JP2004183006A - Steel for nitriding treatment, nitriding treated steel, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide nitriding treated steel in which high strength and elongation properties can be reconciled, steel for nitriding treatment which is suitable as the stock for the nitriding treated steel, and to provide a production method therefor. <P>SOLUTION: The steel for nitriding treatment has a composition(1) comprising, ≤0.01% C, 0.001 to 0.5% Si, 0.01 to 0.5% Mn, 0.001 to 0.1% P, ≤0.01% S, 0.001 to 0.5% Al, 0.04 to 0.2% Ti, 0 to 0.05% Nb, 0 to 0.005% B, and ≤0.005% N, and the balance Fe with impurities, and in which Ti-ä(N/14)+(S/32)+(C/12)}×48 is 0.04 to 0.10%, and is subjected to heat treatment so as to be held at 100 to 200°C for ≥10 min after nitriding treatment. Further, in the nitriding treated steel, the average chemical composition (2) in the central region of the sheet thickness within the range from the center of the sheet thickness to 40% of the sheet thickness toward the surface is the same as the above composition (1) except for ≤0.03% C and 0.08 to 0.25% N, the hardness of the center of the sheet thickness is ≥200 by Hv, the number of pieces of iron nitride with a long side of ≥5 μm in the central region of the sheet thickness is ≤10 pieces per 1×10<SP>-8</SP>m<SP>2</SP>, and the number of pieces of iron nitride with a long length of 0.15 to <5 μm in the center of the sheet thickness is ≥5 pieces per 1×10<SP>-12</SP>m<SP>2</SP>. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００３６】
すなわち、上記板厚０．８ｍｍの冷間圧延鋼板を５８０℃の溶融塩浴に１．５時間浸漬した後、油漕中に浸漬して冷却した。なお、５８０℃から２００℃までの平均冷却速度は、２０℃／秒であった。この窒化処理後、更に１７０℃で２０分保持する低温熱処理も施した。
【００３７】
窒化処理ままの鋼板及び低温熱処理後の鋼板について、引張特性とＨｖ硬さを調査した。
【００３８】
すなわち、各鋼板からＪＩＳ１３号Ｂ引張試験を採取し常温で引張試験を行って引張特性を調査し、又、試験力９．８Ｎで板厚中心のＨｖ硬さ測定を行った。
【００３９】
表２に、上記の調査結果をまとめて示すとともに、図１に、前記 （１）式で表される有効Ｔｉが降伏点及び伸びに及ぼす影響を示す。
【００４０】
【表２】

【００４１】
表１、表２及び図１から、有効Ｔｉが多いほど、窒化処理のままでの降伏点が高くなるが、伸びは低下し、有効Ｔｉが０．０７％以上では伸びは５％以下になることが認められる。なお、Ｔｉ含有量の多い、いわゆる「高Ｔｉ鋼材」を窒化処理すると伸びが極めて小さくなり、適用可能な構造部品が限られてしまうことは従来から知られているところである。
【００４２】
これに対して、窒化処理後に１７０℃で２０分保持する低温熱処理を施した場合は、窒化処理のままと比較して降伏点は低下するもの伸びは著しく向上することが認められる。すなわち、窒化処理を施されたままの鋼板の延性は低いが、これに低温熱処理を施すことにより、伸び（塑性変形能）が回復することが判明した。このように、塑性変形能が回復した窒化処理鋼材を自動車部品などに適用すれば、衝突時に鋼材が脆性的に破壊することを防ぐことができる。
【００４３】
更に、図１から、有効Ｔｉの増加は、窒化処理のままでの降伏点を増加させる効果があると同時に、低温熱処理による降伏点の低下量（すなわち、「窒化処理ままでの降伏点」−「低温熱処理後の降伏点」）を小さく抑える効果を持つことが判明した。このことは、例えば、有効Ｔｉが少ない鋼板を長時間窒化処理してＮ量を増やし、これによって強度を上げたとしても、伸びを回復させるために窒化処理に引き続いて低温熱処理を施せば、降伏点が著しく低下してしまうことを示すものである。
【００４４】
すなわち、高強度と伸び特性との両立のためには、有効Ｔｉを高くすることが本質的に重要であることが明らかとなった。
【００４５】
次に、上述の現象の原因を調べるため、前記のようにして窒化処理と低温熱処理を行った鋼板Ｃについて、組織調査を行った。
【００４６】
すなわち、鏡面研磨した鋼板断面をナイタールでエッチングした後、倍率を５００倍として光学顕微鏡で観察した。
【００４７】
その結果、低温熱処理の有無に関わらず鋼板の組織はフェライトからなっており、表面には厚さ９μｍの化合物層が認められた。なお、表面の化合物層は、フェライトとは明瞭なコントラストを持っており、又、化合物層とフェライトの界面においてフェライト結晶粒界が途切れていることからも、表面の化合物層とフェライトとを判別することが可能である。
【００４８】
なお、窒化処理のままの状態では、フェライト粒内はほとんどエッチングされず、窒化処理前のフェライト粒内と比較すると光沢が若干少ないものの、ほぼ同様な観察結果であった。一方、窒化処理後に低温熱処理を行った場合にはフェライト粒内の光沢が消失し、灰色に観察された。低温熱処理後は、エッチングによりフェライト粒内に微細な凹凸が一様に形成されたため、コントラストに変化が生じたもとと考えられる。
【００４９】
上述のように、光学顕微鏡の観察では不十分であるものの、低温熱処理によって何らかの組織的変化が生じていることが判明した。
【００５０】
そこで次に、板厚の中心から薄膜試料を作製し、加速電圧２００ｋＶの透過型電子顕微鏡を用いて組織観察を行った。
【００５１】
窒化処理のままの鋼板から採取した試料について、倍率４００００倍の写真上で計測した結果、析出物の主体は長辺が０．０５〜０．１５μｍの棒状析出物であり、それは１×１０^−１２ｍ^２当たり３８個認められた。長辺が０．１５μｍ以上の析出物は非常に少なく、多数の視野を平均した場合に、１×１０^−１２ｍ^２当たり２．４個認められた。又、長辺が０．０５〜０．１５μｍの棒状析出物が重なり合い、見かけ上の長辺が０．１５μｍ以上になった状態のものも観察されたが、そのようなものの個数は１×１０^−１２ｍ^２当たり０．５個であった。なお、上記の棒状析出物が重なり合ったものは転位上に優先的に析出したと考えられる。
【００５２】
一方、窒化処理後に更に低温熱処理を行った鋼板から採取した試料では、長辺が約０．２〜０．６μｍで、短辺が約０．１μｍの長方形状の析出物が１×１０^−１２ｍ^２当たり８．２個認められた。この低温熱処理を行った試料の場合には、長辺が０．１５μｍ未満の析出物はほとんど認められなかった。
【００５３】
前記のような窒化処理のままで観察された非常に微細な析出物及び低温熱処理後に観察された微細な析出物をＥＤＸ分析したところＦｅ以外の金属元素のピークは、Ｆｅの１０分の１以下しか検出されなかった。このＥＤＸ分析結果から、上述の窒化処理のままで観察された非常に微細な析出物及び低温熱処理後に観察された微細な析出物は窒化鉄と推定される。
【００５４】
なお、窒化処理前から存在したと考えられるＴｉＮ、ＴｉＳ及びＴｉＣも観察されたが、それらは窒化鉄とは形状が異なるし、ＥＤＸ分析によりＦｅ以外の金属元素が検出されたことから、上述の計数からは除外した。
【００５５】
Ｔｉは強力な炭窒化物形成能を持つので窒化処理中にもＴｉＮやＴｉＣ（又は、Ｔｉ−ＮのクラスターやＴｉ−Ｃのクラスター）が形成されることが知られているが、それらは観察されなかった。５８０℃という窒化処理温度ではＴｉの拡散距離が短いため、こうした析出物やクラスターは極めて微細であり、加速電圧２００ｋＶの透過型電子顕微鏡を用いた組織観察では認められなかったものと考えられる。
【００５６】
引張強さの低下に伴って伸びが著しく改善される理由は明らかではないが、窒化処理のままではＮによる固溶強化の影響が強すぎ、粒内破壊が生じるためと推定される。一方、窒化物が析出し、固溶強化能が減少した場合にはフェライト粒内の塑性変形が可能になり、延性が改善されたと考えられる。
【００５７】
前記（１）〜（８）の本発明は、上記の知見に基づいて完成されたものである。
【００５８】
以下、本発明の各要件について詳しく説明する。なお、各元素の含有量の「％」表示は「質量％」を意味する。
（Ａ）窒化処理用鋼材及び窒化処理鋼材の化学組成
窒化処理を施された窒化処理鋼材の表層には化合物層が形成され、板厚方向に化学組成の不均一が発生するので、窒化処理鋼材の化学組成は、板厚中心から板表面に向かってそれぞれ板厚の４０％までの範囲にある板厚中心領域の平均化学組成、すなわち、板厚中心を挟んで板厚の８０％に相当する内部の平均化学組成を指すものとする。
【００５９】
なお、窒化処理用鋼材と窒化処理鋼材との含有量を区別する必要がある元素については、窒化処理用鋼材の場合を「窒化処理前」、窒化処理鋼材の場合を「窒化処理後」として説明する。
【００６０】
Ｃ（窒化処理前）：
良好な深絞り性すなわちｒ値が高い結晶集合組織を得るためには、Ｔｉ及びＮｂを添加して炭化物を形成させて固溶Ｃ量を減少させた、いわゆる「ＩＦ鋼」にする必要がある。Ｃ量が多い場合には、それを固定するのに必要なＴｉ及びＮｂの量が増加し、それらの炭化物量が増加するため、窒化処理前においては延性を低下させたり、窒化処理後においてはそれらが破壊の起点になる恐れがある。したがって、窒化処理前である窒化処理用鋼材のＣ含有量を０．０１％以下とした。窒化処理用鋼材のＣ含有量の上限は０．００５％とすることが好ましく、０．００３％とすれば一層好ましい。なお、Ｃの含有量が少ないほど成形性が向上するが、０．０００５％未満まで低減しても製鋼コストに見合う効果が期待できないので、下限は０．０００５％とするのが好ましい。
【００６１】
Ｃ（窒化処理後）：
工業的な窒化処理プロセスにおいては、Ｎと同様にＣも富化される場合が多い。窒化処理後のＣは、本発明の効果に大きな影響は与えないので、窒化処理において不可避的に富化されるＣレベルを含有することに問題はない。但し、窒化処理温度におけるフェライト中へのＣの固溶限を大きく超えてＣが富化されると、粗大なセメンタイトが析出して脆性的になる恐れがあるので、窒化処理後である窒化処理鋼材のＣ含有量を０．０３％以下とした。
【００６２】
Ｓｉ：
Ｓｉは、固溶強化作用を有する。この効果は窒化による強化量と比較すれば小さいが、部材（部品）の一部分を意図的に窒化処理せず、且つその部分にも強度を確保させたい場合は、Ｓｉを添加して強化することは有効である。しかし、Ｓｉの多量添加は延性及び深絞り性の低下をきたし、特にその含有量が０．５％を超えると延性及び深絞り性の低下が大きくなる。一方、下限は０％でもよいが、低減に要するコストの観点から０．００１％とする。したがって、Ｓｉの含有量を０．００１〜０．５％とした。なお、強化を必要としない場合には、プレス成形性の観点からＳｉの含有量の上限を０．１％とすることが好ましい。
【００６３】
Ｍｎ：
Ｍｎは、固溶強化作用を有する。この効果は窒化による強化量と比較すれば小さいが、部材（部品）の一部分を意図的に窒化処理せず、且つその部分にも強度を確保させたい場合は、Ｍｎを添加して強化することは有効である。しかし、Ｍｎの多量添加は延性及び深絞り性の低下をきたし、特にその含有量が０．５％を超えると延性及び深絞り性の低下が大きくなる。一方、下限は０％でもよいが、低減に要するコストの観点から０．０１％とする。したがって、Ｍｎの含有量を０．０１〜０．５％とした。なお、強化を必要としない場合には、プレス成形性の観点からＭｎの含有量の上限を０．３％とすることが好ましい。
【００６４】
Ｐ：
Ｐは、固溶強化作用を有する。この効果は窒化による強化量と比較すれば小さいが、部材（部品）の一部分を意図的に窒化処理せず、且つその部分にも強度を確保させたい場合は、Ｐを添加して強化することは有効である。しかし、Ｐの多量添加は延性及び深絞り性の低下をきたし、特にその含有量が０．１％を超えると延性及び深絞り性の低下が大きくなる。一方、下限は０％でもよいが、低減に要するコストの観点から０．００１％とする。したがって、Ｐの含有量を０．００１〜０．１％とした。なお、強化を必要としない場合にはプレス成形性の観点から、Ｐの含有量の上限を０．０３％とすることが好ましい。
【００６５】
Ｓ：
Ｓは鋼中に不可避的不純物として含有される元素である。本発明のようなＴｉが高い鋼では、鋼塊又は鋼片においてＴｉと結合してＴｉＳとして析出する。多量のＴｉＳは延性を劣化させたり、破壊の起点になったりする。又、Ｓの含有量が多いと後述する有効Ｔｉを確保するためにＴｉが多量に必要となるため不経済である。更には、Ｔｉの含有量が一定であるならば、Ｓの含有量が高いほど固溶Ｔｉが減少し、窒化処理後に低温熱処理した場合の強度が低くなる。したがって、Ｓの含有量を０．０１％以下とした。Ｓ含有量は０．００５％以下とするのが好ましい。一方、製鋼コストの観点からは、Ｓ含有量の下限を０．００１％とすることが経済的である。
【００６６】
Ａｌ：
Ａｌは、製鋼工程で脱酸のために添加される元素である。その含有量が０．００１％未満では前記の効果が十分に得られない。ＡｌはＮとの親和力が強いので、窒化処理による強度上昇を目的として添加してもよい。しかし、Ａｌの多量添加は窒化処理前の鋼材（すなわち、窒化処理用鋼材）の延性を低下させ、特にその含有量が０．５％を超えると延性の低下が大きくなる。したがって、Ａｌの含有量を０．００１〜０．５％とした。なお、Ａｌ含有量の上限は０．１％とすることが好ましい。又、Ｔｉ添加の歩留まりをよくするために、Ａｌ含有量の下限は０．００５％とすることが好ましい。
【００６７】
Ｔｉ（窒化処理前）：
窒化処理前である窒化処理用鋼材において、Ｔｉは、Ｃ及びＮを固定して深絞り性を向上させるために必須の元素である。しかし、Ｔｉの含有量が０．０４％未満では添加効果に乏しい。一方、その含有量が０．２％を超えると、再結晶温度が上昇して窒化処理前の鋼材の延性が低下する。したがって、窒化処理前である窒化処理用鋼材のＴｉの含有量を０．０４〜０．２％とした。窒化処理用鋼材のＴｉの含有量は０．０６〜０．２％とすることが好ましく、０．０７〜０．２％とすれば更に好ましい。なお、ＴｉはＣ、Ｎ及びＳと結合するので、窒化処理用鋼材の場合にはＴｉの含有量に加えて前記 （１）式で表される有効Ｔｉ量も適正化する必要がある。このことについては後述する。
【００６８】
Ｔｉ（窒化処理後）：
窒化処理してもＴｉの含有量そのものは変動しないので、窒化処理後である窒化処理鋼材におけるＴｉ含有量の考え方は、窒化処理前である窒化処理用鋼材と同じでよい。つまり、深絞り性を向上させるためにＴｉは０．０４％以上の含有量が必要である。しかし、その含有量が０．２％を超えると、窒化処理後の鋼材の延性が低下する。したがって、窒化処理後である窒化処理鋼材のＴｉの含有量を０．０４〜０．２％とした。窒化処理鋼材のＴｉの含有量は０．０６〜０．２％とすることが好ましく、０．０７〜０．２％とすれば更に好ましい。なお、窒化処理鋼材の場合にはＴｉの含有量を適正化する必要があるが、後述する有効Ｔｉに関して制限する必要はない。
【００６９】
Ｎ（窒化処理前）：
窒化処理前である窒化処理用鋼材において、Ｎは不可避的不純物として含有される元素である。良好な深絞り性すなわちｒ値が高い結晶集合組織を得るためには、Ｔｉを添加してＴｉＮを形成させて固溶Ｎ量を減少させた、いわゆる「ＩＦ鋼」にする必要がある。Ｎ量が多い場合には、それを固定するのに必要なＴｉの量が増加し、粗大なＴｉＮの量が増加するため、窒化処理前においては延性を低下させたり、窒化処理後においてはそれが破壊の起点になる恐れがある。したがって、窒化処理前である窒化処理用鋼材のＮ含有量を０．００５％以下とした。窒化処理用鋼材のＮ含有量の上限は０．００３％とすることが好ましい。なお、製鋼コストの観点からは、Ｎ含有量の下限は０．００１％とすることが経済的である。
【００７０】
Ｎ（窒化処理後）：
窒化処理後である窒化処理鋼材において、Ｎは鋼材の強度を確保するための最も重要な元素である。しかし、過剰に窒化しても、Ｎは粗大な窒化物の生成に費やされるだけで、強度への寄与は飽和する。このため、窒化処理後である窒化処理鋼材のＮ含有量の上限を０．２５％とした。一方、Ｎの含有量が少ないと、強度上昇が小さく、窒化処理のコストに見合う効果が得られないので、窒化処理鋼材には０．０８％以上のＮを含有させるものとした。
【００７１】
Ｎｂ：
Ｎｂは添加しなくてもよい。添加すれば、Ｔｉと同様にＣを固定して、窒化処理前である窒化処理用鋼材の深絞り性を向上させる作用を有する。なお、ＮｂがＣを固定する作用はＴｉより弱いので、微量の固溶Ｃを残留させて２次加工脆性を改善する作用も有する。こうした効果を確実に得るには、Ｎｂは０．００５％以上の含有量とすることが好ましい。しかし、Ｎｂを過剰に添加すると再結晶温度が上昇して延性の低下をきたし、特にその含有量が０．０５％を超えると延性の低下が大きくなる。したがって、Ｎｂの含有量を０〜０．０５％とした。
【００７２】
Ｂ：
Ｂは添加しなくてもよい。添加すれば、粒界に偏析して粒界を強化するため、２次加工脆性を改善する作用を有する。この効果を確実に得るには、Ｂを０．０００２％以上含有させることが好ましい。しかし、Ｂの含有量が増えると深絞り性が低下し、特に０．００５％を超える深絞り性の低下が著しくなる。したがって、Ｂの含有量を０〜０．００５％とした。
【００７３】
有効Ｔｉ：
Ｔｉは、Ｃ、Ｎ及びＳと結合するので、窒化処理用鋼材の場合にはＴｉの含有量に加えて前記 （１）式で表される有効Ｔｉ量も適正化する必要がある。すなわち、延性改善のための低温熱処理における強度の低下を抑制するためには、有効Ｔｉを０．０４％以上とする必要がある。一方、有効Ｔｉが０．１０％を超えると強度が高くなりすぎて脆性的になる。したがって、窒化処理前である窒化処理用鋼材の有効Ｔｉを０．０４〜０．１０％とした。有効Ｔｉは０．０６〜０．１０％とすることが好ましく、０．０７〜０．１０％とすれば更に好ましい。
【００７４】
前記（１）及び（２）の発明に係る窒化処理用鋼材が含有するＦｅと不純物以外の成分元素の規定は、上記のＣから有効Ｔｉまでである。又、前記（３）の発明に係る窒化処理鋼材が含有するＦｅと不純物以外の成分元素の規定は、上記のＣからＢまでである。
【００７５】
上述の化学組成を有する（１）及び（２）の発明に係る窒化処理用鋼材、並びに、（３）の発明に係る窒化処理鋼材は、通常の方法で溶製された後、鋳型に注入する「造塊法」又は「連続鋳造法」のいずれの手段を用いて鋼塊とされたものを素材としてもよい。
（Ｂ）窒化処理鋼材のＨｖ硬さ
窒化処理後の強度が低い場合は、高強度鋼板と比較してわざわざ窒化処理を行う利点が薄れてしまう。そこで、引張強さが７８０ＭＰａ級以上の鋼板を使用した部材（部品）と同等以上の強度特性を確保させるために、（３）の発明に係る窒化処理鋼材の板厚中心のＨｖ硬さを２００以上とした。この板厚中心のＨｖ硬さが過度に高くなると、微細窒化物を制御したとしても伸びが低くなることがあるので、Ｈｖ硬さの上限は４００とすることが好ましい。
【００７６】
窒化処理の際、Ｎは鋼材の表面から内部に拡散して行くため、窒化処理時間が短い場合には、化合物層を除いた表面部の硬さが板厚中心の硬さより大きい硬度分布となるが、表面部の硬さが大きすぎると、硬さの差が大きい界面を起点として破壊が生じることがあるので、表面部と板厚中心のＨｖ硬さの差は２００以下とすることが好ましい。なお、上記の表面部のＨｖ硬さは、鋼材表面から板厚の１０％の深さにおける部位の硬さをいう。
（Ｃ）窒化処理鋼材の板厚中心領域の組織中に存在する粗大窒化鉄：
粗大窒化鉄が析出すると、素地であるフェライト中の固溶Ｎが消費されるのでフェライトの硬さが低下する。一方、粗大な析出物は析出強化能が弱いため、析出強化も期待できず、いたずらに強度が低下するばかりである。窒化処理鋼材に高い強度を付与するには、板厚中心領域の組織中に存在する針状の粗大窒化鉄を１×１０^−８ｍ^２ 当たり１０個以下にする必要がある。
【００７７】
このため、（３）の発明に係る窒化処理鋼材では、板厚中心領域の組織中に存在する粗大窒化鉄を１×１０^−８ｍ^２ 当たり１０個以下とした。この粗大窒化鉄は、Ｎの拡散が速い高温域で析出・成長するものであり、全く存在しないこと（すなわち上記１×１０^−８ｍ^２ 当たり０個であること）が好ましい。
【００７８】
なお、既に述べたように、粗大窒化鉄とはナイタールでエッチングした板厚中心領域の断面を倍率５００倍の光学顕微鏡で観察した場合に認められる析出物のうち、長辺が５μｍ以上のものをいい、必ずしも、純粋な窒化鉄と同定されるものを指すわけではなく、「鉄と窒素とを主成分とする析出物」をいう。
（Ｄ）窒化処理鋼材の板厚中心に存在する微細窒化鉄：
微細窒化鉄の析出も素地であるフェライト中の固溶Ｎが消費されることによるフェライトの硬さ低下を招くが、その一方では析出強化による強化作用が働き、平均としての硬さの低下は緩和される。又、固溶Ｎの減少により素地であるフェライトの延性が改善されるので、積極的に微細窒化鉄を析出させることが必要である。
【００７９】
窒化鉄があまりにも微細で長辺が０．１５μｍ未満の場合には、析出強化が強くなりすぎるため却ってフェライトの脆化を招く。一方、窒化鉄の長辺が５μｍ以上の場合には、十分な強化効果や延性改善効果が得られない。したがって、微細窒化鉄のサイズは長辺が０．１５μｍ以上で５μｍ未満とする必要がある。しかも、窒化処理鋼材の板厚中心に上記長辺が０．１５μｍ以上で５μｍ未満の微細窒化鉄が１×１０^−１２ｍ^２当たり５個以上存在していなければ、顕著な強度低下抑制効果や延性改善効果が得られない。
【００８０】
このため、（３）の発明に係る窒化処理鋼材では、板厚中心に長辺が０．１５μｍ以上で５μｍ未満の微細窒化鉄が１×１０^−１２ｍ^２当たり５個以上存在することとした。上記サイズの微細窒化鉄の１×１０^−１２ｍ^２当たりの個数の上限は特に規定するものではなく、個数が増えるほど強度が低下するので、Ｈｖ硬さで２００以上の硬さが得られる個数でありさえすればよい。
【００８１】
なお、既に述べたように、本発明でいう微細窒化鉄とは、板厚中心から採取した薄膜試料を加速電圧２００ｋＶの透過型電子顕微鏡で観察した場合に認められる析出物のうち、長辺が０．１５μｍ以上５μｍ未満で、しかもＥＤＸ分析でＦｅ以外の金属元素の析出物中への濃化が認められないものをいい、必ずしも、純粋な窒化鉄と同定されるものを指すわけではなく、「鉄と窒素とを主成分とする析出物」をいう。更に、長辺が０．１５μｍ未満の上記意味の「微細窒化鉄」が、転位にそって連珠状に析出した場合、低倍率の観察では長さ０．１５μｍ以上のひも状に見えることがあるが、これは長辺が０．１５μｍ以上とはみなさないことも既に述べたとおりである。
（Ｅ）窒化処理鋼材の表面に形成される化合物層の厚さ
自動車部品には塗装の下地処理として化成処理が施されるが、化合物層が厚くなると化成処理液と地鉄との電気化学反応が阻害され、化成処理が不十分となって塗装後の耐食性が低下する。又、化合物層は地鉄より硬くて脆いため、化合物層が厚いと剥離しやすくなる。特に、化合物層の厚さが３０μｍを超えると、化成処理が不十分となって塗装後の耐食性が低下したり、化合物層が剥離したりすることがあるため、化合物層の厚さは３０μｍ以下とするのがよい。
【００８２】
したがって、（４）の発明においては、表面に形成される化合物層の厚さを３０μｍ以下とした。なお、表面に形成される化合物層の厚さは２５μｍ以下とすることが一層好ましい。この化合物層の厚さの下限は特に限定しないが、窒化処理によって不可避的に形成されるものであることと、化合物層自体が耐食性を有することから、２μｍ程度とするのがよい。
【００８３】
なお、窒化処理鋼材の板厚が大きい場合には、板厚中心までＮが拡散するのに長時間を要するようになり、それに伴って表面に形成される化合物層も厚くなる。したがって、化合物層の発達を抑制しながら、Ｎを板厚中心まで拡散させるために、後述する窒化処理に引き続いて、非窒化雰囲気中で等温保持を行ってもよい。
（Ｆ）窒化処理用鋼材の熱間圧延、巻き取り、冷間圧延及び焼鈍
上述した（Ａ）項のＣから有効Ｔｉまでの規定を満たし残部がＦｅ及び不純物からなる化学組成を有する鋼塊又は鋼片の熱間圧延条件は特に規定するものではなく、通常の方法でよい。更に、熱間圧延の温度範囲は、オーステナイト域でもフェライト域でも構わない。但し、熱間圧延の仕上げ温度が８５０℃を下回ると、熱間圧延中にフェライト変態を生じるため、熱間圧延鋼材の組織が粗粒化して好ましい集合組織が得られない。したがって、深絞り性が要求される場合には、熱間圧延の仕上げ温度を８５０℃以上とすることが好ましい。なお、後述する冷間圧延及び焼鈍の後で、深絞り性に優れた再結晶集合組織を得るためには、熱間圧延した鋼材を細粒化することが有効なため、熱間圧延終了直後から水冷して粒成長を抑制することが好ましい。
【００８４】
熱間圧延後の巻き取りは、ＴｉＣの析出を促進して固溶Ｃ量を減少させるために５００℃以上で行うのがよい。一方、巻き取り温度が高くなり、特に、７００℃を超えると、軟質化して巻き姿が崩れたり、スケールが増加したりすることがあるので、巻き取り温度は７００℃以下とするのがよい。
【００８５】
したがって、（２）の発明においては、熱間圧延後の巻き取り温度を７００〜５００℃と規定した。
【００８６】
上記の温度で巻き取った後は、通常の酸洗によって熱間圧延鋼材のスケールを除去し、更に、冷間圧延を行えばよい。後述の焼鈍後に深絞り性に優れた再結晶集合組織を得るためには、冷間圧延の総圧下率を７０〜９０％とするのがよい。
【００８７】
したがって、（２）の発明においては、総圧下率で７０〜９０％の冷間圧延を施すこととした。
【００８８】
冷間圧延後は、通常の方法によって焼鈍処理すればよい。焼鈍方法は、連続焼鈍でも箱焼鈍でもかまわないが、焼鈍後に深絞り性に優れた再結晶集合組織を得るために、焼鈍温度を６５０〜８８０℃とするのがよい。
【００８９】
したがって、（２）の発明においては、焼鈍温度を６５０〜８８０℃と規定した。焼鈍温度は、箱焼鈍の場合には６５０〜７５０℃、連続焼鈍の場合には７５０〜８８０℃とすることが一層好ましい。
【００９０】
なお、深絞り性が要求されない場合には、上述の冷間圧延及び焼鈍を省略し、熱間圧延後にスケールを除去しただけで、後述の成形及び／又は窒化処理に供してもよい。
（Ｇ）成形加工
鋼材から所定形状の部材（部品）を得るための成形加工方法は、プレス成形、曲げ成形といった塑性加工や切削を初めとする機械加工など手段を問わない。塑性加工を行ってから窒化処理して強化することで、優れた成形性と高強度とを両立させることが可能である。
【００９１】
なお、本発明に係る窒化処理用鋼材のｒ値が高い場合には、窒化処理後の窒化処理鋼材も高いｒ値を示す。このため、窒化処理を行った後に成形する、又は、成形後に窒化処理を行い再度成形する、という工程を採ることも可能である。
【００９２】
したがって、（６）の発明においては、後述する窒化処理の前に成形加工を行うこととした。又、（７）の発明においては、後述する窒化処理と低温熱処理の後に成形加工を行うこととした。
【００９３】
なお、テーラードブランク技術を用い、板厚の異なる鋼板を組み合わせた成形体や、本発明に係る窒化強化に適した鋼板と一般の鋼板を組み合わせた成形体を作り、強度と軽量化を最適化することが可能である。又、部分的にマスキングを施してから窒化処理を行い、部分的に窒化させないことも可能である。例えば、設計者の意図した部材の強度分布を実現するために部分的に窒化していない軟質な部分を配置したり、溶接性の向上の観点から溶接箇所の窒化を避けるといったことも行える。
（Ｈ）窒化処理
窒化処理は、例えば、ガス窒化法、イオン窒化法、ガス軟窒化法や塩浴窒化法など一般に用いられる方法で行えばよい。
【００９４】
窒化処理の温度が高いほどＮの拡散が速くなり、窒化処理時間を短縮できる。しかし、窒化処理温度が６５０℃を超えるとオーステナイトが生成され、窒化処理後の冷却時にオーステナイトからフェライト又はマルテンサイトへの変態が生じ、変態歪みを生じることがある。一方、窒化処理の温度が低いほど鋼中へのＮの固溶限が小さくなり、且つ、鋼中でのＮの拡散が遅くなる。特に、窒化処理温度が５３０℃未満では板厚中心まで窒化できず、且つ、表層近傍の窒化された部位の硬さが低くなることがある。このため、窒化処理は５３０〜６５０℃で行うのがよい。
【００９５】
したがって、（８）の発明においては、窒化処理温度を５３０〜６５０℃とした。なお、Ｎの最大固溶限の温度近傍で窒化処理することが最も効率がよいため、窒化処理の温度は５５０〜６００℃とすることが一層好ましい。
【００９６】
窒化処理後の冷却速度は、できるだけ大きくするのがよい。窒化処理温度からの冷却過程では、Ｎは過飽和状態にあるので、Ｎの拡散速度が大きい温度域においては、粗大な窒化鉄が急速に析出し、固溶Ｎ量が減少することがあるからである。この固溶Ｎ量の減少を防ぐためには、窒化処理温度から２００℃までを４．５℃／秒以上の平均冷却速度で冷却するのがよい。
【００９７】
したがって、（８）の発明においては、５３０〜６５０℃の温度域で窒化した後に平均冷却速度４．５℃／秒以上で２００℃まで冷却することとした。前記の平均冷却速度は、１０℃／秒以上であれば更によい。
【００９８】
なお、上述の平均冷却速度の上限は特に規定するものではなく、窒化処理鋼材のサイズ及び設備面から得られる最大の冷却速度であっても構わない。
（Ｉ）低温熱処理
窒化処理後に１００〜２００℃の温度域で１０分以上保持する低温熱処理を施して析出物の状態を制御すれば伸び特性が大幅に改善されて、窒化処理鋼材に高強度と伸び特性とを兼備させることができる。
【００９９】
すなわち、温度が１００℃未満の場合や保持時間が１０分未満の場合には、Ｎの拡散距離が短いため窒化鉄の析出が不十分となって延性改善の効果が得られず、温度が２００℃を超えると軟化が顕著となって窒化処理鋼材を強化するという目的が達せられない。
【０１００】
このため、（１）及び（２）の発明に係る窒化処理用鋼材は窒化処理後に１００〜２００℃の温度域で１０分以上保持する低温熱処理を施す用途に供されることを必須の条件とした。又、（５）〜（８）の発明においては、窒化処理の後に１００〜２００℃の温度域で１０分以上保持する低温熱処理を施す工程を含むこととした。
【０１０１】
窒化処理後に施す低温熱処理は、窒化処理された鋼材つまり窒化処理鋼材を一旦室温まで冷却してから再加熱して行ってもよいし、窒化処理後の冷却過程において行ってもよい。なお、窒化処理鋼材、なかでも窒化処理部品を塗装する場合には、上記１００〜２００℃の温度域で１０分以上保持する低温熱処理を塗装の乾燥工程と兼用すれば工程を追加する必要がなく経済的である。
【０１０２】
以下、実施例により本発明を更に詳しく説明する。
【０１０３】
【実施例】
（実施例１）
表３に示す化学組成を有する鋼を実験室にて溶解し、各鋼塊を通常の方法で熱間鍛造して厚さ２０ｍｍの鋼片とした。次いで、上記の各鋼片を１２００℃で３０分加熱した後、圧延仕上げ温度を９２０℃とする熱間圧延を行って厚さ４．５ｍｍの鋼板とした。なお、圧延終了後は直ちに水冷を行い、６５０℃まで冷却した後、その温度で３０分保持し、その後２０℃／時の平均冷却速度で室温まで徐冷して巻き取り処理を模擬した。
【０１０４】
【表３】

【０１０５】
上記のようにして得た熱間圧延鋼板を通常の方法で酸洗してスケールを除去した後、通常の方法で総圧下率が８２．２％となる冷間圧延を行って厚さ０．８ｍｍとした。次いで、上記の各冷間圧延鋼板を１０℃／秒の平均加熱速度で加熱して８２０℃で３０秒保持した後、１０℃／秒の平均冷却速度で室温まで冷却し、連続焼鈍を模擬した。更に、伸び率０．１％の調質圧延を行った後、５８０℃で３時間のガス軟窒化処理を施した。その後、１７０℃で２０分保持する低温熱処理を行った。なお、ガス軟窒化処理の条件は、ＮＨ_３ ガスとＲＸガス（吸熱性）との混合ガス（体積比で、ＮＨ_３ ガス：ＲＸガス＝１：１）の雰囲気中で行い、窒化処理後は油漕中に浸漬して冷却した。この冷却における５８０℃から２００℃までの平均冷却速度は、２０℃／秒であった。
【０１０６】
窒化処理前の鋼板及び低温熱処理後の鋼板について引張特性を調査し、低温熱処理後の鋼板についてはＨｖ硬さも調査した。
【０１０７】
すなわち、窒化処理前及び低温熱処理後の各鋼板から圧延方向が引張方向となるようにＪＩＳ１３号Ｂ引張試験を採取して常温で引張特性を調査した。又、低温熱処理後の各鋼板の板厚中心と鋼板表面から板厚の１０％の深さにおける部位の計２ヶ所のＨｖ硬さを試験力９．８Ｎで測定した。
【０１０８】
表４に、窒化処理前及び低温熱処理後の各鋼板の引張特性及び低温熱処理後の各鋼板の板厚中心のＨｖ硬さを示す。なお、いずれの鋼板においても、板厚中心のＨｖ硬さと鋼板表面から板厚の１０％の深さの部位におけるＨｖ硬さの差は５０以下であった。このことから、各鋼板はいずれも板厚方向に一様に窒化されていると判断された。
【０１０９】
【表４】

【０１１０】
Ｃ及びＮは、窒化処理によって含有量が変化するので、窒化処理後の鋼板における平均化学組成の分析を行った。なお、分析の際には鋼板の両表面から０．０８ｍｍ（板厚の１０％に相当）ずつを機械研削して、鋼板表面に形成されるＮ濃度が非常に高い化合物層の影響を除いてから分析に供した。なお、ＣとＮ以外の含有量は窒化処理前のものから変化していなかった。
【０１１１】
又、低温熱処理後の各鋼板についても鋼板の両表面から０．０８ｍｍ（板厚の１０％に相当）ずつを機械研削した後、鋼板断面を鏡面研磨してナイタールでエッチングし、倍率を５００倍として光学顕微鏡で観察した。又、板厚中心から採取した薄膜試料を加速電圧２００ｋＶの透過型電子顕微鏡を用いて観察した。なお、光学顕微鏡観察では、いずれの鋼板においても長辺が５μｍ以上の粗大窒化鉄はほとんど認められず、１×１０^−８ｍ^２ 当たり１個未満であった。
【０１１２】
表４に、窒化処理後におけるＣ及びＮの分析結果、並びに、加速電圧２００ｋＶの透過型電子顕微鏡を用いた長辺が０．１５μｍ以上で５μｍ未満の微細窒化鉄の観察結果を併せて示す。
【０１１３】
表４から明らかなように、本発明で定める化学組成を有する鋼Ｅ〜Ｇ及び鋼Ｋ〜Ｍの窒化処理前の鋼板は、降伏点及び引張強さが低く、伸びが大きい。
【０１１４】
更に、本発明で定める平均化学組成及び板厚中心のＨｖ硬さ、並びに、粗大窒化鉄及び微細窒化鉄の個数を満たす上記の鋼Ｅ〜Ｇ及び鋼Ｋ〜Ｍの各鋼板を窒化処理後に低温熱処理した鋼板の場合には、７８０ＭＰａ級以上の高い引張強さと１０％を超える大きな伸びを確保できていることも明らかである。
【０１１５】
これに対して、窒化処理前の有効Ｔｉが本発明で規定する条件から外れた鋼Ｈの鋼板を窒化処理後に低温熱処理した場合には、窒化後の平均化学組成は満たすものの長辺が０．１５μｍ以上で５μｍ未満の微細窒化鉄の個数が本発明の規定から外れ、かわりに長辺が０．０３〜０．０５μｍの極めて微細な窒化鉄が１×１０^−１２ｍ^２当たり約７０個と非常に多く存在したため、延性が非常に低く加工硬化を示さずに破断した。なお、ｒ値は測定することができなかった。
【０１１６】
窒化処理前の有効Ｔｉ、Ｃ及びＴｉが本発明で規定する条件から外れた鋼Ｊの鋼板を窒化処理後に低温熱処理した場合には、長辺が０．１５μｍ以上で５μｍ未満の微細窒化鉄の個数が本発明の規定から外れ、かわりに長辺が０．０３〜０．０５μｍの極めて微細な窒化鉄が１×１０^−１２ｍ^２当たり約７０個と非常に多く存在し、更に、窒化後の平均化学組成におけるＴｉ含有量も本発明で規定する条件から外れるため、延性が非常に低く加工硬化を示さずに破断した。この場合もｒ値は測定することができなかった。
【０１１７】
有効Ｔｉ及びＳが本発明で規定する条件から外れた鋼Ｉの窒化処理前の鋼板にはＴｉＳが多量に析出しており、その伸びは小さかった。又、鋼Ｉの鋼板を窒化処理後に低温熱処理した場合には、窒化後の平均化学組成におけるＳ含有量が本発明で規定する条件より多いので固溶Ｔｉが減少してしまい、板厚中心のＨｖ硬さが本発明で規定する範囲よりも低くなってしまうため、７８０ＭＰａ級以上という所望の強度レベルが確保できなかった。
（実施例２）
実施例１で製造した鋼Ｇの伸び率０．１％の調質圧延を行った冷間圧延鋼板に、実施例１と同じＮＨ_３ ガスとＲＸガスとの混合ガス雰囲気中で５８０℃で３時間のガス軟窒化処理を施した。その後、１７０℃で２０分保持する低温熱処理を行った。なお、窒化処理後は油漕中に浸漬して冷却したが、油漕に浸漬して冷却するまでの時間を変えることにより、窒化処理温度から２００℃までの平均冷却速度を変化させた。
【０１１８】
低温熱処理後の各鋼板について、前記実施例１におけると同様にして、引張特性調査、Ｈｖ硬さ測定、倍率を５００倍とした光学顕微鏡観察及び加速電圧２００ｋＶの透過型電子顕微鏡を用いた薄膜観察を行った。又、窒化処理後の鋼板における平均化学組成の分析も行った。
【０１１９】
表５に、低温熱処理後の各鋼板の引張特性、板厚中心のＨｖ硬さ、倍率５００倍の光学顕微鏡を用いた長辺が５μｍ以上の粗大窒化鉄の観察結果及び加速電圧２００ｋＶの透過型電子顕微鏡を用いた長辺が０．１５μｍ以上で５μｍ未満の微細窒化鉄の観察結果を示す。なお、いずれの鋼板においても、板厚中心のＨｖ硬さと鋼板表面から板厚の１０％の深さの部位におけるＨｖ硬さの差は５０以下であった。このことから、各鋼板はいずれも板厚方向に一様に窒化されていると判断された。
【０１２０】
表５には窒化処理後におけるＣ及びＮの分析結果も併記した。なお、ＣとＮ以外の含有量は窒化処理前のものから変化していなかった。
【０１２１】
【表５】

【０１２２】
表５から、本発明で定める平均化学組成及び板厚中心のＨｖ硬さ、並びに、粗大窒化鉄及び微細窒化鉄の個数を満たす試験記号Ｇ−１及びＧ−２の場合には、７８０ＭＰａ級以上の高い引張強さと１０％を超える大きな伸びを確保できていることが明らかである。
【０１２３】
これに対して、本発明で定める平均化学組成、板厚中心のＨｖ硬さ及び微細窒化鉄の個数を満たしていても、粗大窒化鉄の個数が本発明の規定から外れた試験記号Ｇ−３及びＧ−４の場合には、強度の低下が大きい。したがって、窒化処理温度から２００℃までの平均冷却速度は４．５℃／秒以上が好ましいことがわかる。
（実施例３）
実施例１で製造した鋼Ｇの伸び率０．１％の調質圧延を行った冷間圧延鋼板に、実施例１と同じＮＨ_３ ガスとＲＸガスとの混合ガス雰囲気中で５８０℃での処理時間を変えてガス軟窒化処理を施した。なお、表面の化合物層に含まれるＮを鋼板内部へ拡散させて化合物層を薄くするために、上記のＮＨ_３ ガスとＲＸガスとの混合ガス雰囲気中で５８０℃で３時間の窒化処理を行った後、雰囲気を１００％Ｎ_２ ガスに変え、同じ５８０℃で１時間保持する処理も行った。いずれの条件で窒化処理したものも窒化処理後は油漕中に浸漬して冷却した。この冷却における５８０℃から２００℃までの平均冷却速度は、２０℃／秒であった。
【０１２４】
更に通常の方法で化成処理と塗装を施して複合腐食サイクル試験に供し、塗装後の耐食性を調査した。
【０１２５】
すなわち、表面を脱脂した後にリン酸亜鉛系の化成処理を施し、その後市販のカチオン電着塗料を約２５μｍの厚さで塗装した後、１７５℃で２５分間の塗装焼き付けを兼用した低温熱処理を施した試験片に「塩水噴霧→乾燥→湿潤」が１サイクル（２４時間）となる複合腐食サイクル試験を行い、２７０サイクル後の腐食深さを測定した。
【０１２６】
なお、上記複合腐食サイクル試験に供した試験片の調査対象面を１０分割して各分割ごとの最大腐食深さを測定した後、上位５点の平均値を腐食深さとした。
【０１２７】
又、塗装・焼き付け後の各鋼板について、塗装を除去後、前記実施例１におけると同様にして、引張特性調査、Ｈｖ硬さ測定、倍率を５００倍とした光学顕微鏡観察及び加速電圧２００ｋＶの透過型電子顕微鏡を用いた薄膜観察を行った。なお、光学顕微鏡観察では、いずれの鋼板においても長辺が５μｍ以上の粗大窒化鉄はほとんど認められず、１×１０^−８ｍ^２ 当たり１個未満であった。又、窒化処理後の鋼板について、平均化学組成分析を行うとともに倍率が５００倍の光学顕微鏡で観察して表面に形成された化合物層の厚さの測定を行った。
【０１２８】
表６に、低温熱処理後の各鋼板の引張特性、板厚中心のＨｖ硬さ、倍率５００倍の光学顕微鏡を用いた長辺が５μｍ以上の粗大窒化鉄の観察結果、加速電圧２００ｋＶの透過型電子顕微鏡を用いた長辺が０．１５μｍ以上で５μｍ未満の微細窒化鉄の観察結果及び複合腐食サイクル試験による耐食性調査結果を示す。表６の耐食性の欄における「◎」、「○」及び「△」それぞれ腐食深さが５０μｍ以下の場合で「非常に良好」、５０μｍを超えて１００μｍ以下の場合で「良好」及び１００μｍを超えて２００μｍ以下の場合で「やや劣る」を意味する。耐食性の目標は腐食深さが１００μｍ以下の場合の「◎」及び「○」とした。
【０１２９】
なお、いずれの鋼板においても、板厚中心のＨｖ硬さと鋼板表面から板厚の１０％の深さの部位におけるＨｖ硬さの差は５０以下であった。このことから、各鋼板はいずれも板厚方向に一様に窒化されていると判断された。
【０１３０】
表６には窒化処理後におけるＣ及びＮの分析結果、並びに倍率が５００倍の光学顕微鏡を用いた化合物層の厚さ測定結果も併記した。なお、ＣとＮ以外の含有量は窒化処理前のものから変化していなかった。
【０１３１】
【表６】

【０１３２】
表６から、本発明で定める平均化学組成及び板厚中心のＨｖ硬さ、並びに、粗大窒化鉄及び微細窒化鉄の個数を満たす試験記号Ｇ−５〜Ｇ−１０のいずれの場合にも７８０ＭＰａ級以上の高い引張強さと１０％を超える大きな伸びを確保できている。しかし、表面に形成された化合物層の厚さが３０μｍを超える試験記号Ｇ−９の場合には耐食性の評価が「△」であり、自動車部品として適用する場合、腐食環境が厳しくない部位に限定されてしまう。
【０１３３】
なお、比較のために、実施例１で製造した鋼Ｇの伸び率０．１％の調質圧延を行った冷間圧延鋼板にそのまま、つまり、ガス軟窒化処理を施さずに、前記の塗装・焼き付け処理を施して複合腐食サイクル試験し、耐食性を調査することも行った。その耐食性調査結果は、腐食深さが２００μｍを超える「劣る」ものであった。
（実施例４）
実施例１で製造した鋼Ｇの伸び率０．１％の調質圧延を行った冷間圧延鋼板に、実施例１と同じＮＨ_３ ガスとＲＸガスとの混合ガス雰囲気中で５８０℃で３時間のガス軟窒化処理を施し、窒化処理後は油漕中に浸漬して冷却した。この冷却における５８０℃から２００℃までの平均冷却速度は、２０℃／秒であった。この後は、温度と保持時間を変化させて熱処理を行った。
【０１３４】
熱処理後の各鋼板について、前記実施例１におけると同様にして、引張特性調査、Ｈｖ硬さ測定、倍率を５００倍とした光学顕微鏡観察及び加速電圧２００ｋＶの透過型電子顕微鏡を用いた薄膜観察を行った。なお、光学顕微鏡観察では、いずれの鋼板においても長辺が５μｍ以上の粗大窒化鉄はほとんど認められず、１×１０^−８ｍ^２ 当たり１個未満であった。窒化処理後の鋼板における平均化学組成の分析も行った。
【０１３５】
表７に、熱処理後の各鋼板の引張特性、板厚中心のＨｖ硬さ及び加速電圧２００ｋＶの透過型電子顕微鏡を用いた長辺が０．１５μｍ以上で５μｍ未満の微細窒化鉄の観察結果を示す。なお、いずれの鋼板においても、板厚中心のＨｖ硬さと鋼板表面から板厚の１０％の深さの部位におけるＨｖ硬さの差は５０以下であった。このことから、各鋼板はいずれも板厚方向に一様に窒化されていると判断された。
【０１３６】
表７には窒化処理後におけるＣ及びＮの分析結果も併記した。なお、ＣとＮ以外の含有量は窒化処理前のものから変化していなかった。
【０１３７】
【表７】

【０１３８】
表７から、本発明で定める平均化学組成及び板厚中心のＨｖ硬さ、並びに、粗大窒化鉄及び微細窒化鉄の個数を満たす試験記号Ｇ−１２〜Ｇ−１４及びＧ−１６の場合には、７８０ＭＰａ級以上の高い引張強さと１０％を超える大きな伸びを確保できている。但し、試験番号Ｇ−１４の場合は、熱処理の温度が２２０℃と本発明の「低温熱処理」の規定から外れるため微細窒化鉄の個数がやや多くなっており、このため軟化の程度が大きい。したがって、窒化処理後に行う熱処理は、１００〜２００℃の温度域で１０分以上保持する低温熱処理とするのが好ましいことがわかる。
【０１３９】
一方、本発明で定める平均化学組成、板厚中心のＨｖ硬さ及び粗大窒化鉄の個数を満たしていても、微細窒化鉄の個数が本発明の規定から外れた試験記号Ｇ−１１及びＧ−１５の場合には、伸びはそれぞれ２．１％と３．１％で著しく低いことが明らかである。なお、これらの場合には伸びが小さいためｒ値は測定することができなかった。
（実施例５）
実施例１で製造した鋼Ｇの伸び率０．１％の調質圧延を行った冷間圧延鋼板を、しわ押さえ圧とダイス肩Ｒの条件を変えることで縦壁の歪み量を変化させてハット形状にプレス成形した。
【０１４０】
プレス成形後に、実施例１と同じＮＨ_３ ガスとＲＸガスとの混合ガス雰囲気中で５８０℃で３時間のガス軟窒化処理を施し、窒化処理後は油漕中に浸漬して冷却した。この冷却における５８０℃から２００℃までの平均冷却速度は、２０℃／秒であった。この後は、１５０℃で２０分保持する低温熱処理を行った。なお、プレス成形しないで上記の窒化処理を施した前記冷間圧延鋼板にも１５０℃で２０分保持する低温熱処理を行った。
【０１４１】
このようにして得たハット形状部材の縦壁部及び冷間圧延鋼板について、前記実施例１におけると同様にして、Ｈｖ硬さ測定、倍率を５００倍とした光学顕微鏡観察及び加速電圧２００ｋＶの透過型電子顕微鏡を用いた薄膜観察を行った。なお、光学顕微鏡観察では、いずれのハット形状部材と冷間圧延鋼板にも長辺が５μｍ以上の粗大窒化鉄はほとんど認められず、１×１０^−８ｍ^２ 当たり１個未満であった。窒化処理後の平均化学組成の分析も行った。
【０１４２】
表８に、低温熱処理後の板厚中心のＨｖ硬さ及び加速電圧２００ｋＶの透過型電子顕微鏡を用いた長辺が０．１５μｍ以上で５μｍ未満の微細窒化鉄の観察結果を示す。なお、いずれのハット形状部材と冷間圧延鋼材においても、板厚中心のＨｖ硬さと鋼板表面から板厚の１０％の深さの部位におけるＨｖ硬さの差は５０以下であった。このことから、各ハット形状部材と冷間圧延鋼板はいずれも板厚方向に一様に窒化されていると判断された。表８には窒化処理後におけるＣ及びＮの分析結果も併記した。なお、ＣとＮ以外の含有量は窒化処理前のものから変化していなかった。
【０１４３】
【表８】

【０１４４】
表８から、試験記号Ｇ−１８〜Ｇ−２０のハット形状にプレス成形した場合は、縦壁の歪み量の増加に伴って板厚中心のＨｖ硬さが若干上昇する傾向があるが、Ｃ及びＮの分析値、並びに微細窒化鉄の個数はプレス成形しない場合の試験記号Ｇ−１７と大差ないことが明らかである。
【０１４５】
この結果から、プレス成形した場合にも、プレス成形を行わない鋼板の状態で評価した実施例１〜４と同じ結果が期待できる。
【０１４６】
【発明の効果】
本発明の窒化処理鋼材は高強度と伸び特性とを兼備しているので自動車の構造部材などに利用することができる。この窒化処理鋼材は、本発明の窒化処理用鋼材を用いた本発明の製造方法によって比較的容易に製造することができる。又、本発明の製造方法によれば、例えば従来の高張力鋼板をプレス成形した場合と比べて複雑な成形体を寸法精度良く製造することができる。
【図面の簡単な説明】
【図１】有効Ｔｉが降伏点及び伸びに及ぼす影響を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a nitriding steel material, a nitriding steel material and a method for producing the same, and in particular, for example, a nitriding steel material requiring high strength such as a structural member of an automobile, a nitriding steel material suitable as the material, and It relates to a method for producing them.
[0002]
[Prior art]
BACKGROUND ART Various high-strength steel materials have been used to improve the collision safety and reduce the weight of automobiles. In the following description, “steel” may be described as “steel” as an example.
[0003]
A high-strength steel sheet is often processed into a predetermined shape by a forming process such as a press process. For this reason, high-strength steel sheets are required to have excellent workability. However, in general, the ductility decreases as the strength of the steel sheet increases, so that it becomes difficult to press-form the steel sheet into a complicated shape. Further, even with a simple shape, the elastic recovery (spring back) of the formed body after press forming increases with the strength of the steel sheet, and it becomes difficult to obtain a predetermined shape accuracy. .
[0004]
As a method for achieving both the press formability and the high strength of a compact, there is known a technique of press-forming a soft steel sheet having excellent ductility and deep drawability, followed by quenching or nitriding. However, when quenching a thin steel plate formed by press forming, quenching distortion is increased due to rapid cooling from a high temperature. On the other hand, in the case of nitriding having a lower treatment temperature than quenching, the distortion is small.
[0005]
However, the main purpose of nitriding a steel material in the prior art is to improve wear resistance by a compound layer formed on the surface or to improve fatigue strength by a diffusion layer formed immediately below the compound layer. Therefore, the inside of the steel material was not strengthened by the nitriding treatment, and the inside was generally not strengthened from the viewpoint of brittleness.
[0006]
In contrast to such a conventional nitriding treatment, Patent Literature 1 and Patent Literature 2 use a thin steel plate to nitrify to the center of the plate thickness and strengthen it, so that only the surface, such as the impact strength of a member at the time of collision, is used. Instead, there is disclosed a technique for improving characteristics governed by the strength in the entire thickness direction. However, if the elongation of the steel sheet after nitriding is small, energy absorption due to plastic deformation cannot be expected at the time of collision, and the member breaks. Therefore, in the case of the technologies proposed in these patent documents, it is necessary to increase the strength while securing the elongation characteristics after nitriding.
[0007]
Patent Literature 1 discloses that it is important to control the amount of solute Ti to achieve both the strength after nitriding and the toughness evaluated by elongation. That is, if the amount of solid-solution Ti is small, the strength cannot be obtained, and if the amount of solid-solution Ti is large, the Vickers hardness of the surface layer (hereinafter, Vickers hardness is referred to as Hv hardness) enters a brittle region of 300 or more. It shows that there is an upper limit to the amount of solid solution Ti. However, as is clear from the examples, the technology proposed in this patent document has a tensile strength at which both strength and elongation can be compatible at most is only 710 MPa, and replaces a 780 MPa class high-tensile steel sheet. Is not strong enough.
[0008]
Patent Literature 2 describes that from the viewpoint of weight reduction, it is necessary that the average hardness in the thickness direction be 300 or more in terms of Hv hardness. In this patent document, a technique is proposed in which a hardness difference between a surface portion and an inner center in a plate thickness direction is set to Hv hardness of 200 or less, thereby avoiding a sharp decrease in elongation property due to high strength. I have.
[0009]
As described above, in order to increase the strength by nitriding, it is a problem to achieve compatibility with elongation characteristics after nitriding. However, the above Patent Documents 1 and 2 simply provide both strength and elongation characteristics. It is merely a technique for clarifying the optimum conditions for the improvement, and is not essentially a technique for improving elongation properties and realizing high strength.
[0010]
[Patent Document 1]
JP-A-11-279686
[Patent Document 2]
JP-A-2002-20853
[0011]
[Problems to be solved by the invention]
The present invention has been made in view of the above situation, and its object is to provide a nitrided steel material that is required to have both high strength and elongation characteristics, such as a structural member of an automobile, and is suitable as a material for the nitrided steel material. An object of the present invention is to provide a steel material for nitriding treatment and a method for producing the same.
[0012]
[Means for Solving the Problems]
The gist of the present invention is as follows: (1) a steel material for nitriding treatment; (2) a method for producing a steel material for nitriding treatment; (3) and (5) a steel material for nitriding treatment; To (8).
[0013]
(1) In mass%, C: 0.01% or less, Si: 0.001 to 0.5%, Mn: 0.01 to 0.5%, P: 0.001 to 0.1%, S: 0.01% or less, Al: 0.001 to 0.5%, Ti: 0.04 to 0.2%, Nb: 0 to 0.05%, B: 0 to 0.005%, N: 0. 005% or less, and the balance consists of Fe and impurities, and the effective Ti represented by the following formula (1) is 0.04 to 0.10%. Nitriding steel used for low-temperature heat treatment for 10 minutes or more in the temperature range.
[0014]
Effective Ti = Ti − {(N / 14) + (S / 32) + (C / 12)} × 48 (1). Here, the symbol of the element on the right side of the equation (1) indicates the content of the element in steel in mass%.
[0015]
(2) In mass%, C: 0.01% or less, Si: 0.001 to 0.5%, Mn: 0.01 to 0.5%, P: 0.001 to 0.1%, S: 0.01% or less, Al: 0.001 to 0.5%, Ti: 0.04 to 0.2%, Nb: 0 to 0.05%, B: 0 to 0.005%, N: 0. 005% or less, and the balance consists of Fe and impurities, and the effective ingot represented by the following formula (1) is 0.04 to 0.10%. The steel material for nitriding treatment according to the above (1), wherein the steel material is wound at a temperature of up to 500 ° C., further cold-rolled at a total reduction of 70 to 90%, and then annealed at 650 to 880 ° C. Manufacturing method.
[0016]
Effective Ti = Ti − {(N / 14) + (S / 32) + (C / 12)} × 48 (1). Here, the symbol of the element on the right side of the equation (1) indicates the content of the element in steel in mass%.
[0017]
(3) The plate thickness center regions each ranging from the plate thickness center to the plate surface up to 40% of the plate thickness are, by mass%, C: 0.03% or less, and Si: 0.001 to 0.5. %, Mn: 0.01 to 0.5%, P: 0.001 to 0.1%, S: 0.01% or less, Al: 0.001 to 0.5%, Ti: 0.04 to 0 0.2%, Nb: 0 to 0.05%, B: 0 to 0.005%, N: 0.08 to 0.25%, the balance being an average chemical composition of Fe and impurities, and Vickers hardness at the thickness center is 200 or more, and coarse iron nitride having a long side of 5 μm or more in the thickness center region is 1 × 10 ^-8 m ² 1 × 10 fine iron nitride with a long side at the center of the plate thickness of 0.15 μm or more and less than 5 μm. ^-12 m ² 5. A nitrided steel material comprising at least 5 pieces per unit.
[0018]
(4) The nitrided steel material according to (3), wherein the thickness of the compound layer formed on the surface is 30 μm or less.
[0019]
(5) A method for producing a nitriding steel material using the nitriding steel material according to the above (1) or the nitriding steel material obtained by the production method according to the above (2), wherein 100% after the nitriding treatment. A method for producing a nitrided steel material, comprising a step of performing a low-temperature heat treatment of maintaining the temperature in a temperature range of 200 to 200 ° C. for 10 minutes or more.
[0020]
(6) The method according to (5), wherein the nitriding treatment is performed after forming the nitriding treatment steel material according to (1) or the nitriding treatment steel material obtained by the manufacturing method according to (2). A method for producing a nitrided steel material according to the above.
[0021]
(7) Nitriding treatment and subsequent temperature range of 100 to 200 ° C. without forming the nitriding treatment steel material according to (1) or the nitriding treatment steel material obtained by the production method according to (2) above. A nitriding steel material subjected to a low-temperature heat treatment for 10 minutes or more, or a nitriding steel material obtained by the manufacturing method according to the above (6), and further performing a forming process. .
[0022]
(8) The above (5) to (7), wherein the nitriding treatment is performed by nitriding in a temperature range of 530 to 650 ° C. and then cooling to 200 ° C. at an average cooling rate of 4.5 ° C./sec or more. The method for producing a nitrided steel material according to any one of the above.
[0023]
The term “nitriding steel” used in the present invention is a steel used as a material of the later-described “nitriding steel”, that is, processed into a required shape by various methods such as rolling, forging, drawing or casting, and subjected to nitriding. Steel.
[0024]
"Nitrided steel" refers to steel that has been subjected to nitriding treatment.In addition to steel sheets that have been subjected to nitriding treatment, they are subjected to nitriding treatment after being formed into a predetermined shape by forming such as press working. Included members (parts).
[0025]
The total rolling reduction in “% unit” in cold rolling is represented by ｛(thickness of material to be rolled before cold rolling−thickness of material to be rolled after cold rolling) / (material to be rolled before cold rolling) Rolled material thickness) 値 × 100.
[0026]
"Coarse iron nitride" means precipitates observed when the cross section of the plate thickness center region etched with nital is observed with an optical microscope with a magnification of 500 times, and has a long side of 5 μm or more. It does not refer to what is identified as pure iron nitride, but refers to "precipitates containing iron and nitrogen as main components".
[0027]
The term “fine iron nitride” refers to a precipitate having a long side of 0.15 μm or more and less than 5 μm among precipitates observed when a thin film sample taken from the center of the plate thickness is observed with a transmission electron microscope at an accelerating voltage of 200 kV. The analysis does not indicate the concentration of metal elements other than Fe in the precipitate, and does not necessarily mean that the iron element is identified as pure iron nitride. "Precipitate".
[0028]
When the “fine iron nitride” having a long side of less than 0.15 μm and having the above meaning is precipitated in a bead shape along dislocations, it may appear as a string having a length of 0.15 μm or more at low magnification. However, this does not consider that the long side is 0.15 μm or more.
[0029]
The “average cooling rate” in the present invention refers to a value obtained by dividing a temperature difference before and after cooling by a cooling time.
[0030]
Hereinafter, the invention relating to the steel material for nitriding treatment of the above (1), the invention relating to the method for producing the steel material for nitriding treatment of (2), the invention relating to the nitriding treatment steel materials of (3) and (4), and (5) The inventions relating to the method for producing a nitrided steel material of (1) to (8) are respectively referred to as the inventions of (1) to (8).
[0031]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors have studied repeatedly to achieve the above object. As a result, it has been found that there is a case where elongation characteristics can be improved by further performing a heat treatment after the nitriding treatment.
[0032]
Therefore, as a result of further study, it has been clarified that the elongation property can be greatly improved by performing a low-temperature heat treatment after the nitriding treatment to control the state of the precipitate.
[0033]
Hereinafter, the experiment which led to the above knowledge will be described in detail.
[0034]
A cold-rolled steel sheet having a chemical composition shown in Table 1 and having a thickness of 0.8 mm was subjected to a molten salt bath nitriding treatment by a usual method, and then to a low-temperature heat treatment.
[0035]
[Table 1]

[0036]
That is, the cold-rolled steel sheet having a thickness of 0.8 mm was immersed in a molten salt bath at 580 ° C. for 1.5 hours, and then immersed in an oil tank and cooled. The average cooling rate from 580 ° C to 200 ° C was 20 ° C / sec. After this nitriding treatment, a low-temperature heat treatment was further performed at 170 ° C. for 20 minutes.
[0037]
The tensile properties and Hv hardness of the as-nitrided steel sheet and the steel sheet after low-temperature heat treatment were examined.
[0038]
That is, a JIS No. 13B tensile test was taken from each steel sheet, a tensile test was conducted at room temperature to investigate the tensile properties, and an Hv hardness measurement at the center of the plate thickness was performed at a test force of 9.8 N.
[0039]
Table 2 summarizes the results of the above investigation, and FIG. 1 shows the effect of the effective Ti represented by the formula (1) on the yield point and elongation.
[0040]
[Table 2]

[0041]
From Table 1, Table 2 and FIG. 1, the more effective Ti, the higher the yield point in the nitridation treatment, but the elongation decreases. When the effective Ti is 0.07% or more, the elongation becomes 5% or less. It is recognized that. It has been known that when nitriding a so-called “high Ti steel material” having a large Ti content, the elongation becomes extremely small, and applicable structural parts are limited.
[0042]
On the other hand, when the low-temperature heat treatment at 170 ° C. for 20 minutes is performed after the nitriding treatment, it is recognized that the yield point is reduced but the elongation is significantly improved as compared with the case of the nitriding treatment. That is, although the ductility of the steel sheet as it was subjected to the nitriding treatment was low, it was found that elongation (plastic deformability) was recovered by subjecting the steel sheet to low-temperature heat treatment. As described above, when the nitriding steel material whose plastic deformability has been restored is applied to an automobile part or the like, it is possible to prevent the steel material from being brittlely broken at the time of collision.
[0043]
Further, from FIG. 1, it can be seen from FIG. 1 that increasing the effective Ti has the effect of increasing the yield point as it is in the nitriding treatment, and at the same time, reducing the yield point by the low-temperature heat treatment (that is, the “yield point as it is in the nitriding treatment”). "Yield point after low-temperature heat treatment") has been found to have an effect of minimizing it. This means that, for example, even if a steel sheet having a small amount of effective Ti is subjected to nitriding for a long time to increase the amount of N and thereby increase the strength, if a low-temperature heat treatment is performed following the nitriding to recover the elongation, the yield can be reduced. This indicates that the points are significantly reduced.
[0044]
That is, it has been clarified that it is essentially important to increase the effective Ti in order to achieve both high strength and elongation characteristics.
[0045]
Next, in order to investigate the cause of the above-mentioned phenomenon, the structure of the steel sheet C subjected to the nitriding treatment and the low-temperature heat treatment as described above was examined.
[0046]
That is, after the mirror-polished cross section of the steel plate was etched with nital, the magnification was set to 500 times and observed with an optical microscope.
[0047]
As a result, the structure of the steel sheet was made of ferrite regardless of the presence or absence of the low-temperature heat treatment, and a 9 μm-thick compound layer was observed on the surface. The compound layer on the surface has a clear contrast with ferrite, and the ferrite crystal grain boundary at the interface between the compound layer and the ferrite is interrupted. It is possible.
[0048]
In the state of the nitriding treatment, the inside of the ferrite grains was hardly etched, and although the gloss was slightly less than that in the ferrite grains before the nitriding treatment, almost the same observation results were obtained. On the other hand, when the low-temperature heat treatment was performed after the nitriding treatment, the gloss inside the ferrite grains disappeared and the ferrite particles were observed in gray. It is considered that after the low-temperature heat treatment, fine irregularities were uniformly formed in the ferrite grains by etching, and thus the contrast was changed.
[0049]
As described above, although observation with an optical microscope was insufficient, it was found that some low-temperature heat treatment caused some structural change.
[0050]
Then, next, a thin film sample was prepared from the center of the plate thickness, and the structure was observed using a transmission electron microscope at an acceleration voltage of 200 kV.
[0051]
As a result of measuring a sample taken from the as-nitrided steel sheet on a photograph at a magnification of 40,000 times, the main component of the precipitate was a rod-like precipitate having a long side of 0.05 to 0.15 μm, which was 1 × 10 5 ^-12 m ² 38 were found per unit. Very few precipitates having a long side of 0.15 μm or more were obtained. ^-12 m ² 2.4 were found per unit. Further, rod-like precipitates having a long side of 0.05 to 0.15 μm overlapped, and a state in which the apparent long side was 0.15 μm or more was observed. ^-12 m ² The number was 0.5 per unit. It is considered that the above-mentioned rod-like precipitates were deposited preferentially on dislocations.
[0052]
On the other hand, in a sample collected from a steel sheet that has been further subjected to a low-temperature heat treatment after nitriding, a rectangular precipitate having a long side of about 0.2 to 0.6 μm and a short side of about 0.1 μm has a size of 1 × 10 3. ^-12 m ² 8.2 were found per unit. In the case of the sample subjected to the low-temperature heat treatment, almost no precipitate having a long side of less than 0.15 μm was observed.
[0053]
When EDX analysis was performed on the very fine precipitates observed as they were in the nitriding treatment and the fine precipitates observed after the low-temperature heat treatment, the peaks of metal elements other than Fe were 1/10 or less of Fe. Only was detected. From the EDX analysis results, it is estimated that the very fine precipitates observed as they were in the above-mentioned nitriding treatment and the fine precipitates observed after the low-temperature heat treatment are iron nitride.
[0054]
Note that TiN, TiS, and TiC, which were considered to have existed before the nitriding treatment, were also observed, but they differed in shape from iron nitride, and metal elements other than Fe were detected by EDX analysis. Excluded from counting.
[0055]
Since Ti has a strong carbonitride forming ability, it is known that TiN and TiC (or Ti-N clusters or Ti-C clusters) are formed even during the nitriding treatment. Was not done. At a nitriding temperature of 580 ° C., since the diffusion distance of Ti is short, such precipitates and clusters are extremely fine, and it is considered that such precipitates and clusters were not observed by microscopic observation using a transmission electron microscope at an acceleration voltage of 200 kV.
[0056]
The reason why the elongation is remarkably improved with a decrease in the tensile strength is not clear, but it is presumed that if the nitriding treatment is used, the influence of solid solution strengthening by N is too strong and intragranular fracture occurs. On the other hand, when the nitride precipitates and the solid solution strengthening ability is reduced, it is considered that plastic deformation in the ferrite grains becomes possible and ductility is improved.
[0057]
The present invention of the above (1) to (8) has been completed based on the above findings.
[0058]
Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" of the content of each element means "% by mass".
(A) Chemical composition of nitriding steel and nitriding steel
A compound layer is formed on the surface layer of the nitrided steel material subjected to the nitriding treatment, and the chemical composition becomes non-uniform in the thickness direction. The average chemical composition in the central region of the thickness in the range of up to 40% of the thickness, that is, the internal average chemical composition corresponding to 80% of the thickness with respect to the center of the thickness.
[0059]
In addition, regarding the elements that need to distinguish the content between the nitriding steel and the nitriding steel, the case of the nitriding steel is described as “before nitriding”, and the case of the nitriding steel is described as “after nitriding”. I do.
[0060]
C (before nitriding):
In order to obtain a good deep drawability, that is, a crystal texture with a high r-value, it is necessary to add Ti and Nb to form a carbide to reduce the amount of solute C, so-called "IF steel". . When the amount of C is large, the amount of Ti and Nb necessary for fixing the amount increases, and the amount of carbide thereof increases. Therefore, ductility is reduced before nitriding, or after nitriding, They can be a starting point for destruction. Therefore, the C content of the steel for nitriding before the nitriding is set to 0.01% or less. The upper limit of the C content of the nitriding steel is preferably 0.005%, more preferably 0.003%. The lower the content of C, the better the formability. However, even if the content is reduced to less than 0.0005%, an effect commensurate with the steelmaking cost cannot be expected. Therefore, the lower limit is preferably set to 0.0005%.
[0061]
C (after nitriding):
In industrial nitriding processes, C as well as N are often enriched. Since C after nitriding does not greatly affect the effect of the present invention, there is no problem in containing a C level which is inevitably enriched in nitriding. However, if C is enriched far beyond the solid solubility limit of C in ferrite at the nitriding temperature, coarse cementite may precipitate and become brittle. The C content of the steel material was set to 0.03% or less.
[0062]
Si:
Si has a solid solution strengthening action. This effect is small compared to the amount of strengthening by nitriding, but if a part of the member (part) is not intentionally nitrided and it is desired to secure the strength in that part, it is necessary to add Si to strengthen the part. Is valid. However, the addition of a large amount of Si causes a decrease in ductility and deep drawability, and particularly when the content exceeds 0.5%, a decrease in ductility and deep drawability becomes large. On the other hand, the lower limit may be 0%, but is set to 0.001% from the viewpoint of cost required for reduction. Therefore, the content of Si is set to 0.001 to 0.5%. If no reinforcement is required, the upper limit of the Si content is preferably set to 0.1% from the viewpoint of press formability.
[0063]
Mn:
Mn has a solid solution strengthening action. Although this effect is small compared to the amount of strengthening by nitriding, if the part of the member (part) is not intentionally nitrided and it is desired to ensure the strength in that part, Mn should be added to strengthen it. Is valid. However, the addition of a large amount of Mn causes a decrease in ductility and deep drawability, and particularly when the content exceeds 0.5%, a decrease in ductility and deep drawability increases. On the other hand, the lower limit may be 0%, but is set to 0.01% from the viewpoint of cost required for reduction. Therefore, the content of Mn is set to 0.01 to 0.5%. If no strengthening is required, the upper limit of the Mn content is preferably set to 0.3% from the viewpoint of press formability.
[0064]
P:
P has a solid solution strengthening action. This effect is small compared to the amount of strengthening by nitriding, but if a part of the member (part) is not intentionally nitrided and it is desired to ensure strength in that part, it is necessary to add P to strengthen it. Is valid. However, the addition of a large amount of P causes a decrease in ductility and deep drawability, and particularly when the content exceeds 0.1%, the decrease in ductility and deep drawability increases. On the other hand, the lower limit may be 0%, but is set to 0.001% from the viewpoint of cost required for reduction. Therefore, the content of P is set to 0.001 to 0.1%. When no reinforcement is required, the upper limit of the P content is preferably set to 0.03% from the viewpoint of press formability.
[0065]
S:
S is an element contained as inevitable impurities in steel. In a steel having a high Ti as in the present invention, it combines with Ti in a steel ingot or a billet and precipitates as TiS. A large amount of TiS deteriorates ductility or becomes a starting point of fracture. On the other hand, if the content of S is large, a large amount of Ti is required to secure effective Ti described later, which is uneconomical. Furthermore, if the content of Ti is constant, the higher the content of S, the lower the amount of solid solution Ti, and the lower the strength when subjected to a low-temperature heat treatment after nitriding. Therefore, the content of S is set to 0.01% or less. The S content is preferably set to 0.005% or less. On the other hand, from the viewpoint of steelmaking cost, it is economical to set the lower limit of the S content to 0.001%.
[0066]
Al:
Al is an element added for deoxidation in the steelmaking process. If the content is less than 0.001%, the above effects cannot be sufficiently obtained. Since Al has a strong affinity for N, it may be added for the purpose of increasing the strength by nitriding. However, the addition of a large amount of Al lowers the ductility of the steel material before nitriding treatment (that is, the steel material for nitriding treatment), and particularly when the content exceeds 0.5%, the decrease in ductility becomes large. Therefore, the content of Al is set to 0.001 to 0.5%. The upper limit of the Al content is preferably set to 0.1%. In order to improve the yield of Ti addition, the lower limit of the Al content is preferably set to 0.005%.
[0067]
Ti (before nitriding):
In the steel material for nitriding treatment before the nitriding treatment, Ti is an essential element for fixing C and N and improving the deep drawability. However, if the content of Ti is less than 0.04%, the effect of addition is poor. On the other hand, if the content exceeds 0.2%, the recrystallization temperature increases, and the ductility of the steel material before nitriding decreases. Therefore, the content of Ti in the steel material for nitriding treatment before the nitriding treatment was set to 0.04 to 0.2%. The content of Ti in the nitriding steel is preferably 0.06 to 0.2%, more preferably 0.07 to 0.2%. Since Ti bonds to C, N, and S, in the case of a steel material for nitriding treatment, it is necessary to optimize the effective Ti amount represented by the formula (1) in addition to the Ti content. This will be described later.
[0068]
Ti (after nitriding):
Since the Ti content itself does not change even after the nitriding treatment, the concept of the Ti content in the nitrided steel material after the nitriding treatment may be the same as that of the nitriding steel material before the nitriding treatment. That is, in order to improve the deep drawability, the content of Ti needs to be 0.04% or more. However, if the content exceeds 0.2%, the ductility of the steel material after nitriding decreases. Therefore, the content of Ti in the nitrided steel material after the nitriding treatment is set to 0.04 to 0.2%. The content of Ti in the nitrided steel is preferably 0.06 to 0.2%, and more preferably 0.07 to 0.2%. In the case of a nitrided steel material, it is necessary to optimize the Ti content, but there is no need to limit the effective Ti described below.
[0069]
N (before nitriding):
In the steel material for nitriding treatment before the nitriding treatment, N is an element contained as an inevitable impurity. In order to obtain good deep drawability, that is, to obtain a crystal texture with a high r-value, it is necessary to form a TiN by adding Ti to form a so-called "IF steel" in which the amount of dissolved N is reduced. When the amount of N is large, the amount of Ti necessary for fixing the N is increased, and the amount of coarse TiN is increased. Therefore, ductility is reduced before the nitriding treatment, or it is decreased after the nitriding treatment. May be the starting point of destruction. Therefore, the N content of the nitriding steel material before the nitriding treatment was set to 0.005% or less. The upper limit of the N content of the nitriding steel is preferably 0.003%. From the viewpoint of steelmaking cost, it is economical to set the lower limit of the N content to 0.001%.
[0070]
N (after nitriding):
In the nitrided steel material after the nitriding treatment, N is the most important element for securing the strength of the steel material. However, even with excessive nitriding, N is only consumed for the generation of coarse nitrides, and the contribution to strength saturates. Therefore, the upper limit of the N content of the nitrided steel material after the nitriding treatment is set to 0.25%. On the other hand, if the content of N is small, the increase in strength is small and an effect commensurate with the cost of the nitriding treatment cannot be obtained. Therefore, the nitriding steel material contains 0.08% or more N.
[0071]
Nb:
Nb may not be added. If added, it has the effect of fixing C similarly to Ti and improving the deep drawability of the steel material for nitriding before nitriding. In addition, since the action of Nb to fix C is weaker than that of Ti, it also has an action to improve the brittleness of secondary working by leaving a small amount of solid solution C. In order to surely obtain such an effect, the content of Nb is preferably set to 0.005% or more. However, when Nb is added excessively, the recrystallization temperature rises and ductility decreases, and especially when the content exceeds 0.05%, ductility decreases significantly. Therefore, the content of Nb was set to 0 to 0.05%.
[0072]
B:
B may not be added. If added, it segregates at the grain boundaries and strengthens the grain boundaries, so that it has the effect of improving the brittleness in secondary processing. In order to surely obtain this effect, it is preferable to contain B in an amount of 0.0002% or more. However, when the content of B increases, the deep drawability decreases, and in particular, the deep drawability exceeding 0.005% significantly decreases. Therefore, the content of B is set to 0 to 0.005%.
[0073]
Effective Ti:
Since Ti bonds with C, N and S, in the case of a steel material for nitriding treatment, it is necessary to optimize the effective Ti amount represented by the above formula (1) in addition to the Ti content. That is, in order to suppress a decrease in strength during low-temperature heat treatment for improving ductility, the effective Ti needs to be 0.04% or more. On the other hand, if the effective Ti exceeds 0.10%, the strength becomes too high and the material becomes brittle. Therefore, the effective Ti of the steel for nitriding before the nitriding is set to 0.04 to 0.10%. The effective Ti is preferably set to 0.06 to 0.10%, and more preferably set to 0.07 to 0.10%.
[0074]
The definition of the component elements other than Fe and impurities contained in the steel material for nitriding treatment according to the inventions of the above (1) and (2) is from the above-mentioned C to effective Ti. Further, the provisions of the component elements other than Fe and impurities contained in the nitrided steel material according to the invention (3) are from C to B described above.
[0075]
The nitriding steel material according to the inventions (1) and (2) and the nitriding steel material according to the invention (3) having the above-described chemical composition are melted by a usual method and then injected into a mold. A steel ingot may be used as a raw material by using any of the “ingot-making method” and the “continuous casting method”.
(B) Hv hardness of nitriding steel
When the strength after the nitriding treatment is low, the advantage of performing the nitriding treatment on purpose is diminished compared to a high-strength steel sheet. Therefore, in order to secure strength characteristics equal to or higher than members (parts) using a steel plate having a tensile strength of 780 MPa or higher, the Hv hardness at the center of the thickness of the nitriding steel material according to the invention of (3) is set to 200. It was above. If the Hv hardness at the center of the plate thickness becomes excessively high, the elongation may be reduced even if the fine nitride is controlled, so the upper limit of the Hv hardness is preferably set to 400.
[0076]
At the time of nitriding, N diffuses from the surface of the steel material into the inside, so if the nitriding time is short, the hardness of the surface portion excluding the compound layer has a hardness distribution larger than the hardness at the center of the plate thickness. However, if the hardness of the surface portion is too large, destruction may occur from the interface where the difference in hardness is large, so the difference in Hv hardness between the surface portion and the center of the plate thickness is preferably 200 or less. . In addition, the Hv hardness of the surface portion refers to the hardness of a portion at a depth of 10% of the plate thickness from the steel material surface.
(C) Coarse iron nitride present in the structure in the central region of the thickness of the nitrided steel:
When coarse iron nitride precipitates, solid solution N in the base ferrite is consumed, so that the hardness of the ferrite decreases. On the other hand, a coarse precipitate has a low precipitation strengthening ability, so that precipitation strengthening cannot be expected, and the strength is unnecessarily reduced. In order to impart high strength to the nitrided steel material, acicular coarse iron nitride existing in the structure in the central region of the plate thickness is reduced to 1 × 10 ^-8 m ² It is necessary to reduce the number to 10 or less.
[0077]
For this reason, in the nitriding steel material according to the invention of (3), the coarse iron nitride existing in the structure in the plate thickness center region is 1 × 10 ^-8 m ² 10 or less. The coarse iron nitride precipitates and grows in a high temperature region where N diffusion is fast, and does not exist at all (that is, the above 1 × 10 ^-8 m ² Is preferably 0).
[0078]
As described above, coarse iron nitride is a precipitate whose long side is 5 μm or more among precipitates observed when a cross section of a plate thickness center region etched with nital is observed by an optical microscope with a magnification of 500 times. In other words, it does not necessarily refer to what is identified as pure iron nitride, but refers to "precipitates containing iron and nitrogen as main components".
(D) Fine iron nitride existing in the center of the thickness of the nitriding steel:
Precipitation of fine iron nitride also causes a decrease in ferrite hardness due to consumption of solute N in ferrite, which is a base material, but on the other hand, strengthening action by precipitation strengthening works, and the decrease in average hardness is moderated. Is done. Further, since the ductility of the base ferrite is improved by the reduction of the solute N, it is necessary to positively precipitate fine iron nitride.
[0079]
If the iron nitride is too fine and the long side is less than 0.15 μm, precipitation strengthening will be too strong, which will rather lead to embrittlement of ferrite. On the other hand, when the long side of iron nitride is 5 μm or more, a sufficient strengthening effect and ductility improving effect cannot be obtained. Therefore, it is necessary that the size of the fine iron nitride has a long side of 0.15 μm or more and less than 5 μm. In addition, fine iron nitride having a long side of 0.15 μm or more and less than 5 μm is 1 × 10 at the center of the thickness of the nitrided steel material. ^-12 m ² If there are no more than 5 pieces per piece, a remarkable strength reduction suppressing effect and ductility improving effect cannot be obtained.
[0080]
For this reason, in the nitriding steel material according to the invention of (3), fine iron nitride having a long side of 0.15 μm or more and less than 5 μm is 1 × 10 5 ^-12 m ² It is determined that there are 5 or more per unit. 1 × 10 of fine iron nitride of the above size ^-12 m ² The upper limit of the number of hits is not particularly defined, and the strength decreases as the number increases, so that the number of hits may be set to a value at which a hardness of 200 or more can be obtained in Hv hardness.
[0081]
As described above, the fine iron nitride referred to in the present invention means that the long side of the precipitates observed when a thin film sample collected from the center of the plate thickness is observed with a transmission electron microscope at an accelerating voltage of 200 kV is used. 0.15 μm or more and less than 5 μm, and refers to those in which no enrichment of metal elements other than Fe in the precipitates is observed by EDX analysis, and does not necessarily indicate what is identified as pure iron nitride, It means "precipitate containing iron and nitrogen as main components". Furthermore, when the “fine iron nitride” having a long side of less than 0.15 μm and having the above meaning is precipitated in a bead shape along dislocations, it may look like a string having a length of 0.15 μm or more at low magnification. However, as described above, the long side is not considered to be 0.15 μm or more.
(E) Thickness of compound layer formed on the surface of nitriding steel
Chemical conversion treatment is applied to automotive parts as a base treatment for painting.If the compound layer is too thick, the electrochemical reaction between the chemical conversion treatment solution and the ground iron is hindered, and the chemical treatment becomes insufficient, resulting in poor corrosion resistance after painting. descend. Further, since the compound layer is harder and more brittle than the ground iron, the thicker the compound layer, the easier it is to peel off. In particular, when the thickness of the compound layer exceeds 30 μm, the chemical conversion treatment becomes insufficient, the corrosion resistance after coating is reduced, or the compound layer may be peeled off, so the thickness of the compound layer is 30 μm or less. It is better to do.
[0082]
Therefore, in the invention of (4), the thickness of the compound layer formed on the surface is set to 30 μm or less. The thickness of the compound layer formed on the surface is more preferably 25 μm or less. The lower limit of the thickness of the compound layer is not particularly limited, but is preferably about 2 μm because it is inevitably formed by nitriding and the compound layer itself has corrosion resistance.
[0083]
When the thickness of the nitrided steel material is large, it takes a long time for N to diffuse to the center of the thickness, and accordingly, the compound layer formed on the surface also becomes thick. Therefore, in order to diffuse N to the center of the plate thickness while suppressing the development of the compound layer, isothermal holding may be performed in a non-nitriding atmosphere following the nitriding treatment described later.
(F) Hot rolling, winding, cold rolling and annealing of nitriding steel
The hot rolling condition of the steel ingot or the billet having the chemical composition of Fe and impurities that satisfies the above-mentioned requirement (A) from C to effective Ti is not particularly specified, and may be an ordinary method. . Further, the temperature range of the hot rolling may be in the austenite range or the ferrite range. However, if the finishing temperature of the hot rolling is lower than 850 ° C., ferrite transformation occurs during the hot rolling, so that the structure of the hot-rolled steel material becomes coarse and a favorable texture cannot be obtained. Therefore, when deep drawability is required, the finishing temperature of hot rolling is preferably set to 850 ° C. or higher. In addition, after cold rolling and annealing described later, in order to obtain a recrystallized texture excellent in deep drawability, it is effective to refine the hot-rolled steel material, so immediately after the end of hot rolling. It is preferable to suppress the grain growth by cooling with water.
[0084]
Winding after hot rolling is preferably performed at 500 ° C. or higher in order to promote precipitation of TiC and reduce the amount of solute C. On the other hand, when the winding temperature is high, and particularly when the winding temperature is higher than 700 ° C., the winding may be softened and the wound shape may be broken or the scale may be increased. Therefore, the winding temperature is preferably set to 700 ° C. or lower.
[0085]
Therefore, in the invention of (2), the winding temperature after hot rolling is specified to be 700 to 500 ° C.
[0086]
After winding at the above temperature, the scale of the hot-rolled steel material is removed by ordinary pickling, and then cold rolling may be performed. In order to obtain a recrystallized texture excellent in deep drawability after annealing described below, the total rolling reduction of the cold rolling is preferably set to 70 to 90%.
[0087]
Therefore, in the invention of (2), cold rolling is performed at a total draft of 70 to 90%.
[0088]
After cold rolling, annealing may be performed by a usual method. The annealing method may be either continuous annealing or box annealing. However, in order to obtain a recrystallized texture excellent in deep drawability after annealing, the annealing temperature is preferably set to 650 to 880 ° C.
[0089]
Therefore, in the invention of (2), the annealing temperature is specified to be 650 to 880 ° C. The annealing temperature is more preferably 650 to 750 ° C for box annealing and 750 to 880 ° C for continuous annealing.
[0090]
If deep drawability is not required, the above-described cold rolling and annealing may be omitted, and the scale may be removed after hot rolling, and then the molding and / or nitriding treatment described below may be provided.
(G) Forming
The forming method for obtaining a member (part) having a predetermined shape from a steel material is not limited to any means such as plastic working such as press forming and bending, and mechanical processing such as cutting. By performing plastic working and then nitriding to strengthen, it is possible to achieve both excellent formability and high strength.
[0091]
In addition, when the r value of the nitriding steel material according to the present invention is high, the nitriding steel material after the nitriding treatment also shows a high r value. For this reason, it is also possible to adopt a process of forming after performing the nitriding process, or performing a nitriding process after the forming and re-forming.
[0092]
Therefore, in the invention of (6), the forming process is performed before the nitriding treatment described later. In the invention of (7), the forming process is performed after the nitriding treatment and the low-temperature heat treatment described later.
[0093]
In addition, using the tailored blank technology, a compact formed by combining steel plates having different thicknesses or a compact formed by combining a steel plate suitable for nitriding strengthening according to the present invention with a general steel plate is used to optimize strength and weight reduction. It is possible. It is also possible to perform nitriding after partial masking and not to partially nitride. For example, it is possible to arrange a soft portion that is not partially nitrided in order to realize the strength distribution of the member intended by the designer, or to avoid nitriding of the welded portion from the viewpoint of improving weldability.
(H) Nitriding treatment
The nitriding treatment may be performed by a generally used method such as a gas nitriding method, an ion nitriding method, a gas soft nitriding method, and a salt bath nitriding method.
[0094]
The higher the temperature of the nitriding treatment, the faster the diffusion of N becomes, and the shorter the nitriding treatment time becomes. However, when the nitriding temperature exceeds 650 ° C., austenite is generated, and transformation from austenite to ferrite or martensite occurs during cooling after the nitriding treatment, which may cause transformation distortion. On the other hand, the lower the nitriding temperature, the lower the solid solubility limit of N in the steel, and the slower the diffusion of N in the steel. In particular, if the nitriding temperature is lower than 530 ° C., nitriding cannot be performed to the center of the sheet thickness, and the hardness of the nitrided portion near the surface layer may be low. Therefore, the nitriding treatment is preferably performed at 530 to 650 ° C.
[0095]
Therefore, in the invention of (8), the nitriding temperature was set to 530 to 650 ° C. Since the nitriding treatment is most efficient near the temperature of the maximum solid solubility limit of N, the nitriding treatment temperature is more preferably 550 to 600 ° C.
[0096]
The cooling rate after the nitriding treatment is preferably as high as possible. In the cooling process from the nitriding temperature, N is in a supersaturated state, so that in a temperature range where the diffusion rate of N is large, coarse iron nitride may be rapidly precipitated and the amount of solute N may decrease. is there. In order to prevent this decrease in the amount of solute N, it is preferable to cool from the nitriding temperature to 200 ° C. at an average cooling rate of 4.5 ° C./sec or more.
[0097]
Therefore, in the invention of (8), after nitriding in the temperature range of 530 to 650 ° C., the cooling is performed to 200 ° C. at an average cooling rate of 4.5 ° C./sec or more. The average cooling rate is more preferably at least 10 ° C./sec.
[0098]
The upper limit of the above-mentioned average cooling rate is not particularly limited, and may be the maximum cooling rate obtained from the size of the nitriding steel and the equipment.
(I) Low temperature heat treatment
If the state of precipitates is controlled by performing a low-temperature heat treatment at a temperature range of 100 to 200 ° C. for 10 minutes or more after the nitriding treatment, the elongation characteristics are greatly improved, and the nitriding steel material has both high strength and elongation characteristics. Can be done.
[0099]
That is, when the temperature is less than 100 ° C. or the holding time is less than 10 minutes, the diffusion distance of N is short, so that the precipitation of iron nitride becomes insufficient, and the effect of improving ductility cannot be obtained. If the temperature exceeds ℃, the softening becomes remarkable, and the purpose of strengthening the nitrided steel cannot be achieved.
[0100]
For this reason, it is an essential condition that the steel material for nitriding treatment according to the inventions of (1) and (2) be subjected to a low-temperature heat treatment of holding at a temperature range of 100 to 200 ° C. for 10 minutes or more after nitriding treatment. did. Further, in the inventions of (5) to (8), a step of performing a low-temperature heat treatment for holding at a temperature range of 100 to 200 ° C. for 10 minutes or more after the nitriding treatment is included.
[0101]
The low-temperature heat treatment performed after the nitriding treatment may be performed by once cooling the nitridized steel material, that is, the nitriding steel material to room temperature, and then reheating it, or may be performed in a cooling process after the nitriding treatment. In the case of coating a nitrided steel material, especially a nitrided component, there is no need to add a step if the low-temperature heat treatment of holding the above-mentioned temperature range of 100 to 200 ° C. for 10 minutes or more is also used as a drying step of the coating. It is economical.
[0102]
Hereinafter, the present invention will be described in more detail with reference to examples.
[0103]
【Example】
(Example 1)
Steel having the chemical composition shown in Table 3 was melted in a laboratory, and each steel ingot was hot forged by a usual method to obtain a steel slab having a thickness of 20 mm. Next, after heating each of the above steel slabs at 1200 ° C. for 30 minutes, hot rolling was performed at a rolling finishing temperature of 920 ° C. to obtain a steel sheet having a thickness of 4.5 mm. In addition, immediately after the rolling was completed, water cooling was performed, and after cooling to 650 ° C., the temperature was maintained for 30 minutes, and then gradually cooled to room temperature at an average cooling rate of 20 ° C./hour to simulate a winding process.
[0104]
[Table 3]

[0105]
The hot-rolled steel sheet obtained as described above is pickled by a normal method to remove scale, and then cold-rolled by a normal method to a total draft of 82.2% to a thickness of 0. 2%. 8 mm. Next, each of the above-mentioned cold-rolled steel sheets was heated at an average heating rate of 10 ° C./sec, held at 820 ° C. for 30 seconds, cooled to room temperature at an average cooling rate of 10 ° C./sec, and simulated continuous annealing. . Further, after temper rolling at an elongation of 0.1%, gas soft nitriding was performed at 580 ° C. for 3 hours. Thereafter, low-temperature heat treatment was performed at 170 ° C. for 20 minutes. The conditions of the gas nitrocarburizing treatment were NH 3 ₃ Mixed gas of gas and RX gas (endothermic) (by volume ratio, NH ₃ It was performed in an atmosphere of gas: RX gas = 1: 1), and after nitriding, was immersed in an oil tank and cooled. The average cooling rate from 580 ° C. to 200 ° C. in this cooling was 20 ° C./sec.
[0106]
The tensile properties of the steel sheet before the nitriding treatment and the steel sheet after the low-temperature heat treatment were examined, and the Hv hardness of the steel sheet after the low-temperature heat treatment was also examined.
[0107]
That is, a JIS No. 13B tensile test was taken from each steel sheet before the nitriding treatment and after the low-temperature heat treatment so that the rolling direction was the tensile direction, and the tensile properties were examined at room temperature. Further, the Hv hardness of a total of two places at a depth of 10% of the plate thickness from the plate thickness center and the plate surface after the low-temperature heat treatment was measured at a test force of 9.8 N.
[0108]
Table 4 shows the tensile properties of each steel sheet before the nitriding treatment and after the low-temperature heat treatment and the Hv hardness at the center of the thickness of each steel sheet after the low-temperature heat treatment. In each of the steel sheets, the difference between the Hv hardness at the center of the sheet thickness and the Hv hardness at a position at a depth of 10% of the sheet thickness from the steel sheet surface was 50 or less. From this, it was determined that each steel sheet was uniformly nitrided in the thickness direction.
[0109]
[Table 4]

[0110]
Since the contents of C and N change due to the nitriding treatment, the average chemical composition of the steel sheet after the nitriding treatment was analyzed. At the time of analysis, 0.08 mm (equivalent to 10% of the plate thickness) was mechanically ground from both surfaces of the steel plate, excluding the effect of the compound layer having a very high N concentration formed on the surface of the steel plate. For analysis. The contents other than C and N did not change from those before the nitriding treatment.
[0111]
Also, for each steel sheet after low-temperature heat treatment, after 0.08 mm (equivalent to 10% of the sheet thickness) was mechanically ground from both surfaces of the steel sheet, the cross section of the steel sheet was mirror-polished and etched with nital, and the magnification was 500 times. And observed with an optical microscope. Further, a thin film sample taken from the center of the plate thickness was observed using a transmission electron microscope at an acceleration voltage of 200 kV. In addition, according to observation with an optical microscope, coarse iron nitride having a long side of 5 μm or more was hardly recognized in any of the steel sheets, and 1 × 10 ^-8 m ² The number was less than one piece.
[0112]
Table 4 also shows the analysis results of C and N after the nitriding treatment, and the observation results of fine iron nitride having a long side of 0.15 μm or more and less than 5 μm using a transmission electron microscope at an acceleration voltage of 200 kV.
[0113]
As is clear from Table 4, the steel sheets having the chemical compositions defined in the present invention before the nitriding treatment of the steels E to G and the steels K to M have a low yield point and a low tensile strength and a large elongation.
[0114]
Furthermore, after nitriding the steel sheets EG and KM satisfying the average chemical composition and the Hv hardness at the center of the sheet thickness determined by the present invention, and the number of coarse iron nitrides and fine iron nitrides, the temperature is lowered after nitriding. In the case of the heat-treated steel sheet, it is apparent that a high tensile strength of 780 MPa or higher and a large elongation of more than 10% can be secured.
[0115]
On the other hand, when the steel sheet of the steel H whose effective Ti before the nitriding treatment is out of the conditions specified in the present invention is subjected to the low-temperature heat treatment after the nitriding treatment, the long side satisfying the average chemical composition after the nitriding is 0.1 mm. The number of fine iron nitrides of 15 μm or more and less than 5 μm is out of the range of the present invention, and instead, extremely fine iron nitride having a long side of 0.03 to 0.05 μm is 1 × 10 5 ^-12 m ² Since there were a very large number of about 70 pieces per piece, the ductility was very low and fractured without showing work hardening. Note that the r value could not be measured.
[0116]
When the steel sheet of the steel J in which the effective Ti, C and Ti are out of the conditions specified in the present invention before the nitriding treatment is subjected to the low-temperature heat treatment after the nitriding treatment, the fine side of the fine iron nitride having a long side of 0.15 μm or more and less than 5 μm is obtained. The number deviates from the definition of the present invention, and instead, extremely fine iron nitride having a long side of 0.03 to 0.05 μm is 1 × 10 ^-12 m ² Since the number of Ti atoms per unit area was as large as about 70, and the Ti content in the average chemical composition after nitriding also fell outside the conditions specified in the present invention, the ductility was extremely low, and fractured without showing work hardening. Also in this case, the r value could not be measured.
[0117]
A large amount of TiS was precipitated in the steel sheet before the nitriding treatment of Steel I in which effective Ti and S were out of the conditions specified in the present invention, and the elongation was small. Further, when the steel sheet of steel I is subjected to low-temperature heat treatment after nitriding treatment, since the S content in the average chemical composition after nitriding is larger than the condition specified in the present invention, solute Ti decreases, and the thickness of the steel sheet at the center of the sheet thickness is reduced. Since the Hv hardness was lower than the range specified in the present invention, a desired strength level of 780 MPa class or higher could not be secured.
(Example 2)
The same NH as in Example 1 was added to the cold-rolled steel sheet subjected to temper rolling at an elongation of 0.1% of the steel G produced in Example 1. ₃ A gas soft nitriding treatment was performed at 580 ° C. for 3 hours in a mixed gas atmosphere of a gas and an RX gas. Thereafter, low-temperature heat treatment was performed at 170 ° C. for 20 minutes. In addition, after nitriding, it was immersed in an oil tank and cooled, but by changing the time until it was immersed in the oil tank and cooled, the average cooling rate from the nitriding temperature to 200 ° C. was changed.
[0118]
For each steel sheet after the low-temperature heat treatment, in the same manner as in Example 1, the tensile properties were investigated, the Hv hardness was measured, the optical microscope was observed at a magnification of 500, and the thin film was observed using a transmission electron microscope at an acceleration voltage of 200 kV. Was done. The average chemical composition of the steel sheet after the nitriding treatment was also analyzed.
[0119]
Table 5 shows the tensile properties of each steel sheet after low-temperature heat treatment, the Hv hardness at the center of the sheet thickness, the observation results of coarse iron nitride having a long side of 5 μm or more using an optical microscope with a magnification of 500 times, and the transmission type with an acceleration voltage of 200 kV. The observation result of the fine iron nitride whose long side is 0.15 μm or more and less than 5 μm using an electron microscope is shown. In each of the steel sheets, the difference between the Hv hardness at the center of the sheet thickness and the Hv hardness at a position at a depth of 10% of the sheet thickness from the steel sheet surface was 50 or less. From this, it was determined that each steel sheet was uniformly nitrided in the thickness direction.
[0120]
Table 5 also shows the analysis results of C and N after the nitriding treatment. The contents other than C and N did not change from those before the nitriding treatment.
[0121]
[Table 5]

[0122]
From Table 5, the average chemical composition and the Hv hardness at the center of the plate thickness determined in the present invention, and the test symbols G-1 and G-2 satisfying the number of coarse iron nitride and fine iron nitride, are 780 MPa class or higher. It is clear that a high tensile strength and a large elongation exceeding 10% can be secured.
[0123]
On the other hand, even if the average chemical composition, the Hv hardness at the center of the plate thickness, and the number of fine iron nitrides defined in the present invention are satisfied, the number of coarse iron nitrides is out of the range specified in the present invention. And in the case of G-4, the decrease in strength is large. Therefore, it is understood that the average cooling rate from the nitriding temperature to 200 ° C. is preferably 4.5 ° C./sec or more.
(Example 3)
The same NH as in Example 1 was added to the cold-rolled steel sheet subjected to temper rolling at an elongation of 0.1% of the steel G produced in Example 1. ₃ The gas soft nitriding treatment was performed in a mixed gas atmosphere of a gas and an RX gas while changing the treatment time at 580 ° C. In order to diffuse the N contained in the compound layer on the surface into the inside of the steel sheet to make the compound layer thin, the above NH is used. ₃ After nitriding at 580 ° C. for 3 hours in a mixed gas atmosphere of a gas and an RX gas, the atmosphere is changed to 100% N. ₂ Instead of using gas, the same process of holding at 580 ° C. for one hour was also performed. After nitriding under any of the conditions, it was immersed in an oil tank and cooled after nitriding. The average cooling rate from 580 ° C. to 200 ° C. in this cooling was 20 ° C./sec.
[0124]
Further, chemical conversion treatment and coating were performed by a usual method, and subjected to a composite corrosion cycle test, and the corrosion resistance after coating was investigated.
[0125]
That is, after degreasing the surface, a zinc phosphate-based chemical conversion treatment is performed, and then a commercially available cationic electrodeposition paint is applied to a thickness of about 25 μm, and then a low-temperature heat treatment is performed at 175 ° C. for 25 minutes, which also serves as a paint bake. A composite corrosion cycle test in which “salt spray → dry → wet” was performed for one cycle (24 hours) was performed on the test specimen, and the corrosion depth after 270 cycles was measured.
[0126]
In addition, the test object surface of the test piece subjected to the composite corrosion cycle test was divided into 10 parts, and the maximum corrosion depth was measured for each division. Then, the average value of the top five points was defined as the corrosion depth.
[0127]
In addition, for each steel sheet after coating and baking, after removing the coating, in the same manner as in Example 1, the tensile properties were investigated, the Hv hardness was measured, observation was performed with an optical microscope at a magnification of 500 times, and transmission at an accelerating voltage of 200 kV was performed. The thin film was observed using a scanning electron microscope. In addition, according to observation with an optical microscope, coarse iron nitride having a long side of 5 μm or more was hardly recognized in any of the steel sheets, and 1 × 10 ^-8 m ² The number was less than one piece. The average chemical composition of the steel sheet after the nitriding treatment was analyzed, and the thickness of the compound layer formed on the surface was measured by observing with an optical microscope having a magnification of 500 times.
[0128]
Table 6 shows the tensile properties of each steel sheet after the low-temperature heat treatment, the Hv hardness at the center of the sheet thickness, the observation results of coarse iron nitride having a long side of 5 μm or more using an optical microscope with a magnification of 500 times, the transmission type with an acceleration voltage of 200 kV. The results of observation of fine iron nitride having a long side of 0.15 μm or more and less than 5 μm using an electron microscope and the results of an investigation of corrosion resistance by a composite corrosion cycle test are shown. In the column of corrosion resistance in Table 6, “◎”, “○”, and “△” are “very good” when the corrosion depth is 50 μm or less, and “good” and more than 100 μm when the corrosion depth is more than 50 μm and 100 μm or less. And 200 μm or less means “slightly inferior”. The targets of the corrosion resistance were “◎” and “○” when the corrosion depth was 100 μm or less.
[0129]
In each of the steel sheets, the difference between the Hv hardness at the center of the sheet thickness and the Hv hardness at a position at a depth of 10% of the sheet thickness from the steel sheet surface was 50 or less. From this, it was determined that each steel sheet was uniformly nitrided in the thickness direction.
[0130]
Table 6 also shows the analysis results of C and N after the nitriding treatment and the results of measuring the thickness of the compound layer using an optical microscope with a magnification of 500 times. The contents other than C and N did not change from those before the nitriding treatment.
[0131]
[Table 6]

[0132]
From Table 6, the average chemical composition and the Hv hardness at the center of the plate thickness determined in the present invention, and the 780 MPa class in any of the test symbols G-5 to G-10 satisfying the number of coarse iron nitride and fine iron nitride. The above high tensile strength and large elongation exceeding 10% can be secured. However, in the case of test symbol G-9 in which the thickness of the compound layer formed on the surface exceeds 30 μm, the evaluation of corrosion resistance is “△”, and when applied as an automobile part, it is limited to a part where the corrosive environment is not severe. Will be done.
[0133]
For comparison, the cold-rolled steel sheet which had been subjected to temper rolling at an elongation of 0.1% of the steel G manufactured in Example 1 was used as it was, that is, without performing the gas nitrocarburizing treatment. -A baking treatment was performed and a composite corrosion cycle test was conducted to investigate corrosion resistance. The results of the corrosion resistance investigation were “poor” with a corrosion depth of more than 200 μm.
(Example 4)
The same NH as in Example 1 was added to the cold-rolled steel sheet subjected to temper rolling at an elongation of 0.1% of the steel G produced in Example 1. ₃ A gas soft nitriding treatment was performed at 580 ° C. for 3 hours in a mixed gas atmosphere of a gas and an RX gas, and after the nitriding treatment, it was immersed in an oil tank and cooled. The average cooling rate from 580 ° C. to 200 ° C. in this cooling was 20 ° C./sec. Thereafter, heat treatment was performed while changing the temperature and the holding time.
[0134]
For each steel sheet after the heat treatment, in the same manner as in Example 1, the tensile properties were investigated, the Hv hardness was measured, the optical microscope was observed at a magnification of 500, and the thin film was observed using a transmission electron microscope at an acceleration voltage of 200 kV. went. In addition, according to observation with an optical microscope, coarse iron nitride having a long side of 5 μm or more was hardly recognized in any of the steel sheets, and 1 × 10 ^-8 m ² The number was less than one piece. The average chemical composition of the steel sheet after the nitriding treatment was also analyzed.
[0135]
Table 7 shows the tensile properties of each steel sheet after heat treatment, the Hv hardness at the center of the sheet thickness, and the observation results of fine iron nitride having a long side of 0.15 μm or more and less than 5 μm using a transmission electron microscope with an acceleration voltage of 200 kV. Show. In each of the steel sheets, the difference between the Hv hardness at the center of the sheet thickness and the Hv hardness at a position at a depth of 10% of the sheet thickness from the steel sheet surface was 50 or less. From this, it was determined that each steel sheet was uniformly nitrided in the thickness direction.
[0136]
Table 7 also shows the analysis results of C and N after the nitriding treatment. The contents other than C and N did not change from those before the nitriding treatment.
[0137]
[Table 7]

[0138]
From Table 7, in the case of the test symbols G-12 to G-14 and G-16 satisfying the average chemical composition and the Hv hardness at the plate thickness center determined by the present invention, and the number of coarse iron nitride and fine iron nitride, 780 MPa or higher, and a large elongation exceeding 10%. However, in the case of test number G-14, the heat treatment temperature is 220 ° C., which is outside the range of the “low-temperature heat treatment” of the present invention, so that the number of fine iron nitrides is slightly large, and therefore the degree of softening is large. Therefore, it is understood that the heat treatment performed after the nitriding treatment is preferably a low-temperature heat treatment in which the heat treatment is maintained in a temperature range of 100 to 200 ° C. for 10 minutes or more.
[0139]
On the other hand, even if the average chemical composition, the Hv hardness at the center of the plate thickness and the number of coarse iron nitrides defined in the present invention are satisfied, the test symbols G-11 and G- In the case of 15, it is clear that the elongation is significantly lower at 2.1% and 3.1% respectively. In these cases, the r value could not be measured because the elongation was small.
(Example 5)
The distortion amount of the vertical wall was changed by changing the condition of the wrinkle pressing pressure and the die shoulder R of the cold-rolled steel sheet subjected to the temper rolling of the steel G manufactured in Example 1 with an elongation of 0.1%. It was press-formed into a hat shape.
[0140]
After press molding, the same NH as in Example 1 was used. ₃ A gas soft nitriding treatment was performed at 580 ° C. for 3 hours in a mixed gas atmosphere of a gas and an RX gas, and after the nitriding treatment, it was immersed in an oil tank and cooled. The average cooling rate from 580 ° C. to 200 ° C. in this cooling was 20 ° C./sec. Thereafter, a low-temperature heat treatment at 150 ° C. for 20 minutes was performed. The cold-rolled steel sheet subjected to the nitriding treatment without press forming was also subjected to a low-temperature heat treatment at 150 ° C. for 20 minutes.
[0141]
With respect to the vertical wall portion of the hat-shaped member and the cold-rolled steel sheet obtained in this manner, Hv hardness measurement, observation with an optical microscope at a magnification of 500 times, and transmission at an acceleration voltage of 200 kV were performed in the same manner as in Example 1. The thin film was observed using a scanning electron microscope. In addition, according to observation with an optical microscope, coarse iron nitride having a long side of 5 μm or more was hardly recognized in any of the hat-shaped members and the cold-rolled steel sheet. ^-8 m ² The number was less than one piece. The average chemical composition after the nitriding treatment was also analyzed.
[0142]
Table 8 shows the observation results of the fine iron nitride having a long side of 0.15 μm or more and less than 5 μm using a transmission electron microscope at an Hv hardness of the center of the thickness of the sheet after low-temperature heat treatment and an acceleration voltage of 200 kV. In each of the hat-shaped members and the cold-rolled steel materials, the difference between the Hv hardness at the center of the sheet thickness and the Hv hardness at a portion at a depth of 10% of the sheet thickness from the steel sheet surface was 50 or less. From this, it was determined that each of the hat-shaped members and the cold-rolled steel sheet were uniformly nitrided in the thickness direction. Table 8 also shows the analysis results of C and N after the nitriding treatment. The contents other than C and N did not change from those before the nitriding treatment.
[0143]
[Table 8]

[0144]
From Table 8, when the hat shape of the test symbols G-18 to G-20 was press-formed, the Hv hardness at the center of the plate thickness tends to slightly increase with the increase in the amount of distortion of the vertical wall. It is clear that the analysis values of N and N, and the number of fine iron nitrides are not much different from the test code G-17 without press molding.
[0145]
From these results, the same results as in Examples 1 to 4 evaluated in the state of a steel sheet not subjected to press forming can be expected also in the case of press forming.
[0146]
【The invention's effect】
Since the nitriding steel material of the present invention has both high strength and elongation characteristics, it can be used for structural members of automobiles and the like. This nitriding steel material can be relatively easily manufactured by the manufacturing method of the present invention using the nitriding steel material of the present invention. Further, according to the manufacturing method of the present invention, it is possible to manufacture a complicated formed body with high dimensional accuracy as compared with, for example, a case where a conventional high-tensile steel plate is press-formed.
[Brief description of the drawings]
FIG. 1 is a diagram showing the effect of effective Ti on yield point and elongation.

Claims (8)

In mass%, C: 0.01% or less, Si: 0.001 to 0.5%, Mn: 0.01 to 0.5%, P: 0.001 to 0.1%, S: 0.01 %, Al: 0.001 to 0.5%, Ti: 0.04 to 0.2%, Nb: 0 to 0.05%, B: 0 to 0.005%, N: 0.005% or less And the balance consists of Fe and impurities, and the effective Ti represented by the following formula (1) is 0.04 to 0.10%. Nitriding steel used for low-temperature heat treatment for 10 minutes or more.
Effective Ti = Ti − {(N / 14) + (S / 32) + (C / 12)} × 48 (1)
Here, the symbol of the element on the right side of the equation (1) indicates the content of the element in steel in mass%. In mass%, C: 0.01% or less, Si: 0.001 to 0.5%, Mn: 0.01 to 0.5%, P: 0.001 to 0.1%, S: 0.01 %, Al: 0.001 to 0.5%, Ti: 0.04 to 0.2%, Nb: 0 to 0.05%, B: 0 to 0.005%, N: 0.005% or less And the balance consists of Fe and impurities, and hot rolling is performed on a steel ingot or a steel slab having an effective Ti represented by the following formula (1) of 0.04 to 0.10% to 700 to 500 ° C. 2. The method for producing a steel material for nitriding treatment according to claim 1, further comprising: performing cold rolling at 70 to 90% at a total reduction ratio, and thereafter performing annealing at 650 to 880C.
Effective Ti = Ti − {(N / 14) + (S / 32) + (C / 12)} × 48 (1)
Here, the symbol of the element on the right side of the equation (1) indicates the content of the element in steel in mass%. The sheet thickness center area ranging from the sheet thickness center to the sheet surface up to 40% of the sheet thickness is, by mass%, C: 0.03% or less, Si: 0.001 to 0.5%, Mn : 0.01 to 0.5%, P: 0.001 to 0.1%, S: 0.01% or less, Al: 0.001 to 0.5%, Ti: 0.04 to 0.2% , Nb: 0 to 0.05%, B: 0 to 0.005%, N: 0.08 to 0.25%, the balance being an average chemical composition of Fe and impurities, and The Vickers hardness is 200 or more, and the number of coarse iron nitrides having a long side of 5 μm or more in the plate thickness center region is not more than 10 per 1 × 10 ⁻⁸ m ² , and the long side in the plate thickness center is 0.15 μm A nitrided steel material characterized in that the number of fine iron nitride particles having a size of less than 5 μm is 5 or more per 1 × 10 ⁻¹² m ² . The nitrided steel material according to claim 3, wherein the thickness of the compound layer formed on the surface is 30 µm or less. A method for producing a nitriding steel material using the nitriding steel material according to claim 1 or the nitriding steel material obtained by the production method according to claim 2, wherein the temperature is 100 to 200 ° C. after the nitriding treatment. A method for producing a nitrided steel material, comprising a step of performing a low-temperature heat treatment for holding for 10 minutes or more in a region. The nitriding treatment according to claim 5, wherein the nitriding treatment is performed after forming the nitriding treatment steel material according to claim 1 or the nitriding treatment steel material obtained by the manufacturing method according to claim 2. Production method. The nitriding steel material according to claim 1 or the nitriding steel material obtained by the manufacturing method according to claim 2 is subjected to nitriding treatment and subsequent holding in a temperature range of 100 to 200 ° C. for 10 minutes or more without forming. A method for producing a nitrided steel material, characterized by further performing a forming process on the nitrided steel material subjected to the low-temperature heat treatment or the nitrided steel material obtained by the production method according to claim 6. The method according to any one of claims 5 to 7, wherein the nitriding treatment is performed by nitriding in a temperature range of 530 to 650 ° C and then cooling to 200 ° C at an average cooling rate of 4.5 ° C / sec or more. Manufacturing method of nitriding steel.

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