JP2004076086A - High-strength steel component superior in delayed fracture resistance, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a component of a high-strength steel which surely improves delayed fracture characteristic, despite having a wide range of a composition. <P>SOLUTION: The component of a high-strength steel superior in delayed fracture resistance includes 0.20-0.55% C (hereinafter by mass%), 0.01-0.10% Ti and 0.02% or less N, wherein grain sizes of austenitic crystals are No.7 or higher, a content of fine TiC having particle diameters of 0.1 μm or less is 0.01% or more, and the content of the above fine TiC (mass%; [fine TiC]) and the content of all Ti (mass%; [Ti]) satisfies the expression (1): [fine TiC]/[Ti] ≥ 0.4 (1). <P>COPYRIGHT: (C)2004,JPO

Description

【００７０】
【表２】

【００７１】
【表３】

【００７２】
実験例２３から明らかなように、焼入れ温度を高くしてＴｉの溶け込み量を増やし、焼戻し後の微細なＴｉＣ量を増大させても、オーステナイト粒が粗大になるため遅れ破壊強さは低くなる。
【００７３】
また実験例２４から明らかなように、オーステナイト粒の粗大化を避けるために低温で焼き入れる条件において、鋼中のＴｉ量を増やすことによって焼入れ時のＴｉの溶け込み量を増やし、焼戻し後の微細なＴｉＣ量を増大させても、鋼中に多量の粗大ＴｉＣが残存しているために、遅れ破壊強さも弱くなっている。
【００７４】
これらに対して、実験例１では高周波によって短時間で焼入れしているため、オーステナイト粒の粗大化を防止しながら鋼中のＴｉを効率よく溶かし込むことができる。そのため、焼戻し丸棒において、結晶粒を小さくでき、且つ粗大ＴｉＣ量を抑制しながらも微細なＴｉＣ量を増大でき（すなわち［微細ＴｉＣ］／［Ｔｉ］を大きくでき）、引張強さ及び遅れ破壊強さを高めることができる。同様に実験例５〜１５でも高周波によって短時間で焼入れしているため、引張強さ及び遅れ破壊強さを高めることができる。
【００７５】
ただし実験例２では高周波焼入れの温度が低すぎてＡ値が低すぎるため、鋼中のＴｉを十分に溶かし込むことができず、微細ＴｉＣ量が不足して引張強さが弱くなっている。実験例３では高周波焼入れ時の入熱量が大きすぎてＡ値が高くなりすぎるため、オーステナイト粒が粗大化して遅れ破壊強さが低下する。実験例４では焼戻し温度が低すぎるために、焼入れ時に溶け込んだＴｉが析出せず、微細ＴｉＣが不足して遅れ破壊強さが小さくなる。
【００７６】
また実験例１６、１９、２１、２２では［Ｔｉ］−３．４［Ｎ］が小さいために、ＴｉがＮと結合してしまう結果、微細なＴｉＣが析出し難くなっている。しかもオーステナイト粒の粗大化を抑止するＴｉＣが不足するために、オーステナイト粒も粗大化している。そのため遅れ破壊強さが低くなる。
【００７７】
実験例１７ではＣ量が少ないために引張強さが低くなり、実験例１８ではＣ量が多いために耐遅れ破壊強さが低下する。実験例２０ではＴｉ量が多すぎるために、微細なＴｉＣが増えるだけでなく粗大なＴｉＣも増えてしまい、遅れ破壊強さが低下する。
【００７８】
【発明の効果】
本発明によれば、結晶粒の粗大化や粗大ＴｉＣ量が抑制されているにも拘わらず、微細ＴｉＣ量が多いために遅れ破壊特性を著しく高めることができる。しかも化学成分的には、Ｃ量、Ｔｉ量、Ｎ量を制御しているだけであるため、幅広い成分組成の高強度鋼部品に適用できる。
【００７９】
前記高強度鋼部品は、高温で速やかに焼入れ、高温で焼き戻すことによって製造できる。
【図面の簡単な説明】
【図１】図１は実験例の引張試験片を示す概略平面図である。
【図２】図２は実験例の遅れ破壊試験片を示す概略平面図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high strength (for example, a tensile strength of 1000 N / mm) used as a high strength bolt, a spring, a PC steel bar, a reinforcing bar, or the like. ² Or more).
[0002]
[Prior art]
Generally, bolts have a tensile strength of 1000 N / mm ² If the strength is higher than about the same, delayed fracture is likely to occur, so that use is restricted. Therefore, there is a demand for high-strength steel parts having excellent delayed fracture characteristics.
[0003]
For example, in JP-A-60-114551, in order to prevent delayed fracture due to alloying elements and impurity elements, in high-strength bolt steel, C: 0.30 to 0.50%, Si: less than 0.15%, Mn: 0.10 to 0.40%, P: 0.015% or less, S: 0.010% or less, Cr: 0.50 to 4.50%, Mo: 0.10 to 0.70% or less, And Si + Mn + 10 (P + S): 0.45% or less. In this publication, it is assumed that adding 0.05 to 0.15% of Ti as an optional component forms a carbonitride, which is effective for refining crystal grains and effective for improving proof stress and toughness and ductility. I have. Further, in this publication, after forging and normalizing the bolt steel, the steel is heated at 950 ° C. for 30 minutes, then oil quenched and tempered.
[0004]
In Japanese Patent Application Laid-Open No. 2-267243, attention is focused on the fact that increasing the amount of Si and Cr can increase the limit of diffusible hydrogen that does not lead to delayed fracture, and C: 0.18 to 0.35% in high-strength bolt steel. It is proposed to control Si: 0.50 to 1.50%, Mn: 0.20 to 0.60%, Cr: 1.50 to 3.50%, and Al: 0.008 to 0.070%. ing. This publication states that the addition of 0.005 to 0.030% of Ti as an optional component is useful for increasing the strength and reducing the size of steel. Further, in this publication, the steel for bolts is formed into a bar having a diameter of 20 mm and then subjected to a normal heat treatment (quenching and tempering).
[0005]
In Japanese Patent Application Laid-Open No. Hei 3-243745, grain boundary segregation is remarkably reduced by lowering P and S to lower Mn, lower Si and lower Cr, thereby greatly strengthening the grain boundaries. Therefore, if Ni and Mo, V, and Nb are added in combination, the refining of steel is remarkably promoted, and the reduction of grain boundary segregation is accompanied by an improvement in delayed fracture resistance. Being effective, the combined addition of Ni and Mo, V and Nb also significantly increases the tempering softening resistance of the steel, thereby making it possible to employ a high tempering temperature and effective in improving delayed fracture resistance. In the steel for machine structural use, C: 0.35 to 0.50%, Si: 0.20% or less, Mn: 0.35% or less, P: 0.012% or less, S: 0 0.01% or less, Ni: 1.0 to 3.0%, Cr: 0 25% or less, Mo: 0.40 to 1.5%, V: 0.05 to 0.50%, Nb: 0.005 to 0.20%, Al: 0.005 to 0.10% Propose that. This publication states that the addition of 0.10% or less of Ti as an optional component is useful for reducing the grain size and increasing the strength of steel. In this publication, a steel slab composed of the above components is heated to 1200 to 1250 ° C., rolled to a thickness of 15 mm, quenched from a temperature of 870 to 1020 ° C., and then tempered to produce steel for machine structural use. ing.
[0006]
However, even with the methods disclosed in these publications, the chemical components are limited and it is difficult to apply them to a wide range of steel parts. In the first place, delayed fracture may occur in a non-corrosive environment or in a corrosive environment, and since various factors are involved in a complicated manner, it is difficult to identify the true cause. For example, as factors affecting the delayed fracture characteristics, reduction of grain boundary segregation and participation of various elements have been pointed out as described above, but other factors such as tempering temperature, structure, and material hardness have also been pointed out. Therefore, it is difficult to establish a true means of preventing delayed fracture, and only various methods are employed by trial and error to improve the delayed fracture resistance of various steel parts.
[0007]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-strength steel part capable of reliably improving delayed fracture characteristics even with a wide component composition, and a method of manufacturing the same. It is in.
[0008]
[Means for Solving the Problems]
The present inventors have focused on Ti in the course of intensive research to solve the above-mentioned problems. That is, a high-strength steel part is manufactured by processing steel into a predetermined shape and then performing quenching and tempering, and when the steel contains Ti, the Ti is in a free state before quenching. In addition, they exist as nitrides (hereinafter sometimes referred to as TiN) and coarse carbides (hereinafter sometimes referred to as coarse TiC). Then, the coarse TiC melts during heating during quenching, and the Ti melted during tempering precipitates as fine TiC, whereby the strength and delayed fracture resistance of the steel component can be increased. In the case of depositing fine TiC, if N is large, TiN is formed and TiC becomes insufficient, so it is necessary to reduce N. However, even if N is reduced, in order to precipitate a large amount of fine TiC and increase the strength and delayed fracture resistance, it is usually necessary to sufficiently increase the amount of Ti, and as a result, it melts during quenching and heating. A large amount of coarse TiC that could not be obtained also remains. This coarse TiC deteriorates the toughness of the steel component and also reduces the delayed fracture characteristics.
[0009]
Therefore, the present inventors have found that the delayed fracture characteristics of a high-strength steel part can be remarkably improved by increasing the fine TiC while suppressing the coarse TiC. However, quenching is usually performed using a heating furnace. In quenching using this heating furnace, heating to a high temperature causes an excessively large heat input, so that austenite grains (hereinafter sometimes simply referred to as crystal grains) are excessively coarse, and there is a concern that delayed fracture characteristics may deteriorate. Therefore, in order to prevent the crystal grains from being coarsened by using a heating furnace, it is necessary to perform heating at a relatively low temperature. However, in this case, insufficient penetration of coarse TiC occurs, and fine TiC precipitates during subsequent tempering. Since the amount is likely to be insufficient, it is necessary to sufficiently increase the amount of Ti. However, when the Ti content is increased, a large amount of coarse TiC remains due to quenching at a relatively low temperature, and it is difficult to sufficiently improve delayed fracture characteristics.
[0010]
And the present inventors, when using a facility capable of rapid heating such as high-frequency heating, to quickly heat the inside in a short time, it is possible to efficiently melt coarse TiC while preventing the coarsening of crystal grains, The present inventors have found that fine TiC can be increased while suppressing the amount of coarse TiC in a steel part, and that delayed fracture characteristics can be remarkably improved, and the present invention has been completed.
[0011]
That is, the high-strength parts having excellent delayed fracture resistance according to the present invention include: 1) C: 0.20 to 0.55% (meaning by mass%; the same applies hereinafter); Ti: 0.01 to 0.10. %, N: 0.02% or less, and 2) the austenite grain size is no. 7) 3) The content of fine TiC having a particle size of 0.1 μm or less is 0.01% or more. 4) The content of fine TiC (% by mass; [fine TiC]) and the total Ti The point is that the content (% by mass; [Ti]) satisfies the following formula (1).
[0012]
[Fine TiC] / [Ti] ≧ 0.4 (1)
The high-strength component includes a first other component (Cr: 2% or less, Mo: 2% or less, V: 1% or less, W: 1% or less, Nb: 1% or less), a second other component. A component (Cu: 1% or less, Ni: 4% or less) may be contained, and P and S, which are impurities, are about P: 0.02% or less and S: about 0.02% or less. Is desirable.
[0013]
The high-strength part having excellent delayed fracture resistance contains C: 0.20 to 0.55%, Ti: 0.01 to 0.10%, and N: 0.02% or less, and contains all Ti. It can be manufactured from steel whose amount (% by mass; [Ti]) and N content (% by mass; [N]) satisfy the following formula (2).
[0014]
[Ti] −3.4 × [N] ≧ 0.01% (2)
That is, after quenching the steel under the condition that the quenching temperature is 900 to 1300 ° C. and the heat input strength A defined by the following formula (3) is 3.0 to 8.0, the steel is tempered at a temperature of 500 ° C. or more. Can be manufactured.
[0015]
A = log [t + (T−700) / (2 × V)] − 22 × 1000 / (T + 273) +20 (3)
[In the formula, T indicates a quenching heating temperature (° C.), V indicates an average heating rate (° C./second) during quenching heating, and t indicates a holding time (second) after heating.]
The heat input strength A is, for example, selected from a range in which the average heating rate during heating in the quenching step is 10 ° C./sec or more, and a holding time after heating is in a range of 60 seconds or less. It can be controlled in the range of 8.0. The quenching is conveniently performed by induction hardening or rapid cooling after electric heating.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
The high-strength steel part of the present invention contains at least C: 0.20 to 0.55%, Ti: 0.01 to 0.10%, and N: 0.02% or less. Hereinafter, the reasons for limiting each component will be described.
[0017]
C: 0.20 to 0.55%
C is an element necessary for securing hardenability and strength of steel. That is, since the high-strength steel part of the present invention is manufactured by tempering at a high temperature as described later, in order to prevent temper softening and secure high strength, C is 0.20% or more, preferably It must be contained at least 0.25%, more preferably at least 0.35%. However, when the addition amount is large, the workability of the steel is reduced, and further, the toughness is deteriorated and the delayed fracture characteristics are deteriorated. Therefore, C is set to 0.55% or less, preferably 0.45% or less, and more preferably 0.40% or less.
[0018]
Ti: 0.01 to 0.10%
Ti is an element useful for improving the strength and delayed fracture resistance of steel parts. That is, the high-strength component of the present invention is manufactured by processing steel into a predetermined shape as described later and then performing high-temperature quenching and high-temperature tempering. Ti melts into steel during high-temperature heating during quenching and precipitates as fine TiC during tempering, so that high strength can be obtained even during high-temperature tempering. In addition, fine TiC has an effect of trapping hydrogen that causes delayed fracture, and can enhance delayed fracture characteristics of steel parts. In order to exert these effects, Ti is made 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more. However, if the amount of Ti is excessive, the amount of coarse TiC existing before quenching is too large, so that even when high-temperature quenching is performed, a large amount of coarse TiC remains undissolved, and the amount of coarse TiC in a steel part increases. For this reason, the toughness of the steel part is deteriorated, and the delayed fracture characteristics are deteriorated. Therefore, Ti is set to 0.10% or less, preferably 0.8% or less, and more preferably 0.7% or less.
[0019]
N: 0.02% or less
N combines with Ti in the solidification stage after steel smelting to form TiN. Since TiN does not dissolve even when heated at a high temperature, it reduces the amount of fine TiC generated during tempering. Further, N is an element harmful to delayed fracture characteristics. Therefore, N is set to 0.02% or less, preferably 0.01% or less, more preferably 0.007% or less, and particularly 0.005% or less. It is difficult to set N to 0%, usually about 0.0005% or more, and often about 0.001% or more.
[0020]
The steel part of the present invention may further contain various other elements as necessary, for example, a first other element such as Cr, Mo, V, W, and Nb; A second other element may be contained. The first and second other elements are useful for further improving the delayed fracture resistance of the steel part. In particular, the second other element reduces the delayed fracture resistance of the steel from the viewpoint of suppressing the intrusion of hydrogen. Useful to improve. The first other element and the second other element may be added alone or in combination. The details will be described below.
[0021]
Cr:
Cr is useful for improving delayed fracture resistance in order to improve corrosion resistance, and is also useful for increasing hardenability and obtaining high strength. The lower limit of the addition amount is not particularly limited, and may be more than 0%. However, in order to remarkably exert the above-mentioned effect, the addition amount is set to 0.2% or more, preferably 0.3% or more. On the other hand, an excessive amount of Cr stabilizes carbides and adversely affects workability. Therefore, the content is, for example, 2% or less, preferably 1.2% or less, and more preferably 0.5% or less.
[0022]
Mo:
Mo is useful for improving delayed fracture resistance by a grain boundary strengthening action, and is also useful for improving hardenability. The lower limit of the addition amount is not particularly limited and may be more than 0%. However, in order to remarkably exert the above-mentioned effect, the addition amount is set to 0.05% or more, preferably 0.1% or more. On the other hand, if Mo is excessive, the workability is impaired. For example, the content is set to 2% or less, preferably 1% or less, and more preferably 0.6% or less.
[0023]
V, W, Nb:
V, W, and Nb, like Ti, form fine precipitates (carbonitrides and the like) and contribute to the improvement of delayed fracture resistance. Further, these carbides and nitrides are also effective elements for refining nitrogen crystal grains. The lower limit of the added amount of V is not particularly limited and may be more than 0%, but is set to 0.03% or more, preferably 0.05% or more in order to remarkably exert the above-mentioned effect. The lower limits of W and Nb are the same as those of V. On the other hand, when V, W, or Nb is excessive, the delayed fracture resistance and toughness are impaired. Therefore, V is, for example, 1% or less, preferably 0.3% or less, and more preferably 0.1% or less. The lower limits of W and Nb are the same as those of V.
[0024]
These first other elements (Cr, Mo, V, W, Nb, etc.) may be added alone or in combination of two or more. Preferably, at least one (particularly both) of Cr and Mo is added.
[0025]
Cu:
Cu is effective in enhancing corrosion resistance and suppressing the intrusion of hydrogen, which has an adverse effect on delayed fracture. The lower limit of the addition amount is not particularly limited and may be more than 0%, but is 0.15% or more, preferably 0.3% or more in order to exert the above-mentioned effect remarkably. On the other hand, when the content of Cu is excessive, the above effect is not only saturated, but also the toughness of the steel is reduced. Therefore, the content is, for example, 1% or less, preferably 0.7% or less, and more preferably 0.6% or less.
[0026]
Ni:
Ni also has the effect of improving corrosion resistance and suppressing hydrogen intrusion, and is also useful for increasing the toughness and hardenability of steel. The lower limit of the addition amount is not particularly limited and may be more than 0%, but is set to 0.05% or more, preferably 0.3% or more in order to exert the above-mentioned effect remarkably. On the other hand, when the amount of Ni is excessive, the effect is saturated and only the cost is increased. Therefore, for example, 4% or less, preferably 3.5% or less, more preferably 1% or less (particularly 0.6% or less). Degree.
[0027]
These second other elements (Cu, Ni, etc.) may be added alone or in combination.
[0028]
Various elements may be contained in addition to the above essential elements (C, Ti, N) and optional elements (first and second other elements), for example, usually containing Mn, B may be contained. Other than the above (remainder) are usually Fe and impurities (Al, Si, P, S, etc.), and may contain unavoidable impurities as long as the effects of the present invention are not impaired. The contents of these and the residual amounts of impurities when Mn and B are added are, for example, as follows.
[0029]
Mn: 2% or less
Since Mn is a hardenability improving element, the addition of Mn makes it easy to increase the strength of the component. The lower limit of the addition amount is not particularly limited, and may be more than 0%, but is 0.3% or more, preferably 0.5% or more in order to exert the above-mentioned effect remarkably. On the other hand, when Mn is excessive, the workability of the steel is reduced, and further, segregation at the grain boundaries is promoted to weaken the grain boundary strength and decrease the delayed fracture resistance. Therefore, the Mn content is, for example, 2% or less, preferably 1.5% or less, and more preferably 0.8% or less.
[0030]
B: 0.003% or less
B is useful for improving the hardenability of steel. The lower limit of the addition amount is not particularly limited, and may be more than 0%. However, in order to remarkably exert the above-mentioned effect, the addition amount is 0.0005% or more, preferably 0.001% or more. On the other hand, if B becomes excessive, the toughness decreases. Therefore, the B content is, for example, 0.003% or less, preferably 0.0025% or less, and more preferably 0.0020% or less.
[0031]
Al: 0.1% or less
Al forms oxide-based inclusions, which reduce the delayed fracture resistance. Therefore, Al content is, for example, 0.1% or less, preferably 0.07% or less, and more preferably 0.05% or less. Although the residual amount of Al is desirably 0%, it is difficult to reduce the amount of Al to 0% because it is used as a deoxidizing agent when smelting steel. Therefore, Al content is often, for example, about 0.02% or more (especially 0.03% or more).
[0032]
Si: 2% or less
Si lowers workability, and further promotes grain boundary oxidation during heat treatment such as quenching, thereby lowering delayed fracture resistance. Therefore, the content of Si is, for example, preferably 2% or less, preferably 1% or less, particularly preferably 0.5% or less. Although the remaining amount of Si is desirably 0%, it is difficult to reduce the amount of Si to 0% because it is used as a deoxidizing agent when smelting steel. Therefore, Si is often, for example, about 0.005% or more (particularly 0.01% or more).
[0033]
P: 0.02% or less
P causes grain boundary segregation and deteriorates delayed fracture resistance. Therefore, P is, for example, 0.02% or less, preferably 0.015% or less, and particularly 0.005% or less. Although the residual amount of P is desirably 0%, it leads to an increase in cost, and thus is often, for example, about 0.001% or more.
[0034]
S: 0.02% or less
Since S forms MnS, which is a stress concentration point, it deteriorates delayed fracture resistance. Therefore, S is set to, for example, 0.02% or less, preferably 0.01% or less, and more preferably 0.005% or less. Although the remaining amount of S is desirably 0%, it is often, for example, about 0.001% or more in order to increase the cost.
[0035]
Among the above impurities (Al, Si, P, S, etc.), it is particularly desirable to control the amounts of P and S.
[0036]
In the high-strength steel part of the present invention, a large amount of fine TiC is present and coarse TiC is suppressed. Fine TiC is useful not only in increasing the tensile strength, but also has an action of trapping hydrogen, and is useful in improving delayed fracture resistance. On the other hand, coarse TiC is detrimental to toughness and delayed fracture resistance. Therefore, by increasing the ratio of fine TiC, the toughness of the steel component can be increased, and the delayed fracture resistance can be significantly increased. In addition, in the steel part of the present invention, TiN is suppressed as described later. Since TiN does not form a solid solution at the time of heating during quenching, it tends to remain as coarse TiN in steel parts and is harmful to toughness and delayed fracture resistance. However, such TiN is suppressed in the steel parts of the present invention. ing. Therefore, a feature of the steel part of the present invention is that desirable Ti (fine TiC) is increased while harmful Ti (coarse TiC, coarse TiN) in steel is reduced. The ratio between fine TiC and harmful Ti (coarse TiC, coarse TiN) can be substantially evaluated by the ratio of fine TiC to total Ti.
[0037]
The ratio of the content of fine TiC (% by mass; hereinafter sometimes referred to as [fine TiC]) to the total content of Ti (% by mass; hereinafter sometimes referred to as [Ti]) ([fine TiC ] / [Ti]) is at least 0.4, preferably at least 0.5, more preferably at least 0.6. The ratio ([fine TiC] / [Ti]) is preferably as large as possible, but is usually about 0.9 or less.
[0038]
The fine TiC means TiC having a particle size of 0.1 μm or less. The fine TiC content in the steel part of the present invention is about 0.01% or more, preferably about 0.02% or more, and more preferably about 0.03% or more. The more fine TiC, the more desirable, but usually about 0.10% or less.
[0039]
The steel part of the present invention needs to have both the requirement of [fine TiC] / [Ti] and the requirement of fine TiC amount.
[0040]
Further, in the high-strength steel part of the present invention, since the crystal grains are not coarsened, the delayed fracture characteristics are also improved from this point. The degree of the coarsening suppression can be evaluated by the grain size number of austenite grains. The crystal grain size number is No. No. 7 or more, preferably no. No. 8 or more, more preferably no. No. 9 or more, especially No. 9 It is about 10 or more (for example, about No. 11 or more). Although the larger the crystal grain size number, the better, It is about 15 or less.
[0041]
The steel part of the present invention has a large amount of fine TiC, a small amount of coarse TiC, and a crystal grain that is not coarse, and thus has a remarkably excellent delayed fracture resistance. In addition, since it contains a predetermined amount of C and has a large amount of fine TiC, a predetermined strength can be secured. For this reason, superior high-strength steel parts (for example, high-strength bolts, springs, PC steel wires, rebars and other linear or rod-like steel materials) that replace conventional high-strength steel parts that have been required to improve delayed fracture strength. It is useful as a processed product.
[0042]
The tensile strength of the steel part of the present invention is, for example, 1000 N / mm. ² Or more (preferably 1200 N / mm ² About the above, more preferably 1400 N / mm ² Above). The tensile strength is 1500 N / mm ² It is often less than the following.
[0043]
Further, the steel component of the present invention has improved delayed fracture resistance by controlling the amounts of C, Ti, and N in terms of chemical components. That is, since there are few types of chemical components to be controlled, it can be applied to a wide range of steel parts. Therefore, even when it is difficult to use various elements (for example, Cr, Mo, V, Nb, Ni, etc.) used for improving the delayed fracture resistance, from the viewpoint of the production method, the use, and the like, According to the present invention, the use of these elements can be easily avoided, and a high-strength component excellent in delayed fracture resistance can be easily provided.
[0044]
The steel part can be manufactured by rapidly quenching steel having the same chemical composition at high temperature and tempering at high temperature. Since the coarse TiC can be efficiently dissolved by increasing the quenching temperature, the amount of the coarse TiC remaining undissolved can be suppressed, and the amount of the fine TiC precipitated during tempering can be increased. Moreover, by quenching promptly, even if the quenching temperature is increased, coarsening of crystal grains can be suppressed.
[0045]
The quenching temperature is specifically about 900 ° C. or more, preferably about 950 ° C. or more, more preferably about 1000 ° C. or more, and particularly about 1100 ° C. or more. If the quenching temperature is too high, even if quenching is performed quickly, the crystal grains are likely to become coarse and the delayed fracture characteristics are likely to be reduced. Therefore, the quenching temperature is usually about 1300 ° C. or lower, preferably about 1250 ° C. or lower, more preferably about 1150 ° C., and particularly about 1100 ° C. or lower.
[0046]
The quickness of quenching can be evaluated by the A value (sometimes referred to as “heat input strength”) represented by the following formula (4). The A value itself is also used as a tempering parameter and serves as a measure of the heat input.
A = logt′−B × [Q / (R × T _a )] + C (4)
[In the formula, t ′ indicates a heating time (Hr), Q indicates an activation energy (cal / mol), R indicates a gas constant (cal / mol), and T _a Represents a heating temperature (K), and B and C represent constants]
In the case of the present invention, the above equation (4) can be arranged as the following equation (5). A = log [t + (T−700) / (2 × V)] − B ′ × (T + 273) + C ′
… (5)
[Wherein T represents the quenching heating temperature (° C.), V represents the average heating rate during quenching heating (° C./second), t represents the holding time after heating (seconds), and B ′ and C ′ Indicates a constant]
When B ′ and C ′ were obtained by regression calculation using values obtained from many experiments, B ′ = 22 × 1000 and C ′ = 20. It can be rewritten as in (3).
A = log [t + (T−700) / (2 × V)] − 22 × 1000 / (T + 273) +20 (3)
The appropriate A value range was determined by changing the quenching speed (heating rate, holding time after heating, etc.), and the A value was about 8.0 or less (preferably about 7.5 or less, more preferably 7 or less). It is desirable to quench under the condition of about 0.0 or less, especially about 6.0 or less. If the A value is too large, quenching cannot be performed quickly, and the crystal grains become coarse, so that the delayed fracture characteristics deteriorate. On the other hand, if the A value is too small, heating will be insufficient even in high-temperature quenching, and Ti cannot be sufficiently dissolved. Therefore, it is desirable to quench under the condition that the A value is about 3.0 or more, preferably about 4.0 or more, more preferably about 4.5 or more, and particularly about 5.0 or more.
[0047]
In order to control the value A to be equal to or less than the predetermined value under the condition of high-temperature heating, it is necessary to increase the heating rate and shorten the holding time after heating. The heating rate can be selected, for example, from a range of about 10 ° C./sec or more, preferably about 20 ° C./sec or more, and more preferably about 50 ° C./sec or more. The holding time can be appropriately set according to the heating rate, and can be selected, for example, from a range of about 60 seconds or less, preferably about 30 seconds or less, and more preferably about 10 seconds or less.
[0048]
On the other hand, in order to control the A value to be equal to or more than the predetermined value, it is necessary that the heating rate is not extremely high and the holding time is not extremely short. The heating rate can be selected, for example, from a range of about 600 ° C./sec or less, preferably about 300 ° C./sec or less, and more preferably about 100 ° C./sec or less. The holding time can be appropriately set according to the heating rate, and can be selected, for example, from a range of about 1 second or more, preferably about 3 seconds or more, and more preferably about 5 seconds or more.
[0049]
For quick quenching, for example, it is convenient to use a rapid heating device such as a high-frequency heating device or an energizing heating device (a heating device utilizing resistance heat generated by energization).
[0050]
Tempering is performed by heating to a temperature of 500 ° C. or higher, preferably 550 ° C. or higher, more preferably 575 ° C. or higher. If the tempering temperature is too low, fine TiC does not precipitate, and the delayed fracture characteristics are not improved. The tempering temperature is usually about 650 ° C. or less (especially about 625 ° C. or less) in many cases.
[0051]
By performing the quenching and tempering as described above, it is possible to manufacture a steel part having a small number of crystal grains and a large amount of fine TiC despite suppression of the total Ti amount. However, if the amount of N in the steel is too large, TiN is formed, so that the amount of fine TiC becomes insufficient even when quenching and tempering are performed as described above. In order to keep the fine TiC amount within the above-mentioned predetermined range, it is necessary to make the Ti amount sufficiently larger than the N amount. For example, when the Ti content in steel is [Ti] (% by mass) and the N content is [N] (% by mass), the numerical value obtained by the formula [Ti] -3.4 × [N] is 0. 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more.
[0052]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to the following Examples, and may be appropriately modified within a range that can be adapted to the purpose of the preceding and the following. It is of course possible to carry out them, and all of them are included in the technical scope of the present invention.
[0053]
Experimental Examples 1 to 22
A test steel having the chemical composition (mass%) shown in Table 1 below was forged and cut to produce a round bar (diameter 12 mm × length 100 mm), which was then heated at 850 ° C. for 60 minutes for normalization. Using a high-frequency heating device, the round bar was rapidly heated under the conditions shown in Tables 2 and 3 for a short time, and then quenched by oil cooling. Then, after heating under the conditions shown in Table 2, it was tempered by water cooling.
[0054]
Across the tempered round bar, the particle size of austenitic crystal grains of D / 4 part (D indicates the diameter) Was measured in accordance with “Austenitic Grain Size Test Method for Steel” (JIS G 0551).
[0055]
The amount of fine TiC in the tempered round bar was determined as shown in (1) to (10) below.
[0056]
(1) The round bar was cut into a measurement sample (diameter: 12 mm × length: 20 mm).
[0057]
(2) The sample was immersed in a methanol solution (electrolyte solution) in which 10% by mass of acetylacetone and 1% by mass of tetramethylammonium chloride were dissolved, and 20 mA / cm ² And a sample is dissolved (electrolysis) by about 0.5 g.
[0058]
(3) After completion of the electrolytic treatment, the sample is immersed in methanol, and the precipitate exposed on the sample surface is peeled off by ultrasonic waves.
[0059]
(4) The above electrolytic solution containing the coarse precipitate and the ultrasonic treatment solution are subjected to suction filtration through a membrane filter having a pore diameter of 0.1 μm to collect the coarse precipitate.
[0060]
(5) The collected coarse precipitate is transferred to a platinum crucible together with the membrane filter, and heated by a gas burner to incinerate.
[0061]
(6) An alkali flux (a mixture of sodium carbonate and sodium tetraborate) is added to the platinum crucible, and the residue is melted by heating again with a gas burner.
[0062]
(7) Hydrochloric acid and pure water are added to a platinum crucible to dissolve the melt and transferred to a volumetric flask. Further, pure water is added until reaching the marked line of the volumetric flask to obtain an analysis solution.
[0063]
(8) The amount of Ti was measured by ICP emission spectrometry, and the amount W of the coarse Ti compound (particle size of more than 0.1 μm) in the analysis solution W _Large-Ti Is measured. On the other hand, the dissolved amount W of the sample during the electrolytic treatment and the ultrasonic treatment is W ₁ Is calculated, and the concentration of the coarse Ti compound in the sample ([coarse Ti compound]) is calculated based on the following equation.
[0064]
[Coarse Ti compound] = W _Large-Ti / W ₁ × 100
(9) About 0.3 g from the round bar (dissolution amount: W ₂ ) Is collected, put into aqua regia, and dissolved by heating. After adding pure water to this solution to adjust the concentration, the amount of Ti (W _total-Ti ) Is measured, and the total Ti concentration ([total Ti]) in the sample is calculated based on the following equation.
[0065]
[All Ti] = W _total-Ti / W ₂ × 100
(10) Based on the following equation, the concentration ([fine TiC]) of fine TiC (particle size: 0.1 μm or less) in the sample is calculated.
[0066]
[Fine TiC] = [Total Ti]-[Coarse Ti compound]
The tempered round bar was cut to produce a tensile test piece shown in FIG. 1 and a delayed fracture test piece shown in FIG. Using these test pieces, tensile strength and delayed fracture strength were examined. The delayed fracture strength is measured by applying a stress while immersing a test piece in distilled water and measuring the time until failure. (100-hour delayed fracture strength).
[0067]
Experimental Examples 23 to 24
The procedure was the same as in Experimental Examples 1 to 22, except that a round bar was put in the heating furnace and quenched.
[0068]
The results are shown in Tables 2 and 3.
[0069]
[Table 1]

[0070]
[Table 2]

[0071]
[Table 3]

[0072]
As is clear from Experimental Example 23, even if the quenching temperature is increased to increase the amount of Ti penetration and the amount of fine TiC after tempering, the austenite grains become coarse and the delayed fracture strength decreases.
[0073]
Further, as is apparent from Experimental Example 24, under the condition of quenching at a low temperature in order to avoid coarsening of austenite grains, the amount of Ti dissolved in the quenching is increased by increasing the amount of Ti in the steel, and the fineness after tempering is increased. Even if the amount of TiC is increased, a large amount of coarse TiC remains in the steel, so that the delayed fracture strength is weak.
[0074]
On the other hand, in Experimental Example 1, quenching is performed in a short time by high frequency, so that Ti in steel can be efficiently dissolved while preventing coarsening of austenite grains. Therefore, in the tempered round bar, the crystal grain can be reduced, and the fine TiC content can be increased while suppressing the coarse TiC content (that is, [fine TiC] / [Ti] can be increased), and the tensile strength and delayed fracture Strength can be increased. Similarly, in Experimental Examples 5 to 15, since quenching is performed in a short time by high frequency, the tensile strength and the delayed fracture strength can be increased.
[0075]
However, in Experimental Example 2, since the temperature of the induction hardening was too low and the A value was too low, Ti in the steel could not be sufficiently dissolved, the amount of fine TiC was insufficient, and the tensile strength was weak. In Experimental Example 3, since the heat input during induction hardening is too large and the A value is too high, austenite grains are coarsened and the delayed fracture strength is reduced. In Experimental Example 4, since the tempering temperature was too low, the dissolved Ti at the time of quenching did not precipitate, and fine TiC was insufficient, and the delayed fracture strength was reduced.
[0076]
In Experimental Examples 16, 19, 21, and 22, since [Ti] -3.4 [N] is small, Ti is bonded to N, and as a result, fine TiC is not easily deposited. Moreover, the austenite grains are also coarse because TiC for suppressing the coarsening of the austenite grains is insufficient. Therefore, the delayed fracture strength decreases.
[0077]
In Experimental Example 17, the tensile strength is reduced due to the small amount of C, and in Experimental Example 18, the delayed fracture strength is reduced due to the large amount of C. In Experimental Example 20, since the amount of Ti is too large, not only the fine TiC increases but also the coarse TiC increases, and the delayed fracture strength decreases.
[0078]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, although the coarsening of a crystal grain and the coarse TiC content are suppressed, since the fine TiC content is large, delayed fracture characteristics can be remarkably improved. Moreover, in terms of chemical components, only the amounts of C, Ti, and N are controlled, so that it can be applied to high-strength steel parts having a wide range of component compositions.
[0079]
The high-strength steel part can be manufactured by rapidly quenching at high temperature and tempering at high temperature.
[Brief description of the drawings]
FIG. 1 is a schematic plan view showing a tensile test piece of an experimental example.
FIG. 2 is a schematic plan view showing a delayed fracture test piece of an experimental example.

Claims (7)

Mass% (hereinafter the same),
C: 0.20 to 0.55%,
Ti: 0.01 to 0.10%,
N: contains 0.02% or less;
The austenite grain size is no. 7 or more,
The content of fine TiC having a particle size of 0.1 μm or less is 0.01% or more;
The content of the fine TiC (% by mass; hereinafter referred to as [fine TiC]) and the content of all Ti (% by mass; hereinafter referred to as [Ti]) satisfy the following formula (1). High strength steel parts with excellent delayed fracture resistance.
[Fine TiC] / [Ti] ≧ 0.4 (1) Still other ingredients,
Cr: 2% or less,
Mo: 2% or less,
V: 1% or less,
The high-strength steel part according to claim 1, which contains at least one selected from W: 1% or less and Nb: 1% or less. Still other ingredients,
The high-strength steel part according to claim 1 or 2, containing at least one selected from Cu: 1% or less and Ni: 4% or less. The high-strength steel part according to any one of claims 1 to 3, wherein P: 0.02% or less and S: 0.02% or less. Mass% (hereinafter the same),
C: 0.20 to 0.55%,
Ti: 0.01 to 0.10%,
N: contains 0.02% or less;
A steel having a total Ti content (% by mass; hereinafter referred to as [Ti]) and an N content (% by mass; hereinafter referred to as [N]) satisfying the following formula (2):
After quenching under the condition that the quenching temperature is 900 to 1300 ° C. and the heat input strength A defined by the following formula (3) is 3.0 to 8.0,
A method for producing a high-strength steel part having excellent delayed fracture resistance, characterized by tempering at a temperature of 500 ° C. or higher.
[Ti] −3.4 × [N] ≧ 0.01% (2)
A = log [t + (T−700) / (2 × V)] − 22 × 1000 / (T + 273) +20 (3)
[In the formula, T indicates a quenching heating temperature (° C.), V indicates an average heating rate (° C./second) during quenching heating, and t indicates a holding time (second) after heating.] The manufacturing method according to claim 5, wherein an average heating rate during heating in the quenching step is selected from a range of 10 ° C / sec or more, and a holding time after heating is selected from a range of 60 seconds or less. The method according to claim 5, wherein the quenching is performed by high frequency and / or electric heating.

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