JP3851534B2 - Method for producing steel material with excellent surface properties - Google Patents
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- JP3851534B2 JP3851534B2 JP2001312802A JP2001312802A JP3851534B2 JP 3851534 B2 JP3851534 B2 JP 3851534B2 JP 2001312802 A JP2001312802 A JP 2001312802A JP 2001312802 A JP2001312802 A JP 2001312802A JP 3851534 B2 JP3851534 B2 JP 3851534B2
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Description
【０００１】
【発明の属する技術分野】
本発明は、表面性状に優れた鋼材の製造方法に関し、詳細には熱間圧延前の鋼素材を２段階加熱したあと、常法により熱間圧延して条鋼材、鋼板などの鋼材を製造する技術に関するものである。
【０００２】
【従来の技術】
鋼材の熱間圧延において、加熱炉にて生成した1次スケールが鋼材表面に残留した状態で圧延を行うとスケール疵と呼ばれる表面欠陥が発生するため、熱間圧延前に通常高圧水デスケーリングによって1次スケールを除去している。
【０００３】
一方、一般にＳｉを含有する鋼材表面に加熱炉内で生成される1次スケールは強固で除去しがたく、高圧水デスケーリングによってもたびたび剥離不良が発生して、スケール疵発生の原因となると言われている。その改善策として、特開２０００−１５３２３号公報には、Ｓｉ含有鋼の1次スケールを除去するに当たり、スケールの上層に位置するＦｅ2Ｏ3、Ｆｅ3Ｏ4を噴射エネルギーを増大させた高圧水でまず除き、その後残留したスケールの下層（気孔を有するＦｅＯ層 及びＦｅ2ＳｉＯ4層）を低い噴射エネルギーの高圧水で除去する方法が提案されている。
【０００４】
上記提案の方法では、高圧水によるデスケーリングの際にスケール上層は容易に吹き飛ばされる。しかしながらスケール下層は層内の気孔が原因となって容易に除去できず、ポーラスなスケール下層部分が鋼材（地鉄）表面に残る。従って、実際はスケールの下層は粒界酸化による地鉄への食い込みがあって剥離性阻害の原因となる上、スケール下層の細かい気孔が剥離性を阻害し、噴射エネルギーが大きい場合であっても十分な脱スケール性は得られない。
【０００５】
また一方、鋼材のスケール起因による表面欠陥の低減策として、熱間圧延時の加熱を制御して行なうことが提案されている。例えば、特開平１１−２８６７１８号公報には、表面欠陥の原因となる楔状の粒界酸化低減のため、加熱時に一定温度、一定時間水蒸気を添加する方法が提案されている。しかしながら、この提案の方法では、水蒸気の添加によりスケール生成量が均一化することで粒界酸化を低減し得る効果は期待されるものの、スケールの剥離性には粒界酸化のほか、スケール内の気孔分布が大きく影響するため、この提案の方法ではスケール剥離性改善効果は必ずしも十分とはいえず、特に炭素量の高い鋼材においては、酸化に伴うＣＯガスの発生が考えられ、その影響でスケールと地鉄の界面部近傍のサブスケール内に微細な気孔が発生し、スケールの剥離性に悪影響を与えることが想定される。
【０００６】
【発明が解決しようとする課題】
本発明は、上記の事情を基になしたものであって、その目的は、熱間圧延前の鋼素材の加熱時に発生するスケールを高圧水デスケーリングで除去し得る形態にすることで、熱間圧延後の表面疵の発生を低減し得る、表面性状に優れた鋼材の製造方法を提供するものである。
【０００７】
【課題を解決するための手段】
上記の目的を達成するために、本発明（請求項１）に係る表面性状に優れた条鋼材の製造方法は、熱間圧延して鋼材を製造する際の熱間圧延前の加熱にあたり、第１段目加熱として加熱温度１２００〜１３５０℃で雰囲気の水蒸気濃度が２０〜３５％となるように水蒸気を燃焼ガスと共に供給して３〜１２分加熱を行ったあと、第２段目加熱として雰囲気の水蒸気濃度を２０％未満、加熱温度を１０００〜１２００℃（但し１２００℃を含まず）で１５〜４０分加熱を行い、その後、高圧水デスケーリングを行うものである。この場合において、特に限定するものではないが、鋼材は０．１質量％以上の炭素、より好ましくは０．３質量％以上の炭素を含有するものを対象とすることが好ましい。
【０００８】
【発明の実施の形態】
本発明者等は、炭素量の比較的高い鋼材（Ｃ≧０．１質量％）においては、酸化に伴うＣＯガスの発生が考えられ、その影響でスケールと地鉄の界面部近傍のサブスケール内に微細な気孔が発生しスケールの剥離性に悪影響を与えるとの認識から、炭素を０．３質量％以上含有する条鋼材を製造する際の鋼素材の加熱炉で生成する1次スケールについて詳細に調査を行い、以下の知見を得た。
【０００９】
すなわち、鋼材においては、一般に加熱炉で生成される1次スケールは地鉄側から順にＳｉ、Ｃｒ等の合金元素が濃化し、かつ微細な気孔を多数含有するサブスケール層、比較的大きい気孔を含有するＦｅＯ層、Ｆｅ3Ｏ4、Ｆｅ2Ｏ3層の各層から構成される。この内、上層のＦｅ2Ｏ3、Ｆｅ3Ｏ4とＦｅＯ層の一部は、高圧水デスケーリングにより急冷され、割れが発生して容易に吹き飛ばされ除去される。しかしながら、気孔を含有するＦｅＯ下層と地鉄界面に生成されるサブスケール層については、微細な気孔と粒界酸化が原因で剥離性が悪化し地鉄表面に残ってしまう。これがそのまま圧延されると、鋼材内部に押し込まれ、スケール疵不良が発生する（図1ｂ参照）。
【００１０】
ところで、上記スケール中の気孔、空隙の生成には、加熱時に脱炭により発生するＣＯガスが関与している。特に炭素を０．１質量％以上、更には０．３質量％以上と多く含有する鋼材においては上層スケール内に空隙層が生成され、下層のサブスケール内には微小な気孔が多数発生する。前記空隙層より上層のスケールは高圧水デスケーリングにより容易に除去されるが、下層のサブスケールは微細な気孔が剥離性を阻害し、地鉄表面にサブスケールが残留する。
【００１１】
そこで、本発明者等は、炭素を比較的多く含有する鋼材（Ｃ≧０．１質量％）においてスケールの剥離性を阻害する気孔に関し、気孔分布を制御可能な加熱方法を検討した結果、第1段目加熱として１２００〜１３５０℃の温度範囲で雰囲気の水蒸気濃度が２０〜３５％になるように水蒸気を燃焼ガスとともに供給して３〜１２分以下加熱を行ったあと、第２段目加熱として水蒸気の供給を遮断して１０００〜１２００℃の温度で１５〜４０分加熱を行うことが有効であることを見出し、本発明を完成させたものである。
【００１２】
上記の本発明方法によれば、まだ十分にメカニズムは解明されてはいないが、第１段目加熱の際の水蒸気の供給により、ＣＯの発生等により生成されるスケール剥離性を阻害するサブスケール層内の微細な気孔が水蒸気の浸透で肥大化、連続化されて、スケールと地鉄の界面近傍に層状の空隙が生成し、第１段目加熱引き続く第２段目加熱中にスケール全体が浮き上がった状態になり（図１ａ参照）、これによりスケール全体が高圧水により容易に吹き飛ばされ、デスケーリング性が大幅に改善されたものと推測される。
【００１３】
以下、本発明が２段階加熱とした理由並びに各段階の加熱条件の限定理由について詳細に説明する。
【００１４】
本発明が２段階加熱とした理由は、水蒸気による気孔の肥大化・連続化効果は高温で短時間水蒸気を供給すれば十分であり、高温で長時間水蒸気を供給すると、スケール発生が急激に増大して、逆にスケール剥離性が悪化する。従って、水蒸気供給により第１段目加熱で高温で短時間加熱を行ったあと、水蒸気を遮断して第２段目加熱で第１段目加熱温度より低い温度で十分均熱することにより、加熱炉抽出時に剥離容易なスケールが生成されるためである。
【００１５】
第１段目の加熱条件について、本発明者等は、鋼材を１２００℃以上で加熱する際に水蒸気を添加すると、気孔の増大・肥大化効果が最大限に発揮されることを見出し、加熱温度の下限を１２００℃以上とした。この１２００℃以上で水蒸気を供給して加熱すると、サブスケール内の気孔が肥大化・連続化してスケール全体を浮き上がらせることができる。水蒸気によりサブスケール内の気孔が肥大化するのは、過剰な水蒸気から解離した水素がＦｅＯ内を拡散、さらにスケールを還元してガス状のＨ2Ｏとなり、このガス状のＨ2Ｏがサブスケール層内を通過するようになり、その結果、ＣＯガスを起因とする微細な気孔の他に、このガス状のＨ2Ｏを起因とする大きな気孔が形成される、又はＣＯガス起因の微細な気孔をガス状のＨ2Ｏが肥大化させていると推論される。そして更に、このように肥大化する過程で気孔同士が合体し、気孔の連続化が生じているものと推論される。なお、１３５０℃を超えて水蒸気を供給すると、スケールの発生が極端に増え、スケール剥離性が悪くなるとともに鋼材の歩留まりが低下する。従って、加熱温度は１２００〜１３５０℃に限定するが、高温での加熱は設備面での制約が大きく、コストも嵩むことになるので、上限はより好ましくは１３００℃以下がよい。
【００１６】
また、水蒸気を供給しての加熱時間は１２分以下で十分であり、１２分を超えるとスケールが厚く付きすぎて剥離性が悪化する。また３分以下では水蒸気の効果は十分ではない。従って、加熱時間は３〜１２分に限定する。
【００１７】
また、雰囲気の水蒸気濃度は２０％以上とする。この濃度を２０％未満にすると、気孔の増大・肥大化効果によるスケール剥離性向上効果が弱く、表面疵の発生率が高くなる。また水蒸気濃度の上限については、水蒸気濃度が３５％を超えると、スケール増大による剥離性悪化、歩留まり低下、また加熱炉内温度の低下によるエネルギー効率悪化といった問題が発生し好ましくない。従って、雰囲気の水蒸気濃度は２０〜３５％に限定する。
【００１８】
第２段目の加熱条件について、第２段目加熱は、第１段目加熱で高温、短時間の水蒸気供給により剥離しやすいスケールを生成させたあと、出来るだけスケールの生成を抑えた上で熱間圧延時の温度確保の目的で実施する。従って、スケールの発生を助長する水蒸気の供給を遮断して雰囲気の水蒸気濃度を２０％未満にするとともに加熱温度を１２００℃未満に下げ、１５〜４０分加熱してスケールの発生を出来るだけ抑制する。なお、加熱温度が１０００℃以下では鋼材の均熱が不十分であり、圧延時の負荷が大きくなって好ましくない。また、加熱時間の下限が１５分未満では材料の均熱が不十分となることが懸念されるので、１５分以上の加熱を行なうが、４０分を超えての加熱では、スケールが大量に発生し、スケール剥離性が悪化する。
【００１９】
なお、本発明方法を適用する鋼材について特に限定するものではないが、炭素量が比較的低い鋼材では、スケール内に気孔がほとんど発生せず、スケール剥離性に問題がないので、本発明方法の効果を期待するのであれば、炭素量０．１質量％以上の鋼材、より効果を期待するのであれば、炭素量０．３質量％以上の鋼材を対象とすることが望ましい。
【００２０】
【実施例】
表１に示す炭素を０．３％、０．７％を含有する鋼素材（１５０ｍｍ角）を、表２の各加熱条件で処理し、通常の高圧水のデスケーリング処理を行ったあと、引き続いて常法により熱間圧延を行って条鋼材（断面寸法：１２ｍｍφ）を製造した。本実験では、加熱炉の燃焼用ガスとしてＬＮＧガスを使用し、水蒸気を別途供給しない場合の炉内雰囲気のガス組成は、７２％Ｎ2−１８％Ｈ2Ｏ−９％ＣＯ2−１％Ｏ2である。
【００２１】
【表１】
【表２】
上記製造によって得られた条鋼材の表面疵の評価を次の要領で行なうと共に、その評価結果を上記表２に併せて示す（表２の疵のランク）。条鋼材の表面疵の評価要領は、条鋼材の圧延方向に垂直な横断面１０箇所以上で観測される鋼材表面から深さ１０μｍ以上に達するスケール起因の表面疵の発生個数を計測してその平均値を算出して行い、発生が全く無いものをランク０、発生個数が１０個以下のものをランク１、１１個以上２０個未満のものをランク２、２０個以上３０個未満のものをランク３、３０個以上のものをランク４とした。なお、ランク１以下であれば、加工性に優れた表面性状となり、製品として全く問題がない。
【００２４】
上記表２から明らかなように、条件１，２，５，６は本発明例（実施例）であって、鋼種Ａ，Ｂともに1段目加熱として雰囲気の水蒸気濃度を２０〜３５％に保持しながら１２００〜１３５０℃の加熱温度で３〜１２分間加熱したあと、２段目加熱として水蒸気の供給を遮断して加熱温度１０００〜１２００℃で１５〜４０分加熱処理を実施したものであって、この場合には、サブスケールと地鉄の界面部近傍に空隙層が生成されて高圧水デスケーリングによりほぼ完全にスケールが除去でき、また熱間圧延が問題なく実施できた。また更に得られた条鋼材はスケール起因のスケール疵がランク１以下であり、疵が極めて少ない。
【００２５】
条件３は比較例であって、第１段目加熱時の加熱時間が１５分と長かったため、水蒸気の影響によりスケールが大量に発生した。このため、スケール剥離性が悪化し、高圧水によるデスケーリングによっても完全に除去できず、鋼材表面に残留したスケールが押し込まれてスケール疵が発生した。
【００２６】
条件４は比較例であって、第２段加熱時の加熱時間が６０分と長かったため、その加熱でスケールが大量に発生した。このため、スケール剥離性が悪化し、高圧水デスケーリングによっても完全に除去できず、残留スケールが押し込まれてスケール疵が発生した。
【００２７】
条件７は比較例であって、第１段加熱時間が２０分と長く、かつ第２段加熱時間も５０分と長かったため、スケールが極めて大量に発生し、スケール剥離性が悪化し、高圧水デスケーリングによっても完全に除去できず、残留スケールが押し込まれてスケール疵が発生した。
【００２８】
条件８は比較例であって、第１段加熱時間が６０分と長かったため、この第１段加熱時に極めて大量のスケールが発生して、スケール剥離性が悪化した。
【００２９】
条件９，１０，１１は比較例であって、第１段加熱時に水蒸気の供給を行なわずに雰囲気のみで加熱したため、サブスケール内気泡が連続化されず、スケール剥離性向上効果が全く無かった。
【００３０】
条件１２，１３は比較例であって、第１段加熱時の雰囲気の水蒸気濃度が４１％、４５％といずれも高かったもので、この水蒸気の影響によりスケールが大量に発生するとともに、加熱炉内温度低下によるエネルギー効率低下といった問題が認められた。
【００３１】
【発明の効果】
以上説明したように、本発明に係る表面性状に優れた鋼材の製造方法によれば、熱間圧延前の鋼材の加熱を２段階で行なうとともに、第１段目の加熱で加熱炉雰囲気へ水蒸気を供給することにより、加熱時に生成するスケールのデスケーリング性を改善することができ、これにより熱間圧延後の鋼材のスケール起因の表面欠陥を低減することができ、表面性状に優れた鋼材を製造することができる。
【図面の簡単な説明】
【図１】加熱後の鋼材のスケール状態を説明するための断面模式図であって、ａは本発明方法の場合、ｂは従来方法の場合の図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a steel material having excellent surface properties. Specifically, after heating a steel material before hot rolling in two stages, it is hot-rolled by a conventional method to produce a steel material such as a strip steel material or a steel plate. It is about technology.
[0002]
[Prior art]
In hot rolling of steel, if rolling is performed with the primary scale generated in the heating furnace remaining on the surface of the steel, surface defects called scale defects occur. The primary scale is removed.
[0003]
On the other hand, the primary scale generated in the furnace in general on the surface of steel containing Si is strong and difficult to remove, and high-pressure water descaling often causes poor peeling, causing scale flaws. It has been broken. As an improvement measure, Japanese Patent Application Laid-Open No. 2000-15323 discloses a high pressure in which the injection energy of Fe 2 O 3 and Fe 3 O 4 located in the upper layer of the scale is increased in removing the primary scale of the Si-containing steel. There has been proposed a method in which water is first removed, and then the remaining lower scale layers (FeO layer having pores and Fe 2 SiO 4 layer) are removed with high-pressure water having a low jet energy.
[0004]
In the proposed method, the scale upper layer is easily blown off when descaling with high-pressure water. However, the scale lower layer cannot be easily removed due to pores in the layer, and a porous scale lower layer portion remains on the surface of the steel material (ground iron). Therefore, in fact, the lower layer of the scale may bite into the iron core due to grain boundary oxidation, which may cause the peelability to be hindered, and the fine pores in the lower layer of the scale inhibit the peelability and are sufficient even when the injection energy is large. Unscalability is not obtained.
[0005]
On the other hand, as a measure for reducing the surface defects due to the scale of the steel material, it has been proposed to control the heating during hot rolling. For example, Japanese Patent Application Laid-Open No. 11-286718 proposes a method of adding water vapor at a constant temperature for a predetermined time during heating in order to reduce wedge-shaped grain boundary oxidation that causes surface defects. However, in this proposed method, the effect of reducing the grain boundary oxidation by homogenizing the amount of scale generated by adding water vapor is expected, but in addition to the grain boundary oxidation, the peelability of the scale is not limited within the scale. Since the pore distribution has a large effect, the proposed method does not necessarily have an effect of improving the scale peelability. In particular, in steel materials with a high carbon content, generation of CO gas due to oxidation is considered. It is assumed that fine pores are generated in the sub-scale near the interface between the steel and the ground iron, adversely affecting the peelability of the scale.
[0006]
[Problems to be solved by the invention]
The present invention is based on the above circumstances, and its purpose is to form a scale that can be removed by high-pressure water descaling when the scale generated during heating of the steel material before hot rolling is heated. The present invention provides a method for producing a steel material having excellent surface properties that can reduce the occurrence of surface defects after cold rolling.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, a method for producing a strip steel material having excellent surface properties according to the present invention (Claim 1) is a method of performing heating before hot rolling when hot rolling to produce a steel material. After heating for 3 to 12 minutes by supplying water vapor with the combustion gas so that the water vapor concentration of the atmosphere becomes 20 to 35% at a heating temperature of 1200 to 1350 ° C. as the first stage heating, the atmosphere as the second stage heating The water vapor concentration is less than 20%, the heating temperature is 1000 to 1200 ° C. (but not including 1200 ° C.) for 15 to 40 minutes, and then high-pressure water descaling is performed. In this case, although it does not specifically limit, it is preferable that steel materials target what contains 0.1 mass% or more carbon, More preferably, it contains 0.3 mass% or more carbon.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors consider that in steel materials having a relatively high carbon content (C ≧ 0.1 mass%), CO gas generation due to oxidation is considered, and as a result, a subscale near the interface between the scale and the steel The primary scale generated in the furnace for steel materials when producing steel bars containing 0.3% by mass or more of carbon, based on the recognition that fine pores are generated in the inside and adversely affect the peelability of the scale We conducted a detailed survey and obtained the following knowledge.
[0009]
That is, in steel materials, the primary scale generally generated in a heating furnace has a subscale layer containing a large number of fine pores and relatively large pores, in which alloy elements such as Si and Cr are concentrated in order from the ground iron side. FeO layer containing consists layers of Fe 3 O 4, Fe 2 O 3 layer. Among them, the upper Fe 2 O 3 , Fe 3 O 4 and part of the FeO layer are rapidly cooled by high-pressure water descaling, cracks are generated, and they are easily blown away and removed. However, the subscale layer generated at the interface between the FeO lower layer containing pores and the ground iron deteriorates due to fine pores and grain boundary oxidation, and remains on the surface of the ground iron. When this is rolled as it is, it is pushed into the steel material and a scale defect occurs (see FIG. 1b).
[0010]
By the way, the generation of pores and voids in the scale involves CO gas generated by decarburization during heating. In particular, in a steel material containing a large amount of carbon of 0.1% by mass or more, and further 0.3% by mass or more, a void layer is generated in the upper scale, and a large number of minute pores are generated in the lower subscale. The scale above the void layer is easily removed by high-pressure water descaling, but in the lower subscale, fine pores impair the peelability, and the subscale remains on the surface of the ground iron.
[0011]
Therefore, the present inventors have studied a heating method capable of controlling the pore distribution as a result of examining the pores that inhibit the peelability of the scale in a steel material (C ≧ 0.1 mass%) containing a relatively large amount of carbon. As the first stage heating, steam is supplied together with the combustion gas so that the steam concentration in the atmosphere becomes 20 to 35% in a temperature range of 1200 to 1350 ° C., and heating is performed for 3 to 12 minutes or less, and then the second stage heating. As a result, the inventors have found that it is effective to perform the heating for 15 to 40 minutes at a temperature of 1000 to 1200 ° C. while cutting off the supply of water vapor, thereby completing the present invention.
[0012]
According to the above-described method of the present invention, the mechanism has not yet been fully elucidated, but the subscale that inhibits the scale peelability generated by the generation of CO or the like by the supply of water vapor during the first stage heating. The fine pores in the layer are enlarged and continuous due to the penetration of water vapor, and a layered void is formed in the vicinity of the interface between the scale and the ground iron, and the entire scale is heated during the second stage heating following the first stage heating. It is assumed that the scale has been lifted (see FIG. 1a), and the entire scale is easily blown away by the high-pressure water, and the descalability is greatly improved.
[0013]
Hereinafter, the reason why the present invention adopts the two-stage heating and the reason for limiting the heating conditions at each stage will be described in detail.
[0014]
The reason why the present invention adopts the two-stage heating is that it is sufficient to supply steam for a short time at a high temperature for the effect of enlargement and continuation of pores by steam, and when steam is supplied for a long time at a high temperature, scale generation increases rapidly. On the contrary, the scale peelability deteriorates. Therefore, after heating for a short time at a high temperature in the first stage heating by supplying steam, the steam is shut off and the temperature is sufficiently soaked at a temperature lower than the first stage heating temperature by the second stage heating. This is because a scale that is easy to peel off is generated during furnace extraction.
[0015]
Regarding the heating conditions of the first stage, the present inventors have found that when steam is added at the time of heating a steel material at 1200 ° C. or higher, the effect of increasing pores and enlarging is exhibited to the maximum. The lower limit was set to 1200 ° C. or higher. When steam is supplied and heated at 1200 ° C. or higher, the pores in the subscale are enlarged and continuous, and the entire scale can be lifted. The pores in the subscale are enlarged by the water vapor because the hydrogen dissociated from the excessive water vapor diffuses in the FeO and further reduces the scale to become gaseous H 2 O. This gaseous H 2 O As a result, in addition to the fine pores caused by CO gas, large pores caused by this gaseous H 2 O are formed, or the fine pores caused by CO gas It is inferred that the gaseous pores are enlarged by gaseous H 2 O. Further, it is inferred that the pores are united in the process of enlargement and the pores are continuous. In addition, when water vapor | steam is supplied over 1350 degreeC, generation | occurrence | production of a scale will increase extremely, scale peelability will worsen, and the yield of steel materials will fall. Therefore, although heating temperature is limited to 1200-1350 degreeC, since the restriction | limiting on an installation surface is large and heating at high temperature will increase cost, More preferably, an upper limit is 1300 degrees C or less more preferably.
[0016]
In addition, the heating time after supplying water vapor is 12 minutes or less, and if it exceeds 12 minutes, the scale becomes too thick and the peelability deteriorates. Further, the effect of water vapor is not sufficient in 3 minutes or less. Therefore, the heating time is limited to 3-12 minutes.
[0017]
The water vapor concentration in the atmosphere is 20% or more. If this concentration is less than 20%, the effect of improving the scale peelability due to the effect of increasing pores and enlarging is weak, and the incidence of surface flaws increases. As for the upper limit of the water vapor concentration, if the water vapor concentration exceeds 35%, problems such as exfoliation deterioration due to scale increase, yield reduction, and energy efficiency deterioration due to temperature reduction in the heating furnace are undesirable. Therefore, the water vapor concentration of the atmosphere is limited to 20 to 35%.
[0018]
Regarding the heating conditions for the second stage, the second stage heating is performed by generating a scale that is easy to peel off by supplying steam at a high temperature for a short time in the first stage heating, and then suppressing the generation of the scale as much as possible. Implemented for the purpose of securing the temperature during hot rolling. Therefore, the supply of water vapor that promotes the generation of scale is cut off to reduce the water vapor concentration of the atmosphere to less than 20%, and the heating temperature is reduced to less than 1200 ° C. and heated for 15 to 40 minutes to suppress the generation of scale as much as possible. . In addition, when heating temperature is 1000 degrees C or less, the soaking | uniform-heating of steel materials is inadequate, and the load at the time of rolling becomes large and is unpreferable. In addition, if the lower limit of the heating time is less than 15 minutes, there is a concern that the soaking of the material will be insufficient, so heating for 15 minutes or more is performed, but heating exceeding 40 minutes causes a large amount of scale. In addition, the scale peelability deteriorates.
[0019]
The steel material to which the method of the present invention is applied is not particularly limited. However, in the steel material having a relatively low carbon content, there are almost no pores in the scale, and there is no problem in the scale peelability. If the effect is expected, it is desirable to target a steel material having a carbon content of 0.1% by mass or more, and if more effect is expected, it is desirable to target a steel material having a carbon content of 0.3% by mass or more.
[0020]
【Example】
A steel material (150 mm square) containing 0.3% and 0.7% of the carbon shown in Table 1 was processed under each heating condition shown in Table 2, followed by normal high-pressure water descaling, followed by Then, hot rolling was performed by a conventional method to produce a steel bar (cross-sectional dimension: 12 mmφ). In this experiment, LNG gas is used as the combustion gas for the heating furnace, and the gas composition in the furnace atmosphere when water vapor is not separately supplied is 72% N 2 -18% H 2 O-9% CO 2 -1%. O 2 .
[0021]
[Table 1]
[Table 2]
The evaluation of the surface defects of the strip obtained by the above production is performed in the following manner, and the evaluation results are also shown in Table 2 (rank of the defects in Table 2). The guideline for the evaluation of surface flaws of steel bars is the average of the number of surface flaws caused by scale reaching a depth of 10 μm or more from the steel surface observed at 10 or more cross-sections perpendicular to the rolling direction of the steel bars. Calculate the value, rank 0 if there is no occurrence, rank 1 if the number is 10 or less, rank 2 if 11 or more and less than 20, rank 20 or more and less than 30 3, 30 or more were ranked 4. In addition, if it is rank 1 or less, it will become the surface property excellent in workability, and there is no problem as a product at all.
[0024]
As is apparent from Table 2 above, Conditions 1, 2, 5 and 6 are examples of the present invention (Examples), and both the steel types A and B are maintained at the water vapor concentration of 20 to 35% as the first stage heating. Then, after heating for 3 to 12 minutes at a heating temperature of 1200 to 1350 ° C., the supply of water vapor was shut off as the second stage heating, and the heat treatment was performed for 15 to 40 minutes at a heating temperature of 1000 to 1200 ° C. In this case, a void layer was generated in the vicinity of the interface between the subscale and the ground iron, and the scale could be removed almost completely by high-pressure water descaling, and hot rolling could be performed without any problems. Furthermore, the obtained bar steel has a scale defect due to scale of rank 1 or less, and the defect is extremely small.
[0025]
Condition 3 is a comparative example, and since the heating time during the first stage heating was as long as 15 minutes, a large amount of scale was generated due to the influence of water vapor. For this reason, the scale peelability deteriorated and could not be completely removed even by descaling with high-pressure water, and the scale remaining on the surface of the steel material was pushed in to generate scale flaws.
[0026]
Condition 4 is a comparative example, and since the heating time in the second stage heating was as long as 60 minutes, a large amount of scale was generated by the heating. For this reason, the scale peelability deteriorated and could not be completely removed even by high-pressure water descaling, and the residual scale was pushed in to generate scale wrinkles.
[0027]
Condition 7 is a comparative example in which the first stage heating time was as long as 20 minutes and the second stage heating time was as long as 50 minutes, so that a large amount of scale was generated, the scale peelability deteriorated, and high pressure water Even with descaling, it could not be removed completely, and the residual scale was pushed in, resulting in scale flaws.
[0028]
Condition 8 is a comparative example, and since the first stage heating time was as long as 60 minutes, a very large amount of scale was generated during the first stage heating, and the scale peelability deteriorated.
[0029]
Conditions 9, 10, and 11 are comparative examples, and since heating was performed only in the atmosphere without supplying water vapor during the first stage heating, the bubbles in the subscale were not continuous, and there was no effect of improving the scale peelability. .
[0030]
Conditions 12 and 13 are comparative examples, in which the water vapor concentration in the atmosphere during the first stage heating was high, both 41% and 45%. A large amount of scale was generated by the influence of this water vapor, and the heating furnace Problems such as reduced energy efficiency due to lower internal temperature were observed.
[0031]
【The invention's effect】
As described above, according to the method for manufacturing a steel material having excellent surface properties according to the present invention, the steel material before hot rolling is heated in two stages, and the first stage heating is performed to bring the steam into the furnace atmosphere. Can improve the descalability of the scale generated during heating, thereby reducing the surface defects caused by the scale of the steel after hot rolling, resulting in a steel with excellent surface properties. Can be manufactured.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view for explaining a scale state of a steel material after heating, in which a is a case of the method of the present invention and b is a case of a conventional method.
Claims (1)
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