JP6844943B2 - Non-microalloyed steel for induction hardening - Google Patents

Description

The present invention relates to non-microalloyed steel, and more particularly to non-microalloyed steel for induction hardening.

Mechanical structural parts used for crankshafts and the like of automobiles and construction vehicles may be subjected to surface hardening treatment in order to improve fatigue strength, wear resistance and the like.

Of the various surface hardening treatments, induction hardening can cure only the required parts. Further, since induction hardening cools after heating at a high temperature, a deeper cured layer depth and higher fatigue strength can be obtained as compared with other surface hardening treatments such as soft nitriding treatment. Therefore, induction hardening is often applied to mechanical structural parts. For example, in order to improve the fatigue strength of a crankshaft, which is one of the mechanical structural parts, a technique of induction hardening of the R portion 1 of the fillet shown in FIG. 1 has been put into practical use.

In recent years, there has been a demand from the industrial world for further increasing the strength of mechanical structural parts, particularly improving fatigue strength. In order to increase the depth of the cured layer by using induction hardening, the heating temperature may be increased by increasing the output of induction power and the heating time. However, when induction hardening is performed at a high temperature, the heating temperature tends to be excessively high at the edge portion of the mechanical structural component (for example, the portion indicated by reference numeral 2 in the case of the crankshaft of FIG. 1). If the heating temperature becomes excessively high and exceeds the melting point of the steel material of the mechanical structural parts, the surface layer or a part of the inside of the steel material melts and cracks occur. Hereinafter, such cracks are referred to as "melt cracks". Melt cracking is a peculiar phenomenon that occurs in induction hardening. Since steel materials with melt cracks are not suitable for practical use, it is required to suppress melt cracks in induction hardening steel.

Induction hardening steel used for machine structural parts is further required to have excellent machinability as well as the above-mentioned fatigue strength. Therefore, S is contained in the induction hardening steel in order to improve the machinability. However, the higher the S content, the more likely the above-mentioned melt cracking will occur. Therefore, induction hardening steel is required to have high fatigue strength and machinability, and to suppress the occurrence of melt cracks.

Examples of techniques related to induction hardening steel are disclosed in JP-A-5-33101 (Patent Document 1), JP-A-2004-27259 (Patent Document 2), and JP-A-2011-26641 (Patent Document 3). Has been done.

The non-microalloyed steel for high frequency hardened crank shaft disclosed in Patent Document 1 has C: 0.40 to 0.52%, Si: 0.10 to 0.40%, Mn: 1.00 to 1.00 on a mass basis. 1.50%, S: 0.010 to 0.070%, Cr: 0.40 to 0.70%, Pb: 0.02 to 0.35%, Ca: 0.0005 to 0.0100%, O : 0.0040% or less, Al: 0.025% or less, N: 0.005 to 0.015%, and the balance is substantially composed of Fe.

The free-cutting steel for machine structure disclosed in Patent Document 2 is C: 0.35 to 0.65%, Si: 0.03 to 1.0%, Mn: 0.30 to 2.50 in mass%. %, S: 0.015 to 0.35%, Al: 0.060% or less, Ca: 0.0005 to 0.01%, and Ni: 0.1 to 3.5%, Cr: 0. .1 to 2.0%, Mo: Contains one or more elements selected from 0.05 to 1.00%, consisting of the balance Fe and unavoidable impurities, and the size of the sulfide in the steel. The major axis is 30 μm or less, and after cutting or forging the material, a part of the part is induction hardened before use.

The non-tamed steel for high frequency quenching disclosed in Patent Document 3 has a mass% of C: 0.35 to 0.45%, Si: more than 0.30% and 0.70% or less, Mn: 1. 00 to 1.50%, P: 0.030% or less, S: 0.035% or less, Cr: 0.10 to 0.30%, Al: 0.005 to 0.050%, V: 0.100 It contains ~ 0.200% and N: 0.0040 to 0.0200%, fn1 represented by the following formula (1) is 50 or less, and fn2 represented by the following formula (2) is 0. It ranges from .80 to 1.00 and the balance consists of Fe and impurities.
fn1 = 80C ² + 55C + 13Si + 4.8Mn + 30P + 30S + 1.5Cr
(1)
fn2 = C + (Si / 10) + (Mn / 5)-(5S / 7) + (5Cr / 22) + 1.65V (2)

Japanese Unexamined Patent Publication No. 5-33101 Japanese Unexamined Patent Publication No. 2004-27259 Japanese Unexamined Patent Publication No. 2011-26641

It is described that the non-tempered steel of Patent Document 1 does not require quenching and tempering treatment, the difference in hardness does not increase due to the difference in cooling rate based on the dimensional difference, and the workability is excellent. However, Patent Document 1 does not study the suppression of melt cracking that may occur during induction hardening.

Patent Document 2 describes that quench cracking that occurs during induction hardening is reduced. However, as in the case of Patent Document 1, suppression of melt cracking has not been studied.

Patent Document 3 considers reduction of melt cracking. However, melt cracking in the case of induction hardening of a steel material containing S, particularly a steel material containing S of more than 0.035% at a high heating temperature of more than 1350 ° C., has not been studied.

An object of the present invention is to suppress the occurrence of melt cracking even if induction hardening is performed, obtain high fatigue strength even after hot forging, and have excellent machinability. Is to provide.

The non-microalloyed steel for high frequency quenching according to the present invention has C: 0.35 to 0.44%, Si: more than 0.30% to 0.70%, Mn: 1.00 to 1.50% in mass%. , P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.10 to 0.30%, Al: 0 to 0.040%, V: 0.1000 to 0.200% , N: 0.0040 to 0.0200%, Ca: 0.0005% to 0.0100%, O: 0.0024% or less, Ti: 0 to 0.020%, Nb: 0 to 0.020%, It contains Pb: 0 to 0.30%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Mo: 0 to 0.20%, and fn1 defined by the formula (1) is 50. It is 0.0 or less, fn2 defined by the formula (2) is 0.80 to 1.00, fn3 defined by the formula (3) is 0.013 to 0.124, and the balance is Fe and Consists of impurities.
fn1 = 80C ² + 55C + 13Si + 4.8Mn + 30P + 30S + 1.5Cr (1)
fn2 = C + (Si / 10) + (Mn / 5)-(5S / 7) + (5Cr / 22) + 1.65V (2)
fn3 = Al + 10.8Ca (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formulas (1) to (3).

In the non-treated steel for induction hardening of the present invention, the occurrence of melt cracking due to induction hardening is suppressed. Further, after hot forging, sufficient fatigue strength and machinability can be obtained.

FIG. 1 is a front view showing a part of a crankshaft which is a mechanical structural part. FIG. 2 shows, in an example, a test piece of non-microalloyed steel for induction hardening, which is a comparative example, was heated to 1380 ° C. at a heating rate of 25 ° C./sec, held for 30 seconds, and then rapidly cooled with He gas. It is a microstructure photographic image when it is done. In FIG. 3, in an example, a test piece of induction-hardened steel according to an example of the present invention is heated to 1380 ° C. at a heating rate of 25 ° C./sec, held for 30 seconds, and then with He gas. It is a microstructure photographic image when quenching.

The present inventors have investigated in detail the sites where melt cracking has occurred in induction-hardened mechanical structural parts. As a result, decarburization did not occur at the site where the melt cracking occurred. On the other hand, the decarburized part was melted.

From the above results, the present inventors considered that the C content affects the melt cracking in the case of induction hardening, and that if the C content is lowered, the melt cracking is suppressed. Therefore, the present inventors further conducted a detailed study on the effects of various elemental contents on the occurrence of melt cracks and the effects on mechanical properties, particularly fatigue strength. As a result, the following findings were obtained.

If the contents of C, Si, Mn, P, S and Cr are reduced, melt cracking that occurs during induction hardening can be suppressed. However, if the contents of C, Si, Mn, P, S and Cr are lowered, the hardenability and the internal hardness of the steel material are lowered even if the occurrence of melt cracking can be suppressed. If hardenability and internal hardness are reduced, high fatigue strength cannot be obtained. Therefore, in order to suppress melt cracking and secure high fatigue strength, it is necessary to appropriately control the contents of C, Si, Mn, P, S and Cr.

The present inventors further investigated the effect of the morphology of inclusions such as MnS on melt cracking, and as a result, obtained the following findings.

When the aspect ratio (maximum length / maximum width) of MnS is large, that is, when the width of MnS is narrow and there is a tip portion, local abnormal heating occurs at the tip portion of MnS during induction hardening. In this case, melt cracking is likely to occur. If the aspect ratio of MnS is reduced to suppress the formation of the tip portion, the occurrence of local abnormal heating can be suppressed. Therefore, the occurrence of melt cracking can be suppressed.

In order to reduce the aspect ratio of MnS, it is effective to include Ca in the steel. However, Ca can produce an Al-based composite oxide having a low melting point. The Al-based composite oxide is easily stretched by hot rolling, hot forging, or the like. If the Al-based composite oxide is stretched to increase the aspect ratio, local abnormal heating is likely to occur and melt cracking is likely to occur, as in MnS. If the total content of Ca and Al, which are elements forming the Al-based composite oxide, is appropriately controlled, the formation of the Al-based composite oxide can be suppressed and the occurrence of melt cracking can be suppressed.

The non-healed steel for high frequency quenching according to the present embodiment completed based on the above findings has a mass% of C: 0.35 to 0.44%, Si: more than 0.30% to 0.70%, and Mn. : 1.00 to 1.50%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.10 to 0.30%, Al: 0 to 0.040%, V : 0.100 to 0.200%, N: 0.0040 to 0.0200%, Ca: 0.0005% to 0.0100%, O: 0.0024% or less, Ti: 0 to 0.020%, Nb: 0 to 0.020%, Pb: 0 to 0.30%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Mo: 0 to 0.20%, and the formula ( The fn1 defined by the formula (1) is 50.0 or less, the fn2 defined by the formula (2) is 0.80 to 1.00, and the fn3 defined by the formula (3) is 0.013 to 0. It is .124, and the balance consists of Fe and impurities.
fn1 = 80C ² + 55C + 13Si + 4.8Mn + 30P + 30S + 1.5Cr (1)
fn2 = C + (Si / 10) + (Mn / 5)-(5S / 7) + (5Cr / 22) + 1.65V (2)
fn3 = Al + 10.8Ca (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formulas (1) to (3).

In the non-tempered steel for induction hardening of the present embodiment, it is possible to suppress the occurrence of melt cracking during heating of induction hardening. In this case, the product yield is improved. The induction-hardened non-microalloyed steel of the present embodiment can further obtain high fatigue strength and machinability even after hot forging in a process of manufacturing mechanical structural parts such as a crankshaft.

The induction-hardened steel may contain one or more selected from the group consisting of Ti: 0.005 to 0.020% and Nb: 0.005 to 0.020%.

The non-induction steel for induction hardening may further contain Pb: 0.10 to 0.30%.

The non-microalloyed steel for induction hardening is further selected from the group consisting of Cu: 0.05 to 0.20%, Ni: 0.05 to 0.20%, and Mo: 0.05 to 0.20%. It may contain one kind or two or more kinds.

Hereinafter, the non-treated steel for induction hardening of the present embodiment will be described in detail. Unless otherwise specified, "%" for an element means mass%.

[Chemical composition]
The chemical composition of the non-hardened steel for induction hardening of the present embodiment contains the following elements.

C: 0.35-0.44%
Carbon (C) increases the hardness of the induction hardened portion and the internal hardness of the steel. If the C content is less than 0.35%, this effect cannot be obtained. On the other hand, if the C content exceeds 0.44%, melt cracking occurs during induction hardening. Therefore, the C content is 0.35 to 0.44%. The preferable lower limit of the C content is 0.37%. The preferable upper limit of the C content is 0.42%.

Si: Over 0.30 to 0.70%
Silicon (Si) reinforces ferrite to increase the internal hardness of steel. If the Si content is 0.30% or less, this effect cannot be obtained. On the other hand, if the Si content exceeds 0.70%, melt cracking is likely to occur. Further, the internal hardness becomes too high and the machinability of the steel is lowered. Therefore, the Si content is more than 0.30 to 0.70%. The preferable lower limit of the Si content is 0.50%. The preferred upper limit of the Si content is 0.68%.

Mn: 1.00 to 1.50%
Manganese (Mn) deoxidizes molten steel during steelmaking. Mn Further, the quenching of steel is enhanced and the internal hardness is enhanced. If the Mn content is less than 1.00%, these effects cannot be obtained. On the other hand, if the Mn content exceeds 1.50%, melt cracking is likely to occur. Further, the internal hardness becomes too high and the machinability decreases. Therefore, the Mn content is 1.00 to 1.50%. The preferable lower limit of the Mn content is 1.05%. The preferred upper limit of the Mn content is 1.48%.

P: 0.030% or less Phosphorus (P) is an impurity. If the P content exceeds 0.030%, the hot forging property is lowered. Further, melt cracking is likely to occur during induction hardening heating. Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is 0.025%.

S: 0.010 to 0.095%
Sulfur (S) produces sulfide-based inclusions and enhances the machinability of steel. If the S content is less than 0.010%, this effect cannot be obtained. On the other hand, if the S content exceeds 0.095%, melt cracking is likely to occur. Therefore, the S content is 0.010 to 0.095%. If Ca is not contained and the S content exceeds 0.035%, melt cracking is likely to occur. However, in the present embodiment, since Ca is contained as described later, if the S content is 0.095% or less, the occurrence of melt cracking can be suppressed and the decrease in hot forging property can also be suppressed. The preferable lower limit of the S content is 0.015%. The preferred upper limit of the S content is 0.070%.

Cr: 0.10 to 0.30%
Chromium (Cr) enhances hardenability and internal hardness of steel. If the Cr content is less than 0.10%, these effects cannot be obtained. On the other hand, if the Cr content exceeds 0.30%, melt cracking is likely to occur. Further, the internal hardness becomes too high and the machinability decreases. Therefore, the Cr content is 0.10 to 0.30%. The preferable lower limit of the Cr content is 0.12%. The preferable upper limit of the Cr content is 0.25%.

Al: 0 to 0.040%
Aluminum (Al) is an optional element. Al can be used as a deoxidizing element. However, if the Al content exceeds 0.040%, Ca and an Al-based composite oxide are formed, causing melt cracking. Therefore, the Al content is 0 to 0.040%. The preferable upper limit of the Al content is 0.030%. In the present specification, the Al content means the total Al content.

V: 0.1000 to 0.200%
Vanadium (V) is deposited in ferrite as V carbonitride in the cooling process after hot forging of steel. The V-carbonitride increases the hardness of the ferrite, and as a result, the internal hardness increases. If the V content is less than 0.100%, this effect cannot be obtained. On the other hand, if the V content exceeds 0.200%, the above effect is saturated and the production cost increases. Therefore, the V content is 0.1000 to 0.200%. The preferable lower limit of the V content is 0.103%. The preferred upper limit of the V content is 0.195%.

N: 0.0040 to 0.0200%
Nitrogen (N) forms nitrides and carbonitrides to make the structure finer and to precipitate and strengthen steel. If the N content is less than 0.0040%, these effects cannot be obtained. On the other hand, if the N content exceeds 0.0200%, the hot forging property is lowered, and therefore, the N content is 0.0040 to 0.0200%. The preferable lower limit of the N content is 0.0060%. The preferred upper limit of the N content is 0.0150%.

O: 0.0024% or less Oxygen (O) is inevitably contained. O forms an Al-based composite oxide having a low melting point. If the O content exceeds 0.0024%, melt cracking is likely to occur due to the Al-based composite oxide. Therefore, the O content is 0.0024% or less. The preferred upper limit of the O content is 0.0020%, more preferably 0.0017%.

Ca: 0.0005-0.0100%
Calcium (Ca) controls the morphology of MnS, suppresses the occurrence of melt cracks even when the S content is in the above range, and suppresses the decrease in hot forging property. If the Ca content is less than 0.0005%, these effects cannot be obtained. On the other hand, if the Ca content exceeds 0.0100%, even if Al is not substantially contained, it _{binds to Al in the refractory-derived component (for example, Al 2} O ₃ ) and has a low melting point Al system. It can form a composite oxide and cause melt cracking. Therefore, the Ca content is 0.0005 to 0.0100%. The preferable lower limit of the Ca content is 0.0010%. The preferable upper limit of the Ca content is 0.0080%.

[About fn1]
Further, in the above chemical composition, fn1 defined by the formula (1) is 50.0 or less.
fn1 = 80C ² + 55C + 13Si + 4.8Mn + 30P + 30S + 1.5Cr (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

fn1 is an index of melt cracking due to the melting point of steel. C, Si, Mn, P, S and Cr all lower the melting point of steel. If the melting point of steel is lowered, melt cracking is likely to occur during induction hardening heating. When fn1 is 50.0 or less, the decrease in the melting point of the steel is suppressed and the occurrence of melt cracks is suppressed. The preferred upper limit of fn1 is 49.4.

On the other hand, C, Si, Mn and Cr in fn1 enhance the hardenability of steel. Therefore, the preferable lower limit of fn1 for improving the hardenability of steel is 40.0.

[About fn2]
Further, in the above chemical composition, fn2 defined by the formula (2) is 0.80 to 1.00.
fn2 = C + (Si / 10) + (Mn / 5)-(5S / 7) + (5Cr / 22) + 1.65V (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (2).

fn2 is an index of the internal hardness of steel. C, Si, Mn, Cr and V increase the internal hardness of the steel material after hot forging. On the other hand, S lowers the internal hardness. If fn2 is less than 0.80, the internal hardness of the steel material is too low and the fatigue strength is lowered. On the other hand, if fn2 exceeds 1.00, the internal hardness is too high and the machinability is lowered. Therefore, fn2 is 0.80 to 1.00. The preferable lower limit of fn2 is 0.84. The preferred upper limit of fn2 is 0.98.

[About fn3]
Further, in the above chemical composition, fn3 defined by the formula (3) is 0.013 to 0.124.
fn3 = Al + 10.8Ca (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (3).

fn3 is an index of melt cracking caused by an Al-based composite oxide having a low melting point. As described above, in order to suppress the formation of a low melting point Al-based composite oxide that causes melt cracking, the upper limit of the Al content is set to 0.040% and the upper limit of the Ca content is set to 0.0100%. .. However, the Al content and the Ca content interact with each other in the formation of the Al-based composite oxide. Specifically, even if the Al content is 0.040% or less and the Ca content is 0.0100% or less, if fn3 exceeds 0.124, an Al-based composite oxide is formed and melted. Cracks occur. On the other hand, if fn3 is lower than 0.013, the deoxidizing power of Ca and Al decreases. Further, instead of the Al-based composite inclusions, a low melting point oxide of Si is remarkably produced. In this case, melt cracking is likely to occur. Therefore, fn3 is 0.013 to 0.124. The preferable lower limit of fn3 is 0.020. The preferred upper limit of fn3 is 0.100.

The balance of the chemical composition of the induction-hardened non-tempered steel according to the present embodiment consists of Fe and impurities. Here, the impurity means an impurity mixed in from ore, scrap, or a manufacturing environment as a raw material when the steel is industrially manufactured.

[About arbitrary elements]
The non-hardened steel for induction hardening of the present embodiment may further contain one or more selected from the group consisting of Ti and Nb instead of a part of Fe. These elements are optional elements and all form carbonitrides to increase the toughness of the steel.

Ti: 0-0.020%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti forms carbonitrides and suppresses coarsening of austenite grains during hot forging. Therefore, the pearlite structure of the steel material after hot forging becomes finer, and the toughness of the steel material is increased. However, if the Ti content exceeds 0.020%, the above effects are saturated and the production cost increases. Therefore, the Ti content is 0 to 0.020%. The preferred lower limit of the Ti content for more effectively increasing toughness is 0.005%, more preferably 0.008%. The preferred upper limit of the Ti content is 0.015%.

Nb: 0 to 0.020%
Niobium (Nb) is an optional element and may not be contained. When contained, Nb forms a carbonitride and suppresses coarsening of austenite crystal grains during hot forging. Therefore, the toughness of the steel material after hot forging is increased. However, if the Nb content exceeds 0.020%, the above effects are saturated and the production cost increases. Therefore, the Nb content is 0 to 0.020%. The preferred lower limit of the Nb content for more effectively increasing toughness is 0.005%, more preferably 0.008%. The preferred upper limit of the Nb content is 0.015%.

The non-hardened steel for induction hardening of the present embodiment may further contain Pb instead of a part of Fe.

Pb: 0 to 0.30%
Lead (Pb) is an optional element and may not be contained. When contained, Pb enhances machinability. However, if the Pb content exceeds 0.30%, the hot forging property is lowered. Therefore, the Pb content is 0 to 0.30%. The preferable lower limit of the Pb content for more effectively enhancing the machinability is 0.10%, more preferably 0.15%. The preferred upper limit of the Pb content is 0.27%.

The non-induction steel for induction hardening of the present embodiment may further contain one or more selected from the group consisting of Cu, Ni and Mo instead of a part of Fe. These elements are arbitrary elements, and all of them increase the fatigue strength of steel.

Cu: 0-0.20%
Copper (Cu) is an optional element and may not be contained. When contained, Cu increases the fatigue strength of steel. However, if the Cu content exceeds 0.20%, the hot forging property of the steel is lowered. Therefore, the Cu content is 0 to 0.20%. The preferable lower limit of the Cu content for more effectively increasing the fatigue strength is 0.05%, and the preferable upper limit of the Cu content is 0.17%.

Ni: 0 to 0.20%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni increases the fatigue strength of steel. However, if the Ni content exceeds 0.20%, the hot forging property is lowered. Therefore, the Ni content is 0 to 0.20%. The preferable lower limit of the Ni content for more effectively increasing the fatigue strength is 0.05%, and the preferable upper limit of the Ni content is 0.17%.

Mo: 0-0.20%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo increases the fatigue strength of steel. However, if the Mo content exceeds 0.20%, the hot forging property is lowered. Therefore, the Mo content is 0 to 0.20%. The preferable lower limit of the Mo content for more effectively increasing the fatigue strength is 0.05%, and the preferable upper limit of the Mo content is 0.17%.

[Production method]
An example of the method for producing the induction-hardened non-tempered steel of the present embodiment is as follows.

A molten steel having the above chemical composition is produced. The molten steel is made into slabs by the continuous casting method. The molten steel may be made into an ingot (steel ingot) by the ingot method. The slab or ingot may be hot-worked into a piece of steel or steel bar. Through the above steps, non-tempered steel for induction hardening is produced.

Non-healing steel materials for induction hardening (shards, ingots, steel pieces or steel bars) are hot forged to produce rough intermediates of machine structural parts (for example, crankshafts). Allow the manufactured intermediates to cool in the air. The intermediate product is machined into a predetermined shape. Induction hardening is performed on the intermediate product after cutting. Through the above steps, mechanical structural parts are manufactured.

In induction hardening, the heating temperature is adjusted according to the desired depth of the hardened layer. When the depth of the hardened layer is increased, the heating temperature becomes high and may exceed 1350 ° C. When manufacturing mechanical structural parts typified by a crankshaft using the non-microalloyed steel for induction hardening of the present embodiment, even if induction hardening is performed at a high temperature exceeding 1350 ° C., melt cracking will occur. Occurrence is suppressed. Further, in the mechanical structural parts after hot forging, excellent fatigue strength can be obtained and machinability is also excellent.

A plurality of induction hardened non-treated steels having various chemical compositions were produced. Using the produced steel, the presence or absence of melt cracking after induction hardening and the internal hardness of the steel after hot forging were evaluated.

[experimental method]
[Manufacturing of non-microalloyed steel for induction hardening]
A 70-ton converter and secondary refining were carried out to produce molten steel having the chemical compositions shown in Tables 1 and 2.

Using the produced molten steel, a slab (bloom) having a cross section of 300 mm × 400 mm was produced by a continuous casting method. The slab was block-rolled to produce a billet having a cross section of 180 mm × 180 mm. The billet was heated to 1250 ° C. and then hot-rolled to produce a steel bar having a diameter of 80 mm (non-tempered steel for induction hardening).

[Melting crack evaluation test]
From the R / 2 part of the manufactured steel bar (R means the radius of the steel bar), a test piece having a diameter of 8 mm and a length of 12 mm was produced by machining.

A mock test of induction hardening was carried out on the above test piece using a test device (trade name "Thermector") manufactured by Fuji Radio Industrial Co., Ltd. Specifically, the test piece was heated to 1380 ° C. at a heating rate of 25 ° C./sec and held at 1380 ° C. for 30 seconds. Then, the test piece was rapidly cooled using He gas.

The cross section (observation surface) of the test piece after quenching was mechanically polished and then corroded with Piclar reagent. The corroded observation surface was observed with a 400x optical microscope to confirm the presence or absence of melt cracking.

FIG. 2 is a microstructure photographic image (test number 4) in which melt cracking occurred, and FIG. 3 is a microstructure photographic image (normal structure: test number 28) in which melt cracking did not occur.

In the structure of the observation surface, when a region clearly corroded at a width of 5 μm or more is observed at the grain boundary (for example, reference numeral 10 in FIG. 2), it is determined that melt cracking has occurred (Tables 1 and Table). In the "melt crack" column in 2, it is indicated by "X"). On the other hand, as shown in FIG. 3, when a corroded region was not observed at the grain boundary, it was determined that no melt cracking occurred (indicated by "A" in the "melt cracking" column in Tables 1 and 2).

[Rockwell hardness test]
The produced steel bars were heat-treated to simulate cooling after hot forging. Specifically, the steel bar was heated to 1100 ° C. and held for 30 minutes. After that, the steel bar was allowed to cool in the atmosphere.

In the R / 2 part of the cross section of the steel bar after the heat treatment, the Rockwell C hardness at four points was measured according to JISZ2245 (2011), and the average value was obtained. Hereinafter, the obtained average value is referred to as HRC hardness.

It has been found that when the HRC hardness, which is the internal hardness, is 20 or more, sufficient fatigue strength can be obtained in the mechanical structural parts after hot forging. On the other hand, if the HRC hardness exceeds 28, the machinability is lowered. Therefore, the obtained HRC hardness was evaluated as shown in Table 3.

[Test results]
The test results are shown in Tables 1 and 2. Test numbers 1-3, 6-8, 11, 12, 15, 17-19, 21, 22, 25, 26, 28, 29, 31-34, 36-38, 40-43, 47, 48, 50- For 53, 56, 57, and 60-65, the chemical composition was appropriate, and fn1 to fn3 were also appropriate. Therefore, no melt cracking was observed. Further, the HRC hardness was in the range of 20 to 28, and it was expected that sufficient fatigue strength and machinability could be obtained.

On the other hand, the C content of Test No. 4 was too high. Therefore, melt cracking was observed. The C content of Test No. 5 was too low. Therefore, it was expected that the HRC hardness was less than 20, and sufficient fatigue strength could not be obtained.

The Si content of Test No. 9 was too high. Therefore, melt cracking occurred. Furthermore, it was expected that the HRC hardness would exceed 28 and sufficient machinability could not be obtained.

The Si content of test number 10 was too low. Therefore, it was expected that the HRC hardness was less than 20, and sufficient fatigue strength could not be obtained.

The Mn content of test number 13 was too high. Therefore, melt cracking occurred. Furthermore, it was expected that the HRC hardness would exceed 28 and sufficient machinability could not be obtained.

The Mn content of test number 14 was too low. Therefore, it was expected that the HRC hardness was less than 20, and sufficient fatigue strength could not be obtained.

The P content of test number 16 and the S content of test number 20 were too high. Therefore, melt cracking occurred in all the test numbers.

The Cr content of test number 23 was too high. Therefore, melt cracking occurred. Furthermore, it was expected that the HRC hardness would exceed 28 and sufficient machinability could not be obtained.

The Cr content of test number 24 was too low. Therefore, it was expected that the HRC hardness was less than 20, and sufficient fatigue strength could not be obtained.

The Al content of Test No. 27 and the O content of Test No. 39 were too high. Therefore, melt cracking occurred in all the test numbers.

The V content of test number 30 and the N content of test number 35 were too low. Therefore, in any of the test numbers, the HRC hardness was less than 20, and it was expected that sufficient fatigue strength could not be obtained.

In test number 44, Ca was not contained, and in test number 46, the Ca content was too low. Therefore, melt cracking occurred. On the other hand, the Ca content of test number 45 was too high. Therefore, melt cracking occurred.

In test number 49, fn1 was too high. Therefore, melt cracking occurred. Furthermore, it was expected that the HRC hardness would exceed 28 and sufficient machinability could not be obtained.

In test number 54, fn2 was too high. Therefore, it was expected that the HRC hardness would exceed 28 and sufficient machinability could not be obtained. On the other hand, in test number 55, fn2 was too low. Therefore, it was expected that the HRC hardness was less than 20, and sufficient fatigue strength could not be obtained.

In test number 58, fn3 was too high. Therefore, melt cracking occurred. On the other hand, in test number 59, fn3 was too low. Therefore, melt cracking occurred.

Test numbers 66-78 did not contain Ca. Therefore, melt cracking occurred in all of these test numbers.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

The non-microalloyed steel for induction hardening of the present invention is used as a mechanical structural part such as a crankshaft by hot forging, and even if induction hardening is performed, the occurrence of melt cracking is suppressed. The non-induction steel for induction hardening of the present invention further has an internal hardness that allows high fatigue strength and machinability to be obtained even when manufactured in the above-mentioned mechanical structural parts. Therefore, the non-microalloyed steel for induction hardening of the present invention can be widely applied to applications in which the above effects are exhibited, and is particularly suitable as a steel for mechanical structural parts where induction hardening is carried out.

1 Ferret R part 2 Crankshaft edge part 10 Melt crack

Claims (4)

By mass%
C: 0.35-0.44%,
Si: Over 0.30 to 0.70%,
Mn: 1.00 to 1.50%,
P: 0.030% or less,
S: 0.010 to 0.095%,
Cr: 0.10 to 0.30%,
Al: 0-0.040%,
V: 0.1000 to 0.200%,
N: 0.0040-0.0200%,
O: 0.0024% or less,
Ca: 0.0012 to 0.0100%,
Ti: 0-0.020%,
Nb: 0 to 0.020%,
Pb: 0 to 0.30%,
Cu: 0-0.20%,
Ni: 0-0.20%,
Mo: Contains 0 to 0.20%,
Fn1 defined by the equation (1) is 50.0 or less, and
Fn2 defined by the formula (2) is 0.80 to 1.00, and is
The fn3 defined by the formula (3) is 0.013 to 0.124, and is
The balance is non-tempered steel for induction hardening, which consists of Fe and impurities.
fn1 = 80C ² + 55C + 13Si + 4.8Mn + 30P + 30S + 1.5Cr (1)
fn2 = C + (Si / 10) + (Mn / 5)-(5S / 7) + (5Cr / 22) + 1.65V (2)
fn3 = Al + 10.8Ca (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formulas (1) to (3). The non-refined steel for induction hardening according to claim 1.
Ti: 0.005 to 0.020% and
Nb: Non-tempered steel for induction hardening containing at least one selected from the group consisting of 0.005 to 0.020%. The non-microalloyed steel for induction hardening according to claim 1 or 2.
Non-tempered steel for induction hardening containing Pb: 0.10 to 0.30%. The non-refined steel for induction hardening according to any one of claims 1 to 3.
Cu: 0.05 to 0.20%,
Ni: 0.05 to 0.20% and
Mo: Non-tempered steel for induction hardening containing one or more selected from the group consisting of 0.05 to 0.20%.