JP3869754B2 - Steel plate for carburizing and quenching with less variation at the time of burring and method for producing the same - Google Patents

Description

（２）（１）に記載の鋼板に、さらに質量比で、Ｍｇ、ＣｅあるいはＲＥＭのうち１種または２種以上を０．００１〜０．０２％含むことを特徴とするバーリング加工時のバラツキが少ない浸炭焼入れ用鋼板。
（３）（１）あるいは（２）に記載の鋼板に、さらに質量比で、Ｔｉ：０．００５〜０．１％を含むことを特徴とするバーリング加工時のバラツキが少ない浸炭焼入れ用鋼板。
（４）（１）乃至（３）のいずれかに記載の鋼板に、さらに質量比で、Ｍｏ：０．０５〜０．５％を含むことを特徴とするバーリング加工時のバラツキが少ない浸炭焼入れ用鋼板。
（５）（１）から（４）のいずれかに記載の成分組成の溶鋼を連続鋳造にてスラブとし、再加熱後あるいは鋳造後直ちに粗圧延を実施し、Ａｒ₃変態点以上の温度域で仕上圧延を終了させかつ、その温度域から冷却を開始するが、５０℃／ｓ以下の冷却速度で冷却し、６００〜７００℃の温度域で巻き取ることを特徴とするバーリング加工時のバラツキが少ない浸炭焼入れ用鋼板の製造方法。
（６）粗圧延を終了し、シートバーを一旦コイルに巻き取り、そのまま仕上圧延に供するか、あるいは先行するシートバーに接続後、仕上圧延を行うことを特徴とする（５）に記載のバーリング加工時のバラツキが少ない浸炭焼入れ用鋼板の製造方法。
（７）１００ｍｍ以下の鋳片に鋳造後、直ちに粗圧延を実施することを特徴とする（５）または（６）に記載のバーリング加工時のバラツキが少ない浸炭焼入れ用鋼板の製造方法。
【０００８】
【発明の実施の形態】
まず、この発明における成分組成の限定理由について述べる。
【０００９】
Ｃは０．１５〜０．２５％とする。過度に添加されると、セメンタイトがグラファイトとして形成されるようになることから、焼入れ処理におけるＣの再固溶が十分に達成されず、部分的な硬度不足となることが懸念されることから、０．２５％を上限とする。一方、焼入れ処理後の硬度を確保するにはＣ量が必要であり、ビッカースで安定して４００を得るには０．１５％以上必要である。
【００１０】
Ｓｉは、過度に添加されると鋼中に圧延方向に長く伸びた酸化物が残存するため、バーリング加工性を低下させるとともに、加工時のバラツキを大きくするため０．５％を上限とする。
【００１１】
Ｍｎは、焼入れ性を確保するには重要な役割を果たす元素ではあるが、熱延段階における仕上圧延後の冷却段階でのパーライトの形成を遅滞させる。すなわち、巻取り後にもパーライト変態が進むため、その体積率が増加すると熱延板での強度が上がり、加工性を劣化させることから０．８％を上限とする。一方、熱間圧延時の割れを回避するためには０．０５％以上の添加が必要である。
【００１２】
Ｐは、本発明においてはとくに限定されるべきものではないが、過度に添加されるとその偏析に起因し、加工性の劣化や二次加工性の劣化をきたすため、０．０３５％を上限とする。
【００１３】
また、Ｓも本発明においてはとくに限定されるべきものではないが、熱間での加工性を確保するには低い方が好ましく、０．０１５％を上限とする。
【００１４】
Ａｌは主として脱酸のために添加されるが、とくにアルミナはその形態からバーリング加工性を劣化させることが懸念される。そのため、少ない方が好ましいことから上限を０．０５％とするが、他の脱酸元素を使用する場合には０．００１％を下限とする。
【００１５】
Ｎは、後述するように固溶Ｂを確保するには極力低い方が良い。そのため、上限を０．００５％とするが、好ましくは０．００３％以下とする。
【００１６】
Ｂは、前述したように焼入れ性を確保することを目的に添加される。ただし、焼入れ性を確保するものは固溶状態のＢであることから、Ｎとのバランスを考慮する必要がある。０．００１５％未満では、鋼中に含有されるＮ量とのバランスからその効果が不十分である。一方、０．００５％を超えると、製鋼段階でスラブ割れが生じることや、熱延板材質として硬質化による加工性劣化も懸念されることから、これを上限とする。
【００１７】
本発明においては、（１）式のようにＢとＮの比率を規定する。前述のとおり、浸炭焼入後の母材硬度をＨｖ：４００以上とするためには、Ｂ／Ｎ（原子比）１．０以上が必要であり、また、Ｂ／Ｎ（原子比）が１．０より小さいと表層部近傍の硬度低下が大きくなる。また、Ｂ／Ｎ（原子比）の上限は、その効果が飽和することとコストアップを極力抑えるため、３．０とする。そのため、（１）式のように規定した。
【００１８】
本発明においては、Ｏ量も重要な規定因子の１つである。すなわち、バーリング加工性のバラツキに対しては、とくに打ち抜き破断面での酸化物分布の影響も大きいことから、極力少ない方が好ましいが、製鋼段階での脱酸コストを考慮して０．００４％を上限とする。
【００１９】
Ｍｇ，ＣｅあるいはＲＥＭは、脱酸のために添加されるものである。脱酸のためには０．００１％以上必要である。一方、過度の添加はコストアップとなるため０．０２％を上限とする。なお、これら元素による脱酸で形成される酸化物は、アルミナ等に比べて微細に晶出する。
【００２０】
Ｔｉは、脱酸のために添加される場合と、ＢＮの析出に先立ちＴｉＮによりＮを固定し、浸炭焼入れ時の焼入れ性に寄与するＢ量を有効に確保するために添加される。０．００５％未満ではその効果が発現できず、０．１％を超えた添加はその効果が飽和するとともに、コストアップとなるためこれを上限とする。
【００２１】
さらに、Ｍｏは溶接熱影響部に生じる軟化抑制のために添加される。その効果を十分に得るには０．０５％以上の添加が必要である。一方、０．５％を超えて添加されてもその効果は飽和するため、これを上限とする。
【００２２】
また、スクラップの利用による微量のＣｕ、Ｎｉ、Ｓｎ及びＣｒの混入は、本発明における効果を何ら損なうものではない。
【００２３】
本発明の熱延工程における加熱温度は、とくに規定されるものではない。しかし、Ｔｉが含有される場合、ＴｉＮは高温域から析出することから、析出するＴｉＮを粗大化させて粒成長性を確保するには高い方が好ましい。
【００２４】
一方、仕上温度についてはＡｒ₃変態点以上とするが、あとに続く冷却工程でのパーライト変態を速やかに進行させるには、Ａｒ₃変態点に極力近い方が好ましい。
【００２５】
仕上圧延後冷却され巻き取られるが、その際、巻取りまでにパーライト変態を終了させておく必要がある。そのためにはランアウトテーブル上での冷却速度を５０℃／ｓ以下とする必要がある。これよりも冷却速度が速くなると、前述したように巻取った後にもパーライト変態が進み、その分率も増加することになる。その結果、熱延板強度が上がることに起因し、加工性の劣化が懸念される。
【００２６】
冷却に引き続いてコイルに巻き取られるが、その際の温度は６００〜７００℃とする。６００℃より低くなるとベイナイトが形成されるようになるため、強度が高くなり加工性を劣化させるため好ましくない。一方、７００℃を超えるとＣがパーライトとして十分に析出しないことから、巻取り後の冷却中に粒界に析出し、バーリング加工性の劣化が懸念される。
【００２７】
上述した熱間圧延を実施する際に、粗圧延を後先行するシートバーにレーザー溶接等を用いて接合し、圧延を実施しても本発明における効果を損なうものではない。
【００２８】
さらにスラブを製造する場合もいわゆるニアネットシェイプとして１００ｍｍ以下の薄スラブを製造し、直ちに前述したような圧延条件で製造することも本発明の効果を損なうものではない。
【００２９】
【実施例】
実施例１
Ｃ：０．２％，Ｓｉ：０．０１％，Ｍｎ：０．５８％，Ｐ：０．０１１％，Ｓ：０．００５％，Ａｌ：０．０２７％，Ｎ：０．００２４％，Ｏ：０．００１７％，Ｂ：０．００３８％を含む鋼を転炉出鋼し、連続鋳造にてスラブとした。熱延は１２００℃で加熱後、粗圧延を実施してから表１に示す条件で熱間圧延を終了し、２．０ｍｍの熱延板とした。なお、ここでＡｒ₃変態点は９１６−５０［Ｃ（％）］＋２７［Ｓｉ（％）］−６４［Ｍｎ］で概算すると約８６９℃である。
【００３０】
材質評価は、ＪＩＳ Ｚ ２２０１記載の５号試験片に加工し、ＪＩＳ Ｚ ２２４１記載の試験方法にしたがって引張試験を行った。また、バーリング加工特性の評価として穴拡げ性の調査を実施した。その方法は、直径１０ｍｍ（ｄ₀）の穴を打ち抜き、６０度円錐ポンチを使用してバリが外側になるようにその穴を押し広げ、割れが板厚を貫通した時点での穴径（ｄ）を測定し、ｄ／ｄ₀で評価するものである。なお、試験片は幅方法及び長手方向３列に採取し、ｄ／ｄ₀は平均値と最小値を求めた。さらに、浸炭焼入れ相当の熱処理として、９００℃×１ｈ→油焼入れを実施し、ビッカース（荷重：１００ｇ）による硬度測定を行った。
【００３１】
表１に得られた熱延板の機械的性質、ｄ／ｄ₀及び熱処理後の硬度を示す。本発明にしたがったＮｏ．２，３，４，５，及び６では、バラツキが少なくかつ、２．２以上の穴拡げ性が得られているとともに、３０％を超える高い延性が得られている。一方、仕上温度が低く外れたＮｏ．１、Ｒｏｔ冷速が本発明の範囲より速く外れたＮｏ．７、さらに巻取温度が本発明の範囲から外れたＮｏ．８では延性が低い。また、Ｎｏ．９では、穴拡げ性のバラツキが大きくなっており、最小値がとくに小さい。いずれもミクロ組織に起因するものであり、Ｎｏ．１ではフェライトの不均一組織に起因したものである。Ｎｏ．７ではＲｏｔ冷速が本発明の範囲から高く外れたことにより、巻取り中にパーライト変態が進行するためである。Ｎｏ．８ではベイナイトの形成に起因するものであり、Ｎｏ．９は主として粒界にセメンタイトが形成されるようになることに起因するものと推察される。なお、Ｎｏ．６は本発明の範囲ではあるが、仕上温度が高めであることと、Ｒｏｔ冷速がやや速めであることから強度が高めとなっている。なお、いずれの条件においても熱処理後の硬度としてビッカースで４００を超える硬度が得られている。
【００３２】
【表１】

【００３３】
実施例２
表２に示す種々の鋼を転炉出鋼し、連続鋳造でスラブとした。熱延は１１５０〜１２００℃で加熱後、粗圧延及び仕上圧延を実施して表３に示すような板厚の熱延板を製造した。なお、仕上圧延はいずれもＡｒ₃変態点以上の温度域で終了した。さらに、仕上圧延後の冷却速度は本発明の範囲内の条件となるよう、冷却ゾーンにおける水量を調整した。冷却後、６００〜７００℃で巻取を行い、実施例１と同様に引張試験による材質評価と穴拡げ性評価を実施した。また、浸炭焼入れ相当の熱処理と、各熱処理材について行った硬度測定は実施例１と同条件で実施した。
【００３４】
結果を同表に示す。本発明にしたがったＡ，Ｂ，Ｃ，Ｄ，Ｅ，Ｎ，Ｏ及びＰ鋼では、高い延性と、バラツキが少なくかつ２．２を超える穴拡げ性が得られている。とくに、脱酸時にＴｉ，Ｍｇ，Ｃｅを使用したＤ，Ｉ，Ｎ及びＯ鋼ではとくに穴拡げ性におけるバラツキが極めて小さい。一方、Ｈ，Ｋ及びＭ鋼では穴拡げ性のバラツキが大きく、最小値でみると２．０を下回っている。Ｃ量が低く外れたＦ鋼では焼入れ処理による硬度が４００を下回っており、特性としては不十分である。一方、高く外れたＧ鋼は、延性が低いため加工性が不十分である。さらにＭｎが高く外れたＩ鋼では、強度が高いため加工性が不十分である。Ｊ鋼ではＰ量が高く外れたため偏析に起因して延性が低い。また、Ｂ／Ｎが本発明の範囲から低く外れたＬ鋼は、焼入れ性が不十分となるため、焼入れ処理後の硬度が低い。
【００３５】
【表２】

【００３６】
【表３】

【００３７】
実施例３
実施例２の本発明の範囲にしたがったＣ及びＮ鋼について、薄スラブ連鋳法による鋳造後直ちに熱延工程に送る製造工程と、熱延工程で粗圧延終了後に先行材と接続して圧延を実施する、いわゆる連続熱延による工程で製造した。表４に製造工程を示す。なお、仕上温度、冷却条件、巻取温度は実施例２と同じとした。得られた材質を同表に示す。得られた材質も実施例２でのものとほぼ同様の特性である。
【００３８】
【表４】

【００３９】
【発明の効果】
本発明により、プレス等による加工時には優れた成形性を有しかつ、浸炭焼入れ処理を施す場合には、十分な焼入れ性も具備していることから、板厚中心部での強度を十分に確保した良成形性の熱処理用鋼板を提供することができる。
【図面の簡単な説明】
【図１】鋼中酸素量と穴拡げ試験におけるバラツキとの関係を示す図である。
【図２】鋼に含まれるＢ及びＮ量における原子比と浸炭焼入れ処理後の鋼硬度（Ｈｖ）との関係を示す図である。
【図３】浸炭焼入れ処理後の板厚方向の硬度分布に及ぼすＢ／Ｎの影響を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention has good formability with little variation at the time of burring, which affects the defect rate of the processed part, particularly when press forming, especially burring, and at the same time, the strength and resistance to heat by the heat treatment applied thereafter. The present invention relates to a steel plate for carburizing and quenching that has wearability and has little variation during burring, and a method for manufacturing the same. The steel plate according to the present invention is particularly applicable to strength members used in automobiles, motorcycles and bicycles, gears and clutch plates that require wear resistance, and the like.
[0002]
[Prior art]
As a method of providing wear resistance by heat treatment, there is a method of carburizing and nitriding using an atmosphere by liquid immersion or gas immersion. However, in this method, the hardness of the core part may be lower than the target value due to a change in hardenability due to a change in steel composition. In addition, from the viewpoint of workability, the burring workability of the steel sheet needs to be stabilized at a high level, especially in the burring portion because the processing variation greatly affects the product yield. In Japanese Patent Application Laid-Open No. 7-97662 considering improvement of workability, particularly bendability, it is assumed that there is no consideration of the amount of dissolved B that contributes to hardenability. Is not sufficient, and no consideration is given to burring workability. Japanese Patent Application Laid-Open No. 2000-34542 defines the total oxygen amount and the composition ratio of non-viscous inclusions formed in steel, and has cold workability and surface characteristics after nitriding treatment. Therefore, the present invention is clearly different from the present invention considering the cleanliness in steel and the hardenability by carburizing and quenching treatment. Furthermore, since none of them considers B / N, the idea is fundamentally different from the present invention.
[0003]
[Problems to be solved by the invention]
Therefore, the object of the present invention is to improve the product yield by reducing the variation in burring processing in component molding, and to ensure hardenability so that sufficient hardness can be imparted to the component core by carburizing and quenching treatment. It is aimed to do.
[0004]
[Means for Solving the Problems]
The present inventors conducted basic experiments in order to solve the above problems, and obtained the following knowledge leading to the present invention by controlling the amounts of O, B and N and optimizing the cooling conditions during hot rolling. That is, steels with various amounts of O, B and N were melted with the basic composition of C: 0.19%, Si: 0.01%, and Mn: 0.56%. These steels were hot-rolled at a heating temperature of 1200 ° C. and a finishing temperature of 850 ° C., cooled at a cooling rate of 20 ° C./s, and wound at 680 ° C. In addition, the winding process conditions at this time were furnace-cooled after heat-retaining at 680 degreeC for 2 hours. FIG. 1 shows the relationship between the amount of O in steel and the hole expandability. The hole expandability here is when a hole with a diameter of 10 mm (d ₀ ) is punched out, and the hole is expanded using a 60-degree conical punch so that the burr is on the outside, and the crack penetrates the plate thickness. The hole diameter (d) was measured and evaluated by d / d ₀ . The test piece was taken as a representative sample from the three places for the entire width and a longitudinal, d / d ₀ was obtained and the average value, the maximum and minimum values. As d / d ₀ decreases with an increase in the amount of oxygen in the steel, the minimum value becomes less than 2 especially when the content exceeds 0.004%.
[0005]
In order to ensure hardness by quenching, it is necessary to increase the hardness of martensite itself and at the same time improve the hardenability. Moreover, about the influence of B amount, the result shown in FIG. 2 was discovered. That is, in order to ensure that the hardness of the base material after carburizing and quenching is Hv: 400 or more, it is important to secure solid solution B, and the amount ratio of B and N expressed by atomic ratio is 1.0 or more. is necessary. It was also found that the atomic ratio of B and N has an influence on the hardness distribution in the vicinity of the surface layer after carburizing and quenching. That is, as shown in FIG. 3, it was found that when B / N (atomic ratio) is smaller than 1.0, the decrease in the vicinity of the surface layer portion is increased in the hardness distribution in the thickness direction. Such a phenomenon is considered to be because the hardenability by B is lowered because Fe (C, B) is formed during carburizing. In addition, when it has such a characteristic, we are anxious about the fall of rolling fatigue strength.
[0006]
Based on the above knowledge, a steel plate with little variation during burring and excellent carburizing hardenability and a manufacturing method thereof were established.
[0007]
The gist of the present invention is that
(1) By mass ratio, C: 0.15-0.25%, Mn: 0.05-0.8%, Al: 0.001-0.05%, N: 0.0015-0.005% B: 0.0015 to 0.005% satisfying the formula (1), Si: 0.5% or less, P: 0.035% or less, S: 0.015% or less, O: 0.004 % Carburizing and quenching steel sheet with less variation during burring, characterized by being made up of the remaining Fe and inevitable impurity elements.

(2) The variation during burring, characterized in that the steel sheet described in (1) further contains 0.001 to 0.02% of one or more of Mg, Ce or REM by mass ratio. Steel plate for carburizing and quenching with a small amount.
(3) A steel plate for carburizing and quenching with less variation at the time of burring, wherein the steel plate according to (1) or (2) further contains Ti: 0.005 to 0.1% by mass ratio.
(4) The steel plate according to any one of (1) to (3), further containing, by mass ratio, Mo: 0.05 to 0.5%, and carburizing and quenching with less variation during burring. Steel plate.
(5) The molten steel having the composition described in any one of (1) to (4) is made into a slab by continuous casting, and rough rolling is performed immediately after reheating or immediately after casting, in a temperature range above the Ar ₃ transformation point. Finishing rolling is finished and cooling is started from the temperature range, but cooling is performed at a cooling rate of 50 ° C./s or less, and winding is performed at a temperature range of 600 to 700 ° C. A method for producing a small number of carburized and quenched steel sheets.
(6) The burring according to (5), wherein the rough rolling is finished and the sheet bar is once wound on a coil and subjected to finish rolling as it is or connected to the preceding sheet bar and then finished rolling. A method for manufacturing steel plates for carburizing and quenching with less variation during processing.
(7) The method for producing a carburized and quenched steel sheet with less variation in burring processing according to (5) or (6), wherein rough rolling is performed immediately after casting on a slab of 100 mm or less.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
First, the reasons for limiting the component composition in this invention will be described.
[0009]
C is 0.15 to 0.25%. When added excessively, cementite will be formed as graphite, so re-solidification of C in the quenching process is not sufficiently achieved, and there is a concern that partial hardness will be insufficient, The upper limit is 0.25%. On the other hand, the amount of C is necessary to secure the hardness after quenching, and 0.15% or more is necessary to obtain 400 stably with Vickers.
[0010]
When Si is excessively added, an oxide elongated in the rolling direction remains in the steel, so that the burring workability is lowered, and the upper limit is set to 0.5% in order to increase the variation during processing.
[0011]
Mn is an element that plays an important role in ensuring hardenability, but delays the formation of pearlite in the cooling stage after finish rolling in the hot rolling stage. That is, since the pearlite transformation proceeds even after winding, if the volume ratio increases, the strength of the hot-rolled sheet increases and the workability deteriorates, so 0.8% is made the upper limit. On the other hand, 0.05% or more of addition is necessary to avoid cracking during hot rolling.
[0012]
P is not particularly limited in the present invention, but if added excessively, it causes segregation and causes deterioration of workability and secondary workability, so 0.035% is the upper limit. And
[0013]
Further, S is not particularly limited in the present invention, but is preferably lower in order to ensure hot workability, and the upper limit is 0.015%.
[0014]
Al is mainly added for deoxidation, but alumina in particular has a concern that its burring workability is deteriorated due to its form. Therefore, the lower limit is preferable, so the upper limit is set to 0.05%. However, when other deoxidizing elements are used, 0.001% is set as the lower limit.
[0015]
N is preferably as low as possible to ensure solid solution B as will be described later. Therefore, the upper limit is made 0.005%, preferably 0.003% or less.
[0016]
B is added for the purpose of ensuring hardenability as described above. However, since it is B in solid solution that ensures hardenability, it is necessary to consider the balance with N. If it is less than 0.0015%, the effect is insufficient from the balance with the amount of N contained in the steel. On the other hand, if it exceeds 0.005%, slab cracking occurs in the steelmaking stage, and workability deterioration due to hardening as a hot-rolled sheet material is concerned, so this is the upper limit.
[0017]
In the present invention, the ratio of B and N is defined as shown in equation (1). As described above, B / N (atomic ratio) of 1.0 or more is required to make the base metal hardness after carburizing and quenching Hv: 400 or more, and B / N (atomic ratio) is 1 If it is less than 0.0, the hardness decrease near the surface layer will increase. Moreover, the upper limit of B / N (atomic ratio) is set to 3.0 in order to suppress the increase in cost and the saturation of the effect. Therefore, it is defined as in equation (1).
[0018]
In the present invention, the amount of O is one of important defining factors. That is, for the variation in burring workability, the influence of the oxide distribution on the punched fracture surface is particularly large, so it is preferable to reduce the burring workability as much as possible. However, considering the deoxidation cost in the steelmaking stage, 0.004% Is the upper limit.
[0019]
Mg, Ce or REM is added for deoxidation. For deoxidation, 0.001% or more is necessary. On the other hand, excessive addition increases the cost, so 0.02% is made the upper limit. Note that oxides formed by deoxidation with these elements crystallize more finely than alumina or the like.
[0020]
Ti is added for deoxidation and to fix N by TiN prior to precipitation of BN, and to effectively secure an amount of B that contributes to hardenability during carburizing and quenching. If the amount is less than 0.005%, the effect cannot be exhibited. If the amount exceeds 0.1%, the effect is saturated and the cost is increased.
[0021]
Furthermore, Mo is added to suppress softening that occurs in the weld heat affected zone. In order to obtain the effect sufficiently, addition of 0.05% or more is necessary. On the other hand, even if added over 0.5%, the effect is saturated, so this is the upper limit.
[0022]
Further, mixing of trace amounts of Cu, Ni, Sn, and Cr due to the use of scrap does not impair the effects of the present invention.
[0023]
The heating temperature in the hot rolling process of the present invention is not particularly defined. However, when Ti is contained, TiN precipitates from a high temperature range, so that it is preferable that the TiN precipitate is coarsened to ensure grain growth.
[0024]
On the other hand, finish but for temperature and Ar ₃ transformation point or higher, in order to proceed rapidly the pearlite transformation in the cooling process that follows, it is preferable as close as possible to the Ar ₃ transformation point.
[0025]
Although it is cooled and wound after finishing rolling, it is necessary to finish the pearlite transformation before winding. For this purpose, the cooling rate on the runout table needs to be 50 ° C./s or less. If the cooling rate becomes faster than this, the pearlite transformation proceeds even after winding as described above, and the fraction also increases. As a result, there is a concern about deterioration of workability due to an increase in hot-rolled sheet strength.
[0026]
The coil is wound around the coil following cooling, and the temperature at that time is 600 to 700 ° C. When the temperature is lower than 600 ° C., bainite is formed, which is not preferable because strength is increased and workability is deteriorated. On the other hand, when C exceeds 700 ° C., C does not sufficiently precipitate as pearlite, and thus precipitates at grain boundaries during cooling after winding, and there is concern about deterioration of burring workability.
[0027]
When the above-described hot rolling is performed, the effect of the present invention is not impaired even if the rolling is performed by joining the sheet bar preceded by rough rolling using laser welding or the like.
[0028]
Furthermore, when manufacturing a slab, manufacturing a thin slab of 100 mm or less as a so-called near net shape and immediately manufacturing it under the rolling conditions as described above does not impair the effects of the present invention.
[0029]
【Example】
Example 1
C: 0.2%, Si: 0.01%, Mn: 0.58%, P: 0.011%, S: 0.005%, Al: 0.027%, N: 0.0024%, O : Steel containing 0.0017% and B: 0.0038% was converted into a steel from a converter and made into a slab by continuous casting. Hot rolling was performed at 1200 ° C., followed by rough rolling, and then hot rolling was terminated under the conditions shown in Table 1 to obtain a 2.0 mm hot rolled sheet. Here, the Ar ₃ transformation point is approximately 869 ° C. when roughly calculated by 916-50 [C (%)] + 27 [Si (%)] − 64 [Mn].
[0030]
The material evaluation was processed into a No. 5 test piece described in JIS Z 2201, and a tensile test was performed according to the test method described in JIS Z 2241. In addition, hole expansibility was investigated as an evaluation of burring characteristics. The method is to punch a hole with a diameter of 10 mm (d ₀ ), use a 60 degree conical punch to expand the hole so that the burr is on the outside, and the hole diameter (d ) And measured by d / d ₀ . The test specimens were taken in the width methods and longitudinal three rows, d / d ₀ is an average value and the minimum value. Furthermore, as a heat treatment equivalent to carburizing quenching, 900 ° C. × 1 h → oil quenching was performed, and hardness was measured with Vickers (load: 100 g).
[0031]
Table 1 shows the mechanical properties, d / d ₀ and hardness after heat treatment of the hot-rolled sheet obtained. No. according to the present invention. In 2, 3, 4, 5, and 6, there is little variation, a hole expandability of 2.2 or more is obtained, and a high ductility exceeding 30% is obtained. On the other hand, no. No. 1, Rot cold speed deviated faster than the range of the present invention. No. 7, and the winding temperature deviated from the scope of the present invention. In No. 8, ductility is low. No. In No. 9, the variation in hole expansibility is large, and the minimum value is particularly small. Both are attributed to the microstructure. No. 1 is caused by the heterogeneous structure of ferrite. No. 7 because the Rot cold speed deviates from the range of the present invention and the pearlite transformation proceeds during winding. No. No. 8 is caused by the formation of bainite. 9 is presumably due to the fact that cementite is formed at the grain boundaries. In addition, No. Although 6 is within the scope of the present invention, the strength is higher because the finishing temperature is higher and the Rot cooling speed is slightly higher. Under any conditions, the hardness after heat treatment is over 400 Vickers.
[0032]
[Table 1]

[0033]
Example 2
Various steels shown in Table 2 were converted into steel from the converter and made into slabs by continuous casting. Hot rolling was carried out at 1150 to 1200 ° C., followed by rough rolling and finish rolling to produce hot rolled plates having a thickness as shown in Table 3. Incidentally, finish rolling are all finished in a temperature range of not lower than Ar ₃ transformation point. Furthermore, the amount of water in the cooling zone was adjusted so that the cooling rate after finish rolling was within the range of the present invention. After cooling, winding was performed at 600 to 700 ° C., and material evaluation and hole expansibility evaluation by a tensile test were performed in the same manner as in Example 1. Further, the heat treatment corresponding to carburizing and quenching and the hardness measurement performed on each heat-treated material were performed under the same conditions as in Example 1.
[0034]
The results are shown in the same table. A, B, C, D, E, N, O, and P steels according to the present invention have high ductility, little variation, and hole expansibility exceeding 2.2. In particular, the D, I, N, and O steels using Ti, Mg, and Ce at the time of deoxidation have extremely small variations in hole expansibility. On the other hand, the H, K, and M steels have large variations in hole expansibility, which is less than 2.0 when viewed at the minimum value. In steel F, which has a low C content, the hardness by quenching treatment is less than 400, which is insufficient as a characteristic. On the other hand, since the steel G which deviated highly has low ductility, workability is inadequate. Furthermore, in steel I with a high Mn, the workability is insufficient due to the high strength. In steel J, the amount of P is high, so the ductility is low due to segregation. In addition, L steel with B / N deviating from the range of the present invention has insufficient hardenability, and therefore has low hardness after quenching.
[0035]
[Table 2]

[0036]
[Table 3]

[0037]
Example 3
For C and N steels according to the scope of the present invention of Example 2, a manufacturing process that is sent to the hot rolling process immediately after casting by the thin slab continuous casting method, and a rolling process that is connected to the preceding material after the rough rolling in the hot rolling process It manufactured by the process by what is called continuous hot rolling. Table 4 shows the manufacturing process. The finishing temperature, cooling conditions, and winding temperature were the same as in Example 2. The obtained materials are shown in the same table. The obtained material has almost the same characteristics as those in Example 2.
[0038]
[Table 4]

[0039]
【The invention's effect】
According to the present invention, it has excellent formability at the time of processing with a press and the like, and when carburizing and quenching is performed, it has sufficient hardenability, so sufficient strength at the center of the plate thickness is secured. It is possible to provide a heat-formed steel sheet with good formability.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the amount of oxygen in steel and variation in a hole expansion test.
FIG. 2 is a graph showing the relationship between the atomic ratio of B and N contained in steel and the steel hardness (Hv) after carburizing and quenching.
FIG. 3 is a diagram showing the influence of B / N on the hardness distribution in the plate thickness direction after carburizing and quenching treatment.

Claims (7)

By mass ratio, C: 0.15-0.25%, Mn: 0.05-0.8%, Al: 0.001-0.05%, N: 0.005% or less, B: 0.0015 -0.005% satisfying the formula (1), Si: 0.5% or less, P: 0.035% or less, S: 0.015% or less, O: 0.004% or less, the balance A steel plate for carburizing and quenching with less variation during burring, characterized by comprising Fe and inevitable impurity elements.

The steel plate according to claim 1, further containing 0.001 to 0.02% of one or more of Mg, Ce, or REM by mass ratio, and carburizing with less variation during burring. Hardened steel sheet. A steel plate for carburizing and quenching with less variation during burring, wherein the steel plate according to claim 1 or 2 further contains Ti: 0.005 to 0.1% by mass. A steel plate for carburizing and quenching with less variation at the time of burring, wherein the steel plate according to any one of claims 1 to 3 further contains Mo: 0.05 to 0.5% by mass ratio. The molten steel having the component composition according to any one of claims 1 to 4 is made into a slab by continuous casting, rough rolling is performed immediately after reheating or after casting, and finish rolling is finished in a temperature range above the Ar ₃ transformation point. In addition, the steel plate for carburizing and quenching, which starts cooling from that temperature range, is cooled at a cooling rate of 50 ° C./s or less, and is wound at a temperature range of 600 to 700 ° C., and has less variation during burring. Manufacturing method. The rough rolling is finished, the sheet bar is once wound on a coil, and subjected to finish rolling as it is or connected to the preceding sheet bar, and then finish rolling is performed. A method for producing a carburized and quenched steel sheet with little variation. The method for producing a steel sheet for carburizing and quenching with less variation during burring according to claim 5 or 6, wherein rough rolling is performed immediately after casting on a slab of 100 mm or less.

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