【発明の詳細な説明】[Detailed description of the invention]
本発明は高Si系ばね用鋼線材の製造法、さらに
詳しくは鋼材表層部のフエライト脱炭層の生成を
防止できる高Si系ばね用鋼線材の製造法に関す
る。
高Si系鋼、典型的にはC0.35〜0.75％、Si1〜３
％、Mn0.3〜1.5％を含む高Si系鋼は主としてば
ね用鋼線材として多く使用されているが、一般に
知られるようにSi含量が高いため、高温加熱中に
鋼材表層部が脱炭し易い。ばね用鋼線材の場合、
脱炭層が存在すると、ばねの主要特性である疲労
強度が低下する。特に、鋼線材表層部にフエライ
トのみで、パーライトの殆んどない層（以下、フ
エライト脱炭層という）が存在すると、強度付与
のため焼入焼戻し処理を行つても強度が出ず、疲
労破壊の起点となるなど、ばね特性を著しく損な
う原因となる。
したがつて、従来、高Si系ばね用鋼線材を製造
するにあたつては、熱間圧延に先立つ加熱炉での
鋼片加熱は可能な限り、低い温度で短時間行うよ
うにしたり、低温加熱した鋼片を熱間圧延し巻き
取り後の冷却速度を速くする方法（特開昭55−
100931号公報）等フエライト脱炭防止策が講じら
ているが、未だ充分な成果は得られていず、通常
ばねに成形する前に鋼線材表面を切削または研削
して脱炭層を除去することが一般に採用れる。
そこで、本発明者らは高Si系ばね用鋼線材の製
造において有効なフエライト脱炭防止策を講ずる
べく、種々研究の結果、熱間圧延に先立つ加熱炉
での鋼片表面温度は従来の低温、短時間加熱法を
反対に加熱温度を高くし、表面炭素濃度が低下し
てもオーステナイト粒として安定に存在し得る温
度以上に加熱すると結果的にフエライト脱炭層の
生成が防止できることを見い出し、本発明を完成
するに至つた。
すなわち、本発明はSi1〜３％を含有するばね
用鋼線材を製造するにあたり、熱間圧延に先立つ
た加熱炉での鋼片の表面温度を1050℃を超え、
1250℃以下に加熱し、熱間圧延後、鋼線材を巻取
つて冷却することを特徴とする高Si系ばね用鋼線
材の製造法を提供することを目的とする。
一般に熱間圧延に先立つ加熱温度が高いほど、
脱炭反応が著しく、総脱炭は大きくなり、それと
共にフエライト脱炭層も生成する等々不利な条件
であると考えられるのに反し、加熱炉での鋼片表
面温度を従来法におけるよりも高い加熱温度1050
℃を超え、とする理由は、鋼中炭素の減少量は炭
素の拡散と脱炭反応の相対速度によつて決まると
考えられ、高温加熱によつて表面から脱炭は大き
くなるが、同時に内部からの炭素の拡散供給も容
易となるので、結果的には高温加熱によつて表面
炭素濃度の低下は従来の低温加熱ほど著しいもの
とならない。添付の第１図に示すように加熱温度
が上昇するにつれて鋼中での炭素拡散が活発とな
つて表面部から内部に至る炭素濃度勾配がゆるや
かになる傾向が見られるからである。
すなわち、第１図より明らかなごとく、900、
1000℃の加熱温度では表面層の炭素濃度が低いた
めフエライト層が生成しやすく、一方1100℃以上
では表面層の炭素濃度が0.1％以上ありフエライ
ト層が生成しないと考えられる。ここでフエライ
ト層生成の臨界加熱温度を一層明らかにするた
め、加熱温度のフエライト脱炭層厚さに及ぼす影
響についてさらに詳細に検討した結果を添付の第
４図に示す。第４図より明らかなごとく、加熱温
度1050℃を超え、でフエライト脱炭層は全く存在
せず安定している。他方、表面炭素濃度が低下し
てもオーステナイト粒として安定に存在し得る状
態まで加熱されることがフエライト脱炭防止に必
要であるが、第２図に模式的に示すように、表面
炭素濃度の低下とともにオーステナイト領域は著
しく減少する（(A)領域から(B)領域）ため、特に高
Si系鋼としては加熱時の脱炭層程度を考慮してSi
含有量との関係に鑑みて、少なくとも鋼材表層部
がオーステナイト領域に至る温度に加熱る必要が
あるためである。
なお、加熱温度は1250℃を越えると、全脱炭深
さが大きくなり、疲労強度に影響が出てくるとと
もに、酸化スケール中に含れる低融点酸化物
（2FeO・SiO2）が溶融して操業上のトラブルの
原因となる。また、省エネの関係からもこの温度
以下で実施されるのが望ましい。
本発明方法は通常ばね用鋼線材として使用され
るC0.35〜0.75％、Si1〜３％、Mn0.3〜1.5％を含
むSi―Mn鋼に限らず、Siを多量に含有する低合
金鋼、例えばSi―Cr鋼、Si―Cr―Ｖ鋼、Si―Cr
―Ｖ―Nb鋼について適用できる。これらの場合、
通常Cr1.5％、Ｖ0.5％、および／またはNb、
Ti、Al0.1％程度を含むであろう。
本発明方法は熱間圧延に先立つ加熱温度を制御
する以外は通常の方法と同様に実施されてよい。
そのため、ここでは特に他の工程については詳述
しない。
以下、実施例にもとずき、本発明をさらに詳細
に説明する。
下記第１表に示す化学組成の通常ばね用鋼線材
を熱間圧延に先立つ加熱炉での鋼片表面温度を
950℃〜150℃に変える以外は同様に熱間圧延し、
巻取り後冷却した。冷却後のコイルより採取した
サンプルを検鏡し、フエライト脱炭層の発生の有
無を判定る。結果をフエライト脱炭発生サンプル
数の全検査サンプル数比で示す。また、各実験No.
１〜３において得られた線材の断面組織の顕微鏡
写真（×90）を第３図に示す。
この結果から明らかなように、加熱温度が低い
場合（950℃）、フエライト脱炭発生率も高く、第
３図ａに示すように、圧延後の製品表面に見られ
るフエライト脱炭層深さも大きい。加熱温度を少
し下げた1000℃の場合、フエライト脱炭発生率は
少し低下し、第３図ｂに示すようにフエライト脱
炭層深さも小さくはなつているが、フエライト脱
炭防止効果は完全ではない。しかしながら、加熱
温度が1050℃となると、フエライト脱炭の発生は
殆んどなく、第３図ｃに示すように断面組織も極
めて健全な製品が得られた。
The present invention relates to a method for manufacturing a high-Si spring steel wire rod, and more particularly to a method for manufacturing a high-Si spring steel wire rod that can prevent the formation of a ferrite decarburized layer in the surface layer of the steel material. High Si steel, typically C0.35-0.75%, Si1-3
High-Si steel containing 0.3% to 1.5% Mn is mainly used as steel wire rod for springs, but it is generally known that due to the high Si content, the surface layer of the steel material decarburizes during high-temperature heating. easy. In the case of steel wire for springs,
The presence of a decarburized layer reduces fatigue strength, which is the main characteristic of springs. In particular, if a layer containing only ferrite and almost no pearlite exists in the surface layer of the steel wire (hereinafter referred to as a ferrite decarburized layer), even if quenching and tempering treatment is performed to impart strength, the strength will not be obtained and fatigue fracture will occur. This can cause a significant loss of spring characteristics, such as becoming a starting point. Therefore, conventionally, when manufacturing high-Si spring steel wire rods, the heating of the steel billet in a heating furnace prior to hot rolling is carried out at a low temperature for a short time as much as possible, or at a low temperature. A method of hot rolling a heated steel billet to increase the cooling rate after winding (Unexamined Japanese Patent Publication No. 1983-
Measures to prevent ferrite decarburization have been taken, such as (No. 100931), but sufficient results have not yet been obtained.The decarburized layer is usually removed by cutting or grinding the surface of the steel wire before forming it into a spring. Generally adopted. Therefore, in order to take effective measures to prevent ferrite decarburization in the production of high-Si spring steel wire rods, the present inventors conducted various studies and found that the surface temperature of the steel billet in the heating furnace prior to hot rolling was lower than the conventional low temperature. found that the formation of a ferrite decarburized layer could be prevented as a result by increasing the heating temperature to a temperature at which austenite grains can stably exist even if the surface carbon concentration decreases, as opposed to the short-time heating method. The invention was completed. That is, in producing a spring steel wire rod containing 1 to 3% Si, the present invention raises the surface temperature of a steel piece in a heating furnace to a temperature exceeding 1050°C prior to hot rolling.
The object of the present invention is to provide a method for manufacturing a high-Si spring steel wire rod, which is characterized by heating to 1250°C or lower, hot rolling, winding the steel wire rod, and cooling the steel wire rod. Generally, the higher the heating temperature prior to hot rolling, the
Although the decarburization reaction is significant, the total decarburization is large, and a ferrite decarburized layer is also formed, which is considered to be a disadvantageous condition, the steel billet surface temperature in the heating furnace is heated to a higher temperature than in the conventional method. temperature 1050
The reason for this is that the amount of carbon reduction in steel is thought to be determined by the relative speed of carbon diffusion and decarburization reaction, and high-temperature heating increases decarburization from the surface, but at the same time the internal Since the diffusion and supply of carbon from the surface becomes easy, as a result, the decrease in surface carbon concentration due to high-temperature heating is not as remarkable as with conventional low-temperature heating. This is because, as shown in the attached FIG. 1, as the heating temperature rises, carbon diffusion in the steel becomes more active, and the carbon concentration gradient from the surface to the interior tends to become gentler. In other words, as is clear from Figure 1, 900,
At a heating temperature of 1000°C, a ferrite layer is likely to form because the carbon concentration in the surface layer is low, whereas at a heating temperature of 1100°C or higher, the carbon concentration in the surface layer is 0.1% or more and a ferrite layer is not likely to be formed. In order to further clarify the critical heating temperature for ferrite layer formation, the effects of heating temperature on the thickness of the ferrite decarburized layer were studied in more detail, and the results are shown in the attached FIG. 4. As is clear from Fig. 4, when the heating temperature exceeds 1050°C, there is no ferrite decarburized layer at all and it is stable. On the other hand, in order to prevent ferrite decarburization, it is necessary to heat the ferrite to a state where it can stably exist as austenite grains even if the surface carbon concentration decreases. As the austenite region decreases (from region (A) to region (B)),
As Si-based steel, Si
This is because, in view of the relationship with the content, it is necessary to heat at least the surface layer of the steel material to a temperature that reaches the austenite region. If the heating temperature exceeds 1250℃, the total decarburization depth will increase, affecting fatigue strength, and the low melting point oxides (2FeO・SiO 2 ) contained in the oxide scale will melt. This may cause operational troubles. Also, from the perspective of energy saving, it is desirable to carry out the process at a temperature below this temperature. The method of the present invention is applicable not only to Si-Mn steel containing 0.35 to 0.75% C, 1 to 3% Si, and 0.3 to 1.5% Mn, which is normally used as steel wire for springs, but also to low alloy steel containing a large amount of Si. , for example, Si-Cr steel, Si-Cr-V steel, Si-Cr
-Applicable to V-Nb steel. In these cases,
Usually Cr1.5%, V0.5%, and/or Nb,
It will contain about 0.1% of Ti and Al. The method of the present invention may be carried out in the same manner as conventional methods, except that the heating temperature prior to hot rolling is controlled.
Therefore, other steps will not be described in detail here. Hereinafter, the present invention will be explained in more detail based on Examples. The surface temperature of the steel billet in the heating furnace prior to hot rolling of the regular spring steel wire rod with the chemical composition shown in Table 1 below is
Hot rolling was carried out in the same manner except that the temperature was changed from 950℃ to 150℃,
After winding, it was cooled. A sample taken from the coil after cooling is examined under a microscope to determine the presence or absence of a ferrite decarburized layer. The results are shown as the ratio of the number of samples in which ferrite decarburization occurred to the total number of samples tested. Also, each experiment no.
FIG. 3 shows a micrograph (×90) of the cross-sectional structure of the wire rods obtained in Examples 1 to 3. As is clear from these results, when the heating temperature is low (950°C), the rate of ferrite decarburization is high, and as shown in Figure 3a, the depth of the ferrite decarburization layer seen on the product surface after rolling is also large. When the heating temperature was slightly lowered to 1000℃, the rate of ferrite decarburization decreased slightly, and the depth of the ferrite decarburization layer became smaller, as shown in Figure 3b, but the effect of preventing ferrite decarburization was not complete. . However, when the heating temperature reached 1050° C., almost no ferrite decarburization occurred, and a product with an extremely sound cross-sectional structure was obtained as shown in FIG. 3c.
【表】【table】
【表】
上記第２表はSi量を広範囲に変えた諸種の高Si
系鋼につき、表面加熱温度とフエライト脱炭発生
率との関係を検討した結果を示す。この結果から
明らかなように、いずれの場合にもフエライト脱
炭の発生がほとんど見られない。
以上の説明で明らかなように、本発明方法によ
れば、フエライト脱炭のない高Si系ばね用鋼線材
を製造することができるので、従来のようにばね
成形前に表面切削または研削することを要さず、
工業的に極めて利用価値の高いものである。[Table] Table 2 above shows various types of high-Si products with a wide range of Si content.
The results of examining the relationship between surface heating temperature and ferrite decarburization incidence for steel series are shown below. As is clear from these results, almost no ferrite decarburization occurs in any case. As is clear from the above explanation, according to the method of the present invention, it is possible to produce a high-Si spring steel wire rod without ferrite decarburization, so there is no need to perform surface cutting or grinding before spring forming as in the conventional method. without requiring
It has extremely high utility value industrially.
【図面の簡単な説明】[Brief explanation of drawings]
第１図は第１表に示す化学組成のばね用鋼線材
を各温度に30分加熱後急冷したサンプルの断面を
表面部から中心にかけてEPMA（Ｘ線マイクロア
ナライザー）を用いて求めた炭素濃度の分布変化
を示すグラフ、第２図は鋼中炭素量が減少した場
合にオーステナイト領域が減少する状況を示す模
式図、第３図は加熱温度を変えて圧延し、冷却し
て得られる線材の表面部に見られる断面組織の顕
微鏡写真、第４図は加熱温度とフエライト脱炭層
厚さの関係を示すグラフである。
Figure 1 shows the carbon concentration measured using an EPMA (X-ray microanalyzer) from the surface to the center of a cross section of a sample of spring steel wire with the chemical composition shown in Table 1 heated to each temperature for 30 minutes and then rapidly cooled. A graph showing distribution changes. Figure 2 is a schematic diagram showing how the austenite region decreases when the carbon content in the steel decreases. Figure 3 shows the surface of a wire rod obtained by rolling at different heating temperatures and cooling. FIG. 4 is a graph showing the relationship between heating temperature and ferrite decarburized layer thickness.