JP2000026933A - Hot forging steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot forging steel in which machinability is improved by S and the occurrence of grinding crack caused by sulfide inclusions is inhibited and further the occurrence of a suspended pattern of flaw at the time of magnetic particle inspection of a product is prevented. SOLUTION: This steel has a composition which consists of, by weight, 0.2-0.6% C, 0.05-1.5% Si, 0.1-3.0% Mn, <=0.08% P, 0.01-0.2% S, 0.04-1.0% Ti, <=0.008% N, 0-0.1% Cu, 0-2% Ni, 0-5% Cr, 0-1.0% Mo, 0-1% Al, 0-0.005% B, 0-1% V, 0-0.5% Nb, 0-0.5% Pb, 0-0.01% Ca, 0-0.5% Bi, and the balance Fe with inevitable impurities and in which respective contents of Ti and S are regulated so that the value of fn 1, represented by equation fn 1=Ti(%)-1.2×S(%), is plug quantity (fn 1>0).

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hot forging steel such as die forging and free forging. 2. Description of the Related Art Forged products such as crankshafts, connecting rods, knuckles and the like, or hot forged products such as shafts and mechanical parts, are used to reduce the cutting allowance as much as possible. Is hot forged, but mechanical processing such as cutting and grinding is inevitably added in order to secure the accuracy when joining or combining with other parts. As an example of a manufacturing process for a hot forged product, as shown in FIG. 1, a raw material ingot (a billet) is heated, hot forged into a desired shape, and then subjected to a heat treatment such as quenching and tempering. Temper to the required mechanical properties of steel. When using non-heat treated steel, required mechanical properties are obtained by controlling cooling after forging. These forged products are machined, such as cutting and drilling, at the joints and attachments with other parts that require dimensional accuracy.Furthermore, induction hardening and carburizing are performed to improve wear resistance and fatigue strength. ,
Surface treatment such as surface hardening or surface modification such as nitriding, and finish grinding are performed. In general, S in steel deteriorates hot workability,
It becomes an inclusion and deteriorates the toughness and ductility of steel. Therefore, for plastic working such as rolling and forging of steel, and for use as a strength member, it is considered that the lower the S content, the better. However, when the amount of S is small, there is a problem that the machinability such as the cutting resistance and the life of the cutting tool is reduced, and the finish of the cut surface or the ground surface is deteriorated. For steel used in hot forging, it is desirable that the S content be as low as possible from the viewpoint of the performance of the forged product, but that cutting and drilling must be performed in a high strength state such as after tempering. Therefore, the content of S is often increased in steel as a material in consideration of machinability. S usually forms sulfide-based inclusions containing MnS as a main component, and the sulfide-based inclusions expand with the deformation of the steel and are dispersed in the steel. Then, it becomes the starting point of cracks near the cutting edge of tools such as cutting, reducing the cutting resistance,
It is said to provide a beautiful finish and provide a lubricating effect between the tool and the steel. However, in the case of a hot forged product, when the content of S is increased in order to improve machinability, the MnS-based inclusions increase, which may be a starting point of grinding cracks during finish grinding,
In a commonly used magnetic particle inspection, this inclusion may be recognized as if it were a flaw. In the magnetic particle flaw detection, when the steel is magnetized, if there is a flaw, the magnetization is disturbed, and the flaw is detected due to a different accumulation of the magnetic powder. Since MnS is non-magnetic, the magnetization is disturbed and a fake pattern of flaws is generated. Sometimes, there is no defect as a forged product, but it may be scrapped down as a defective product. In such a case, additional inspection must be performed to determine whether the defect has a problem with the opening property, and the number of inspection steps has been increased. SUMMARY OF THE INVENTION It is an object of the present invention to improve the machinability by using S, suppress the occurrence of grinding cracks due to sulfide-based inclusions, and simulate flaws during magnetic particle inspection of products. An object of the present invention is to provide a hot forging steel that does not generate a pattern. [0007] To improve the machinability of steel, the addition of more S, the addition of elements that improve machinability such as Pb and Ca, and other adjustment of the structure And graphitization. However, when targeting hot forged products, it does not hinder hot workability, improves the machinability after high-strength tempering, and does not deteriorate the toughness or fatigue strength of the forged product, and If the method is not disadvantageous in terms of cost, means other than the addition of S can hardly be considered. The present inventors have reduced the S content of carbon steel or low-alloy steel having a C content of 0.2 to 0.6%, which is generally applied to parts manufactured by hot forging. Without doing so, that is, with the machinability unchanged or further improved, the possibility of using steel that suppresses grinding cracks and does not generate false scratch patterns was examined. Usually S
Are sulfide-based inclusions mainly composed of MnS in the steel, which are viscously deformed and elongated in the metal flow direction of the steel. Recent advances in steelmaking technology have reduced the amount of inclusions in steel and reduced oxygen, etc., and this tendency has become even stronger, resulting in the occurrence of grinding cracks and simulated flaws due to expanded sulfides. Is coming. As a method of changing the form of sulfide inclusions in steel, various attempts have been made to add specific elements. As the additional elements that can change the form of the sulfide-based inclusions, Ca, rare earth elements, Ti, Zr, V, Nb, and the like are conventionally known. Therefore, we first investigated the hot forgeability, machinability, stability of the effect, economic efficiency, etc. of these elements together with the occurrence of grinding cracks and pseudo patterns of scratches. Was not enough. However, further investigation was conducted on Ti, which was considered to be relatively significant among them, and especially when the nitrogen content was reduced, the effect of improving machinability, suppressing grinding cracks and reducing pseudo-patterns of scratches was observed. Was found to appear remarkably. [0010] The addition of Ti is relatively low in C used in steel sheets.
It is often used for low carbon steels with low carbon content. For example, in cold rolled steel sheets, the carbon in the steel is set to TiC
If fixed, a steel sheet which is non-ageing and extremely excellent in deep drawability can be obtained. In the case of producing a steel sheet having a high strength while being hot-rolled from a steel having C of 0.25% or less, a small amount is added. However, in a high C steel of which C is about 0.4%, which is often used for hot forging, etc., the effect of the addition is scarcely known except that the above-mentioned Mn can be used to suppress hot brittleness due to S instead of Mn. This is because TiC is combined with C to form TiC, and the effect is hardly produced. Further, it is combined with N to form hard TiN inclusions and deteriorate machinability. So I thought. On the other hand, when the N is made as low as possible, the above-mentioned effect appears at a high temperature immediately after solidification. When the N is high, it becomes TiN. If Ti is present, it is presumed that TiS is formed first and changes to carbon sulfide with decreasing temperature. The carbosulfide is relatively large when formed, but is broken and dispersed by processing without viscous deformation unlike MnS. As a result, grinding cracks are suppressed, and false patterns of flaws during magnetic particle flaw detection are reduced. And, similarly to MnS, it has an effect of lowering the cutting resistance, and since it is relatively soft as an inclusion, it has a lubricating effect more than MnS, and it is thought that the machinability is further improved. The reduction of N has been made easier by the progress of steel smelting techniques such as molten steel vacuum processing. With regard to the effect of the addition of Ti in such a steel for hot forging, the present invention has been completed by clarifying the application limit for sufficiently exerting the effect. The gist of the present invention is that, by weight%, C: 0.2 to 0.6%, S:
i: 0.05 to 1.5%, Mn: 0.1 to 3.0%, P: 0.08% or less, S: 0.01 to 0.2%, Ti: 0.04 to 1.0%, N: 0.008
%, Cu: 0 to 1.0%, Ni: 0 to 2%, Cr: 0 to 5
%, Mo: 0 to 1.0%, Al: 0 to 1%, B: 0 to 0.005%,
V: 0 to 1%, Nb: 0 to 0.5%, Pb: 0 to 0.5%, Ca:
0-0.01% and Bi: 0-0.5%, with the balance being F
e and unavoidable impurities, and wherein fn1 represented by the following formula is a content of Ti and S in which fn1 is positive (fn1> 0): fn1 = Ti (%)- 1.2 × S (%) The reasons for limiting the steel composition in the present invention are as follows. C: 0.2-0.6% C is a basic element that determines the properties of steel such as strength.
After hot forging, in order to obtain the required strength regardless of whether it is tempered or not, it is necessary to contain 0.2% or more, but if it is too large, the workability of forging decreases, and cracking during heat treatment occurs. Problems such as reduced machinability, etc.
Up to 0.6%. Si: 0.05-1.50% Si is added for deoxidation during steelmaking. Content is 0.05%
If the amount is less than the above, deoxidation becomes insufficient, and the soundness of the slab decreases.
However, if the amount is too large, surface decarburization tends to occur during forging, and the appearance such as scale residue on the surface of the forged product is deteriorated. Mn: 0.1 to 3.0% Mn has effects such as deoxidation of steel, suppression of hot working brittleness by S, and improvement of hardenability during heat treatment. In order to obtain such an effect, the content of 0.3% or more is usually required. However, in the steel of the present invention, a sufficient amount of Ti is contained with respect to the amount of S. The effect is obtained.
If it is less than 0.1%, effects such as improvement of hardenability become insufficient.
However, excessive addition causes crazing during heat treatment,
Further, since the machinability is reduced, the content is set to at most 3.0%. P: not more than 0.08% P is one of the inevitable impurities and degrades the toughness of the steel. Therefore, usually, the smaller the content, the better. However, depending on the forged product, increasing the content of P may increase the fatigue strength, and is added as necessary. However, if the content is too large, the toughness is deteriorated. S: 0.01-0.2 In order to obtain sufficient machinability with a hot forged product, at least 0.
It is necessary to contain more than 01%. However, excessive addition causes cracking at the time of hot forging and deteriorates the toughness of the obtained forged product, so it is at most 0.2%. That is, the content range of S is set to 0.01 to 0.2%. Desirably, the content is 0.04% or more. Ti: 0.04% to 1.0% Ti is an important element that improves the machinability after forging, suppresses grinding cracks, and reduces flaws in scratches by changing the form of sulfide. In order to obtain this effect, the content must be at least 0.04% or more. But 1.0
%, The effect is saturated, the toughness of the steel is reduced, and the cost increases. fn1 = Ti (%) − 1.2 × S (%): positive (fn1> 0) The number obtained by subtracting 1.2 times the S content from the Ti content is fn1.
And fn1 indicates the amount of Ti required to sufficiently combine S with Ti, and in the case of the present invention, fn1> 0. When fn1 ≦ 0, S other than bonding to Ti is M
It becomes a sulfide bonded to n. In this case, if S is sufficiently contained, the suppression of grinding cracks and pseudo patterns of scratches becomes insufficient even if the machinability can be ensured. Further, if Ti is sufficiently contained in S, hot working cracks can be prevented even if the amount of Mn is small. N: 0.008% or less The content of N is set to 0.008% or less. In the present invention, the smaller the content, the more preferable. If N exceeds 0.008%, even if the amount of Ti satisfies the expression, sufficient effects of improving machinability, preventing grinding cracks, and suppressing false scratch patterns cannot be obtained. This is because Ti is easier to bond with N than S, and when N is present in a large amount, TiN is first formed, and Ti for forming S and sulfide is insufficient. Desirably, N is set to 0.006% or less. Cu: 0 to 1.0% Cu need not be added, but is added as necessary for improving hardenability and precipitation hardening. Even if it is added in a large amount, the effect is saturated. However, the addition of more than 0.3% causes hot working cracks during forging, so that the amount of N of 1/2 or more of the amount of Cu
It is preferable that i is simultaneously contained. Ni: 0 to 2% Ni may not be added, but may be added if necessary for the purpose of improving the toughness of steel products, improving the hardenability, and preventing hot work cracking when Cu is added. When added, the content is at most 2% or less from the viewpoint of saturation of effects and cost increase. Cr: 0 to 5% Cr need not be added. However, it has the effect of improving the hardenability, and is added as necessary. When adding a large amount, the machinability deteriorates if the amount is large, so that the content should be at most 5%. Mo: 0 to 1.0% Mo may not be added. When added, it has the effects of improving hardenability, improving temper softening resistance, and improving toughness. However, even if a large amount is added, the effect saturates and the cost also increases. Al: 0-1% Al does not need to be added, but has a strong deoxidizing effect.
In order to ensure the soundness of the slab, it is included as necessary. In the case where surface nitriding is performed, it is contained in order to obtain sufficient hardening due to nitriding. However, even if it is contained in a large amount, the curing is saturated and only the surface flaws are increased, so the content is set to at most 1%. B: 0 to 0.005% B may not be added. The addition of a small amount has the effect of improving the hardenability, and is added as necessary. When added, if the content is several ppm or more, the hardenability improvement effect is almost constant regardless of the amount. However, 0.00
Content of more than 5% embrittles steel, so at most 0.005
%. V: 0-1% V may not be added. However, when the steel is made to be a non-heat treated steel, it is added in order to precipitate finely as carbide or nitride in the cooling process after forging and to improve the strength. In the case of quenching and tempering, there is an improvement in hardenability and a secondary hardening during tempering. Excessive addition causes a reduction in hot workability and an increase in cost, so the content should be up to 1%. Nb: 0 to 0.5% Nb may not be added. When steel is used as a non-heat treated steel, the effect is the same as that of V when added. Further, the crystal structure of steel may be refined to improve toughness. However, excessive addition delays recrystallization during hot working of steel and makes hot working difficult, so the content should be at most 0.5%. Pb: 0-0.5% Pb may not be added. However, if added, the machinability of the steel can be improved, so it is included when more machinability is required. However, excessive addition deteriorates the hot workability of steel, so the content should be up to 0.5%. Ca: 0 to 0.01% Ca may not be added. Ca has an effect of improving the machinability of a carbide tool, and Ca is contained as necessary. However, even if it is added excessively, the effect is saturated and coarse inclusions are formed to lower toughness and fatigue strength. Therefore, the upper limit is made 0.01%. Bi: 0 to 0.5% Bi, like Pb, is an element for improving the machinability of steel and need not be added, but is contained when more machinability is required. However, if the content exceeds 0.5%, it causes the generation of a false pattern at the time of magnetic particle flaw detection. EXAMPLE Steels having the chemical compositions shown in Table 1 were melted in a 1-ton high-frequency vacuum melting furnace. Steel Nos. 1 to 6 contain Ti, and Steel Nos. 7 to 12 do not contain Ti. In these, the S content was changed, and the other compositions were made as identical as possible. Where Ti
When Ti is added, the Ti content is also increased with the S content so that fn1 in the formula becomes positive (fn1> 0). In the case where Ti is not added, Mn is used to suppress hot brittleness. Was increased. The obtained steel ingot was rolled into a billet of 100 m square. 1250 of these billets
After heating to 0 ° C., the launch temperature was set to 1100 ° C., and the mixture was forged into a crankshaft having a pin journal diameter of 60 mmφ and air-cooled. After forging, quenching and tempering are performed under normal conditions, and the hardness is HB
250. A round bar test piece having a diameter of 55 mm and a length of 60 mm was cut out from the pin portion, and the machinability was evaluated by drilling a 6 mmφ through hole in the length direction and the number of holes that could be drilled. The circumferential surface portion of such a round bar test piece was induction hardened, and after polishing, the presence or absence of occurrence of a pseudo pattern on the circumferential surface portion in magnetic particle flaw detection by a shaft current method of a current value of 2500 A was investigated. The total length was measured. In addition, the circumferential surface of the round bar specimen was subjected to high-frequency quenching and then ground under heavy grinding conditions, and the presence or absence of grinding cracks was checked by magnetic particle flaw detection with a current value of 1000 A. Was measured. [Table 1] The results of these investigations are shown in FIGS. 2 to 4 in which the S content is plotted on the horizontal axis and the measured values are plotted on the vertical axis. In FIG. 2, when the S content is small, the number of drilled holes is small and the machinability is not good, but the number of drilled holes increases as the S content increases, and the machinability is improved. Comparing with the same S content, it can be seen that the steel with the addition of Ti of the present invention has better machinability than the steel without the addition. FIG. 3 or FIG.
As shown in Fig. 5, the pseudo pattern at the time of flaw detection and the crack at the time of grinding also increase with the increase of the S content. And has declined significantly. The steel of the present invention, when applied to a hot forged product, exhibits excellent machinability as it is after forging or when it is made into a non-heat treated steel, as it is forged. There are no false patterns at the time of magnetic particle flaw detection, and there are few cracks during grinding. By using such steel, it becomes possible to significantly reduce the manufacturing cost of forged products.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing a manufacturing process of a hot forged product. FIG. 2 is a diagram showing the relationship between the S content of steel and the machinability of forged steel. FIG. 3 is a diagram showing a relationship between the S content of steel and the occurrence of a pseudo-defect pattern during magnetic particle flaw detection. FIG. 5 is a graph showing the relationship between the S content of steel and the occurrence of grinding cracks when grinding after surface hardening.

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[Procedural amendment] [Date of submission] September 3, 1998 (1998.9.3) [Procedure amendment 1] [Document name to be amended] Specification [Item name to be amended] Brief explanation of drawings [Method of amendment] FIG. 1 is a view showing a manufacturing process of a hot forged product. FIG. 2 is a diagram showing the relationship between the S content of steel and the machinability of forged steel. FIG. 3 is a diagram showing a relationship between the S content of steel and the occurrence of a pseudo-defect pattern during magnetic particle flaw detection. FIG. 4 is a diagram showing the relationship between the S content of steel and the occurrence of grinding cracks when grinding after surface hardening.

Claims (1)

[Claims] C: 0.2-0.6%, Si: 0.05-1.5%, M
n: 0.1 to 3.0%, P: 0.08% or less, S: 0.01 to 0.2%,
Ti: 0.04 to 1.0%, N: 0.008% or less, Cu: 0 to 1.0
%, Ni: 0 to 2%, Cr: 0 to 5%, Mo: 0 to 1.0%, A
1: 0-1%, B: 0-0.005%, V: 0-1%, Nb: 0-0
0.5%, Pb: 0 to 0.5%, Ca: 0 to 0.01%, and B
i: 0 to 0.5%, the balance being Fe and unavoidable impurities, and the content of Ti and S is such that fn1 represented by the following formula is positive (fn1> 0). Steel for forging. fn1 = Ti (%)-1.2 x S (%)