JP2014047356A - Bar steel or wire material - Google Patents

Abstract

The present invention provides a steel bar or wire rod for mechanical structure capable of suppressing the coarsening of austenite grains during heating at 1100 ° C. or higher in hot forging or carburizing.
SOLUTION: C: 0.05-0.3%, sol.Si: 0.001-0.3%, insol.Si: 0.001-0.01%, sol.Mn: 0.5-2 0.0%, insol.Mn: 0.001 to 0.01%, P: 0.03% or less, S: 0.001 to 0.02%, N: 0.02% or less, sol.Al: 0. A steel bar or wire containing 0001 to 0.0005%, insol.Al: 0.0001% or more and less than 0.005%, and O: 0.001 to 0.005%, the balance being Fe and impurities. Instead of a part of the Fe, one or more of a specific amount of Cu, Ni, Cr, Mo, V, B, Ca, REM, and Nb may be included.
[Selection figure] None

Description

  The present invention relates to a steel bar or a wire. More specifically, the present invention relates to a steel bar or wire rod for a machine structure having excellent anti-roughening properties during high-temperature heating, and is particularly suitable as a component material that is hot forged and / or carburized at 1100 ° C. or higher. The present invention relates to steel bars or wires for machine structures.

  Steel parts such as rolling parts, bearing parts, gears, and crankshafts of automobiles and industrial machines are often manufactured by the following process.

  Step (i): A rolled steel bar or wire made of machine structural steel is prepared. Examples of the machine structural steel include JIS standard SCr420, SCM420 and SNCM420.

  Step (ii): The rolled steel bar or wire obtained in step (i) is roughly formed by hot forging. In this hot forging, forging is generally performed after heating to 1100 ° C. or higher.

  Step (iii): The rough molded product obtained in step (ii) is subjected to cutting directly or after normalizing as necessary.

  Step (iv): If necessary, the cutting product obtained in step (iii) is subjected to surface hardening treatment such as carburizing and quenching. In this case, after carburizing at 900 to 1000 ° C. for a long time, the oil is cooled. In addition, after the surface curing treatment, tempering is further performed as necessary.

  In the hot forging in step (ii), since it is usually heated to 1100 to 1300 ° C., the austenite grains are coarsened, and the structure after hot forging is also coarsened. For this reason, when it is used in a “non-tempered” state as hot forging, the toughness and ductility deteriorate.

  Moreover, although the carburizing process of a process (iv) is normally performed on the heating conditions for 2 to 5 hours at 900-1000 degreeC, shortening of carburizing time is an important subject from a viewpoint of energy saving.

  Increasing the carburizing temperature is effective for shortening the carburizing time. Currently, carburizing treatment at 1100 ° C. or higher is possible by high-frequency heating, but in this case, austenite grains become significantly coarse. In carburized parts, coarsening of austenite grains causes a large amount of heat treatment strain during quenching and a decrease in fatigue strength as a part. Therefore, there is a problem of noise reduction and vibration when used as a part. It is easy to produce.

  As described above, if it is possible to prevent coarsening of austenite grains during heating by hot forging or carburizing at a high temperature, it is very effective industrially.

  For this reason, various studies have been made on the prevention of the coarsening of the austenite grains described above.

  Patent Document 1 discloses a technique that can suppress the coarsening of austenite grains by the pinning effect of TiC by carburizing using steel having a Ti content stoichiometrically higher than the N content. It is disclosed.

  In Patent Document 2, even when a steel containing a specific amount of Al and N is heated at 1200 ° C. or higher and then hot-worked to perform carburizing treatment at 980 ° C. for 6 hours, a pin made of AlN is used. A technique that can suppress the coarsening of austenite grains by a stopping effect is disclosed.

  Patent Document 3 specifies the contents of Nb, Al, N, Ti, and O for the purpose of suppressing the austenite grain coarsening during high-temperature carburizing of parts that omit the normalizing treatment after hot forging. By finely precipitating Nb (CN) more than a specific amount after forging, while suppressing the precipitation amount of AlN, optimizing the structure after hot forging, and finely precipitating AlN and Nb (CN) during carburizing heating A technique for expressing a pinning effect and suppressing coarsening of austenite grains is disclosed.

  Patent Document 4 specifies the amount of precipitation of Nb (CN) and AlN after hot rolling or hot forging of steel containing specific amounts of Al, Nb and N, and Nb (CN) A technique is disclosed in which the pinning effect is expressed by finely dispersing AlN to suppress the austenite grain coarsening.

JP-A-61-2217553 JP-A-56-75551 JP 2001-303174 A International Publication No. 99/05333

  In the technique disclosed in Patent Document 1, when carburized at a high temperature of 1100 ° C. or higher, the pinning effect is reduced by the solidification of TiC or coarsening by Ostwald growth, and austenite grains become coarse.

  Also in the technique disclosed in Patent Document 2, when carburizing at a high temperature of 1100 ° C. or higher, the pinning effect is reduced by the solidification of AlN or coarsening by Ostwald growth, and the austenite grains become coarse.

  The technique disclosed in Patent Document 3 does not necessarily provide a pinning effect sufficient for suppressing the austenite grain coarsening during high-temperature carburization.

  In the technique disclosed in Patent Document 4, as described in the examples, spheroidizing annealing is performed after hot rolling, or normalizing treatment is performed after hot forging. For this reason, when carburizing is performed at a high temperature of 1100 ° C. or higher in the state of hot forging or hot rolling, austenite grains become coarse.

  The present invention has been made in view of such problems, and can suppress austenite grain coarsening during heating at 1100 ° C. or higher in hot forging or heating at 1100 ° C. or higher during carburizing. An object of the present invention is to provide a steel bar or wire rod for machine structure suitable as a material for steel parts such as rolling parts, bearing parts, gears and crankshafts of automobiles and industrial machines.

  The present inventors conducted various examinations and experiments in order to develop the effect of suppressing the austenite grain coarsening. As a result, the knowledge shown in the following (a) to (d) was obtained.

(A) By dispersing fine Al ₂ O ₃ and fine MnAl ₂ O ₄ in a steel bar or wire, coarsening of austenite grains due to heating during hot forging or carburizing can be suppressed.

(B) For that purpose, the content of acid-soluble Al (hereinafter referred to as “sol.Al”) and O (oxygen) content for dispersing fine Al ₂ O ₃ , and fine MnAl ₂ O ₄ The content of acid-insoluble Al (hereinafter referred to as “insol.Al”) for dispersion must be within a specific range. Specifically, these contents are sol.Al: 0.0001 to 0.0005%, O (oxygen): 0.001 to 0.005%, and insol.Al: 0.0001% or more, respectively. The range needs to be less than 005%.

(C) Further, in order to sufficiently secure fine Al ₂ O ₃ and fine MnAl ₂ O ₄ , it is necessary to limit Si deoxidation during steelmaking. For this reason, acid-insoluble Si (hereinafter “insol.Si”) is required. ")") Must be 0.001 to 0.01%. This is because when the content of insol.Si exceeds 0.01%, the production of Si oxide increases, and the amount of O (oxygen) necessary for the production of fine Al ₂ O ₃ and fine MnAl ₂ O ₄ is reduced. This is because it cannot be secured.

  (D) Sol.Al: 0.0001% to 0.0005% and insol.Al: 0.0001% or more and less than 0.005%, each content range is satisfied, and acid-soluble Mn (hereinafter referred to as “sol.Mn”). ) And acid-insoluble Mn (hereinafter referred to as “insol.Mn”), respectively, sol.Mn: 0.5 to 2.0% and insol.Mn: 0.001 to 0.001, respectively. By controlling in the range of 0.01%, the effect of suppressing austenite grain coarsening during carburization is further improved. This is considered to be because fine MnS can be deposited on the Al-based oxide by using steel that satisfies the above conditions, and the effect of refining the austenite initial grain size immediately after austenite transformation can be expressed. .

  The acid-soluble Si is referred to as “sol.Si” in the same manner as Al and Mn, and the fractional determination of “sol.” And “insol.” Of Al, Si, and Mn is inductively coupled plasma (ICP) emission. It can be determined by the following method by spectroscopic analysis.

  First, the sample is acid-decomposed with aqua regia and the solution is filtered with filter paper. At this time, what is dissolved in the solution is referred to as “sol.” Moreover, after putting the filter paper after the filtration into a platinum crucible, the filter paper is burned and incinerated, what remains in the platinum crucible is melted in potassium pyrosulfate (potassium disulfate), and the resulting melt is diluted with dilute hydrochloric acid. The solution dissolved in is called “insol.” Obtaining the values of sol.Al, insol.Al, sol.Si, insol.Si, sol.Mn and insol.Mn for the solution thus obtained by using an ICP emission spectroscopic analyzer. Can do.

  This invention is completed based on such knowledge, The summary exists in the steel bar or wire shown to following (1)-(5).

(1) In mass%,
C: 0.05-0.3%
sol.Si: 0.001 to 0.3%,
insol.Si: 0.001 to 0.01%,
sol.Mn: 0.5 to 2.0%,
insol.Mn: 0.001 to 0.01%,
P: 0.03% or less,
S: 0.001 to 0.02%,
N: 0.02% or less,
sol.Al: 0.0001 to 0.0005%,
insol.Al: 0.0001% or more and less than 0.005% and O: 0.001 to 0.005%
A steel bar or wire rod, characterized in that the balance is Fe and impurities.

(2) Instead of a part of Fe, in mass%,
The steel bar or wire according to (1) above, containing one or more of Cu: 1.0% or less and Ni: 1.5% or less.

(3) Instead of part of Fe, in mass%,
Cr: 2.5% or less,
Mo: 0.5% or less,
The bar or wire according to (1) or (2) above, which contains at least one of V: 0.2% or less and B: 0.005% or less.

(4) In place of part of Fe, in mass%,
The bar or wire according to any one of (1) to (3) above, which contains at least one of Ca: 0.005% or less and REM: 0.05% or less.

(5) Instead of a part of Fe, in mass%,
The steel bar or wire according to any one of (1) to (4) above, which contains Nb: 0.05% or less.

  According to the present invention, when heating at 1100 ° C. or higher in hot forging or heating at 1100 ° C. or higher during carburizing, coarsening of austenite grains can be suppressed, and rolling parts and bearings for automobiles and industrial machines. A steel bar or wire rod for machine structure suitable as a material for steel parts such as parts, gears and crankshafts can be provided.

  Hereinafter, the chemical composition of the steel bar or wire according to the present invention will be described. In addition, "%" regarding content means "mass%".

C: 0.05-0.3%
C is an element effective for ensuring carburizing hardenability and steel strength. If the content is less than 0.05%, the above effects are insufficient. On the other hand, if it exceeds 0.3%, the strength becomes too high, and the machinability is significantly reduced. Therefore, the C content is set to 0.05 to 0.3%. Moreover, since C has the effect of forming a carbide by combining with Cr and complementing the pinning effect, the preferable lower limit of the C content is 0.15%. The upper limit of preferable C content is 0.25%.

sol.Si: 0.001 to 0.3%, insol.Si: 0.001 to 0.01%
Si is an effective element for preliminary deoxidation of molten steel. Si in steel is classified into acid-soluble sol.Si that exists as Si (solid solution Si) in the matrix and acid-insoluble insol.Si that exists as inclusions such as Si ₂ O _3. .

  If the sol.Si content is less than 0.001%, a sufficient preliminary deoxidation effect cannot be obtained, so that O remains excessively in the molten steel, resulting in an increase in cleanliness and a decrease in cleanliness. Therefore, the sol.Si content is set to 0.001% or more. On the other hand, if the sol.Si content is excessive, the strength of the steel material becomes too high and the machinability deteriorates. Therefore, the upper limit of the sol.Si content is set to 0.3%. The upper limit with preferable sol.Si content is 0.1%, and a preferable minimum is 0.01%.

Further, insol.Si is Si that forms Si-based inclusions, and exhibits the effect of refining the austenite initial grain size immediately after austenite transformation. This complements the suppression of austenite grain coarsening due to the dispersion of fine Al ₂ O ₃ and fine MnAl ₂ O ₄ . However, if the insol.Si content is less than 0.001%, the above effect is insufficient, so the lower limit of the content is set to 0.001%. On the other hand, if the insol.Si content is excessive, the Si inclusions become agglomerated and coarsened, which becomes the starting point of destruction, and also reduces the amount of O necessary for the generation of desired Al ₂ O ₃ and MnAl ₂ O _4. Invite. Therefore, the upper limit of the insol.Si content is set to 0.01%. A preferable upper limit of the insol.Si content is 0.008%, and a preferable lower limit is 0.004%.

sol.Mn: 0.5 to 2.0%, insol.Mn: 0.001 to 0.01%
Mn has an effect of improving hardenability and is an element necessary for ensuring strength and toughness. In the present invention, Mn is a main element for promoting the production of fine Al ₂ O ₃ and MnAl ₂ O ₄ . Mn in steel is classified into acid-soluble sol.Mn that exists as Mn (solid solution Mn) dissolved in the matrix and acid-insoluble insol.Mn that exists as inclusions such as MnS.

  Furthermore, under the conditions satisfying the sol.Al content and the insol.Al content described later, the sol.Mn content is controlled to be in the range of 0.5 to 2.0% on the Al-based oxide. MnS is precipitated, and the austenite grain coarsening suppression effect is improved.

The upper limit of the sol.Mn content is set to 2.0% because if the content exceeds 2.0%, the hardenability is increased and the strength becomes too high, and the machinability is lowered. It is. In addition, the lower limit of the sol.Mn content is set to 0.5% because if the content is less than 0.5%, the strength cannot be ensured by improving the hardenability, and MnAl ₂ O ₄ is also generated. This is because it becomes insufficient. A preferable upper limit of the sol.Mn content is 1.5%, and a preferable lower limit is 1.0%.

On the other hand, insol.Mn generates MnS by controlling its content in the range of 0.001 to 0.01%, and exhibits the effect of refining the austenite initial grain size immediately after austenite transformation. This promotes suppression of coarsening of austenite grains by fine Al ₂ O ₃ and fine MnAl ₂ O ₄ . However, if the insol.Mn content is less than 0.001%, the above effect cannot be obtained. Therefore, the lower limit is made 0.001%. On the other hand, if the insol.Mn content exceeds 0.01%, MnS becomes agglomerated and coarsened, which becomes the starting point of fracture. Furthermore, the amount of O necessary for the generation of desired fine Al ₂ O ₃ and fine MnAl ₂ O ₄ Incurs reduction. Therefore, the upper limit of the insol.Mn content is set to 0.01%. A preferable upper limit of the insol.Mn content is 0.008%, and a preferable lower limit is 0.005%.

P: 0.03% or less P is present in steel as an inevitable impurity causing grain boundary segregation. If the content exceeds 0.03%, the grain boundary segregation of P becomes prominent, the grain boundary becomes brittle and becomes a starting point of fracture, so that the fatigue strength decreases. Therefore, the content of P is set to 0.03% or less. The upper limit with preferable P content is 0.01%, and it is so preferable that it is small. In addition, the minimum with preferable P content when considering economical efficiency is 0.005%.

S: 0.001 to 0.02%
S is an element necessary for precipitating fine MnS on the Al-based oxide. However, if the S content is less than 0.001%, sufficient MnS cannot be formed, and the generation of MnAl ₂ O ₄ becomes insufficient, so the lower limit is made 0.001%. On the other hand, when there is too much content of S, MnS will produce | generate excessively and will coarsen and fatigue strength will fall. Therefore, the upper limit of the content is 0.02%. A desirable upper limit of the S content is 0.01%, and a desirable lower limit is 0.003%.

N: 0.02% or less N is an element effective in preventing austenite grain coarsening during carburization heating by forming AlN, NbN and TiN by combining with Al, Nb and Ti. However, since it is difficult to stably contain a large amount of N in mass production in the steelmaking process, the upper limit of the content is set to 0.02%. The upper limit with preferable N content is 0.007%, and a preferable minimum is 0.002%.

sol.Al: 0.0001 to 0.0005%, insol.Al: 0.0001% or more and less than 0.005% Al is a typical deoxidizing element, and in the present invention, austenite grains are coarsened. It is an important element for supplying an Al-based oxide for suppression. Al in steel is classified into acid-soluble sol.Al that exists as solute Al or Al nitride, and acid-insoluble insol.Al that exists mainly as Al ₂ O ₃ and MnAl ₂ O ₄ .

The most important point in the present invention is how to disperse MnAl ₂ O ₄ in a fine and large amount in addition to Al ₂ O ₃ .

When sol.Al content is high, it becomes a negative factor with respect to the fine dispersion of Al ₂ O ₃ and MnAl ₂ O _4. That is, when sol.Al increases and the sol.Al content as a whole exceeds 0.0005%, generation of MnAl ₂ O ₄ does not occur, and only Al ₂ O ₃ is generated. It is difficult to ensure the fine precipitation of MnAl ₂ O ₄ that can withstand the carburizing treatment of. Therefore, the upper limit of the sol.Al content is set to 0.0005%. However, if the sol.Al content is less than 0.0001%, a sufficient deoxidation effect cannot be obtained, which is a problem in production. Therefore, the lower limit is made 0.0001%. The upper limit with preferable sol.Al content is 0.0004%, and a preferable minimum is 0.0003%.

On the other hand, a large insol.Al content means that fine dispersion of Al ₂ O ₃ and MnAl ₂ O ₄ is sufficiently generated. When the insol.Al content is less than 0.0001%, the distribution of fine Al ₂ O ₃ and fine MnAl ₂ O _{4 in} the steel becomes extremely coarse, and a sufficient austenite grain refinement effect cannot be obtained. Therefore, the lower limit of the insol.Al content is 0.0001%. However, when the insol.Al content is 0.005% or more, the Al content increases as a whole, and the fine Al ₂ O ₃ and MnAl ₂ O ₄ aggregate and coarsen, making it impossible to obtain the desired austenite grain refinement effect. . Therefore, the insol.Al content is less than 0.005%. The insol.Al content is preferably less than 0.0045%, and more preferably 0.001% or less. Moreover, the minimum with preferable insol.Al content is 0.0025%.

O: 0.001 to 0.005%
The content of O (oxygen) is preferably low from the viewpoint of the cleanliness of the steel. However, since it is necessary to obtain desired Al ₂ O ₃ and MnAl ₂ O ₄ , the lower limit of the content is set to 0.001%. On the other hand, increasing the O content not only increases the cleanliness of the steel and lowers the cleanliness, but also causes agglomeration and coarsening of Al ₂ O ₃ and MnAl ₂ O ₄ , resulting in an Al-based oxide having a desired particle size. It can no longer be obtained. Therefore, the upper limit of the content is made 0.005%. The upper limit with preferable O content is 0.0015%, and a preferable minimum is 0.0025%.

  The steel bar or wire according to the present invention contains the above elements, with the balance being Fe and impurities. Here, an impurity means the component mixed by various factors of a manufacturing process including raw materials, such as an ore and a scrap, when manufacturing steel industrially.

The steel bar or wire according to the present invention contains one or more elements selected from the elements shown in the following first group to fourth group, instead of a part of the above-described Fe if necessary. Can be made.
(1) First group: one or more of Cu and Ni,
(2) Second group: one or more of Cr, Mo, V and B,
(3) Third group: one or more of Ca and REM,
(4) Fourth group: Nb.

(1) First group: one or more of Cu and Ni Since the elements of the first group, Cu and Ni, have an effect of improving fatigue strength, one or both of them are contained as necessary. be able to. Hereinafter, the elements of the first group will be described in detail.

Cu: 1.0% or less Cu is an element that has an effect of improving hardenability and can further improve fatigue strength. However, when the content exceeds 1.0%, the hot ductility is lowered and the hot workability is remarkably deteriorated. Therefore, the Cu content is 1.0% or less. The upper limit with preferable content of Cu is 0.7%. In addition, when acquiring the effect by Cu, it is preferable to contain 0.1% or more of Cu, and it is more preferable to contain 0.2% or more.

Ni: 1.5% or less Ni, like Cu, is an effective element for improving fatigue strength. However, if the Ni content exceeds 1.5%, the strength of the steel material becomes too high and the machinability deteriorates. Therefore, the Ni content is 1.5% or less. The upper limit with preferable Ni content is 0.7%. In addition, when obtaining the effect by Ni, it is preferable to contain 0.1% or more of Ni, and it is more preferable to contain 0.2% or more.

  Coexistence of Cu and Ni, which are elements of the first group, can avoid an excessive increase in hardenability by containing a large amount of Cu or Ni alone, making it easy to balance desired fatigue strength and machinability. . In this case, it is desirable to control the total content of Cu and Ni to 2.0% or less.

(2) Group 2: One or more of Cr, Mo, V, and B All elements of Group 2, Cr, Mo, V, and B all increase the hardenability of the steel material, and strength during carburizing and quenching Therefore, if necessary, one or more of these elements can be contained. Hereinafter, the second group of elements will be described in detail.

Cr: 2.5% or less Cr is effective for increasing the hardenability of the steel material and ensuring the strength. However, if the Cr content exceeds 2.5%, the strength becomes too high and the machinability deteriorates, so the Cr content is set to 2.5% or less. The upper limit with preferable Cr content is 1.5%. When it is desired to obtain the effect of Cr, the Cr content is preferably 0.1% or more, and more preferably 0.2% or more.

Mo: 0.5% or less Mo is effective for increasing hardenability and ensuring strength. Specifically, Mo is effective in improving bending fatigue strength and surface fatigue strength, and has a large effect of increasing temper softening resistance. However, if the Mo content exceeds 0.5%, the strength becomes too high and the machinability deteriorates, so the Mo content is set to 0.5% or less. The upper limit with preferable Mo content is 0.4%. In addition, when acquiring the effect by Mo, it is preferable to make Mo content into 0.1% or more, and it is more preferable to set it as 0.2% or more.

V: 0.2% or less V is effective in increasing the hardenability of the steel material and ensuring strength, particularly strength during quenching carburization. However, if the V content exceeds 0.2%, the strength becomes too high and the machinability deteriorates, so the V content is set to 0.2% or less. In addition, when obtaining the effect by V, it is preferable to make content of V 0.02% or more.

B: 0.005% or less B is effective in increasing the hardenability of the steel material and ensuring strength, particularly strength during quenching carburization. However, if the B content exceeds 0.005%, the strength becomes too high and the machinability deteriorates, so the B content is set to 0.005% or less. In addition, when obtaining the effect by B, it is preferable to make B content 0.001% or more.

  When two or more elements of the second group are contained, excessive hardenability increase due to containing a large amount of each element alone can be avoided, and adverse effects on strength can be suppressed to a minimum. It becomes easy to balance machinability. In this case, it is desirable to control the total content of these elements to 2.0% or less.

(3) Third group: one or more of Ca and REM Necessary because the third group element Ca and REM both control the form of sulfides and increase hot workability Depending on, one or more of these elements can be included. Hereinafter, the elements of the third group will be described in detail.

Ca: 0.005% or less Ca is effective for controlling the form of sulfide and increasing hot workability. However, if the Ca content exceeds 0.005%, large inclusions and clusters are generated to increase the cleanliness of the steel and impair the cleanliness, so the Ca content is set to 0.005% or less. In addition, when acquiring the effect by Ca, it is preferable to make content of Ca 0.001% or more.

REM: 0.05% or less REM, like Ca, is effective for controlling the form of sulfide and increasing hot workability. However, if the content of REM exceeds 0.05%, large inclusions and clusters are generated and the cleanliness of the steel is increased to impair cleanliness, so the REM content is set to 0.05% or less. In addition, when obtaining the effect by REM, it is preferable to make content of REM 0.01% or more. Here, REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. REM may be contained by adding misch metal which is a mixture of REM. Note that the content of REM means the total content of these elements.

  When combined with Ca and REM, which are elements of the third group, excessive hardenability is prevented by containing a large amount of Ca or REM alone, and adverse effects on machinability are minimized. This makes it easy to balance the desired strength and machinability. In this case, it is desirable to control the total content of Ca and REM to 0.05% or less.

(4) Group 4: Nb
Nb, which is an element of the fourth group, generates nitrides and has an effect of suppressing the coarsening of austenite grains during carburizing, and thus can be contained as necessary. This will be described in detail below.

Nb: 0.05% or less Nb easily forms NbC, NbN, Nb (CN) by combining with C and N, and austenite coarsening during carburization due to the pinning effect of Al ₂ O ₃ and MnAl ₂ O ₄ Complement suppression. However, if the Nb content exceeds 0.05%, the strength becomes too high and the machinability deteriorates, so the Nb content is set to 0.05% or less. The upper limit with preferable Nb content is 0.04%. In addition, when obtaining the effect by Nb, the Nb content is preferably 0.005% or more, and more preferably 0.008% or more.

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

  Steel AZ, A1 and B1 having chemical components shown in Table 1 were adjusted in a converter, and then continuously cast to obtain a 400 mm × 300 mm square slab, which was cooled to 800 ° C.

  Steels A to R in the table are all steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels S to Z, A1 and B1 are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention.

  Next, each slab was heated to 1280 ° C., and a steel piece of 180 mm × 180 mm square was produced by split rolling and cooled to room temperature.

  Furthermore, after heating said steel piece to 1200 degreeC, hot rolling was performed on the conditions whose finish rolling temperature is 950-1050 degreeC, and obtained the bar steel of 35 mm in diameter.

In order to simulate heating by carburization using a cylindrical test piece obtained in this way, a cylindrical test piece having a diameter of 30 mm and a height of 80 mm was cut out from the obtained steel bar having a diameter of 35 mm. About each steel, after hold | maintaining at the temperature of Table 2 for 1 hour, it cooled to room temperature by water cooling.

  Each of the specimens after cooling was cut into two equal parts in the height direction, mirror-polished, corroded with a saturated aqueous solution of picric acid to which a surfactant was added, and then randomly 10 fields at a magnification of 100 using an optical microscope. By observing, the presence or absence of abnormal grain growth of austenite grains and the occurrence of coarsening were investigated.

  The austenite crystal grain size was measured by a cutting method according to JIS G 0551.

  When there are 20% or more of visual fields having particle size numbers different by 3 or more in the range of 10 visual observations at random, it was determined as “mixed grains” and “abnormal grain growth” was considered to have occurred. Further, when the average grain size number of 10 fields of view was less than 2.0, it was judged that “grain coarsening” had occurred.

  In the case where neither “abnormal grain growth” or “grain coarsening” as the above judgment occurred, the target prevention of austenite grain coarsening could be achieved.

  On the other hand, when at least one of “abnormal grain growth” and “crystal grain coarsening” that is determined as described above has occurred, the target austenite grain coarsening prevention could not be achieved. Indicated with "x".

  The steels of the examples of the present invention (steel A to R) within the range of the chemical composition according to the present invention are all “O” in the “Achieving austenite grain coarsening prevention” column in Table 2, and 10 fields of view. It can be seen that the average particle size number of the particles satisfies 2.0 or more, abnormal grain growth does not occur, and excellent anti-roughening properties are exhibited.

  On the other hand, the comparative steels (steel S to Z, A1 and B1) outside the range of the chemical composition according to the present invention have all the “achievement of austenite grain coarsening prevention” columns in Table 2. It can be seen that at least one of coarsening of crystal grains and abnormal grain growth occurs and the coarsening resistance property is inferior.

The steel bar or wire of the present invention can suppress austenite grain coarsening during heating at 1100 ° C. or higher in hot forging or heating at 1100 ° C. or higher during carburizing. For this reason, the steel bar or wire of the present invention can be suitably used as a material for steel parts such as rolling parts, bearing parts, gears, crankshafts and the like of automobiles and industrial machines.

Claims (5)

% By mass
C: 0.05-0.3%
sol.Si: 0.001 to 0.3%,
insol.Si: 0.001 to 0.01%,
sol.Mn: 0.5 to 2.0%,
insol.Mn: 0.001 to 0.01%,
P: 0.03% or less,
S: 0.001 to 0.02%,
N: 0.02% or less,
sol.Al: 0.0001 to 0.0005%,
insol.Al: 0.0001% or more and less than 0.005% and O: 0.001 to 0.005%
A steel bar or wire rod, characterized in that the balance is Fe and impurities. Instead of part of Fe, in mass%,
The steel bar or wire according to claim 1, characterized by containing at least one of Cu: 1.0% or less and Ni: 1.5% or less. Instead of part of Fe, in mass%,
Cr: 2.5% or less,
Mo: 0.5% or less,
The steel bar or wire according to claim 1 or 2, comprising at least one of V: 0.2% or less and B: 0.005% or less. Instead of part of Fe, in mass%,
The steel bar or wire according to any one of claims 1 to 3, characterized by containing at least one of Ca: 0.005% or less and REM: 0.05% or less. Instead of part of Fe, in mass%,
The steel bar or wire according to any one of claims 1 to 4, characterized by containing Nb: 0.05% or less.