JP4259145B2

JP4259145B2 - Abrasion resistant steel plate with excellent low temperature toughness and method for producing the same

Info

Publication number: JP4259145B2
Application number: JP2003065177A
Authority: JP
Inventors: 康宏室田; 照輝貞末; 和秀高橋
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2003-03-11
Filing date: 2003-03-11
Publication date: 2009-04-30
Anticipated expiration: 2023-03-11
Also published as: JP2004270001A

Description

【０００１】
【発明の属する技術分野】
本発明は、産業機械、運搬機器等に用いられる耐磨耗鋼板およびその製造方法に関する。
【０００２】
【従来の技術】
建設現場、土木工事現場、鉱山等で使用される例えば、パワーショベル、ブルドーザー、ホッパー、バケット等の産業機械、運搬機器等およびこれらの部品には、それらの寿命を確保するために耐磨耗性に優れた鋼が用いられる。鋼の耐磨耗性を向上させるためには、鋼の表面を焼入れ組織にして表面硬度を高くする必要がある。
【０００３】
一般に、鋼中のＣ含有量を増加させることで鋼の焼入れ硬さは確保することができるが、一方で硬度が増すと材質が脆くなって低温靭性が劣化するという問題が生ずる。マイナス２０℃以下の低温域での作業に用いることを考えると、鋼は耐磨耗性が良くても低温靭性が低ければ、脆性破壊（遅れ破壊）を生じて作業に重大な支障を与える。このため、耐磨耗性を有するとともに低温靭性にも優れる耐磨耗鋼が望まれてきた。
【０００４】
このような要求に対して、いくつかの方法が検討されてきている。例えば、特許文献１、特許文献２、または特許文献３では、ＣｒやＭｏなどの合金元素を多量に添加することで耐磨耗鋼板の靭性を向上させる技術が開示されている。これらの技術においてＣｒは焼入れ性を向上させる目的で、またＭｏは焼入れ性を向上させると同時に粒界強度を改善する目的で添加されている。
【０００５】
また、特許文献４では耐磨耗鋼板の製造プロセスを改良した技術として、熱間圧延工程でオースフォームを利用し、旧オーステナイト（γ）粒を展伸させて鋼板の靭性を改善する技術が開示されている。
【０００６】
【特許文献１】
特開平８−４１５３５号公報
【０００７】
【特許文献２】
特開平２−１７９８４２号公報
【０００８】
【特許文献３】
特開昭６１−１６６９５４号公報
【０００９】
【特許文献４】
特開２００２−２０８３７号公報
【００１０】
【発明が解決しようとする課題】
しかしながら特許文献１、特許文献２、または特許文献３のように、鋼に合金元素を多量に添加することにより粒界強度を強化して靭性を向上させる場合、合金元素添加コストが大きくなるという問題がある。
【００１１】
また、特許文献４のように熱間圧延工程でオースフォームを利用する場合、鋼を安定に製造するためには別途工夫が必要であり、実製造上必ずしも容易なプロセスとは言えない。
【００１２】
このように従来技術では、安価に製造することができ、さらに作り込みが容易で、良好な遅れ破壊特性を有し低温靭性に優れた耐磨耗鋼を提供することは困難である。
【００１３】
本発明は、上記の課題を解決するためになされたものであり、表面硬度を低下させることなく耐磨耗性を安定的に有し、さらに低温靭性にも優れた耐磨耗鋼板およびその製造方法を提供することを目的とする。
【００１４】
【課題を解決するための手段】
本発明者らは、低温靭性に優れた耐磨耗鋼板を提供すべく鋭意検討を重ねた結果、次のような知見を得た。すなわち、成分指標値Ｈａを所定の値に調整し、鋼片を１２００〜１２５０℃の温度範囲に加熱することが、再加熱焼入れ後の鋼板の耐磨耗性を確保し、さらに靭性を改善する上で有効である。本発明はこの知見に基づいてなされたものである。
【００１５】
本発明の低温靭性に優れた耐磨耗鋼板は、質量％で、Ｃ：０．２３〜０．３５％、Ｓｉ：０．０５〜１．０％（ただし、０．５０％超を除く）、Ｍｎ：０．１〜２．０％、Ｐ：０．０２０％以下、Ｓ：０．００５％以下、Ｎｂ：０．００５〜０．０３％、Ｔｉ：０．００５〜０．１％、Ｂ：０．０００３〜０．００２％を含有し、さらに質量％で、Ｃｕ：０．０３〜２．０％、Ｎｉ：０．０３〜２．０％、Ｃｒ：０．０３〜２．０％、Ｍｏ：０．０３〜１．０％、Ｖ：０．００５〜０．１％からなる群より選択される１種または２種以上を含有し、式（１）で規定される成分指標値Ｈａが２．５以上であり、前記式（１）中のＣ，Ｍｎ，Ｃｕ，Ｎｉ，Ｃｒ，Ｍｏ，Ｖ，Ｂは鋼中に含まれる各元素の質量％での含有量であり、Ｃｕが含まれない場合にはＣｕ＝０、Ｎｉが含まれない場合にはＮｉ＝０、Ｃｒが含まれない場合にはＣｒ＝０、Ｍｏが含まれない場合にはＭｏ＝０、Ｖが含まれない場合にはＶ＝０とし、残部がＦｅおよび不可避不純物からなり、粒径１５μｍ以下の焼入れままのマルテンサイトを９０％以上含有することを特徴とする。
Ｈａ＝Ｃ×（１＋３×Ｍｎ）×（１＋０．５×Ｃｕ）×（１＋２×Ｎｉ）
×（１＋３×Ｃｒ）×（１＋２×Ｍｏ）×（１＋Ｖ）
×（１＋３００×Ｂ） …（１）
本発明の低温靭性に優れた耐磨耗鋼板の製造方法は、質量％で、Ｃ：０．２３〜０．３５％、Ｓｉ：０．０５〜１．０％（ただし、０．５０％超を除く）、Ｍｎ：０．１〜２．０％、Ｐ：０．０２０％以下、Ｓ：０．００５％以下、Ｎｂ：０．００５〜０．０３％、Ｔｉ：０．００５〜０．１％、Ｂ：０．０００３〜０．００２％を含有し、さらに質量％で、Ｃｕ：０．０３〜２．０％、Ｎｉ：０．０３〜２．０％、Ｃｒ：０．０３〜２．０％、Ｍｏ：０．０３〜１．０％、Ｖ：０．００５〜０．１％からなる群より選択される１種または２種以上を含有し、式（１）で規定される成分指標値Ｈａが２．５以上であり、前記式（１）中のＣ，Ｍｎ，Ｃｕ，Ｎｉ，Ｃｒ，Ｍｏ，Ｖ，Ｂは鋼中に含まれる各元素の質量％での含有量であり、Ｃｕが含まれない場合にはＣｕ＝０、Ｎｉが含まれない場合にはＮｉ＝０、Ｃｒが含まれない場合にはＣｒ＝０、Ｍｏが含まれない場合にはＭｏ＝０、Ｖが含まれない場合にはＶ＝０とし、残部がＦｅおよび不可避不純物からなる鋼片を、１２００℃〜１２５０℃の温度範囲に加熱し、板厚が５ｍｍ〜５０ｍｍの範囲になるまで熱間圧延を行い、その後８５０℃〜９５０℃の温度範囲に再加熱し、焼入れ、粒径１５μｍ以下の焼入れままのマルテンサイトを９０％以上含有する低温靭性に優れた耐磨耗鋼板を得ることを特徴とする。
【００１６】
【発明の実施の形態】
以下、本発明の化学成分および製造方法の限定理由について述べる。
【００１７】
（１）Ｃ：０．２３〜０．３５質量％
Ｃは鋼の硬度を高め、耐磨耗性を向上させるために重要な元素である。Ｃ含有量が０．２３質量％未満では十分な硬度が得られず、一方、０．３５質量％を超えて添加すると、溶接性、靭性および加工性を劣化させる。このため、Ｃ含有量は０．２３〜０．３５質量％の範囲に規定する。
【００１８】
（２）Ｓｉ：０．０５〜１．０％（ただし、０．５０％超を除く）
Ｓｉは脱酸元素として有効な元素であり、また、固溶強化に対しても有効な元素である。Ｓｉ含有量が０．０５質量％未満では脱酸効果が十分に得られず、一方、１．０質量％（ただし、０．５０％超を除く）を超えて添加すると、延靭性が低下する、介在物が増加するといった問題が生ずる。このため、Ｓｉ含有量は０．０５〜１．０％（ただし、０．５０％超を除く）に規定する。
【００１９】
（３）Ｍｎ：０．１〜２．０質量％
Ｍｎは焼入れ性確保の観点から有効な元素である。Ｍｎ含有量が０．１質量％未満ではその効果が十分に得られず、一方、２．０質量％を超えて添加すると溶接性が劣化する。このため、Ｍｎ含有量は０．１〜２．０質量％の範囲に規定する。
【００２０】
（４）Ｐ：０．０２０質量％以下
Ｐは鋼中に多量に含まれると靭性の劣化を招くが、その含有量が０．０２０質量％以下であれば問題にならない。このため、Ｐ含有量の上限を０．０２０質量％とする。
【００２１】
（５）Ｓ：０．００５質量％以下
Ｓは鋼中に多量に含まれるとＭｎＳとして析出し、これが介在物として高強度鋼の破壊発生起点となり靭性の劣化を招く。しかし、その含有量が０．００５質量％以下であれば問題にならない。このため、Ｓ含有量の上限を０．００５質量％とする。
【００２２】
（６）Ｎｂ：０．００５〜０．０３質量％
Ｎｂは再加熱焼入れ時に析出物として存在し、粒径を微細化する効果を有し、結果的に靭性の向上に役立つ元素である。Ｎｂ含有量が０．００５質量％未満ではこの効果を発揮することができず、一方、０．０３質量％を超えて添加すると溶接性が劣化する。このため、Ｎｂ含有量は０．００５〜０．０３質量％の範囲に規定する。また、微細化効果を有効に得るためには、Ｎｂ含有量を０．０１２〜０．０３質量％の範囲とすることが望ましい。
【００２３】
（７）Ｔｉ：０．００５〜０．１質量％
Ｔｉは靭性に有害な固溶ＮをＴｉＮとして固定することにより靭性を向上させるとともに、焼入れ性の向上に有効な固溶Ｂを確保する効果を有する。Ｔｉ含有量が０．００５質量％未満ではこの効果を発揮することができず、一方、０．１質量％を超えて添加すると靭性が劣化する。このため、Ｔｉ含有量は０．００５〜０．１質量％の範囲に規定する。また、よりコストを低減するためには、Ｔｉ含有量を０．００５〜０．０３質量％の範囲とすることが望ましい。
【００２４】
（８）Ｂ：０．０００３〜０．００２質量％
Ｂは微量添加で焼入れ性を高める元素である。Ｂ含有量が０．０００３質量％未満ではこの効果を発揮することができず、一方、０．００２質量％を超えて添加すると靭性が劣化する。このため、Ｂ含有量は０．０００３〜０．００２質量％の範囲に規定する。また、耐磨耗鋼板に一般的に実施されるＣＯ₂溶接などの低入熱溶接部における低温割れを抑制する観点から、Ｂ含有量を０．０００３〜０．００１５質量％の範囲とすることが望ましい。
【００２５】
本発明では、強度、低温靭性および耐磨耗性等をさらに向上する目的で、以下に示すＣｕ、Ｎｉ、Ｃｒ、ＭｏおよびＶのうちの１種または２種以上を含有する。
【００２６】
（９）Ｃｕ：０．０３〜２．０質量％
Ｃｕは焼入れ性を高める元素である。Ｃｕ含有量が０．０３質量％未満ではこの効果を十分に発揮することができず、一方、２．０質量％を超えて添加すると熱間加工性が低下するとともに、コストも上昇する。このためＣｕを添加する場合には、その含有量を０．０３〜２．０質量％の範囲に規定する。また、よりコストを低減するためにはＣｕ含有量を０．０３〜０．５質量％の範囲とすることが望ましい。
【００２７】
（１０）Ｎｉ：０．０３〜２．０質量％
Ｎｉは焼入れ性を高めるとともに、低温靭性を向上させる元素である。Ｎｉ含有量が、０．０３質量％未満ではこの効果を十分に発揮することができず、一方、２．０質量％を超えて添加するとコストが上昇する。このためＮｉを添加する場合には、その含有量を０．０３〜２．０質量％の範囲に規定する。また、よりコストを低減するためにはＮｉ含有量を０．０３〜０．５質量％の範囲とすることが望ましい。
【００２８】
（１１）Ｃｒ：０．０３〜２．０質量％
Ｃｒは焼入れ性を高める元素である。Ｃｒ含有量が０．０３質量％未満ではこの効果を十分に発揮することができず、一方、２．０質量％を超えて添加すると、溶接性が劣化するとともに、コストが上昇する。このためＣｒを添加する場合には、その含有量を０．０３〜２．０質量％の範囲に規定する。
【００２９】
（１２）Ｍｏ：０．０３〜１．０質量％
Ｍｏは焼入れ性を高める元素である。Ｍｏ含有量が０．０３質量％未満ではこの効果を十分に発揮することができず、一方、１．０質量％を超えて添加すると、溶接性が劣化するとともに、コストが上昇する。このためＭｏを添加する場合には、その含有量を０．０３〜１．０質量％の範囲に規定する。
【００３０】
（１３）Ｖ：０．０５〜０．１質量％
Ｖは析出強化に有効な元素であり、鋼の硬度を上昇させ、耐磨耗性を向上させる効果を有する。Ｖ含有量が、０．０５質量％未満ではこの効果を十分に発揮することができず、一方、０．１質量％を超えて添加すると溶接性が劣化する。このためＶを添加する場合には、Ｖ含有量を０．０５〜０．１質量％の範囲に規定する。
【００３１】
（１４）成分指標値Ｈａ：２．５以上
成分指標値Ｈａは下式（１）で規定され、焼入れ後の組織と関係があり、鋼の硬度、すなわち耐磨耗性に大きな影響を与える。
【００３２】
Ｈａ＝Ｃ×（１＋３×Ｍｎ）×（１＋０．５×Ｃｕ）×（１＋２×Ｎｉ）
×（１＋３×Ｃｒ）×（１＋２×Ｍｏ）×（１＋Ｖ）
×（１＋３００×Ｂ） …（１）
ここで、式（１）中、Ｃ，Ｍｎ，Ｃｕ，Ｎｉ，Ｃｒ，Ｍｏ，Ｖ，Ｂは鋼中に含まれる各元素の質量％での含有量であり、Ｃｕが含まれない場合にはＣｕ＝０、Ｎｉが含まれない場合にはＮｉ＝０、Ｃｒが含まれない場合にはＣｒ＝０、Ｍｏが含まれない場合にはＭｏ＝０、Ｖが含まれない場合にはＶ＝０とする。
【００３３】
成分指標値Ｈａが２．５未満であると、組織が完全な焼入れ組織とならない恐れがあり、また、表面の組織が完全な焼入れ組織となっていても、表層から板厚中心部にかけて完全な焼入れ組織とならず、硬さが低下する。さらに、成分指標値Ｈａが２．５未満の場合には、板厚中央部付近のマルテンサイト分率が低下し、靭性が極端に劣化する。従って、成分指標値Ｈａを２．５以上と規定する。なお、成分指標値Ｈａが１２．０を超えると溶接性が劣化するため、成分指標値Ｈａは１２．０以下とすることが好ましい。
【００３４】
（１５）組織：粒径１５μｍ以下のマルテンサイトが９０％以上
鋼組織において、焼入れままのマルテンサイトの分率が９０％未満であると、十分な靭性が得られない。また、マルテンサイトが９０％以上含まれていたとしても、マルテンサイト粒径が１５μｍを超えて粗大であれば、やはり靭性が劣化する。従って、粒径１５μｍ以下の焼入れままのマルテンサイトを９０％以上含有するものとする。なお、マルテンサイト分率を１００％にすることは実際の製造では困難であり、表２に示すように９８％程度が上限である。
【００３５】
（１６）加熱温度：１２００〜１２５０℃
鋼片（スラブ）の加熱温度は、再加熱時のオーステナイト粒径と相関性を有する。鋼片加熱温度を１２００℃以上とすることにより、再加熱時のオーステナイト粒径、すなわち焼入れ後のマルテンサイト粒径を１５μｍ以下にまで微細化することができ、その結果、靭性を改善することができる。しかし、１２５０℃を超えて鋼片を加熱すると、鋼板の表面に疵が発生する。従って、鋼片加熱温度は１２００〜１２５０℃の範囲とする。
【００３６】
（１７）板厚：５〜５０ｍｍ
耐磨耗鋼板の板厚が５ｍｍ未満では、熱間圧延での製造が困難となり、生産性が低下する。一方、板厚が５０ｍｍを超えると、板厚中央部付近の冷却速度が低下し、完全な焼入れ組織とならずに硬さが低下して耐磨耗性が劣化する。従って、板厚は５〜５０ｍｍの範囲とする。
【００３７】
（１８）再加熱焼入れ温度：８５０〜９５０℃
再加熱焼入れ温度が８５０℃未満では、微細なオーステナイトへの変態が完全には終了せず、そのまま焼入れたとしても不完全な焼入れ組織となり、硬さが低下して耐磨耗性が劣化する。一方、再加熱焼入れ温度が９５０℃を超えると、再加熱時のオーステナイト粒径が粗大化し、その結果、焼入れままのマルテンサイト粒径が粗大化するために靭性が劣化する。従って、再加熱焼入れ温度は８５０〜９５０℃の範囲とする。
【００３８】
【実施例】
＜実施例１〜８、比較例１〜２２＞
種々の化学成分を有する供試鋼片Ａ〜Ｆを用いて鋼板を製造した。用いた供試鋼片の化学成分（質量％）を表１に示す。１００ｍｍの厚さを有する供試鋼片を加熱し、板厚１２ｍｍまたは５０ｍｍまで熱間圧延を行い室温まで放冷した後に、再加熱し焼入れて実施例１〜８および比較例１〜２２の鋼板を得た。このときの製造条件としてスラブ加熱温度（℃）および再加熱焼入れ温度（℃）を表２に示し、また鋼板の板厚（ｍｍ）も表２に併記する。
【００３９】
得られた鋼板について、組織観察を実施しマルテンサイトの分率と粒径を以下のように測定した。
【００４０】
すなわち、マルテンサイト分率は、板厚中央部（１／２ｔ）付近より採取した薄膜状のサンプルを透過型電子顕微鏡により、２万倍の倍率で２０視野観察し、セメンタイトの析出していない領域の面積を測定し、その測定面積の全体に対する割合に基づいて、マルテンサイト分率として求めた。また、粒径は板厚中央部（１／２ｔ）付近を光学顕微鏡により２００倍の倍率で１０視野観察し、その平均粒径を測定することにより求めた。この結果を表２に併記する。
【００４１】
また、得られた鋼板の耐磨耗性および低温靭性を調べた。耐磨耗性の特性値として、表面硬度（ＨＢ）をＪＩＳ規格のＺ２２４３に準拠して、鋼板表面のランダムに選んだ５点で測定し、その結果の平均値を算出した。低温靭性（ｖＴｓ）の特性値として、破面遷移温度（℃）をＪＩＳ規格のＺ２２４２に準拠してシャルピー衝撃試験を行うことにより測定した。これらの結果を表２に併せて示す。
【００４２】
【表１】

【００４３】
【表２】

【００４４】
表２に示すように、供試鋼片として本発明の範囲内の化学成分を有する鋼種Ａ〜Ｄを用い、本発明に従う製造条件で製造した実施例１〜８の鋼板は、板厚が１２ｍｍおよび５０ｍｍのいずれであっても、耐磨耗鋼板として有効なＨＢ４５０以上の高硬度を有し、耐磨耗性が優れているとともに、シャルピー衝撃試験における破面遷移温度が−２０℃以下と低く、良好な低温靭性を有していた。
【００４５】
これに対して、供試鋼片の化学成分は本発明範囲内であるものの、スラブ加熱温度が１２００℃を下回って低かった比較例１、２、４、５、７、８、１０、１１の鋼板は、結果として得られたマルテンサイト粒径が１５μｍを超えて粗大となり、靭性が劣化していた。
【００４６】
供試鋼片の化学成分は本発明範囲内であるものの、再加熱焼入れ温度が９５０℃を超えて高かった比較例３、６、９、１２の鋼板は、結果として得られたマルテンサイト粒径が１５μｍを超えて粗大となり、靭性が劣化していた。
【００４７】
Ｂを含有せず、Ｈａ値が２．５を下回る鋼種Ｅを供試鋼片として用い、スラブ加熱温度が１２００℃を下回って低かった比較例１３、１４の鋼板は、結果としてマルテンサイト分率が９０％を下回り、マルテンサイト粒径が１５μｍを超えていたため、靭性が劣化していた。
【００４８】
製造条件は本発明範囲内であるものの、Ｂを含有せず、Ｈａ値が２．５を下回る鋼種Ｅを用いた比較例１５、１６の鋼板は、板厚１２ｍｍ、５０ｍｍともに、結果としてマルテンサイト分率が９０％を下回り、靭性が劣化していた。
【００４９】
Ｂを含有せず、Ｈａ値が２．５を下回る鋼種Ｅを用い、再加熱焼入れ温度が９５０℃を超えて高かった比較例１７の鋼板は、結果としてマルテンサイト分率が９０％を下回り、マルテンサイト粒径が１５μｍを超えて粗大となり、靭性が劣化していた。
【００５０】
Ｎｂを含有しない鋼種Ｆを供試鋼片として用い、スラブ加熱温度が１２００℃下回って低かった比較例１８、１９の鋼板は、結果として得られたマルテンサイト粒径が１５μｍを上回って粗大となり、靭性が劣化していた。
【００５１】
製造条件は本発明範囲内であるものの、Ｎｂを含有しない鋼種Ｆを用いた比較例２０、２１の鋼板は、板厚１２ｍｍ、５０ｍｍともに、結果としてマルテンサイト粒径が１５μｍを超えて粗大となり、靭性が劣化していた。
【００５２】
Ｎｂを含有しない鋼種Ｆを用い、再加熱焼入れ温度が９５０℃を超えて高かった比較例２２の鋼板は、結果としてマルテンサイト粒径が１５μｍを超えて粗大となり、靭性が劣化していた。
【００５３】
＜実施例９〜１３、比較例２３〜２９＞
種々の化学成分を有する供試鋼片Ｇ〜Ｒを用いて鋼板を製造した。用いた供試鋼片の化学成分（質量％）を表３に示す。
【００５４】
【表３】

【００５５】
１００ｍｍの厚さを有する供試鋼片Ｇを１２００℃まで加熱し、板厚２０ｍｍまで熱間圧延を行い、室温まで放冷した後に、９００℃まで再加熱し焼入れを行って実施例９の鋼板を得た。同様に供試鋼片Ｈ、Ｉ、Ｊ、Ｋを用いてそれぞれ順に実施例１０、１１、１２、１３の鋼板を製造し、供試鋼片Ｌ、Ｍ、Ｎ、Ｏ、Ｐ、Ｑ、Ｒを用いてそれぞれ順に比較例２３、２４、２５、２６、２７、２８、２９の鋼板を製造した。
【００５６】
得られた鋼板について、実施例１〜８および比較例１〜２２の鋼板と同様にマルテンサイト分率（％）を測定し、低温靭性の特性値として破面遷移温度（℃）を調べた。
【００５７】
これらの結果から、供試鋼片の成分指標値Ｈａと得られた鋼板のマルテンサイト分率との関係を図１の（ａ）に示し、供試鋼片の成分指標値Ｈａと得られた鋼板の低温靭性（破面遷移温度）との関係を図１の（ｂ）に示す。図１中、（ａ）は横軸が成分指標値Ｈａを示し、縦軸がマルテンサイト分率（％）を示し、（ｂ）は横軸が成分指標値Ｈａを示し、縦軸が低温靭性（℃）を示す。また、図１の（ａ）、（ｂ）ともに点線より右側は、成分指標値Ｈａが２．５以上で本発明範囲内である供試鋼を用いた実施例９〜１３の鋼板の結果を示す領域、点線より左側は、成分指標値Ｈａが２．５未満と本発明の範囲から外れる供試鋼を用いた比較例２３〜２９の鋼板の結果を示す領域である。
【００５８】
図１の（ａ）より、Ｈａ値が２．５以上の本発明範囲内の領域ではマルテンサイト分率が９０％以上となり、良好な鋼組織が得られた。このときＨａ値が大きければ大きいほど、マルテンサイト分率は増大していた。図１の（ｂ）より、Ｈａ値が２．５以上の本発明範囲内の領域では、低温靭性はマイナス４０℃を下回って優れた値を示していた。また、低温靭性もＨａ値が大きいほど良好となる傾向があった。
【００５９】
一方、図１の（ａ）より、Ｈａ値が２．５未満と本発明範囲から外れる領域では、本発明範囲内の領域に比べてマルテンサイト分率が低く鋼組織が劣化していた。またＨａ値が小さいほどマルテンサイト分率が低下していた。図１の（ｂ）より、Ｈａ値が２．５未満と本発明範囲から外れる領域では、マルテンサイト分率の低下により低温靭性がマイナス１０℃を上回って極端に劣化していた。また、Ｈａ値が小さいほど低温靭性が劣化する傾向にあった。
【００６０】
＜実施例１４、実施例１５、比較例３０〜３３＞
次に、鋼種Ｋを用いて、スラブ加熱温度を１０００℃、１０５０℃、１１００℃、１１５０℃と変えてそれぞれ順に比較例３０、３１、３２、３３の鋼板を製造し、スラブ加熱温度を１２００℃、１２５０℃と変えてそれぞれ順に実施例１４、１５の鋼板を製造した。スラブ加熱温度を変えた以外は、実施例９〜１３および比較例２３〜２９と同様の製造条件とした。
【００６１】
得られた鋼板について、実施例１〜８および比較例１〜２２の鋼板と同様にマルテンサイト粒径（μｍ）を測定し、低温靭性の特性値として破面遷移温度（℃）を調べた。
【００６２】
これらの結果から、スラブ加熱温度と得られた鋼板のマルテンサイト粒径（旧γ粒径）との関係を図２の（ａ）に示し、および、スラブ加熱温度と得られた鋼板の低温靭性（破面遷移温度）との関係を図２の（ｂ）に示す。図２中、（ａ）は横軸がスラブ加熱温度（℃）を示し、縦軸がマルテンサイト粒径（μｍ）を示し、（ｂ）は横軸がスラブ加熱温度（℃）を示し、縦軸が低温靭性（℃）を示す。また、図２の（ａ）、（ｂ）ともに点線より右側は、スラブ加熱温度が１２００℃以上で本発明範囲内である製造条件で製造した実施例１４および１５の鋼板の結果を示す領域、点線より左側は、スラブ加熱温度が１２００℃未満と本発明の範囲から外れる製造条件で製造した比較例３０〜３３の鋼板の結果を示す領域である。
【００６３】
図２の（ａ）より、スラブ加熱温度が１２００℃以上の本発明範囲内の領域では、マルテンサイト粒径が１５μｍ以下となり、良好な鋼組織が得られた。図２の（ｂ）より、スラブ加熱温度が１２００℃以上の本発明範囲内の領域では、低温靭性もマイナス４０℃を下回って優れた値を示した。
【００６４】
一方、図２の（ａ）より、スラブ加熱温度が１２００℃未満と本発明範囲から外れる領域では、本発明範囲内の領域に比べてマルテンサイト粒径が極端に粗大化していた。また、図２の（ｂ）より、スラブ加熱温度が１２００℃未満と本発明範囲から外れる領域では、マルテンサイトの粗大化により低温靭性も本発明範囲内の領域に比べて大きく劣化していた。
【００６５】
【発明の効果】
以上詳述したように本発明によれば、表面硬度を低下させることなく耐磨耗性を安定的に有し、さらに低温靭性にも優れる耐磨耗鋼板を容易に、かつ廉価に製造することができ、工業的に非常に有意である。
【図面の簡単な説明】
【図１】（ａ）は成分指標値Ｈａとマルテンサイト分率との関係を示すグラフ図、（ｂ）は成分指標値Ｈａと低温靭性との関係を示すグラフ図。
【図２】（ａ）はスラブ加熱温度とマルテンサイト粒径との関係を示すグラフ図、（ｂ）はスラブ加熱温度と低温靭性との関係を示すグラフ図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wear-resistant steel plate used for industrial machines, transportation equipment, and the like, and a method for manufacturing the same.
[0002]
[Prior art]
Construction site, civil engineering construction site, for example, are used in a mine or the like, excavators, bulldozers, hoppers, industrial machines such as a bucket, the handling equipment and the like, and these components, abrasion resistance in order to ensure their lifetime Excellent steel is used. In order to improve the wear resistance of steel, it is necessary to increase the surface hardness by making the steel surface hardened.
[0003]
In general, the quenching hardness of steel can be ensured by increasing the C content in the steel. However, when the hardness increases, the material becomes brittle and the low temperature toughness deteriorates. Considering that it is used for work in a low temperature range of minus 20 ° C. or less, if steel has good wear resistance and low temperature toughness, it causes brittle fracture (delayed fracture) and seriously hinders the work. Therefore, abrasion steel excellent in low temperature toughness and has a wear resistance has been desired.
[0004]
Several methods have been examined for such a demand. For example, Patent Literature 1, Patent Literature 2, or Patent Literature 3 discloses a technique for improving the toughness of a wear-resistant steel sheet by adding a large amount of an alloy element such as Cr or Mo. In these techniques, Cr is added for the purpose of improving hardenability, and Mo is added for the purpose of improving hardenability and at the same time improving the grain boundary strength.
[0005]
Patent Document 4 discloses a technique for improving the toughness of a steel sheet by using ausfoam in a hot rolling process and expanding old austenite (γ) grains as a technique for improving the manufacturing process of a wear-resistant steel sheet. Has been.
[0006]
[Patent Document 1]
JP-A-8-41535 [0007]
[Patent Document 2]
Japanese Patent Laid-Open No. 2-179842
[Patent Document 3]
Japanese Patent Laid-Open No. 61-166554
[Patent Document 4]
Japanese Patent Laid-Open No. 2002-20837
[Problems to be solved by the invention]
However, as in Patent Document 1, Patent Document 2 or Patent Document 3, when adding a large amount of alloy elements to steel to enhance the grain boundary strength and improve toughness, the problem of increased alloy element addition costs There is.
[0011]
Moreover, when using ausfoam in a hot rolling process like patent document 4, another device is needed in order to manufacture steel stably, and it cannot necessarily be said that it is an easy process on actual manufacture.
[0012]
As described above, according to the conventional technology, it is difficult to provide a wear-resistant steel that can be manufactured at low cost, is easy to manufacture, has good delayed fracture characteristics, and is excellent in low-temperature toughness.
[0013]
The present invention has been made to solve the above-mentioned problems, and has a wear-resistant steel sheet having stable wear resistance without reducing surface hardness, and having excellent low-temperature toughness, and production thereof. It aims to provide a method.
[0014]
[Means for Solving the Problems]
As a result of intensive studies to provide a wear-resistant steel sheet having excellent low-temperature toughness, the present inventors have obtained the following knowledge. That is, adjusting the component index value Ha to a predetermined value and heating the steel slab to a temperature range of 1200 to 1250 ° C. ensures the wear resistance of the steel sheet after reheating and quenching, and further improves the toughness. Effective above. The present invention has been made based on this finding.
[0015]
The wear-resistant steel sheet having excellent low-temperature toughness according to the present invention is, by mass, C: 0.23-0.35%, Si: 0.05-1.0 % (excluding over 0.50%) Mn: 0.1 to 2.0%, P: 0.020% or less, S: 0.005% or less, Nb: 0.005 to 0.03%, Ti: 0.005 to 0.1%, B: 0.0003-0.002% is contained, and further, by mass, Cu: 0.03-2.0%, Ni: 0.03-2.0%, Cr: 0.03-2.0 %, Mo: 0.03 to 1.0%, V: One or more selected from the group consisting of 0.005 to 0.1%, and a component index defined by the formula (1) The value Ha is 2.5 or more, and C, Mn, Cu, Ni, Cr, Mo, V, and B in the formula (1) are the contents in mass% of each element contained in the steel, Does not contain Cu When Cu = 0, Ni is not included, Ni = 0, when Cr is not included, Cr = 0, when Mo is not included, Mo = 0, when V is not included Is characterized in that V = 0, the balance is Fe and inevitable impurities, and 90% or more of as-quenched martensite with a particle size of 15 μm or less is contained.
Ha = C × (1 + 3 × Mn) × (1 + 0.5 × Cu) × (1 + 2 × Ni)
× (1 + 3 × Cr) × (1 + 2 × Mo) × (1 + V)
× (1 + 300 × B) (1)
The method for producing a wear-resistant steel sheet having excellent low-temperature toughness according to the present invention is mass%, C: 0.23-0.35%, Si: 0.05-1.0 % (however, more than 0.50%) except for), Mn: 0.1~2.0%, P : 0.020% or less, S: 0.005% or less, Nb: 0.005~0.03%, Ti: 0.005~0. 1%, B: 0.0003-0.002%, and further by mass: Cu: 0.03-2.0%, Ni: 0.03-2.0%, Cr: 0.03- Contains one or more selected from the group consisting of 2.0%, Mo: 0.03-1.0%, V: 0.005-0.1%, and is defined by Formula (1) The component index value Ha is 2.5 or more, and C, Mn, Cu, Ni, Cr, Mo, V, and B in the formula (1) are the contents in mass% of each element contained in the steel. And Cu When Cu is not included, Cu = 0, when Ni is not included, Ni = 0, when Cr is not included, Cr = 0, when Mo is not included, Mo = 0, V is included. If not, V = 0, and the steel slab consisting of Fe and inevitable impurities is heated to a temperature range of 1200 ° C. to 1250 ° C. and hot-rolled until the plate thickness is in the range of 5 mm to 50 mm, Thereafter, the steel sheet is reheated to a temperature range of 850 ° C. to 950 ° C., and a wear-resistant steel sheet excellent in low-temperature toughness containing 90% or more of as-quenched martensite having a grain size of 15 μm or less is obtained .
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the reasons for limiting the chemical components and the production method of the present invention will be described.
[0017]
(1) C: 0.23 to 0.35 mass%
C is an important element for increasing the hardness of the steel and improving the wear resistance. If the C content is less than 0.23% by mass, sufficient hardness cannot be obtained. On the other hand, if the C content exceeds 0.35% by mass, weldability, toughness and workability are deteriorated. For this reason, C content is prescribed | regulated in the range of 0.23-0.35 mass%.
[0018]
(2) Si: 0.05 to 1.0% (excluding over 0.50%)
Si is an effective element as a deoxidizing element, and is also an effective element for solid solution strengthening. If the Si content is less than 0.05% by mass, a sufficient deoxidation effect cannot be obtained. On the other hand, if the Si content exceeds 1.0% by mass (excluding more than 0.50%) , ductility decreases. The problem of increased inclusions arises. For this reason, Si content is prescribed | regulated to 0.05 to 1.0% (however, excluding more than 0.50%) .
[0019]
(3) Mn: 0.1 to 2.0% by mass
Mn is an effective element from the viewpoint of ensuring hardenability. If the Mn content is less than 0.1% by mass, the effect cannot be obtained sufficiently. On the other hand, if the Mn content exceeds 2.0% by mass, the weldability deteriorates. For this reason, Mn content is prescribed | regulated in the range of 0.1-2.0 mass%.
[0020]
(4) P: 0.020% by mass or less P, if contained in a large amount in steel, causes deterioration of toughness, but if the content is 0.020% by mass or less, there is no problem. For this reason, the upper limit of the P content is 0.020% by mass.
[0021]
(5) S: 0.005% by mass or less When S is contained in a large amount in steel, it precipitates as MnS, which becomes an origin of fracture of high-strength steel as inclusions, leading to deterioration of toughness. However, there is no problem if the content is 0.005 mass% or less. For this reason, the upper limit of S content shall be 0.005 mass%.
[0022]
(6) Nb: 0.005 to 0.03 mass%
Nb is an element that exists as a precipitate during reheating and quenching, has the effect of reducing the particle size, and consequently helps to improve toughness. If the Nb content is less than 0.005% by mass, this effect cannot be exhibited. On the other hand, if the Nb content exceeds 0.03% by mass, the weldability deteriorates. For this reason, Nb content is prescribed | regulated in the range of 0.005-0.03 mass%. Further, in order to effectively obtain the fine effect, it is desirable that the Nb content is in the range of 0.012 to 0.03 mass%.
[0023]
(7) Ti: 0.005 to 0.1% by mass
Ti has the effect of securing solid solution B effective in improving hardenability as well as improving toughness by fixing solid solution N harmful to toughness as TiN. If the Ti content is less than 0.005% by mass, this effect cannot be exhibited. On the other hand, if the Ti content exceeds 0.1% by mass, the toughness deteriorates. For this reason, Ti content is prescribed | regulated in the range of 0.005-0.1 mass%. Moreover, in order to reduce cost more, it is desirable to make Ti content into the range of 0.005-0.03 mass%.
[0024]
(8) B: 0.0003 to 0.002 mass%
B is an element that enhances hardenability by adding a small amount. If the B content is less than 0.0003% by mass, this effect cannot be exhibited. On the other hand, if the B content exceeds 0.002% by mass, the toughness deteriorates. For this reason, B content is prescribed | regulated in the range of 0.0003-0.002 mass%. Further, from the viewpoint of suppressing low temperature cracking in a low heat input weld such as CO ₂ welding that is generally carried out on wear-resistant steel plates, the B content should be in the range of 0.0003 to 0.0015% by mass. Is desirable.
[0025]
In the present invention, one or more of Cu, Ni, Cr, Mo and V shown below are contained for the purpose of further improving strength, low temperature toughness, wear resistance and the like.
[0026]
(9) Cu: 0.03 to 2.0% by mass
Cu is an element that enhances hardenability. If the Cu content is less than 0.03% by mass, this effect cannot be exhibited sufficiently. On the other hand, if the Cu content exceeds 2.0% by mass, the hot workability decreases and the cost also increases. For this reason, when adding Cu, the content is prescribed | regulated to the range of 0.03-2.0 mass%. In order to further reduce the cost, it is desirable that the Cu content is in the range of 0.03 to 0.5 mass%.
[0027]
(10) Ni: 0.03 to 2.0% by mass
Ni is an element that improves hardenability and improves low temperature toughness. If the Ni content is less than 0.03% by mass, this effect cannot be exhibited sufficiently. On the other hand, if the Ni content exceeds 2.0% by mass, the cost increases. For this reason, when adding Ni, the content is prescribed | regulated to the range of 0.03-2.0 mass%. In order to further reduce the cost, it is desirable that the Ni content is in the range of 0.03 to 0.5% by mass.
[0028]
(11) Cr: 0.03 to 2.0% by mass
Cr is an element that enhances hardenability. If the Cr content is less than 0.03% by mass, this effect cannot be sufficiently exerted. On the other hand, if the Cr content exceeds 2.0% by mass, the weldability deteriorates and the cost increases. For this reason, when adding Cr, the content is prescribed | regulated to the range of 0.03-2.0 mass%.
[0029]
(12) Mo: 0.03-1.0 mass%
Mo is an element that enhances hardenability. If the Mo content is less than 0.03% by mass, this effect cannot be exhibited sufficiently. On the other hand, if the Mo content exceeds 1.0% by mass, the weldability deteriorates and the cost increases. For this reason, when adding Mo, the content is prescribed | regulated to the range of 0.03-1.0 mass%.
[0030]
(13) V: 0.05 to 0.1% by mass
V is an element effective for precipitation strengthening, and has the effect of increasing the hardness of the steel and improving the wear resistance. If the V content is less than 0.05% by mass, this effect cannot be exhibited sufficiently. On the other hand, if the V content exceeds 0.1% by mass, the weldability deteriorates. For this reason, when adding V, V content is prescribed | regulated in the range of 0.05-0.1 mass%.
[0031]
(14) Component index value Ha: 2.5 or more The component index value Ha is defined by the following formula (1) and is related to the structure after quenching, and greatly affects the hardness of the steel, that is, the wear resistance.
[0032]
Ha = C × (1 + 3 × Mn) × (1 + 0.5 × Cu) × (1 + 2 × Ni)
× (1 + 3 × Cr) × (1 + 2 × Mo) × (1 + V)
× (1 + 300 × B) (1)
Here, in Formula (1), C, Mn, Cu, Ni, Cr, Mo, V, and B are the contents in mass% of each element contained in the steel, and when Cu is not contained. Cu = 0, when Ni is not included, Ni = 0, when Cr is not included, Cr = 0, when Mo is not included, Mo = 0, when V is not included, V = 0.
[0033]
If the component index value Ha is less than 2.5, the structure may not be a completely quenched structure, and even if the surface structure is a completely quenched structure, it is not completely formed from the surface layer to the center of the plate thickness. It does not become a hardened structure and hardness decreases. Furthermore, when the component index value Ha is less than 2.5, the martensite fraction in the vicinity of the center portion of the plate thickness is lowered, and the toughness is extremely deteriorated. Therefore, the component index value Ha is defined as 2.5 or more. In addition, since weldability will deteriorate when component index value Ha exceeds 12.0, it is preferable that component index value Ha shall be 12.0 or less.
[0034]
(15) Structure: 90% or more of martensite having a particle size of 15 μm or less In a steel structure, if the fraction of as-quenched martensite is less than 90%, sufficient toughness cannot be obtained. Even if 90% or more of martensite is contained, if the martensite particle size exceeds 15 μm and is coarse, the toughness is also deteriorated. Accordingly, 90% or more of as-quenched martensite having a particle size of 15 μm or less is contained. Note that it is difficult to make the martensite fraction 100% in actual production, and as shown in Table 2, the upper limit is about 98%.
[0035]
(16) Heating temperature: 1200-1250 ° C
The heating temperature of the steel slab (slab) has a correlation with the austenite grain size at the time of reheating. By setting the billet heating temperature to 1200 ° C. or more, the austenite grain size during reheating, that is, the martensite grain size after quenching can be refined to 15 μm or less, and as a result, toughness can be improved. it can. However, when the steel piece is heated above 1250 ° C., wrinkles are generated on the surface of the steel plate. Accordingly, the billet heating temperature is in the range of 1200 to 1250 ° C.
[0036]
(17) Plate thickness: 5-50mm
If the thickness of the wear-resistant steel sheet is less than 5 mm, the production by hot rolling becomes difficult and the productivity is lowered. On the other hand, if the plate thickness exceeds 50 mm, the cooling rate in the vicinity of the central portion of the plate thickness is lowered, and the hardness is lowered and the wear resistance is deteriorated without becoming a completely quenched structure. Accordingly, the plate thickness is in the range of 5 to 50 mm.
[0037]
(18) Reheating quenching temperature: 850 to 950 ° C
If the reheating quenching temperature is less than 850 ° C., the transformation to fine austenite is not completed completely, and even if quenched as it is, an incomplete quenching structure is formed, the hardness is lowered, and the wear resistance is deteriorated. On the other hand, when the reheating quenching temperature exceeds 950 ° C., the austenite grain size at the time of reheating becomes coarse, and as a result, the as-quenched martensite grain size becomes coarse so that toughness deteriorates. Therefore, the reheating quenching temperature is set to a range of 850 to 950 ° C.
[0038]
【Example】
<Examples 1-8, Comparative Examples 1-22>
Steel plates were manufactured using test steel pieces A to F having various chemical components. Table 1 shows the chemical composition (mass%) of the test steel piece used. Steel sheets of Examples 1 to 8 and Comparative Examples 1 to 22 were heated by heating a test piece having a thickness of 100 mm, hot rolling to a plate thickness of 12 mm or 50 mm, allowing to cool to room temperature, reheating and quenching. Got. As manufacturing conditions at this time, the slab heating temperature (° C.) and the reheating quenching temperature (° C.) are shown in Table 2, and the thickness (mm) of the steel sheet is also shown in Table 2.
[0039]
About the obtained steel plate, structure | tissue observation was implemented and the fraction and particle size of the martensite were measured as follows.
[0040]
That is, the martensite fraction was determined by observing 20 thin-film samples collected from the vicinity of the center of the plate thickness (1 / 2t) with a transmission electron microscope at 20 magnifications at a magnification of 20,000 times. Was measured as a martensite fraction based on the ratio of the measured area to the whole. The particle size was determined by observing the vicinity of the center of the plate thickness (1 / 2t) with an optical microscope at 10 magnifications and measuring the average particle size. The results are also shown in Table 2.
[0041]
The obtained steel sheet was also examined for wear resistance and low temperature toughness. As the wear resistance characteristic value, the surface hardness (HB) was measured at 5 points selected at random on the surface of the steel sheet in accordance with JIS standard Z2243, and the average value of the results was calculated. As a characteristic value of low temperature toughness (vTs), the fracture surface transition temperature (° C.) was measured by conducting a Charpy impact test according to JIS standard Z2242. These results are also shown in Table 2.
[0042]
[Table 1]

[0043]
[Table 2]

[0044]
As shown in Table 2, the steel plates A to D having chemical components within the scope of the present invention were used as test steel pieces, and the steel plates of Examples 1 to 8 manufactured under the manufacturing conditions according to the present invention had a plate thickness of 12 mm. And 50 mm, it has a high hardness of HB450 or more, which is effective as a wear-resistant steel sheet , has excellent wear resistance, and has a low fracture surface transition temperature of −20 ° C. or less in the Charpy impact test. And had good low temperature toughness.
[0045]
On the other hand, although the chemical composition of the test steel slab is within the range of the present invention, the slab heating temperature was lower than 1200 ° C., and the comparative examples 1, 2, 4, 5, 7, 8, 10, 11 were used. As a result, the resulting martensite grain size was larger than 15 μm, and the toughness was deteriorated.
[0046]
Although the chemical composition of the test steel slab is within the range of the present invention, the steel sheets of Comparative Examples 3, 6, 9, and 12 in which the reheating quenching temperature was higher than 950 ° C. were obtained as a result of the martensite particle size Was over 15 μm, and the toughness was deteriorated.
[0047]
The steel sheet E containing no B and having a Ha value of less than 2.5 was used as a test steel slab, and the steel sheets of Comparative Examples 13 and 14 in which the slab heating temperature was lower than 1200 ° C were low, resulting in a martensite fraction. Was less than 90%, and the martensite particle size exceeded 15 μm, so the toughness was deteriorated.
[0048]
Although the manufacturing conditions are within the scope of the present invention, the steel sheets of Comparative Examples 15 and 16 using steel type E that does not contain B and the Ha value is less than 2.5 are both martensite as a result of the plate thicknesses of 12 mm and 50 mm. The fraction was below 90% and the toughness was degraded.
[0049]
Steel plate E containing no B and having a Ha value of less than 2.5 and the reheat quenching temperature was higher than 950 ° C., the martensite fraction was less than 90% as a result, The martensite particle size exceeded 15 μm and became coarse, and the toughness was deteriorated.
[0050]
Steel plate F containing no Nb was used as a test steel slab, and the steel plates of Comparative Examples 18 and 19 where the slab heating temperature was lower than 1200 ° C. were low, and the resulting martensite grain size was coarser than 15 μm, The toughness was degraded.
[0051]
Although the manufacturing conditions are within the scope of the present invention, the steel plates of Comparative Examples 20 and 21 using the steel type F not containing Nb have a plate thickness of 12 mm and 50 mm, resulting in a coarse martensite particle size exceeding 15 μm, The toughness was degraded.
[0052]
The steel sheet of Comparative Example 22, which used steel type F not containing Nb and had a high reheating quenching temperature exceeding 950 ° C., had a martensite grain size exceeding 15 μm, resulting in coarseness and deteriorated toughness.
[0053]
<Examples 9 to 13 and Comparative Examples 23 to 29>
Steel plates were manufactured using test steel pieces G to R having various chemical components. Table 3 shows the chemical composition (mass%) of the test steel pieces used.
[0054]
[Table 3]

[0055]
The test steel slab G having a thickness of 100 mm is heated to 1200 ° C., hot-rolled to a plate thickness of 20 mm, allowed to cool to room temperature, reheated to 900 ° C. and quenched, and then the steel plate of Example 9 Got. Similarly, steel plates of Examples 10, 11, 12, and 13 were manufactured in order using test steel slabs H, I, J, and K, respectively, and test steel slabs L, M, N, O, P, Q, R The steel plates of Comparative Examples 23, 24, 25, 26, 27, 28, and 29 were produced in this order.
[0056]
About the obtained steel plate, the martensite fraction (%) was measured similarly to the steel plates of Examples 1 to 8 and Comparative Examples 1 to 22, and the fracture surface transition temperature (° C.) was examined as a characteristic value of low temperature toughness.
[0057]
From these results, the relationship between the component index value Ha of the test steel slab and the martensite fraction of the obtained steel sheet is shown in FIG. 1 (a), and the component index value Ha of the test steel slab was obtained. FIG. 1B shows the relationship with the low temperature toughness (fracture surface transition temperature) of the steel sheet. In FIG. 1, (a) shows the component index value Ha, the vertical axis shows the martensite fraction (%), (b) shows the component index value Ha, and the vertical axis shows the low temperature toughness. (° C.). 1A and 1B, the right side of the dotted line shows the results of the steel plates of Examples 9 to 13 using the test steel having the component index value Ha of 2.5 or more and within the scope of the present invention. The left side of the region to be shown, the dotted line, is a region showing the results of the steel plates of Comparative Examples 23 to 29 using the sample steel having a component index value Ha of less than 2.5 and out of the scope of the present invention.
[0058]
From FIG. 1 (a), the martensite fraction was 90% or more in the region within the range of the present invention where the Ha value was 2.5 or more, and a good steel structure was obtained. At this time, the higher the Ha value, the higher the martensite fraction. As shown in FIG. 1 (b), the low temperature toughness was less than minus 40 ° C. in the region within the range of the present invention where the Ha value was 2.5 or more. Also, the low temperature toughness tended to be better as the Ha value was larger.
[0059]
On the other hand, as shown in FIG. 1A, in the region where the Ha value is less than 2.5 and out of the scope of the present invention, the martensite fraction is lower than that in the region of the present invention, and the steel structure has deteriorated. Further, the martensite fraction decreased as the Ha value decreased. As shown in FIG. 1B, in the region where the Ha value is less than 2.5 and deviates from the scope of the present invention, the low temperature toughness was extremely deteriorated by exceeding −10 ° C. due to the decrease in the martensite fraction. Further, the lower the Ha value, the lower the low temperature toughness.
[0060]
<Example 14, Example 15, Comparative Examples 30 to 33>
Next, using the steel type K, the slab heating temperature was changed to 1000 ° C., 1050 ° C., 1100 ° C., and 1150 ° C., and the steel sheets of Comparative Examples 30, 31, 32, and 33 were manufactured in this order, and the slab heating temperature was 1200 ° C. The steel plates of Examples 14 and 15 were manufactured in this order at 1250 ° C. The production conditions were the same as in Examples 9 to 13 and Comparative Examples 23 to 29 except that the slab heating temperature was changed.
[0061]
About the obtained steel plate, the martensite particle size (micrometer) was measured similarly to the steel plate of Examples 1-8 and Comparative Examples 1-22, and the fracture surface transition temperature (degreeC) was investigated as a characteristic value of low temperature toughness.
[0062]
From these results, the relationship between the slab heating temperature and the martensite grain size (old γ grain size) of the obtained steel sheet is shown in FIG. 2 (a), and the slab heating temperature and the low temperature toughness of the obtained steel sheet are shown. The relationship with (fracture surface transition temperature) is shown in FIG. In FIG. 2, (a) shows the slab heating temperature (° C.) on the horizontal axis, the martensite particle size (μm) on the vertical axis, and (b) shows the slab heating temperature (° C.) on the horizontal axis. The axis indicates low temperature toughness (° C). Moreover, the area | region which shows the result of the steel plate of Examples 14 and 15 manufactured on the manufacturing conditions which are slab heating temperature 1200 degreeC or more and it exists in the scope of the present invention on the right side of the dotted line in both (a) and (b) of FIG. On the left side of the dotted line is a region showing the results of the steel sheets of Comparative Examples 30 to 33 manufactured under manufacturing conditions where the slab heating temperature is less than 1200 ° C. and deviates from the scope of the present invention.
[0063]
From FIG. 2 (a), in the region within the scope of the present invention where the slab heating temperature is 1200 ° C. or higher, the martensite particle size was 15 μm or less, and a good steel structure was obtained. As shown in FIG. 2 (b), the low temperature toughness was less than minus 40 ° C. in the region within the range of the present invention where the slab heating temperature was 1200 ° C. or higher.
[0064]
On the other hand, from FIG. 2 (a), the martensite particle size was extremely coarse in the region outside the scope of the present invention when the slab heating temperature was less than 1200 ° C., compared to the region within the scope of the present invention. Further, as shown in FIG. 2B, in the region where the slab heating temperature is less than 1200 ° C. and deviates from the range of the present invention, the low temperature toughness is greatly deteriorated as compared with the region within the range of the present invention due to coarsening of martensite.
[0065]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to easily and inexpensively manufacture a wear-resistant steel plate that has stable wear resistance without lowering surface hardness and is excellent in low-temperature toughness. Is industrially very significant.
[Brief description of the drawings]
FIG. 1A is a graph showing the relationship between component index value Ha and martensite fraction, and FIG. 1B is a graph showing the relationship between component index value Ha and low temperature toughness.
FIG. 2A is a graph showing the relationship between slab heating temperature and martensite particle size, and FIG. 2B is a graph showing the relationship between slab heating temperature and low temperature toughness.

Claims

In mass%, C: 0.23-0.35%, Si : 0.05-1.0% (excluding more than 0.50%) , Mn: 0.1-2.0%, P: 0.020% or less, S: 0.005% or less, Nb: 0.005 to 0.03%, Ti: 0.005 to 0.1%, B: 0.0003 to 0.002%, Further, by mass%, Cu: 0.03-2.0%, Ni: 0.03-2.0%, Cr: 0.03-2.0%, Mo: 0.03-1.0%, V : Containing one or more selected from the group consisting of 0.005 to 0.1%, the component index value Ha defined by the formula (1) is 2.5 or more, the formula (1 C, Mn, Cu, Ni, Cr, Mo, V, and B are the contents in mass% of each element contained in the steel. When Cu is not contained, Cu = 0 and Ni are contained. Not included Ni = 0, Cr = 0 when Cr is not included, Mo = 0 when Mo is not included, V = 0 when V is not included, and the balance is Fe and inevitable impurities. A wear-resistant steel sheet having excellent low-temperature toughness, characterized by containing 90% or more of as-quenched martensite having a particle size of 15 μm or less.
Ha = C × (1 + 3 × Mn) × (1 + 0.5 × Cu) × (1 + 2 × Ni)
× (1 + 3 × Cr) × (1 + 2 × Mo) × (1 + V)
× (1 + 300 × B) (1)

In mass%, C: 0.23-0.35%, Si : 0.05-1.0% (excluding more than 0.50%) , Mn: 0.1-2.0%, P: 0.020% or less, S: 0.005% or less, Nb: 0.005 to 0.03%, Ti: 0.005 to 0.1%, B: 0.0003 to 0.002%, Further, by mass%, Cu: 0.03-2.0%, Ni: 0.03-2.0%, Cr: 0.03-2.0%, Mo: 0.03-1.0%, V : Containing one or more selected from the group consisting of 0.005 to 0.1%, the component index value Ha defined by the formula (1) is 2.5 or more, the formula (1 C, Mn, Cu, Ni, Cr, Mo, V, and B are the contents in mass% of each element contained in the steel. When Cu is not contained, Cu = 0 and Ni are contained. Not included Ni = 0, Cr = 0 when Cr is not included, Mo = 0 when Mo is not included, V = 0 when V is not included, and the balance is Fe and inevitable impurities. The resulting steel slab is heated to a temperature range of 1200 ° C. to 1250 ° C., hot-rolled until the plate thickness reaches a range of 5 mm to 50 mm, and then reheated to a temperature range of 850 ° C. to 950 ° C. and quenched. A method for producing a wear-resistant steel sheet excellent in low-temperature toughness containing 90% or more of as-quenched martensite having a particle size of 15 μm or less .
Ha = C × (1 + 3 × Mn) × (1 + 0.5 × Cu) × (1 + 2 × Ni)
× (1 + 3 × Cr) × (1 + 2 × Mo) × (1 + V)
× (1 + 300 × B) (1)