element in determining the balance between austenite and delta ferrite, but 1
If it exceeds 1%, it will promote σ embrittlement of delta ferrite and the amount of Ni required to control the amount of ferrite will become large, leading to an increase in the cost of the material, so it is limited to 1% or less. Mn: Mn is an element necessary to adjust the austenite-delta ferrite balance, but if it exceeds 3%, it will cause problems in steel manufacturing, especially damage to refractories and increase in manufacturing costs. , the upper limit is set at 3%. Ni: Ni is the main component of austenitic stainless steel, but if it is less than 5%, the corrosion resistance inherent in stainless steel cannot be maintained, and if it exceeds 16%, the amount of Cr and Si required to maintain the austenite-delta ferrite balance is limited to a range of 5 to 16% because it causes economic problems and also tends to cause σ embrittlement in the manufacturing process. Cr: Cr is a major component of stainless steel along with Ni, but if it is less than 15%, the corrosion resistance inherent in stainless steel will be insufficient, and if it is more than 30%, σ embrittlement will easily occur during the manufacturing process, and austenite -
Ni to maintain the balance of delta ferrite
It becomes necessary to increase the amount of C, C, and Mn, which causes economic problems and problems with properties, so it is limited to a range of 15 to 30%. Mo: Mo is an element necessary to improve the corrosion resistance of stainless steel, but it also has a strong ability to form ferrite and σ phase, so adding a large amount will increase the amount of Ni required and is not economical.
Furthermore, since σ embrittlement is likely to occur, the upper limit is set at 3%. Al: Al is an essential element for deoxidizing and imparting precipitation hardening properties, but at the same time, it has the strongest ferrite forming ability among all the components, and if it exceeds 1.5%, the amount of Ni required increases, resulting in high costs. Become. Also high
If the austenite-delta ferrite balance in Al is achieved with C, the original corrosion resistance and mechanical properties will be impaired, so the upper limit is limited to 1.5%. Nb: Nb is an element necessary to fix C and ensure corrosion resistance, but excessive addition leads to higher costs and an increase in inclusions due to nitrides, so it is limited to 1.5% or less. Furthermore, the delta ferrite-containing austenitic stainless steel that is the object of this invention has a delta ferrite content of 10% or more and less than 40% at the start of hot rolling, but the reason for this limitation is as follows. It is. That is, when the amount of delta ferrite at the start of hot rolling is less than 10%, edge cracks and surface microcracks do not substantially occur during the hot rolling process, and therefore, as in the present invention, S content is reduced. There is no need to reduce it extremely. On the other hand, even when the amount of delta ferrite at the start of hot rolling is 40% or more, the hot workability improves as the amount of ferrite increases, so there is no need to reduce the S content extremely, and the conventional known low S
is sufficient. Therefore, the amount of delta ferrite at the start of hot rolling was set within a range of 10% or more and less than 40%. Furthermore, when the amount of delta ferrite in the product stage exceeds 15%, as mentioned above, the amount of delta ferrite at the start of hot rolling is usually 40% or more, and therefore it is necessary to reduce S to an extremely low level. Because it will disappear,
The amount of delta ferrite in the product stage was specified to be 15% or less. As mentioned above, in austenitic stainless steels in which the amount of delta ferrite is in the range of 10 to 40% at the start of hot rolling, the conventional S content in steel or the conventional method for improving hot workability is Hot workability is extremely poor at the common-sense level of low S content, and it is only by reducing the S content to 0.0010% or less that the occurrence of edge cracks and surface microcracks during hot rolling can be almost completely eliminated. Therefore, the S content was set to 0.0010% or less. Here, it is preferable to set the S content to a level of 0.0008% or less, which has recently started to be put into practical use due to advances in desulfurization technology, among 0.0010% or less. can be prevented from occurring. Furthermore, in the steel of the second invention, at the same time as reducing the S content, one or more of Ca and REM is added. To explain the reason for limiting the components in this case, Ca is an effective element for fixing S and rendering it harmless, and exhibits its effect at twice or more the S content in weight percent. On the other hand, excessive addition of Ca may cause surface flaws and microcracks on the steel strip, but 0.01
There is no problem up to 0.01% of the amount of S added.
shall be. REM also has the same effect as Ca, but the effect is exhibited at a weight percent that is more than four times the S content. Also, excessive addition of REM causes surface defects, but up to 0.08% there is virtually no effect;
The upper limit of REM addition amount is set at 0.08%. Here, Ca,
Usually, one of the REMs may be added alone, but in some cases, both may be added in combination. When Ca or REM, which fixes S, is added in this way, the upper limit of the allowable amount of S is slightly expanded, and together with the presence of Ca or REM, ear cracking is completely prevented up to S0.0015%. . S is 0.0015
%, even if Ca is added at least twice the S content or REM is added at least 4 times the S content, the occurrence of ear cracks cannot be completely prevented and the cracks of about 1 mm may occur. Not only will ear cracking occur,
Since the amount of Ca and REM increases and causes problems in the production of steel billets, the upper limit of the amount of S when Ca or REM is added is set to 0.0015%. However, even when Ca or REM is added, it is desirable to keep the S content to 0.0010% or less. As mentioned above, the S content can be suppressed to a very small amount of 0.001% or less, or the S content can be reduced to 0.0015%.
At the same time as suppressing trace amounts of Ca or REM
By adding , it is possible to effectively prevent the occurrence of edge cracks and surface microcracks during the hot rolling process of austenitic stainless steel with a delta ferrite content of 10 to 40% at the start of hot rolling. . In other words, it is possible to obtain a steel strip with no edge cracks or surface microcracks from a slab as cast (for example, a continuously cast slab) through normal slab heating and hot rolling, and without forging, sizing, etc. during rolling. This eliminates the need for any operations such as preliminary treatment or strict control of hot rolling temperature. Note that the amount of delta ferrite (%δ) at the hot rolling start stage is determined by the hot rolling start temperature and component composition (content of austenite-forming elements and ferrite-forming elements). At the rolling start temperature of 1250°C, it can be calculated using the following formula. (%δ) = 3.2Cr _eq −2.5Ni _eq −24.7 where Cr _eq = (%Cr) + 1.5 × (%Si) + 2 × (%Mo
)+0.5×(%Nb)+6×(%Al) Ni _eq =30×{(%C)+(%N)}+
(%Ni)+0.5×(%Mn) Examples and comparative examples of the present invention will be described below. By adjusting the ratio of slab basicity and alumina in slag to an appropriate value during steelmaking, various low-S steels were produced. Specifically, in a 100 ton electric furnace, the slag basicity was set to 1.6 and CaO/CaF ₂ was set to 3.0. The temperature of the molten steel is set to 1650℃ or higher, and the oxygen in the molten steel is removed by strong stirring in a top-bottom blowing converter and by adding Al (in the case of 17-7PH) or deoxidizing Al.
After reducing the slag composition to 0.008 or less, change the slag composition to CaO/SiO ₂ :
Desulfurization was carried out by adjusting the ratio of Al ₂ O ₃ to 1:0.3. By adjusting the desulfurization conditions, low S steel in the following examples was produced. A continuous cast slab having a thickness of 200 mm and having the composition shown in Table 1 was heated to 1250° C. and subjected to rough rolling and finish rolling on a normal hot rolling line to obtain a hot rolled steel strip having a thickness of 4 mm. Table 1 also shows the results of investigating the occurrence of edge cracks and surface flaws (such as surface microcracks) on the obtained hot rolled steel strips. All steels have a delta ferrite content in the range of 10 to 40% at the start of hot rolling (approximately 1240°C), and a delta ferrite content of 15% or less at the product stage. Regarding the determination of cracked ears,
As a criterion for determining whether hot-rolled steel strips can be sent to the cold rolling process without trimming, those with no edge cracks are considered acceptable, and those with edge cracks that require further trimming are determined by the length of the edge cracks. I looked into it. On the other hand, regarding the determination of surface flaws, pass/fail was determined based on whether a coil grinder process was required or not. As is clear from Table 1, in the case of austenitic stainless steels outside the scope of this invention, at least one or both of edge cracks and surface flaws occur, whereas in the case of austenitic stainless steels of this invention, at least one or both of edge cracks and surface flaws occur. In all cases, there was no occurrence of ear cracks or surface flaws. As described above, the austenitic stainless steel of the present invention has a delta ferrite content at the start of hot rolling, which was conventionally considered to have unavoidable edge cracks and surface microcracks in the normal slab heating-hot rolling method. In 10-40% austenitic stainless steel, normal slab heating and rolling can cause edge cracking and cracking without applying any special heating or rolling methods.
It is possible to completely prevent the occurrence of surface microcracks, and therefore, the austenitic stainless steel of the present invention can significantly improve the yield in the hot rolling process compared to the conventional one, and There is no need to employ special processes to prevent the occurrence of cracks or surface microcracks, or to perform work to remove or repair cracks that have occurred, resulting in a significant reduction in manufacturing costs.

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【table】

Claims (1)

[Claims] 1. Less than 0.1% C, 1% or less Si, in weight percent;
Contains the following components: Mn 3% or less, Ni 5-16%, Cr 15-30%, Mo 3% or less, Al 1.5% or less, Nb 1.5% or less,
And in austenitic stainless steel that contains delta ferrite in the range of 10% or more and less than 40% at the stage of starting hot rolling, and the amount of delta ferrite in the product is 15% or less, the S content is 0.0010.
% or less, and the balance for each of the above elements consists of Fe and unavoidable impurities. 2. Less than 0.1% C, less than 1% Si by weight,
Contains the following components: Mn 3% or less, Ni 5-16%, Cr 15-30%, Mo 3% or less, Al 1.5% or less, Nb 1.5% or less,
And in austenitic stainless steel that contains delta ferrite in the range of 10% or more and less than 40% at the stage of starting hot rolling, and in which the amount of delta ferrite in the product is 15% or less, the S content is 0.0015.
% or less and more than twice the S content, 0.01%
Ca below, more than 4 times the S content, less than 0.08%
1. An austenitic stainless steel with excellent hot workability, characterized in that it contains at least one kind of REM, and the balance for each of the above elements consists of Fe and unavoidable impurities.