JP4190617B2

JP4190617B2 - Method for producing hot rolled sheet of stainless steel

Info

Publication number: JP4190617B2
Application number: JP19096998A
Authority: JP
Inventors: 公正杉村; 環藤山; 正純平居
Original assignee: 大平洋金属株式会社
Priority date: 1998-06-23
Filing date: 1998-06-23
Publication date: 2008-12-03
Anticipated expiration: 2018-06-23
Also published as: JP2000005803A

Description

【０００１】
【発明の属する技術分野】
本発明は表面疵のないステンレス鋼の熱延鋼板を製造する方法に関するものである。
【０００２】
【従来の技術】
ステンレス鋼は高価なＮｉ、Ｃｒ、Ｍｏ、Ｃｕなどを多量に含有しているため、歩留向上は製造コスト低減の最重要項目であり、製造工程での疵防止による歩留向上が望まれる。しかし、耐食性、耐酸化性、および強度の点からＮｉ、Ｃｒ、Ｍｏ、Ｃｕなど多量に含有したステンレス鋼は熱間での加工性が劣り、熱間加工中にデンドライト粒界で割れが発生し、へげ状疵となって歩留低下の原因となっていた。
連続鋳造鋳片の熱間加工性向上対策として、例えば、特公昭５４−２４３６４、特公平２−１４４１９、特公平５−７４５７、特開平４−１１０４１９でＣａ、Ｍｇ、希土類元素（Ｙ、Ｌａ、Ｃｅなどで以下ＲＥＭと呼ぶ）、Ｂなどを添加する方法が開示されている。
【０００３】
しかし、Ｃａ、ＭｇやＲＥＭは酸化されやすい元素のため、添加歩留が不安定であり、ＣａＯ系、ＭｇＯ系やＲＥＭ系介在物による表面疵や地疵などの内部欠陥が多発し、製品歩留低下および品質低下の問題があった。また、溶接材料用のステンレス鋼では、Ｃａ、Ｍｇ、ＲＥＭなどの元素を添加した場合、溶接性が悪くなるとされている。
一方、Ｂについても、耐食性など材質特性への影響から添加できない製品があるため、すべての品種に適用できないことや、他製品の不純物元素管理対策として、Ｂ添加品種の発生屑に対し、煩雑な特別管理が必要となる欠点がある。
【０００４】
【発明が解決しようとする課題】
本発明は、上記のような問題点を解決し、熱間加工性が悪い成分系の鋼種も熱間加工で割れ疵がなく、かつ、溶接性や耐食性などの材質特性の良好なステンレス鋼製品を経済的に歩留良く製造することを目的とするものである。
【０００５】
【課題を解決するための手段】
本発明者らは、この問題を解決するために種々の製造実験を行い、検討を重ねた結果、連続鋳造されたδフェライト量の多いステンレス鋼鋳片は、鋳片表面の成分偏析が小さく微細結晶のチル晶を残した無手入れのまま加熱し熱間圧延した方が、熱間圧延鋼板の表面疵が少ないことを見い出した。
また、熱間加工性が特に悪い鋼種においては、必要に応じてブレークダウン圧延を行ったが、圧延比（圧延スラブ厚み／鋳片厚み）が３０〜６５％のブレークダウン圧延を行い、連続鋳造鋳片（以下ＣＣ鋳片と呼ぶ）の鋳造組織を破壊した後に、該ブレークダウンスラブを再加熱し、再結晶による結晶の微細化と粒界偏析の拡散を行った後に、無手入れでブレークダウン圧延によるスラブ表層の圧延組織を残したままで仕上圧延を行うことによって、仕上圧延時の表面割れや耳割れなどの表面欠陥のない高δフェライト含有ステンレス鋼の熱間圧延製品を安定して歩留良く製造することができることを見い出した。
【０００６】
また、該ブレークダウン圧延時のＣＣ鋳片の加熱温度は１０００〜１２７０℃の比較的低温で加熱し、鋳造結晶の破壊のためのブレークダウンによる圧延効果を大きくし、仕上圧延前の加熱は１１００〜１２７０℃で加熱し、粒界偏析の拡散促進と熱延仕上がり温度を高くすることによって、より表面割れや耳割れなどの表面欠陥のない高δフェライト含有ステンレス鋼の熱延鋼板が安定して歩留良く製造することができた。
ＣＣ鋳片をブレークダウン圧延した後、該ブレークダウンスラブの表面は有害疵がある場合は、再加熱・仕上圧延する前に、有害疵のみを部分手入れしてもよいが、ＣＣ鋳片は、鋳片表面の微細結晶かつ成分偏析の少ないチル晶を残したままの無手入れの状態でブレークダウン圧延することによって、ブレークダウン圧延スラブの表面疵低減または手入れ省略が可能となった。
【０００７】
すなわち、本発明は、下記手段をとるものである。
重量％でＣ：０．２％以下、Ｓｉ：０．１〜２％、Ｍｎ：０．３〜３％、Ｐ：０．０４％以下、Ｃｒ：１５〜３０％、Ｎｉ：３〜３０％、Ｎ：０．０１〜０．４％、Ｓ：０．００５％以下、Ｏ：０．００７％以下、Ａｌ：０．０２％以下を含有し、また、必要に応じて、さらにＭｏ：４％以下、Ｃｕ：３％以下、Ｎｂ：２％以下、Ｔｉ：２％以下を含有し、残部はＦｅ、および不可避的不純物成分からなり、かつ、
Ｃｒｅｑ＝（Ｃｒ％）＋１．５（Ｓｉ％）＋（Ｍｏ％）＋０．５（Ｎｂ％）
Ｎｉｅｑ＝（Ｎｉ％）＋３０（Ｃ％＋Ｎ％）＋０．５（Ｍｎ％）
δ％＝−０．０８１６（Ｃｒｅｑ）^２＋５．９７５（Ｃｒｅｑ）−３．７８６（Ｎｉｅｑ）＋０．０５８７（Ｃｒｅｑ）・（Ｎｉｅｑ）−４６．２３
で計算されるδが７％以上となるステンレス鋼の熱間圧延板を製造するに際し、連続鋳造された連続鋳造鋳片を無手入れのまま１０００〜１２７０℃に加熱し圧下比（圧延スラブ厚み／鋳片厚み）が３０〜６５％のブレークダウン圧延を行った後に、該ブレークダウンスラブを無手入れのまま１１００〜１２７０℃に再加熱し仕上げ圧延することを特徴とするステンレス鋼の熱間圧延板を製造する方法。
【０００８】
【発明の実施の形態】
以下、本発明を詳細に説明する。
まず、本発明が含有する合金元素の含有量を限定する理由を以下に説明する。
Ｃは強力なオーステナイト相形成元素で、かつ固溶している場合は強度を高める元素であるが、耐食性の点から低い方が良く０．２％以下とする。
【０００９】
Ｓｉはステンレス鋼溶製時の脱酸のために添加し、０．１％未満の時Ｃｒ歩留が著しく低下するので、その下限を０．１％とする。また、ステンレス鋼の耐食性、耐酸化性を増すが、２％を超えると高温での熱間加工性を害するとともに、熱間圧延での圧延負荷が増大し製造上好ましくないので、上限を２％とする。
【００１０】
Ｍｎはオーステナイト相安定化元素で、かつ脱酸に役立ち、また鋼中Ｓと結びつきＭｎＳとして熱間加工性を改善するが、耐食性を低下させるので０．３〜３％とする。
Ｐは耐食性、熱間加工性の点では少ない方が良好で０．０４％以下とする。
Ｓは熱間加工性改善のため低い方が良く０．００５％以下とする。
【００１１】
Ｃｒはステンレス鋼の基本成分で耐食性、耐酸化性の点から１５％以上とするが、高価となるので１５〜３０％とする。
Ｎｉは強力なオーステナイト相形成元素で、かつ耐食性、耐熱性を向上させる。３％未満では耐食性、耐熱性が低く実用的でなく、３０％を超えると高価となるため、３〜３０％とする。
【００１２】
Ｎはオーステナイト相安定化元素であり、かつ、ステンレス鋼の強度と耐食性を改善し、０．０１％以上で効果を示すが、０．４％を超えると熱間加工性を著しく低下させ、また、オーステナイト相固溶限界を超え気泡が発生するので０．０１〜０．４％とする。
Ｏは熱間加工性および介在物性欠陥防止に対し低い方が良好であり、０．００７％以下とする。
【００１３】
Ｍｏは耐食性を向上させるために、必要に応じて添加するが、熱間加工性を低下させ、また熱間変形が大きくなり圧延負荷を増大させ、高価であり４％以下とする。
Ｃｕはオーステナイト安定化元素であり、冷鍛性の改善や耐食性を向上させるために、必要に応じて添加するが、３％を超えて含有させても効果が飽和し、むしろ溶接部の割れの原因になるほか、高温における粒界脆化によって熱間加工性を低下させるので３％以下とする。
【００１４】
Ａｌは強力な脱酸剤として添加されるが、Ａｌ₂ Ｏ₃ 介在物の生成による品質上の問題、かつ連続鋳造におけるノズル閉塞や耐食性低下の防止のため０．０２％以下とする。
Ｔｉ、Ｎｂはいずれも炭窒化物形成元素であり、必要に応じて１種または２種を２％以下添加することによって微細な炭窒化物を析出せしめ、ステンレス鋼の耐食性、耐酸化性、強度を向上させる。しかし、前記含有量を超えた添加は、効果が飽和するのみならず、鋼の熱間加工性を低下させるのでそれぞれ２％以下とする。
【００１５】
また、Ｃｒｅｑ＝（Ｃｒ％）＋１．５（Ｓｉ％）＋（Ｍｏ％）＋０．５（Ｎｂ％）
Ｎｉｅｑ＝（Ｎｉ％）＋３０（Ｃ％＋Ｎ％）＋０．５（Ｍｎ％）
δ％＝−０．０８１６（Ｃｒｅｑ）² ＋５．９７５（Ｃｒｅｑ）−３．７８６（Ｎｉｅｑ）＋０．０５８７（Ｃｒｅｑ）・（Ｎｉｅｑ）−４６．２３
で計算されるδが７％未満では熱間加工性は特に問題がないので、δは熱間加工性の悪い７％以上を対象とする。
【００１６】
次に、ブレークダウン圧延を行う場合の圧延比（圧延スラブ厚み／鋳片厚み）は、ＣＣ鋳片の鋳造組織を壊しスラブ表層の組織を圧延組織にするとともに粒界偏析の拡散を促進するため大きい方が良いが、３０％未満ではその効果はなく、６５％超では仕上圧延前のスラブ厚みが薄くなりすぎ、仕上圧延製品のコイル単重が小さくなるとともに、歩留が悪くなるため３０〜６５％とする。
ＣＣ鋳片のブレークダウン圧延前の加熱温度は低い方が鋳造組織の破壊に対し良好であるが、１０００℃未満では熱間変形抵抗が高くなるとともに、熱間延性が悪くなるので１０００〜１２７０℃とする。
【００１７】
一方、仕上熱延時のブレークダウンスラブの加熱温度は熱間加工性の点から高い方がよいが、粒界酸化による割れへげ発生の防止のため１１００〜１２７０℃とする。
かくして前記の如くδが７％以上である高δフェライト含有ステンレス鋼の熱間圧延割れ欠陥の問題点が解決され、表面品質、内部品質ともに安定して歩留良く製造することが可能になる。
なお、本発明組成範囲内で連続鋳造によりビレットに鋳造した後、線材圧延により製品化に適用することも可能である。
【００１８】
【実施例】
次に、本発明の実施例について説明する。
表１は本発明鋼と比較鋼の取鍋下化学成分組成を示すもので、電気炉−ＡＯＤ炉で溶製し、連続鋳造の通常条件でスラブ鋳片に鋳造した。また、該鋳片をホットストリップ、または厚板にそれぞれ通常のステンレス鋼用の条件で熱間圧延し、熱間圧延後の鋼板の表面疵の結果を表２に示した。
【００１９】
【表１】

【００２０】
【表２】

【００２１】
実施例１〜３は板厚４ｍｍの熱延鋼板の結果である。
これに対し比較例１、２はブレークダウン圧延後にブレークダウンスラブの全表面を手入れし仕上圧延を行った例であり、比較例３、４はブレークダウン圧延または仕上圧延の温度が本発明範囲外で製造され、また、比較例５、６はブレークダウン圧延なしで、ＣＣ鋳片の全面手入後に圧延されたもので、本発明範囲外で製造されている。
表２から明らかな如く、実施例は比較例に比べ、いずれも圧延表面疵が少ない熱間圧延鋼板が歩留良く製造できた。
【００２２】
【発明の効果】
この発明に従って製造された高δフェライトステンレス鋼は、経済的に表面品質および内部品質ともに良好な製品が安定して得られるので、産業上に及ぼす効果は極めて大きい。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a stainless steel hot rolled steel sheet having no surface defects.
[0002]
[Prior art]
Since stainless steel contains a large amount of expensive Ni, Cr, Mo, Cu, etc., yield improvement is the most important item for reducing manufacturing costs, and yield improvement by preventing wrinkles in the manufacturing process is desired. However, stainless steel containing a large amount of Ni, Cr, Mo, Cu, etc. in terms of corrosion resistance, oxidation resistance, and strength is inferior in hot workability and cracks occur at the dendrite grain boundaries during hot working. It became a bald ridge and caused a decrease in yield.
As measures for improving the hot workability of continuously cast slabs, for example, Japanese Patent Publication No. 54-24364, Japanese Patent Publication No. 2-14419, Japanese Patent Publication No. 5-7457, Japanese Patent Application Laid-Open No. 4-110419, Ca, Mg, rare earth elements (Y, La, A method of adding B or the like is disclosed.
[0003]
However, Ca, Mg, and REM are oxidizable elements, so the addition yield is unstable, and internal defects such as surface defects and ground defects due to CaO-based, MgO-based, and REM-based inclusions occur frequently, resulting in product yield. There was a problem of yield reduction and quality reduction. Moreover, in stainless steel for welding materials, when elements such as Ca, Mg, and REM are added, the weldability is said to deteriorate.
On the other hand, there are products that cannot be added to B due to the effects on the material properties such as corrosion resistance, so that it cannot be applied to all varieties, and it is complicated for the waste generated from the B-added varieties as countermeasures for impurity elements of other products. There are drawbacks that require special management.
[0004]
[Problems to be solved by the invention]
The present invention solves the above-mentioned problems, and stainless steel products that have good material properties such as weldability and corrosion resistance, as well as component steel types with poor hot workability, are free from cracks during hot working. The purpose is to produce the product economically with a high yield.
[0005]
[Means for Solving the Problems]
In order to solve this problem, the present inventors have conducted various manufacturing experiments, and, as a result of repeated studies, the continuously cast stainless steel slab with a large amount of δ ferrite has a small component segregation on the slab surface and is fine. It has been found that the surface rolling of the hot-rolled steel sheet is less when heated and hot-rolled with the chilled crystal remaining untreated.
Moreover, in steel types with particularly poor hot workability, breakdown rolling was performed as necessary, but breakdown rolling (rolling slab thickness / slab thickness) was performed at a rolling ratio of 30 to 65%, and continuous casting was performed. After breaking the cast structure of the slab (hereinafter referred to as CC slab), the breakdown slab is reheated, the crystal is refined by recrystallization and grain boundary segregation is diffused, and then the maintenance is broken down without maintenance. By performing finish rolling while leaving the rolling structure of the slab surface layer by rolling, it is possible to stably produce hot rolled products of high δ ferritic stainless steel without surface defects such as surface cracks and ear cracks during finish rolling. I found out that it can be manufactured well.
[0006]
Further, the heating temperature of the CC slab during breakdown rolling is heated at a relatively low temperature of 1000 to 1270 ° C. to increase the rolling effect due to breakdown for breaking the cast crystal, and heating before finish rolling is 1100. By heating at ˜1270 ° C., promoting diffusion of grain boundary segregation and increasing the hot rolling finish temperature, the hot-rolled steel sheet of high δ ferrite-containing stainless steel without surface defects such as surface cracks and ear cracks is more stable. It was possible to manufacture with good yield.
After the CC slab has been subjected to breakdown rolling, if the surface of the breakdown slab has harmful flaws, it may be partially cleaned before reheating and finish rolling. By carrying out breakdown rolling in a state in which fine crystals on the surface of the slab and chill crystals with little component segregation are left untreated, it is possible to reduce the surface defects of the breakdown rolled slab or to omit maintenance.
[0007]
That is, the present invention takes the following means.
C: 0.2% or less by weight%, Si: 0.1-2%, Mn: 0.3-3%, P: 0.04% or less, Cr: 15-30%, Ni: 3-30% N: 0.01 to 0.4%, S: 0.005% or less, O: 0.007% or less, Al: 0.02% or less, and if necessary, Mo: 4 % Or less, Cu: 3% or less, Nb: 2% or less, Ti: 2% or less, the balance consisting of Fe and inevitable impurity components, and
Creq = (Cr%) + 1.5 (Si%) + (Mo%) + 0.5 (Nb%)
Nieq = (Ni%) + 30 (C% + N%) + 0.5 (Mn%)
δ% = − 0.0816 (Creq) ² +5.975 (Creq) −3.786 (Nieq) +0.0587 (Creq) · (Nieq) −46.23
When producing a stainless steel hot-rolled sheet in which δ calculated in (7) is 7% or more, the continuously cast slab continuously cast is heated to 1000 to 1270 ° C. without maintenance, and the reduction ratio (rolling slab thickness / A stainless steel hot-rolled sheet, wherein after the breakdown rolling with a slab thickness of 30 to 65% is performed, the breakdown slab is reheated to 1100 to 1270 ° C. without being treated and finish-rolled. How to manufacture.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
First, the reason for limiting the content of alloy elements contained in the present invention will be described below.
C is a strong austenite phase-forming element and is an element that increases the strength when it is in solid solution, but it is preferably lower in terms of corrosion resistance and is 0.2% or less.
[0009]
Si is added for deoxidation when melting stainless steel, and when it is less than 0.1%, the Cr yield is remarkably lowered, so the lower limit is made 0.1%. In addition, the corrosion resistance and oxidation resistance of stainless steel are increased, but if it exceeds 2%, hot workability at high temperatures is impaired, and the rolling load in hot rolling increases, which is not preferable for production. Therefore, the upper limit is 2%. And
[0010]
Mn is an austenite phase stabilizing element and is useful for deoxidation, and is combined with S in the steel to improve hot workability as MnS, but it decreases the corrosion resistance, so is 0.3 to 3%.
P is preferably as small as possible in terms of corrosion resistance and hot workability, and is 0.04% or less.
S is preferably low for improving hot workability, and is 0.005% or less.
[0011]
Cr is a basic component of stainless steel, and is made 15% or more from the viewpoint of corrosion resistance and oxidation resistance.
Ni is a strong austenite phase-forming element and improves corrosion resistance and heat resistance. If it is less than 3%, the corrosion resistance and heat resistance are low and impractical, and if it exceeds 30%, it becomes expensive, so 3-30%.
[0012]
N is an austenite phase stabilizing element and improves the strength and corrosion resistance of stainless steel, and shows an effect at 0.01% or more, but when it exceeds 0.4%, hot workability is remarkably lowered. , Since the austenite solid solution limit is exceeded and bubbles are generated, the content is made 0.01 to 0.4%.
The lower O is better for hot workability and prevention of inclusion physical defect, and it is 0.007% or less.
[0013]
Mo is added as necessary in order to improve the corrosion resistance. However, the hot workability is reduced, the hot deformation is increased, the rolling load is increased, the cost is high, and the amount is 4% or less.
Cu is an austenite stabilizing element, and is added as necessary to improve cold forgeability and corrosion resistance. However, even if it exceeds 3%, the effect is saturated, rather the crack of the weld In addition to causing this, the hot workability is reduced by grain boundary embrittlement at high temperatures, so the content is made 3% or less.
[0014]
Al is added as a strong deoxidizer, but is made 0.02% or less in order to prevent quality problems due to the formation of Al ₂ O ₃ inclusions and to prevent nozzle clogging and deterioration of corrosion resistance in continuous casting.
Ti and Nb are both carbonitride-forming elements. If necessary, fine carbonitrides are precipitated by adding one or two of 2% or less, and the corrosion resistance, oxidation resistance and strength of stainless steel. To improve. However, addition exceeding the above content not only saturates the effect, but also reduces the hot workability of the steel, so each content is made 2% or less.
[0015]
Creq = (Cr%) + 1.5 (Si%) + (Mo%) + 0.5 (Nb%)
Nieq = (Ni%) + 30 (C% + N%) + 0.5 (Mn%)
δ% = − 0.0816 (Creq) ² +5.975 (Creq) −3.786 (Nieq) +0.0587 (Creq) · (Nieq) −46.23
Since the hot workability is not particularly problematic when δ calculated in (5) is less than 7%, δ is intended for 7% or more with poor hot workability.
[0016]
Next, the rolling ratio (rolling slab thickness / slab thickness) when performing breakdown rolling is to break the cast structure of the CC slab to make the slab surface structure a rolled structure and promote diffusion of grain boundary segregation. Larger is better, but if it is less than 30%, there is no effect, and if it exceeds 65%, the slab thickness before finish rolling becomes too thin, the coil unit weight of the finished rolled product becomes smaller, and the yield becomes worse. 65%.
The lower the heating temperature of the CC slab before breakdown rolling is, the better the fracture of the cast structure is. However, when it is less than 1000 ° C, the hot deformation resistance increases and the hot ductility deteriorates, so 1000-1270 ° C. And
[0017]
On the other hand, the heating temperature of the breakdown slab at the time of finish hot rolling is preferably high from the viewpoint of hot workability, but is set to 1100 to 1270 ° C. in order to prevent cracking due to grain boundary oxidation.
Thus, as described above, the problem of hot rolling crack defects in high δ ferritic stainless steel having δ of 7% or more is solved, and it becomes possible to manufacture both the surface quality and the internal quality stably and with high yield.
In addition, after casting into a billet by continuous casting within the composition range of this invention, it is also possible to apply to product production by wire rod rolling.
[0018]
【Example】
Next, examples of the present invention will be described.
Table 1 shows the chemical composition of the steel of the present invention and the comparative steel under the ladle, melted in an electric furnace-AOD furnace, and cast into a slab slab under normal conditions of continuous casting. Further, the slab was hot-rolled on a hot strip or a thick plate under normal conditions for stainless steel, and the results of surface defects of the steel plate after hot rolling are shown in Table 2.
[0019]
[Table 1]

[0020]
[Table 2]

[0021]
Examples 1 to 3 are the results of a hot-rolled steel sheet having a thickness of 4 mm.
On the other hand, Comparative Examples 1 and 2 are examples in which the entire surface of the breakdown slab was subjected to finish rolling after breakdown rolling. In Comparative Examples 3 and 4, the temperature of breakdown rolling or finishing rolling was outside the scope of the present invention. In addition, Comparative Examples 5 and 6 were rolled after the entire surface of the CC slab was prepared without breakdown rolling, and were manufactured outside the scope of the present invention.
As is apparent from Table 2, all of the examples were able to produce hot rolled steel sheets with a reduced rolling surface flaw with a higher yield than the comparative examples.
[0022]
【The invention's effect】
The high δ ferritic stainless steel manufactured according to the present invention has an extremely large effect on the industry since a product with good surface quality and internal quality can be stably obtained economically.

Claims

C: 0.2% or less by weight%, Si: 0.1-2%, Mn: 0.3-3%, P: 0.04% or less, Cr: 15-30%, Ni: 3-30% N: 0.01 to 0.4%, S: 0.005% or less, O: 0.007% or less, Al: 0.02% or less, and if necessary, Mo: 4 % Or less, Cu: 3% or less, Nb: 2% or less, Ti: 2% or less, the balance consisting of Fe and inevitable impurity components, and
Creq = (Cr%) + 1.5 (Si%) + (Mo%) + 0.5 (Nb%)
Nieq = (Ni%) + 30 (C% + N%) + 0.5 (Mn%)
δ% = − 0.0816 (Creq) ² +5.975 (Creq) −3.786 (Nieq) +0.0587 (Creq) · (Nieq) −46.23
When producing a stainless steel hot-rolled sheet in which δ calculated in (7) is 7% or more, the continuously cast slab continuously cast is heated to 1000 to 1270 ° C. without maintenance, and the reduction ratio (rolling slab thickness / A stainless steel hot-rolled sheet, wherein after the breakdown rolling with a slab thickness of 30 to 65% is performed, the breakdown slab is reheated to 1100 to 1270 ° C. without being treated and finish-rolled. How to manufacture.