JPS58107474A - Hot forging steel for machine structure - Google Patents

Abstract

PURPOSE:To provide hot forging steel for a machine structure enabled in the omission of direct hardening and annealing after hot forging, by a method wherein C, Si, Mn, Al and N are contained in Fe in a specific ratio and a specific amount of AlN with a specific particle size is made present. CONSTITUTION:Steel containing 0.12-0.8% C, 0.1-1.0% Si, 0.3-2.5% Mn, 0.01- 0.1% Al, 0.002-0.012% N among which Al and N are present in a form of AlN with an average particle size of 0.08-1.0mu in an amount of 0.01-0.03% and comprising the remainder Fe and inevitable impurities is prepared. By this composition, the growth can be suppressed. In addition, in order to attain to enhance strength, according to necessity, one kind or more of 2.0% or less Cr, 0.5% or less V, 2% or less Ni, 0.3% or less Cu and 0.3% or less Mo is contained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a forging machine structural steel that enables hot forging @ 0 direct quenching and omitting standards after hot forging.

Steel for machine structures is usually used after hot forging, reheating, and quenching. After hot forging, it is often further subjected to standard treatment and then reheated and quenched. In recent years, for the purpose of energy saving, attempts have been made to omit these heat treatments after hot forging and to directly quench the material after hot forging, a so-called forging direct quenching method. However, the hot forging temperature is generally 1200℃ or higher, and when conventional steels are heated to such high temperatures, the crystal grains become significantly coarsened, so machine parts that are forged and quenched directly have excellent toughness. It's hard to get anything.

Furthermore, steel for machine structures is often used as a standard after hot forging. In order to save energy, there are attempts to omit this standard processing. However, as in the case of direct forging and quenching, the hot forging temperature is high, so the crystal grains become coarse. Therefore, if the standard is omitted, the toughness will be extremely low.

Furthermore, in recent years, carbon steel with an appropriate amount of V added thereto and used as hot-forged has begun to be used. However, although strength can be obtained through carbide precipitation strengthening in (2), since it is used as hot forged, the crystal grains become coarse and it is difficult to obtain excellent toughness, which limits the scope of application of this type of steel.

As mentioned above, the grains of mechanical structural steel subjected to hot forging become coarse due to heating at high temperatures for hot forging, so if various heat treatments after hot forging are omitted, The toughness of mechanical parts decreases. This decrease in toughness can be resolved by lowering the heating temperature for hot forging, but if the temperature is lowered too much, it will become impossible to form or the life of the mold will be extremely shortened. Therefore, it is necessary to heat to at least 1150°C. However, in conventional steels, the crystal grains become considerably coarse even at 1000°C, and a decrease in toughness is inevitable due to omission of heat treatment after hot forging.

From the above, if we can supply a mechanical structural steel with excellent performance in suppressing grain growth during hot forging, it will greatly contribute to reducing energy consumption in the processing process.

Now, prevention of grain coarsening in steel at high temperatures is a major challenge in metallography, and much research has been carried out. It is also known that precipitates of Aj, Nb, T1, etc. have an effect of inhibiting crystal grain coarsening. In other words, kl has a strong deoxidizing effect, and the heating temperature for normal reheating quenching is 9
It is widely used because it can easily achieve a finer graining effect at temperatures around 00°C.

However, in the conventional grain size control using AtN, AtN is precipitated as finely and as much as possible before heating, and the temperature range at which the precipitated particles do not dissolve, that is, 900
The aim was to refine grains at relatively low temperatures, around ℃.

Therefore, although hot forging conditions have become lower in temperature and shorter in time in recent years, the above-mentioned fine AtN has been completely dissolved into solid solution and has not been useful in preventing coarsening of crystal grains.

By the way, the present inventors added Aj which is usually added to this steel.
As a result of various investigations into whether there is a way to suppress grain growth after hot forging by making effective use of It was discovered that grain growth after hot forging can be suppressed by skillfully utilizing this.

That is, the gist of the present invention is that C0,12~o
, s*, SiO, 1-1. On, Mn 0.3~2
．． 5111, AtO, 01-0.1%, No, 0'
O'2 ~ 0.012-, and since these are AtN0 type and have an average particle size of O,OS ~ 1.0 fi, it is 0.01 ~ 0.
．． 03, or in addition to these basic components, Cr
2 flb or less, v O, 5n or less, N129i or less,
The hot forging machine structural steel contains one or more of Cu 0.3 III or less and Mo 0.3 - or less, with the remainder being F and impurities.

The present invention will be explained in more detail below.

First, the average particle size of AAN was limited to 0.08 to ton based on the experiment shown below. i.e. 0.45% C-0,
25 regrets 5i-0,75%M! 1 as the basic component, At,
By changing the amount of N and the heating rate, temperature, and time of rolling heating, steel with an average grain size of AtN of 0.01 to 8 μm was produced.

These steel pieces were high-frequency heated to 1200° C., and the amount of AtN in the steel immediately after forging was determined.

Figure 1 shows the relationship between the average grain size of AjH in the material and the amount of AAN immediately after forging. AAN effective in preventing crystal grain coarsening
is hardly detected when the average particle size of AtN in the material is less than O,OSμ. That is, in order for AAN, which is effective in coarsening grains, to exist in the steel after hot forging, the average grain size of AAN in the material must be 0.081 or more. Furthermore, if the average grain size of AjN exceeds 1 μ, the amount of AAN after forging decreases, so it is appropriate that the average grain size is 1 μ or less.

Next, we will discuss the main reason for limiting the amount of AtN precipitated to 0.601 - 0.03C.
-0,25181-1,3516Mn as basic component A
This figure shows the relationship between the grain size after forging and finishing of steel bars produced with varying amounts of AAN precipitation within the range of 01 to 0.15 μm in average grain size. If the amount of precipitation is less than 0.0111, the crystal grains will rapidly become coarse and the effects of the present invention will not be apparent. If it is 0.01- or more, the crystal grains become finer as the amount of precipitation increases, or 0.03- or more is sufficient from a practical point of view.

In addition, to explain in detail the reasons for limiting other ingredients, first of all, At
is the ninth 6bt' element that precipitates mu α,
If it is less than 11, it is insufficient and should be added, and if it exceeds 1, the effect commensurate with the addition cannot be obtained, so it is set to be 0.01 to 0.01.

Next, N is a necessary element to precipitate AtN like A/, but if it is less than 9.002-, it is insufficient;
If it exceeds 01211, there is a risk of embrittlement of the steel, so 0.00
2 to 0.0121.

In addition, C is an essential element for obtaining strength in machine structural steel, but if it is less than 0.12, the strength will be insufficient as a machine rod part, and if it exceeds 0.08, the ductility will be extremely low.
．． The temperature was 12 to 0.8 minutes.

Furthermore, 81 is added for the purpose of deoxidizing, but if it is less than 0.1%, it is not sufficient (for deoxidation), and if it is more than 1.0%, the effect commensurate with the amount added cannot be obtained, so the addition range is limited to 0, 1. -1.0
It was a fight.

Finally, Mn is added to increase hardenability, as well as to desulfurize and fix sulfur, but if it is less than 03, the effect is insufficient, and if it is more than 2.5, the effect of increasing hardenability will not be noticeable. Therefore, it was set as 0.3 to 2.5 skin.

The above is the basic composition system of the steel of the present invention, and by using this composition system, grain growth after hot forging can be suppressed. is Cr l V I Nl l Cu, M.

One type or 211 or more of these can be contained in an appropriate amount.

First, Cr has the effect of strengthening steel by increasing hardenability and temper softening resistance. But 2g! Addition of more than 1 is expensive and coexistence with Kn tends to cause deterioration of toughness, so the content is set to 2 Is or less.

Next, ■ is a useful element because it works to precipitate carbides and strengthen steel materials when added in small amounts; Therefore, the upper limit of addition was set at 0.5-.

Further, Nl is an element that has the effect of toughening the base of steel, but if it exceeds 2, the effect commensurate with its addition cannot be obtained, so it is set to 2 or less.

Furthermore, Ca is an element that strengthens the base of steel and increases its corrosion resistance, but if added in a large amount, it causes melt flow and cracking during manufacturing and rolling, so it was set to 0.3 or less.

Finally, MO increases the hardenability and temper softening resistance of steel,
It is an element that gives strength to steel, but it is expensive and
If the amount of addition exceeds 1, the effect commensurate with the amount added cannot be obtained.

As explained above, the mechanical structural steel of the present invention has the effect of suppressing grain growth due to hot forging after high-temperature short-time heating.

The effects of the present invention will be described in more detail below with reference to Examples.

Example Steel having the chemical composition shown in Table 1 was melted, heated at a moderate temperature after blooming, and then rolled to a diameter of 40 mm. A 40φ x 60t steel material cut out from this was heated by induction heating to the maximum heating source f in Table 1 for the time indicated in the heating time column in Table 1, and upturned to a thickness of 15 mm in 20 seconds. carry out forging,
After 10 seconds, it was quenched in water. The cross section was examined under a microscope to determine the average particle size.

Crystal grains in the same table [as shown in column No. 11, the end of the symbol is A
The steel of the present invention, indicated by 7, is 2
．． The effect of the present invention is obvious when the grains are 5 to 5 times finer.

As mentioned above, the steel of the present invention has a great effect of suppressing grain growth during hot forging, and has high industrial value as it enables energy saving in the manufacturing process of mechanical parts.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the average grain size of AtN in the material and the amount of ktN precipitated immediately after hot forging, and Figure 2 shows the relationship between the amount of AAN precipitated before forging and the grain size number after forging. This is a diagram showing this. o/ 10 Material I) AIN average grain size, j Rod 2 side 0 6.0/

Claims (2)

[Claims] (1) C0,12~o,s*, at O,1~1
．． 01G, MlO93~2.5min, AJ! 0.01～G
1 tragedy, NO, 002 ~ 0.01! 0.02, containing particles and having an average particle size of 0.08 to 1.0μ in the form of AtN.
．． 01 to 0.03- present, with the remainder consisting of F and impurities. (2) CG, 12~0.81G, 810.1~1. O
哄, M+aO, 3~2.5 Ill, A40.01~0
1%, NO,002~9.01211, and these are present in the form of MUAH with average grain @O,OS~1.0μ, 0.01~0.03*, and further Cr2. 0
gk and above 1;, vo, s- and below, N12Is and below, Cm
O, 3 or more capsules 7. A steel for machine structural use for hot forging, characterized in that it contains one or more types of O below Mo0.3, and the remainder consists of F and impurities.