JPS63183157A - Warm-forging steel - Google Patents

Abstract

PURPOSE:To prevent internal grain coarsening at the time of carburizing treatment after warm forging, by adding Ni in combination with specific amounts of Al and N to a steel with a specific composition. CONSTITUTION:This case-hardending steel for warm forging has a composition consisting of, by weight, 0.10-0.30% C, <=0.50% Si, 0.3-2.0% Mn, 0.10-0.40% Ni, 0.60-2.0% Cr, 0.02-0.05% Al, 0.0010-0.0060% N, and the balance Fe and containing, if necessary, 0.10-0.30% Mo and also has a structure in which internal grains after carburizing treatment is formed into graded fine grains of grain size No. 6-10. In this steel, Ni is necessary in order to form initial austenite grains into graded fine grains of the above grain size number, neither coarse grains nor fine grains, at the time of carburizing heating by the combined addition with prescribed amounts of Al and N. For this purpose, >=0.10% is required as additive quantity and the above effect increases with the increase of the additive quantity, but, even if the additive quantity exceeds 0.40%, the above effect is saturated and steel manufacturing costs are needlessly increased.

Description

[Detailed Description of the Invention] The present invention relates to a case-hardened steel for warm forging, and more specifically, for warm forging in which internal crystal grains do not become coarse during carburizing treatment after warm forging. Regarding case hardening steel.

BACKGROUND OF THE INVENTION In recent years, warm forging has been increasingly adopted in the production of mechanical structural parts for the purpose of eliminating process steps and saving energy. Warm forging is a forging method that incorporates the advantages of conventional hot forging and cold forging, and because its processing temperature range is lower than that of hot forging, it causes problems such as scale formation and thermal distortion. dimensional accuracy close to that of cold forging can be obtained, and on the other hand, since the work hardening that occurs in cold forging hardly occurs, post-forging annealing treatment required in cold forging can be omitted. It has the following advantages.

On the other hand, mechanical structural parts such as gears are often carburized after forming to improve wear resistance, but this carburizing process requires heating at high temperatures for a long time. Because of this treatment, crystal grains become coarser inside the component, causing strain or reducing toughness. Conventionally, in order to solve this problem, nitrides and carbides are formed by adding grain refining elements such as AjSNb and Ti and a large amount of N, as in the case of cold forging steel. Although it has been proposed to prevent coarsening of crystal grains during heating, sufficient effects have not yet been achieved.

Therefore, conventional countermeasures have been to perform normalizing treatment after warm forging to prevent coarsening of crystal grains during carburizing, or to change the warm forging temperature to a hot temperature that does not cause coarsening of crystal grains. Although measures such as bringing the temperature close to the forging temperature range have been adopted, it is clear that adopting such measures significantly impairs the advantages of warm forging described above.

Problems to be Solved by Japan The present invention was made to solve the above-mentioned problems with the conventional warm forging steel.In the carburizing treatment after warm forging, the internal crystal grains The purpose of the present invention is to provide case hardening steel that does not harden.

Means for solving the problems The warm forging steel according to the present invention has a C0.10 to 0 in weight%.
．． 30%, Si 0.50% or less, Mn 0.3-2.0%, Ni 0.10-0.40%, Cr 0.60-2.0%1. It consists of 10.02~0.05%, N 0.0010~0.0060%, the balance iron and inevitable impurities, and is characterized by being fine grains with an internal grain size number of 6~10 after carburizing. shall be.

The reason for limiting the chemical composition of the warm forging steel according to the present invention will be explained.

C needs to be added in an amount of at least 0.10% in order to provide strength necessary for mechanical structural parts. When it is less than 0.10%, the required strength cannot be imparted, and in order to obtain a constant C concentration,
Requires significantly longer carburizing times. However, when adding too much, sufficient internal toughness cannot be obtained depending on the low-temperature tempering performed on the carburized and quenched material, so the addition amount is set to be within a range of 0.30% or less.

Si is added as a deoxidizing element for steel, and as a hardenability improving element and solid solution strengthening element to obtain the required strength as a mechanical structural component. However, when added in excess, silicate-based inclusions increase, inhibiting warm workability, and increasing the hardness after warm forging, resulting in poor finishing cold workability and cutting workability. The upper limit of the amount added is set at 0.50% because it lowers the internal hardness after carburizing and lowers the internal toughness.

Mn has the effect of deoxidizing steel and improving hardenability. In order to effectively obtain such an effect, at least 0.3
% addition is required. However, when adding too much, bainite and martensitic structures are generated during cooling after warm forging, which impairs subsequent finishing cold working and machinability. The upper limit of the amount added is set at 2.0% because it promotes coarsening.

By adding Ni in combination with a predetermined amount of AJ and N, it is possible to make the initial austenite grains into fine grains with a grain size number of about 6 to 9 without making them coarse or fine during carburizing heating. A necessary element, in the present invention, 0.1
It is necessary to add 0% or more. Such an effect is
It increases as the amount of Ni added increases, but on the other hand, even if it is added in excess of 0.4%, the above effect will be saturated and the manufacturing cost of steel will increase unnecessarily.

Cr has the effect of improving the hardenability of steel and increasing the strength and toughness in a well-balanced manner, and in order to effectively obtain this effect, it is necessary to add 0.60% or more. However, when adding too much, it increases the deformation resistance during warm forging, shortens the life of the mold, and generates a bainite structure during cooling after forging, which reduces the subsequent finish cold workability and cutting process. The amount added should be 2.0% or less since it impedes the performance.

A2 has the effect of deoxidizing the steel, and when combined with a limited amount of N, it precipitates an appropriate amount of AIN, and furthermore, when combined with Ni, which will be described later, the initial austenite crystal grains during carburizing heating are reduced. It has the effect of preventing coarsening of the grains. To effectively obtain such effects, it is necessary to add 0.02% or more. However, when adding too much,
Since this may have harmful effects such as an increase in the amount of nonmetallic inclusions such as AhOl and inhibition of toughness, the amount added should be 0.05% or less.

As mentioned above, N precipitates an appropriate amount of AIN, and N
The combined addition of i prevents the initial austenite crystal grains from growing into coarse grains during carburizing heating, and also prevents them from becoming fine grains, thus increasing the initial grain size to the grain size number. This has the effect of making the particles evenly sized to about 6 to 9. In order to effectively obtain these effects, a small amount (
It is necessary to add 0°0010% for both. However, if too much is added, too much /IN will precipitate, and the initial grains will become fine grains or mixed grains containing fine grains, and the grain size after carburization will become coarser, so the amount added is 0.00
60% or less.

In addition to the above-mentioned elements, the case hardening steel for warm forging according to the present invention may contain Mo, if necessary, in order to further improve the strength and toughness of the steel. In order to effectively obtain the above effects, it is necessary to add 0.10% or more. However, addition of MO in an amount exceeding 0.3% causes significant generation of bainite and martensite during the cooling process after warm forging, and MO is an expensive element, so it is difficult to use from a practical standpoint. , an addition amount of 0.3% or less is sufficient.

As described above, the case-hardened steel for warm forging according to the present invention is particularly capable of removing initial austenite grains during carburizing and heating by adding Ni together with a predetermined amount of Al and N. Since the grain size is not made into coarse grains or fine grains, but is made into fine grains with a grain size number of about 6 to 9, the internal crystal grains do not become coarse during the carburizing treatment after warm forging, and thus, A carburized product having fine grains with a grain size number of 6 to 9 and excellent strength and toughness can be obtained.

Oshi ■ Steel having the chemical composition shown in Table 1 was melted and hot-rolled into a 4011-diameter steel bar, which was then machined into a test material with a diameter of 40mm and a length of 601m, which was then used for warm installation. Fully forged,
Warm workability such as warm deformation resistance, microstructure and hardness after warm forging were investigated. Furthermore, after warm forging, a carburizing treatment was carried out at 950° C. for 3 hours, and the characteristics of the crystal grain size were investigated, and the austenite crystal grain size at the initial stage of heating was also investigated to show the grain growth process. The initial grain size was determined by heating the test material after warm forging at 900° C. for 30 minutes and then water-quenching it. The above results are shown in Table 2.

In Table 1, the steels of the present invention are indicated with an asterisk.

Comparative steel 1 has a low surface hardness (Hv) of 5 due to the low C content.
It is only 10, which is considerably lower than that of the steel of the present invention.

Since Comparative Steel 5 does not contain Ni, both the initial grain size and the grain size after carburizing treatment are coarse grains of 6 or less, and the internal toughness is also inferior to the steel of the present invention.

Comparative Steels 6 and 7 are steels with a high N content, which is conventionally widely used to prevent coarsening. but,
In either case, the effect of coarsening the crystal grains due to N is poor, and the internal toughness is also low. Comparative Steel 9 shows an example in which the amount of Si is excessive, and it is clear that the deformation resistance during warm forging is considerably greater than that of the steel of the present invention, and that the workability is inferior. In addition, since the hardness after warm forging is high, the finish cold forgeability and finish machinability in the subsequent process are low.

Comparative steel 10 shows an example in which the amount of Mn is significantly lower than the amount specified in the present invention, and the internal hardness is low (lack of strength as a part. On the other hand, comparative steel 12 has an excessive amount of Mn. As an example, problems arise in terms of warm deformation resistance and hardness after warm forging, and the crystal grains also become coarser after carburizing.Furthermore, this comparative steel 12 has a low Ni and N content. , is within the range specified in the present invention, but due to the excessive Mn content, the structure after forging partially becomes fine bainite, so the initial grains become fine grains and eventually become coarse. This has led to the promotion of

Comparative steel 13 shows an example with a high amount of Ni, and there is no difference in properties compared to inventive steels 3 and 4, but even if excessive Ni is added, particularly, further effects can be obtained. It has been shown that this is not possible. Comparative steels 14 and 15 show examples in which the Cr content is outside the range specified by the present invention, and the same problems occur when the Mn content is out of the range specified by the present invention.

Comparative steels 16 and 19 are examples in which the /l amount is outside the specified range according to the present invention. Comparative Steel 16 shows an example in which the amount of Al is excessive, and although grain coarsening does not occur due to the effects of Ni addition and low N, the internal toughness is extremely deteriorated due to the increase in the amount of alumina inclusions. There is. On the other hand, comparative steel 1
9 is a case where the Ajli amount is extremely low, and as is clear from the initial grain size of 4.5, coarsening of the grains is observed at the initial stage of heating, and the grains continue to coarsen even after carburizing, resulting in poor toughness. It's hindering. Comparative steel 18 shows an example with a high amount of C,
The hardness after warm forging is high, and the internal hardness after carburizing is significant, resulting in significant deterioration of toughness.

Comparative Steel 20 is an example in which a large amount of MO is added, and compared to Invention Steel 17, it has higher deformation resistance and hardness after warm forging;
It is shown that adding a large amount of MO is rather harmful. Furthermore, due to the bainitic structure, the crystal grains during carburization also become coarse.

Comparative Steel 21 shows an example in which Nb, which is conventionally used as a refining element, is added, and due to the coarsening prevention effect of Nb carbonitride, the initial grain is extremely small with a grain size number of 10. However, after carburizing treatment at high temperatures and over a long period of time, there is no effect of preventing this, and coarsening is evident.

In contrast to the comparative steel described above, the steel of the present invention has an internal grain size of 6 to 9 during carburizing, and has excellent internal toughness.

Claims (2)

[Claims] (1) C0.10-0.30%, Si0.50% or less, Mn0.3-2.0%, Ni0.10-0.40%, Cr0.60-2.0%, Al0. 02~0.05%, N0.0010~0.0060%, balance iron and unavoidable impurities, and is characterized by being fine grains with an internal grain size number of 6~10 after carburizing. Case hardened steel for forging. (2) C0.10-0.30%, Si0.50% or less, Mn0.3-2.0%, Ni0.10-0.40%, Cr0.60-2.0%, Al0. 02~0.05%, N0.0010~0.0060%, Mo0.10~0.30%, balance iron and unavoidable impurities, and the internal grain size number after carburizing is 6~10. A case hardening steel for warm forging characterized by grains.