CN-121986180-A - Steel material

Abstract

The steel material has a predetermined chemical composition, and when d/2 is the length of a line segment connecting the center of gravity of the cross section and the surface of the cross section closest to the center of gravity, and d/4 is the position of d/4 in the direction from the surface to the center of gravity, the number density of MnS of 1.0-10.0 [ mu ] m 2 in the d/4 of the cross section is 70.0/mm 2 or less, the number density of MnS of more than 10.0 [ mu ] m 2 in the area is 4.00/mm 2 or less, and the number density of Nb-based precipitates of 20.0 [ mu ] m 2 or more in the area is 0.20/mm 2 or less.

Inventors

• He Tianai
• YUYA MASATO
• JIANG TOUCHENG
• SUZUKI TAKAHISA

Assignees

• 日本制铁株式会社

Dates

Publication Date

20260505

Application Date

20240925

Priority Date

20231011

Claims (2)

1. 1. A steel material characterized by comprising, in mass%, C：0.10~0.30%、Si：0.05~1.50%、Mn：0.40~2.00%、P：0.002~0.025%、S：0.002~0.050%、Cr：0.05~2.30%、Al：0.010~0.100%、N：0.010~0.030%、Nb：0.020~0.080%、Mo：0~0.40%、Ti：0.100% or less, 0 to 0.15% of V, 0 to 0.0050% of B, 0 to 0.40% of Cu, 0 to 0.30% of Ni, 0 to 0.100% of Sn, 0 to 0.0050% of Ca, 0 to 0.0050% of Mg, 0 to 0.0050% of O, and the balance of Fe and impurities, When d/2 is the length of a line segment connecting the center of gravity of the cross section and the surface of the cross section closest to the center of gravity in a cross section perpendicular to the rolling direction, and d/4 is the position of d/4 in the direction of the center of gravity from the surface, the number density of MnS having an area of 1.0 to 10.0 [ mu ] m 2 is 70.0/mm 2 or less, the number density of MnS having an area exceeding 10.0 [ mu ] m 2 is 4.00/mm 2 or less, and the number density of Nb-based precipitates having an area of 20.0 [ mu ] m 2 or more is 0.20/mm 2 or less in the d/4 of the cross section.
2. 2. A steel product as claimed in claim 1, wherein, The chemical composition contains at least one selected from the group consisting of 0.03 to 0.40% by mass of Mo, more than 0% by mass of Ti, not more than 0.100% by mass of 、V：0.01~0.15%、B：0.0001~0.0050%、Cu：0.01~0.40%、Ni：0.01~0.30%、Sn：0.010~0.100%、Ca：0.0001~0.0050%、Mg：0.0001~0.0050%、% by mass, and 0.0001 to 0.0050% by mass of O.

Description

Steel material Technical Field The present invention relates to steel. The present application claims priority based on 2023, 10,11 and japanese patent application No. 2023-175730, invented in japan, the contents of which are incorporated herein by reference. Background Parts such as gears, pulleys, shafts, etc. of automobiles and industrial machines are often roughly molded by hot forging or cold forging. In addition, such a mechanical component is required to have not only bending fatigue strength but also surface fatigue strength (pitting corrosion property). The carburizing and quenching are effective for improving the surface fatigue strength of the mechanical component. Therefore, in general, the above-mentioned machine parts are often manufactured by processing steel materials such as steel bars as raw materials by forging or the like to form a work material, cutting the work material into a part shape as needed, and then performing carburizing treatment (carburizing and quenching). As the carburizing treatment, a vacuum carburizing treatment for reducing the CO 2 discharge amount is used instead of the gas carburizing treatment which has been conventionally used in many cases. The gas carburizing treatment is a surface treatment performed by heating an object to a carburizing temperature in a carburizing gas. The vacuum carburization is a surface treatment performed by heating an object to a carburization temperature under vacuum, and then introducing a carburizing gas into an ambient gas at a low pressure and holding the object in a vacuum atmosphere for a diffusion period in which carbon is diffused more than once. In the vacuum carburization treatment, a hydrocarbon gas is used as a carbon supply source. Therefore, the vacuum carburization process does not generate CO 2 in the carburization reaction. Therefore, the vacuum carburization treatment can produce a member in a process friendly to the earth environment. On the other hand, it is known that in the carburizing treatment, since the object is heated to the carburizing temperature, abnormal grain growth occurs in the steel material, which becomes a cause of degradation of fatigue characteristics and the like. Therefore, suppression of abnormal grain growth (improvement of coarse grain resistance) at the time of carburization is required. In particular, in vacuum carburization, denitrification and NbC dissolution occur, and therefore, compared with gas carburization, the inhibition of abnormal grain growth is a very strict condition. Further, as described above, the member is roughly formed by hot forging or cold forging, but cold forging has an advantage that the cutting amount after forging can be reduced because of higher dimensional accuracy than hot forging. On the other hand, when carburized steel (engineering material) after cold forging is subjected to carburization, abnormal grain growth is very likely to occur. The reason for this is not clear, but it is considered that since the strain amount of the steel after cold forging is very high, recrystallization occurs at the time of carburizing and temperature rising, and since the gamma crystal grains are miniaturized due to gamma phase transformation from the recrystallized crystal grains, the grain growth driving force becomes very high, and abnormal grain growth easily occurs. After forging, the occurrence of abnormal grain growth can be suppressed to some extent by performing heat treatment such as normalizing for the purpose of grain sizing of γ grains, but this is not preferable because of an increase in cost and an increase in CO 2 emission due to an increase in the number of steps. Therefore, in recent years, for the purpose of improving environmental awareness, reducing cost, and the like, there has been a demand for a technique capable of suppressing abnormal grain growth during carburization even when vacuum carburization is performed without heat treatment such as normalizing after cold forging. In order to solve the problem of abnormal grain growth during carburization, patent document 1 discloses a cold forging steel which is excellent in cold forgeability and machinability after cold forging and can prevent coarsening of austenite grains during a surface hardening treatment after cold forging. In patent document 1, in order to improve chip disposability, the number density of sulfides having an average equivalent circular diameter of less than 2 μm is set to 300 pieces/mm 2 or more, and a steel for cold forging having a predetermined chemical composition containing a small amount of Bi is produced, whereby solid solution and coarsening of pinning particles (AlN or the like) in a surface hardening treatment after cold forging are suppressed, and generation of coarse austenite grains is suppressed. Patent document 2 discloses a hot rolled steel bar or wire rod which can realize excellent cold forgeability, i.e., low deformation resistance and high deformability after spheroi