Steels for cold forging and process for producing the same

TECHNICAL FIELD

This invention relates to a structural steel that is subjected to cold forging, either as-rolled or after rolling and annealing, and a method of producing such a steel.

BACKGROUND ART

Steels used for structural members are passed through various forming processes in order to impart required properties to them. Radio-frequency hardening, for hardening the surface layer, is one of these processes. Since such structural members are required to have only a high surface layer hardness, in most cases, an increase in the number of processes results in an increase of the cost of production, and this has been one of the problems in the past. Since as-rolled materials of the conventional structural steels have a low cooling rate, they have a ferrite-pearlite structure in most cases. However, their surface layer hardness is low and never reaches the level achievable by radio-frequency hardening. More often than not, the surface layer hardness is lower than the internal hardness due to the influence of decarburization, and so forth. Though ordinary members need not always have a maximum hardness corresponding to the C (carbon) content brought forth by radio-frequency hardening, it is undeniable that some of the members are required to have a hardness higher than that of the annealed materials. Therefore, the provision of steels having, as-rolled, a higher surface layer hardness than the internal hardness has been another problem.

When complicated shapes are required, the steel materials are passed through forging and cutting processes. Because hot forging needs heating and has a low forming accuracy, cold forging, having higher forming accuracy, has been preferred. Nonetheless, conventional as-rolled materials are not suitable for cold forging because the hardness is too high. Ordinary steels for cold forging are generally softened by spheroidizing cementite. The annealing time is extremely long and is as much as about 20 hours.

The prior art references such as Japanese Unexamined Patent Publication (Kokai) No. 3-140411 describe that cold formability and cuttability of even a steel having a carbon content equivalent to the level of carbon steels for cold forging can be improved by graphitizing carbon and converting the steel structure to a ferrite-graphite dual phase. However, annealing for a long time is necessary to achieve such a structure, and the problems of production efficiency and production cost are left unsolved. In other words, the problem of shortening the annealing time is yet to be solved.

In order to reduce the graphitization annealing time, a technique has been suggested which adds B and uses BN as precipitation nuclei. However, when such a specific precipitate is used, a temperature-retaining process, in the BN precipitation temperature range, is necessary before annealing is conducted, and an additional annealing process becomes necessary. If this heat-treatment is conducted conjointly by rolling or hot forging, temperature control must be conducted extremely strictly until annealing, and this is virtually impossible.

In other words, the precipitation temperature of BN is believed to be from about 850 to about 900° C., but rolling and hot forging are actually carried out at a temperature higher than 1,000° C. in many cases. Therefore, in order to use such a graphite-containing steel for cold forging, rolling and hot forging, as prior processes, must be conducted at a temperature below 1,000° C. Hot forming at such a temperature lowers the service life of tools such as rolls and punches. The increase of the number of limitations on the processes leads to the drop of production efficiency, and must be therefore avoided to restrict the increase of the production cost. From the aspects of steel making and hot forging, as a prior process to cold forging, steel materials that do not need strict temperature control and can be annealed and softened within a short time have been required.

Japanese Unexamined Patent Publication (Kokai) No. 2-111842 teaches shortening the annealing time by restricting the graphite content within a short time. However, this technology does not provide a fundamental solution because cold forgeability and cuttability are deteriorated in proportion to the amount of cementite that remains in the steel materials as a result of suppression of the graphite content.

As described above, the conventional as-rolled materials are not entirely satisfactory because their surface layer hardness is not sufficient when they are used as such, but it is too high when they are subjected to cold forging and cutting. From the viewpoint of production, on the other hand, there is the fundamental problem that the steels should preferably be produced collectively by reducing the number of their kinds in order to reduce the cost of production. Therefore, it has been desired that the as-rolled materials have a sufficient surface hardness, the annealing time can be shortened when the as-rolled materials are subjected to cold forging, and they can exhibit excellent cold forgeability after annealing.

When strength is also further required, it may be possible, in principle, to add those elements which do not impede graphitization for improving hardenability but can improve hardenability. Particularly when the surface hardness by radio-frequency hardening is necessary, hardenability becomes more different problem because of increase the thickness of the hardened layer. However, since ordinary hardenability improving elements such as Cr, Mn, Mo, etc, hinder graphitization, the amounts of addition are limited. When the graphitization annealing time is shortened by forming BN, B cannot be used as the hardenability improving element, and the hardening depth cannot be sufficiently secured, either.

Under the above-described condition, a steel which makes it possible to reduce the annealing time, and is excellent in cold forgeability after annealing, hardenability and cuttability, has been required.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a steel that has, as-rolled, excellent surface hardness, by regulating the chemical components of the steel and its microstructure, and can impart excellent cold forgeability within an extremely short softening/annealing time before cold forging and cutting, and to provide a method of producing the steel.

It is another object of the present invention to provide a steel, for cold forging after annealing, that can shorten the annealing time, by regulating the chemical components of the steel, is excellent in cold formability and cuttability after annealing and has excellent strength and toughness after hardening and tempering.

To accomplish these objects, the present invention provides the following inventions.

(1) The first invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, that contains, in terms of wt %, C: 0.1 to 1.0%, Si: 0.1 to 2.0%, Mn: 0.01 to 1.50%, P: not greater than 0.100%, S: not greater than 0.500%, sol. N: being limited to not greater than 0.005%, and the balance consisting of Fe and unavoidable impurities, wherein a pearlite ratio in the steel structure (pearlite occupying area ratio in microscope plate/microscope plate area) is not greater than 120×(C %) % (with the maximum being not greater than 100%), and the outermost surface layer hardness is at least 450×(C %)+90 in terms of the Vickers hardness HV.

(2) The second invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains at least one of Cr: 0.01 to 0.70% and Mo: 0.05 to 0.50%, in addition to the chemical components of the first invention (1) described above, wherein a pearlite ratio in the steel structure (pearlite occupying area ratio in microscope plate/microscope plate area) is not greater than 120×(C %) %, and the outermost surface layer hardness is at least 450×(C %)+90 in terms of the Vickers hardness HV.

(3) The third invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains at least one of Ti: 0.01 to 0.20%, V: 0.05 to 0.50%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.30% and Al: 0.001 to 0.050% in addition to the chemical components of the paragraph (1) or (2) described above, wherein a pearlite ratio in the steel structure (pearlite occupying area ratio on microscope plate/microscope plate area) is not grater than 120×(C %) %, and the outermost surface layer hardness is at least 450×(C %)+90 in terms of the Vickers hardness Hv.

(4) The fourth invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains B: 0.0001 to 0.0060% in addition to the chemical components of any of the paragraphs (1) to (3), wherein a pearlite ratio in the steel structure (pearlite occupying area ratio on microscope plate/microscope plate area) is not greater than 120×(C %) %, and the outermost layer surface hardness is at least 450×(C%)+90 in terms of the Vickers hardness Hv.

(5) The fifth invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains Pb: 0.01 to 0.30%, Ca: 0.0001 to 0.0020%, Te: 0.001 to 0.100%, Se: 0.01 to 0.50% and Bi: 0.01 to 0.50% in addition to the chemical components of any of the paragraphs (1) to (4), wherein a pearlite ratio in the steel structure (pearlite occupying area ratio in microscope plate/microscope plate area) is not greater than 120×(C %) %, and the outermost layer hardness is at least 450×(C %)+90 in terms of the Vickers hardness Hv.

(6) The sixth invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains Mg: 0.0005 to 0.0200% in addition to said chemical components according to any of claims

1

through

6

, wherein a pearlite ratio in the steel structure (pearlite occupying area ratio on microscope plate/microscope plate area) is not greater than 120×(C %) %, and the outermost surface layer hardness is at least 450×(C %)+90 in terms of the Vickers hardness HV.

(7) The seventh invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains, in terms of wt %, C: 0.1 to 1.0%, Si: 0.1 to 2.0%, Mn: 0.01 to 1.50%, P: not greater than 0.100%, S: not greater than 0.500, sol. N: being limited to not greater than 0.005% and the balance consisting of Fe and unavoidable impurities, and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10×(C %)

⅓

μm and the maximum crystal grain diameter is not greater than 20 μm.

(8) The eighth invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains at least one of Cr: 0.01 to 0.70% and Mo: 0.05 to 0.50%, and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10×(C %)

⅓

μm , and a maximum crystal grain diameter is not greater than 20 μm.

(9) The ninth invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains at least one of Ti: 0.01 to 0.20%, V: 0.05 to 0.50%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.30% and Al: 0.001 to 0.050% in addition to the chemical components described in the paragraph (7) or (8), and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10×(C %)

⅓

μm, and a maximum crystal grain diameter is not greater than 20 μm.

(10) The tenth invention provides a steel for cold forging, which contains B: 0.0001 to 0.0060% in addition to the chemical components of any of the paragraphs (7) to (9), and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10×(C %)

⅓

μm and a maximum crystal grain diameter is not greater than 20 μm.

(11) The eleventh invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains Pb: 0.01 to 0.30%, Ca: 0.0001 to 0.0020%, Te: 0.001 to 0.100%, Se: 0.01 to 0.50% and Bi: 0.01 to 0.50% in addition to the chemical components of any of the paragraphs (7) to (10), and has a structure wherein a ratio of a graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of graphite is not greater than 10×(C %)

⅓

μm, and a maximum crystal grain diameter is not greater than 20 μm.

(12) The twelfth invention provides a steel for cold forging, excellent in cold formability, cuttability and radio-frequency hardenability, which contains Mg: 0.0005 to 0.0200% in addition to the chemical components of any of the paragraphs (7) to (11), and has a structure wherein a ratio of graphite amount to the carbon content in the steel (graphitization ratio: amount of carbon precipitated as graphite/carbon content in the steel) exceeds 20%, a mean crystal grain diameter of the graphite is not greater than 10×(C %)

⅓

μm, and a maximum crystal grain diameter is not greater than 20 μm.

(13) A method of producing a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which comprises the steps of rolling the steel having the chemical components of any of the paragraphs (1) to (6) described above in an austenite temperature zone or in an austenite-ferrite dual phase zone so that a pearlite ratio in the steel structure (pearlite occupying area ratio in microscope plate/microscope plate area) is not greater than 120×(C %) % and the outermost surface layer hardness is at least 450×(C %)+90 in terms of the Vickers hardness Hv; rapidly cooling the steel immediately after the finish of rolling at a rate of at least 1° C./s; and controlling a recuperative temperature to 650° C. or below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is an explanatory view showing the outline of a pearlite ratio measuring method.

FIG. 2

is a graph showing the relation between a pearlite area ratio and an annealing time until softening in an embodiment of a 0.20% class.

FIG. 3

is a graph showing the relation between the pearlite area ratio and the annealing time until softening in an embodiment of a 0.35% class.

FIG. 4

is a graph showing the relation between the pearlite area ratio and the annealing time until softening in an embodiment of a 0.45% class.

FIG. 5

is a graph showing the relation between the pearlite area ratio and the annealing time until softening in an embodiment of 0.55% class.

FIG. 6

is a graph showing the relation between a recuperative temperature and a surface layer hardness.

FIG. 7

is a graph showing the relation between the recuperative temperature and the pearlite area ratio.

FIG. 8

is a graph showing the relation between solid solution nitrogen and the annealing time until softening.

FIG. 9

is a graph showing the relation between a maximum crystal grain diameter and a hardening time by radio-frequency heating in an embodiment of a 0.55% C class.

FIG. 10

is a graph showing the relation between a mean crystal grain diameter and the hardening time by radio-frequency heating in an embodiment of the 0.55 C class.

FIG. 11

is a graph showing the relation between the mean crystal grain diameter and the hardening time by radio-frequency heating in an embodiment of the 0.35% C class.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be explained in detail.

Initially, the steel structure used for the steel for cold forging according to the present invention, and its contents, will be explained.

At least 0.1% of C (carbon) must be contained in order to secure strength as components after hardening and tempering. The upper limit is set to 1.0% to prevent firing cracking.

Si (silicon) has the function of promoting graphitization by increasing carbon activity in the steel. Its lower limit is preferably at least 0.1% from the aspect of graphitization. If the Si content exceeds 2.0%, problems such as the increase of ferrite hardness and the loss of toughness of the steel become remarkable. Therefore, the upper limit is 2.0%. Si can be used as the element that regulates the graphitization ratio. The smaller its content, the smaller becomes the graphitization ratio after annealing. When the graphitization ratio is lowered by decreasing the Si content, the hardness of the ferrite phase drops. Therefore, the hardness of the steel material does not increase within the range described above, and cold forgeability is not lowered.

Mn (manganese) must be added in the total amount of the amount required for fixing and dispersing S in the steel as MnS and the amount required for securing the strength after hardening by causing Mn to undergo solid solution in the matrix. Its lower limit value is 0.01%. The hardness of the base becomes higher with the increase of the Mn content, and cold formability drops. Mn is also a graphitization-impeding element. When the amount of addition increases, the annealing time is likely to become longer. Therefore, the upper limit is set to 1.50%.

P (phosphorus) increases the hardness of the base metal in the steel and lowers cold formability. Therefore, its upper limit must be 0.1000%.

S (sulfur) exists as MnS inclusions as it combines with Mn. From the aspect of cold formability, its upper limit must be set to 0.500%.

Solid solution nitrogen, that does not exist as nitrides, dissolves in cementite and impedes decomposition of cementite. Therefore, it is a graphitization-impeding element. Therefore, the present invention stipulates N as sol. N. If the sol. N content exceeds 0.005%, the annealing time necessary for graphitization becomes extremely long. Therefore, the upper limit of sol. N is 0.005%. This is because sol. N hinders the diffusion of C, retards graphitization and enhances the ferrite hardness.

Cr (chromium) is a hardenability-improving element and at the same time, a graphitization-impeding element. Therefore, when the improvement of hardenability is required, at least 0.01% of Cr must be added. When added in a large amount, Cr impedes graphitization and prolongs the annealing time. Therefore, the upper limit is 0.70%.

Mo (molybdenum) is the element that increases the strength after hardening, but is likely to form carbides and impedes graphitization. Therefore, the upper limit is set to 0.50% at which the graphitization-impeding effect becomes remarkable, and the Mo content is set to the addition amount that does not greatly impede the formation of the graphite nuclei. In comparison with other hardenability-improving elements, however, the degree of impeding of graphitization by Mo is smaller. For this reason, the Mo addition amount may be increased so as to improve hardenability within the range stipulated above.

Ti (titanium) forms TiN in the steel and reduces the γ grain diameter. Graphite is likely to precipitate at the γ grain boundary and precipitates, or in other words, “non-uniform portions” of the lattice, and carbonitrides of Ti bear the role of the precipitation nuclei of graphite and the role of creation of the graphite precipitation nuclei due to the reduction of the γ grain diameters to fine diameters. Furthermore, Ti fixes N as the nitrides and thus reduces sol. N. If the Ti content is less than 0.01%, its effect is small, and if the Ti content exceeds 0.20%, the effect gets into saturation and at the same time, a large amount of TiN is precipitated and spoil the mechanical properties.

V (vanadium) forms carbonitrides, and shortens the graphitization annealing time from both the aspect of fining of the γ grains and of the precipitation nuclei. It reduces sol. N at the time of the formation of carbonitrides. If the V content is less than 0.05%, its effect is small, and if the V content exceeds 0.50%, the effect gets into saturation and at the same time, large amounts of non-dissolved carbides remain with the result being deterioration of the mechanical properties.

Nb (niobium) forms carbonitrides and shortens the graphitization annealing time from both the aspect of fining of the γ grain diameters to fine diameters and of the precipitation nuclei. It also lowers sol. N at the time of the formation of the nitrides. If the Nb content is less than 0.01%, the effect is small and if it exceeds 0.10%, the effect gets into saturation and at the same time, large amounts of non-dissolved carbides remain with the result being deterioration of the mechanical properties.

Mo (molybdenum) increases the strength after hardening. However, it is the element that is likely to form carbides, lowers carbon activity, and impedes graphitization. Therefore, the upper limit is set to 0.5% at which the graphitization-impeding effect becomes remarkable, and the addition amount is limited to the level at which the graphite nucleus formation is not greatly impeded. Since the degree of the graphitization-impeding effect of Mo is lower than that of other hardenability-improving elements, however, the Mo addition amount may be increased so as to improve hardenability within the range stipulated above.

Zr (zirconium) forms oxides, nitrides, carbides and sulfides, which shorten the graphitization annealing time as the precipitation nuclei. Zr reduces sol. N at the time of the formation of the nitrides. Furthermore, Zr spheroidizes the shapes of the sulfides such as MnS, and can mitigate rolling anisotropy as one of the mechanical properties. Furthermore, Zr can improve hardenability. If the Zr content is less than 0.01%, the effect is small and if it exceeds 0.30%, the effect gets into saturation and at the same time, large amounts of non-dissolved carbides remain with the result being deterioration of the mechanical properties.

At least 0.001% of Al (aluminum) is necessary for deoxidizing the steel and for preventing surface scratches during rolling. The deoxidizing effect gets into saturation when the Al content exceeds 0.050% and the amounts of aluminum type inclusions increase. Therefore, the upper limit is 0.050%. When precipitated as AlN, aluminum plays the role of the precipitation nuclei of graphite and the role of creating the graphite precipitation nuclei due to fining of the y grain diameters to fine diameters. Furthermore, because Al fixes N as the nitrides, it reduces sol. N.

B (boron) reacts with N and precipitates as BN in the austenite crystal grain boundary. It is therefore useful for reducing sol. N. BN has a hexagonal system as its crystal structure in the same way as graphite, and functions as the precipitation nuclei of graphite. Furthermore, sol. B is the element that improves hardenability, and is preferably added when hardenability is required. Its lower limit value must be 0.0001%. The effects of precipitating BN and improving hardenability get into saturation when the B content exceeds 0.0060%. Therefore, the upper limit is 0.0060%.

Pb (lead) is a cuttability-improving element, and at least 0.01% is necessary when cuttability is required. If the Pb content exceeds 0.30%, Pb impedes graphitization and invites problems during production such as rolling scratches. Therefore, the upper limit is 0.30%.

Ca (calcium) is effective when mitigation of rolling anisotropy by spheroidizing of MnS and the improvement of cuttability are required. If the Ca content is less than 0.0001%, the effect is small, and if it exceeds 0.0020%, the precipitates will deteriorate the mechanical properties. Therefore, the upper limit is 0.0020%.

Te (tellurium) is a cuttability-improving element and helps mitigate rolling anisotropy by spheroidizing of MnS. If the Te content is less than 0.001%, the effect is small and if it exceeds 0.100%, problems such as impediment of graphitizing and rolling scratches occur. Therefore, the upper limit is 0.100%.

Se (selenium) is effective for improving cuttability. If the Se content is less than 0.01%, the effect is small, and if it exceeds 0.50%, the effect gets into saturation. Therefore, the upper limit is 0.50%.

Bi (bismuth) is effective for improving cuttability. If the Bi content is less than 0.01%, the effect is small, and if it exceeds 0.50%, the effect gets into saturation. Therefore, the upper limit is 0.50%.

Mg (manganese) is an element that forms oxides such as MgO and also forms sulfides. MgS is co-present with MnS in many cases and such oxides and sulfides function as the graphite precipitation nuclei and are effective for finely dispersing graphite and for shortening the annealing time. If the Mg content is less than 0.0005%, the effect cannot be observed and if it exceeds 0.0200%, Mg forms large amounts of oxides and lowers the strength of the steel. Therefore, the Mg content is limited to the range of 0.0005 to 0.0200%.

Next, the as-rolled steel structure of the steel for cold forging according to the present invention will be explained.

The hardness of the surface layer of the steel for cold forging can be increased by rapidly cooling the steel from a temperature above a transformation point, but is affected by the C content. When the surface layer hardness is too low, the steel cannot be used for the application that requires the surface layer hardness. For example, those steels for which wear resistance is required must have hardness at least higher than the strength of ordinary annealed steel materials. The present invention can provide a steel having hardness of at least 450×(C %)+90 in terms of the Vickers hardness Hv in accordance with the C content.

Next, the reason why the pearlite ratio in the steel structure, that is, (pearlite occupying area ratio in microscope plate/microscope plate), is limited to not greater than 120×(C %) % (with the proviso that the value is not greater than 100%; and hereinafter the same) will be explained. When carbon in the steel is graphitized in the component system of the present invention, cementite is generally formed if the steel is cooled from the austenite region at an atmospheric cooling rate or a rate higher than the former. In order to impart excellent cold formability after annealing, however, carbon (C) must be graphitized by annealing. The graphitization process by annealing is believed to comprise decomposition of cementite→diffusion of C→formation and growth of graphite nuclei. From the viewpoint of the decomposition of cementite, a long time is necessary for the decomposition of cementite if the size of cementite is great and it is stable energy-wise, that is, if C forms pearlite on the lamella. In consequence, the annealing time cannot be shortened.

From the viewpoint of the growth of graphite, graphite at positions having a small diffusion distance for C are likely to be formed and to grow. In other words, graphite is likely to be formed near the positions of previous pearlite. This means that the graphite so formed is coarse and is non-uniformly dispersed. The deformation quantity till breakage after annealing is decreased, decomposition of graphite by radio-frequency hardening and diffusion of C are time-consuming, and hardening properties by radio-frequency hardening are lowered. In this way, in the steel according to the present invention, the formation of pearlite is restricted as much as possible so that the annealing time can be shortened and excellent deformation properties can be imparted after annealing.

Next, the outline of the method of measuring the pearlite ratio is shown in FIG.

1

. The calculation method of the pearlite ratio by the pearlite ratio measuring method is made in accordance with the following equation.

(P %) = \sum_{i = 1}^{n} {(Pi %) \cdot 2 π w \cdot ri}

Here,

ri=(i−1)·w+w/2, w =R/n

(P%)=pearlite ratio,

w: measurement representative width,

n: number of splitting

(Pi%): pearlite proportion at measurement position,

ri: measurement representative radius,

i: argument at the time of splitting (I=1, 2, . . . , n) from inside),

R: radius of steel bar or wire material.

This method is a simple method. The greater the number of splitting n, the smaller becomes w. Therefore, the pearlite ratio of the steel can be calculated as a correct area ratio.

The present invention stipulates n to ≧5. More concretely, a polished sample for microscope inspection, which is etched in a sectional direction by a nital reagent, is inspected in a 1 mm pitch from the surface layer to the center through a 1,000× optical microscope (n=10 in a 20 mm wire material). The pearlite area ratio inside the visual field is measured by an image processor, and the pearlite area occupying ratio inside the section is calculated using the area ratio as a representative value w of a 1 mm width in the radial direction of the steel bar or the wire material.

In this case, the samples in which the lamella structure can be observed by etching by the nital reagent are defined as pearlite. When this area ratio exceeds 120×(C %) %, the annealing time is extremely extended. The influences on the annealing time vary with the C content of the raw material. However, if the C content is great and the pearlite area occupying ratio is greater than 120×(C %) %, the material cannot be practically used from the aspect of the production cost. Therefore, the upper limit of the pearlite area ratio is limited to 120×(C %) %. However, this value does not exceed 100%.

FIGS. 2

to

5

show the relation between the pearlite area ratio before annealing and the annealing time when the C content is different, respectively. The steel is softened more easily when the C content is smaller, but the annealing time is extremely prolonged outside the range of the present invention, as can be seen from these graphs.

Next, the steel structure of the steel for cold forging according to the present invention, after it is hardened or annealed, will be explained.

The majority of C in the steel exists as cementite or graphite. Graphite can easily undergo deformation because it has cleavages. If the matrix is soft, cold forgeability is excellent. When the steel is cut, cuttability can be improved by the functions of both an internal lubricant and a breaking starting point. If the graphite content is smaller than 20%, the steel cannot exhibit sufficient deformation/lubricating functions. Therefore, the graphite content must exceed 20%. When deformation properties are preferentially required, the graphitization is increased. In order to secure excellent radio-frequency hardenability, on the other hand, it is effective to intentionally leave a part of C without being graphitized and to leave it as cementite.

Furthermore, the present invention stipulates that the mean crystal grain diameter of graphite is not greater than 10×(C%)

⅓

μm and the maximum grain diameter is not greater than 20 μm, in consideration of radio-frequency hardenability. In other words, when radio-frequency hardening is conducted, the hardening properties are governed by decomposition/diffusion of C in graphite. In this instance, if the graphite grain diameter is great, a large quantity of energy and much time are necessary for the decomposition/diffusion, and a stable hardened layer cannot be obtained easily by radio-frequency hardening. In order to stably obtain the hardened layer corresponding to the C content contained in the steel by radio-frequency hardening, the process of which can be finished within a short time, the mean grain diameter of graphite must be not greater than 10×(C %)

⅓

μm. If the mean grain diameter exceeds this limit, the amount of non-dissolved graphite is great even after radio-frequency hardening, or the amount of a mixed structure of a layer containing C in the diffusion process and ferrite that does not yet contain diffused C becomes great. As a result, not only hardening becomes difficult, but a stabilized hardened layer cannot be obtained.

FIGS. 10 and 11

show the relation between the mean grain diameter of graphite and the hardening time by radio-frequency hardening, and

FIG. 9

shows the relation between the maximum grain diameter of graphite and the hardening time by radio-frequency hardening.

Next, the production method when the steel for cold forging according to the present invention is used as-rolled will be explained.

After the steel having the steel composition described above is rolled in the austenite temperature range, the formation quantity of pearlite will become great if the cooling rate is low, and the annealing time till softening gets prolonged. Because the surface layer hardness is not sufficient, either, the steel is so soft that it cannot be used directly as such and is too hard for cold forging. To solve these problems, the steel is preferably cooled rapidly. If the cooling rate of the surface layer from the end of rolling to 500° C. is at least 1° C./s, the hardness at the surface layer can be increased in comparison with the hardness of the inside that is gradually cooled. In order to keep the pearlite area ratio on the steel section at 120×(C %) % or below, too, cooling must be carried out at a cooling rate of at least 1° C./s. The austenite amount can be decreased by once cooling the steel, heating it again to the austenitization temperature, and then cooling it by water. However, on-line treatment is more preferred from the aspects of the production cost and the production process.

In connection with the internal structure of the steel, the main object of the present invention is not to increase the hardness by rapid cooling as in the case of ordinary hardening but is to prevent the formation of pearlite so that decomposition easily develops during annealing. For this reason, the cooling capacity need not particularly be increased. In the practical production process of the steel materials, products having diameters of 5 to 150 mm are shipped in most cases, and the present invention may be directed to restrict the formation of pearlite in such products. In other words, the steel structure need not particularly comprise the martensite structure, and even the structure having the bainite structure can shorten the annealing time for softening much more than the steels having the ferrite and pearlite structures. Concrete means pass the steel material immediately after rolling through a cooling apparatus such as a cooling trough or a water tank that is installed at the rearmost part of the rolling line.

In the on-line process, the steel material is passed through the cooling means and is then cooled in the open atmosphere. It is hereby important that even when the surface layer is once cooled, it is heated recuperatively by the heat inside the steel material. It is necessary to limit this recuperative temperature to 650° C. or below.

If the recuperative temperature is higher than 650° C., the surface layer hardness drops, and pearlite is formed at a part of the structure during cooling of the steel material in the open atmosphere. Therefore, it becomes difficult to limit the pearlite amount to 120×(C %) %. The cooling rate and the recuperative properties are greatly affected by the diameters of the rods and the wires that are rolled. Cooling means is not limited to water cooling, and any means capable of achieving the cooling rate of at least 1° C./sec and the recuperative temperature of not higher than 650° C. may be employed, such as oil cooling, air cooling, and so forth.

As described above, the steel material is cooled immediately after rolling by the cooling means mounted to the rolling line, and the recuperative temperature is limited to 650° C. or below. In this way, the surface layer hardness can be increased and the pearlite area occupying ratio can be limited to 120×(C %) % or below.

FIG. 6

shows the relation between the recuperative temperature and the surface layer hardness. As shown in

FIG. 6

, the surface layer hardness cannot be secured when the recuperative heat becomes high.

FIG. 7

shows the relation between the recuperative temperature and the pearlite area ratio. It can be seen from

FIG. 7

that the pearlite area ratio increases when the recuperative temperature becomes high. It can be thus appreciated from

FIGS. 6 and 7

that restriction of the recuperative temperature after rapid cooling is of importance.

Next, the annealing condition when the steel for cold forging, that is produced in accordance with the present invention and is used for cold forming after annealing, will be explained.

In order to obtain graphite in the amount stipulated by the present invention for using the steel for cold forming, annealing is further necessary. Since graphite is a stable phase of the steels in Fe—C type steels, the steels may be kept at a temperature lower than the transformation temperature A, for a long time. However, since it is practically necessary to precipitate graphite within a limited time, the steels are preferably kept at a temperature within the range of 600 to 710° C. at which graphite precipitates more quickly. In this case, graphitization can be completed within 1 to 50 hours.

When such a condition is employed, the structure, in which the existence ratio of C as graphite in the steel exceeds 20%, the mean grain diameter of graphite is not greater than 10×(C %)

⅓

μm and the maximum grain diameter is not greater than 20 μm, as stipulated in the present invention, can be acquired.

EXAMPLES

Example 1

Steels having the chemical components shown in Tables 1 to 8 were melted. In this example, the steels were rolled into a diameter of 50 mm or 20 mm in the austenite temperature zone and were immediately cooled with water. The rolling temperatures were within the range of 800 to 1,100° C. falling within the austenite temperature zone. Water cooling was conducted using a cooling trough installed at the rearmost part of the rolling line. Some of test specimens inclusive of Comparative Examples were rolled to a diameter of 500 mm or 20 mm at temperatures higher than 1,200° C. and were then cooled by air.

A specimen for optical microscope study was collected from each test steel in the sectional direction and, after being polished into a mirror surface, each specimen was etched using nital. Pearlite was isolated from other structures at a magnification of 1,000×, and the pearlite area ratio was quantitatively determined by an image processor. In this case, the number of visual fields, as the object, was 50.

Such heat-treated materials were annealed at 680° C. To determine the hardness, the hardness was measured every four hours up to the annealing time of 16 hours, every 8 hours up to the annealing time of 48 hours and every 24 hours after the annealing time of longer than 48 hours. The Vickers hardness was determined by the annealing time at which the hardness dropped below HV: 130. As to the temperature, the surface temperatures of the steel materials were measured by a radiation pyrometer. The cooling rate was obtained by dividing the temperature difference between the temperature immediately before cooling and the temperature after recuperation, by the time required for recuperation.

Tables 1 to 6 illustrate examples of the present invention (Nos. 1 to 42) and Tables 7 and 8 show Comparative Examples (Nos. 43 to 62). All of the examples of the present invention had a high surface hardness, and the softening annealing time was short, too. In Comparative Examples 43 to 54, however, the annealing time for softening was prolonged when the sol. N amount was outside the range of the present invention. In Comparative Examples 55 to 59, the pearlite fraction was great because the cooling rate was insufficient, and the annealing time was long. In Comparative Examples 60 to 62, the recuperative temperature was high and the annealing time was long, too. It could be appreciated that the surface layer hardness was insufficient when the cooling rate and the recuperative temperature were outside the respective ranges stipulated by the present invention.

TABLE 1

chemical components

No.

section

C

Si

Mn

P

S

sol. N

Cr

Ti

V

Nb

Zr

Mo

1

Example of

0.51

1.23

0.32

0.023

0.017

0.0020

this invention

2

Example of

0.54

1.87

0.82

0.023

0.017

0.0021

this invention

3

Example of

0.56

1.43

1.21

0.008

0.008

0.0019

0.021

this invention

4

Example of

0.52

1.17

0.45

0.012

0.030

0.0042

0.20

this invention

5

Example of

0.51

1.23

0.32

0.023

0.017

0.0020

0.11

this invention

6

Example of

0.54

1.87

0.82

0.023

0.017

0.0021

0.022

this invention

7

Example of

0.56

1.43

1.21

0.008

0.008

0.0019

0.023

this invention

8

Example of

0.52

1.17

0.45

0.012

0.030

0.0042

0.035

this invention

9

Example of

0.51

1.16

0.45

0.027

0.028

0.0035

0.12

this invention

10

Example of

0.48

1.26

0.28

0.024

0.021

0.0019

0.11

this invention

11

Example of

0.54

1.82

0.54

0.024

0.021

0.0029

0.05

this invention

12

Example of

0.48

1.26

0.36

0.029

0.018

0.0037

this invention

13

Example of

0.51

1.29

0.38

0.021

0.015

0.0032

this invention

14

Example of

0.53

1.25

0.36

0.029

0.018

0.0037

this invention

TABLE 2

anneal-

Cool-

recuper-

pear-

anneal-

ing

ing

ative

surface

lite

ing

hard-

chemical components

rate

temp.

layer

ratio

time

ness

No.

section

Al

B

Pb

Ca

Te

Se

Bi

Mg

(° C./s)

(° C.)

(HV)

(%)

(hr)

(HV)

1

Example of

0.027

15

100

652

0

8

121

this invention

2

Example of

0.023

8

489

429

0

8

124

this invention

3

Example of

0.017

3

520

364

25

16

127

this invention

4

Example of

10

560

410

23

16

126

this invention

5

Example of

0.027

3

510

319

12

8

121

this invention

6

Example of

0.023

15

380

621

0

8

124

this invention

7

Example of

0.017

8

490

510

11

8

127

this invention

8

Example of

8

420

565

7

8

126

this invention

9

Example of

0.022

8

380

589

0

8

119

this invention

10

Example of

0.034

0.0021

5

510

405

10

16

126

this invention

11

Example of

0.029

0.13

5

410

425

32

16

127

this invention

12

Exampleof

0.027

0.0021

0.0013

10

530

385

15

16

124

this invention

13

Example of

0.021

0.0025

0.031

10

550

398

35

8

125

this invention

14

Example of

0.023

0.0024

0.23

15

620

320

53

32

120

this invention

TABLE 3

chemical components

No.

section

C

Si

Mn

P

S

sol. N

Cr

Ti

V

Nb

Zr

Mo

15

Example of

0.32

1.23

0.42

0.013

0.027

0.0021

this invention

16

Example of

0.32

1.27

0.54

0.023

0.012

0.0022

this invention

17

Example of

0.26

1.83

0.51

0.003

0.015

0.0037

this invention

18

Example of

0.32

1.17

0.45

0.020

0.025

0.0012

this invention

19

Example of

0.25

1.20

0.60

0.026

0.020

0.0042

0.21

this invention

20

Example of

0.34

1.32

0.25

0.022

0.025

0.0032

0.25

0.022

this invention

21

Example of

0.35

1.21

0.36

0.019

0.022

0.0022

0.023

this invention

22

Example of

0.35

1.19

0.81

0.027

0.023

0.0038

0.035

0.25

this invention

23

Example of

0.23

1.16

0.52

0.028

0.023

0.0045

0.040

this invention

24

Example of

0.35

1.26

0.55

0.027

0.019

0.0025

0.048

this invention

25

Example of

0.31

1.26

0.75

0.028

0.025

0.0033

0.22

this invention

26

Example of

0.38

1.46

0.18

0.025

0.029

0.0015

0.10

this invention

27

Example of

0.32

1.31

0.91

0.030

0.022

0.0042

this invention

28

Example of

0.32

1.20

0.34

0.021

0.026

0.0042

this invention

29

Example of

0.33

1.26

0.36

0.028

0.018

0.0037

this invention

30

Example of

0.38

1.34

0.45

0.029

0.017

0.0026

this invention

TABLE 4

anneal-

Cool-

recuper-

pear-

anneal-

ing

ing

ative

surface

lite

ing

hard-

chemical components

rate

temp.

layer

ratio

time

ness

No.

section

Al

B

Pb

Ca

Te

Se

Bi

Mg

(° C./s)

(° C.)

(HV)

(%)

(hr)

(HV)

15

Example of

0.025

15

100

652

32

4

119

this invention

16

Example of

0.022

3

390

385

12

4

125

this invention

17

Example of

0.022

3

280

275

19

12

124

this invention

18

Example of

15

100

398

0

4

122

this invention

19

Example of

0.021

15

100

310

0

4

128

this invention

20

Example of

0.018

8

330

416

0

4

124

this invention

21

Example of

0.030

3

390

402

2

4

118

this invention

22

Example of

0.031

3

360

420

0

4

125

this invention

23

Example of

0.029

3

480

295

5

4

126

this invention

24

Example of

0.027

3

530

361

10

8

119

this inventionn

25

Example of

0.017

3

480

311

9

8

120

this invention

26

Example of

0.023

0.0028

3

390

451

0

8

118

this invention

27

Example of

0.026

0.0025

0.021

3

470

338

17

8

131

this invention

28

Example of

0.022

0.0022

0.0016

3

610

306

25

16

125

this invention

29

Example of

0.022

0.0023

0.25

3

500

318

17

8

109

this invention

30

Example of

0.025

3

510

298

20

8

121

this invention

TABLE 5

chemical components

No.

section

C

Si

Mn

P

S

sol. N

Cr

Ti

V

Nb

Zr

Mo

31

Example of

0.55

0.75

0.31

0.023

0.017

0.0020

this invention

32

Example of

0.44

0.65

0.72

0.023

0.017

0.0021

this invention

33

Example of

0.36

0.50

1.01

0.008

0.008

0.0019

this invention

34

Example of

0.22

0.42

0.52

0.012

0.030

0.0042

this invention

35

Example of

0.54

0.46

0.42

0.021

0.019

0.0022

0.25

this invention

36

Example of

0.54

0.21

0.51

0.024

0.021

0.0042

0.21

0.021

this invention

37

Example of

0.55

0.55

0.36

0.022

0.024

0.0022

0.025

this invention

38

Example of

0.48

0.64

0.24

0.024

0.021

0.0048

0.025

0.21

this invention

39

Example of

0.52

0.43

0.37

0.022

0.022

0.0035

0.031

this invention

40

Example of

0.65

0.51

0.38

0.017

0.012

0.0025

0.053

this invention

41

Example of

0.51

0.35

0.48

0.027

0.028

0.0035

0.12

this invention

42

Example of

0.48

0.65

0.19

0.024

0.021

0.0019

0.11

this invention

TABLE 6

anneal-

Cool-

recuper-

pear-

anneal-

ing

ing

ative

surface

lite

ing

hard-

chemical components

rate

temp.

layer

ratio

time

ness

No.

section

Al

B

Pb

Ca

Te

Se

Bi

Mg

(° C./s)

(° C.)

(HV)

(%)

(hr)

(HV)

31

Example of

0.027

10

430

521

0

8

121

this invention

32

Example of

0.023

15

100

521

0

8

124

this invention

33

Example of

0.017

3

500

320

0

4

127

this invention

34

Example of

3

380

325

0

8

126

this invention

35

Example of

0.029

10

370

596

0

12

125

this invention

36

Example of

0.019

0.0021

10

440

562

36

16

122

this invention

37

Example of

0.029

8

430

545

37

12

120

this invention

38

Example of

0.030

0.0021

3

550

320

42

24

128

this invention

39

Example of

0.036

0.0025

3

560

410

45

16

124

this invention

40

Example of

0.021

0.0024

8

440

495

35

16

126

this invention

41

Example of

0.022

3

470

452

31

16

119

this invention

42

Example of

0.034

8

390

495

2

12

126

this invention

TABLE 7

chemical components

No.

section

C

Si

Mn

P

S

sol. N

Cr

Ti

V

Nb

Zr

Mo

43

Comparative

0.55

1.23

0.34

0.019

0.017

0.0059

Example

44

Comparative

0.49

1.19

0.40

0.021

0.020

0.0070

Example

45

Comparative

0.35

1.18

0.35

0.021

0.026

0.0062

Example

46

Comparative

0.53

0.75

0.41

0.029

0.027

0.0057

Example

47

Comparative

0.46

0.69

0.41

0.022

0.021

0.0061

Example

48

Comparative

0.36

0.72

0.34

0.024

0.021

0.0057

Example

49

Comparative

0.58

1.28

0.50

0.021

0.026

0.0082

0.01

Example

50

Comparative

0.46

0.73

0.34

0.023

0.019

0.0059

Example

51

Comparative

0.36

0.72

0.34

0.024

0.021

0.0057

Example

52

Comparative

0.58

1.21

0.32

0.024

0.026

0.0068

0.11

Example

53

Comparative

0.48

1.06

0.35

0.021

0.022

0.0063

0.014

Example

54

Comparative

0.48

0.71

0.50

0.029

0.021

0.0065

Example

55

Comparative

0.53

1.12

0.36

0.022

0.027

0.0035

Example

56

Comparative

0.51

1.21

0.35

0.019

0.019

0.0038

Example

57

Comparative

0.54

1.87

0.82

0.023

0.017

0.0021

Example

58

Comparative

0.46

1.43

1.21

0.008

0.008

0.0019

0.021

Example

59

Comparative

0.35

1.23

0.42

0.021

0.016

0.0045

Example

60

Comparative

0.22

1.17

0.45

0.012

0.030

0.0042

0.20

Example

61

Comparative

0.51

1.23

0.32

0.023

0.017

0.0020

Example

62

Comparative

0.54

1.87

0.82

0.023

0.017

0.0021

0.022

Example

TABLE 8

anneal-

Cool-

recuper-

pear-

anneal-

ing

ing

ative

surface

lite

ing

hard-

chemical components

rate

temp.

layer

ratio

time

ness

No.

section

Al

B

Pb

Ca

Te

Se

Bi

Mg

(° C./s)

(° C.)

(HV)

(%)

(hr)

(HV)

43

Example of

0.028

10

450

586

0

120

138

Example

44

Comparative

0.019

0.0026

10

550

546

0

120

141

Example

45

Comparative

0.021

6

560

405

10

120

145

Example

46

Comparative

0.028

8

540

486

10

120

145

Example

47

Comparative

0.019

8

500

456

40

120

141

Example

48

Comparative

0.021

0.0024

10

450

385

55

120

135

Example

49

Comparative

0.010

10

450

367

25

120

150

Example

50

Comparative

0.019

10

570

341

20

120

141

Example

51

Comparative

0.021

0.0024

10

570

345

15

120

135

Example

52

Comparative

0.015

10

400

520

0

120

152

Example

53

Comparative

0.027

0.0021

10

420

512

0

120

148

Example

54

Comparative

0.021

0.0021

10

440

465

16

32

148

Example

55

Comparative

0.028

0.0025

0.5

770

265

86

48

125

Example

56

Comparative

0.027

0.0028

0.5

700

253

90

32

126

Example

57

Comparative

0.023

0.5

780

243

82

120

124

Example

58

Comparative

0.017

0.5

760

225

75

70

127

Example

59

Comparative

0.024

0.5

770

205

36

48

124

Example

60

Comparative

2

780

211

36

120

126

Example

61

Comparative

0.027

2

750

254

92

72

151

Example

62

Comparative

0.023

2

720

259

81

96

164

Example

Example 2

Steels having the chemical components shown in Tables 9 to 16 were melted, and were rolled into a diameter of 50 mm or 30 mm at 750 to 850° C. Some of the test specimens inclusive of Comparative Examples were forged at a temperature above 1,200° C. Rolled materials, as examples of the present invention, were cooled with water by an on-line water cooling apparatus from 800 to 900° C. immediately after rolling. The forged materials were heated to 850° C. by a heating furnace. The examples of the present invention were cooled by water while the Comparative Examples were cooled by air or water. When air cooling was conducted, the grain diameter of graphite became great. The size of the test specimens in this case was 30 mm in diameter and 40 mm in length. After cooling, the heat-treated materials were heated again to 680° C. and annealed. The graphitization ratio was measured in accordance with JIS G 1211.

The polished samples were prepared, and the graphite grain diameter was measured in the number of 50 visual fields and in magnification of at least 400 times by an image processor. After graphitization annealing, a measurement of the hardness, a cutting test and a radio-frequency hardening test were conducted. The cutting test was carried out by boring using a high-speed steel drill having a diameter of 3 mmφ. This test was done while the cutting speed was changed, and the drill peripheral speed at which the tool life of at least 1,000 mm, or so-called VL 1,000 (m/min), was reached, and this value was used as the index. This was wet cutting using a water-soluble oil at a feed quantity of 0.33 mm/rev.

The results are shown in Tables 17 to 19.

These tables show the hardness before and after annealing and the hardening time by radio-frequency hardening. The examples of the present invention (Nos. 1 to 59) had a hardness around HV: 120 before annealing and could be hardened to around HV: 600 after annealing. Hardenability by radio-frequency hardening was evaluated by a transformation point automatic measuring equipment (“Formaster”). When heating to 1,000° C. and rapid cooling were conducted by the Formaster, variance occurred in the hardness after radio-frequency annealing because graphite had a slow diffusion time. Therefore, the time before this variance of the hardness due to hardening disappeared was measured by changing the heating time and conducting rapid cooling, and hardenability was evaluated by this time. The size of each test specimen was 3 mm in diameter and 10 mm in length. Here, the variance of hardness was regarded as having disappeared when the variance of hardness of five test specimens fell below HV: 200.

The steels of the examples of the present invention could be softened sufficiently within the short annealing time, and had excellent machinability. Since machinability VL1,000=150 m/min was the limit of the tester, the steels had the possibility of further improvement. Though soft, they were hardened without variance by radio-frequency annealing. The annealing time was 3 seconds, and the steels could be annealed sufficiently by radio-frequency annealing without variance in the shortest time that could be controlled by the Formaster tester. These tendencies did not change fundamentally even when elements such as Ti and Cr were added, and these elements could be added whenever machinability and hardenability were further required.

Comparative Examples Nos. 57 to 70 were test specimens the N content of which exceeded the range of the present invention, and the graphite grain diameter of which exceeded the range of the present invention. In order to further clarify the effect of sol. N,

FIG. 8

shows the influences of sol. N on the graphite annealing time and the hardness. Numerals in circles in

FIG. 8

represent the Example No., and the hardness obtained thereby is added.

The annealing time necessary for achieving HV: 120 or below could be remarkably shortened when sol. N was decreased. Generally, the hardness of the steel materials was affected by the C content, and the influence of ferrite hardness became remarkable when graphite was formed. When large amounts of sol. N were contained, the hardness was not lowered sufficiently at any C contents even when the annealing time was extended up to 120 hours. It could be appreciated also that that even when the total N content was at the same level, the annealing time changed greatly depending on the sol. N amount (Examples Nos. 7 and 26 and Comparative Examples Nos. 57 and 60).

Minimum hardness could be lowered by lowering sol. N. The steels having such a lowered amount of sol. N could be made softer than the steels having a large sol. N content. It could be thus appreciated that when the sol. N amount exceeded the limit of the present invention, the annealing time became long, though there are certain differences in the addition elements. When annealing was cut halfway as in Comparative Examples Nos. 65 to 67, the graphitization ratio became insufficient, so that the hardness after annealing did not lower and cold forgeability became inferior. When the hardness was high, cuttability fell, as well. Even if a process that was economically disadvantageous was conducted by extending the annealing time, variance of the hardness was likely to occur in radio-frequency hardening unless the graphite grain diameter was small enough to fall within the range of the present invention.

Since the maximum grain diameter was great and diffusion of C by radio-frequency hardening was difficult in Comparative Examples Nos. 68 to 71, a long heating time was necessary for obtaining a uniform hardness.

As could be seen from Comparative Examples 71 to 73, the radio-frequency annealing heating time had to be extended so as to eliminate the variance when the mean grain diameter was great. This became the same as overall heating by radio-frequency heating. In consequence, control of the thickness of the hardened layer became difficult, and firing cracks were likely to occur.

TABLE 9

chemical components

No.

section

C

Si

Mn

P

S

sol. N

total N

Cr

Ti

V

Nb

Zr

Mo

1

Example of

0.51

1.23

0.32

0.023

0.017

0.0020

0.0025

this invention

2

Example of

0.54

1.87

0.82

0.023

0.017

0.0029

0.0035

this invention

3

Example of

0.56

1.43

1.21

0.008

0.008

0.0019

0.0026

this invention

4

Example of

0.52

1.17

0.45

0.012

0.030

0.0032

0.0036

this invention

5

Example of

0.54

1.20

0.30

0.021

0.019

0.0022

0.0042

0.25

this invention

6

Example of

0.54

1.22

0.35

0.024

0.021

0.0018

0.0052

0.21

0.021

this invention

7

Example of

0.55

1.21

0.32

0.022

0.024

0.0022

0.0062

0.015

this invention

8

Example of

0.55

1.19

0.41

0.024

0.021

0.0038

0.0068

0.025

0.21

this invention

9

Example of

0.52

1.16

0.50

0.022

0.022

0.0035

0.0055

0.031

this invention

10

Example of

0.65

1.26

0.35

0.017

0.012

0.0025

0.0057

0.053

this invention

11

Example of

0.51

1.16

0.45

0.027

0.028

0.0035

0.0045

0.12

this invention

12

Example of

0.48

1.26

0.28

0.024

0.021

0.0019

0.0047

0.11

this invention

13

Example of

0.54

1.82

0.54

0.024

0.021

0.0029

0.0032

this invention

14

Example of

0.52

1.09

0.36

0.029

0.018

0.0037

0.0055

this invention

15

Example of

0.51

1.29

0.38

0.021

0.015

0.0032

0.0050

this invention

16

Example of

0.53

1.25

0.36

0.029

0.018

0.0037

0.0047

this invention

17

Example of

0.54

1.31

0.46

0.027

0.012

0.0017

0.0026

this invention

18

Example of

0.54

1.31

0.46

0.027

0.012

0.0017

0.0036

this invention

19

Example of

0.52

1.20

0.32

0.015

0.010

0.0027

0.0060

this invention

TABLE 10

graphiti-

zation

mean

maximum

chemical components

ratio

grain

grain

No.

section

Al

B

Pb

Ca

Te

Se

Bi

Mg

(%)

diameter

10 × C

⅓

diameter

1

Example of

0.027

79

4.2

7.99

13.2

this invention

2

Example of

0.023

85

4.5

8.14

11.5

this invention

3

Example of

0.017

82

5.5

8.24

10.6

this invention

4

Example

82

4.8

8.04

14.2

this invention

5

Example of

0.029

72

4.2

8.14

12.6

this invention

6

Example of

0.019

85

5.9

8.14

8.9

this invention

7

Example of

0.029

82

5.0

8.19

12.5

this invention

8

Example of

0.030

76

4.6

8.19

10.3

this invention

9

Example of

0.036

73

4.1

8.04

14.3

this invention

10

Example of

0.021

85

3.9

8.66

14.5

this invention

11

Example of

0.022

86

4.8

7.99

13.5

this invention

12

Example of

0.034

0.0021

93

4.2

7.83

12.6

this invention

13

Example of

0.029

0.13

91

4.6

8.14

14.5

this invention

14

Example of

0.027

0.0021

0.0013

86

5.0

8.04

18.3

this invention

15

Example of

0.021

0.0025

0.031

88

4.7

7.99

12.0

this invention

16

Example of

0.023

0.0024

0.23

79

5.8

8.09

11.9

this invention

17

Example of

0.027

0.30

86

5.5

8.14

13.5

this invention

18

Example of

0.017

0.0060

86

5.5

8.14

13.5

this invention

19

Example of

0.0045

86

5.5

8.04

13.5

this invention

TABLE 11

chemical components

No.

section

C

Si

Mn

P

S

sol. N

total N

Cr

Ti

V

Nb

Zr

Mo

20

Example of

0.32

1.23

0.42

0.013

0.027

0.0021

0.0036

this invention

21

Example of

0.32

1.27

0.54

0.023

0.012

0.0022

0.0040

this invention

22

Example of

0.26

1.83

0.51

0.003

0.015

0.0037

0.0048

this invention

23

Example of

0.32

1.17

0.45

0.020

0.025

0.0012

0.0020

this invention

24

Example of

0.25

1.20

0.60

0.026

0.020

0.0032

0.0042

0.21

0.022

this invention

25

Example of

0.34

1.32

0.25

0.022

0.025

0.0032

0.0065

0.25

0.023

this invention

26

Example of

0.35

1.21

0.36

0.019

0.022

0.0023

0.0065

0.035

0.25

this invention

27

Example of

0.35

1.19

0.81

0.027

0.023

0.0038

0.0055

0.040

this invention

28

Example of

0.23

1.16

0.52

0.028

0.023

0.0041

0.0050

0.048

this invention

29

Example of

0.35

1.26

0.55

0.027

0.019

0.0025

0.0046

0.22

this invention

30

Example of

0.31

1.26

0.75

0.028

0.025

0.0033

0.0047

0.10

this invention

31

Example of

0.38

1.46

0.18

0.025

0.029

0.0015

0.0040

this invention

32

Example of

0.24

1.32

0.50

0.026

0.025

0.0039

0.0038

this invention

33

Example of

0.32

1.31

0.91

0.030

0.022

0.0042

0.0051

this invention

34

Example of

0.32

1.20

0.34

0.021

0.026

0.0042

0.0055

this invention

35

Example of

0.33

1.26

0.36

0.028

0.018

0.0037

0.0057

this invention

36

Example of

0.38

1.34

0.45

0.029

0.017

0.0026

0.0036

this invention

37

Example of

0.32

1.24

0.32

0.022

0.012

0.0030

0.0045

this invention

TABLE 12

graphiti-

zation

mean

maximum

chemical components

ratio

grain

grain

No.

section

Al

B

Pb

Ca

Te

Se

Bi

Mg

(%)

diameter

10 × C

⅓

diameter

20

Example of

0.025

81

3.5

6.84

10.1

this invention

21

Example of

0.022

76

3.7

6.84

9.8

this invention

22

Example

0.022

85

2.8

6.38

10.6

this invention

23

Example of

88

2.4

6.84

12.3

this invention

24

Example of

0.021

76

3.5

6.30

9.8

this invention

25

Example of

0.018

77

2.2

6.98

8.3

this invention

26

Example of

0.030

88

3.5

7.05

9.6

this invention

27

Example of

0.031

75

3.8

7.05

10.6

this invention

28

Example of

0.029

74

3.7

6.13

10.2

this invention

29

Example of

0.027

85

3.6

7.05

13.2

this invention

30

Example of

0.017

89

3.5

6.77

12.5

this invention

31

Example of

0.023

0.0028

91

2.8

7.24

9.8

this invention

32

Example of

0.021

0.21

89

3.8

6.21

12.2

this invention

33

Example of

0.026

0.0025

0.0016

86

3.6

6.84

9.6

this invention

34

Example of

0.022

0.0022

0.021

87

3.0

6.84

8.9

this invention

35

Example of

0.022

0.0023

0.25

95

3.6

6.91

7.9

this invention

36

Example of

0.025

0.29

92

3.0

7.24

9.0

this invention

37

Example of

0.001

0.0055

92

3.0

6.84

9.0

this invention

TABLE 13

chemical components

No.

section

C

Si

Mn

P

S

sol. N

total N

Cr

Ti

V

Nb

Zr

Mo

38

Example of

0.55

0.75

0.31

0.023

0.017

0.0020

0.0032

this invention

39

Example of

0.44

0.65

0.72

0.023

0.017

0.0021

0.0034

this invention

40

Example of

0.36

0.50

1.01

0.008

0.008

0.0019

0.0025

this invention

41

Example of

0.22

0.42

0.52

0.012

0.030

0.0042

0.0056

this invention

42

Example of

0.54

0.46

0.42

0.021

0.019

0.0022

0.0038

0.25

this invention

43

Example of

0.54

0.21

0.51

0.024

0.021

0.0032

0.0052

0.007

this invention

44

Example of

0.55

0.55

0.36

0.022

0.024

0.0022

0.0061

0.025

this invention

45

Example of

0.55

0.64

0.24

0.024

0.021

0.0048

0.0078

0.025

0.21

this invention

46

Example of

0.52

0.43

0.37

0.022

0.022

0.0035

0.0049

0.031

this invention

47

Example of

0.65

0.51

0.38

0.017

0.012

0.0025

0.0051

0.053

this invention

48

Example of

0.51

0.35

0.48

0.027

0.028

0.0035

0.0045

0.12

this invention

49

Example of

0.48

0.65

0.19

0.024

0.021

0.0019

0.0056

0.11

this invention

50

Example of

0.54

0.78

0.62

0.024

0.021

0.0029

0.0043

this invention

51

Example of

0.52

0.25

0.25

0.029

0.018

0.0037

0.0062

this invention

52

Example of

0.51

0.35

0.54

0.021

0.015

0.0032

0.0055

this invention

53

Example of

0.33

0.45

0.27

0.029

0.018

0.0037

0.0058

this invention

54

Example of

0.44

0.32

0.29

0.027

0.012

0.0027

0.0064

0.11

this invention

55

Example of

0.54

0.62

0.29

0.027

0.012

0.0021

0.0048

invention

56

Example of

0.52

0.32

0.29

0.027

0.012

0.0024

0.0058

0.010

this invention

TABLE 14

graphiti-

zation

mean

maximum

chemical components

ratio

grain

grain

No.

section

Al

B

Pb

Ca

Te

Se

Bi

Mg

(%)

diameter

10 × C

⅓

diameter

38

Example of

0.027

79

4.7

8.19

12.5

this invention

39

Example of

0.023

85

4.0

7.61

13.1

this invention

40

Example of

82

3.6

7.11

10.5

this invention

41

Example of

0.017

92

3.5

6.04

10.2

this invention

42

Example of

0.029

72

4.9

8.14

11.3

this invention

43

Example of

0.019

85

5.6

8.14

11.8

this invention

44

Example of

0.029

82

5.8

8.19

14.5

this invention

45

Example of

0.030

76

5.5

8.19

13.0

this invention

46

Example of

0.036

73

4.6

8.04

12.7

this invention

47

Example of

0.021

85

4.2

8.66

14.5

this invention

48

Example of

0.022

86

4.3

7.99

12.5

this invention

49

Example of

0.034

0.0021

93

4.4

7.83

13.6

this invention

50

Example of

0.029

0.13

91

5.2

8.14

15.2

this invention

51

Example of

0.027

0.0021

0.0013

86

5.4

8.04

14.4

this invention

52

Example of

0.021

0.0025

33

4.3

7.99

11.9

this invention

53

Example of

0.023

0.0024

46

4.1

6.91

12.0

this invention

54

Example of

0.027

67

4.8

7.61

14.3

this invention

55

Example of

0.027

0.0035

86

4.8

8.14

14.3

this invention

56

Example of

0.0041

86

4.8

8.04

14.3

this invention

TABLE 15

chemical components

No.

section

C

Si

Mn

P

S

sol. N

total N

Cr

Ti

V

Nb

Zr

Mo

57

Comparative

0.55

1.23

0.34

0.019

0.017

0.0059

0.0068

Example

58

Comparative

0.49

1.19

0.40

0.021

0.020

0.0070

0.0091

Example

59

Comparative

0.52

1.20

0.29

0.015

0.012

0.0068

0.0095

Example

60

Comparative

0.35

1.18

0.35

0.021

0.026

0.0062

0.0075

Example

61

Comparative

0.35

1.21

0.31

0.011

0.019

0.0082

0.0105

Example

62

Comparative

0.53

0.75

0.41

0.029

0.027

0.0057

0.0067

Example

63

Comparative

0.46

0.69

0.41

0.022

0.021

0.0061

0.0101

Example

64

Comparative

0.36

0.75

0.34

0.024

0.021

0.0057

0.0069

Example

65

Comparative

0.58

0.35

0.50

0.021

0.026

0.0082

0.0124

0.01

Example

66

Comparative

0.46

0.38

0.34

0.023

0.019

0.0059

0.0079

Example

67

Comparative

0.36

0.40

0.34

0.024

0.021

0.0057

0.0084

Example

68

Comparative

0.55

1.21

0.32

0.024

0.026

0.0068

0.0083

0.11

Example

69

Comparative

0.44

1.06

0.35

0.021

0.022

0.0063

0.0092

0.014

Example

70

Comparative

0.47

0.71

0.50

0.029

0.021

0.0065

0.0087

Example

71

Comparative

0.53

1.12

0.36

0.022

0.027

0.0035

0.0045

Example

72

Comparative

0.51

1.21

0.35

0.019

0.019

0.0038

0.0058

Example

73

Comparative

0.36

1.22

0.35

0.014

0.022

0.0037

0.0049

Example

TABLE 16

graphiti-

zation

mean

maximum

chemical components

ratio

grain

grain

No.

section

Al

B

Pb

Ca

Te

Se

Bi

Mg

(%)

diameter

10 × C

⅓

diameter

57

Comparative

0.028

65

3.4

8.19

13.8

Example

58

Comparative

0.019

0.0026

58

3.2

7.88

11.7

Example

59

Comparative

0.028

52

3.4

8.19

14.8

Example

60

Comparative

55

4.2

7.05

8.7

Example

61

Comparative

0.021

54

4.7

7.05

12.7

Example

62

Comparative

0.028

48

4.6

8.09

10.5

Example

63

Comparative

0.019

42

4.5

7.72

12.9

Example

64

Comparative

0.021

0.0024

41

4.7

7.11

13.8

Example

65

Comparative

0.010

15

4.4

8.34

10.5

Example

66

Comparative

0.019

18

4.5

7.72

12.5

Example

67

Comparative

0.021

0.0024

16

1.5

7.11

10.0

Example

68

Comparative

0.015

85

4.3

8.19

25.1

Example

69

Comparative

0.027

0.0021

64

4.6

7.61

26.9

Example

70

Comparative

0.021

0.0021

79

3.6

7.78

31.0

Example

71

Comparative

0.028

0.0025

78

9.1

8.09

21.6

Example

72

Comparative

0.027

0.0028

89

9.4

7.99

14.8

Example

73

Comparative

0.022

0.0021

45

7.7

7.11

16.8

Example

TABLE 17

annealing

annealing

heating

No.

section

machinability

time

hardness (HV)

time

hardness

1

Example of

150

8

121

3

645

this invention

2

Example of

150

8

124

3

657

this invention

3

Example of

150

8

127

3

721

this invention

4

Example of

150

14

126

3

581

this invention

5

Example of

150

12

125

3

594

this invention

6

Example of

150

8

120

3

679

this invention

7

Example of

150

12

122

3

702

this invention

8

Example of

150

6

128

3

712

this invention

9

Example of

150

6

124

3

680

this invention

10

Example of

150

8

126

3

750

this invention

11

Example of

150

8

119

3

654

this invention

12

Example of

150

16

126

3

621

this invention

13

Example of

150

16

127

3

655

this invention

14

Example of

150

8

124

6

649

this invention

15

Example of

150

8

125

3

635

this invention

16

Example of

150

8

120

3

681

this invention

17

Example of

150

8

123

3

678

this invention

18

Example of

150

8

123

3

678

this invention

19

Example of

150

8

123

3

678

this invention

20

Example of

150

4

119

3

452

this invention

21

Example of

150

4

125

3

458

this invention

22

Example of

150

6

124

3

432

this invention

23

Example of

150

4

122

3

452

this invention

24

Example of

150

4

128

3

401

this invention

25

Example of

150

4

124

3

459

this invention

26

Example of

150

6

118

3

481

this invention

27

Example of

150

4

125

3

446

this invention

28

Example of

150

4

126

3

385

this invention

TABLE 18

annealing

annealing

heating

No.

section

machinability

time

hardness (HV)

time

hardness

29

Example of

150

6

119

3

446

this invention

30

Example of

150

6

120

3

450

this invention

31

Example of

150

6

118

3

521

this invention

32

Example of

150

6

125

3

385

this invention

33

Example of

150

6

131

3

450

this invention

34

Example of

150

6

125

3

461

this invention

35

Example of

150

6

109

3

463

this invention

36

Example of

150

6

121

3

501

this invention

37

Example of

150

6

121

3

501

this invention

38

Example of

150

8

121

3

681

this invention

39

Example of

150

8

124

3

592

this invention

40

Example of

150

8

127

3

450

this invention

41

Example of

150

8

126

3

392

this invention

42

Example of

150

12

125

3

681

this invention

43

Example of

150

8

122

3

702

this invention

44

Example of

150

12

120

3

721

this invention

45

Example of

150

6

128

3

681

this invention

46

Example of

150

6

124

3

677

this invention

47

Example of

150

8

126

3

730

this invention

48

Example of

150

8

119

3

624

this invention

49

Example of

150

16

126

3

623

this invention

50

Example of

150

16

127

3

592

this invention

51

Example of

150

8

124

3

681

this invention

52

Example of

150

8

125

3

653

this invention

53

Example of

150

8

120

3

693

this invention

54

Example of

150

8

123

3

672

this invention

55

Example of

150

8

123

3

672

this invention

56

Example of

150

8

123

3

672

this invention

TABLE 19

annealing

annealing

heating

No.

section

machinability

time

hardness (HV)

time

hardness

57

Comparative

60

60

138

3

648

Example

58

Comparative

70

60

141

3

589

Example

59

Comparative

70

120

135

7

631

Example

60

Comparative

100

72

145

3

460

Example

61

Comparative

90

120

132

3

454

Example

62

Comparative

70

120

145

3

659

Example

63

Comparative

60

120

141

3

601

Example

64

Comparative

100

120

135

3

452

Example

65

Comparative

50

16

152

3

720

Example

66

Comparative

50

16

141

3

601

Example

67

Comparative

60

8

145

3

452

Example

68

Comparative

100

120

152

15

759

Example

69

Comparative

80

120

148

12

589

Example

70

Comparative

100

120

148

10

592

Example

71

Comparative

120

48

125

12

625

Example

72

Comparative

120

32

126

12

752

Example

73

Comparative

120

24

126

8

453

Example

INDUSTRIAL APPLICABILITY

The steel for cold forging according to the present invention has excellent surface hardness, excellent deformation properties and machinability, and can be used either as-rolled or under an annealed state for a short time. Moreover, because the steel contains C, the strength can be remarkably improved by heat-treatment, and mechanical components can be produced easily and highly efficiently. Furthermore, the steel for cold forging according to the present invention can shorten the annealing time for softening.

Number	Date	Country	Kind
10 067642	Mar 1998	JP
10 067641	Mar 1998	JP

Number	Name	Date	Kind
5476556	Hoshino et al.	Dec 1995	A
5830285	Katayama et al.	Nov 1998	A

Number	Date	Country
62-23929	Jan 1987	JP
62-196327	Aug 1987	JP
2-111842	Apr 1990	JP
2-145745	Jun 1990	JP
3-146618	Jun 1991	JP
5-26850	Apr 1993	JP
7-5960	Jan 1995	JP
7-242990	Sep 1995	JP
8-283847	Oct 1996	JP
8-291366	Nov 1996	JP
9-157786	Jun 1997	JP

Steels for cold forging and process for producing the same

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (2)

PCT Information

US Referenced Citations (2)

Foreign Referenced Citations (11)

Non-Patent Literature Citations (1)