JPS5852459A - Case hardened steel and its manufacture - Google Patents

Abstract

PURPOSE:To reduce the amount of a strain produced during surface hardening and to reduce the disperion by manufaturing a case hardened steel contg. specified percentages of C, Si, Mn, Cr, Ni and Mo in Fe from an ingot having a specified central segregation degree of C. CONSTITUTION:A steel consisting of, by weight, <=0.4% C, <=1% Si, <=2% Mn, <=3% Cr, <=5% Ni, <=1% Mo and the balance Fe is prepared. The desired composition of the steel is composed of 0.25-0.35% C, <=0.5% Si, 0.7-1.5% Mn, 0.35-1% Cr, 0.01-0.06% Al, 0.01-0.025% N and the balance Fe or further contains <=2% Ni and/or <=0.3% Mo. The O content of the steel is adjusted to <=30ppm. When the steel is continuously cast, by regulating the superheating temp. during casting to <=40 deg.C and magnetic intensity during electromagnetic agitation at the solidification interface to >=40G, the central segregation degree of C is made 1.1-1.0.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low strain case hardening steel suitable for use in carburized or carbonitrided stations, and a method for manufacturing the same.

Machines that perform surface hardening treatments such as carburizing, carbonitriding, and vacuum carburizing on parts or products that require surface wear resistance or fatigue strength, such as gears, are used to Since the material is quenched after being heated to a temperature above a point, deformation occurs due to K such as thermal stress during volume change due to transformation. The amount of deformation is large and the variation is large (i'); a great deal of effort is required to correct it in the subsequent process. For example, in the case of main shafts for transmissions, it is necessary to straighten each shaft after surface hardening treatment, and in the case of differential ring gears, breath quenching is essential.

Therefore, there is an urgent need to develop a low-strain case hardening steel that exhibits a small amount of strain during surface hardening treatment and has small variations thereof, in order to simplify the process of correcting strain after surface hardening treatment.

The present invention has been made by focusing on the above-mentioned conventional problems.
Therefore, it is an object of the present invention to provide a low-strain case hardening steel that has a small amount of strain and small variations in strain during surface hardening treatments such as carburizing, carbonitriding, and vacuum carburizing.

The present invention focuses on the center segregation degree of C (carbon) in the slab stage of steel having chemical composition suitable for station areas (center part of steel slab C1/C
ladle analysis value) of 1.1 to i.

It is characterized by being manufactured from slabs in the range of
More preferably, the steel suitable for the station area has the following properties by weight: C: 0.4% or less, Si: 1% or less, Mn: 2
% or less, Cr: 3% or less, Ni: 5% or less, Mo:
1% or less, with the remainder substantially consisting of F., and even more preferably, C: 0.25 to 0.
35%, St: Q, 5% or less, Mn: 0.
7-1.5chi, Cr: 0.35-19b, At:
0.01-0.06%.

N: 0.01-0.025chi, Ni: if necessary
Less than 2 inches! vlo: Select one containing one or two of the following: 0.3 p or less, with the remainder substantially consisting of F, and more desirably, reduce the O content in the steel to 39 p.
Select one that is kept below pm. In addition, in producing steel with a chemical composition suitable for use in station areas, the steel is produced by a continuous casting method. The difference in -) was set to 40°C or less, and the stirring intensity during electromagnetic stirring was set to 40 Gauss or more at the solidification interface, so that the center segregation degree of C (carbon) was 1.1 to 1.0. It is a feature.

Steels suitable for station areas include carbon steel for machine structures, nickel-chromium-containing low-alloy steel, nickel-chromium-containing low-alloy steel, chromium-containing low-alloy steel, chromium-containing molybdenum low-alloy steel, manganese-containing low-alloy steel, There are various types of steel such as manganese chromium low alloy steel, but among these types of steel, it is more desirable to have a heavy duty, C: 0.4% or less, Si: 1
S or less, Mn: 2 inches or less, Cr: 3% or less, N
15% or less, Mo: 1- or less, and A as appropriate as necessary.
j: 0.01-0.06%, N: 0.0
1 to 0.025%%N1: 2- or less, Mo: 0.
3% or less, and other Nb#T'
It is preferable to use a material to which e Z' e V + Ta or the like is added, and the reason for this will be explained below.

C; C is an effective element for securing the necessary depth of hardened layer in surface hardening treatment. For example, when forming gears from case hardened steel material, C is an element that is effective in maintaining the required base hardness while maintaining the required hardness. By lowering the hardness of the part, distortion caused by hardening treatments such as carburizing, carbonitriding, and vacuum carburizing and quenching can be minimized, and the required effective hardened layer depth can be secured. However, if it exceeds 0.44i, the toughness and stiffness will decrease, so it is more preferable to keep it below 0.4i, and even more preferably, to maximize the above-mentioned effect, the toughness and stiffness will decrease. It is better to set it as a range.

St:Si is an effective element for deoxidizing during melting and as a desulfurizing element, but if the content is too large, carburizability and machinability tend to decrease, so it is more desirable to keep it at 1% or less. It is even more desirable that the sail length be 5 inches or less.

Mn: Mn is an effective element as a deoxidizing and desulfurizing element during melting, but if the content is too large, the stiffness decreases, so it is more preferably 2- or less, and even more preferably, taking into account the above effects. and 0.7-1.5
It is best to keep it within the range of

Cr: Cr is an effective element for securing a predetermined effective hardening layer depth in surface hardening treatment and maintaining good carburizing and carbonitriding properties, but if it is too large, the 6-part hardness increases; Since K causes high distortion, it is more preferably 3% or less, and more preferably K is in the range of 0.35 to 1 kan, taking into consideration the above-mentioned effects.

AA: An element effective in suppressing the growth of austenite crystal grains and preventing the occurrence of distortion during AAu surface hardening treatment, and for this purpose it is necessary to contain 0.014 or more. However, if it exceeds 0.06%, the cleanliness of the steel decreases. Therefore, when it is contained, it is desirable that it be in the range of 0.01 to 0.0616.

N: N combines with the above-mentioned At to form AtNt, thereby forming an austenitic structure of 0.01-
If it is less than 0.025 St, the effect is small, and if it exceeds 0.025 St, blowing tends to occur. Therefore, when it is included, it is preferably in the range of 0.01 to 0.025%.

Ni: Ni is effective in improving carburizing properties and monocarbonitriding properties, and in maintaining the hardness of the tooth root of the gear, for example, when the station part is a gear. However, if it is too large, the hardness of the 6th part becomes large, which causes high distortion, so it is more desirable to set it to 5mm or less, and still more preferably to set it to 2mm or less.

Mo: Like Ni, Mo improves carburizing and carbonitriding properties, and is effective in maintaining root hardness in the case of gears. However, if the amount is too high, the hardness at the 15 core increases, causing high strain, so it is more preferably 1- or less, and even more preferably 0.3% or less.

If the 0:0 content is too high, the cleanliness of the steel will deteriorate and the fatigue strength will decrease, so 3020 pp is more desirable.
It is better to set it to less than m.

In addition, Nb, Ti, Zr, as necessary.
Naturally, elements such as V and Ta may also be contained.

C center segregation degree: 1.1 to 1.0 If the center segregation degree of C in the slab state is large, center segregation of C will remain in the rolled material made from this material, and this will cause distortion during surface hardening treatment. Therefore, it is necessary to reduce center segregation of C in the slab state. In this case, the ct of the central part of the slab is 1.5% compared to the dollar analysis value of the slab.
If the degree of segregation exceeds 1 ffl, it will be difficult to suppress the occurrence of strain, so it is limited to i, i times or less.

Thus, C, Si, Mn, Cr and Ni.

Carburizing by adjusting the content of MO etc.

In surface hardening treatments such as carbonitriding and vacuum carburizing, it is possible to reduce the occurrence of distortion without deteriorating the surface hardening properties, and by containing specified amounts of AA and N, the austenite during the surface hardening treatment can be reduced. By suppressing the growth of crystal grains, the amount of strain can be reduced, and in addition, the center segregation degree t-1 of C (carbon) at the slab stage can be reduced.
, 1-1. By doing so, the occurrence of distortion due to surface hardening treatment can be significantly reduced.

The present inventors focused on the phenomenon that the C content increases due to segregation in the center of the thickness of nine cast slabs manufactured by continuous casting method, that is, center segregation, and found that this center segregation is caused by surface hardening treatment. Examine the effect of thickness change (strain amount) on K.

In this case, as shown in FIG.
The degree of central segregation of C was determined by measuring the concentration and determining the C concentration at the center of the slab, and expressing the C concentration at the center as a ratio to CaI2 (0%) in one dollar. Further, the thickness of the surface hardened product was changed as follows. First, as shown in Figures 2 and 3, a material 2 with a diameter D = 60-9 and a height H of -94 m is hot upset forged, and as shown in Figure 3 (
An intermediate material 6 having a diameter D2 = 95cm and a height = 42cm as shown in b) was manufactured, and further the diameter D2 = 961111 #height H3 = 40wm as shown in Fig. 3(c).
.

After processing into a ring material 4 with a center thickness T1 = 7 mm, and normalizing this ring material 4'', the outer diameter shown in Fig. 3(d) is 90mm, inner diameter D1 = 45gIIM, thickness T4
It was processed into a ring-shaped test piece 5 with a diameter of 30 mm (target).

A large number of such ring-shaped test pieces 5 were prepared, and the finish of each test piece 5 was accurately measured to have a thickness T4, and then a surface hardening treatment 31t as shown in FIG. 4 was performed. That is, the test piece was heated to 5-900C and carburized for 2 hours,
A pot that undergoes diffusion treatment for 1 hour, holds at 850°C for 30 minutes, cools in oil, then heats at 180°C for 2 hours, and then performs tempering by cooling in air. Thereafter, the thickness T4 of each test piece 5 was measured to determine the amount of thickness change. That is, the thickness change amount (pm) = ((Thickness after carburizing, quenching and tempering treatment, )') - (Thickness before carburizing and quenching and tempering treatment -)))X100O. In this way,
When we investigated the relationship between the center segregation degree of C and the amount of thickness change in slabs, we obtained the results shown in Figure 5. As shown in FIG. 5, it has become clear that if the center segregation degree of C is between 1.1 and 1.0, the amount of thickness change, that is, the amount of strain caused by surface hardening treatment can be significantly reduced. The ring-shaped test piece 5 was used here because it was assumed that a gear would be used as a station component, and there is a clear correlation between the distortion of this ring-shaped test piece 5 and the distortion of the actual gear. This is because it has been confirmed that there is a relationship.

Next, the present inventors further conducted experimental research to determine what casting conditions should be set in order to maintain the center segregation degree of C in the slab between 1.1 and 1.0. .

First of all, in the continuous casting method, the superheating temperature (superheat; the difference between the casting temperature and the solidification degree) during pouring of molten steel and the C
When we investigated the relationship between the degree of central segregation and the degree of central segregation, we obtained the results shown in Figure 6. Note that the ladle value of C at this time was 0.28 weight tS, and the stirring intensity during electromagnetic stirring (EMS) was 50 Gauss at the solidification interface. From the results shown in FIG. 6, it is clear that in this case, by setting the superheating temperature to 40° C. or lower, the center segregation degree of C can be made sufficiently smaller than 1.1.

Next, 1 ji 1 part (
When the central segregation degree of C between the (Top) and the bottom (Rot) was investigated, the results shown in FIG. 7 were obtained. 7th
As shown in the figure, in the case of ingots made by the usual melting method, the proportion of the portion where the center segregation degree of C is 1.1 to 1.0 is quite small, so the yield is different. In addition, when using the continuous casting method (CC), the superheating temperature is set to 45°C, and the strength of electromagnetic stirring (EMS) is set to 35 Gauss at the solidification interface, and the central segregation degree of C in all slabs is 1. 1
exceeds. On the other hand, in slabs where the superheating temperature was 40°C and the stirring intensity of electromagnetic stirring was 40 Gauss at the t-solidification interface, the center segregation degree of C was less than 1.1, and the superheating temperature was as small as 30°C. Moreover, when the electromagnetic stirring strength was increased to 45 Gauss at the solidification interface, the center segregation degree of C became smaller. Therefore, it is preferable that the stirring strength of the electromagnetic stirring is 40 Gauss or more at the solidification interface. and,
As is clear from the results shown in Figure 7, by adopting the continuous casting method, the center segregation degree of C is almost constant between parts of the same charge compared to the steel ingot casting method, and as a result, the Continuous casting is advantageous in that variations in the amount of strain between parts within the charge are reduced and the amount of strain can be made constant.

Example Steel having the chemical composition A- shown in Table 1 was melted and then continuously cast at the superheating temperature shown in Table 1 at the time of casting. At this time, electromagnetic stirring was applied, and the stirring intensity [also shown in Table 1] was determined at the solidification interface.
The magnetic flux density was set as shown in . Further, the center segregation degree of C- was measured in the same manner as explained based on FIG. 1 above. Furthermore, the amount of change in thickness of the ring-shaped test piece was measured in the same manner as explained based on FIGS. 2 to 4 above. These results are shown in Table 1.

Comparative Example A steel having a chemical composition of L-8 shown in Table 2 was melted and then a steel ingot was manufactured by a steel ingot casting method. In addition, T to Y are superheating temperatures rl during casting! Slab f: Steel manufactured by continuous casting method with L as shown in Table 2. At this time, add electromagnetic stirring,
The stirring intensity was set to the magnetic flux density shown in Table 2 at the solidification interface. In addition, for the steel ingot casting method and the continuous casting method, the center segregation If of C was measured in the same manner as explained based on FIG. The measurement was carried out in the same manner as explained based on FIGS. 2 to 4. These results are shown in Table 2.

91 and Table 2, the central segregation degree of C is 1.1.
In the case of the present invention, which is in the range of ~1.0, the thickness change of the ring-shaped specimen is small and the variation is also small, whereas in comparison, the center segregation degree of C exceeds 1.1. In the example, the thickness change of the ring-shaped test piece is large and the variation is also large, and even when the average value of the thickness change of the ring-shaped test piece is relatively small (R, U), the variation is large. is larger than that according to the present invention, and it is clear that the strain according to the present invention is small and a constant strain can be obtained.

Furthermore, as shown in Tables 1 and 2, when the superheating temperature during casting is higher than 40°C and the intensity of electromagnetic stirring is lower than 40 Gauss, the center segregation degree of C exceeds 1.1. It is clear that this is true, and it has also been confirmed that when attempting to obtain a part with a C central segregation degree of 1.1-1.0 by the steel ingot casting method, the yield is quite small.

As explained above, the present invention provides a case-hardened steel and a method for producing the same, which are characterized by low strain generation during surface hardening treatment and small variation in strain generated, and have industrial value. It is enormous.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the procedure for measuring the degree of central segregation of C;
Figures 3 and 3 (a) to 3 (d) are process diagrams and explanatory diagrams of processing conditions, respectively, showing the process of creating test pieces for measuring thickness changes of light surface hardened products, and Figure 4 is surface hardening. An explanatory diagram of processing conditions. Figure 5 is a graph showing an example of the relationship between the central segregation degree of C and the amount of thickness change. Figure 6 is an example of the relationship between the superheating temperature during casting and the central segregation degree of C. FIG. 7 is a graph showing an example of the results of investigating changes in the center segregation degree of C depending on the location of the slab and ingot. Patent Applicant Daido Steel Co., Ltd. Representative Patent Attorney Yutaka Oshio 1 @ Figure 2 Figure 3 (a) (1)) (C) (Mountain Figure 4)

Claims (8)

[Claims] (1) Case-hardened steel manufactured by continuous casting,
A low-strain case-hardening steel characterized in that it is manufactured from a piece having a center segregation degree of C (carbon) in the range of 1.1-i,o. (2) The case hardening steel has a weight of -, C: 0.4 S or less, 3
1: 1% or less, Mn: 2S or less, Cr: 3-
Below, Ni: 5- or less, MO: 1% or less t-containing->
, the balance substantially consisting of F. according to claim (1). (3) Case hardening steel has a weight of -C: 0.25 to 0.35
%. 3i: 0.5 S or less, Mn: 0.7 to 1.5
%%Cr: 0.35-1%, ht: 0
．． 01-0.06%, N: 0.01-0.025
The low strain case-hardening steel according to claim 1, which comprises F. and the remainder substantially F. (4) Hada fi steel d! , weight t*i”, C: 0.2
5-0.35S. St: 0.5% or less, Mn: 0.7 to 1.5
%, (:r: 0.35-1 qlb%*t:
0.01-0.06%, N: 0.01-0.02
5, Nl: 2- or less, and the remainder substantially consists of KF. (5) Case hardening steel has C: 0.25 to 0.35 in weight%
%. St: 0.5 S or less, Mn: 0.7 to 1.5
%%Cr: 0.35-1-1AA: 0.0
1~0.06chi, N: 0.01~0.025%, M
o: 0.3'14 or less, the rest is substantially F・
A low strain case hardening steel according to claim (1). (6) Case hardening steel has a weight of -C: 0.25 to 0.35
%. si: o, s or less, Mn: 0.7 to 1.5
%, Cr: 0.35~1%,)L: 0.01~
0.06%%N: 0.01-0.025chi, Ni
2% or less, Mo: 0.316 or less, and the remainder substantially consists of Fe. (7) The low strain case hardening steel according to any one of claims (1) to (6), wherein the case hardening steel has an o: a o ppm or less. (8) In producing low-strain case hardening steel, the steel is produced by continuous casting, and at this time, the superheating temperature during casting is set to 40°C.
A method for producing a low strain case hardening steel, characterized in that the stirring intensity during electromagnetic stirring is 40 Gauss or more at the solidification interface.