JP3842864B2

JP3842864B2 - Carburized steel plate

Info

Publication number: JP3842864B2
Application number: JP09448197A
Authority: JP
Inventors: 政幸立山; 孝士藤本; 由多可平賀; 利昭沖村
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1997-03-31
Filing date: 1997-03-31
Publication date: 2006-11-08
Anticipated expiration: 2017-03-31
Also published as: JPH10273753A

Description

【０００１】
【発明の属する技術分野】
本発明は，浸炭特性および焼入れ特性に優れたいわゆる肌焼用鋼板（浸炭用鋼板）に係り，特にリムド鋼とほぼ同等の特性を有する肌焼用鋼板を連鋳工程を経て製造する方法に関する。
【０００２】
【従来の技術】
自動車や産業機械の部品用として製造されている熱延鋼板の一部には，成形加工後，浸炭−焼入れ処理（肌焼処理）を行なうことにより表面硬さや耐摩耗性を付与することが要求される場合がある。従来より，このような目的に使用される炭素鋼には，リムド鋼あるいは低窒素Ａｌ弱脱酸鋼を用いて対応している。
【０００３】
リムド鋼は，表層では不純物が少なくγ粒は大きくなり，内層では介在物が分散しγ粒は細粒になるため，浸炭−焼入れ性に優れている。しかし，造塊・分塊工程を経て製造されるリムド鋼は，生産性の向上，特に連々比の向上が求められている今日の状況にそぐわないので徐々にＡｌキルド鋼に代替されつつある。
【０００４】
しかし，通常のＡｌキルド鋼はＡｌを０．０２％以上含有しており，ＡｌＮの析出によるピンニング効果により一般にγ粒が微細化し，このため，焼入れ性が低下し，十分な浸炭深さが得られない。すなわち，リムド鋼と同様の熱処理を採用する限り，所要の表面硬さおよび耐摩耗性が得られない。
【０００５】
このようなことから，例えば特開昭５７−１４５９３４号公報に記載のように，Ａｌ：０．００１〜０．００５％，Ｎ：０．００４％以下とし，かつ仕上温度をＡｒ₁点以上Ａｒ₃点以下，巻取り温度を６５０℃以上とし，α＋γの２相領域での低温仕上げ圧延後，高温巻取りする方法が提案されている。この方法は，Ａｌ量とＮ量を低下させてＡｌＮの絶対量を少なくし，α粒の微細化を防止する一方，二相領域圧延を行ってα粒の生成・粗大化を図り，高温巻取りによってα粒の均一成長を促すというものである。
【０００６】
また，特公平５−５３８４５号公報には，Ａｌを０．０１〜０．０４％，Ｎを０．００６％以下の通常通りとし，連続鋳造より得られた熱鋳片を７５０℃以上Ａｒ₃点以下に降温して積極的にＡｌＮを粗大化させ，再び１０００〜１１５０℃に加熱し，析出したＡｌＮを粗大化させてからＡｒ₃点以上で仕上げ圧延を行い，２５０〜４８０℃で巻取という，ＡｌＮを粗大化させて無害化を図る方法が提案されている。
【０００７】
【発明が解決しようとする課題】
前記のように，リムド鋼から肌焼鋼を得る場合には，造塊・分塊工程を経て製造されるため，生産性の向上が求められている状況に適していない。そのため，連続鋳造において製造が可能なＡｌキルド鋼に代替されているが，上述のごとく通常のＡｌキルド鋼にはＡｌＮが存在しており，十分な硬化深さが得られない。このため，ＡｌＮの絶対量を低減するためにＡｌ量およびＮ量を可及的に低下させる方法が採られているが，Ａｌ脱酸鋼のＡｌ量を現状の手段で０．００５％以下に制御することは困難であり，また二次精錬炉における特別な処理を必要とするため製造コストの上昇を招くことは否めない。
【０００８】
特公平５−５３８４５号公報のようにＡｌＮを粗大化させる方法も，特別な条件での製造となり，他の鋼種との生産管理上の問題があるうえ，ＡｌとＮが存在する以上，浸炭特性および焼入れ特性が十分に改善される保証はない。
【０００９】
このようなことから，ＣｒやＢ等の焼入れ性向上元素を添加することにより，焼入れ性を補償する方法も考えられるが，合金コストの上昇を招くため好ましくない。
【００１０】
したがって，本発明は，Ａｌの添加を極力控え，しかも焼入れ性向上元素の無添加の状態で，リムド鋼と同等の特性を有する肌焼鋼板を通常の連続鋳造工程を経て製造する技術の確立を課題としたものである。
【００１１】
【課題を解決するための手段】
本発明によれば，重量％において，Ｃ：０.０１〜０.３０％，Ｓｉ：０.４０％以下，Ｍｎ：０.１〜１.２％，Ｐ：０.０２５％以下，Ｓ：０.０３０％以下，Ｍｇ：０.００５％以下，Ｔ.Ｏ：５０ｐｐｍ以下，Ａｌ：０.００５％以下，Ｎ：８０ｐｐｍ以下，残部がＦｅおよび不可避的不純物からなる鋼板であって，脱酸を金属Ｍｇで行うことによって製造された浸炭用鋼板を提供する。ここで，Ｔ.Ｏはトータル酸素量であり，鋼中に溶存しているもののほか，ＭｇやＡｌなどと反応して酸化物を形成している酸化物中の酸素量を含む。
【００１２】
本発明の前記の浸炭用鋼板は，転炉溶製鋼を脱酸処理して連続鋳造し，得られた連鋳スラブを熱間圧延して熱延鋼板とするさいに，該脱酸処理時に，金属Ｍｇを脱酸剤として行うことによって製造することができる。そのさいの脱酸処理としては，金属Ｍｇ５〜３０mass％と残部がＣａＯ系フラックスからなる脱酸剤を鉄シース内に封入した金属Ｍｇ含有ワイヤを，取鍋内溶鋼に対して，Ｍｇ純分の供給速度が溶鋼トン当り０.０２〜０.２０kg/minとなるように供給し，好ましくは，このワイヤの供給と同時におよび／または供給のあと，窒素ガスまたはＡｒガスを溶鋼中にバブリングさせる。
【００１３】
【発明の実施の形態】
本発明は，前記のように，Ａｌの添加を極力控え且つ焼入れ性向上元素の無添加の状態で，リムド鋼と同等の特性を有する肌焼用鋼板を得ることを目的としたものであり，この目的が達成されるべく本発明は鋼中成分と組成を総合的にバランスさせ且つ相互に満足するように定めている。以下に，各成分ごとにその組成を規定する理由を個別に説明する。なお，各成分の含有量は重量％として表示する。
【００１４】
通常の焼入れ性は母材Ｃ％に支配されるが，浸炭−焼入れ性は低・中炭素鋼ではほとんど影響されない。Ｃ量が０．０１％より少ない鋼を得る場合には，真空脱炭等の精錬を要するため，溶鋼温度が低下する。他方，Ｍｇワイヤ投入時に所要の溶鋼温度を得るには出鋼温度を高くしなければならず，この場合には耐火物が短命になる。このような理由からＣ量の下限を０．０１％とする。一方，Ｃ量が０．３％を越えると加工性が劣化するので，各種の部品に加工される浸炭用鋼としては不向きとなるので，その上限を０．３％とする。
【００１５】
Ｓｉは浸炭−焼入れ性には影響のない元素であるが，Ｓｉ量が増加するとＢ系介在物が急激に増え，このために浸炭処理前の原板の機械的特性が不利となる。このため本発明鋼のＳｉ量の上限を０．４０％とする。
【００１６】
Ｍｎ量が低いと十分な浸炭深さが得られないので，Ｍｎ量の下限を０．１％とし，他方，加工性の観点からＭｎは１．２％を上限とした。
【００１７】
ＰとＳはいずれも鋼のじん性を低下させる。このため，Ｐの上限を０．０２５％，Ｓの上限を０．０３０％とした。
【００１８】
Ｍｇは本発明鋼では特徴的な元素である。前記目的のために，転炉溶製鋼の脱酸を金属Ｍｇを用いて行うために，ＭｇＯが脱酸生成物として生成する。本発明鋼のＭｇ含有量は，このＭｇＯ中のＭｇと，鋼中に固溶しているＭｇとの総和として検出される量を言うが，後者の固溶Ｍｇ濃度が高い場合，相対的にＭｇＯ濃度が高くなっている。この場合，ＭｇＯは鋼中に均一分散していることが多く，このＭｇＯはＡｌＮと同様の作用，すなわちピンニング効果によりγ粒を微細化し，浸炭−焼入れ性を低下させる。しかし，金属Ｍｇによる脱酸を行う関係上，Ｍｇの微量の残存は不可避である。したがって，本発明鋼は不可避にＭｇを含有するが，その上限を０．００５％とする。
【００１９】
トータル酸素（Ｔ．Ｏ）量は，前記のように酸化物を形成している酸素も含む量であるが，Ｍｇ量が上限の０．００５％であるとすると，そのすべてのＭｇがＭｇＯの形で存在しているものと仮定した場合，ＭｇＯ中の酸素は３３ｐｐｍとなる。他方，ＭｇＯ以外の酸化物例えばＡｌ₂Ｏ₃ などが存在すると鋼の清浄性が悪くなり，このような他の酸化物は可及的に少ない方が望ましい。したがってＭｇＯ以外の酸化物による清浄性の劣化を考慮し，トータル酸素量の上限を５０ｐｐｍとする。
【００２０】
ＡｌとＮはγ粒の成長を阻止し，過剰に存在すると浸炭−焼入れ性を劣化させる。したがって，本発明に従って金属Ｍｇによる脱酸を行う場合でも，ＡｌとＮのレベルは低いほうが好ましい。このため，Ａｌは０．００５％以下，Ｎは８０ｐｐｍ以下とする。
【００２１】
本発明鋼の製造にあたっては，造塊・分塊で製造したリムド鋼から肌焼鋼を得るのとは異なり，転炉溶製鋼を脱酸処理して連続鋳造し，得られた連鋳スラブを熱間圧延する。しかし，通常の連鋳操業のようにＡｌ脱酸を行うとＡｌＮの析出の問題が付随する。この問題は，金属Ｍｇをワイヤで供給する脱酸法を採用すると解決できることがわかった。すなわち，転炉溶製条件，連続鋳造条件および熱間圧延条件は従来のＡｌ脱酸鋼の場合と実質的に同様の条件で行ない，脱酸を金属Ｍｇで行うことに本発明法の特徴がある。したがって，以下に，この金属Ｍｇによる脱酸処理について具体的に説明する。
【００２２】
本発明法の脱酸処理では金属Ｍｇ含有ワイヤを使用する。このワイヤは，金属ＭｇとＣａＯ系フラックスの混合物を鉄シース材で被覆したものである。ここで言う金属Ｍｇは，いわゆるＭｇ金属そのものを言う。合金化したＭｇよりも，金属Ｍｇの方が反応性に富むので脱酸処理時間が短縮し，また合金化のための処理が不要であるから廉価でもある。したがって，本発明では金属Ｍｇを脱酸剤として使用するが，これをＣａＯ系フラックスと共に鉄シース材で被覆しワイヤ状としたものを，ワイヤの軸方向に沿って連続的に溶鋼中に供給する。
【００２３】
鉄シース内に金属Ｍｇと共にＣａＯを共存させると，金属Ｍｇを溶鋼中に安定して供給させることができ，且つ脱酸反応の効率を高めるとともに脱酸生成物であるＭｇＯを溶鋼中で結合・凝集させる作用を供し，その浮上分離を促進させることができる。
【００２４】
脱酸処理にあたっては，ワイヤ中に充填される金属Ｍｇの配合率を５〜３０％mass％とし，Ｍｇ純分の供給速度を溶鋼トンあたり０．０２〜０．２０kg/minとなるように設定することが望ましいことがわかった。ワイヤ中の金属Ｍｇが３０mass％を越えると残部のフラックス量が相対的に不足し，脱酸反応により生成したＭｇＯを溶鋼中で直ちに結合・凝集させる効果が充分でなく，５％未満では，充分な脱酸速度が得られないうえ，フラックス量が相対的に多くなりすぎ，スラグ量の無用な増大をまねくようになる。
【００２５】
図１は，本発明に従う脱酸処理を図解的に示したもので，前記の金属Ｍｇ含有ワイヤ１を，そのコイル４からワイヤフィーダ３によりガイドパイプ２を介して取鍋８内の溶鋼９に浴面上からワイヤ軸方向に連続的に供給する。このようにしてワイヤ１を連続供給する場合，そのワイヤ投入速度をＭｇ純分の供給速度に換算して，溶鋼トンあたり０．０２〜０．２kg/minにコントロールすると，最大の効果が得られることがわかった。
【００２６】
マグネシウムは非常に反応性が大きいので，或る供給速度以上では安全面から精錬プロセスに使用するのが困難であるが，マグネシウム純分の供給速度に換算して溶鋼１ｔあたり０．２kg/min以下にすることにより過度の発煙，攪拌，スプラッシュの発生を防止でき，安定して安全な操業が可能であることがわかった。しかし０．０２kg/min未満では，Ｍｇの供給不足となり，本発明の効果が薄れ，長期処理となり，温度低下が大きくなる。
【００２７】
金属Ｍｇの反応効率を考慮すると，ワイヤ１を取鍋８の炉底近傍で溶解するような速度で送り込むことが望ましい。このワイヤ１の送り込み速度はワイヤ１の溶解速度と浴深から求められる。上述したＭｇ純分の供給速度とこのワイヤ１の送り込み速度から，ワイヤ１中のＭｇ配合率が前記範囲内となるようにワイヤ１の太さや本数等（例えばワイヤ中の脱酸剤単重量ｇ／ｍ）を適正に決定すればよい。
【００２８】
溶鋼中にＭｇが投入されると，溶鋼中から直ちにＭｇガス７が発生するのでバブリングの役割も果たす。Ｍｇ供給速度が小さい場合はバブリング能力も小さくなる。この場合には，インジェクションランス５より，Ｎ₂ガスもしくはＡｒガス６を吹込むことが有効となる。ワイヤ１の投入中にバブリングを行えばＭｇガス７の攪拌が促進され，反応速度が向上する。また，ワイヤ１の投入処理が終える直前またはあとでＮ₂もしくはＡｒガス６を導入すれば，鋼中の脱酸生成物や懸濁物の浮上・分離が促進される。
【００２９】
【実施例】
〔実施例１〕
転炉溶製鋼９０トンを取鍋に受鋼し，取鍋で金属Ｍｇによる脱酸処理を行ったあと通常の方法で連続鋳造し，熱延仕上温度８４０℃，巻取温度６８０℃で熱間圧延して，表２に示す化学成分値の熱延鋼板（板厚６ｍｍ）を製造した。
【００３０】
取鍋で行った金属Ｍｇによる脱酸処理条件を表１に示した。使用した金属Ｍｇ含有ワイヤは，マグネシウム３０mass％と石灰７０mass％の混合物（表１でコア剤と表示）を鉄シース内に充填したものである。該ワイヤの初期供給本数は１本であり，ワイヤの供給速度については，マグネシウム純分の投入速度が溶鋼１ｔあたり０.０８kg/minとなるように決定した。この場合，ワイヤの供給速度は３００ｍ/minである。
【００３１】
得られた熱延鋼板（原板）の引張試験値と浸炭−焼入れ特性を表３に示した。浸炭処理は，カーボンポテンシャル０．９％のガス浸炭を９００℃×９０分の条件で行い，１６０℃の油中に焼入れた。得られた浸炭材の表面硬さＨｖと，Ｈｖ５５０以上の有効浸炭深さ（ＥＣＤ）を測定し，また金属間摩耗試験（大越式回転摩耗試験機）により回転金属に試験片を接触させて摩耗量（ｍｍ²/ｍ−ｋｇ×１０⁴）を測定した。この摩耗試験は，摩耗距離６００ｍ，最終荷重２.１ｋｇ，摩耗速度０.９４ｍ／ｓｅｃ，対金属ＳＵＪ２で行った。また，オーステナイト結晶粒を測定し，その結晶粒番号（ＡＧＳ）を表３に併記した。
【００３１】
〔実施例２〕
表１の脱酸処理条件で表２に示す化学成分値の熱延鋼板を実施例１と同様にして製造し，得られた熱延鋼板の原板の引張試験値，浸炭−焼入れ特性およびＡＧＳを実施例１と同様に測定し，その結果を表３に示した。本例では，脱酸処理時にワイヤを供給すると同時にＡｒガスをインジエクションランスから２Ｎｍ³/ｍｉｎの流量でバブリングさせた。
【００３２】
〔実施例３〕
表１の脱酸処理条件で表２に示す化学成分値の熱延鋼板を実施例１と同様にして製造し，得られた熱延鋼板の原板の引張試験値Ｐ浸炭−焼入れ特性およびＡＧＳを実施例１と同様に測定し，その結果を表３に示した。本例では，脱酸処理時にワイヤを供給し終えたあとにＡｒガスをインジエクションランスから２Ｎｍ³/ｍｉｎの流量で２分間バブリングさせた。
【００３３】
〔比較例１〕
造塊・分塊工程を経て熱間圧延することにより，表２に示す化学成分値のリムド鋼の熱延鋼板を製造し，該原板の引張試験値，浸炭−焼入れ特性およびＡＧＳを実施例１と同様に測定し，その結果を表３に示した。
【００３４】
〔比較例２〕
表１の脱酸処理条件で表２に示す化学成分値の熱延鋼板を実施例１と同様にして製造し，得られた熱延鋼板の原板の引張試験値，浸炭−焼入れ特性およびＡＧＳを実施例１と同様に測定し，その結果を表３に示した。本例では，ワイヤ内に充填したコア剤は金属Ｍｇが３mass％，ＣａＯが９７mass％であり，マグネシウム純分の供給速度は０.００８kg/t.minである。なお，脱酸処理時に２Ｎｍ³/ｍｉｎのＡｒガスをバブリングさせた。
【００３５】
【表１】

【００３６】
【表２】

【００３７】
【表３】

【００３８】
表３の結果から，本発明実施例のものは，比較例１のリムド鋼のものと同等の浸炭−焼入れ特性を有することがわかる。またトータル酸素量の多い比較例２のものは浸炭−焼入れ特性が劣っていることがわかる。
【００３９】
【発明の効果】
本発明によれば，造塊・分塊工程を実施することなく，リムド鋼とほぼ同等の特性を有する浸炭焼入れ材を連鋳工程を経て製造することができる。
【図面の簡単な説明】
【図１】本発明に従って溶鋼を脱酸処理する状態を図解した図である。
【符号の説明】
１ワイヤ
２ガイドパイプ
３ワイヤフイーダ
４ワイヤコイル
５インジェクションランス
６ＡｒもしくはＮ₂ガス
７Ｍｇガス
８取鍋
９溶鋼[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a so-called case-hardening steel plate (carburizing steel plate) excellent in carburizing characteristics and quenching characteristics, and more particularly to a method for manufacturing a case-hardening steel sheet having substantially the same characteristics as rimmed steel through a continuous casting process.
[0002]
[Prior art]
Some hot-rolled steel sheets manufactured for automotive and industrial machine parts must be given surface hardness and wear resistance by carburizing and quenching (skin hardening) after forming. May be. Conventionally, carbon steel used for such purposes has been dealt with using rimmed steel or low nitrogen Al weakly deoxidized steel.
[0003]
The rimmed steel has excellent carburization and hardenability because there are few impurities in the surface layer and γ grains are large, and inclusions are dispersed in the inner layer and the γ grains become fine. However, the rimmed steel produced through the ingot-making and smashing process is not suitable for today's situation where improvement of productivity, particularly improvement of the ratio is required, and is gradually being replaced by Al killed steel.
[0004]
However, ordinary Al-killed steel contains 0.02% or more of Al, and γ grains are generally refined by the pinning effect due to the precipitation of AlN, which reduces hardenability and provides a sufficient carburization depth. I can't. In other words, the required surface hardness and wear resistance cannot be obtained as long as heat treatment similar to that of rimmed steel is employed.
[0005]
Therefore, for example, as described in JP-A-57-145934, Al: 0.001 to 0.005%, N: 0.004% or less, and the finishing temperature is Ar ₁ point or more and Ar A method has been proposed in which the winding temperature is set to ₃ points or less, the winding temperature is set to 650 ° C. or more, and high temperature winding is performed after low temperature finish rolling in the α + γ two-phase region. This method reduces the amount of AlN and the amount of AlN by reducing the amount of Al and N, and prevents the refinement of α grains. On the other hand, the two-phase rolling is performed to produce and coarsen the α grains. The removal promotes uniform growth of α grains.
[0006]
In Japanese Patent Publication No. 5-53845, Al is 0.01 to 0.04%, N is 0.006% or less as usual, and a hot slab obtained by continuous casting is 750 ° C. or higher Ar _3. The temperature is lowered below the point to actively coarsen AlN, heated again to 1000 to 1150 ° C, and the precipitated AlN is coarsened, and then finish rolling is performed at the Ar ₃ point or higher, and wound at 250 to 480 ° C. There has been proposed a method for making AlN coarse and detoxifying.
[0007]
[Problems to be solved by the invention]
As described above, when the case-hardened steel is obtained from the rimmed steel, it is manufactured through the ingot-making / bundling process, so that it is not suitable for the situation where improvement in productivity is required. Therefore, it is replaced by Al killed steel that can be manufactured by continuous casting. However, as described above, AlN exists in normal Al killed steel, and a sufficient hardening depth cannot be obtained. For this reason, in order to reduce the absolute amount of AlN, a method of reducing the Al amount and the N amount as much as possible has been adopted, but the Al amount of Al deoxidized steel is reduced to 0.005% or less by current means. It is difficult to control and requires special treatment in the secondary smelting furnace, which inevitably increases production costs.
[0008]
The method of coarsening AlN as described in Japanese Patent Publication No. 5-53845 is also a production under special conditions, and there are problems in production control with other steel types, and as long as Al and N exist, carburizing characteristics. There is no guarantee that the quenching properties will be sufficiently improved.
[0009]
For this reason, a method of compensating the hardenability by adding a hardenability improving element such as Cr or B can be considered, but this is not preferable because the alloy cost increases.
[0010]
Therefore, the present invention establishes a technique for producing a case-hardened steel sheet having characteristics equivalent to those of rimmed steel through a normal continuous casting process, with the addition of Al as much as possible and without the addition of a hardenability improving element. It is a problem.
[0011]
[Means for Solving the Problems]
According to the present invention, in weight percent, C: 0.01 to 0.30%, Si: 0.40% or less, Mn: 0.1 to 1.2%, P: 0.025% or less, S: 0.030% or less, Mg: 0.005% or less, T: O: 50 ppm or less, Al: 0.005% or less, N: 80 ppm or less, the balance being Fe and inevitable impurities steel plate, deoxidation The steel plate for carburizing manufactured by performing with metal Mg is provided. Here, T.O is the total amount of oxygen, and includes the amount of oxygen in oxides that react with Mg and Al to form oxides in addition to those dissolved in steel.
[0012]
The carburized steel sheet according to the present invention is obtained by deoxidizing the converter molten steel and continuously casting it. When the obtained continuous cast slab is hot-rolled into a hot-rolled steel sheet, It can manufacture by performing metal Mg as a deoxidizer. At that time, as the deoxidation treatment, a metal Mg-containing wire in which a deoxidizer composed of 5-30 mass% of metal Mg and the balance of CaO-based flux is enclosed in an iron sheath is used for the pure Mg content relative to the molten steel in the ladle. Supply is performed so that the supply rate is 0.02 to 0.20 kg / min per ton of molten steel. Preferably, nitrogen gas or Ar gas is bubbled into the molten steel simultaneously with and / or after the supply of this wire.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
As described above, the present invention aims to obtain a case-hardening steel sheet having the same characteristics as rimmed steel in a state where the addition of Al is kept to a minimum and no hardenability improving element is added. In order to achieve this object, the present invention is defined so that the composition and composition of steel are balanced in total and satisfy each other. The reason why the composition is specified for each component will be individually described below. The content of each component is expressed as% by weight.
[0014]
Normal hardenability is governed by the base metal C%, but carburizing and hardenability is hardly affected by low and medium carbon steels. When obtaining a steel having a C content of less than 0.01%, refining such as vacuum decarburization is required, so the molten steel temperature decreases. On the other hand, in order to obtain the required molten steel temperature when the Mg wire is charged, the steel output temperature must be increased, and in this case, the refractory becomes short-lived. For this reason, the lower limit of the C amount is set to 0.01%. On the other hand, if the C content exceeds 0.3%, the workability deteriorates, so it is not suitable as a carburizing steel processed into various parts, so the upper limit is made 0.3%.
[0015]
Si is an element that does not affect carburizing and hardenability. However, when the amount of Si increases, B-based inclusions increase rapidly, which disadvantageously impairs the mechanical properties of the original plate before carburizing. For this reason, the upper limit of the Si content of the steel of the present invention is set to 0.40%.
[0016]
If the Mn amount is low, a sufficient carburization depth cannot be obtained. Therefore, the lower limit of the Mn amount is set to 0.1%, while Mn is set to an upper limit of 1.2% from the viewpoint of workability.
[0017]
Both P and S reduce the toughness of the steel. Therefore, the upper limit of P is 0.025%, and the upper limit of S is 0.030%.
[0018]
Mg is a characteristic element in the steel of the present invention. For this purpose, MgO is produced as a deoxidized product in order to perform deoxidation of converter molten steel using metallic Mg. The Mg content of the steel of the present invention refers to the amount detected as the sum of Mg in MgO and Mg dissolved in the steel. When the latter solid solution Mg concentration is high, The MgO concentration is high. In this case, MgO is often uniformly dispersed in the steel, and this MgO refines the γ grains by the same action as that of AlN, that is, the pinning effect, and lowers the carburizing and quenching properties. However, a small amount of Mg is unavoidable due to deoxidation with metallic Mg. Accordingly, the steel of the present invention inevitably contains Mg, but its upper limit is made 0.005%.
[0019]
The total oxygen (T.O) amount includes the oxygen that forms the oxide as described above, but if the Mg amount is 0.005% of the upper limit, all the Mg is MgO. Assuming that it exists in a form, the oxygen in MgO is 33 ppm. On the other hand, the presence of oxides other than MgO, such as Al ₂ O ₃ , deteriorates the cleanliness of the steel, and it is desirable that such other oxides be as few as possible. Therefore, the upper limit of the total oxygen amount is set to 50 ppm in consideration of deterioration of cleanliness due to oxides other than MgO.
[0020]
Al and N prevent the growth of γ grains, and if they are present in excess, the carburizing and quenching properties are deteriorated. Therefore, even when deoxidizing with metal Mg according to the present invention, it is preferable that the levels of Al and N are low. For this reason, Al is 0.005% or less, and N is 80 ppm or less.
[0021]
In the production of the steel according to the present invention, unlike the case-hardened steel produced from the rimmed steel produced by ingot-making / splitting, the converter molten steel is deoxidized and continuously cast. Hot rolled. However, when Al deoxidation is carried out as in a normal continuous casting operation, the problem of precipitation of AlN is accompanied. It was found that this problem can be solved by adopting a deoxidation method in which metallic Mg is supplied by wire. That is, converter melting conditions, continuous casting conditions, and hot rolling conditions are substantially the same as in the case of conventional Al deoxidized steel, and deoxidation is performed with metallic Mg. is there. Therefore, the deoxidation treatment with the metal Mg will be specifically described below.
[0022]
In the deoxidation treatment according to the present invention, a metal Mg-containing wire is used. This wire is obtained by coating a mixture of metal Mg and CaO-based flux with an iron sheath material. The metal Mg referred to here is a so-called Mg metal itself. Metal Mg is more reactive than alloyed Mg, so the deoxidation time is shortened, and it is inexpensive because no treatment for alloying is required. Therefore, in the present invention, metal Mg is used as a deoxidizer, and this is coated with an iron sheath material together with a CaO-based flux to form a wire shape, and is continuously fed into the molten steel along the axial direction of the wire. .
[0023]
Coexistence of CaO together with metal Mg in the iron sheath enables stable supply of metal Mg into the molten steel, and increases the efficiency of the deoxidation reaction and binds the deoxidation product MgO in the molten steel. It can act to agglomerate and promote its floating separation.
[0024]
In the deoxidation treatment, the metal Mg filling ratio is set to 5 to 30% mass%, and the supply rate of pure Mg is set to 0.02 to 0.20 kg / min per ton of molten steel. I found it desirable to do. When the metal Mg in the wire exceeds 30 mass%, the remaining flux is relatively insufficient, and the effect of immediately bonding and agglomerating MgO produced by the deoxidation reaction in the molten steel is insufficient. In addition, a high deoxidation rate cannot be obtained, and the amount of flux becomes relatively large, leading to an unnecessary increase in the amount of slag.
[0025]
FIG. 1 schematically shows a deoxidation treatment according to the present invention. The metal Mg-containing wire 1 is transferred from the coil 4 to the molten steel 9 in the ladle 8 via the guide pipe 2 by the wire feeder 3. It is continuously supplied in the wire axial direction from above the bath surface. When the wire 1 is continuously supplied in this way, the maximum effect can be obtained by converting the wire charging rate into a pure Mg supply rate and controlling it to 0.02 to 0.2 kg / min per ton of molten steel. I understood it.
[0026]
Magnesium is very reactive, so it is difficult to use it in the refining process from a safety point of view at a certain supply rate or more, but it is 0.2kg / min or less per ton of molten steel in terms of the supply rate of pure magnesium. Therefore, it was found that excessive smoke generation, stirring, and splashing can be prevented, and stable and safe operation is possible. However, if it is less than 0.02 kg / min, the supply of Mg becomes insufficient, the effect of the present invention is diminished, long-term treatment is performed, and the temperature drop is increased.
[0027]
In consideration of the reaction efficiency of Mg metal, it is desirable to feed the wire 1 at a speed that melts in the vicinity of the bottom of the ladle 8. The feeding speed of the wire 1 is obtained from the dissolution speed of the wire 1 and the bath depth. Based on the supply rate of the pure Mg and the feed rate of the wire 1, the thickness and number of the wires 1 and the like (for example, the deoxidizer single weight g in the wire) / M) may be determined appropriately.
[0028]
When Mg is introduced into the molten steel, Mg gas 7 is immediately generated from the molten steel, so that it also plays a role of bubbling. When the Mg supply rate is low, the bubbling ability is also small. In this case, it is effective to blow N ₂ gas or Ar gas 6 from the injection lance 5. If bubbling is performed while the wire 1 is being charged, stirring of the Mg gas 7 is promoted and the reaction rate is improved. Further, if N ₂ or Ar gas 6 is introduced immediately before or after the wire 1 is charged, the deoxidation products and suspensions in the steel are promoted to float and separate.
[0029]
【Example】
[Example 1]
90 tons of converter molten steel is received in a ladle, deoxidized with metal Mg in the ladle, then continuously cast in the usual way, hot rolled at a finish temperature of 840 ° C and at a coiling temperature of 680 ° C. It rolled and manufactured the hot-rolled steel plate (sheet thickness 6mm) of the chemical component value shown in Table 2.
[0030]
Table 1 shows the deoxidation treatment conditions with metal Mg performed in a ladle. The metal Mg-containing wire used is one in which an iron sheath is filled with a mixture of 30 mass% magnesium and 70 mass% lime (indicated as a core agent in Table 1). The initial supply number of the wires was one, and the supply rate of the wires was determined so that the input rate of pure magnesium was 0.08 kg / min per ton of molten steel. In this case, the wire supply speed is 300 m / min.
[0031]
Table 3 shows the tensile test values and carburization-quenching characteristics of the obtained hot-rolled steel sheet (original sheet). The carburizing treatment was performed by gas carburizing with a carbon potential of 0.9% under the conditions of 900 ° C. × 90 minutes and quenching in 160 ° C. oil. The surface hardness Hv of the obtained carburized material and the effective carburized depth (ECD) of Hv550 or more are measured, and the test piece is brought into contact with the rotating metal by the intermetal wear test (Ogoshi type rotary wear tester). The amount (mm ² / m-kg × 10 ⁴ ) was measured. This wear test was carried out with a wear distance of 600 m, a final load of 2.1 kg, a wear rate of 0.94 m / sec, and a metal SUJ2. Further, austenite crystal grains were measured, and the crystal grain numbers (AGS) were also shown in Table 3.
[0031]
[Example 2]
A hot-rolled steel sheet having the chemical composition values shown in Table 2 under the deoxidation treatment conditions shown in Table 1 was produced in the same manner as in Example 1. Measurements were made in the same manner as in Example 1, and the results are shown in Table 3. In this example, the wire was supplied during deoxidation, and at the same time, Ar gas was bubbled from the injection lance at a flow rate of 2 Nm ³ / min.
[0032]
Example 3
A hot-rolled steel sheet having the chemical composition values shown in Table 2 under the deoxidation conditions shown in Table 1 was produced in the same manner as in Example 1, and the tensile test value P carburization-quenching characteristics and AGS of the obtained hot-rolled steel sheet were determined. Measurements were made in the same manner as in Example 1, and the results are shown in Table 3. In this example, Ar gas was bubbled from the injection lance at a flow rate of 2 Nm ³ / min for 2 minutes after supplying the wire during the deoxidation treatment.
[0033]
[Comparative Example 1]
A hot rolled steel sheet of rimmed steel having the chemical composition values shown in Table 2 was manufactured by hot rolling through the ingot-making and ingot-making processes, and the tensile test value, carburization-quenching characteristics and AGS of the original sheet were measured in Example 1. The results are shown in Table 3.
[0034]
[Comparative Example 2]
A hot-rolled steel sheet having the chemical composition values shown in Table 2 under the deoxidation conditions shown in Table 1 was produced in the same manner as in Example 1, and the tensile test value, carburization-quenching characteristics, and AGS of the obtained hot-rolled steel sheet were determined. Measurements were made in the same manner as in Example 1, and the results are shown in Table 3. In this example, the core agent filled in the wire is 3 mass% of metal Mg and 97 mass% of CaO, and the supply rate of pure magnesium is 0.008 kg / t.min. In addition, 2Nm ³ / min Ar gas was bubbled during the deoxidation treatment.
[0035]
[Table 1]

[0036]
[Table 2]

[0037]
[Table 3]

[0038]
From the results of Table 3, it can be seen that the examples of the present invention have carburizing and quenching characteristics equivalent to those of the rimmed steel of Comparative Example 1. Moreover, it turns out that the thing of the comparative example 2 with much total oxygen amount is inferior in the carburizing-quenching characteristic.
[0039]
【The invention's effect】
According to the present invention, a carburized and quenched material having substantially the same characteristics as rimmed steel can be manufactured through a continuous casting process without performing the ingot forming / bundling process.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a state where molten steel is deoxidized according to the present invention.
[Explanation of symbols]
1 Wire 2 Guide Pipe 3 Wire Feeder 4 Wire Coil 5 Injection Lance 6 Ar or N ₂ Gas 7 Mg Gas 8 Ladle 9 Molten Steel

Claims

In weight%, C: 0.01 to 0.30%, Si: 0.40% or less, Mn: 0.1 to 1.2%, P: 0.025% or less, S: 0.030% or less, Mg: 0.005% or less, T.O: 50 ppm or less, Al: 0.005% or less, N: 80 ppm or less, the balance being Fe and unavoidable impurities , and deoxidation with metallic Mg Carburized steel plate manufactured by

In weight%, C: 0.01 to 0.30%, Si: 0.40% or less, Mn: 0.1 to 1.2%, P: 0.025% or less, S: 0.030% or less, Mg: 0.005% or less, T: O: 50 ppm or less, Al: 0.005% or less, N: 80 ppm or less, the balance being Fe and unavoidable impurities, the balance being 5-30 mass % of metal Mg metallic Mg containing wire but encapsulating deoxidizer consisting CaO-based flux in the iron sheath for BOF smelting steel in the ladle, feed rate per ton of the molten steel of Mg purity from 0.02 to 0. A carburized steel plate manufactured through a process of continuous casting by deoxidation treatment by a method of supplying at 20 kg / min and hot rolling the obtained continuous cast slab .

In weight%, C: 0.01 to 0.30%, Si: 0.40% or less, Mn: 0.1 to 1.2%, P: 0.025% or less, S: 0.030% or less, Mg: 0.005% or less, T: O: 50 ppm or less, Al: 0.005% or less, N: 80 ppm or less, the balance being Fe and unavoidable impurities, the balance being 5-30 mass % of metal Mg metallic Mg containing wire but encapsulating deoxidizer consisting CaO-based flux in the iron sheath for BOF smelting steel in the ladle, feed rate per ton of the molten steel of Mg purity from 0.02 to 0. It was supplied so that 20 kg / min, continuous after simultaneously and / or supply and the supply of the wire, the continuous casting processes deoxidation method bubbling nitrogen gas or Ar gas into molten steel, resulting A carburized steel plate manufactured through a process of hot rolling a cast slab .