JPS5852424A - Manufacture of steel for carburizing at high temperature - Google Patents

Abstract

PURPOSE:To enable carburizing at a high temp. in a short time without forming abnormal coarse grains, etc. by hot rolling a steel contg. specified amounts of N and Al under specified conditions to precipitate fine AlN and by using the resulting steel as a case hardening steel for carburizing at a high temp. CONSTITUTION:When steel parts are carburized, to reduce the consumption of energy they are carburized at a high temp. such as >=980 deg.C in a short time, so mixed grains and abnormal coarse grains are formed in the carburized steel structure to reduce the toughness. In order to prevent the formation of such grains, a case hardening carbon steel or a case hardening low alloy steel contg. 0.0040-0.0200% N and 0.020-0.100% Al is heated to 700-1,050 deg.C and hot rolled to precipitate solubilized AlN as 20X10<-12>/cm<2> fine AlN grains as rolled. The resulting steel is used as a steel for carburizing at a high temp. The steel can be carburized at a high temp. in a short time with no reduction in the toughness and little burning strain.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing case hardening steel for high 1 carburization, which can prevent the generation of granules or coarse grains even when carburized at a high temperature such as 980°C. It is.

Gears used in automobile transmissions, differentials, etc., as well as mechanical parts such as pins, screws, and bolts, are manufactured using complex processes such as bar steel or no-in coils, multi-forging, cutting, annealing, etc. After that, it is finally manufactured by carburizing. This heat treatment for carburizing is a heat treatment that requires a long time ranging from several hours to several tens of hours, and is extremely energy-consuming.

By the way, by raising the carburizing heat treatment temperature, it is possible to shorten the carburizing treatment time and achieve effective energy saving. However, by raising the treatment temperature, crystal grains such as mixed grains and abnormal grains Coarsening may occur, resulting in material deterioration such as a decrease in toughness and an increase in sintering strain, and the reality is that carburization occurs at temperatures below 920°C to 930°C and 11°C. A number of measures have been proposed to prevent the coarsening of crystal grains, but no perfect solution has yet been found.

Conventionally, it has been known that the fine precipitation of AtN can be used to prevent crystal grain coarsening during heat treatment. When it comes to how to achieve this in an industrially stable manner, it is difficult to know exactly when it will be in good condition, i.e., a salt of 1200 t or more, in which AtN will be dissolved as a solid solution. Hot forged at 0°C, then annealed,
At what stage and how should we achieve fine precipitation of AtN for gears that are manufactured through the gear cutting and carburizing processes? In the case of carburized parts, which are made by immediately molding steel materials by cold extrusion, the question of when to do so is extremely complex and varied. Currently, no countermeasures have been taken.

The present inventors have discovered that the At
The precipitation state of N was studied in detail, and it was found that the number of ktN (more than 20
By aiming for 011JII precipitation, an effective grain coarsening prevention effect can be obtained, and in order to achieve this, the optimum steel composition and rolling conditions were discovered, and 2*a were completed.

That is, 0.0040-0.0200XN, 0.01-0
．． In the production of carbon steel containing 100XAtt or low joint skin cast steel, steel pieces containing AN in solid solution ttooc or more1
After heating to 000°C or less, hot rolling is performed to finely precipitate 20XlO~ or more of tN in the rolling ott, which effectively prevents coarsening of crystal grains during carburizing. .

No. 15A is At precipitated in the process immediately before carburizing heat treatment.
N 12) This is a diagram showing the relationship between density and crystal grain coarsening temperature during 4-hour heating.
It has been found that the crystal grain coarsening temperature increases to about 990° C. by precipitating 0x1G or more than cJ.

Furthermore, the present inventors should secure a precipitation basin for AjN of 20X10 ke/C## or more in the state of pressure IA], and under normal secondary processing conditions, sufficiently immerse [before treatment]. It was confirmed that this precipitation state was maintained at t. In other words, annealing and simple heat treatment performed during the parts manufacturing process are performed at temperatures below 950°C, and AAN precipitates during rolling.
It remains well above the rest. Also, large gears,
The shape is formed by hot forging, but in this case, the AAN that has precipitated in the as-rolled state becomes a solid solution at the hot forging temperature, but since annealing is usually performed afterwards, the amount of AtN is adjusted. If this is done, fine precipitation of AtN can be sufficiently ensured. That is, we studied the conditions for achieving the number of AtN deposits of 20 x 10 or more under pressure, and the following results were obtained:
Aj, steel billet with NO solid solution t"700p, or more 1
What can be achieved by heating and rolling after heating to 050 degrees Celsius or less The present invention aims at saving energy by shortening the carburizing time on the steel user's side, and saving energy by shortening the carburizing time on the steel material manufacturer's side, and saving energy by low-temperature hot rolling on the steel material manufacturer's side. It is also a beneficial method that can enjoy the industrial benefits of energy saving.

Here, the reasons for limiting the chemical composition and hot rolling conditions will be described.

N combines with At as muAN to exert a crystal grain refining effect. If it is less than 0.004N, the amount of precipitated Nc) is small and a sufficient effect cannot be obtained, and if it exceeds 0.0200Xt However, it was excluded from the scope of the claims because no further effect could be obtained.

Aj is a concentrated substance that combines with N and is used as ALN. If it is less than 0.020%, the number of AtNQ precipitated will be small and a sufficient effect will not be obtained, and even if it is added in an amount exceeding 0.100N, , was excluded from the scope of the claims because no further effect could be expected.

A steel billet subjected to bar-in-coil rolling is a state in which Aj and N are dissolved in Kn, and even if the following rolling heating conditions are regulated, sufficient fine precipitation of AtN cannot be achieved, especially when the steel billet is 0.
If AjN is incompletely precipitated in this state, during the first heating of the steel material, the remaining p will be A
It must be avoided because it precipitates as J, N and becomes a coarse precipitated O state.

Heating the steel billet to 11 degrees Celsius must be avoided because if the temperature exceeds 1050 degrees Celsius, fine precipitation of AN will not be obtained.The base steel billet to be subjected to rolling may be heated by the steel ingot method if AtN is dissolved in solid solution. It does not matter whether the slab was manufactured by a continuous casting method or by a continuous casting method, and the slab is manufactured by blooming a continuously cast slab without cooling it, or by blooming the slab after cooling once. It does not matter whether it was manufactured by

Steels to which the present invention can be applied include carbon steel that is carburized and used;
It is a case hardening steel of low alloy steel, usually 0:0.05X~0.3
%, Si: 0.4X or less, Mllo, carbon steel containing 3 to L7X, low-in gun steel, and ○rlo of tz% or less
, 3X or less MO, 2% or less Nj, etc.

In addition to the above, by adding Nb with a K of 0.02 to 0.10X, it is possible to manufacture a steel material whose crystal grains do not become coarse at even higher temperatures.

Further explanation will be given below with reference to Examples.

[Example 1] 80R420H steel having the components shown in Table 1 was melted,
150-square steel billets were produced by continuous casting and blooming rolling.0-minute flow rolling was performed by heating at 1250°C, and At the steel billet stage.
Nt was dissolved in solid solution.

These steel pieces were rolled into round bars with a diameter of 40 cm under the heating conditions shown in Table 2, and then the steel bars were cut and annealed.

Cold forging, simple, gears were manufactured by carburizing at 990℃ for 3 hours after gear cutting.All carburizing depths were 1.
An effective depth of 2-1.3- was obtained.

Samples of each part of this gear were cut out under a microscope and the crystal grains were observed. The results are shown in Table 2.

It can be seen that in the case of a gear with a higher H content than the steel material according to the method of the present invention, coarsening of crystal grains does not occur.

[Example 2] 50M420H steel having the components shown in the FA3 table was melted, and a 150-square steel billet was produced by continuous casting and blooming.0-blooming was performed at 12sO ℃ heating, and at the billet stage. λ
We attempted to make AN a solid solution.

These O steel pieces were rolled into steel bars with a diameter of 36 mm under the heating conditions shown in Table 4. After cutting this steel bar, part of it is cold extruded,
After hot forging the remaining 9 parts, annealing and gear cutting]%990 after processing
A gear was manufactured by carburizing at ℃ for 2 hours. An effective carburizing depth of 0.7 to 0.8 was obtained for all cases.

Microscopic samples were taken from each part of the earth and crystal grains were observed. The results are shown in Table 4.

It can be seen that when the steel material according to the method of the present invention was produced, coarsening of crystal grains did not occur.

[Example 3] Perfume 4.5 80M42 having the chemical components shown in Table 4
0H#t-Example 1 As in Example 2, a 150-square steel piece was used.

The steel pieces were rolled into round steel with a diameter of 90 m1 under the heating conditions shown in Table 5. After cutting this steel bar, hot forging, annealing, and gear cutting]
It was processed and carburized at 990°C for 4 hours to produce a gear. Effective carburization depths of 1.7 to 1.8 m were obtained in all cases.

This gear was cut and microscopic samples were taken from each part to observe the crystal grains. The results are shown in Table 5.

It can be seen that when the steel material according to the method of the present invention is manufactured, coarsening of crystal grains does not occur if the 4N is within the range of non-invention steel.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between AtN precipitation density and grain coarsening temperature. Agent: Patent attorney Masamitsu Akizawa and 2 others

Claims (1)

[Claims] (1) N: 0.0040-0.0200%,
A/, : Steel bar 7 for case hardening of carbon steel or low alloy steel containing 0.020 to 0.1OOX, in the production of bar-in coil, steel billet t-700℃ or higher with tN solid solution 1050
Production of high-temperature carburizing steel that prevents coarsening of grains during carburizing by performing hot rolling after heating to 112 °C or less, and finely precipitating AAN of 20 x 10 K or more as it is rolled. Method.