JP5397308B2 - Hot-worked steel for case hardening - Google Patents

Description

  The present invention relates to a hot-worked steel material (hereinafter referred to as “skin-hardening”) that is a material of a part subjected to carburizing or carbonitriding (hereinafter, “carburizing or carbonitriding” may be simply referred to as “carburizing”). In particular, the surface to be carburized after being formed by cold forging and excellent in austenite grain coarsening prevention characteristics during carburizing. The present invention relates to a case-working hot-worked steel suitable for use as a material for hardened parts.

  Conventionally, surface-hardened parts that have been subjected to carburizing or carbonitriding treatment such as shafts and gears (hereinafter referred to as “carburizing-hardened parts”) use case-hardened steel as a material, and “spheroidizing annealing-drawing” or “ Steel wire production by "drawing-spheroidizing annealing-skin pass", forming into a predetermined shape by cold forging and machining, then carburizing, quenching and tempering to improve properties such as fatigue strength and wear resistance It is manufactured by sequentially performing each process.

  In the case-hardened steel material cold-forged, it is a problem to prevent austenite grains from coarsening during carburizing. For this reason, for example, Patent Document 1 and Patent Document 2 propose a technique for preventing the austenite grains from coarsening during carburization.

  Specifically, in Patent Document 1, the Nb (CN) precipitate after hot rolling is 0.005% or more, the precipitation amount of AlN is limited to 0.015% or less, and the structure fraction of bainite. Has been disclosed as "high temperature carburizing steel excellent in high temperature carburizing characteristics and hot forged members for high temperature carburizing".

  Patent Document 2 discloses “a case hardening steel excellent in crystal grain coarsening characteristics and cold workability” containing Nb and Ti and a manufacturing method thereof.

JP 2001-279383 A JP 2006-307270 A

  Even if the steel disclosed in Patent Document 1 and Patent Document 2 described above is used as a raw material, it is not possible to stably and reliably prevent coarsening of austenite grains during carburizing in a carburized and hardened part formed by cold forging. It is enough.

  The present invention has been made in view of the above-mentioned present situation, and can stably prevent coarsening of austenite grains during carburization, has excellent cold forgeability, and is carburized and hardened part formed by cold forging. An object of the present invention is to provide a hot-worked steel material for case hardening suitable for use as a raw material.

  In order to achieve the above-mentioned object, the present inventors have made various studies on the coarsening of austenite grains in the carburizing process after cold forging for the case-work hot-worked steel. As a result, it became clear that the occurrence of coarsening of austenite grains has a correlation with the amount of Nb solid solution and the area ratio of bainite after hot working.

  However, the “Nb solid solution amount” refers to an amount calculated by subtracting “the amount of Nb forming precipitates in the steel material” from the “amount of Nb contained in the steel material”.

  Measurement of the above-mentioned “amount of Nb forming precipitates in steel” is the residue collected by a filter having a mesh opening of 0.2 μm by the extraction residue method according to the following procedures [1] to [4]. This is done by measuring the amount of Nb in the medium.

  [1] The test piece is dissolved by electrolysis using a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution to expose Nb precipitates on the surface.

  [2] Nb and Ti precipitates are peeled off in ethanol by ultrasonic waves and suction filtered to collect Nb precipitate residues on the filter.

  [3] Put the collected residue in a platinum crucible and bake with a burner, add an alkali flux of sodium carbonate and sodium tetraborate, then bake with a burner to obtain a glass containing Nb and Ti precipitate residues. Make it.

  [4] The glass produced in [3] above is dissolved in an aqueous solution containing hydrochloric acid, and the amount of Nb in the extraction residue is determined by ICP emission analysis.

  Specific findings obtained by the present inventors are the following (a) to (d).

  (A) When the amount of Nb solid solution is too low, the amount of NbC that finely precipitates during spheroidizing annealing (including the Ostwald growth of NbC that is too fine to be captured by the residue analysis described above) is small, so the austenite grains are coarse As a result, the austenite grain is coarsened during carburizing.

  (B) When the area ratio of bainite is too low, the structure in the cross section is non-uniform, so that the dispersion state of precipitates such as NbC during spheroidizing annealing becomes non-uniform, and the precipitates are not carburized during carburization. Since it coarsens, it is difficult to suppress the coarsening of austenite grains.

  (C) When the area ratio of bainite is too high, the hardness after spheroidizing annealing does not decrease, so the cold forgeability is low.

  (D) There is a correlation between the area ratio of bainite and the Nb solid solution amount. When the area ratio of bainite is high, the Nb solid solution amount also increases.

  Therefore, the present inventors conducted a more detailed study on the influence of the Nb solid solution amount and the area ratio of bainite on the austenite grain coarsening and cold forgeability. As a result, the following knowledge (e) was obtained.

  (E) After hot working, when the area ratio of bainite is 75 to 90% and [Nb solid solution amount x area ratio of bainite] is 1.1 to 3.0, hot working steel for case hardening In the carburizing step after cold forging, coarsening of austenite grains is suppressed, and cold forgeability is also good. In the above case, if the Nb solid solution amount is mass% and 0.015% or more, the austenite grain coarsening suppressing action is further increased.

  This invention is completed based on said knowledge, The summary exists in the hot-working steel materials for skin hardening shown to following (1) and (2).

(1) In mass%,
C: 0.10 to 0.30%,
Si: 0.50% or less,
Mn: 0.15 to 1.5%,
P: 0.04% or less,
S: 0.005 to 0.07%,
Cr: 1.5 to 3.0%,
Al: 0.01 to 0.05%,
N: 0.0035 to 0.010%,
Ti: 0.005 to 0.10%,
Nb: 0.02-0.07% and B: 0.0005-0.0050%
And the balance is a chemical composition consisting of Fe and impurities,
It is a metal structure having an area ratio of bainite of 75 to 90%, and
1.1 ≦ [Nb solid solution amount × bainite area ratio] ≦ 3.0,
A hot-worked steel for case hardening, characterized by satisfying

(2) Instead of a part of Fe, in mass%,
Containing one or more of Mo: 0.50% or less and V: 0.20% or less,
The hot-worked steel for case hardening according to (1) above, characterized in that.

  The “impurities” in the remaining “Fe and impurities” refer to those mixed from ore, scrap, or the environment as raw materials when industrially producing steel materials.

  The unit of “Nb solid solution amount” in the formula [Nb solid solution amount × bainite area ratio] is “mass%”. The unit of “area ratio of bainite” is “%”.

  The hot-worked steel for case hardening of the present invention is excellent in cold forgeability, and can stably prevent coarsening of austenite grains during carburizing after cold forging. It can be suitably used as a material for carburized and hardened parts formed by forging.

It is a figure which shows the relationship between [the amount of Nb solid solution x area ratio of bainite] and the coarsening temperature of austenite. It is a figure which shows the relationship between the area ratio of a bainite, and a limit compression rate. It is a figure which shows the relationship between [the amount of Nb solid solution x area ratio of bainite] and the limit compression rate.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition:
C: 0.10 to 0.30%
C is an element necessary for securing the strength of the carburized and hardened parts such as shafts and gears made of the steel material of the present invention. When the content is less than 0.10%, the effect of addition is poor. On the other hand, if the content exceeds 0.30%, the toughness of the dough of the carburized and hardened part is lowered. Therefore, the content of C is set to 0.10 to 0.30%. The C content is preferably 0.12% or more and 0.25% or less.

Si: 0.50% or less Si is an element having a deoxidizing action, but if the content is excessive, workability is lowered. Further, Si generates a grain boundary oxide layer on the surface portion of the carburized component, thereby causing a decrease in fatigue strength. Therefore, the Si content is set to 0.50% or less. Desirable Si content is 0.35% or less.

Mn: 0.15 to 1.5%
Mn has the effect | action which improves hardenability. In order to obtain this effect, a Mn content of 0.15% or more is required. However, if the Mn content exceeds 1.5%, the machinability decreases. Therefore, the Mn content is set to 0.15 to 1.5%. The Mn content is desirably 0.30% or more and 1.2% or less.

P: 0.04% or less P is an impurity that lowers toughness, and as its content increases, the toughness significantly decreases. Therefore, its content is set to 0.04% or less. The P content is preferably 0.03% or less.

S: 0.005-0.07%
S has an effect of improving machinability. In order to obtain this effect, an S content of 0.005% or more is necessary. However, if the S content exceeds 0.07%, cold forgeability and hot workability are lowered, and further, the toughness of the surface hardened layer in the carburized hardened parts such as shafts and gears is lowered. Therefore, the content of S is set to 0.005 to 0.07%. The S content is desirably 0.01% or more and 0.05% or less.

Cr: 1.5-3.0%
Cr has the effect of improving the hardenability of the dough of carburized and hardened parts such as shafts and gears made of the steel material of the present invention. Further, Cr has an effect of stabilizing cementite in bainite, so that the spheroidizing ratio in spheroidizing annealing is improved and good cold forgeability can be obtained. In order to acquire said effect, it is necessary to contain 1.5% or more of Cr. However, if the Cr content exceeds 3.0%, the machinability deteriorates. Therefore, the Cr content is set to 1.5 to 3.0%. The Cr content is preferably 1.6% or more and 2.5% or less.

Al: 0.01 to 0.05%
Al is an element having a deoxidizing action, and is contained in an amount of 0.01% or more as a deoxidizing agent. However, if the Al content becomes excessive and exceeds 0.05%, huge alumina inclusions that become the starting point of fatigue fracture may be generated, and the fatigue strength may be reduced. Therefore, the Al content is set to 0.01 to 0.05%. The Al content is preferably 0.02% or more and 0.04% or less.

N: 0.0035 to 0.010%
N, together with C, combines with Nb and Ti to form carbonitrides, and has the effect of suppressing grain coarsening in the austenite region. In order to obtain this effect, the N content is set to 0.0035% or more. However, if the N content becomes excessive and exceeds 0.010%, not only the cold forgeability is lowered, but also BN is formed by combining with B, so that the hardenability of B described later is improved. The effect will be insufficient. Therefore, the content of N is set to 0.0035 to 0.010%. The N content is preferably 0.0040% or more and 0.0090% or less.

Ti: 0.005-0.10%
Ti combines with N to form TiN and has an effect of ensuring the hardenability of B. Ti further forms carbonitride with C and N, and has the effect of suppressing the coarsening of austenite grains by its grain boundary pinning action. However, if the Ti content is less than 0.005%, the effect of suppressing the austenite grain coarsening is poor, while if it exceeds 0.10%, the cold forgeability is reduced. Therefore, the content of Ti is set to 0.005 to 0.10%. The Ti content is preferably 0.020% or more and 0.075% or less.

Nb: 0.02 to 0.07%
Nb forms carbonitride with C and N, and has the effect | action which suppresses the coarsening of an austenite grain by the grain boundary pinning effect | action. However, if the Nb content is less than 0.02%, the effect of suppressing the austenite grain coarsening is poor, while if it exceeds 0.07%, the cold forgeability is reduced. Therefore, the Nb content is set to 0.02 to 0.07%. The Nb content is preferably 0.025% or more and 0.06% or less.

  Regarding Nb, not only the content but also the “Nb solid solution amount” needs to satisfy the formula of “1.1 ≦ [Nb solid solution amount × bainite area ratio] ≦ 3.0” described later.

B: 0.0005 to 0.0050%
B has the effect | action which improves hardenability. However, if the B content is less than 0.0005%, a sufficient effect of improving hardenability cannot be obtained. On the other hand, even if B exceeding 0.0050% is contained, the effect is saturated, and only the cost is increased. Therefore, the content of B is set to 0.0005 to 0.0050%. The B content is preferably 0.0010% or more and 0.0035% or less.

  One of the chemical compositions of the hot-worked steel for case hardening according to the present invention is that the balance is Fe and impurities in addition to the above elements.

  Another chemical composition of the hot-worked steel for case hardening of the present invention contains at least one of Mo and V in place of part of Fe.

  Since both Mo and V have the effect of improving the hardenability, these elements may be contained as necessary. Hereinafter, the above Mo and V will be described.

Mo: 0.50% or less Mo has an effect of improving hardenability. Furthermore, Mo also has the effect | action which raises the hardness of the material | dough of carburizing hardening components, such as a shaft and a gear which use the steel materials of this invention as a raw material. Therefore, in order to acquire said effect, you may contain Mo. However, if the Mo content becomes excessive and exceeds 0.50%, the cold forgeability is lowered. Therefore, the amount of Mo in the case of inclusion is set to 0.50% or less. When Mo is contained, the amount of Mo is preferably 0.40% or less.

  On the other hand, in order to surely obtain the effect of Mo described above, the amount of Mo in the case of inclusion is desirably 0.05% or more.

V: 0.20% or less V has an effect of improving hardenability. Furthermore, V also has the effect of increasing the temper softening resistance. Therefore, in order to acquire said effect, you may contain V. However, if the V content becomes excessive and exceeds 0.20%, the cold forgeability deteriorates. Therefore, when V is included, the amount of V is set to 0.20% or less. When V is contained, the amount of V is desirably 0.10% or less.

  On the other hand, in order to surely obtain the effect of V described above, the amount of V when contained is preferably 0.005% or more.

  Said Mo and V can be contained only in any 1 type or 2 types of composites. The total content of Mo and V may be 0.70% or less, but is preferably 0.50% or less.

(B) Metal structure:
The hot-worked steel for case hardening of the present invention has the chemical composition described in the section (A), and the metal structure should have an area ratio of bainite of 75 to 90%. That is, in order to ensure excellent cold forgeability after spheroidizing annealing, the area ratio of bainite in the metal structure must be 75 to 90%. The phases other than bainite are ferrite and / or pearlite.

(C) Nb solid solution amount x area ratio of bainite:
Even if it has the chemical composition described in the section (A) and has a metal structure having a bainite area ratio of 75 to 90%, “Nb solid solution amount × bainite area ratio” is less than 1.1. The effect of suppressing the austenite grain coarsening is small, and the austenite grains become coarse during carburization. On the other hand, when “the amount of Nb solid solution × the area ratio of bainite” exceeds 3.0, the cold forgeability becomes low. Therefore, “Nb solid solution amount × bainite area ratio” needs to be 1.1 or more and 3.0 or less.

  When the amount of Nb dissolved is less than 0.015%, the amount of NbC finely precipitated during spheroidizing annealing (including the Ostwald growth of NbC that is too fine to capture by residue analysis) is small, and austenite during carburizing The effect of suppressing grain coarsening is reduced. Therefore, it is desirable that the Nb solid solution amount be 0.015% or more.

  The above-mentioned “solid solution Nb amount”, “area ratio of bainite” and “Nb solid solution amount × area ratio of bainite” are obtained by melting a steel having the chemical composition described in the item (A) and continuously casting it. A slab or ingot is obtained by the method or the steel ingot method, and can be obtained, for example, by applying the following steps <1> to <4>.

  <1> A soaking treatment is performed at 1250 to 1350 ° C. for 3 to 12 hours.

  <2> The soaked slab or ingot is batch-rolled at a finish rolling temperature of 1000 to 900 ° C. and cooled to room temperature.

  <3> After the rolled material is heated in the temperature range of 1200 to 1350 ° C. for 20 to 90 minutes, it is hot rolled at a finish rolling temperature of 950 to 800 ° C.

  <4> Cool from the hot rolling finish temperature to room temperature at a cooling rate of 0.5 to 3 ° C./second.

The NbC precipitate can be dissolved in the base material by soaking at 1250 to 1350 ° C. for 3 to 12 hours in the step <1>, and in the hot rolling and rolling step <3>, 1200 By heating to ˜1350 ° C. for 20 to 90 minutes, the NbC precipitated by cooling after the bulk rolling of <2> is re-dissolved, and at the cooling rate of 0.5 to 3 ° C./second after completion of hot rolling By cooling to room temperature, precipitation of NbC can be suppressed.
The temperature and cooling rate in each of the above treatments refer to the temperature and cooling rate based on the surface.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels a to h having the chemical composition shown in Table 1 were melted in a 50 kg vacuum melting furnace to produce an ingot.

  Among the above steels, steels a to f are steels in which the content of each component element is within the range defined by the present invention. On the other hand, steel g and steel h are steels whose chemical compositions deviate from the conditions defined in the present invention.

  The ingot thus obtained was heated to 1300 ° C. and held for 10 hours, and then hot forged so that the finishing temperature was 950 ° C. After the hot forging was completed, it was allowed to cool in the atmosphere, A round bar with a diameter of 70 mm was used.

  Next, the 70 mm diameter round bar is divided in the length direction, and each divided material is heated at the temperature shown in Table 2 for 30 minutes, and then hot forged at the finishing temperature shown in Table 2, and the diameter is 30 mm. A round bar was prepared. At the end of hot forging, the product was cooled to room temperature at the cooling rate shown in Table 2.

  A test piece was cut out using a part of each round bar with a diameter of 30 mm thus obtained, and the metal structure and the Nb solid solution amount were investigated.

The metal structure survey was conducted as follows. That is, after cutting a cross section perpendicular to the longitudinal direction of a round bar, R / 2 part (“R”) of a test piece that was embedded in a resin and mirror-polished so that the cross section became a test surface and corroded with nital For the radius of a round bar.) Using an optical microscope, a photograph of 4 fields of view was randomly taken at a magnification of 400 times and a size of 1 field of view of 30000 μm ² to identify phases in the metal structure, and Then, image analysis was performed for each of the above-mentioned visual fields to determine the area ratio of bainite, and the area ratio of bainite was determined by arithmetically averaging the values of four visual fields.

  The Nb solid solution amount was investigated by cutting out a 10 mm × 10 mm × 10 mm test piece from the R / 2 part of the round bar, and using the extraction residue method consisting of the steps [1] to [4] described above to obtain an opening of 0.2 μm. The amount of Nb in the residue collected by the mesh filter is measured to determine “the amount of Nb that forms precipitates in the steel”, and the above “steel” from the “amount of Nb contained in the steel”. The amount was calculated by subtracting the “amount of Nb forming precipitates”.

  On the other hand, the remaining round bars with a diameter of 30 mm are held at 760 ° C. for 3 hours, and then gradually cooled to a temperature range up to 660 ° C. at a cooling rate of 15 ° C./hour, and then allowed to cool in the atmosphere. Spheroidizing annealing is performed to measure the Vickers hardness (hereinafter referred to as “HV hardness”), and the cold forging property is investigated by performing a test simulating cold forging after spheroidizing annealing. did.

  Furthermore, a test simulating carburization performed after cold forging after spheroidizing annealing was conducted to investigate the coarsening characteristics of austenite grains.

  HV hardness measurement is performed by cutting a section perpendicular to the longitudinal direction of a round bar, embedding in a resin so that the section becomes a test surface, and mirror polishing, and then “Vickers hardness in JIS Z 2244 (2009)” Based on "Test-Test Method", the test force was set to 9.807 N for the R / 2 part 8 points of the test piece, and the value of the above 8 points was obtained by arithmetic averaging.

  The cold forgeability was investigated for “deformability” by the following method.

  That is, a cylindrical test piece having a diameter of 14 mm and a height of 21 mm was cut out from the center portion of each round bar subjected to the spheroidizing annealing, and a depth of 0.8 mm was formed on the side surface of each of the cylindrical test pieces. A V-notch having a tip R of 0.15 mm was formed in the vertical direction, an upsetting test was performed, and the deformability was evaluated based on the limit compression rate (%) at which cracking occurred.

  The austenite grain coarsening characteristics when carburizing after cold forging were investigated by the following method.

  That is, a cylindrical test piece having a diameter of 14 mm and a height of 21 mm was cut out from the center of each round bar subjected to the spheroidizing annealing described above, and first, 75% in the height direction was simulated in order to simulate cold forging. The compression process was performed.

  Next, in order to simulate carburization, the test piece subjected to 75% compression in the height direction was heated to each temperature of 920 to 1010 ° C. and held for 3 hours, and then cooled to room temperature by water cooling. .

  After cutting out the longitudinal section including R / 2 part of each test piece obtained as described above, the cut surface was mirror-polished and corroded with a saturated aqueous solution of picric acid to which a surfactant was added. The maximum austenite grain size number was examined by visual observation of the entire cut surface at a magnification of 200 times.

  The maximum heating temperature at which coarse austenite grains having a grain size number of 4 or less did not exist was evaluated as the “coarse graining temperature” by the above optical microscope observation.

  The results of the above investigations are also shown in Table 2. FIG. 1 shows the relationship between [Nb solid solution amount × bainite area ratio] and the austenite coarsening temperature, FIG. 2 shows the relation between the bainite area ratio and the critical compressibility, and FIG. Shows the relationship between [Nb solid solution amount × area ratio of bainite] and the critical compressibility.

  From Table 2 and FIGS. 1-3, in the case of “examples of the present invention” of test numbers 1, 5, 7, 10, 12, and 14 that satisfy the conditions specified in the present invention, the deformability after spheroidizing annealing is high. It is excellent in cold forgeability, and further, it is clear that the “roughening temperature” at the time of carburizing performed after cold forging is 980 ° C. or more, and a sufficient austenite grain coarsening inhibiting effect is obtained. .

  On the other hand, even when the steel a to f in which the content of each component element is within the range defined in the present invention is used, among the area ratio of bainite and [Nb solid solution amount × bainite area ratio] In the case of “Comparative Examples” of test numbers 2 to 4, 6, 8, 9, 11, 13, and 15 of comparative examples in which at least one of these is out of the conditions defined in the present invention, the cooling after spheroidizing annealing is performed. One or both of the forgeability and the coarsening resistance characteristics of austenite grains when carburizing after cold forging are inferior.

  In the case of “Comparative Examples” with test numbers 16 to 20 using steels g and h whose chemical composition deviated from the range specified in the present invention, the area ratio of bainite and [Nb solid solution amount × bainite area ratio] In addition to being inferior in the effect of inhibiting the coarsening of austenite grains, the “coarse grain resistance temperature” is 920 to 940 ° C., the deformability after spheroidizing annealing is low, and the cold forgeability is improved. Is also inferior.

  The hot-worked steel for case hardening of the present invention is excellent in cold forgeability, and can stably prevent coarsening of austenite grains during carburizing after cold forging. It can be suitably used as a material for carburized and hardened parts formed by forging.

Claims (2)

% By mass
C: 0.10 to 0.30%,
Si: 0.50% or less,
Mn: 0.15 to 1.5%,
P: 0.04% or less,
S: 0.005 to 0.07%,
Cr: 1.5 to 3.0%,
Al: 0.01 to 0.05%,
N: 0.0035 to 0.010%,
Ti: 0.005 to 0.10%,
Nb: 0.02-0.07% and B: 0.0005-0.0050%
And the balance is a chemical composition consisting of Fe and impurities,
It is a metal structure having an area ratio of bainite of 75 to 90%, and
1.1 ≦ [Nb solid solution amount × bainite area ratio] ≦ 3.0,
A hot-worked steel for case hardening, characterized by satisfying Instead of part of Fe, in mass%,
Containing one or more of Mo: 0.50% or less and V: 0.20% or less,
The hot-worked steel for case hardening according to claim 1.

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