JPH0219183B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for carburizing steel, in which steel in a furnace is heat treated while a carburizing atmosphere gas is continuously passed through the furnace. Conventionally, the atmospheric gas used in steel carburizing treatment is C 3 H 8 , C ₃ H ₈ ,
A gas that is generally referred to as an endothermic atmospheric gas and is produced by a reaction from a gas generating furnace by introducing a fuel gas such as C ₄ H ₁₀ and air to an extent that causes incomplete combustion of the fuel gas. is frequently used. The typical composition of the above atmospheric gas is _N2 : 43% by volume, _H2 : 32% by volume, CO: 25% by volume, _CO2 : 0.1% by volume in the gas generating furnace, and the dew point in the furnace is D = -5°C. This atmospheric gas contains _CO2 , which hinders carburization.
It contains many oxidizing components such as H ₂ O. Therefore, when using it, H ₂ in the gas,
The amount of CO is increased to an extent sufficient to suppress the oxidation and decarburization effects of CO ₂ and H ₂ O in the gas, so that the gas can perform the role of carburizing the steel. On the other hand,
When the amount of H ₂ O and CO ₂ is large, so-called grain boundary oxidation occurs, which has an oxidizing effect on metals that have a particularly strong affinity for O ₂ , such as Cr and Mn, among the alloying elements contained in steel. Hardenability near the field decreases. To prevent this, it is necessary to separately reduce the amount of H ₂ O and CO ₂ or use other methods such as vacuum carburization that require complicated and expensive equipment.
Furthermore, in the above method using atmospheric gas,
It requires a gas generating furnace that causes a catalytic reaction between a fuel gas such as C ₃ H ₈ or C ₄ H ₁₀ and air, and the drawback is that it takes time for the furnace to stabilize the generation of atmospheric gas, that is, it takes a long time to start up. be.
Furthermore, since the gas used is generated in a generating furnace, it is difficult to change the composition and flow rate of the required atmospheric gas. Therefore, in general, the operating conditions of the gas generating furnace are kept constant, and the composition is controlled by directly adding hydrocarbons such as C ₃ H ₈ and C ₄ H ₁₀ to the carburizing furnace through a separate system, or by controlling the required flow rate. Changes are made by setting the flow rate to the required maximum amount in advance, and when reducing the flow rate, the excess amount is released into the atmosphere, which wastes gas. The present invention was made in view of the above circumstances, and its purpose is to be economical, have a high carburizing effect, and prevent metals that have a strong affinity for O ₂ such as Cr and Mn in steel from oxidizing during carburizing treatment. Our objective is to provide a method for carburizing steel that can eliminate oxidizing components such as O ₂ , H ₂ O, and CO ₂ in the furnace by controlling the composition and flow rate of atmospheric gas within a predetermined range. _CH4 , which has carburizing and reducing effects, effectively reduces
This makes it possible to make maximum use of the effect of H ₂ , which has a reducing effect. This invention will be explained in detail below. The carburizing atmosphere gas of the present invention is nitrogen gas _N2 and hydrogen gas _H2.
It is prepared by _{mixing a carrier} gas _consisting of _H
+V _C ) becomes 2% by volume < V < 40% by volume, and the ratio K of V _C to V _H (= V _C /V _H ) is 0.5K.
They are mixed at a ratio of 5. The above numerical ranges of 0.5K5 and 2% by volume<V<40% by volume were determined based on the reasons and methods described below. First, we investigated the relationship between the rate of increase in surface carbon concentration of steel and K. At this time, the treatment temperature T = 925°C, the total content of hydrogen and methane V = 8% by volume, the dew point in the furnace D = -70°C, the oxygen concentration in the furnace V ₀ = 0 (ppm), and the results are It became as shown in Figure 1. As is clear from the figure, the rate of increase in surface carbon concentration △W ₀
shows the maximum value of 0.215% by weight/hour when the ratio of hydrogen to methane (V _C /V _H ), K = 1.5, and the above increase rate △W ₀
It can be seen that the amount decreases. Therefore, when V _C +V _H = constant, it can be seen that the rate of increase in surface carbon concentration ΔW ₀ is fastest when K = 1.5 or 1K2. However, in practice, it is necessary to take into account fluctuations in the reaction rate depending on the magnitude of the K value, as well as the price difference between CH ₄ gas and H ₂ gas, so the optimum K value for practical use was set at 0.5K5. Note that the increasing speed △W ₀ when K = 0.5 or K = 5 is about 30% smaller than the maximum value of △W ₀ , that is, △W ₀ when K = 1.5. The decrease in the rate of increase in carburization can be easily compensated for by changing other carburizing conditions, such as increasing the treatment temperature by about 20° C., so it does not pose much of a problem. Next, the total content of hydrogen and methane (V _C +V _H ), V
In order to find the numerical range of , the amount of increase in surface carbon concentration ΔW (wt %) was determined when factors such as in-furnace dew point D, processing temperature T, processing time H, and in-furnace oxygen concentration V ₀ were used as variables. First, the amount of increase in surface carbon concentration △W 1 at treatment temperature T = 925°C, treatment time H = 4 hours, in-furnace oxygen concentration V ₀ = 0, and K = _1.5 is expressed as in-furnace dew point D and CH ₄ concentration.
When we calculated the relationship between them using V _C as a variable, we found that
The result is as shown in equation (1) below. △W ₁ = −0.025×(D+70)+0.25×V _C −0.29 …(1) Next, the carburizing reaction rate R _T of steel with treatment temperature T (℃) as a variable is R when T = 925℃ _T is 1, 850℃
When calculated in the range of ＜T＜1050℃, the following
Equation (2) was obtained. R _T = 1.62 × 10 ^-2 T - 14.02 ... (2) In addition, the carburizing reaction rate R _H with the processing time H (hours) as a variable was determined by setting R _H when H = 4 (hours) to 1. However, the following equation (3) was obtained. R _H = 0.25H ...(3) Furthermore, the carburizing reaction rate R _p with the oxygen concentration V ₀ (ppm) in the furnace as a variable is calculated as V at treatment temperature T = 925°C, treatment time H = 4 hours, and K = 1.5. R ₀ when ₀ = 0 (ppm)
When calculated with 1 as 1, the following equation (4) was obtained. R ₀ = -8.83×10 ^-3 ×V _p /V _C +1 ...(4) Therefore, the carburizing reaction rate R when the factors T, H, V _p , and V _C are variables is R= R _T ×R _H ×R _p ...(5), and the increase in surface carbon concentration △W with each factor as a variable
(% by weight) can be determined as △W=△W ₁ ×R. However, regarding the influence of atmospheric gas flow rate, carbon concentration pre-contained in steel, and reaction inhibiting factors,
It is difficult to find a quantitative relationship like the one above because it largely depends on the performance and usage conditions of the carburizing furnace used in actual operations. Therefore, in consideration of such fluctuations in actual operation, it was considered appropriate to provide a tolerance of ±50% for the estimated increase in surface carbon concentration, and the correction coefficient A was set at 0.5 to 1.5. Therefore, the surface carbon concentration increase amount ΔW can be expressed as in the following equation (6). △W=△W ₁ ×R _T ×R _H ×R _p ×A ...(6) Using this formula (6), V=V _C +
The numerical range of _VH was determined. First, under the conditions where the carburizing reaction rate is the fastest, treatment is performed for H = 16 hours, which is on the long side of the normal carburizing time range, and from this point on, the increase in surface carbon concentration △W is set as the purpose of carburizing, and surface hardening is achieved. be done
The V required to make it 0.8% by weight was determined, and this value was taken as the minimum value of V. Therefore, processing temperature T = 1050°C, processing time H = 16
time, oxygen concentration in the furnace V ₀ = 0 ppm, dew point in the furnace D = -
To make △W=0.8% by weight under the conditions of 70℃, K=1.5, and A=1.5, from equation (6), 0.8=(0.25×V _C −0.29)×2.99×4.0×1×1.5. , V _C =1.35 From the above-mentioned relationship K=V _C /V _H =1.5, V _H =0.9. Therefore, V=V _C +V _H =2.25>2 (weight %) (7). Next, to find the maximum value of V, first, for V ₀ , the maximum O ₂ concentration (approximately 1000 ppm) contained in N ₂ gas produced by the PSA method, or commercially available N ₂
Considering the O ₂ concentration in the cylinder (approximately 50 ppm)
V ₀ was set to 1100ppm. As for the amount of H ₂ O, it is better to keep it as low as possible in order to fully utilize the carburizing effect, so the amount of H ₂ O allowed in the carburizing atmosphere gas in the conventional method is more than 10 times lower.
It was defined as 300 ppm, that is, a range lower than D=-32°C in terms of the dew point inside the furnace. Under these conditions, the treatment temperature is set to T = 925°C, which is the usual value, the treatment time H is set to H = 2.5 hours, which is close to the short time side of the normal carburizing time range, and the increase in surface carbon concentration △W is set to 0.8% by weight. The required amount of V was determined and this value was taken as the maximum value of V. Therefore, processing temperature T = 925°C processing time H = 2.5
Time, oxygen concentration in the furnace V ₀ = 1100ppm, dew point D in the furnace
△W=0.8 under the conditions of =-33℃, K=1.5, A=0.5
To calculate weight%, from the above formula (7), 0.8=(0.25×V _C −1.22)×1×0.63×(1−9.71/
V _C )×0.5, V _C =23=1.5V _H , that is, V=V _C V _H =38.3<40 (weight %)...(8). In this way, the composition concentration range of the carburizing atmosphere gas that is most effective in actual operation was determined to be 0.5
K=V _C /V _H 5 and 2 volume%<V=V _H +V _C
<40% by volume is specified. Next, change this numerical range to
If plotted on a coordinate system with V _C on the horizontal axis and V _H on the vertical axis, it will look like Figure 2. In other words, if the atmospheric gas is configured by selecting the V _C value and V _H so that they belong to the area surrounded by the four lines A, B, C, and D in the figure, efficient carburizing treatment can be achieved in actual operation. It will be possible to do it. In addition, line A in the figure is a line when K=0.5, line B is a line when V=40 (weight%), line C is a line when K=5, and line D is a line when K=1.5 and V=2.0 ( The carburizing ability is equivalent to that of point b). That is, for example, the following three conditions show equivalent carburizing ability. K=0.5 and V=3 (point a) K=1.5 and V=2 (point b) K=5 and V=3 (point C) The method of introducing atmospheric gas in the present invention is as shown in FIG. After charging the steel M into the furnace 1, the flow rate regulating valve 2 is opened and _N2 is introduced into the furnace 1 through the line 3 to sufficiently purge the inside of the furnace 1. Furnace dew point D<-32℃, furnace oxygen concentration V ₀ 1100ppm
After reaching , open flow regulating valve 4 and mix N ₂ from line 3 and H ₂ from line 5 to (N ₂ + H ₂ ).
The mixed gas is introduced into the furnace 1 and the temperature is started to rise. After reaching a predetermined temperature, open the flow rate adjustment valve 6,
N ₂ from line 3, H ₂ from line 5, and CH ₄ from line 7 are mixed (N ₂ +H ₂ +CH ₄ ) and introduced into the furnace as a mixed gas for carburizing. For the amount of CH ₄ and H ₂ at this time, the K value is 0.5 ~
5, and the dew point D in the furnace 1,
This can be done by adjusting the V amount within 2<V<40 so as to obtain the required carburizing ability while measuring the atmosphere composition, or conversely, by keeping the V amount constant and changing the K value. You can do it later. but,
In order to effectively utilize the atmospheric gas, it is preferable to perform the process with K=constant. As explained above, according to the carburizing method of the present invention, not only can nitrogen gas, hydrogen gas, and methane gas constituting the carburizing atmosphere gas be flowed separately and mixed, but also the composition concentration of each can be adjusted to achieve the carburizing effect. Since it is a method that can set the concentration within the range where it can be increased the most, (i) the amount of oxidizing components such as O ₂ , CO ₂ , H ₂ O, etc. can be kept low, and moreover, the amount of CH ₄ required for carburization can be kept low;
and the amount of _H2 can be reduced. (ii) The composition of the atmospheric gas can be changed by adjusting the flow rate of each composition gas, and it can also be used as a purge operation, which is economical. (iii) Since the amounts of O ₂ , CO ₂ , and H ₂ O were reduced as much as possible,
Oxidation of alloying elements that have a strong affinity for O ₂ can be suppressed, and the formation of grain boundary oxides can be significantly reduced compared to conventional gas carburizing methods. (iv) Since a furnace for generating atmospheric gas is not required,
Operation is easy and it is easy to respond to fluctuations in composition flow rate. In order to quantitatively confirm the effects of the present invention, a steel test piece (length 45 mm x width 60 mm x thickness
0.3mm) under various conditions, and conducted an experiment to determine the amount of increase in surface carbon concentration at that time. The initial carbon concentration in the steel is 0.004wt%, and the total gas flow rate is 800N.
ml/min. As a result, the values shown in the table below were obtained.

[Table] As is clear from the above table, the increase in surface carbon concentration was more than 0.8wt% in all cases, and it can be seen that the carburizing ability was equal to or higher than that of the conventional method. Moreover, when treated with the method of the present invention, N ₂ used in the carburizing treatment,
Since both CH ₄ and H ₂ gases have high purity and low dew points, _the oxygen concentration in the furnace and the dew point in the furnace can be maintained considerably lower.
This not only reduces the amount of expensive active gases such as H ₂ used, but also reduces the amount of H ₂ , CO,
Because it contains 50 to 60 vol% of active gases such as hydrocarbons and approximately 4000 ppm of water (DD≒-5℃),
This moisture forms grain boundary oxides, but the method of the present invention allows extremely good carburizing treatment without causing this undesirable phenomenon.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the relationship between surface carbon concentration increase rate △W ₀ and K, Fig. 2 is a diagram showing the concentration range of hydrogen and methane gas in the carburizing atmosphere gas used in the present invention, and Fig. 3 is a diagram showing the concentration range of hydrogen and methane gas in the carburizing atmosphere gas used in the present invention. 1 is a schematic configuration diagram of a carburizing treatment apparatus suitable for carrying out. 1...Furnace, 2...Nitrogen gas flow rate adjustment valve, 3...Nitrogen gas line, 4...Hydrogen gas flow rate adjustment valve, 5...Hydrogen gas line, 6...Methane gas flow rate adjustment valve, 7...Methane gas line.

Claims (1)

[Claims] 1 Steel is placed in a furnace with the furnace dew point D set to D<-32°C and the furnace oxygen concentration V ₀ set to V ₀ 1100 ppm, and carburizing gas consisting of nitrogen gas, hydrogen gas, and methane gas is placed in the furnace. This is a method of heat treating steel at 850°C to 1050°C while continuously circulating atmospheric gas, and the total volume V of hydrogen gas and methane gas in the carburizing atmospheric gas in the above furnace is 2% by volume < V < 40
A method for carburizing steel, comprising adjusting the flow rates of each of the nitrogen, hydrogen and methane gases so that the volume ratio K of methane gas to hydrogen gas is 0.5K5.