JPS6154844B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing steel for forging machine structures that enables direct quenching after hot forging, omitting normalization after hot forging, or shortening carburizing time. In general, steel for machine structures requires a lot of thermal energy for hot forging, heat treatment, especially carburizing treatment, etc., and many of them are made into machine parts. It is a major social demand to reduce this as much as possible. Hot forging of mechanical structural steel is generally performed at temperatures of 1200°C or higher. When conventional steels are heated to such high temperatures, their crystal grains become significantly coarsened and their structures become coarse. For this reason, mechanical parts that require toughness even after hot forging are subject to the effects of hot forging, and are then directly quenched (also called forge quenching) by using the residual heat after hot forging. quenching without heating), the required performance cannot be obtained. There it is reheated and hardened. Further, in order to adjust the coarse structure after hot forging, normalization is often performed after hot forging. These problems can be resolved by lowering the current heating temperature during forging, but if the temperature is lowered too much, it will not be possible to form, so the temperature should be at least 1150℃.
requires heating. However, with conventional steels, the structure becomes quite coarse even at 1000°C. In addition, so-called case-hardened steel, which is used after carburizing, is usually heated at a temperature of around 930°C after forging, and is generally used at a temperature of 4~5°C.
Carburized for 50 to 60 hours. In these cases, the carburizing time can be shortened by increasing the carburizing temperature. If the carburizing temperature is increased by 50℃, the time will be approx.
It can be done in 1/2. Therefore, even if the carburizing temperature is increased, total energy is saved. However, with conventional case hardening steel, even if the carburizing temperature is increased to 930°C or higher, the structure becomes coarser and the performance deteriorates. Moreover, it is known that such coarsening of the structure and deterioration of performance are particularly noticeable when cold forging is performed before carburizing. As described above, if the steel for machine structures used for forging has a fine structure even at high temperatures, it will greatly contribute to reducing energy consumption in the processing process. Now, the coarsening of the structure of steel at high temperatures is based on the coarsening of crystal grains, and preventing this grain coarsening is a major challenge in metallography, and much research has been carried out. And A, Nb,
It is known that precipitates such as Ti have the effect of inhibiting crystal grain coarsening. In particular, it is known that TiN is thermally stable and has the effect of inhibiting crystal grain coarsening at higher temperatures than other precipitates. However, in fact,
Conventional knowledge alone does not necessarily yield the desired steel. Sometimes it works, but it is not reproducible. Therefore, even though it is known that TiN has a large grain coarsening inhibiting effect up to high temperatures,
Very few have been put into practical use. Furthermore, Zr, Hf
It is known that like Ti, it has the effect of inhibiting crystal grain coarsening, but its effect is poorly reproducible and few have been put into practical use. The present invention clarifies the interrelationship between alloy components of steel and manufacturing conditions in order to take advantage of the crystal grain coarsening inhibiting effect of Ti, Zr, and Hf nitrides, and embodies this in steel for machine structural use for forging. It has become. In other words, the gist of the present invention is C0.12
~0.8%, Si0.1~1.0%, Mn2 (C%) ~2.5%,
A 0.01 to 0.05%, containing one or more of Ti, Zr and Hf, (Ti%) + 0.53 (Zr%) +
0.27 (Hf%) is 0.01~0.05%, N is 0.29 (Ti%) +
0.15 (Zr%) + 0.08 (Hf%) to 0.015%, and further contains one or more of Cr2% or less, Ni3% or less, Mo0.5% or less,
Continuously cast slabs with the remainder consisting of Fe and impurities
A method for producing a steel for machine structural use for forging which still has a fine structure even at high temperatures, which comprises heating to a temperature of 950 to 1120°C and then rolling or forging. The present invention will be explained in detail below. Ti, N using A and B shown in Table 1 as basic components
Steel with various additive amounts is melted, formed into ingots by continuous casting method and ingot method, and heated to 1100℃ to form 20φ
It was extended to. After heating these steel pieces to various temperatures for 1 hour, we observed the prior austenite grains when cooled, measured the temperature at which approximately 50% coarse grains were visible in terms of the area ratio of the observed surface, and determined this as crystallization. This was taken as the grain coarsening temperature.

[Table] The relationship between the grain coarsening temperature and the amount of Ti is shown in Figure 1. In Figure 1, regardless of whether the component system A or B is used as the basic component, if the ingot is made into an ingot,
Regardless of the amount of Ti, the crystal grain coarsening temperature is not improved at all. On the other hand, when a continuously cast steel ingot is used, a remarkable improvement in grain coarsening temperature is observed as the amount of Ti increases. Therefore,
In order to use Ti to improve the grain coarsening temperature, ingot formation by continuous casting is essential. Also,
When Zr and Hf were added, the same results as in FIG. 1 were obtained when the amounts of Zr and Hf were converted to the amount of Ti based on the atomic weight ratio. Therefore, the addition of Ti, Zr, and Hf can be handled uniformly by converting the atomic weight ratio into the amount of Ti and using the sum as the Ti equivalent. i.e.
Ti equivalent can be expressed as Ti% + 0.53 (Zr%) + 0.27 (Hf%). Next, using S53C and SMn443 steel as basic components, the amounts of Ti, Zr, Hf, and N were varied to produce steel materials having various grain coarsening temperatures. Prior austenite grains were observed when this steel material was directly quenched after hot forging at 1250 to 1300°C (forging ratio 5), and the area ratio of coarse grains on the observed surface was measured. This is defined as the coarse particle mixing rate, and the results are shown in FIG. Furthermore, by using SCM420 steel and SCr420 steel as basic components and varying the amounts of Ti, Zr, Hf, and N, steel materials with various grain coarsening temperatures were manufactured. This steel material was annealed to form a spheroid, then cold forged at compression ratios of 0, 30, and 80%, and carburized at 930°C for 5 hours.
The prior austenite grains of this carburized material were observed, and the area ratio of coarse grains on the observed surface was measured, which was defined as the coarse grain mixing ratio. The results are shown in Figure 3. From these Figures 2 and 3, it is clear that in order to save energy during hot forging or carburizing of forging machine structural steel mentioned at the beginning, the grain coarsening temperature must be 1050°C or higher. It turns out to be desirable. Therefore, as can be seen from FIG. 1, in order to obtain a grain coarsening temperature of 1050° C. or higher, the Ti content or Ti equivalent must be 0.01% or higher. However, it is almost saturated at 0.05%. On the other hand, in order to utilize N more effectively than Ti, Zr and Hf, it is necessary to at least
Sufficient N is required so that Ti, Zr and Hf all become nitrides. For that purpose, M+N→MN(M
(Ti, Zr or Hf) reaction, the amount of N commensurate with the amount of Ti, i.e. 0.29 (Ti%) + 0.15 (Zr
%)+0.08 (Hf%) or more is required. In fact, the high grain coarsening temperature is 0.29 (Ti%) +
Excess N is better than 0.15 (Zr%) + 0.08 (Hf%).
Obtained stably. However, even when more than 0.015% was added, no effect commensurate with the added amount was observed. Next 0.25, 0.45, 0.75%C-0.01%Ti-0.01%
Based on Zr-0.01%Hf-0.009%N, Si1% or less,
Mn2.5% or less, Cr2% or less, Ni3% or less, Mo0.5%
Hereinafter, steel to which various amounts of one or more of each element (A 0.05% or less) were added was melted and cast into slabs by continuous casting, and experiments were conducted in the same manner as described above. Then, the grain coarsening temperatures of these steels were determined, and the effects of Si, Mn, Cr, Ni, Mo, and A on these steels were determined.
clarified the impact of As a result, it was found that other elements except Mn had almost no effect. i.e. normally used in mechanical structural steel
Even if one or more of the following elements are added: Si 1% or less, Cr 2% or less, Ni 3% or less, Mo 0.5% or less, A 0.05% or less, there will be no adverse effect on the crystal grain coarsening temperature. . Therefore, in order to impart performance as a mechanical structural steel, each of these elements is added as necessary within the above-mentioned range. If the above range is exceeded,
Not required for general mechanical structural steel. However, if the molten steel is not sufficiently deoxidized, the yield of Ti, Zr and Hf will not be stabilized, so in order to stabilize this, Si0.1% or more of Si and A, which have a strong deoxidizing effect, and A
0.01% or more is required. As can be seen from Figure 4, which shows the relationship between Mn content and crystal grain coarsening temperature in the above experiment, Mn
As the amount increases, the crystal grain coarsening temperature also increases. Also, from the same figure, in order to obtain a grain coarsening temperature of 1050℃ or higher, for 0.25% C steel,
0.5% or more, 0.9% or more for steel with 0.45%C,
It can be seen that 0.75% C steel requires 1.5% or more of Mn. In order to obtain a grain coarsening temperature of 1050° C. or higher, steel with a large amount of C requires a larger amount of Mn. Generalizing this relationship, 2(C%)≦Mn. However, 2%
It is almost saturated with Mn and does not require more than 2.5%. Not only that, but exceeding 2.5% actually impairs the toughness of the steel. Although we have conducted a large number of experiments on the effects of alloying elements on grain coarsening temperatures and clarified their interrelationships, sometimes things still don't work out as expected. That is,
Even with steel with appropriate composition, it is often not possible to obtain a grain coarsening temperature of 1050°C or higher. It was found that this was due to the appropriate or inappropriate heating temperature when forging the test steel ingot. That is, 0.45%C-0.25%Si-1.0%Mn-
Steel whose main components are 0.01%Ti-0.015%Zr-0.02%Hf-0.008%N is melted and cast into 5.5 ton ingots and continuously cast slabs, and these steel ingots are heated to various temperatures. , stretched. The grain coarsening temperature of each of the thus drawn steel pieces was determined in the same manner as described above. The result is the fifth
Shown in the figure. The figure illustrates the grain coarsening temperature of each steel slab with respect to the heating temperature of the steel ingot. As is clear from the figure, the target grain coarsening temperature of 1050°C or higher can be obtained only when the casting method is continuous casting and the ingot heating temperature is 1120°C or lower or 1300°C or higher. .
That is, in the present invention, the means for forging the slab must be continuous casting, and the heating temperature for drawing the slab is also important;
In a range below 1300°C, the desired goal cannot be achieved. In addition, the heating temperature range of the slab is 950 to 950 in the present invention based on the data shown in FIG.
The temperature was limited to 1120℃. That is, if the heating temperature is less than 950°C, the deformation load during rolling or casting will only increase, and the degree of improvement in the effect will be small. On the other hand, if the heating temperature exceeds 1120°C, the crystal grain coarsening temperature will become low, and the object of the present invention will not be achieved. does not match. Note that when the temperature exceeds 1300°C, although the crystal grain coarsening temperature rises again, the heating temperature is high and the consumption of thermal energy becomes large, which is disadvantageous from an industrial perspective. To summarize what has been explained above, C0.12~
0.8%, Si0.1~1.0%, A0.01~0.05%, and
In general mechanical structural steels consisting of one or more of Cr2% or less, Ni3% or less, and Mo0.5% or more, Mn2 (C %)~2.5%, Ti, Zr and Hf
Contains one or more of the following, Ti + 0.53 (Zr%)
+0.27 (Hf%) is 0.01~0.05%, N is 0.29 (Ti
%) + 0.15 (Zr%) + 0.08 (Hf%) to 0.015% it is necessary to draw the continuous cast steel ingot at a temperature of 950-1120 ° C. Thus, FIGS. 2 and 3
As shown in the figure, it is possible to produce forging machine structural steel that still has a fine structure even at high temperatures. Hereinafter, the effects of the present invention will be described in more detail with reference to Examples. Example Conventional steel 1, 4, having the chemical composition shown in Table 2
7, 10 and Mn, Ti, Zr, Hf, N within the scope of the present invention.
Steel 2, 3, 5, 6, 8, 9, 11, 1 with adjusted
2, 13, and 14 were melted, some of them were made into steel ingots, and some of them were made into continuously cast slabs.
Heated to 1100℃ and approximately 1250℃ and rolled.
We made a 120□ square steel. These square steel bars were cut to determine the grain coarsening temperature. This crystal is the third
Shown in the table. In the same table, the numbers marked with a circle are examples of the present invention, and the others are comparative examples. Comparing these grain coarsening temperatures, it is clear that the method of the present invention is superior. The forging machine structural steel produced by the method of the present invention contributes to energy savings in the secondary processing process, and has great industrial value.

【table】

【table】

【table】

【table】 [Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between Ti content and crystal grain coarsening temperature, Figure 2 is a diagram showing the relationship between grain coarsening temperature of the material in hot forging and coarse grain mixing rate, and Figure 3 is a diagram showing the relationship between grain coarsening temperature and coarse grain mixing rate of the material in hot forging. Diagram 4 showing the relationship between the grain coarsening temperature of the material and the coarse grain mixing rate during carburizing after cold compression.
The figure shows the relationship between Mn content and grain coarsening temperature, and FIG. 5 shows the relationship between steel ingot heating temperature and grain coarsening temperature.

Claims (1)

[Claims] 1 C0.12-0.8%, Si0.1-1.0%, Mn2 (C%)
~2.5%, A0.01~0.05%, Ti, Zr, Hf
(Ti%) + 0.53 (Zr
%) + 0.27 (Hf%) is 0.01 to 0.05%, N is 0.29 (Ti
%) + 0.15 (Zr%) + 0.08 (Hf%) to 0.015%, with the balance consisting of Fe and impurities. After heating the continuous cast slab to a temperature of 950 to 1120°C, rolling or forging is performed. A method for producing steel for forging machine structural use that still has a fine structure even at high temperatures, characterized by: 2 C0.12-0.8%, Si0.1-1.0%, Mn2 (C%)
~2.5%, A0.01~0.05%, Ti, Zr, Hf
(Ti%) + 0.53 (Zr
%) + 0.27 (Hf%) is 0.01 to 0.05%, N is 0.29 (Ti
%) + 0.15 (Zr%) + 0.08 (Hf%) to 0.015%, and further contains one or more of Cr2% or less, Ni3% or less, Mo0.5% or less, and the balance is Fe and A method for producing a steel for machine structural use for forging which still has a fine structure even at high temperatures, the method comprising heating a continuously cast slab containing impurities to a temperature of 950 to 1120°C and then rolling or forging.