JP2004250768A - Steel for machine structural having excellent cold workability and low decarburizing property, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel as hot-rolled for machine structural in which the thickness of a decarburizing layer on the surface is suppressed, and which has cold workability equal to that of the conventional steel subjected to softening annealing after hot rolling, and to provide a production method therefor. <P>SOLUTION: The steel has a composition comprising, by mass, 0.1 to 1.2% C, 0.01 to 2.5% Si, 0.1 to 1.5% Mn, ≤0.04% P, 0.0005 to 0.05% S, ≤0.2% Al, 0.0005 to 0.05% Te or/and 0.0005 to 0.05% Se and 0.0005 to 0.03% N, and in which the total content of S and Te or/and Se is 0.005 to 0.05%, and the balance Fe with inevitable impurities, and has a structure essentially consisting of ferrite and pearlite, and in which ferrite grain size number prescribed in JIS G 0552 is ≥11. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００５６】
【表２】

【００５７】
表３には熱間圧延材と軟化焼鈍材の組織、加工性と脱炭特性を示す。加工性の評価には引張強度（ＴＳ）、絞り（ＲＡ）及び限界圧縮率を用い、脱炭性の評価にはＪＩＳ Ｇ ０５５８の方法でフェライト脱炭層厚みを用いた。引張試験はＪＩＳ２号引張試験片を用い、限界圧縮率は、線材をφ１０に伸線加工後、文献「冷間据込み性試験方法」（塑性と加工、２２（１９８１）、１３９．）に示されている１号試験片（圧縮試験片）に準拠したφ１０×１５ｍｍの圧縮試験片を用い端面拘束圧縮試験により評価した。
【００５８】
【表３】

【００５９】
表３に示すように、本発明の請求範囲を満たす鋼は、いずれも限界圧縮率及び低脱炭性がともに優れている。ＴｅまたはＳｅの含有量が本発明の下限未満である鋼種ＳはＩ〜ＩＩＩの圧延条件のいずれで製造してもフェライト脱炭が認められ低脱炭性が劣化する。またＴｅ含有量が本発明の上限を超える鋼Ｕ、及びＳｅ含有量が本発明の上限を超える鋼Ｖはいずれもほぼ同等の成分である鋼Ｊ、Ｌと比べ著しく限界圧縮率が低下し、加工性が劣化する。また熱間圧延後の冷却速度が請求項６で示した製造条件の下限未満である１８はフェライト粒度番号が本発明を外れ延性が低下する。また熱間圧延後の冷却速度が請求項６で示した製造条件の上限を超える１９では組織が粗大化しフェライト粒度番号が本発明の範囲を外れ加工性が劣化するまた本発明は圧延後に軟化焼鈍を行った２３と比較しても同等以上の加工性が得られることがわかる。
【００６０】
【発明の効果】
本発明によれば、軟質化焼鈍することなしに焼鈍材と同等以上の加工性を有し、さらに鋼材表面の脱炭層厚みが抑制された機械構造用鋼が提供可能であり、産業上極めて大きな効果を有する。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to steel for mechanical structures used as a material for automobile parts and industrial machine parts, and particularly to steel for mechanical structures with improved cold workability such as drawing, cutting and cold forging, and a method for producing the same. It is about.
[0002]
[Prior art]
For automotive parts and industrial machine parts, hot-rolled materials of machine structural steel such as carbon steel and alloy steel are generally softened and annealed to ensure cold workability, and then drawn, cold forged and rolled. It is manufactured by molding or cutting into a predetermined shape and imparting strength by quenching and tempering.
[0003]
Softening annealing requires a long-term heat treatment of about 20 hours in the case of, for example, spheroidizing annealing, and thus accounts for a large proportion of the component manufacturing cost. For this reason, in recent years, the omission of soft annealing has been studied for the purpose of reducing the cost of parts, and a method of obtaining workability equivalent to that of a soft annealed material while hot rolling has been proposed.
[0004]
As a method of gradually cooling after hot rolling, a method of slowly cooling until the ferrite transformation is completed at a cooling rate of 1 ° C./sec or less (for example, see Patent Document 1), or a cooling rate of 0.02 to 0.3 ° C./sec. A method of slow cooling (for example, see Patent Document 2) is disclosed. According to the investigations of the present inventors, in these methods, the cooling rate is low, so that there is no hard structure such as bainite, and the ferrite fraction is high, so that the steel is softened. A region having a low carbon concentration, that is, a decarburized layer is formed. If such a decarburized layer exists, the surface strength of the final product may not be ensured in some cases.
[0005]
Further, as a method of raising the temperature immediately after hot rolling, a method of controlling cooling at a cooling rate of 5 ° C./sec or more and immediately maintaining the temperature in a furnace atmosphere temperature range of 500 to 700 ° C. for 15 minutes to less than 1 hour (see Patent Document 3) ) Is disclosed. In this method, although the steel is softened by reheating immediately after hot rolling, a decarburized layer is formed on the surface depending on the type of steel. This is considered to be because the pearlite transformation or the bainite transformation is not completed at the time of cooling in the steel type having high hardenability, and the austenite / ferrite two-phase region is maintained.
[0006]
In addition, as a method for producing a soft wire having no decarburized layer in the surface layer, a steel slab is heated to 900 ° C to 1250 ° C so that rolling before finish rolling is not exposed to a temperature range of 650 to 750 ° C for more than 60 seconds. And finish rolling at 700 to 900 ° C. with a surface reduction rate of 30% or more, and after finishing rolling, cooling to a temperature exceeding the Ms point to 850 ° C. or less, and on a conveyor at a ring density of 10 to 50 rings / m. A method is disclosed in which the film is developed, cooled to 650 ° C. or more and 800 ° C. or less, and then cooled to 500 ° C. at a cooling rate of 0.15 to 2 ° C./s by changing the ring density to 150 to 500 pieces / m. (See Patent Document 4). In this method, strict control of temperature and cooling rate and complicated production conditions are required. On the other hand, although the strength of the rolled material is softened, the strength is higher than that of the annealed material. This is because although the cooling rate from the finish rolling to 500 ° C. is high, the incorporation of a hard structure such as bainite can be prevented, but the softening of the pearlite portion is insufficient.
[0007]
As described above, in order to soften a hot-rolled material, it is generally necessary to maintain the temperature in a temperature range of about 650 to 750 ° C. immediately after hot rolling or to perform controlled cooling such as gradually cooling around a ferrite transformation temperature. However, it is known that decarburization of medium carbon steel is more likely to occur when the medium carbon steel is kept in the ferrite / austenite two-phase region. As described above, when the hot-rolled material is softened by controlled cooling, the residence time in a temperature range where decarburization occurs remarkably increases, and therefore, in the conventional method, the same strength as the soft-annealed material can be obtained as it is in the hot-rolled state. If so, there remains a problem that decarburization of the surface layer is inevitable.
[0008]
[Patent Document 1]
Japanese Patent Publication No. 2-13004 (page 3, line 5, line 23)
[Patent Document 2]
Japanese Patent Application Laid-Open No. 2000-336456 (Claim 4)
[Patent Document 3]
Japanese Patent Application Laid-Open No. 2000-336460 (Claim 4)
[Patent Document 4]
Japanese Patent Application Laid-Open No. 2000-256740 (Claim 1)
[0009]
[Problems to be solved by the invention]
The present invention has been devised in order to solve the above-described problems, and the thickness of the decarburized layer on the surface of the steel material is suppressed, and has a cold workability equivalent to that of a steel material subjected to softening and annealing after conventional hot rolling. An object of the present invention is to provide a steel for machine structural use as hot rolled and a method for producing the steel.
[0010]
[Means for Solving the Problems]
In order to solve the above problems, the present inventors investigated the effect of steel material composition and temperature history on decarburization, and even if the hot-rolled material was softened by controlled cooling, to suppress the occurrence of decarburization, It was considered effective to suppress the decarburization reaction on the steel material surface by the segregation of trace elements, and the effect of suppressing surface segregation and decarburization by various trace elements was evaluated. Cu and P are generally known as surface segregation elements. However, these elements lower hot ductility and cause rolling flaws or embrittle grain boundaries to deteriorate toughness. Te, Se, and S are found to be effective as trace elements that suppress decarburization with little effect on the material properties, and by adding these elements in specific amounts, the softening and decarburization of hot-rolled steel can be achieved. It has been found that both suppression of charcoal can be achieved.
[0011]
In steel materials, these elements are present in the form of Te or Se, such as MnTe or MnSe, in the form of Te compounds, Se compounds, or composite inclusions of these and S. S exists as a sulfide such as MnS. Since Te, Se, and S lower the interfacial energy, they tend to segregate on the steel material surface and crystal grain boundaries. The segregation of these elements on the surface of the steel material prevents carbon in the steel from segregating on the surface. As a result, it is considered that the reaction between carbon and oxygen or water vapor on the steel surface layer is inhibited, and decarburization is suppressed.
[0012]
Further, in order to concentrate Te, Se or S on the steel material surface and suppress decarburization, the heat history after hot rolling is 0.1 ° C / sec or more up to a temperature range of 500 to 700 ° C after finish rolling. After cooling at a cooling rate of less than 1 ° C / sec and once completing the pearlite transformation, the heat history of reheating to a temperature range of 650 ° C to 750 ° C is effective for improving workability and suppressing decarburization. I found it.
[0013]
As described above, a hot-rolled material having the same strength and ductility as the conventional softened and annealed material and having a reduced thickness of the surface decarburized layer has been invented.
The gist of the present invention is as follows.
[0014]
(1) In mass%,
C: 0.1 to 1.2%,
Si: 0.01 to 2.5%,
Mn: 0.1-1.5%,
P: 0.04% or less (including 0%),
S: 0.0005 to 0.05%,
Al: 0.2% or less,
One or two of Te: 0.0005 to 0.05% and Se: 0.0005 to 0.05%,
N: 0.0005 to 0.03%
Wherein the total content of S and one or two of Te and Se is 0.005% to 0.05%, with the balance being Fe and unavoidable impurities, A steel for machine structural use comprising a structure mainly composed of ferrite and pearlite and having a ferrite crystal grain size number of 11 or more specified in JIS G 0552, and having excellent cold workability and low decarburization.
[0015]
(2) Further, in mass%, Sb: 0.001 to 0.05%
The steel for machine structural use according to the above (1), which is excellent in cold workability and low decarburization property.
[0016]
(3) Further, Cr: 0.2 to 2.0% by mass%,
Mo: 0.1-1.0%,
Ni: 0.3-1.5%,
Cu: 1.0% or less,
B: The steel for machine structural use excellent in cold workability and low decarburization described in the above (1) or (2), which contains one or more of 0.005% or less.
[0017]
(4) Further, in mass%,
Ti: 0.002% to 0.05%,
Nb: 0.005 to 0.1%,
V: 0.03-0.3%,
The steel for machine structural use according to any one of the above (1) to (3), further comprising one or two or more of the above.
[0018]
(5) Further, Mg: 0.0002 to 0.01% by mass%,
Zr: 0.0001-0.01%
Ca: 0.0002 to 0.008%,
The steel for machine structural use according to any one of (1) to (4), which is excellent in cold workability and low decarburization property, comprising one or more of the above.
[0019]
(6) The steel having the steel component described in any of (1) to (5) above is hot rough-rolled in a temperature range of 850 ° C to 1000 ° C and finished in a temperature range of 700 ° C to 1000 ° C. After rolling, it is cooled to a temperature of 500 ° C. or more and 700 ° C. or less at a cooling rate of 0.1 ° C./second or more and less than 5 ° C./second, and immediately thereafter, is heated to a furnace atmosphere temperature of 650 ° C. or more and 750 ° C. or less for 15 minutes or more. A method for producing steel for machine structural use having excellent cold workability and low decarburization properties, wherein the steel is held for not more than one minute and then left to cool.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
The reasons for limiting the chemical components of the steel for machine structural use of the present invention will be described below.
[0021]
C is added to secure the strength as a component for machine structure. If it is less than 0.1%, the strength required as a mechanical component cannot be secured, and if it exceeds 1.2%, ductility and toughness deteriorate, so the content was made 0.1 to 1.2%.
[0022]
Si functions as a deoxidizing element and is an element effective for imparting necessary strength and hardenability to steel and improving tempering softening resistance. If it is less than 0.01%, these effects are insufficient, and if it exceeds 2.5%, toughness and ductility are deteriorated and hardness is increased, and cold forgeability is deteriorated. %.
[0023]
Mn is an element effective for imparting necessary strength and hardenability to steel. If it is less than 0.1%, the effect is insufficient, and if it exceeds 1.5%, the toughness is deteriorated, the hardness is increased, and the cold forgeability is deteriorated.
[0024]
P increases the deformation resistance during cold forging and deteriorates toughness. In addition, it is desirable to reduce segregation at the grain boundaries and embrittle the crystal grain boundaries after quenching and tempering to deteriorate toughness. Therefore, the upper limit is set to 0.04%.
[0025]
S reacts with alloying elements such as Mn, Cu, and Mg and exists as sulfide. These sulfides function as pinning particles, refine crystal grains, improve ductility and toughness, and improve machinability. It also segregates on the surface of the steel material and effectively functions to suppress decarburization. If it is less than 0.0005%, the effect of suppressing decarburization is not seen, and if it exceeds 0.05%, the cold forgeability is deteriorated, and the grain boundaries after quenching and tempering are embrittled to deteriorate toughness. Therefore, the content is set to 0.0005 to 0.05%.
[0026]
Al is a strong deoxidizing element and combines with O to form an oxide. Alternatively, it exists as AlN by combining with N. These oxides and nitrides function as pinning particles and reduce crystal grains. If it exceeds 0.2%, the cold workability is deteriorated, so the upper limit is made 0.2%.
[0027]
Te segregates on the surface and suppresses decarburization. If it is less than 0.0005%, the effect of suppressing decarburization is small, and if it exceeds 0.05%, hot workability and cold workability are deteriorated.
[0028]
Se segregates on the surface and suppresses decarburization. If it is less than 0.0005%, the effect of suppressing decarburization is small, and if it exceeds 0.05%, hot workability and cold workability are deteriorated.
[0029]
Further, if the total content of S and one or two of Te and Se is less than 0.005%, the effect of suppressing decarburization is not recognized, and if it exceeds 0.05%, hot workability is deteriorated. Inhibit. Therefore, the total content of S, Te, and Se is set to 0.005 to 0.05%.
[0030]
N combines with Al, Ti and the like to form nitrides, function as pinning particles, and refine crystal grains. If it is less than 0.0005%, the amount of nitride precipitation is insufficient, crystal grains are coarsened and workability is deteriorated. Further, if added in excess of 0.03%, the hot ductility is deteriorated, so the content was made 0.0005 to 0.03%.
[0031]
Next, the reasons for limiting the components of claims 2 to 5 will be described. One or more of these elements can be contained for the purpose of improving the characteristics described below.
[0032]
Sb segregates on the surface and suppresses decarburization. If it is less than 0.001%, the effect of suppressing decarburization is small, and if it exceeds 0.05%, hot workability and cold workability are deteriorated.
[0033]
Cr, Mo, Ni, Cu, and B are effective in improving hardenability and increasing the strength of steel.
[0034]
Cr is added for the purpose of increasing strength and hardenability. If it is less than 0.2%, the effect is insufficient, and if it exceeds 2.0%, the cold workability deteriorates.
[0035]
Mo is added for the purpose of improving hardenability. If it is less than 0.1%, the effect is insufficient, and if it exceeds 1.0%, the production cost is increased.
[0036]
Ni is added for the purpose of improving hardenability. If the content is less than 0.3%, the effect is insufficient. If the content exceeds 1.5%, the production cost is increased.
[0037]
Cu is added for the purpose of improving hardenability. If added in excess of 1.0%, the hot ductility is significantly reduced and causes surface flaws, so the upper limit was made 1.0%.
[0038]
B is added for the purpose of improving hardenability, but the effect is saturated even if added over 0.005%, so the upper limit was made 0.005%. Note that, for hardenability, the lower limit of the amount of B added is preferably set to 0.0003%.
[0039]
Ti, Nb, and V form carbonitrides. These carbonitrides are dispersed in the steel and function as pinning particles, which suppress the coarsening of crystal grains and improve workability.
[0040]
Ti combines with C or N and exists as TiC or TiN. These carbonitrides are effective as pinning particles. If the content is less than 0.002%, no effect is exhibited, and if it exceeds 0.05%, the effect is saturated and the hardness is increased, and the cold forgeability is deteriorated.
[0041]
Nb combines with N or C to form NbN, NbC or a composite inclusion thereof, and functions effectively to suppress the coarsening of crystal grains. If the content is less than 0.005%, the effect is insufficient, and if the content exceeds 0.1%, the effect is saturated.
[0042]
V combines with N or C to form VN, VC or a composite inclusion thereof, and functions effectively to suppress the coarsening of crystal grains. If it is less than 0.03%, the effect is insufficient, and if it exceeds 0.3%, the effect is saturated.
[0043]
Mg, Zr, and Ca are contained as deoxidizing elements.
[0044]
Mg is present as oxides, sulfides or composite inclusions containing these by combining with S or O, and effectively functions as pinning particles. If the content is less than 0.0002%, no effect is exhibited, and if it exceeds 0.01%, the production cost is increased.
[0045]
Zr combines with O and exists as an oxide, and also combines with N or C to form ZrN, ZrC or a composite inclusion thereof, and effectively functions to suppress the coarsening of crystal grains. If the amount is less than 0.0001, the effect is insufficient, and if the amount exceeds 0.01%, the effect is saturated.
[0046]
Ca is a powerful deoxidizing element and is effective in controlling the form of sulfide. It prevents MnS from elongation in the rolling direction and improves the deterioration of workability and toughness. If it is less than 0.0002%, the effect is insufficient, and if it exceeds 0.008%, the effect is saturated, and a coarse oxide is formed to lower the fracture toughness value.
[0047]
O is inevitably contained in steel and exists as an oxide such as Al or Ti. If the O content is high, a coarse oxide is formed, which causes fatigue fracture, so it is desirable to suppress the content to 0.01% or less.
[0048]
Further, the steel of the present invention has a specific structure.
[0049]
The structure mainly composed of ferrite and pearlite is because if hard bainite or martensite is mixed in the structure, the strength increases and the cold workability deteriorates. If the ferrite crystal grain size number is less than 11, the ductility decreases.
[0050]
Next, the manufacturing method of the present invention will be described.
[0051]
If the hot rough rolling temperature exceeds 1000 ° C., the austenite crystal grains become coarse, so that a ferrite grain size of 11 or more after rolling cannot be obtained. On the other hand, if the rough rolling temperature is lower than 850 ° C., rolling becomes difficult from the viewpoint of the rolling mill load. If the finish rolling temperature exceeds 1000 ° C., a ferrite grain size of 11 or more cannot be obtained. If the finish rolling temperature is less than 700 ° C., rolling occurs in the two-phase region of austenite and ferrite, and a uniform ferrite-pearlite structure cannot be obtained after rolling. , Deteriorating workability.
[0052]
Next, if the cooling rate after rolling is less than 0.1 ° C./sec, the pearlite transformation temperature increases and pearlite having a large lamella interval, that is, pearlite having high thermal stability, is formed, and the strength increases. On the other hand, at a temperature of 5 ° C./sec or more, in the case of a steel type having high hardenability, the transformation is not completed during cooling after rolling, the steel remains untransformed, the structure is coarsened, the workability is deteriorated, and the scale layer on the steel material surface and The Te and Se concentrations in the Te and Se enriched layers formed at the interface of the base iron are reduced, and the decarburization suppressing effect is reduced. If the cooling end temperature exceeds 650 ° C., the pearlite transformation is partially incomplete, the structure becomes non-uniform, and the workability deteriorates. If the cooling end temperature is lower than 500 ° C., it takes a long time to reach a temperature of 650 ° C. or higher in the subsequent reheating, which leads to an increase in cost due to a decrease in productivity.
[0053]
When the reheating temperature is lower than 650 ° C., granulation of cementite is insufficient, and softening is insufficient. On the other hand, when the temperature exceeds 750 ° C., a part of the structure becomes austenite and the strength is increased. On the other hand, if the reheating time is less than 15 minutes, the temperature inside the coil does not rise sufficiently and softening becomes insufficient. If it is longer than 90 minutes, the productivity is impaired.
[0054]
【Example】
Hereinafter, the present invention will be described specifically with reference to examples. Table 1 shows the chemical components of the test steel. These steels were continuously cast from converter steel, slab-rolled into slabs having a cross section of 162 × 162 mm, and then hot-rolled into a wire having a diameter of 11 mm under the rolling conditions shown in Table 2. In hot rolling, extraction is performed at 1100 ° C. in a heating furnace, rough rolling and finish rolling are performed at a predetermined temperature, and the resultant is rolled into a ring. I, II and III are cooled to a predetermined temperature at a predetermined cooling rate using a blast cooling, slow cooling cover, heat treatment furnace, water tank, etc. on the transfer line, and then converged on a coil to move on the line While heating in a heat treatment furnace for 30 minutes, it was allowed to cool outside the furnace. Rolling No. IV corresponds to the step of performing softening annealing after conventional hot rolling. After hot rolling, it was cooled by applying a slow cooling cover on the transport line, and then allowed to cool. After cooling, softening annealing at 700 ° C. × 3 h was performed in an annealing furnace.
[0055]
[Table 1]

[0056]
[Table 2]

[0057]
Table 3 shows the structures, workability and decarburization characteristics of the hot-rolled material and the soft-annealed material. The tensile strength (TS), the drawing (RA) and the critical compressibility were used for the evaluation of the workability, and the thickness of the ferrite decarburized layer according to JIS G 0558 was used for the evaluation of the decarburizing property. For the tensile test, a JIS No. 2 tensile test piece was used, and the critical compressibility was shown in the document "Cold Upsetting Test Method" (plasticity and processing, 22 (1981), 139.) after drawing a wire into φ10. Using a compression test piece of φ10 × 15 mm based on No. 1 test piece (compression test piece), evaluation was performed by an end face constraint compression test.
[0058]
[Table 3]

[0059]
As shown in Table 3, any steel satisfying the claims of the present invention is excellent in both the critical compression ratio and the low decarburization property. Regarding steel type S in which the content of Te or Se is less than the lower limit of the present invention, ferrite decarburization is observed and low decarburization is deteriorated even when manufactured under any of the rolling conditions I to III. Further, the steel U having a Te content exceeding the upper limit of the present invention and the steel V having a Se content exceeding the upper limit of the present invention have significantly lower critical compressibility than the steels J and L, which are almost the same components, Workability deteriorates. Further, when the cooling rate after hot rolling is less than the lower limit of the production condition shown in claim 6, the ferrite grain size number deviates from the present invention and the ductility is reduced. If the cooling rate after hot rolling exceeds the upper limit of the manufacturing conditions as set forth in claim 6, the microstructure becomes coarse, the ferrite grain size number falls outside the range of the present invention, and the workability is deteriorated. It can be seen that even when compared with 23 which was performed, the same or higher workability was obtained.
[0060]
【The invention's effect】
According to the present invention, it is possible to provide a steel for machine structural use having a workability equal to or more than that of an annealed material without softening annealing and further suppressing the thickness of a decarburized layer on the surface of a steel material, which is extremely large in industry. Has an effect.

Claims (6)

In mass%,
C: 0.1 to 1.2%,
Si: 0.01 to 2.5%,
Mn: 0.1-1.5%,
P: 0.04% or less (including 0%),
S: 0.0005 to 0.05%,
Al: 0.2% or less,
One or two of Te: 0.0005 to 0.05% and Se: 0.0005 to 0.05%,
N: 0.0005 to 0.03%
Wherein the total content of S and one or two of Te and Se is 0.005% to 0.05%, with the balance being Fe and unavoidable impurities, A steel for machine structural use comprising a structure mainly composed of ferrite and pearlite and having a ferrite crystal grain size number of 11 or more specified in JIS G 0552, and having excellent cold workability and low decarburization. Furthermore, Sb: 0.001 to 0.05% by mass%
The steel for machine structural use excellent in cold workability and low decarburization property according to claim 1, characterized by containing. Further, Cr: 0.2 to 2.0% by mass%,
Mo: 0.1-1.0%,
Ni: 0.3-1.5%,
Cu: 1.0% or less,
B: The steel for machine structural use excellent in cold workability and low decarburization according to claim 1 or 2, which contains one or more of 0.005% or less. Furthermore, in mass%,
Ti: 0.002% to 0.05%,
Nb: 0.005 to 0.1%,
V: 0.03-0.3%,
The steel for machine structural use excellent in cold workability and low decarburization according to any one of claims 1 to 3, wherein the steel contains one or more of the following. Further, Mg: 0.0002 to 0.01% by mass%,
Zr: 0.0001-0.01%
Ca: 0.0002 to 0.008%,
The steel for machine structural use excellent in cold workability and low decarburization according to any one of claims 1 to 4, further comprising one or more of the following. The steel having the steel component according to any one of claims 1 to 5 is hot rough-rolled in a temperature range of 850 ° C or more and 1000 ° C or less, and is finish-rolled in a temperature range of 700 ° C or more and 1000 ° C or less, and then 500 ° C or more. Cool to a temperature of 700 ° C or less at a cooling rate of 0.1 ° C / sec or more and less than 5 ° C / sec, and immediately maintain the furnace atmosphere temperature of 650 ° C or more and 750 ° C or less for 15 minutes or more and 90 minutes or less. A method for producing steel for machine structural use having excellent cold workability and low decarburization characteristics, which is characterized by cooling.