JP4350257B2 - Materials for hot rolling of Fe-Cr and Fe-Cr-Ni stainless steels and methods for preventing ear cracks during hot rolling - Google Patents

Description

【００２１】
【表２】

【００２２】
この熱間圧延用素材を加熱炉で加熱し、熱間圧延機により5mmの熱延鋼帯に圧延した。その時の熱延鋼帯の耳割れ状況も合わせて表２に示す。また、比較例として肉盛り溶接無しの素材についても熱延を行った。
表２から明らかなように、本発明に従ったNo.1〜9では耳割れは鋼種Bを除いて耳割れは発生せず、鋼種Bでも耳割れは5mm以下と軽微であった。
【００２３】
肉盛り溶接なしの場合は、5鋼種とも耳割れが全長にわたって多数生じ、耳割れ深さも21mm以上であった（No.10，13，15，18，19）。肉盛り溶接した場合でも溶接金属厚みが3mmと薄いNo.12，16では耳割れ深さは若干浅くなったものの6mm以上の耳割れが全長にわたって生じていた。No.11，14，17は耳割れの数は少ないものの、生じた耳割れ深さは21mm以上であった。この耳割れは溶接金属のCあるいはＣｒが高くなり溶接金属に割れが発生し、この割れを起点として生じたと推察された。
【００２４】
【発明の効果】
本発明に従えば熱間圧延中にオーステナイトとフェライトが共存する耳割れが発生しやすいＦｅ-Ｃｒ系、Ｆｅ-Ｃｒ-Ｎｉ系ステンレス鋼スラブから耳割れのない、あるいは耳割れが生じても軽微な熱延鋼帯を得る事が出来る。
【図面の簡単な説明】
【図１】 溶接金属層のＣ,Ｃｒ含有量と熱延時の割れ個数との関係[0001]
BACKGROUND OF THE INVENTION
The present invention is for hot rolling mainly used when hot-rolling Fe-Cr-based or Fe-Cr-Ni-based stainless steel in which processability is reduced by hot rolling and ear cracks occur during hot rolling. The present invention relates to a material and a hot rolling method.
[0002]
[Prior art]
Among stainless steels, Fe-Cr-Ni stainless steels such as austenitic stainless steels with 10% by volume or more of ferrite in the austenite in the solidified state and ferrite + austenitic duplex stainless steels are hot-rolled. Had a problem of cracking ears. In addition, even in Fe-Cr stainless steels such as ferritic stainless steel or martensitic steel stainless steel, ear cracks occur during hot rolling even when two phases of ferrite and austenite are formed during hot rolling. There was a problem to do. The term “ear crack” as used herein refers to a crack of several mm to several tens mm in the width direction from both edges of the steel strip, and is also referred to as “ear cut” or “end cut”. The above-mentioned Fe-Cr or Fe-Cr-Ni stainless steels are steel types in which the ferrite phase and austenite phase coexist in the temperature range where hot rolling is performed, but heat is generated in the temperature range where the ferrite phase and austenite phase coexist. The reason why ear cracks are likely to occur is that the impurity element such as S is segregated at the interface between the austenite phase and the ferrite phase and the interface becomes brittle, or the difference in deformation resistance between the austenite phase and the ferrite phase is large. Explained.
If ear cracks occur during hot rolling, the cracked pieces will not only lead to flaws and holes in the healthy part, leading to a decrease in the yield of the hot-rolled steel strip and an increase in the grinder process. The steel strip may break as a starting point, which may hinder hot rolling operations.
[0003]
Up to now, in order to prevent the occurrence of edge cracking during rolling in "Plastic processing science, Precision engineering course No.10, Corona, (1978)", the slab side surface can be rolled by joining a material with high ductility. It is disclosed. Japanese Patent Application Laid-Open No. 59-150603 discloses a method in which a mild steel plate is attached to the slab side surface of an electromagnetic steel plate or is welded and hot rolled using a welding rod made of a material equivalent to mild steel. Japanese Patent Laid-Open No. 9-269398 discloses overlay welding of austenitic stainless steel so that delta ferrite is 3 to 12% by volume on the side surface of the slab of austenitic stainless steel to which B is added.
[0004]
[Problems to be solved by the invention]
However, in order to adjust the volume of ferrite in the weld metal layer to a volume% that does not cause ear cracks by overlay welding by strip electrode welding using SUS304 hoops on the slab side of various Fe-Cr-Ni stainless steels It is necessary to change the Cr% in the flux in accordance with the composition of each stainless steel slab. Further, even if the weld metal layer was formed so that the ferrite content was in the range of 5 to 10%, the ear crack could not be completely prevented.
[0005]
On the other hand, when a SUS304 hoop-like welding material is applied to the Fe—Cr stainless steel slab, the ferrite content of the weld metal layer increases to 10% or more at which ear cracks occur. Also, in the case of Fe-Cr-Ni stainless steel slabs where the delta ferrite% of the as-solidified slab is 10% or more, adjust the delta ferrite of the weld metal layer to 5 to 10% to form a weld metal layer with a thickness of 5 mm. Although it was formed and hot-rolled, the ear cracking could not be completely prevented although it was slightly improved.
[0006]
The method of attaching a mild steel plate to the side of a continuous cast slab with a thickness of 200 mm and a length of 8 m in the Fe-Cr and Fe-Cr-Ni systems that cause ear cracks is applied to the plate or slab so that the mild steel plate does not peel during hot rolling It is necessary to create a groove by machining and weld the interface between the slab and the plate without any gaps, and there is a problem that it takes time to join and increases the construction cost of machining and the like. On the other hand, even when the side surface of the continuously cast slab is build-up welded with a welding rod using a welding rod made of a mild steel equivalent material, there is a problem that a great amount of time and cost increase.
[0007]
Therefore, when welding is performed on the side of a continuously cast slab by simply using a hoop made of a material equivalent to mild steel by the strip electrode welding method, which is significantly more productive than the welding rod, cracks can be made on the surface of the weld metal, although it can be constructed in a relatively short time. When this slab was hot-rolled, there was a problem that an ear crack was caused due to a crack on the surface of the weld metal.
[0008]
As described above, although a method for preventing the ear cracking by covering the side surface of the material to be rolled with a material having a high hot ductility is disclosed, an Fe-Cr-based or Fe-Cr-Ni-based stainless steel that easily causes the ear cracking is disclosed. Appropriate coating conditions and ear cracks that allow overlay welding to be performed in a relatively short time and at a low cost, and cracks do not occur in the weld metal layer when hot-rolling continuous cast slabs or slabs of steel on an industrial scale There has been no disclosure of a hot rolling method in which no occurrence occurs. For this reason, the hot-rolled steel strip of Fe—Cr or Fe—Cr—Ni stainless steel, which is prone to ear cracks, could not be economically produced on an industrial scale without the occurrence of ear cracks.
[0009]
[Means for Solving the Problems]
The present invention relates to a slab of Fe—Cr or Fe—Cr—Ni stainless steel that becomes two phases of ferrite and austenite during hot rolling, and overlay welding formed on a side surface of the slab by a strip electrode welding method. A metal layer, and the thickness of the build-up weld metal layer is 4 mm or more, and the Cr content in the range of 1 mm from the surface of the weld metal layer is 3.0 mass% or less, and the C content is 0.15 mass% or less. There are provided a material for hot rolling, and a method for producing a steel strip by hot rolling the material for hot rolling.
[0010]
[Action]
The inventors of the present invention have a thickness of 50 mm, a width of 100 mm, and a length of 150 mm of SUS309S (delta ferrite is about 23% by volume) in which remarkable ear cracks occur among Fe-Cr and Fe-Cr-Ni stainless steels. On the side of the test piece, welding materials of various steel types were build-up welded by a strip electrode welding method, and a hot rolling experiment was conducted. The evaluation of the ear crack was performed by multi-pass rolling with the rolling reduction per pass being constant 25%, and the number of passes where the ear crack occurred. That is, it shows that the effect of improving the ear cracking is higher as the number of passes where the ear cracks are generated is larger than that of the test piece that is not welded.
[0011]
Using SUS430 as the welding material, overlay welding with a thickness of about 5 mm was performed on both sides of the SUS309S test piece by strip electrode welding. However, weld cracks occurred on the surface of the weld metal layer, and cracks occurred starting from the weld cracks during hot rolling.
[0012]
Mild steel generally refers to steel with a C level of 0.18 to 0.30 mass% (Steel Handbook IV (1981) p5) .When using mild steel containing 0.20 mass% of C as the welding material, the specimen sample of the size described above is welded. Cracks did not occur on the surface of the metal layer, and cracks did not occur up to 10 passes in hot rolling experiments. Therefore, we used a mild steel containing 0.20mass% C as a welding material, stripped on the side of a continuous cast slab of SUS309S (delta ferrite: about 23% by volume) with a thickness of 200mm, a width of 1000mm and a length of 8m with the side oxide scale removed. Overlay welding was performed by an electrode welding method. However, many cracks occurred on the surface of the weld metal layer. Since the crack occurs after the weld metal solidifies and cools, this crack is considered to be a cold crack. In general, cold cracking is said to occur due to a combination of factors that cause the weld metal to become a martensite structure and harden, the amount of diffusible hydrogen to increase, and the binding force to increase.
[0013]
On the other hand, the continuous cast slab has a larger mass than the small piece sample, so the cooling rate of the weld metal layer is remarkably increased, the amount of hard martensite structure increases, and cold cracking occurs due to insufficient hydrogen diffusion. It was. In general, preheating and postheating are effective for low temperature cracking by promoting hardening of weld metal and removing hydrogen, but a large heating furnace is required to preheat and postheat continuous casting slabs. Will be forced to make capital investment.
[0014]
Therefore, in order to prevent the weld metal from cracking, a method for reducing the crack sensitivity of the weld metal was studied. First, the effect of C content in the weld material on cracking was investigated. The component values in the weld metal are the values obtained by overlaying welds on continuous casting slabs with various amounts of C welding steel under various conditions, and chemical analysis of samples taken from a 1 mm area from the surface of the weld metal. The relationship between this component value and the number of cracks per slab surface (per 1.6 m ² ) was investigated. The crack was investigated by the penetrant testing method.
[0015]
As a result, the component in the weld metal partially melts the base material and diffuses into the weld metal layer, so that the concentration of components such as Cr and Ni as the base material component increases. As a result of investigating the relationship between components and cracks in the weld metal, it was found that cracks in the weld metal mainly depend on C in the weld metal and Cr, which is the main component of the base metal. In other words, weld metal cracking mainly increases the amount of Cr that melts and diffuses partially in the welding material C and the base metal, and the martensitic structure of the weld metal hardens, increasing the cracking susceptibility. Caused by It has been found that cracking of this weld metal can be prevented by making the weld metal layer C 0.15 mass% or less and Cr 3.0 mass% or less without depending on the Fe—Cr and Fe—Cr—Ni steel types. It was.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
The amount of C in the weld metal is as small as 0.15% at the maximum for the target Fe-Cr or Fe-Cr-Ni stainless steel, so the influence from the base metal is small. Almost corresponds to the amount of C or slightly lower than the amount of C in the welding material. For this reason, in order to reduce the C content in the weld metal to 0.15 mass% or less, the C content is 0.15 mass% or less, which is a lower C content than mild steel, and the component made of steel consisting of the remainder Fe and inevitable impurities is made. It is sufficient to use a welding material.
[0017]
On the other hand, Cr in the weld metal layer increases when Cr in the Fe—Cr-based or Fe—Cr—Ni-based stainless steel as a base material melts and diffuses in the weld metal. In order to reduce the Cr amount to 3.0 mass% or less, it can be achieved by setting the welding rate to the base metal side, a so-called dilution rate to be 30% or less, and the number of overlay layers being two or more. . As an example of appropriate welding conditions with a dilution rate of 30% or less, when the width of the hoop-like welding material is 50 mm, it can be achieved at a current of 700 A, a voltage of 24 V, and a welding speed of 200 mm / min.
[0018]
Next, a hot rolling experiment was performed by changing the thickness of the weld metal layer. As a result, it was found that the thickness of the weld metal layer is required to be 4 mm or more in order to prevent the ear cracks. In actual hot rolling, thinning of the weld metal due to rolling and oxide scale loss occur. In particular, the weld metal layer was not stainless steel such as SUS304 but steel, and it was found that there was 1 mm oxide scale loss. This weld metal layer having a thickness of 4 mm or more can be achieved by setting welding current to 700 A, voltage constant to 24 V, and overlay welding of two layers or more under welding conditions of 150 to 250 mm / min. The greater the number of overlay layers, the higher the effect of suppressing the ear cracks. However, if the thickness exceeds 10 mm, the cost is unnecessarily increased, so a thickness of 4 to 10 mm is appropriate as the thickness of the weld metal.
[0019]
【Example】
Fe—Cr and Fe—Cr—Ni stainless steels having the components shown in Table 1 were cast into slabs of 200 mm thickness × 1000 mm width by a continuous casting method. After removing the oxide scale on the side surface of this slab by grinding with a grindstone, overlay welding is performed on the entire side surface of the slab by a strip electrode method using a hoop-shaped welding material with a C content of 0.4 mm and a width of 50 mm as shown in Table 2. went. As welding conditions, the voltage was kept constant at 24 V, the current was changed in the range of 600 to 750 A, and the welding speed was changed in the range of 100 to 300 mm / min. The thickness of the weld metal was changed by changing the welding speed and the number of overlay layers. The thickness of the weld metal was determined from the difference in slab width before and after build-up welding, and was the average value measured for each 1 m length. Cracks on the surface of the build-up weld metal were performed by the penetration inspection method, and the results are shown in Table 2.
[0020]
[Table 1]

[0021]
[Table 2]

[0022]
This material for hot rolling was heated in a heating furnace and rolled into a 5 mm hot-rolled steel strip by a hot rolling mill. Table 2 also shows the ear cracking condition of the hot-rolled steel strip at that time. Further, as a comparative example, hot rolling was performed on a material without overlay welding.
As apparent from Table 2, in Nos. 1 to 9 according to the present invention, the ear cracks did not occur except for steel type B, and even in steel type B, the ear cracks were as small as 5 mm or less.
[0023]
When there was no build-up welding, many of the five steel types had many ear cracks over the entire length, and the ear crack depth was 21 mm or more (No. 10, 13, 15, 18, 19). Even when overlay welding was performed, weld cracks of No. 12 and 16 with a weld metal thickness of 3 mm were slightly shallower, but ear cracks of 6 mm or more occurred over the entire length. In Nos. 11, 14, and 17, although the number of ear cracks was small, the depth of the generated ear cracks was 21 mm or more. It was speculated that these ear cracks originated from the cracks in the weld metal due to an increase in the C or Cr of the weld metal.
[0024]
【The invention's effect】
According to the present invention, there is no ear cracks from the Fe-Cr-based or Fe-Cr-Ni-based stainless steel slab that is liable to generate an ear crack in which austenite and ferrite coexist during hot rolling. A hot-rolled steel strip can be obtained.
[Brief description of the drawings]
Fig. 1 Relationship between C and Cr content of weld metal layer and number of cracks during hot rolling

Claims (2)

A slab of Fe-Cr or Fe-Cr-Ni stainless steel that becomes two phases of ferrite and austenite during hot rolling, and a buildup of two or more layers formed on the side of the slab by strip electrode welding A weld metal layer, the thickness of the build-up weld metal layer is 4 mm or more, and the Cr content in the range of 1 mm from the surface of the weld metal layer is 3.0 mass% or less, and the C content is 0.15 mass% or less The material for hot rolling characterized by being. On the side of the slab of Fe-Cr or Fe-Cr-Ni stainless steel that becomes two phases of ferrite and austenite during hot rolling, the thickness of the weld metal layer consisting of two or more layers by the strip electrode welding method is 4mm or more And forming a build-up weld metal layer having a Cr content of 3.0 mass% or less and a C content of 0.15 mass% or less in a range of 1 mm from the surface of the weld metal layer. Ear crack prevention method.

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