JP5858204B2 - Steel material for hot forging, method for producing the same, and method for producing hot forged raw material using the steel material - Google Patents

Description

  The present invention relates to a steel material for hot forging, a method for producing the steel material, and a hot forging material using the steel material.

  In many cases, parts such as gears, pulleys, and shafts of automobiles and industrial machines are manufactured by rough forming by hot forging or cold forging, then cutting, and then surface hardening by carburizing and quenching. However, when the austenite grains before quenching are coarsened by heating during the carburizing process (hereinafter referred to as “carburizing heating” for the sake of simplicity), fatigue strength decreases, and heat treatment distortion during quenching increases. Problems are likely to occur.

  In this way, when austenite grains become coarse, in carburized parts such as gears, fatigue strength as parts cannot be ensured, or noise and vibration due to increased heat treatment strain may be caused, resulting in deterioration of part characteristics. cause. For this reason, steel materials that do not generate coarse grains in carburized parts are strongly demanded.

  In general, in hot forging, in order to form precisely, a material (forging material) is often heated to a high temperature of about 1250 ° C. or more and softened before being processed. However, such high-temperature heating may cause problems such as material loss due to generation of scale, reduction in mold life, and sometimes deterioration in quality. Furthermore, it is difficult to avoid high energy costs.

  Therefore, in recent years, for the purpose of reducing the problems caused by high temperature heating as described above, improving the quality and reliability of carburized parts, reducing costs by saving energy and materials and improving the environment, etc. Attempts have been made to perform hot forging in a temperature range lower than conventional at a heating temperature of 900 to 1100 ° C.

  On the other hand, compared to cold forged parts, hot forged parts have been thought to be difficult to coarsen austenite grains at the time of carburizing heating, but are often hot forged in various temperature ranges as described above, The number of hot forged parts in which austenite grains become coarse during carburizing heating is increasing.

  Therefore, there is a strong demand for a steel for hot forging that can stably prevent coarsening of austenite grains during carburizing heating even when hot forging in various temperature ranges. For example, the following technologies are disclosed: Has been.

  JP-A-2004-59969 discloses, in mass%, C: 0.10 to 0.40%, Si: 0.01 to 1.0%, Mn: 0.20 to 2.0%, Al: 0.00. 015 to 0.050%, N: 0.010 to 0.025%, Al / N: 1.0 to 3.0, and further selected from Ni, Cr and Mo as necessary A carburizing steel material that suppresses grain coarsening, comprising the above, comprising the balance Fe and inevitable impurities, and satisfying the bainite index ≦ 0.2 or the bainite index ≧ 0.8 in a state before carburizing. It is disclosed.

  JP-A-2006-257482 contains, in mass%, Al: 0.010-0.060%, Nb: 0.030-0.070%, N: 0.010-0.030%, Further, if necessary, a steel material that is rolled at a heating temperature of 900 to 1100 ° C. and a finishing temperature of 800 to 950 ° C. after casting a steel containing C, Si, Mn, and Cr and the balance Fe and inevitable impurities. A forged member excellent in crystal grain coarsening prevention characteristics during carburizing, characterized by using parts forging.

JP-A-2007-162128 discloses, in mass%, C: 0.05 to 0.30%, Si: 2.0% or less (excluding 0%), Mn: 1.0% or less (0% Not included), P: 0.03% or less (including 0%), S: 0.03% or less (including 0%), Cr: 2.0% or less (not including 0%), Al: 0 0.1% or less (not including 0%), Nb: 0.05 to 0.30%, Ti: 0.05 to 0.10%, N: 0.0080% or less (not including 0%), O : Satisfying 0.0020% or less (including 0%), and further including one or more selected from Cu, Ni, Mo, B, Ca, Pb, Bi, V, Zr and W as necessary, The balance is made of iron and inevitable impurities, and the maximum particle size of the composite nitride containing Nb and Ti in the steel is 20 μm or less, and the particle size is 1 μm. There is disclosed a case-hardening steel excellent in forgeability and crystal grain coarsening prevention characteristics, characterized in that the average number of nitrides of m or more and 20 μm or less is 50 or less in 1 mm ² .

In JP-A-2007-321211, the components in steel are mass%, C: 0.10 to 0.30%, Si: 0.01 to 1.0%, Mn: 0.2 to 2.0%. , P: 0.03% or less, S: 0.2% or less, Al: 0.01-0.10%, N: 0.003-0.030%, Nb and / or Ti: 0.010-0 .20%, further including one or more selected from Cr, Mo, Ni, Cu, B, Pb, Bi, Mg, Ca, Te, Zr, Hf, and V, if necessary, the balance: Fe and inevitable It is an impurity, and in ferrite, (1) average particle size of precipitates (nm) / area ratio of precipitates <5.0 × 10 ⁴ (nm), (2) average particle size of precipitates: 6 nm or more, (3) Density of precipitates: 20 pieces / μm ² or more and (4) (Nb amount and / or Ti amount in precipitates) / (Nb amount in steel) And / or Ti content): Nb and / or Ti-containing precipitates satisfying the requirement of 95% or more, and the Nb and / or Ti-containing precipitates are Nb carbide, Nb carbonitride, A hot-rolled material excellent in prevention of grain coarsening during high-temperature carburization, which is at least one selected from the group consisting of Ti carbide, Nb-Ti composite carbide, and Nb-Ti composite carbonitride Is disclosed.

  JP, 2013-234354, C: 0.1-0.25%, Si: 0.01-0.5%, Mn: 0.5-1.5%, S: 0.003-0. 05%, Cr: 0.7-2.0%, Mo: 0.4% or less (including 0%), Al: 0.02-0.10% and N: 0.004-0.025%, And, if necessary, one or more selected from Cu, Ni, B, Ti and Nb, the balance being Fe and impurities, and P and O in the impurities being P: 0. It has a chemical composition of 025% or less and O: 0.002% or less, and the structure is composed of ferrite pearlite or ferrite pearlite bainite, and the ferrite fraction is 55 to 80% and the pearlite fraction is 20 to 35. %, Bainite fraction is 10% or less, and 0.50 ≦ 100 / (100-ferrite fraction)] × C ≦ 0.80, and the total concentration of Mn, Cr and Mo in cementite is 2.8 to 4.5%. A hot rolled steel bar or wire rod is disclosed.

JP 2004-59969 A JP 2006-257482 A JP 2007-162128 A JP 2007-321211 A JP 2013-234354 A

  The technique disclosed in Japanese Patent Application Laid-Open No. 2004-59969 cannot always stably prevent austenite grain coarsening during carburizing heating when hot forged in various temperature ranges. That is, the carburizing steel material disclosed in Japanese Patent Application Laid-Open No. 2004-59969 is defined in a state before carburizing heating, in other words, an Al / N balance and a bainite index in a state after hot working. However, it is not the state of the steel material before heating in hot working. And in this Unexamined-Japanese-Patent No. 2004-59969, the round bar of (phi) 30 used as the raw material of the test piece of (phi) 8x12mm used for the hot upsetting (hot forging) before performing a carburizing simulation test in an Example. As a specific method of hot working, it is only described that the heating temperature is 1200 ° C. In other words, there is no description of the microstructure before heating at the time of hot upsetting, and the case where hot forging is performed in a temperature range lower than the conventional temperature range of 900 to 1100 ° C. has not been studied. For this reason, as shown in Table 2 of Japanese Patent Application Laid-Open No. 2004-59969, for example, after the steel materials A to D are heated to 1200 ° C., they are hot upset (hot forging) at the same temperature of 1000 ° C. Even in the case of the same steel material A, it is not possible to achieve stable prevention of coarsening of crystal grains (austenite grains) in the carburizing simulation test. When installed, austenite grain coarsening during carburization cannot always be prevented stably.

  The technique disclosed in Japanese Patent Application Laid-Open No. 2006-257482 also does not always stably prevent austenite grain coarsening during carburizing heating when hot forged in various temperature ranges. That is, the basic idea of the technique disclosed in Japanese Patent Application Laid-Open No. 2006-257482 is that fine AlN and Nb are effective in preventing the coarsening of crystal grains (austenite grains) by setting the rolling heating temperature of the steel material to 900 to 1100 ° C. Although the (C, N) precipitate is not dissolved in the matrix, there is no specific description of the microstructure after rolling. Generally, if coarse precipitates remain in the matrix without being dissolved in the stage before rolling and heating, the precipitates are likely to be aggregated and further coarsened by heating during hot rolling. For this reason, as stipulated in Japanese Patent Application Laid-Open No. 2006-257482, in the steel matrix obtained by hot rolling at a rolling heating temperature of 900 to 1100 ° C., fine AlN and Nb (C N) Precipitates are not always deposited. Therefore, when the heating temperature at the subsequent forging is 900 to 1100 ° C., it is highly likely that coarse AlN remains in the steel material without being dissolved, and the austenite grain coarseness during carburization is not necessarily high. Cannot be stably prevented.

The technique disclosed in Japanese Patent Application Laid-Open No. 2007-162128 also does not always stably prevent austenite grain coarsening during carburizing heating when hot forged in various temperature ranges. That is, the basic idea of the technique disclosed in Japanese Patent Application Laid-Open No. 2007-162128 is to specify the form and number of composite nitrides containing Nb and Ti in the steel material, and in addition, optimization of Ti and Nb To produce a larger amount of precipitates than before. However, the N content is 0.0080% or less. The maximum amount of nitride precipitated in the steel material is limited by the N content. Therefore, as specified in Japanese Patent Application Laid-Open No. 2007-162128, N: 0.0080% or less, and the particle size of the composite nitride containing Nb and Ti in the steel is 1 μm or more and 20 μm or less When the average number of nitrides is limited to 50 or less in 1 mm ² , austenite grain coarsening during carburization cannot be prevented after hot forging in various temperature ranges.

  The technique disclosed in Japanese Patent Application Laid-Open No. 2007-321211 also does not always stably prevent austenite grain coarsening during carburizing heating when hot forged in various temperature ranges. That is, in the hot-rolled material disclosed in Japanese Patent Application Laid-Open No. 2007-321211, the size, area ratio, density, and precipitation amount of Nb and / or Ti-containing precipitates are defined, but the microstructure is studied. Not. The non-uniformity of the microstructure of the forging material produced by hot working continues as a tendency even after hot forging. Therefore, for example, when bainite is mixed in the microstructure, austenite grain coarsening during carburization cannot be prevented. In addition, workability such as shear cutting before hot forging deteriorates with increasing hardness.

  The present invention has been made in view of the above situation, and its purpose is to stably prevent coarsening of austenite grains during carburizing heating even when hot forging in various temperature ranges. It is to provide a hot forging steel material, a manufacturing method thereof, and a hot forging raw material using the steel material. More specifically, the object of the present invention is to heat at a temperature of 950 ° C. or less, especially at a temperature of 950 ° C. or less, even when hot forging after heating at 900 to 1250 ° C. Steel for hot forging that can stably prevent coarsening of austenite grains and is suitable as a material for parts such as gears that are coarsely formed by hot forging and pulleys for automobiles such as CVT pulleys. And a method for producing the same, and a hot forging material using the steel.

In the present invention, it is assumed that the austenite grains are coarsened when there are two or more austenite crystal grains having a grain size number of 5 or less as defined in JIS G 0551 (2013) within an area of 10 mm ² .

  In order to solve the above-described problems, the present inventors have made various studies. As a result, the following findings (a) to (f) were obtained.

  (A) In the case of rough forming by hot forging, if the material (forging material) is heated to a high temperature of about 1250 ° C. or more, which is a general hot forging heating temperature, regardless of the state before heating for hot forging. In addition, there is no AlN that remains without dissolving in the matrix (insoluble AlN). In other words, since AlN is dissolved in the matrix, AlN is finely dispersed and precipitated when carburized and heated, and has a pinning effect. Demonstrated. For this reason, in a normal carburizing temperature range of about 920 to 950 ° C., austenite grains are difficult to coarsen. In the following description, assuming that the forging material is produced by rolling steel bars, the above “before heating for hot forging” is referred to as “after rolling steel bars”, for example, “before heating for hot forging” "Stage of stage" is sometimes referred to as "stage after steel bar rolling".

  (B) However, when performing rough forming by hot forging, the heating temperature of the forging material is set to 900 to 1100 ° C., and when hot forging is performed in a temperature range lower than conventional, coarse AlN at the stage after the steel bar rolling. May remain undissolved in the matrix. When hot forging is performed in such a state, the remaining AlN aggregates during heating before hot forging, and the AlN tends to be further coarsened. As a result, since the pinning effect of AlN becomes extremely insufficient during carburizing heating, it is difficult to avoid the coarsening of the austenite grains even in the carburizing temperature range of about 920 to 950 ° C.

(C) Therefore, in order to suppress the coarsening of austenite grains during carburizing heating even if hot forging in various temperature ranges, the stage before heating of hot forging, that is, the stage after steel bar rolling. Therefore, it is necessary to build it in as a steel material suitable for carburizing applications. Although it is necessary to prevent the coarsening of austenite grains during carburizing heating, it is necessary to disperse a large amount and finely of pinning particles of AlN at the stage after rolling the steel bar, but the Al and N contained in the steel Depending on the amount, the maximum precipitation amount of AlN varies. For this reason, it is necessary to limit the content of Al and N to a specific range to precipitate an appropriate amount of AlN in the steel and to control the precipitation / dispersion state of AlN in the steel. Specifically, the element symbol in the formula is the content in mass% of the element, and the value of E represented by the following formula (i) or formula (ii) is in the range of 0.012 to 0.022. Further, the coarsening of the austenite grains can be stably prevented by controlling the size and dispersion state of AlN.
In the case of Al / N ≧ 1.93: E = 0.965 × N (i),
In the case of Al / N <1.93: E = 0.5 × Al (ii).

  (D) In the slab after continuous casting with a large cross section, which is general as a mass production process, coarse AlN is generated, and if this remains in the stage after steel bar rolling, austenite grains during carburizing heating Is prone to coarsening. Therefore, in order to stably suppress the coarsening of austenite grains during carburizing heating, it is preferable to heat the cast slab, that is, the heating during the block rolling, to 1250 ° C. or higher, and once dissolve AlN in the steel. .

(E) In order to finely disperse AlN at the stage after rolling the steel bar and stably prevent the austenite grains from coarsening during carburizing heating, the steel slab heating temperature before rolling the steel bar is set to the following (iii) It is preferable to set it to T ° C. or less represented by the formula.
T = −7397 / [log {(E1 ² −E2 ² ) /1.93} −1.71] −273 (iii).
However, in the formula (iii)
E1 = (0.5 × Al + 0.965 × N−E) (iv),
E2 = (0.5 × Al−0.965 × N) (v)
E in the formula (iv) is
In the case of Al / N ≧ 1.93: E = 0.965 × N (i),
In the case of Al / N <1.93: E = 0.5 × Al (ii).
In addition, the element symbol in each formula represents content in the mass% of the element.

  (F) If the slab heating temperature before rolling the steel bar is set to T ° C. or lower as described in the section (e), not only AlN is finely dispersed but also the structure after the steel bar rolling is easily mixed with bainite. It becomes a mixed structure of ferrite and pearlite. This structure is relatively soft and is useful for securing workability such as shear cutting later.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in the following steel materials for hot forging, a method for producing the same, and a hot forging shaped material using the steel.

(1) By mass%, C: 0.1 to 0.3%, Si: 0.16 to 0.50%, Mn: 0.3 to 1.0%, S: 0.030% or less, Cr: 0.8 to 1.8%, Al: 0.02 to 0.060 %, N: 0.010 to 0.0250 %, Cu: 0 to 0.50% and Ni: 0 to 0.50%,
The balance consists of Fe and impurities,
P, Ti and O in the impurities are respectively P: 0.020% or less, Ti: 0.005% or less and O: 0.0020% or less,
Furthermore, the steel material for hot forging having a chemical composition in which the value of E represented by the following formula (i) or formula (ii) is in the range of 0.012 to 0.022,
In the microstructure in the cross section perpendicular to the rolling direction, the matrix is composed of bainite having an area ratio of 0 to 10%, ferrite, and pearlite, and in an area of 1 μm ² , AlN having an equivalent circle diameter of 10 nm or more. Among them, a steel material for hot forging in which the equivalent circle diameter is 10 to 100 nm is 85% or more.
In the case of Al / N ≧ 1.93: E = 0.965 × N (i),
In the case of Al / N <1.93: E = 0.5 × Al (ii).
The element symbols in the above formulas (i) and (ii) mean the content in mass% of the element.

  (2) The chemical composition described above in (1), wherein the chemical composition contains one or more selected from Cu: 0.05 to 0.50% and Ni: 0.05 to 0.50% by mass%. Steel for hot forging.

(3) A slab or steel ingot obtained by casting the steel having the chemical composition described in (1) or (2) above is heated at a temperature of 1250 ° C. or higher for 50 minutes or more and then subjected to block rolling. Then, the steel slab was heated at a temperature of T ° C. or lower and 900 ° C. or higher represented by the following formula (iii) for 60 minutes or longer, and then hot-rolled at a finishing temperature of 800 ° C. or higher, A step of cooling to 600 ° C. at a cooling rate of 2.0 ° C./second or less after the end of rolling ,
In the microstructure in the cross section perpendicular to the rolling direction, the matrix is composed of bainite having an area ratio of 0 to 10%, ferrite, and pearlite, and in an area of 1 μm ² , AlN having an equivalent circle diameter of 10 nm or more. Among them, a method for producing a steel material for hot forging in which a steel material for hot forging having an equivalent circle diameter of 10 to 100 nm is 85% or more and 96% or less .
T = −7397 / [log {(E1 ² −E2 ² ) /1.93} −1.71] −273 (iii).
However, in the formula (iii)
E1 = (0.5 × Al + 0.965 × N−E) (iv),
E2 = (0.5 × Al−0.965 × N) (v)
E in the formula (iv) is
In the case of Al / N ≧ 1.93: E = 0.965 × N (i),
In the case of Al / N <1.93: E = 0.5 × Al (ii).
In addition, the element symbol in each formula represents content in the mass% of the element.

(4) the hot forging steel according to (1) or (2), after heating at 900-1,250 ° C., forging at a temperature range of 900 to 1100 ° C., the production of hot forged and fabricated material Way .

  The “impurities” in the remaining “Fe and impurities” refer to those mixed from ore as a raw material, scrap, or the manufacturing environment when the steel material is industrially produced.

  The steel material for hot forging according to the present invention is not limited to 3 at a temperature of 950 ° C. or less, especially during carburizing heating, even when hot forging after heating in various temperature ranges, particularly in a wide temperature range of 900 to 1250 ° C. Since austenite grains can be prevented from coarsening when heated for a long time, they can be suitably used as materials for parts such as gears that are roughly formed by hot forging and automobile transmissions such as CVT pulleys. it can.

  Moreover, the steel for hot forging according to the present invention can be easily obtained by the production method of the present invention.

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

(A) Chemical composition:
C: 0.1 to 0.3%
C is an essential element for securing the core strength of the parts when carburized and quenched, and the above effect is insufficient when the content is less than 0.1%. On the other hand, when the content of C increases, the machinability after hot forging decreases significantly. Therefore, the content of C is set to 0.1 to 0.3%. The C content is preferably 0.18% or more and 0.25% or less.

Si: 0.16-0.50%
Si has an action of improving hardenability and a deoxidizing action. Si is an element that has a large effect of increasing the temper softening resistance and also has an effect of improving fatigue strength. If the Si content is less than 0.16%, the above effects are insufficient. However, since Si is an oxidizing element, when its content increases, Si is selectively oxidized by a small amount of H ₂ O or CO ₂ contained in the carburizing gas, and Si oxide is generated on the steel surface. Therefore, the depth of the carburizing abnormal layer becomes large. And if the depth of a carburizing abnormal layer becomes large, the fall of fatigue strength will be caused. Furthermore, when the Si content increases, the machinability after hot forging decreases significantly. Therefore, the Si content is set to 0.16 to 0.50%. In addition, it is preferable that content of Si is 0.17% or more and 0.35% or less.

Mn: 0.3 to 1.0%
Mn is an element that has a large effect of improving hardenability and resistance to temper softening and also has an effect of improving fatigue strength. However, if the Mn content is less than 0.3%, the above effect is insufficient. On the other hand, when the content of Mn increases, not only the effect of increasing the fatigue strength is saturated, but also the machinability after hot forging becomes remarkable. Therefore, the Mn content is set to 0.3 to 1.0%. In addition, it is preferable that content of Mn is 0.5% or more and 0.95% or less.

S: 0.030% or less S is contained as an impurity. Further, when S is positively contained, it combines with Mn to form MnS, and is an element that improves machinability. However, when the S content is increased, coarse MnS is easily generated, and hot forgeability and fatigue strength are reduced. Therefore, the content of S is set to 0.030% or less. The S content is preferably 0.025% or less.

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

Cr: 0.8 to 1.8%
Cr is an element that has a large effect of improving hardenability and resistance to temper softening and also has an effect of improving fatigue strength. If the content is less than 0.8%, the above effect is insufficient. On the other hand, when the content of Cr increases, not only the effect of increasing the fatigue strength is saturated, but also the machinability after hot forging decreases significantly. Moreover, since Cr is an oxidizing element like Si, when the content is increased, the depth of the carburized abnormal layer becomes deep, and the bending fatigue strength is reduced. Therefore, the Cr content is set to 0.8 to 1.8%. The Cr content is preferably 0.9% or more and 1.7% or less.

Al: 0.02 to 0.06%
Al is an element effective for preventing austenite grain coarsening at the time of carburizing heating because Al has a deoxidizing action and is easily combined with N in steel to precipitate as AlN. However, if the Al content is less than 0.02%, the effect is not sufficient. On the other hand, when the Al content is excessive, AlN is likely to aggregate, so that fine precipitates effective in preventing austenite grain coarsening are reduced, and heating at a temperature of 950 ° C. or lower, which will be described later, for 3 hours. In this case, the austenite grain coarsening prevention effect targeted by the present invention, that the austenite grains do not become coarse, is not obtained. In addition, if the Al content is large, the machinability is lowered due to the formation of hard and coarse Al ₂ O ₃ and the fatigue strength is also lowered. Therefore, the content of Al is set to 0.02 to 0.06%. The Al content is preferably 0.025% or more and 0.05% or less.

N: 0.010 to 0.025%
N is an element necessary for bonding to Al and precipitating AlN, and has an effect of preventing austenite grain coarsening during carburizing heating. However, if the N content is less than 0.010%, the effect is not sufficient. On the other hand, when the content of N is excessive, AlN tends to aggregate, so that fine precipitates effective in preventing austenite grain coarsening are reduced, and the above-described target austenite grain coarsening prevention is achieved. The effect is not obtained. Therefore, the N content is set to 0.010 to 0.025%. In addition, it is preferable that content of N is 0.012% or more and 0.023% or less.

Cu: 0 to 0.50%
Cu has the effect of increasing the hardenability and is an effective element for increasing the fatigue strength. Therefore, Cu may be contained as necessary. However, when the Cu content is increased, not only the effect of increasing the fatigue strength by improving the hardenability is saturated, but also the machinability after hot forging becomes remarkable. Therefore, the amount of Cu in the case of containing is 0.50% or less. In addition, it is preferable that the quantity of Cu in the case of making it contain is 0.30% or less.

  On the other hand, in order to stably obtain the effects of Cu described above, the amount of Cu in the case of inclusion is preferably 0.05% or more.

Ni: 0 to 0.50%
Ni has an effect of increasing the hardenability and is an element effective for increasing the fatigue strength. Therefore, Ni may be contained as necessary. However, when the content of Ni increases, not only the effect of increasing the fatigue strength by improving the hardenability is saturated, but also the machinability after hot forging becomes remarkable. Therefore, the amount of Ni in the case of inclusion is set to 0.50% or less. In addition, when Ni is contained, the amount of Ni is preferably 0.30% or less.

  On the other hand, in order to stably obtain the above-described effect of Ni, the amount of Ni when contained is preferably 0.05% or more.

  Said Cu and Ni can be contained only in any 1 type or 2 types of composite. In addition, it is preferable that the total amount of these elements in the case where two types are combined is 0.8% or less.

  The steel for hot forging of the present invention is composed of the above-described elements from C to Ni, the balance being Fe and impurities, and P, Ti and O in the impurities are respectively P: 0.020% or less, Ti: 0.005% or less and O: 0.0020% or less, and a chemical composition in which the value of E represented by the above formula (i) or (ii) is in the range of 0.012 to 0.022. It is what you have. As described above, “impurities” in “Fe and impurities” refers to those mixed from ores, scraps, or production environments as raw materials when industrially producing steel materials.

  Hereinafter, P, Ti, and O in the impurities will be described.

P: 0.020% or less P is an impurity contained in the steel, segregates at the grain boundaries, embrittles the steel, and reduces fatigue strength. Therefore, the content of P in the impurities is set to 0.020% or less. In addition, it is preferable that content of P in an impurity shall be 0.015% or less.

Ti: 0.005% or less Ti combines with N to easily form TiN, which is a hard and coarse non-metallic inclusion, and reduces fatigue strength. Furthermore, hot forgeability and machinability are also reduced. Therefore, the content of Ti in the impurities is set to 0.005% or less. Note that the content of Ti as an impurity element is preferably 0.003% or less.

O: 0.0020% or less O (oxygen) is easy to form Al ₂ O ₃ which is a hard oxide inclusion by combining with Al in steel, lowers machinability, and lowers fatigue strength. Also invite. Therefore, the content of O in the impurities is set to 0.0020% or less. Note that the content of O as an impurity element is preferably 0.0015% or less.

0.012 ≦ E ≦ 0.022
The steel for hot forging according to the present invention is
When Al / N ≧ 1.93: E = 0.965 × N (i)
Or
In the case of Al / N <1.93: E = 0.5 × Al (ii)
The value of E represented by the formula
0.012 ≦ E ≦ 0.022
The chemical composition must satisfy This will be described below. In addition, as already stated, the element symbol in said (i) Formula and (ii) Formula means content in the mass% of the element.

The above E is an appropriate amount of Al precipitated as AlN effective in preventing austenite grain coarsening. Al and N combine at a 1: 1 atomic ratio to form AlN. The atomic weight of Al is 26.98, and the atomic weight of N is 14. Here, the maximum precipitation amount of Al that can be precipitated as AlN (hereinafter referred to as Alm) can be obtained from the stoichiometric ratio by the following equation.
When Al is excessive compared to N, that is, when Al / N ≧ 26.98 / 14 = 1.93: Alm = (Al atomic weight ÷ N atomic weight) × N
That is, the AlN that can be precipitated is limited by the amount of N contained in the steel.
On the other hand, when N is excessive as compared with Al, that is, when Al / N <1.93: Alm = Al
That is, the precipitated AlN is limited by the amount of Al contained in the steel.

  On the other hand, as described above, since the steel for hot forging undergoes heating processes such as rolling, forging, and carburizing, AlN is dissolved in the steel in an amount corresponding to the heating temperature. Therefore, in order to reliably precipitate AlN necessary for preventing coarsening of austenite grains in the steel, even if the Al and N contents are dissolved at the time of carburizing heating, a sufficient amount of AlN is still precipitated. It is necessary to control to an appropriate value.

Therefore, the present inventors have investigated and studied the amount of Al precipitated as AlN effective in preventing the austenite grains from coarsening during carburizing in the steel for hot forging. As a result, the content of Al and N in the steel is controlled, and by making the amount (E) half of the maximum precipitation amount of Al that can be precipitated as AlN an appropriate value, it is effective in preventing austenite grain coarsening. I found out. Based on this idea, E was obtained by the following equation.
In the case of Al / N ≧ 1.93: E = 0.5 × Alm = 0.965 × N (i)
Or
In the case of Al / N <1.93: E = 0.5 × Alm = 0.5 × Al (ii)
Here, in order to stably suppress the coarsening of austenite grains, it is necessary to disperse AlN in a large amount and finely, and a larger amount is preferable for pinning the crystal grain boundaries.

  When the value of E is small and less than 0.012, the amount of AlN that precipitates in the steel and is effective for pinning is reduced, so that the coarsening of austenite grains is insufficient. On the other hand, if the value of E is too large and exceeds 0.022, the amount of AlN that can be precipitated in the steel becomes too large, so that AlN tends to aggregate and coarsen, and the pinning effect is reduced. The prevention of coarsening of austenite grains becomes insufficient. Therefore, the coarsening of the austenite grain can be stably prevented by setting the value of E in the range of 0.012 to 0.022.

(B) Microstructure:
In the steel for hot forging of the present invention having the chemical composition described in the previous section, the matrix has an area ratio of 0 to 0 in a microstructure in a cross section perpendicular to the rolling direction (hereinafter sometimes referred to as “cross section”). It is composed of 10% bainite, ferrite and pearlite, and in an area of 1 μm ² , AlN having an equivalent circle diameter of 10 nm or more must be 85% or more if the equivalent circle diameter is 10 to 100 nm. Don't be.

  The non-uniformity of the microstructure of the forging material produced by hot working continues as a trend even after hot forging for rough forming into parts such as gears.

For this reason, it is necessary to make the microstructure in a cross section appropriate. And in the microstructure in the cross section, the matrix is composed of bainite having an area ratio of 0 to 10%, ferrite, and pearlite, and in an area of 1 μm ² , AlN having an equivalent circle diameter of 10 nm or more. Among them, when the equivalent circle diameter of 10 to 100 nm is 85% or more, austenite grains can be prevented from coarsening during carburizing heating.

  When martensite is included in the matrix, due to the fact that martensite is hard and has low ductility, for example, cracks are likely to occur during the transport of hot forging steel, and shear cutting before hot forging, etc. The workability of is deteriorated.

  In addition, when bainite is mixed into the matrix, it causes coarse austenite grains during carburizing heating, and the workability such as shear cutting before hot forging deteriorates as the hardness increases. If the ratio exceeds 10%, the tendency becomes remarkable.

In addition, AlN having an equivalent circle diameter of less than 10 nm easily dissolves and disappears during carburizing and heating, so that the effect as pinning particles is extremely small. On the other hand, coarse AlN has a small effect as a pinning particle at the grain boundary and tends to grow and become coarse during carburizing heating. In particular, AlN having a circle equivalent diameter exceeding 100 nm grows during carburizing heating.・ Since it becomes coarse, its effect as a pinning particle is small. And, in the area of 1 μm ² in the cross section, when AlN having an equivalent circle diameter of 10 nm or more is 85% or more with an equivalent circle diameter of 10 to 100 nm, the austenite grains are stable and coarse during carburizing heating. Can be prevented. Of the AlN having an equivalent circle diameter of 10 nm or more in the cross-sectional area of 1 μm ² , those having an equivalent circle diameter of 10 to 100 nm are preferably 90% or more, and extremely preferably 100%.

  The “phase” of the matrix in the microstructure is, for example, a test piece that is perpendicular to the rolling direction (longitudinal direction) of the steel for hot forging and that includes a central portion and then mirror-polished and corroded with nital. Can be identified by observing 5 visual fields at random, excluding the surface, at a magnification of 400 times and a visual field size of 250 μm × 250 μm. Moreover, the area ratio of bainite can be calculated | required as an arithmetic average value of five visual fields by calculating the area ratio of bainite in each visual field using image analysis software from the obtained microstructure photograph, for example.

Moreover, in the area 1 μm ² of the cross section (transverse section) perpendicular to the rolling direction, the ratio of the equivalent circle diameter of 10 to 100 nm in AlN having an equivalent circle diameter of 10 nm or more is An extraction replica sample is prepared by a conventional method, and an EDS is used using a transmission electron microscope (hereinafter referred to as “TEM”) equipped with an energy dispersive X-ray detector (hereinafter referred to as “EDS”). The content and shape of Al contained in the precipitates were confirmed by elemental analysis, and the magnification was 30000 times, the area per field of view was 6.4 μm ^2, and 5 fields were randomly observed except for the surface, and the equivalent circle diameter However, the number of AlN having a thickness of 10 to 100 nm and the number of AlN having a thickness of 100 nm or more can be counted and converted into a ratio per area of 1 μm ² based on this. At this time, the size of each AlN may be converted into a circle equivalent diameter by image analysis. However, in the pearlite and bainite regions, since cementite is extracted in a large amount and it is difficult to measure the size and number of AlN, observation is preferably performed in a field where ferrite accounts for 2/3 or more in area ratio.

(C) Manufacturing method:
The steel for hot forging of the present invention is obtained by, for example, heating a slab or a steel ingot obtained by casting steel having the chemical composition described in the above section (A) at a temperature of 1250 ° C. or more for 50 minutes or more. From the above, the steel slab is heated to a temperature of T ° C. or lower and 900 ° C. or higher represented by the following formula (iii) for 60 minutes or longer, and then heated to a finish temperature of 800 ° C. or higher. It can manufacture by carrying out hot rolling and cooling to 600 degreeC with the cooling rate of 2.0 degrees C / second or less after completion | finish of hot rolling.
T = −7397 / [log {(E1 ² −E2 ² ) /1.93} −1.71] −273 (iii).
However, in the formula (iii)
E1 = (0.5 × Al + 0.965 × N−E) (iv),
E2 = (0.5 × Al−0.965 × N) (v)
E in the formula (iv) is
In the case of Al / N ≧ 1.93: E = 0.965 × N (i),
In the case of Al / N <1.93: E = 0.5 × Al (ii). In addition, the element symbol in each formula represents content in the mass% of the element.

  By heating the obtained slab or steel ingot at a temperature of 1250 ° C. or more for 50 minutes or longer, coarse AlN produced in the slab or steel ingot can be dissolved in the steel. In the steel slab after ingot rolling, AlN is likely to precipitate finely during the heating process in the subsequent hot rolling such as bar rolling. The slab or steel ingot is heated more preferably at a temperature of 1250 to 1300 ° C. for 240 minutes or longer. However, if the heating is performed for an extremely long time, the manufacturing cost is increased, so the upper limit is 600 minutes or less. It is preferable that Further, the cooling after the partial rolling may be performed by an appropriate method such as slow cooling or cooling in the air (hereinafter simply referred to as “cooling”).

  In addition, by controlling the heating temperature of the steel slab obtained by split rolling under the above conditions by heating at the time of hot rolling such as bar rolling, the temperature is controlled to T ° C. or less represented by the above formula (iii), It becomes possible to disperse AlN in steel in a large amount and finely, it is possible to stably prevent coarsening of austenite grains during carburizing heating, stable hot rolling structure, shear cutting, etc. Therefore, it is possible to control the mixed structure of ferrite and pearlite, which is beneficial for ensuring the workability of the steel and contains less bainite. When the heating temperature exceeds T ° C., AlN starts to dissolve, but the AlN remaining without being dissolved does agglomeration and coarsening by being held at the heating temperature. A relatively small AlN having a circle-equivalent diameter of 10 to 100 nm decreases by solid solution, or aggregates and coarsens to become a large AlN of 100 nm. However, if the heating temperature is too low, the deformation resistance at the time of rolling increases, which hinders the production. Therefore, the lower limit of the heating temperature may be 900 ° C, and more preferably 1000 ° C. If the heating time of the steel slab is 60 minutes or more, the temperature distribution does not occur from the surface to the center of the steel slab, but if the heating time is too long, the finely dispersed AlN aggregates and coarsens as pinning particles. The effect becomes smaller. For this reason, it is preferable that the heating time of the steel slab is 240 minutes or less.

  Further, in addition to the heating temperature of the steel slab, if the finishing temperature during hot rolling is 800 ° C. or less, the deformation resistance during rolling may increase, which may hinder manufacturing, so the lower limit of the finishing temperature is 800 It is better to exceed ℃.

And after finishing the hot rolling at the above finishing temperature, by cooling to 600 ° C. at a cooling rate of 2.0 ° C./second or less, in the microstructure in the cross section (transverse cross section) perpendicular to the rolling direction, the matrix , Composed of bainite having an area ratio of 0 to 10%, ferrite and pearlite, and in an area of 1 μm ² , AlN having an equivalent circle diameter of 10 nm or more and having an equivalent circle diameter of 10 to 100 nm A steel material of 85% or more can be obtained.

  The cooling rate after completion of hot rolling is preferably 1.5 ° C./second or less, but since extreme slow cooling leads to an increase in production cost, the lower limit is about 0.1 ° C./second. Preferably there is. In addition, after completion | finish of hot rolling, it is not necessary to cool to room temperature with the above-mentioned cooling rate, and when reaching 600 degreeC, you may cool by appropriate means, such as standing_to_cool, mist cooling, and water cooling.

  The heating temperature in this specification means the average value of the furnace temperature of the heating furnace, and the heating time means the in-furnace time.

  Moreover, the finishing temperature of hot rolling refers to the surface temperature of the steel material immediately after finishing rolling. Furthermore, the cooling rate after completion of hot rolling also refers to the cooling rate of the steel surface.

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

  Steels A to L having the chemical composition shown in Table 1 were melted by a converter to produce a slab.

  Specifically, after adjusting the components in a 70-ton converter, continuous casting was performed to produce a 400 mm × 300 mm square slab (bloom).

  In addition, all of steels A to F in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels G and H are steels of comparative examples in which the content of each element and the value of E represented by formula (i) or (ii) deviate from the conditions defined in the present invention. ~ L is a comparative example in which the value of E represented by formula (i) or (ii) deviates from the conditions defined by the present invention, although the content of each element is within the range defined by the present invention. It is steel.

  The slab produced as described above was heated to 1250 ° C. and then rolled into pieces to produce a 180 mm × 180 mm square steel slab and cooled to room temperature. Furthermore, after heating the above 180 mm × 180 mm square steel pieces, hot rolling was performed to obtain a steel bar having a radius R of 20 mm.

  Table 2 shows the manufacturing conditions I to V. When finishing a 400 mm × 300 mm slab into a steel bar having a radius R of 20 mm, the slab heating condition, the cooling condition after the block rolling, the slab heating condition, the steel bar The detail of the finishing temperature of rolling is shown. After the end of rolling, it was allowed to cool. At that time, the surface temperature of the steel material was measured with a radiation thermometer, and the cooling rate from the end of the steel bar rolling to 600 ° C. was obtained. The values are also shown in Table 2.

  The following tests were performed on each steel bar having a radius R of 20 mm obtained as described above.

Investigation 1: Observation of the matrix phase in the microstructure From each steel bar, the resin was buried so that the cross section perpendicular to the rolling direction (longitudinal direction), that is, the cross section was the test surface, mirror-polished and corroded with nital. For the test piece, 5 fields were randomly observed except for the surface at a magnification of 400 times to identify “tissue”. The size of each field of view was 250 μm × 250 μm. Moreover, the area ratio of bainite in each visual field was calculated from the obtained microstructure photograph using image analysis software, and the arithmetic average value of five visual fields was defined as the area ratio of bainite.

Investigation 2: Vickers hardness test The above resin-filled test piece is mirror-polished again, and the Vickers hardness (HV) of 5 points in total, 1 point at the center of the surface to be tested and 4 points at R / 2, is measured according to JIS Z. In accordance with “Vickers Hardness Test—Test Method” described in 2244 (2009), the test force was measured with a Vickers hardness tester at 9.8 N. The arithmetic average value of the above five points was HV.

Investigation 3: Investigation of AlN Size and Dispersion State in Microstructure Extracted replica samples were prepared by a general method from a portion excluding the cross section surface of each steel bar, and observed by TEM. A TEM equipped with EDS was used, and the Al content and shape of the precipitate were confirmed by elemental analysis by EDS. The magnification is 30000 times, the area per field of view is 6.4 μm ² , and each of the 5 fields of view is randomly observed, and the number of AlN having an equivalent circle diameter of 10 to 100 nm and AlN of 100 nm or more is counted. Based on this, among AlN having an equivalent circle diameter of 10 nm or more, a ratio per 1 μm ² of AlN having an equivalent circle diameter of 10 to 100 nm was determined. At this time, the dimensions of individual AlN were converted to equivalent circle diameters by image analysis. However, in the pearlite and bainite regions, a large amount of cementite was extracted, and it was difficult to measure the size and number of AlN. Therefore, the observation was performed in a field where ferrite accounted for 2/3 or more in area ratio.

Investigation 4: Investigation of occurrence of coarsening of austenite grains A test piece having a length of 60 mm was cut out from each steel bar and heated at 1250 ° C., 1080 ° C. and 900 ° C. for 30 minutes in order to simulate hot forging, Ten seconds after taking out from the furnace, 60% compression processing was performed in the height direction of the columnar shape, and then cooled to room temperature by cooling.

  Next, after cutting each test piece obtained as described above into four equal parts on the surface including the central axis, three of them are used to simulate heating by carburization. After holding at 920 ° C., 950 ° C. and 980 ° C. for 3 hours, the mixture was cooled to room temperature by water cooling. In the following description, the temperature for simulating the heating in the above carburizing is referred to as “pseudo carburizing temperature”.

  The cut surface of each test piece thus obtained was removed by 1 mm in thickness, the surface was mirror-polished, corroded with a saturated aqueous solution of picric acid to which a surfactant was added, and then magnified 100 times using an optical microscope. Then, 10 fields of view were randomly observed except for the surface, and the occurrence of coarsening of austenite grains was investigated.

The size of one field of view in the above investigation is 1.0 mm × 1.0 mm. By this observation, austenite crystal grains having a grain size number of 5 or less as defined in JIS G 0551 (2013) are ² in an area of 10 mm ² . When there were more than one, it was determined that the austenite grains were coarsened.

  The target of the effect of preventing the austenite grain coarsening was to prevent the austenite grains from coarsening when heated at a temperature of 950 ° C. or lower for 3 hours.

  Tables 3 and 4 show the results of the above investigations in order to simulate the heating temperature of the steel slab, T ° C expressed by the formula (iii), the cooling rate up to 600 ° C after the end of the steel bar rolling, and hot forging. Together with the measured temperature. The manufacturing condition numbers in Tables 3 and 4 correspond to the manufacturing condition numbers described in Table 2 above.

  From Tables 3 and 4, the chemical composition is within the range specified by the present invention, and the microstructure in the cross section (of AlN having a phase and equivalent circle diameter of 10 nm or more, and the equivalent circle diameter of 10 to 100 nm. In the case of test numbers 1 to 6 of “examples of the present invention” satisfying the conditions specified in the present invention, the carburization heating simulation is performed even when hot forging is performed by heating to various temperatures of 900 to 1250 ° C. It is clear that no coarse particles are generated at a temperature of 950 ° C., and the effect of preventing austenite grain coarsening is obtained.

  On the other hand, in the case of the “comparative example” that does not satisfy all of the chemical composition and microstructure conditions specified in the present invention, the target austenite grain coarsening prevention characteristic is not obtained.

  In the case of test number 7, since the N content of steel G and the value of E represented by the formula (i) are out of the conditions defined in the present invention, heating to various temperatures of 900 to 1250 ° C. When hot forging, coarse grains are generated at a pseudo carburizing temperature of 920 ° C. at any heating temperature, and the target austenite grain coarsening preventing property is not obtained.

In the case of test number 8, the chemical composition of steel H, that is, the contents of Al, N and O, and the value of E represented by the formula (i) deviate from the conditions defined in the present invention. Since the proportion of the microstructure having an equivalent circle diameter of 10 to 100 nm out of AlN having an equivalent circle diameter of 10 nm or more in an area of 1 μm ² is also outside the conditions specified in the present invention, it is 900 and 1080 ° C. When heated forging and hot forging, coarse grains are generated at a pseudo carburizing temperature of 950 ° C., and the target austenite grain coarsening preventing characteristic is not obtained.

In the case of test number 9 and test number 10, the value of E represented by the formula (i) of steel I and steel J in each test number deviates from the conditions defined in the present invention, and in an area of 1 μm ² Of AlN having an equivalent circle diameter of 10 nm or more, the ratio of the equivalent circle diameter of 10 to 100 nm is also out of the conditions defined in the present invention. Therefore, when heated to 900 and 1080 ° C. and hot forging, coarse grains are generated at a pseudo carburizing temperature of 950 ° C. for test number 9 and at a pseudo carburizing temperature of 920 ° C. for test number 10, The target austenite grain coarsening prevention characteristic is not obtained.

  In the case of test number 11, since the value of E represented by the formula (ii) of steel K is out of the conditions specified in the present invention, when heated to 900 and 1080 ° C. and hot forged, pseudo carburizing temperature 950 ° C. Coarse grains are generated at, and target austenite grain coarsening prevention characteristics are not obtained.

In the case of the test number 12, the value of E represented by the formula (i) of the steel L is out of the conditions specified in the present invention, and in an area of 1 μm ² , AlN having an equivalent circle diameter of 10 nm or more Since the ratio of the circle equivalent diameter of 10 to 100 nm is also outside the conditions defined in the present invention, when heated to 900 ° C. and hot forging, coarse grains are generated at a pseudo carburizing temperature of 950 ° C. The austenite grain coarsening preventing property is not obtained.

In the case of Test No. 13 and Test No. 14, the chemical composition of Steel A is within the range specified by the present invention, but the Al equivalent diameter is 10 out of AlN having an equivalent circle diameter of 10 nm or more in an area of 1 μm ^2. Since the ratio of ˜100 nm is out of the conditions defined in the present invention, when hot forging by heating to 900 and 1080 ° C., coarse grains are generated at a pseudo carburizing temperature of 920 ° C., and the target austenite grain coarseness Anti-oxidation characteristics are not obtained.

  Each steel bar having a radius of 20 mm was also shear-cut. As a result, shear cutting was possible without any problems except for the steel bar of test number 14 which contained 25% bainite in the matrix and deviated from the conditions specified in the present invention.

  The steel material for hot forging of the present invention has a temperature of not more than 950 ° C., particularly at the temperature of 950 ° C. or less, even when hot forging after heating in various temperature ranges, particularly in a wide temperature range of 900 to 1250 ° C. Since austenite grains can be prevented from coarsening when heated for a long time, they can be suitably used as materials for parts such as gears that are roughly formed by hot forging and automobile transmissions such as CVT pulleys. it can.

  Moreover, the steel for hot forging according to the present invention can be easily obtained by the production method of the present invention.

Claims (4)

In mass%, C: 0.1 to 0.3%, Si: 0.16 to 0.50%, Mn: 0.3 to 1.0%, S: 0.030% or less, Cr: 0.8 ~1.8%, Al: 0.02~ 0.060% , N: 0.010~ 0.0250%, Cu: 0~0.50% and Ni: and 0 to 0.50%,
The balance consists of Fe and impurities,
P, Ti and O in the impurities are respectively P: 0.020% or less, Ti: 0.005% or less and O: 0.0020% or less,
Furthermore, the steel material for hot forging having a chemical composition in which the value of E represented by the following formula (i) or formula (ii) is in the range of 0.012 to 0.022,
In the microstructure in the cross section perpendicular to the rolling direction, the matrix is composed of bainite having an area ratio of 0 to 10%, ferrite, and pearlite, and in an area of 1 μm ² , AlN having an equivalent circle diameter of 10 nm or more. Among them, a steel material for hot forging having an equivalent circle diameter of 10 to 100 nm is 85% or more and 96% or less.
In the case of Al / N ≧ 1.93: E = 0.965 × N (i),
In the case of Al / N <1.93: E = 0.5 × Al (ii).
The element symbols in the above formulas (i) and (ii) mean the content in mass% of the element.   The hot forging according to claim 1, wherein the chemical composition contains one or more selected from Cu: 0.05 to 0.50% and Ni: 0.05 to 0.50% by mass%. Steel material. The slab or steel ingot obtained by casting the steel having the chemical composition according to claim 1 or 2 is heated at a temperature of 1250 ° C. or more for 50 minutes or more and then rolled into a steel slab, The steel slab was heated for 60 minutes or more at a temperature of T ° C. or lower and 900 ° C. or higher represented by the following formula (iii), and then hot-rolled at a finishing temperature of 800 ° C. or higher. Comprising a step of cooling to 600 ° C. at a cooling rate of ° C./second or less ,
In the microstructure in the cross section perpendicular to the rolling direction, the matrix is composed of bainite having an area ratio of 0 to 10%, ferrite, and pearlite, and in an area of 1 μm ² , AlN having an equivalent circle diameter of 10 nm or more. Among them, a method for producing a steel material for hot forging in which a steel material for hot forging having an equivalent circle diameter of 10 to 100 nm is 85% or more and 96% or less .
T = −7397 / [log {(E1 ² −E2 ² ) /1.93} −1.71] −273 (iii).
However, in the formula (iii)
E1 = (0.5 × Al + 0.965 × N−E) (iv),
E2 = (0.5 × Al−0.965 × N) (v)
E in the formula (iv) is
In the case of Al / N ≧ 1.93: E = 0.965 × N (i),
In the case of Al / N <1.93: E = 0.5 × Al (ii).
In addition, the element symbol in each formula represents content in the mass% of the element. A method for producing a hot forged shape material , wherein the steel for hot forging according to claim 1 or 2 is heated at 900 to 1250 ° C and then forged in a temperature range of 900 to 1100 ° C.