JP2013139604A - Hot-rolled bar steel or wire rod - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-rolled bar steel or wire rod which, even when hot-forged after being heated to various temperature regions, can stably prevent the austenite grains from coarsening during heating for carburizing or carbonitriding.SOLUTION: The hot-rolled bar steel or wire rod comprises 0.1-0.3% C, 0.05-1.5% Si, 0.4-2.0% Mn, 0.003-0.05% S, 0.5-3.0% Cr, 0.03-0.06% sol. Al, and 0.010-0.025% N, with the balance comprising Fe and unavoidable impurities, wherein in the impurities, P≤0.025%, Ti≤0.003%, and O≤0.002%, and wherein in each of the region covering from the surface of a cross section to 1/5 of the radius and the region covering from the center of the cross section to 1/5 of the radius, the amount of Al deposited as AlN is 0.015% or more, the value calculated from either of specified formulas using the Al amount is 0.008% or more, the numerical density of AlN particles with diameters of 100-300 nm is 10 particles/100 μmor more.

Description

  The present invention relates to a hot-rolled steel bar or wire, and in particular, excellent as a grain coarsening prevention property during carburizing or carbonitriding, suitable as a material for parts such as gears and shafts that are roughly formed by hot forging. It relates to hot rolled steel bars or wires.

  Steel parts such as gears and shafts of automobiles and industrial machines are roughly formed by hot forging or cold forging of steel bars or wire rods, and then cut into parts, and then carburized or carbonitrided. In many cases, the surface is hardened and manufactured. However, when the austenite grains before quenching are coarsened, problems such as a decrease in fatigue strength as a part and an increase in deformation during quenching are likely to occur.

  In general, it has been considered that hot forged parts are less likely to coarsen austenite grains during carburizing or carbonitriding than cold forging parts. However, in recent years, due to advancement in hot forging technology, hot forging is often performed in various temperature ranges, and hot forged parts in which austenite grains become coarse during carburizing or carbonitriding are increasing. Therefore, there is a need for a hot-rolled steel bar or wire that can stably prevent coarsening of austenite grains during heating in the carburizing or carbonitriding process even when hot forging in various temperature ranges.

  Therefore, for example, Patent Documents 1 to 3 propose technologies relating to steel and its manufacturing method.

  Specifically, Patent Document 1 discloses sol. A “grain-stabilized carburizing steel” is disclosed in which a steel with limited amounts of Al, N and “sol.Al/N” is heated to 1200 ° C. or higher and then hot-worked. .

  In Patent Document 2, the amount of precipitation of AlN, the size and number of AlN are limited, and the hardness and decarburization depth of the steel material are defined. “Skin burning excellent in cold workability and coarse grain prevention characteristics during carburizing” Steel materials and manufacturing methods thereof are disclosed. The technique proposed in Patent Document 2 is based on the premise that it is cold-worked as it is rolled and roughly formed, and then carburized, as described in the title of the invention and the purpose of the invention. Is.

  Patent Document 3 discloses “a case hardening steel excellent in coarse grain prevention characteristics and a method for producing the same” that defines the amount of AlN precipitation, the bainite structure fraction, the ferrite band, and the like. The technique proposed in Patent Document 3 is also premised on rough forming by cold forging and then carburizing and quenching as described in paragraph [0002].

  Patent Document 4 discloses a “hot rolled steel bar or wire rod” proposed by the present inventors, in which the precipitation amount of AlN, the size and number of AlN are limited, and the structure is also defined.

JP-A-56-75551 JP 2004-204263 A JP-A-11-106866 International Publication No. 2011/055651

  In the techniques disclosed in Patent Documents 1 to 3, when hot forging is performed in various temperature ranges, the austenite grains are not necessarily coarsened during heating in the carburizing or carbonitriding process. I could not say.

  That is, the technique proposed in Patent Document 1 is hot working after heating steel to 1200 ° C or higher, but in hot forging in mass production, there are many parts whose heating temperature is not 1200 ° C or higher. . For this reason, even when hot forged in various temperature ranges, it is not a technique that can stably prevent austenite grain coarsening during carburizing.

  In the technique proposed in Patent Document 2, no consideration is given to the center of the heating temperature of the material. Also, the amount of AlN precipitated in the process after hot rolling is not considered, and since AlN is fine, austenite grain coarsening during carburizing heating when hot forged in various temperature ranges It is not always possible to prevent this stably.

  The technique proposed in Patent Document 3 also does not consider the center of the heating temperature of the material. In addition, since the amount of AlN precipitated in the process after hot rolling is not considered, when hot forging is performed in various temperature ranges, austenite grain coarsening during carburizing heating can be always stably prevented. It is not a thing.

  In the case of the technique proposed by the present inventors in Patent Document 4, even if hot forging after heating to various temperature ranges, heating is performed in the carburizing or carbonitriding process, particularly heating to a temperature of 980 ° C. or less within 3 hours. When this is done, coarsening of the austenite grains can be stably prevented. However, when manufacturing steel bars and wire rods by hot rolling, for example, a steel slab as a rolling material obtained by split rolling may have to be heated to a high temperature. It becomes bulky.

  The present invention has been made in view of the above situation, and its purpose is to produce a heating temperature of a steel slab as a rolling material, for example, a steel slab obtained by split rolling in the production of hot rolled steel bars and wires. It is a steel bar or wire obtained without making it too high, and it can be carburized or carbonitrided even if it is hot forged after being heated to 900-1200 ° C in various temperature ranges using the steel bar or wire. Since it can stably prevent the austenite grains from coarsening when heated in the process, particularly at a temperature of 980 ° C. or less for 3 hours or less, it is a hot rolled steel bar suitable as a material for parts to be roughly formed by hot forging. Or to provide wire.

  In the present invention, an optical microscope is used to observe 10 visual fields with a size of each visual field of 1.0 mm × 1.0 mm, and when there are two or more austenite crystal grains having a grain size number of 5 or less. The austenite grains are coarsened.

  Until now, as in Patent Document 2 and Patent Document 3, by reducing the precipitation amount of AlN at the stage of hot rolled material, during carburizing heating in the case of rough forming by cold working (cold forging) Is known to be capable of preventing coarsening of austenite grains, but when hot forged in various temperature ranges, even if the precipitation amount of AlN is reduced at the stage of hot rolled material, 980 ° C. Austenite grain coarsening when carburizing and heating at the following temperatures is not necessarily prevented stably.

  For this reason, in the production of hot rolled steel bars and wire rods, the present inventors have obtained steel bars as rolling materials, for example, steel bars obtained without excessively heating the heating temperature of steel pieces obtained by split rolling. When the steel bar or wire is hot forged in various temperature ranges, the coarsening of austenite grains is stabilized even when heated to a temperature of 980 ° C. or lower in the carburizing or carbonitriding process. We investigated and studied the effects that can be prevented, especially the effects of precipitation amount, dispersion state and microstructure of AlN in hot rolled steel bars or wires. As a result, the following findings (a) to (e) were obtained. In the following description, “carburization or carbonitriding” may be simply referred to as “carburization”. Unless otherwise specified, “carburizing heating” refers to “heating at a temperature of 980 ° C. or less for carburizing”.

  (A) In a part roughly formed by hot forging, when AlN is precipitated to some extent at the stage of hot rolled material, the amount of AlN deposited is a specific amount or more, and the size of AlN is not too fine. And if it is the magnitude | size of the specific range which is not too coarse, it will be hard to produce variation in austenite grain growth at the time of carburizing heating.

  (B) If the amount of Al that can be precipitated as AlN in the process after hot rolling is a specific amount or more, the austenite grain coarsening is stabilized during carburizing heating by superimposing action with (a) above. Can be prevented.

  (C) Coarse AlN is produced in the slab after continuous casting with a large cross section, which is a general mass production process, and if this remains in the hot rolled material, the precipitation amount of AlN is At least, austenite grains are likely to become coarse during carburizing heating.

  (D) In heating of a slab and a steel slab obtained by performing ingot rolling on a slab, since the temperature is raised from the surface side, it takes a long time for the temperature of the central part to be equal to the temperature of the surface. Therefore, in the case of general heating, the amount of precipitated AlN and coarse AlN grains are larger in the center of the hot rolled material than in the surface layer, so austenite grain coarsening during carburizing heating is not necessarily stable. Cannot be prevented. For this reason, in order to stably prevent coarsening of austenite grains during carburizing heating, it is necessary to consider not only the vicinity of the surface layer portion but also the center portion.

  (E) The amount of deposited AlN is generally determined by analyzing the residue obtained by electrolytic extraction from the surface layer. For this reason, the amount of AlN precipitation determined by general extraction residue analysis, and the amount of Al that can be precipitated in a step after hot rolling calculated using the amount of AlN precipitation, is determined by carburizing heating near the center. It does not serve as an index for preventing the coarsening of austenite grains. In order to achieve the prevention of coarsening of austenite grains during carburizing heating in the vicinity of the center, the amount of AlN precipitated in the vicinity of the center and the amount of Al that can be precipitated in the process after hot rolling also satisfy a predetermined amount. There is a need.

  This invention is completed based on said knowledge, The summary exists in the hot-rolled steel bar or wire shown to following (1)-(3).

(1) Hot-rolled steel bar or wire,
C: 0.1 to 0.3%
Si: 0.05 to 1.5%,
Mn: 0.4 to 2.0%,
S: 0.003-0.05%,
Cr: 0.5 to 3.0%
sol. Al: 0.03 to 0.06% and N: 0.010 to 0.025%,
The balance consists of Fe and impurities,
P, Ti and O in the impurity are respectively
P: 0.025% or less,
Having a chemical composition of Ti: 0.003% or less and O: 0.002% or less;
In the cross section of the steel bar or wire, the amount of Al precipitated as AlN in each of the region from the surface of the steel bar or wire to 1/5 of the radius and the region from the center of the cross section to 1/5 of the radius Is 0.015% or more, X represented by the formula (1) or (2) is 0.008% or more, and the number density of AlN having a diameter of 100 to 300 nm is 10/100 μm ² or more. Is,
A hot-rolled steel bar or wire, characterized in that
[(14/27) sol. In the case of Al + (14 / 47.9) Ti ≧ N],
X = [{N− (14 / 47.9) Ti} × (27/14)] − [Al precipitated as AlN] (1)
[(14/27) sol. In the case of Al + (14 / 47.9) Ti <N],
X = sol. Al- [Al deposited as AlN] (2)
Here, each element symbol in the formula represents the content (% by mass) of the corresponding element.

(2) Instead of a part of Fe, in mass%,
Cu: 0.4% or less,
The hot-rolled steel bar or wire according to (1) above, which contains at least one selected from Ni: 1.5% or less and Mo: 0.8% or less.

(3) Instead of part of Fe, in mass%,
The hot-rolled steel bar or wire according to (1) or (2) above, which contains one or more selected from Nb: 0.08% or less and V: 0.2% or less.

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

  The “diameter” of AlN refers to the arithmetic average of the major axis and minor axis of AlN, which is obtained by preparing an extracted replica sample by a general method and observing it using a transmission electron microscope.

  Furthermore, X represented by Formula (1) or Formula (2) refers to the amount of Al that can be precipitated as AlN in a step after hot rolling.

  The hot-rolled steel bar or wire of the present invention can be obtained without increasing the heating temperature of the steel slab as a rolling material. Moreover, even if this steel bar or wire is heated to various temperatures, particularly 900 to 1200 ° C. and then hot forged, it is heated in the carburizing or carbonitriding step, particularly heated to a temperature of 980 ° C. or less within 3 hours. In this case, the austenite grains can be prevented from coarsening, so that they can be suitably used as materials for parts such as gears and shafts that are roughly formed by hot forging.

  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: 0.1 to 0.3%
C is an essential element for securing the core strength of the parts when carburizing and quenching or carbonitriding and quenching, and if the content is less than 0.1%, the above effect is insufficient. On the other hand, if the C content exceeds 0.3%, the machinability after hot forging is significantly reduced. 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: 0.05 to 1.5%
Si is an element that has a large effect of increasing hardenability and temper softening resistance, and also has an effect of improving fatigue strength. However, when the Si content is less than 0.05%, the above effects are insufficient. On the other hand, when the Si content exceeds 1.5%, not only the effect of increasing the fatigue strength is saturated, but also the machinability after hot forging becomes remarkable. Therefore, the Si content is set to 0.05 to 1.5%. When the Si content is 0.4% or more, the effect of improving the fatigue strength becomes remarkable. Therefore, the Si content is preferably 0.4% or more. Note that the Si content is preferably 0.8% or less.

Mn: 0.4 to 2.0%
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 content is less than 0.4%, the above effect is insufficient. On the other hand, when the content of Mn exceeds 2.0%, 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 0.4 to 2.0%. In addition, it is preferable that content of Mn is 0.8% or more and 1.2% or less.

S: 0.003-0.05%
S combines with Mn to form MnS and improves machinability. However, if the content is less than 0.003%, it is difficult to obtain the above effect. On the other hand, when the content of S increases, coarse MnS tends to be generated, and the fatigue strength tends to decrease. In particular, when the content exceeds 0.05%, the decrease in fatigue strength becomes significant. . Therefore, the content of S is set to 0.003 to 0.05%. In addition, it is preferable that content of S is 0.01% or more and 0.03% or less.

Cr: 0.5 to 3.0%
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. However, if the content is less than 0.5%, the above effect is insufficient. On the other hand, if the content of Cr exceeds 3.0%, not only the effect of increasing the fatigue strength is saturated, but also the machinability after hot forging becomes remarkable. Therefore, the Cr content is set to 0.5 to 3.0%. When the Cr content is 1.3% or more, the effect of improving the fatigue strength becomes remarkable, so the Cr content is preferably 1.3% or more. Note that the Cr content is preferably 2.0% or less.

sol. Al: 0.03-0.06%
Al is an element that has a deoxidizing action and is easily combined with N to form AlN and is effective in preventing austenite grain coarsening during carburizing heating. However, the content of Al is sol. If the Al content is less than 0.03%, even if the other requirements are satisfied, the austenite grains targeted by the present invention, which will be described later, that “coarse grains do not occur when heated at a temperature of 980 ° C. or lower for 3 hours” are described. The effect of preventing coarsening cannot be obtained. Further, the content of Al is sol. Similarly, even when Al exceeds 0.06%, even if other requirements are satisfied, the target austenite grain coarsening preventing effect cannot be obtained. Therefore, the content of Al is sol. It was 0.03 to 0.06% with Al. The content of Al is sol. It is preferable that it is 0.035% or more and 0.05% or less with Al. “Sol.Al” means acid-soluble Al, which means Al other than Al oxide.

N: 0.010 to 0.025%
N is an element that easily forms AlN, NbN, VN, and TiN by combining with Al, Nb, V, and Ti. In the present invention, among the above nitrides, AlN, NbN, and VN have an effect of preventing austenite grain coarsening during carburizing heating. However, if the N content is less than 0.010%, the target effect of preventing austenite grain coarsening in the present invention cannot be obtained even if other requirements are satisfied. On the other hand, when the N content exceeds 0.025%, it is difficult to stably mass-produce particularly in the steelmaking process. Therefore, the N content is set to 0.010 to 0.025%. The N content is preferably 0.013% or more and 0.020% or less.

  One of the chemical compositions of the hot-rolled steel bar or wire of the present invention is that, in addition to the above elements, the balance consists of Fe and impurities, and P, Ti, and O (oxygen) in the impurities are each P: 0.025% Hereinafter, Ti: 0.003% or less and O: 0.002% or less.

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

P: 0.025% or less P is an element that easily segregates at the grain boundary and easily embrittles the grain boundary. When it exceeds 0.025%, the fatigue strength is reduced. Therefore, the content of P in the impurities is set to 0.025% or less. In addition, it is preferable that content of P in an impurity shall be 0.015% or less.

Ti: 0.003% or less Ti combines with N to easily form hard and coarse TiN, and reduces fatigue strength. In particular, when the Ti content exceeds 0.003%, the fatigue strength is significantly reduced. Therefore, the Ti content in the impurities is set to 0.003% or less. In addition, it is preferable to make content of Ti as an impurity element 0.002% or less, and it is still more desirable to make it as small as possible in the range which does not raise the cost in a steelmaking process.

O (oxygen): 0.002% or less O is liable to form hard oxide inclusions by bonding with Al, and lowers the surface fatigue strength. In particular, when the O content exceeds 0.002%, the surface fatigue strength is significantly reduced. Therefore, the O content in the impurities is set to 0.002% or less. In addition, it is preferable to make content of O as an impurity element 0.001% or less, and it is further more desirable to reduce as much as possible in the range which does not raise the cost in a steelmaking process.

  Another one of the chemical compositions of the hot-rolled steel bar or wire of the present invention contains one or more elements of Cu, Ni, Mo, Nb and V in place of part of Fe. .

  Hereafter, the effect of said Cu, Ni, Mo, Nb and V which are arbitrary elements and the reason for limitation of content are demonstrated.

  Cu, Ni, and Mo all have an effect of improving hardenability. For this reason, when it is desired to obtain greater hardenability, these elements may be contained. Hereinafter, the above Cu, Ni, and Mo will be described.

Cu: 0.4% or less Cu has an effect of improving hardenability, and is an element effective for increasing fatigue strength. Therefore, Cu may be contained as necessary. However, when the Cu content exceeds 0.4%, the hot ductility is lowered, and the hot workability is significantly lowered. Therefore, the Cu content when contained is set to 0.4% or less. In addition, it is preferable that content of Cu in the case of making it contain is 0.3% or less.

  On the other hand, in order to stably obtain the effect of increasing the fatigue strength by improving the hardenability of Cu described above, the content of Cu in the case of inclusion is preferably 0.1% or more.

Ni: 1.5% or less Ni has an effect of improving the hardenability and is an element effective for increasing the fatigue strength. Therefore, Ni may be contained as necessary. However, if the Ni content exceeds 1.5%, 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 Ni content when contained is set to 1.5% or less. In addition, when Ni is contained, the content of Ni is preferably 0.8% or less.

  On the other hand, in order to stably obtain the effect of increasing the fatigue strength by improving the hardenability of Ni as described above, the Ni content when contained is preferably 0.1% or more.

Mo: 0.8% or less Mo has an effect of increasing hardenability and also has an effect of increasing resistance to temper softening, and is an element effective for increasing fatigue strength. May be. However, when the content of Mo exceeds 0.8%, not only the effect of increasing the fatigue strength is saturated, but also the machinability after hot forging becomes remarkable. Therefore, the Mo content in the case of inclusion is set to 0.8% or less. In addition, it is preferable that content of Mo in the case of making it contain is 0.4% or less.

  On the other hand, in order to stably obtain the effect of increasing the fatigue strength by improving the hardenability and temper softening resistance of Mo described above, the Mo content in the case of being included is 0.05% or more. preferable.

  Said Cu, Ni, and Mo can be contained only in any one of them, or 2 or more types of composites. The total content of these elements may be 2.7% or less, but is preferably 1.2% or less.

  Since both Nb and V have the effect of supplementing the above-described prevention of coarsening of austenite grains during carburizing heating with AlN, these elements may be contained. Hereinafter, Nb and V will be described.

Nb: 0.08% or less Nb easily forms NbC, NbN, Nb (C, N) by combining with C and N, and supplements the above-described prevention of austenite grain coarsening during carburizing heating with AlN. It is an effective element. However, if the Nb content exceeds 0.08%, the effect of preventing austenite grain coarsening is rather lowered. For this reason, the cost of the alloy increases and the economy is impaired. Therefore, when Nb is contained, the Nb content is set to 0.08% or less. When Nb is included, the Nb content is preferably 0.04% or less.

  On the other hand, in order to stably obtain the above-described effect of preventing the austenite grain coarsening of Nb, the content of Nb when contained is preferably 0.01% or more.

V: 0.2% or less V is easy to combine with C and N to form VN and VC. Among these, VN is effective in supplementing the above-described prevention of coarsening of austenite grains during carburizing heating with AlN. is there. However, if the V content exceeds 0.2%, the effect of preventing austenite grain coarsening is rather lowered. For this reason, the cost of the alloy increases and the economy is impaired. Therefore, when V is included, the content of V is set to 0.2% or less. In addition, when V is contained, the content of V is preferably 0.1% or less.

  On the other hand, in order to stably obtain the effect of preventing the austenite grain coarsening of V described above, the content of V when contained is preferably 0.02% or more.

  Said Nb and V can be contained only in any 1 type or 2 types of composites. The total content of these elements may be 0.28% or less, but is preferably 0.14% or less.

(B) In a cross section of a steel bar or wire, when a region from the surface of the steel bar or wire to 1/5 of the radius and a region from the center of the cross section to 1/5 of the radius are observed, in each region, AlN The amount of Al precipitated as, the amount of Al that can be precipitated as AlN in the process after hot rolling, and the number density of AlN with a diameter of 100 to 300 nm Since the slab and steel slab are large in cross section, up to the center It takes a long time to reach a predetermined temperature. Therefore, when the slab and the steel slab are heated, the temperature of the central part is generally lower than that of the surface layer part, or the time during which the center part is kept at a predetermined temperature is short. Therefore, at the stage of hot-rolled steel bar or wire rod in a hot-worked state, the precipitation amount and dispersion state of AlN are different between the surface layer portion and the central portion, and there is a difference in the coarsening of austenite grains.

However, in the cross section of the hot rolled steel bar or wire, it is precipitated as AlN in each of the region from the surface of the steel bar or wire to 1/5 of the radius and the region from the center of the cross section to 1/5 of the radius. The amount of Al is 0.015% or more, and X expressed by the formula (1) or (2), that is, the amount of Al that can be precipitated as AlN in the process after hot rolling is 0.008. If the number density of AlN having a diameter of 100 to 300 nm is 10 pieces / 100 μm ² or more, the steel bar or wire is hot forged after being heated to various temperatures between 900 and 1200 ° C. Even in the entire region from the surface layer to the central portion, austenite grain coarsening during carburizing heating can be suppressed.
[(14/27) sol. In the case of Al + (14 / 47.9) Ti ≧ N],
X = [{N− (14 / 47.9) Ti} × (27/14)] − [Al precipitated as AlN] (1)
[(14/27) sol. In the case of Al + (14 / 47.9) Ti <N],
X = sol. Al- [Al deposited as AlN] (2)
Here, each element symbol in the formula represents the content (% by mass) of the corresponding element.

Therefore, in the present invention, in the cross section, Al precipitated as AlN in each of the region from the surface of the steel bar or wire to 1/5 of the radius and the region from the center of the cross section to 1/5 of the radius. The amount is 0.015% or more, X represented by the formula (1) or (2) is 0.008% or more, and the number density of AlN having a diameter of 100 to 300 nm is 10/100 μm ^2. That is the above.

The amount of Al deposited as AlN is a general condition after, for example, collecting an appropriate test piece and masking the cross section of the test piece with a resin so as not to be electropolished. Using a% AA-based electrolyte, extraction (electrolysis) is performed at a current density of 250 to 350 A / m ² , the extracted solution is filtered through a filter having a mesh size of 0.2 μm, and a general chemical analysis is performed on the filtrate. Can be determined by The 10% AA electrolyte solution described above is a 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol solution.

Moreover, about 100-300 nm AlN in the said 2 area | region, for example, an extraction replica sample is produced from each area | region by a general method, 20,000 times magnification and the area per visual field are 10 micrometers using a transmission electron microscope. ² , the number density per area of 100 μm ² can be obtained by observing 10 fields of view at random.

In each of the two regions, the amount of Al precipitated as AlN is preferably 0.018% or more and 0.030% or less. Moreover, it is preferable that X represented by Formula (1) or Formula (2) is 0.012% or more. Furthermore, the number density of AlN having a diameter of 100 to 300 nm is preferably 13 pieces / 100 μm ² or more.

  Therefore, as an example of the method for obtaining the amount of Al precipitated as AlN, the amount of Al that can be precipitated as AlN in a step after hot rolling, and the number density (dispersed state) of AlN, A steel containing 0.20-0.25% C, 0.1-0.5% Si, 0.7-1.0% Mn and 1.0-1.5% Cr was used. Show the case. Of course, the method for producing a hot-rolled steel bar or wire according to the present invention is not limited to this.

-When steel is melted and slabs are manufactured, a reduction is applied to the slabs during solidification,
-When the slab is rolled into pieces, the slab is subjected to heating at a heating temperature of 1250 to 1300 ° C and a heating time of 5 hours or more, and then rolled into pieces.
・ Slow cooling of the steel slab after the rolling
When the steel slab is hot-rolled, the heating time is 60 minutes or more when the heating temperature of the steel slab is less than 1100 to 1150 ° C, and the heating time is 60 when the heating temperature of the steel slab is 1150 to 1200 ° C. Hot rolling as ~ 80 minutes,
The hot rolling finish temperature is 980 to 1050 ° C., and after finish rolling, the steel is cooled to a temperature of 600 ° C. or less at a cooling rate in the air (hereinafter simply referred to as “cooling”).
-The rolling ratio from steel slab to steel bar and wire (cross-sectional area of steel slab / cross-sectional area of steel bar and wire) is 8 or more.

  In addition, after finish rolling in hot rolling, it is not necessary to cool to room temperature at a cooling rate equal to or lower than that of cooling, and when it reaches a temperature of 600 ° C. or lower, it is cooled by appropriate means such as air cooling, mist cooling, or water cooling. May be.

  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 bar or wire immediately after finish rolling, and the cooling rate after finishing rolling also refers to the surface cooling rate of the steel bar or wire.

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

Example 1
Steel α and steel β having the chemical compositions shown in Table 1 were adjusted for components in a 70-ton converter, and then continuously cast to produce a 400 mm × 300 mm square slab (bloom), which was gradually cooled to 600 ° C. . Note that reduction was applied during the solidification of continuous casting. Both the steel α and the steel β are steels whose chemical compositions are within the range defined by the present invention.

  The slab produced in this manner was heated from the above 600 ° C. to 1280 ° 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 diameter of 40 mm.

  In Table 2, as production conditions <1> to <8>, when finishing a 400 mm × 300 mm slab into a steel bar with a diameter of 40 mm, the slab heating conditions, the cooling conditions after the block rolling, and the slab heating conditions The details of the rolling finishing temperature of the steel bar rolling and the cooling conditions after rolling are shown.

  For each steel bar having a diameter of 40 mm obtained as described above, a region from the surface to 1/5 of the radius and a region from the center of the cross section to 1/5 of the radius in the cross section were observed and precipitated as AlN. In addition to investigating the amount of Al, X represented by the formula (1) or (2) using the Al amount, that is, the amount of Al that can be precipitated as AlN in the process after hot rolling Asked. Further, the number density of AlN having a diameter of 100 to 300 nm in the above two regions was investigated. In addition, a test simulating hot forging and carburizing heating was conducted to investigate the presence or absence of coarse grains. The specific investigation method will be described below.

First, since a steel bar having a diameter of 40 mm has a scale on the surface, extraction residue analysis cannot be performed as it is. For this reason, a test piece having a diameter of 39 mm and a length of 10 mm and a test piece having a diameter of 8 mm and a length of 20 mm were collected from the concentric position by turning. The cross sections of these test pieces were masked with a resin so as not to be electropolished, and then extracted at a current density of 250 to 350 A / m ² using 10% AA electrolyte, which is a general condition (electrolysis). )did. The extracted solution was filtered through a filter having a mesh size of 0.2 μm, and a general chemical analysis was performed on the filtrate to determine the amount of Al deposited as AlN.

In addition, in the cross section of a steel bar having a diameter of 40 mm, an extraction replica sample was prepared by a general method from a region from the surface to 1/5 of the radius and a region from the center of the cross section to 1/5 of the radius, respectively. Using a transmission electron microscope, 10 fields of view were randomly observed at a magnification of 20000 times and an area of 10 μm ² per field, and the number density per 100 μm ² area was determined for AlN having a diameter of 100 to 300 nm.

  Further, a 60 mm long test piece was cut out from a steel bar having a diameter of 40 mm, heated at 1200 ° C., 1100 ° C., 1000 ° C., and 900 ° C. for 30 minutes in order to simulate hot forging, and then taken out from the furnace. 10 seconds later, 60% compression processing was performed in the height direction of the cylindrical shape, and then cooled to room temperature by cooling. The test piece thus obtained was further heated at 930 ° C. for 1 hour and then allowed to cool to room temperature.

  Next, after cutting each test piece obtained as described above into four equal parts in the longitudinal cross-sectional direction, in order to simulate heating by carburizing, 950 ° C., 980 ° C., 1010 ° C. and 1040 ° C. After holding at each temperature for 3 hours, it was cooled to room temperature by water cooling. 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. Each of 10 visual fields was randomly observed to investigate the occurrence of austenite grain coarsening. The size of each visual field in the above investigation was 1.0 mm × 1.0 mm. By this observation, when there were two or more austenite crystal grains having a grain size number of 5 or less, it was determined that the austenite grains were coarsened. In addition, the target of the austenite grain coarsening prevention effect was that the austenite grain did not become coarse when heated at a temperature of 980 ° C. or lower for 3 hours.

  Tables 3 and 4 collectively show the results of the above investigations together with the manufacturing conditions of the steel bars and the temperatures heated to simulate hot forging. The manufacturing condition symbols in Table 3 and Table 4 correspond to the manufacturing condition symbols described in Table 2 above.

  From Table 3 and Table 4, the chemical composition is within the range defined by the present invention, and in the cross section, the area from the surface to 1/5 of the radius and the area from the center of the cross section to 1/5 of the radius. The amount of Al deposited as AlN in each of the above, and the value of X determined from the formula (1) or formula (2) using the amount of Al and the number density of AlN having a diameter of 100 to 300 nm are all In the case of “examples of the present invention” that satisfy the conditions defined in the present invention (that is, when manufactured by manufacturing condition symbols <2> and <8>), they are heated to various temperatures of 900 to 1200 ° C. Even if forging, coarse grains are not generated up to the carburized heating simulation temperature of 980 ° C., and it is clear that the effect of preventing austenite grain coarsening is obtained.

  On the other hand, even if steel α and steel β whose chemical composition is within the range defined in the present invention are used, in the case of “Comparative Example” that deviates from other conditions defined in the present invention, the target coarse Anti-granulation properties are not obtained.

(Example 2)
Components of steels a to k having chemical compositions shown in Table 5 were adjusted in a 70-ton converter, and then continuous casting was performed to produce a 400 mm × 300 mm square slab (bloom), which was cooled to 600 ° C. Note that reduction was applied during the solidification of continuous casting.

  In addition, all of steel a, steel b, and steels f to k in Table 5 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels c to e are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention.

  The slab produced in this manner was heated from the above 600 ° C. to 1280 ° 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 diameter of 40 mm.

  For each steel bar having a diameter of 40 mm obtained as described above, in the same manner as in the above (Example 1), in the cross section, the region from the surface to 1/5 of the radius and the center of the cross section of the radius 1 The region up to / 5 is observed to investigate the amount of Al deposited as AlN, and X expressed by the formula (1) or (2) using the Al amount, that is, hot rolling The amount of Al that can be deposited as AlN in a later process was determined. Further, the number density of AlN having a diameter of 100 to 300 nm in the above two regions was investigated. In addition, a test simulating hot forging and carburizing heating was conducted to investigate the presence or absence of coarse grains.

That is, by turning a steel bar having a diameter of 40 mm, a test piece having a diameter of 39 mm and a length of 10 mm and a test piece having a diameter of 8 mm and a length of 20 mm were collected from the concentric position. The cross sections of these test pieces were masked with a resin so as not to be electropolished, and then extracted at a current density of 250 to 350 A / m ² using 10% AA electrolyte, which is a general condition (electrolysis). )did. The extracted solution was filtered through a filter having a mesh size of 0.2 μm, and a general chemical analysis was performed on the filtrate to determine the amount of Al deposited as AlN.

In addition, in the cross section of a steel bar having a diameter of 40 mm, an extraction replica sample was prepared by a general method from a region from the surface to 1/5 of the radius and a region from the center of the cross section to 1/5 of the radius, respectively. Using a transmission electron microscope, 10 fields of view were randomly observed at a magnification of 20000 times and an area of 10 μm ² per field, and the number density per 100 μm ² area was determined for AlN having a diameter of 100 to 300 nm.

  Further, a 60 mm long test piece was cut out from a steel bar having a diameter of 40 mm, heated at 1200 ° C., 1100 ° C., 1000 ° C., and 900 ° C. for 30 minutes in order to simulate hot forging, and then taken out from the furnace. 10 seconds later, 60% compression processing was performed in the height direction of the cylindrical shape, and then cooled to room temperature by cooling. The test piece thus obtained was further heated at 930 ° C. for 1 hour and then allowed to cool to room temperature.

  Next, after cutting each test piece obtained as described above into four equal parts in the longitudinal cross-sectional direction, in order to simulate heating by carburizing, 950 ° C., 980 ° C., 1010 ° C. and 1040 ° C. After holding at each temperature for 3 hours, it was cooled to room temperature by water cooling. 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. Each of 10 visual fields was randomly observed to investigate the occurrence of austenite grain coarsening. The size of each visual field in the above investigation was 1.0 mm × 1.0 mm. By this observation, when there were two or more austenite crystal grains having a grain size number of 5 or less, it was determined that the austenite grains were coarsened. In addition, the target of the austenite grain coarsening prevention effect decided that austenite grain does not coarsen when it heats for 3 hours at the temperature of 980 degrees C or less similarly to the case of (Example 1).

  Tables 6 to 8 collectively show the above-described investigation results together with the manufacturing conditions of the steel bars and the temperatures heated to simulate hot forging. In addition, the manufacturing condition symbols in Tables 6 to 8 also correspond to the manufacturing condition symbols described in Table 2 above.

  From Tables 6 to 8, the chemical composition is within the range defined by the present invention, and in the cross section, the area from the surface to 1/5 of the radius and the area from the center of the cross section to 1/5 of the radius. The amount of Al deposited as AlN in each, the value of X obtained from the formula (1) or formula (2) using the amount of Al, and the number density of AlN having a diameter of 100 to 300 nm are all present. In the case of the “examples of the present invention” that satisfy the conditions specified in the invention, even when heated to various temperatures of 900 to 1200 ° C. and hot forging, coarse grains are not generated up to a carburizing heating simulated temperature of 980 ° C. It is clear that the effect of preventing coarsening of austenite grains is obtained.

  On the other hand, in the case of the “comparative example” that does not satisfy all the conditions defined in the present invention, the target coarsening prevention characteristic is not obtained.

  The hot-rolled steel bar or wire of the present invention can be obtained without increasing the heating temperature of the steel slab as a rolling material. Moreover, even if this steel bar or wire is heated to various temperatures, particularly 900 to 1200 ° C. and then hot forged, it is heated in the carburizing or carbonitriding step, particularly heated to a temperature of 980 ° C. or less within 3 hours. In this case, the austenite grains can be prevented from coarsening, so that they can be suitably used as materials for parts such as gears and shafts that are roughly formed by hot forging.

Claims (3)

Hot-rolled steel bar or wire,
C: 0.1 to 0.3%
Si: 0.05 to 1.5%,
Mn: 0.4 to 2.0%,
S: 0.003-0.05%,
Cr: 0.5 to 3.0%
sol. Al: 0.03 to 0.06% and N: 0.010 to 0.025%,
The balance consists of Fe and impurities,
P, Ti and O in the impurity are respectively
P: 0.025% or less,
Having a chemical composition of Ti: 0.003% or less and O: 0.002% or less;
In the cross section of the steel bar or wire, the amount of Al precipitated as AlN in each of the region from the surface of the steel bar or wire to 1/5 of the radius and the region from the center of the cross section to 1/5 of the radius Is 0.015% or more, X represented by the formula (1) or (2) is 0.008% or more, and the number density of AlN having a diameter of 100 to 300 nm is 10/100 μm ² or more. Is,
A hot-rolled steel bar or wire, characterized in that
[(14/27) sol. In the case of Al + (14 / 47.9) Ti ≧ N],
X = [{N− (14 / 47.9) Ti} × (27/14)] − [Al precipitated as AlN] (1)
[(14/27) sol. In the case of Al + (14 / 47.9) Ti <N],
X = sol. Al- [Al deposited as AlN] (2)
Here, each element symbol in the formula represents the content (% by mass) of the corresponding element. Instead of part of Fe, in mass%,
Cu: 0.4% or less,
The hot-rolled steel bar or wire according to claim 1, comprising at least one selected from Ni: 1.5% or less and Mo: 0.8% or less. Instead of part of Fe, in mass%,
The hot-rolled steel bar or wire according to claim 1 or 2, comprising at least one selected from Nb: 0.08% or less and V: 0.2% or less.