JP4164185B2 - Method for producing Cu-containing stainless steel slab - Google Patents

Description

【００１３】
【発明の効果】
以上に説明したように、本発明においては、Ｃｕ含有ステンレス鋼を連続鋳造する際、二次冷却時にクレータエンド部の冷却を強化し、スラブ中心部に析出するＣｕ合金相の粒径を５０μｍ以下に抑えている。粒径制御されたＣｕ合金相は、熱延時に結晶粒界で液膜化することがないので、熱延割れ，鰐口割れ等の原因にならない。このようにして、本発明によるとき、従来熱延困難な材料であったＣｕ含有ステンレス鋼スラブを操業上のトラブルなく熱延でき、Ｃｕ添加による加工性，耐食性が改善された素材が製造される。[0001]
[Industrial application fields]
The present invention relates to a method for producing a stainless steel slab containing Cu, which is effective for improving workability and corrosion resistance.
[0002]
[Prior art and problems]
Cu is an alloy component that improves the workability and corrosion resistance of stainless steel, and SUS304J1, SUS304J2, SUS304J3, SUS316J1, SUS316J1L, SUS317J5L, SUSXM7, SUS630, and the like are used as the Cu-containing stainless steel.
Although Cu is effective in improving workability and corrosion resistance, it is a typical component that adversely affects the hot workability of steel. For example, in the case of Cu-containing steel, slight surface defects are likely to occur even when Cu is contained in an amount of less than 0.3% by mass . The adverse effect of Cu on hot workability becomes prominent with an increase in Cu content, and there is a tendency for small cracks to grow at 0.3% by mass or more, and cracks to grow significantly at 0.8% by mass . In fact, especially in the case of steel types containing 1% by mass or more and a large amount of Cu, troubles such as cracks and double cracks (hole cracks) are likely to occur during hot rolling, and the manufacturability is hindered.
[0003]
[Means for Solving the Problems]
The present invention has been devised to solve such a problem, and by strengthening the cooling of the crater end portion, the coarsening of the Cu alloy phase is suppressed, and cracking at the time of hot rolling, cracking at the neck, etc. It aims at obtaining the continuous cast slab which does not generate | occur | produce.
In order to achieve the object of the manufacturing method of the present invention, when continuously casting an austenitic stainless steel containing 1 to 5% by mass of Cu, the grain size of the Cu alloy phase in the center of the slab is 50 μm or less. Cooling water is sprayed on the crater end portion at a specific water amount of 0.2 to 0.5 liter / kg-steel .
[0004]
[Action]
Various investigations and researches were conducted on cracks and spout cracks that occurred during hot rolling of Cu-containing stainless steel. As a result of investigating the crack generation part, it was found that Cu exists in the crystal grain boundary of the part where the crack started and Cu is distributed along the crack.
Therefore, in order to elucidate the factors inherent in the slab where the hot-rolled crack occurred, the slab of the charge where the hot-rolled crack occurred and the slab of the charge showing good hot-rolled results were investigated. Normally, solute elements such as C, P, and S are centrally segregated at the center of the slab, but Cu is also an element that is centrally segregated. That is, at the center of the slab, when the contraction due to solidification begins, the molten steel enriched with Cu is sucked, so the Cu concentration increases, and there exists a low melting point precipitation phase (Cu alloy phase) enriched with solute elements such as Cu. To do.
[0005]
As a result of investigating the charge slab in which hot rolling cracks occurred, coarse grain boundary precipitation of the Cu alloy phase was detected in the center of the slab. On the other hand, in a charge slab in which hot rolling cracks did not occur, a small-sized Cu alloy phase was precipitated in the grains. The occurrence of hot rolling cracks when coarse Cu alloy phases precipitated at the grain boundaries means that the Cu alloy phases became liquid films at the grain boundaries due to heating during hot rolling, and cracking occurred due to stress during hot rolling. To do.
Therefore, the cause of the difference in the form of the Cu alloy phase was investigated, and a production method for refining the Cu alloy phase was studied. The slab of a charge in which hot-rolling cracking has occurred is a slab manufactured under conditions where the cooling of the crater end portion is weak compared to a slab of a charge that has shown good hot-rolling results. The crater end portion refers to a range in which solute concentrated steel is likely to be sucked by solidification shrinkage, that is, a range in which the solid phase ratio in the center portion of the slab is 0.1 to 1.0.
[0006]
If the cooling conditions are weak, the solidified shell temperature at the crater end portion is high, so the strength is low, and bulging is likely to occur between rolls during continuous casting. Bulging further promotes suction of molten steel enriched with Cu in addition to solidification shrinkage. As a result, center segregation of Cu is promoted, and a coarse Cu alloy phase is precipitated at the center of the slab.
Since the coarse Cu alloy phase is a cause of hot cracking, Cu segregation of Cu is reduced and coarsening of the Cu alloy phase is prevented when bulging is prevented by optimizing the cooling conditions of the crater end portion. It is expected that. The range in which solute-concentrated molten steel can be attracted by solidification shrinkage under normal continuous casting conditions is in the range of a solid fraction of 0.4 to 1.0 at the crater end, that is, the slab center. Therefore, as a result of investigating the relationship between the particle size of the Cu alloy phase and the cooling condition in this range, when the cooling condition is strengthened so that the Cu alloy phase at the center of the slab has a particle size of 50 μm or less, cracks occur during hot rolling. Clarified that it does not occur.
[0007]
The stainless steel targeted by the present invention contains 1 to 5 mass % and a large amount of Cu, and is therefore a steel type in which a Cu alloy phase is likely to precipitate at the center of the slab. The occupied area ratio of the Cu alloy phase by microscopic observation increases as the Cu content increases. The effect of improving the workability and corrosion resistance due to the addition of Cu becomes significant when the content is 1% by mass or more. However, when a large amount of Cu is added exceeding 5% by mass , the hot workability is remarkably lowered and it is impossible to manufacture industrially. Stainless steel to which the present invention is directed, a 1 to 5 mass% of Cu is including austenitic stainless steels. As other optional components, steel types including Mo, Nb, Ti, Zr, V, B, rare earth metals (REM), Y, Ca, Mg, W, N and the like are also targeted.
[0008]
The maximum particle size of 50 μm of the Cu alloy phase for preventing hot rolling cracks is a value found from the results of investigations and studies by the present inventors. The maximum particle size of the Cu alloy phase is obtained as a maximum measured value by observing the central portion in the thickness direction of the slab cross section with an optical microscope and measuring the particle size of 100 or more Cu alloy phases.
When the cooling of the crater end portion is strengthened and the occurrence of bulging is suppressed, the grain size of the Cu alloy phase becomes 50 μm or less and hardly precipitates at the grain boundaries. Even when precipitated at the grain boundary, the Cu alloy phase has a small particle size, so that it is easily re-dissolved in the matrix by heating during hot rolling. Therefore, it becomes difficult to form a liquid film at the grain boundary, and hot rolling cracks are suppressed. On the other hand, when bulging occurs, the Cu alloy phase is coarse with a maximum particle size of 60 to 100 μm and is precipitated at the grain boundary. Is likely to occur.
[0009]
The particle size of the Cu alloy phase is controlled by the cooling conditions of the crater end portion when the slab is cooled by spray cooling, mist cooling or the like. For controlling the cooling conditions, a method of adjusting the flow rate of the cooling water sprayed on the crater end portion of the slab is adopted. However, the amount of cooling water at the crater end for making the Cu alloy phase 50 μm or less is not uniquely determined because it varies depending on the cross-sectional area of the slab, the casting speed and the Cu concentration in the steel. As a guide, when the cooling water is sprayed at a specific water amount of 0.2 to 0.5 liter / kg-steel, the particle size of the Cu alloy phase is suppressed to 50 μm or less.
[0010]
[Example 1]
Austenitic stainless steel (70 tons / charge) based on SUS XM7 (Cu content 3% by mass ) is melted through an electric furnace, converter, and VOD process, and continuously cast into a slab with a thickness of 200mm and a width of 1000mm. did. Table 1 shows the casting speed during continuous casting and the specific water amount at the crater end.
A sample was cut out from the center of the slab, and the particle size of the Cu alloy phase was measured. Further, a sample for hot rolling evaluation was cut out from the center of the slab, and hot rolled to a plate thickness of 3 mm by hot rolling in a laboratory. The presence or absence of hot-rolled cracks was determined by observing the hot-rolled sheet, and evaluations were given as ◯ for those without hot-rolled cracks, Δ for those with fine cracks, and X for those with large cracks.
As can be seen from the investigation results in Table 1, when the Cu alloy phase was adjusted to a particle size of 50 μm or less by controlling the cooling conditions, it was manufactured into a hot-rolled sheet having a predetermined thickness without causing hot-rolling cracks. On the other hand, in the comparative example with weak cooling conditions, a large Cu alloy phase exceeding 50 μm was precipitated, and cracking occurred during hot rolling.
[0011]
[Example 2]
Austenitic stainless steel (70 tons / charge) having a basic composition of SUS316J1 (Cu content 2 mass %) was melted and continuously cast in the same manner as in Example 1. And like Example 1, while measuring the particle size of Cu alloy phase in the center part of the obtained slab, it hot-rolled to plate | board thickness 3mm by the hot rolling in the laboratory.
Also in this case, when the Cu alloy phase was adjusted to a particle size of 50 μm or less, it was produced into a hot-rolled sheet having a predetermined thickness without causing hot-rolling cracks. On the other hand, in the comparative example with weak cooling conditions, a large Cu alloy phase exceeding 50 μm was precipitated, and cracking occurred during hot rolling.
[0012]

[0013]
【The invention's effect】
As described above, in the present invention, when Cu-containing stainless steel is continuously cast, the cooling of the crater end portion is strengthened at the time of secondary cooling, and the particle size of the Cu alloy phase precipitated in the center portion of the slab is 50 μm or less. It is suppressed to. Since the Cu alloy phase whose grain size is controlled does not form a liquid film at the grain boundary during hot rolling, it does not cause hot rolling cracks, throat cracks, and the like. Thus, according to the present invention, a Cu-containing stainless steel slab, which has been difficult to hot-roll conventionally, can be hot-rolled without any trouble in operation, and a material with improved workability and corrosion resistance due to addition of Cu is produced. .

Claims (1)

When continuously casting austenitic stainless steel containing 1 to 5% by mass of Cu , 0.2 to 0.5 liter / kg at the crater end so that the grain size of the Cu alloy phase at the center of the slab is 50 μm or less. -Manufacturing method of Cu containing stainless steel slab characterized by spraying cooling water with specific water amount of steel.