JP3836766B2 - High strength steel parts with excellent delayed fracture resistance and manufacturing method thereof - Google Patents

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]
BACKGROUND OF THE INVENTION
The present invention provides high strength (for example, tensile strength 1000 N / mm) used as high strength bolts, springs, PC steel bars, reinforcing bars, etc. ² Or more) steel parts.
[0002]
[Prior art]
Generally, bolts have a tensile strength of 1000 N / mm ² When the strength is higher than the above level, delayed fracture is likely to occur, which limits use. 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 alloy elements and impurity elements, 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): it is proposed to control to 0.45% or less. In this publication, if Ti: 0.05 to 0.15% is added as an optional component, carbonitrides are formed, which is effective in refining crystal grains and effective in improving yield strength and toughness. Yes. In this publication, the bolt steel is forged and normalized, heated at 950 ° C. for 30 minutes, then oil-quenched and tempered.
[0004]
In JP-A-2-267243, focusing on the fact that by increasing Si and Cr, it is possible to increase the limit of diffusible hydrogen that does not lead to delayed fracture, C: 0.18 to 0.35% in steel for high-strength bolts, Proposed to control Si: 0.50 to 1.50%, Mn: 0.20 to 0.60%, Cr: 1.50 to 3.50%, Al: 0.008 to 0.070% ing. In this publication, adding 0.005 to 0.030% Ti as an optional component is useful for increasing the strength and refining of steel. In this publication, the bolt steel is made into a bar steel of φ20 mm, and then normal heat treatment (quenching / tempering) is performed.
[0005]
In Japanese Patent Laid-Open No. 3-243745, by reducing P and S and further reducing Mn, Si and Cr, grain boundary segregation is remarkably reduced, and thereby the grain boundary is extremely strengthened. Therefore, the delayed fracture resistance is greatly improved, and if Ni, Mo, V, and Nb are added in combination, the refinement of the steel is remarkably promoted, and the accompanying reduction in grain boundary segregation improves the delayed fracture resistance. Effectiveness, the combined addition of Ni and Mo, V and Nb also significantly increases the temper softening resistance of the steel, thereby making it possible to adopt a high tempering temperature and effective in improving delayed fracture resistance. Paying attention to the above, 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. In this publication, it is said that if Ti: 0.10% or less is added as an optional component, it is useful for making steel finer and stronger. Further, in this publication, a steel for machine structure is manufactured by heating a steel slab comprising the above components to 1200 to 1250 ° C., rolling it to a thickness of 15 mm, quenching it from a temperature of 870 to 1020 ° C., and tempering it. ing.
[0006]
However, even the methods disclosed in these publications have limited chemical components and are difficult to apply to a wide range of steel parts. In the first place, delayed fractures occur in a non-corrosive environment and in a corrosive environment. Since various factors are involved in a complicated manner, it is difficult to identify the true cause. For example, as described above, reduction of grain boundary segregation and involvement of various elements have been pointed out as factors affecting delayed fracture characteristics, but tempering temperature, structure, material hardness, etc. are also pointed out. Therefore, it is difficult to establish a true delayed fracture prevention means, and various methods are merely employed by trial and error in order to improve the delayed fracture resistance of various steel parts.
[0007]
[Problems to be solved by the invention]
The present invention has been made paying attention to the circumstances as described above, and its object is to provide a high-strength steel part capable of reliably improving delayed fracture characteristics even with a wide component composition, and a method for producing the same. There is.
[0008]
[Means for Solving the Problems]
The present inventors paid attention to Ti in the process of intensive research in order to solve the above problems. That is, high-strength steel parts are manufactured by processing steel into a predetermined shape, followed by quenching and tempering. When the steel contains Ti, this 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). And coarse TiC melt | dissolves in the case of the heating of quenching, and Ti melt | dissolved in the case of tempering precipitates as fine TiC, and can improve the intensity | strength and delayed fracture resistance of steel parts. When fine TiC is deposited, if N is large, TiN is formed and TiC is 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 increase the amount of Ti sufficiently, and as a result, it dissolves during quenching heating. A large amount of coarse TiC that could not be obtained also remains. This coarse TiC degrades the toughness of steel parts and also reduces delayed fracture characteristics.
[0009]
Therefore, the present inventors have found that if the fine TiC is increased while suppressing coarse TiC, the delayed fracture characteristics of high strength steel parts can be remarkably improved. However, quenching is usually performed using a heating furnace. When heating to a high temperature in this quenching using a heating furnace, the amount of heat input becomes too large, so that there is a concern that austenite grains (hereinafter sometimes referred to simply as crystal grains) become too coarse and delayed fracture characteristics deteriorate. Therefore, although it is necessary to heat at a relatively low temperature in order to prevent coarsening of the crystal grains using a heating furnace, in this case, the coarse TiC is not sufficiently dissolved and fine TiC is precipitated during subsequent tempering. Since the amount tends to be insufficient, it is necessary to sufficiently increase the amount of Ti. However, if the amount of Ti is increased, a large amount of coarse TiC remains because of quenching at a relatively low temperature, and it is difficult to sufficiently improve delayed fracture characteristics.
[0010]
And, the present inventors can rapidly melt coarse TiC while preventing coarsening of crystal grains when heated rapidly in a short time using facilities capable of rapid heating such as high-frequency heating, The present inventors have found that fine TiC can be increased while suppressing the amount of coarse TiC in steel parts, and that delayed fracture characteristics can be remarkably improved, and the present invention has been completed.
[0011]
That is, the high-strength parts excellent in delayed fracture resistance according to the present invention are as follows: 1) C: 0.20 to 0.55% (meaning mass%, hereinafter the same), Ti: 0.01 to 0.10 %, N: 0.02% or less, 2) The austenite grain size is No. 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 (mass%; [fine TiC]) and the total Ti The content (mass%; [Ti]) has a gist in that the following formula (1) is satisfied.
[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, etc.), Components (Cu: 1% or less, Ni: 4% or less, etc.) may be contained, and impurities P and S are P: 0.02% or less and S: 0.02% or less. Is desirable.
[0013]
The high-strength parts having excellent delayed fracture resistance contain C: 0.20 to 0.55%, Ti: 0.01 to 0.10%, N: 0.02% or less, and contain all Ti The amount (mass%; [Ti]) and the content of N (mass%; [N]) can be produced from steel satisfying the following formula (2).
[0014]
[Ti] -3.4 × [N] ≧ 0.01% (2)
That is, after quenching the steel under a 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)
[Wherein T represents quenching heating temperature (° C.), V represents average heating rate during quenching heating (° C./second), and t represents retention time (seconds) 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./second or more, and the holding time after heating is in the range of 60 seconds or less. It can be controlled within the range of 8.0. In the quenching, induction quenching or rapid cooling after energization heating is convenient.
[0016]
DETAILED DESCRIPTION OF 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 reason for limitation of each component is demonstrated.
[0017]
C: 0.20 to 0.55%
C is an element necessary for ensuring the 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 will be described later, in order to prevent temper softening and ensure high strength, C is preferably 0.20% or more, preferably It must be contained at 0.25% or more, more preferably 0.35% or more. However, if the added amount is large, the workability of the steel is lowered, and further, the toughness is deteriorated and the delayed fracture property is deteriorated. Therefore, C is 0.55% or less, preferably 0.45% or less, more preferably 0.40% or less.
[0018]
Ti: 0.01-0.10%
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 will be described later, followed by high-temperature quenching / high-temperature tempering treatment. Since Ti dissolves in steel during high-temperature heating during quenching and precipitates as fine TiC during tempering, high strength can be obtained even during high-temperature tempering. Moreover, fine TiC has an action of trapping hydrogen which causes delayed fracture, and can enhance delayed fracture characteristics of steel parts. In order to exert these effects, Ti is 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more. However, when Ti is excessive, the amount of coarse TiC existing before quenching increases too much, so that even if high-temperature quenching is performed, a large amount of coarse TiC remains undissolved and the amount of coarse TiC in the steel part increases. Therefore, the toughness of steel parts deteriorates and the delayed fracture characteristics deteriorate. Therefore, Ti is 0.10% or less, preferably 0.8% or less, more preferably 0.7% or less.
[0019]
N: 0.02% or less
N is combined with Ti in the solidification stage after the steel is melted to form TiN. Since TiN does not dissolve even when heated at a high temperature, the amount of fine TiC produced during tempering is reduced. Further, N is an element harmful to delayed fracture characteristics. Therefore, N is 0.02% or less, preferably 0.01% or less, more preferably 0.007% or less, and particularly 0.005% or less. Note that it is difficult to reduce 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 required, for example, first other elements such as Cr, Mo, V, W, Nb; Cu, Ni, etc. A second other element may be contained. The first and second other elements are useful for further improving the delayed fracture resistance of steel parts, and in particular, the second other element reduces the delayed fracture resistance of the steel from the point of suppressing hydrogen penetration. Useful to improve. The first other element and the second other element may be added alone or in combination. This will be described in detail below.
[0021]
Cr:
Cr is useful for improving delayed fracture resistance because it improves corrosion resistance, and is also useful for increasing hardenability and obtaining high strength. The lower limit of the addition amount is not particularly limited as long as it exceeds 0%, but in order to exert the above-described effect remarkably, it is 0.2% or more, preferably 0.3% or more. On the other hand, if Cr is excessive, the carbide is stabilized and the workability is adversely affected. For example, it is 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 the grain boundary strengthening action, and is also useful for improving hardenability. The lower limit of the addition amount is not particularly limited as long as it exceeds 0%. However, in order to exert the above-described effect remarkably, it is 0.05% or more, preferably 0.1% or more. On the other hand, when Mo is excessive, workability is hindered, so, for example, 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, etc.) 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 addition amount of V is not particularly limited and may be more than 0%. However, in order to exert the above-described effect remarkably, it is 0.03% or more, preferably 0.05% or more. The lower limits of W and Nb are the same as V. On the other hand, if V, W, or Nb is excessive, delayed fracture resistance and toughness are hindered. Therefore, V is, for example, 1% or less, preferably 0.3% or less, more preferably 0.1% or less. The lower limits of W and Nb are the same as V.
[0024]
These first other elements (Cr, Mo, V, W, Nb, etc.) may be added singly or in combination. 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 that adversely affects 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-described effect remarkably. On the other hand, when Cu is excessive, not only is the above effect saturated, but also the toughness of the steel is lowered. For example, it is 1% or less, preferably 0.7% or less, and more preferably 0.6% or less.
[0026]
Ni:
Ni also has an action of improving corrosion resistance and suppressing hydrogen penetration, and is also useful for enhancing the toughness and hardenability of steel. The lower limit of the addition amount is not particularly limited and may be more than 0%. However, in order to exert the above-described effect remarkably, it is 0.05% or more, preferably 0.3% or more. On the other hand, if Ni is excessive, the above-described effect is saturated and only the cost is increased. For example, 4% or less, preferably 3.5% or less, more preferably 1% or less (particularly 0.6% or less). To the extent.
[0027]
These second other elements (Cu, Ni, etc.) may be added alone or in combination.
[0028]
In addition to the above essential elements (C, Ti, N) and optional elements (first and second other elements), various elements may be contained, for example, usually containing Mn, B may be contained. Other than the above (remainder) is usually Fe and impurities (Al, Si, P, S, etc.), and may contain inevitable impurities as long as the effects of the present invention are not impaired. The contents of Mn and B and the residual amount of impurities 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-described effect remarkably. On the other hand, when Mn is excessive, the workability of the steel is lowered, and further segregation to the grain boundary is promoted, the grain boundary strength is weakened and the delayed fracture resistance is lowered. Therefore, the amount of Mn 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 the steel. The lower limit of the addition amount is not particularly limited and may be more than 0%. However, in order to exert the above-described effect remarkably, it is 0.0005% or more, preferably 0.001% or more. On the other hand, if B is excessive, toughness is reduced. Accordingly, the B amount 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 generates oxide inclusions, and these inclusions deteriorate the delayed fracture resistance. Therefore, Al is, for example, 0.1% or less, preferably 0.07% or less, and more preferably 0.05% or less. Although the remaining amount of Al is preferably 0%, Al is used as a deoxidizer during the melting of steel, so it is difficult to reduce it to 0%. Therefore, Al is often about 0.02% or more (particularly 0.03% or more), for example.
[0032]
Si: 2% or less
Si lowers workability and further promotes grain boundary oxidation during heat treatment such as quenching, thereby reducing delayed fracture resistance. Therefore, Si is, for example, desirably 2% or less, preferably 1% or less, particularly 0.5% or less. Although the remaining amount of Si is preferably 0%, Si is used as a deoxidizer during the melting of steel, so it is difficult to reduce it to 0%. Therefore, Si is often about 0.005% or more (particularly 0.01% or more), for example.
[0033]
P: 0.02% or less
P causes segregation at the grain boundary and deteriorates the delayed fracture resistance. Therefore, P is, for example, 0.02% or less, preferably 0.015% or less, particularly 0.005% or less. Although the remaining amount of P is desirably 0%, it leads to high costs, and for example, is often about 0.001% or more.
[0034]
S: 0.02% or less
Since S forms MnS which becomes a stress concentration portion, the delayed fracture resistance is deteriorated. Therefore, S is, for example, 0.02% or less, preferably 0.01% or less, more preferably 0.005% or less. Although the residual amount of S is preferably 0%, it leads to high costs, and is often about 0.001% or more, for example.
[0035]
Of the impurities (Al, Si, P, S, etc.), it is desirable to control the amount of P and S in particular.
[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 for increasing tensile strength but also for trapping hydrogen and improving delayed fracture resistance. On the other hand, coarse TiC is detrimental to toughness and delayed fracture resistance. Therefore, by increasing the proportion of fine TiC, the toughness of the steel part can be increased, and the delayed fracture resistance can be significantly increased. In addition, TiN is suppressed in the steel part of the present invention as described later. Since TiN does not dissolve during quenching heating, it tends to remain as coarse TiN in steel parts and is harmful to toughness and delayed fracture resistance. However, in the steel parts of the present invention, such TiN is suppressed. ing. Therefore, the steel part of the present invention is characterized in that the harmful Ti (coarse TiC, coarse TiN) in the steel is reduced while the desired Ti (fine TiC) is increased. The ratio of fine TiC to harmful Ti (coarse TiC, coarse TiN) can be substantially evaluated by the ratio of fine TiC to total Ti.
[0037]
Ratio of fine TiC content (mass%; hereinafter referred to as [fine TiC]) and total Ti content (mass%; hereinafter referred to as [Ti]) ([fine TiC ] / [Ti]) is 0.4 or more, preferably 0.5 or more, more preferably 0.6 or more. Although this ratio ([fine TiC] / [Ti]) is preferably as large as possible, it is usually about 0.9 or less.
[0038]
The fine TiC means TiC having a particle size of 0.1 μm or less. The amount of fine TiC 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. Although more fine TiC is desirable, it is usually about 0.10% or less.
[0039]
In addition, the steel part of this invention needs to have both the requirements of the said [fine TiC] / [Ti] and the requirements of the amount of fine TiC.
[0040]
Furthermore, since the high-strength steel parts of the present invention are not coarsened, the delayed fracture characteristics are improved from this point. The degree of coarsening suppression can be evaluated by the grain size number of austenite grains. The grain size number is No. No. 7 or more, preferably no. About 8 or more, more preferably No. No. 9 or more, especially No. It is about 10 or more (for example, about No. 11 or more). Although the larger the grain size number, the better. It is about 15 or less.
[0041]
The steel part according to the present invention has remarkably excellent delayed fracture resistance because there are many fine TiCs, few coarse TiCs, and no crystal grains are coarsened. In addition, since it contains a predetermined amount of C and has a large amount of fine TiC, a predetermined strength can be ensured. Therefore, excellent high-strength steel parts (e.g., high-strength bolts, springs, PC steel wires, rebars and other linear or bar-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 About above (preferably 1200 N / mm ² About the above, more preferably 1400 N / mm ² Or more). The tensile strength is 1500 N / mm ² Often it is about the following.
[0043]
Further, the steel component of the present invention has improved delayed fracture resistance by controlling the amount of C, Ti and N in terms of chemical composition. That is, since there are few kinds 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 to improve the delayed fracture resistance, from the viewpoint of the production method and application, According to the present invention, the use of these elements can be easily avoided, and a high-strength part having excellent delayed fracture resistance can be easily provided.
[0044]
The steel part can be produced by rapidly quenching steel having an equivalent 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 coarse TiC that remains undissolved can be suppressed, and the amount of fine TiC that precipitates during tempering can be increased. In addition, by quenching quickly, coarsening of crystal grains can be suppressed even when the quenching temperature is increased.
[0045]
The quenching temperature is specifically about 900 ° C. or higher, preferably about 950 ° C. or higher, more preferably about 1000 ° C. or higher, particularly about 1100 ° C. or higher. If the quenching temperature is too high, the crystal grains are likely to be coarsened even if quenching is performed quickly, and the delayed fracture characteristics are liable to deteriorate. Accordingly, the quenching temperature is usually about 1300 ° C. or less, preferably about 1250 ° C. or less, more preferably about 1150 ° C., particularly about 1100 ° C. or less.
[0046]
The rapidity 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 is a measure of the amount of heat input.
A = logt′−B × [Q / (R × T _a ]] + C (4)
[Wherein t ′ represents the heating time (Hr), Q represents the activation energy (cal / mol), R represents the gas constant (cal / mol), T _a Indicates heating temperature (K), and B and C indicate constants]
In the case of the present invention, the formula (4) can be arranged as the following formula (5). A = log [t + (T−700) / (2 × V)] − B ′ × (T + 273) + C ′
... (5)
[In the formula, T represents the quenching heating temperature (° C.), V represents the average heating rate during quenching heating (° C./second), t represents the holding time (second) after heating, and B ′ and C ′ Indicates a constant]
B ′ and C ′ were obtained by performing regression calculation using values obtained from a number of experiments. As a result, B ′ = 22 × 1000 and C ′ = 20. It can be rewritten as (3).
A = log [t + (T−700) / (2 × V)] − 22 × 1000 / (T + 273) +20 (3)
When an appropriate A value range was determined by changing the speed of quenching (heating rate, holding time after heating, etc.), the A value was about 8.0 or less (preferably about 7.5 or less, more preferably 7). It is desirable to quench at a 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 delayed fracture characteristics deteriorate. On the other hand, if the A value is too small, even if it is high-temperature quenching, heating is insufficient and Ti cannot be sufficiently dissolved. Therefore, it is desirable that quenching is performed 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, particularly about 5.0 or more.
[0047]
In order to control the A value below the predetermined value under high temperature heating conditions, it is necessary to increase the heating rate and shorten the holding time after heating. The heating rate can be selected from the range of, for example, about 10 ° C./second or more, preferably about 20 ° C./second or more, more preferably about 50 ° C./second or more. The holding time can be appropriately set according to the heating rate, and can be selected from a range of, for example, 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 higher than the predetermined value, it is necessary not to extremely increase the heating rate and not to extremely shorten the holding time. The heating rate can be selected from the range of, for example, about 600 ° C./second or less, preferably about 300 ° C./second or less, more preferably about 100 ° C./second or less. The holding time can be appropriately set according to the heating rate, but can be selected from a range of, for example, about 1 second or more, preferably about 3 seconds or more, and more preferably about 5 seconds or more.
[0049]
In order to quench rapidly, it is convenient to use a rapid heating apparatus such as a high-frequency heating apparatus or an energization heating apparatus (a heating apparatus using resistance heat generation accompanying 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 delayed fracture characteristics are not improved. The tempering temperature is usually about 650 ° C. or less (particularly about 625 ° C. or less) in many cases.
[0051]
By quenching and tempering as described above, it is possible to produce a steel part with a large amount of fine TiC despite having small crystal grains and suppressing the total amount of Ti. However, if the amount of N in the steel is too large, TiN will be formed, so that even if quenching and tempering are performed as described above, the amount of fine TiC will be insufficient. In order to make the fine TiC amount within the 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] (mass%) and the N content is [N] (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】
EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all 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), and then heated at 850 ° C. for 60 minutes for normalization. Using a high-frequency heating device, the round bar was quenched by oil cooling after rapid heating under the conditions shown in Tables 2-3. Subsequently, after heating on the conditions shown in Table 2, it tempered by cooling with water.
[0054]
Crossing the tempered round bar, the grain size No. of the austenite crystal grain of D / 4 part (D indicates the diameter). Was measured according to “Austenite 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 to obtain a measurement sample (diameter 12 mm × length 20 mm).
[0057]
(2) The sample was immersed in a methanol solution (electrolytic solution) in which 10% by mass of acetylacetone and 1% by mass of tetramethylammonium chloride were dissolved, and 20 mA / cm. ² The sample is dissolved (electrolyzed) by supplying a constant current of about 0.5 g.
[0058]
(3) After completion of the electrolytic treatment, the sample is immersed in methanol, and the deposit exposed on the sample surface is peeled off by ultrasonic waves.
[0059]
(4) The electrolytic solution containing the coarse precipitate and the ultrasonic treatment liquid are suction filtered through a membrane filter having a pore size 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 is ashed by heating with a gas burner.
[0061]
(6) Add an alkali flux (mixed solution of sodium carbonate and sodium tetraborate) to the platinum crucible, and heat again with a gas burner to melt the residue.
[0062]
(7) Add hydrochloric acid and pure water to a platinum crucible to dissolve the melt, and transfer to a volumetric flask. Furthermore, pure water is added to reach the marked line of the volumetric flask to obtain an analysis solution.
[0063]
(8) The amount of Ti is measured by ICP emission analysis, and the amount of coarse Ti compound (particle size greater than 0.1 μm) in the analysis solution W _Large-Ti Measure. On the other hand, the dissolution amount W of the sample during the above electrolytic treatment and ultrasonic treatment ₁ And the coarse Ti compound concentration ([coarse Ti compound]) in the sample is calculated based on the following formula.
[0064]
[Coarse Ti compound] = W _Large-Ti / W ₁ × 100
(9) About 0.3 g from the round bar (dissolution amount: W ₂ ) Is taken, put into aqua regia and dissolved by heating. After adjusting the concentration by adding pure water to this solution, the Ti amount (W _total-Ti ) And the total Ti concentration ([total Ti]) in the sample is calculated based on the following formula.
[0065]
[All Ti] = W _total-Ti / W ₂ × 100
(10) Based on the following formula, the concentration of fine TiC (particle size of 0.1 μm or less) in the sample ([fine TiC]) 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 specimens, the tensile strength and delayed fracture strength were examined. The delayed fracture strength is a method in which stress is applied while immersing the test piece in distilled water and the time until failure is measured, and when the stress is applied at multiple levels and the fracture time is 100 hours. The stress was evaluated by determining the stress (100-hour delayed fracture strength).
[0067]
Experimental Examples 23-24
Except for putting a round bar in a heating furnace and quenching, it was the same as Experimental Examples 1-22.
[0068]
The results are shown in Tables 2-3.
[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 dissolved Ti, and the amount of fine TiC after tempering is increased, the austenite grains become coarse and the delayed fracture strength decreases.
[0073]
Further, as apparent from Experimental Example 24, in order to avoid coarsening of austenite grains, the amount of Ti in the quenching is increased by increasing the amount of Ti in the steel under the conditions of quenching at a low temperature, and the fineness after tempering is increased. Even if the amount of TiC is increased, the delayed fracture strength is also weak because a large amount of coarse TiC remains in the steel.
[0074]
On the other hand, in Experimental Example 1, since quenching is performed in a short time by high frequency, Ti in the steel can be efficiently dissolved while preventing austenite grains from coarsening. Therefore, in tempered round bars, crystal grains can be reduced and the amount of fine TiC can be increased while suppressing the amount of coarse TiC (that is, [fine TiC] / [Ti] can be increased), tensile strength and delayed fracture Strength can be increased. Similarly, in Experimental Examples 5 to 15, since quenching is performed in a short time with a high frequency, the tensile strength and delayed fracture strength can be increased.
[0075]
However, in Experimental Example 2, since the induction hardening temperature is too low and the A value is too low, Ti in the steel cannot be sufficiently dissolved, the amount of fine TiC is insufficient, and the tensile strength is weakened. In Experimental Example 3, the amount of heat input during induction hardening is too large and the A value becomes too high, so the austenite grains become coarse and the delayed fracture strength decreases. In Experimental Example 4, since the tempering temperature is too low, Ti melted at the time of quenching does not precipitate, the fine TiC becomes insufficient, and the delayed fracture strength becomes small.
[0076]
In Experimental Examples 16, 19, 21, and 22, [Ti] -3.4 [N] is small, and as a result of Ti bonding with N, fine TiC is difficult to precipitate. Moreover, since the TiC that suppresses the coarsening of the austenite grains is insufficient, the austenite grains are also coarsened. Therefore, the delayed fracture strength is lowered.
[0077]
In Experimental Example 17, the tensile strength is low because the amount of C is small, and in Experimental Example 18, the delayed fracture resistance is low because of the large amount of C. In Experimental Example 20, since the amount of Ti is too large, not only fine TiC increases, but also coarse TiC increases, and the delayed fracture strength decreases.
[0078]
【The invention's effect】
According to the present invention, the delayed fracture characteristics can be remarkably enhanced because the amount of fine TiC is large, although the coarsening of crystal grains and the amount of coarse TiC are suppressed. In addition, as the chemical components, only the C content, Ti content, and N content are controlled, and therefore, it can be applied to high-strength steel parts having a wide composition.
[0079]
The high-strength steel part can be manufactured by rapidly quenching at a high temperature and tempering at a 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 (6)

% By mass (hereinafter the same), C: 0.20 to 0.55%, Si: 2% or less, Mn: 2% or less, Al: 0.1% or less, P: 0.02% or less, S: 0.02% or less, Ti: 0.01 to 0.10%, N: 0.02% or less,
Furthermore, it contains at least one selected from Cr: 2% or less, Mo: 2% or less, V: 1% or less, W: 1% or less, and Nb: 1% or less, and the balance is Fe and inevitable impurities ,
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 fine TiC content (mass%; hereinafter referred to as [fine TiC]) and the total Ti content (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) As other ingredients,
The high-strength steel part according to claim 1, containing at least one selected from Cu: 1% or less and Ni: 4% or less. Furthermore, B: The high strength steel component of Claim 1 or 2 containing 0.003% or less. % By mass (hereinafter the same),
C: 0.20 to 0.55%,
Ti: 0.01-0.10%,
N: 0.02% or less,
A steel in which the total Ti content (% by mass; hereinafter referred to as [Ti]) and the N content (% by mass; hereinafter referred to as [N]) satisfy the following formula (2):
After quenching at a quenching temperature of 900 to 1300 ° C. and a heat input strength A defined by the following formula (3) of 3.0 to 8.0,
A method for producing a high-strength steel part excellent in 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)
[Wherein T represents quenching heating temperature (° C.), V represents average heating rate during quenching heating (° C./second), and t represents retention time (seconds) after heating] The manufacturing method according to claim 4 , wherein an average heating rate during heating in the quenching step is selected from a range of 10 ° C / second or more, and a holding time after heating is selected from a range of 60 seconds or less. The manufacturing method according to claim 4 or 5 , wherein the quenching is performed by high frequency and / or energization heating.

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