JPS59177352A - Low-decarburization spring steel for continuous casting - Google Patents

Abstract

PURPOSE:To obtain a low-decarburization spring steel for continuous casting having a significant effect of preventing local decarburization by adding specified amounts of C, Si, Mn, Cr and one or more among V, Nb, Mo, W, Ta, Zr and Ti to Fe. CONSTITUTION:The composition of a lowdecarburization spring steel for continuous casting is composed of, by weight, 0.4-0.75% C, 1-2.5% Si, 0.5-1% Mn, 0.1-1% Cr, one or more among 0.03-0.3% V, 0.01-0.3% Nb, 0.01-0.3% Mo, 0.01-1.5% W, 0.01-1.5% Ta, 0.01-0.3% Zr and 0.01-0.3% Ti, and the balance Fe with inevitable impurities. When the steel is continuously cast into a bloom, carbon diffuses slowly in the side and pin corner parts of the bloom, so decarburized layers are not formed up to a large depth especially in the pin corner parts.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low decarburization spring steel for continuous casting that has a large effect of preventing local decarburization that tends to occur during continuous casting.

Conventionally, JIS 5UP6, 5UP7 and 5AE9254
The spring steel specified in the above has a high St composition in order to obtain sufficient strength and hardness, but this high St spring steel is a steel type that is extremely susceptible to decarburization.

On the other hand, in recent years, in order to improve steel productivity, the continuous casting method has been adopted in the field of special steel instead of the conventional steel ingot manufacturing method using an ingot mold. However, blooms manufactured by the continuous casting method have significantly sharper corners than ingots manufactured by the conventional steel ingot manufacturing method. That is, in a normal ingot, the corner radius is at least about 50 mm, whereas in a continuous casting billet, the corner part has a sharp pin corner shape with almost no roundness.

Therefore, billets manufactured by the continuous casting method have the problem of significant decarburization at the corner portions of the pins. This decarburization is caused by the diffusion of C (carbon) to the outside, but the diffusion rate of C is higher in the case of pin corners than in the case of round pins, so the decarburization of the pin corners is is expected to become significant.

Therefore, when high-St spring steel is manufactured by the highly productive continuous casting method, it is inevitable that a relatively deep decarburized layer will be formed at the pin corners of the bloom, so hot scarf ink is applied after continuous casting. It is necessary to remove the decarburized layer at the corner of the bloom bin, but the decarburized layer at the corner of the pin of high St spring steel cannot be completely removed with a normal amount of hot scarf, and may remain in spots. However, this spot-remaining localized decarburized layer has a very negative effect on the spring properties, so it must be completely eliminated, but such removal work is extremely troublesome, and it is difficult to produce spring steel. There was a problem in that it greatly impeded the performance and also caused a large decrease in wading retention.

This invention was made to solve these conventional problems, and has a great effect of preventing local decarburization, and even when high St spring steel is manufactured by continuous casting, it does not occur at the bottle corners of the bloom. The low decarburized spring steel for continuous casting that can prevent the formation of deep decarburized layers, that is, the low decarburized spring steel for continuous casting according to the present invention, has a C: 0.40 to 0.75 in weight%.
%, Si:1. O~2.5%, Mn:0.5~1.0%
, Cr: 0.1-1.0%, and V: 0.03-0.
3%, Nb: 0.01-0.3%, M.

:O, O2N2.3%, W: 0.01-1.5%, Ta
:0.01~1.5%, Zr:0.01~0.3%, T
It is characterized by containing one or more of i: 0.01 to 0.3%, with the balance consisting of Fe and impurities, and when producing bloom by continuous casting method,
It is characterized in that the diffusion of C in the side portions of the bloom and the pin corner portions is slow, so that a deep decarburized layer is not formed particularly in the pin corner portions.

Next, the reason for limiting the composition range (weight %) of the low decarburization spring steel for continuous casting according to the present invention will be explained.

C (carbon); C is an element effective in increasing the strength of steel.

However, if it is less than 0.40%, it will not be possible to obtain the necessary strength as a spring, and if it exceeds 0.75%, reticular cementite will tend to form, impairing the fatigue strength of the spring.
,40 to 0.75%.

St (silicon): St is an element effective in improving the strength of steel and the setting resistance of springs, but if it is less than 1.0%, it will not provide the necessary setting resistance for springs. If the content exceeds 2.5%, the toughness deteriorates, so the content was set in the range of 1.0 to 2.5%.

Mn (manganese): Mn is an element that is effective in deoxidizing steel as well as preventing damage caused by S, and for this purpose it is necessary to contain it in an amount of 0.5% or more. If it exceeds 0%, segregation tends to occur during agglomeration, and this segregation may cause uneven strength, so it is set in the range of 0.5 to 1.0%.

Cr (Chromium) Cr is an effective element for preventing decarburization and graphitization of high carbon steel, but if it is less than 0.1%, these effects cannot be fully expected; If it exceeds 0%, the toughness deteriorates, so it is set in the range of 0.1 to i,o%.

■ (vanadium), Nb (niobium), M6 (molybdenum), W (tungsten), Ta (tantalum), Zr (zirconium), Ti (titanium) V, N b
, M o , W , T a , Z r ,
T i is an element that has a large affinity with C,
By adding one or more of these, stable carbides are precipitated, and since these carbides are difficult to decompose, the decarburization resistance of steel can be significantly improved. Among these, V and Nb have a large precipitation hardening effect during quenching and tempering of steel, and also have the effect of improving spring characteristics.

In each of these elements, ■ is less than 0.03%, the above effects cannot be expected, and if it exceeds 0.3%, it becomes difficult to handle in steel manufacturing, so 0.03 to 0.3%. range.

In addition, if Nb is less than 0.015%, the above effects cannot be expected, and if it exceeds 0.3%, carbide (Nb) is formed during agglomeration.
bC) is formed in the form of stringers, which do not become solution during normal blooming rolling and are difficult to dissolve during subsequent heat treatment, reducing the spring properties of the product. O1~0.
The range was set at 3%.

Furthermore, if Mo, W, Ta, Zr, and Ti are all less than 0.01%, the above-mentioned effects cannot be expected.
In addition, if too much is added, the carbide becomes coarse and sufficient spring characteristics cannot be obtained, so M o , Z r ,
0.3% or less for Ti, W.

Ta was set to 1.5% or less.

In addition, B is an effective element for increasing the hardenability of steel, and should be added depending on the purpose of use to obtain the necessary spring characteristics, but if it is less than 0.0005%, the above It is difficult to obtain the above effect, and even if it exceeds 0.01%, the above-mentioned effect does not increase significantly, so when adding 0.01%.
The range is preferably from 0.0005% to 0.01%.

Further, S is an element that tends to impair the fatigue strength of the spring, and the lower the S content, the higher the reliability of the spring, so it is more desirable to regulate its upper limit depending on the purpose of use. If it is 0.010% or less, there will be almost no harm caused by S, so it is more desirable that it be 0.010% or less.

Furthermore, since 0 produces oxide-based inclusions, which may become the starting point of fatigue failure, it is more desirable to regulate the content depending on the purpose of use. in this case,
If it is 0.0015% or less, it is less likely to become a starting point for fatigue fracture, so it is more desirable that it is 1.0.0015% or less.

In springs made of steel with such a large amount of ingredients,
In order to improve the spring properties such as adhesion to the sake and fatigue strength, the material after continuous casting and product rolling is subjected to tempering such as quenching and tempering, controlled rolling, etc. to improve the crystal grain size of the structure. It is more desirable that the number is 9 or higher.

Examples of the present invention will be described below along with comparative examples.

Example 1 A steel having the chemical composition shown in the table was melted and then formed into an ingot.The steel ingot was then subjected to blooming rolling and wire rod rolling, and a sample having a diameter of 10 mm and a length of 50IIIn was cut from the obtained wire rod. Next, each sample was kept in a heat treatment furnace heated and maintained at 1000° C. for a predetermined time (taken out every hour) to form a decarburized layer, and then the depth of the decarburized layer of each sample was measured. The results are shown in Figure 1. Figure 1 shows 5UP6 (No, l) as a comparison steel, and 5UP6 (No, l) as the invention steel.
Steel in which the three components of n are relatively similar (No. 4).

5, 6, and 7), and show the results of examining the total decarburization layer depth after heating and holding in a heat treatment furnace. As shown in Figure 1, the first two
It is clear that the decarburized layer formed very quickly over time, and that the rate of increase in the decarburized layer subsequently slowed down. On the other hand, in the steel of the present invention, it is clear that the formation of a decarburized layer within 2 hours is slow, and the thickness of the decarburized layer itself is small over a long period of time. This means that V, Nb, Mo, W, T
a.

This shows that the addition of one or more of Zr and Ti is very effective in preventing decarburization. In addition,
The spring properties were rather improved by the addition of these elements, and no negative factors were observed for the spring properties.

FIG. 2 takes up 5UP7 (No. 2) as a comparison steel, and vI4 (No. 8), which has relatively similar values of 31 & of C1Si and Mn to the above-mentioned 5UP7 as an inventive steel.

9.10, 11, 12, 13, 14.15), and shows the results of investigating the total decarburization layer depth after heating and holding in a heat treatment furnace. As shown in FIG. 2, the same results as in FIG. 1 were obtained in this case as well.

It was confirmed that the addition of one or more of Nb, Mo, W, Ta, Zr, and Ti is very effective in preventing decarburization. Moreover, in this case as well, the spring properties were rather improved1, and there were no problems caused by the addition of these substances.

Furthermore, Figure 3 shows SAE 9254 as a comparison steel.
(No. 3) and the above SAE as the invention steel.
Steels (No', 16, 17, 18.19) whose three components of C, Si, and Mn are relatively similar to 9254 were taken up, and the total decarburization layer depth was investigated after each was heated and held in a heat treatment furnace. This shows the results. As shown in FIG. 3, results similar to those in FIGS. 1 and 2 were obtained in this case as well.

Drilling 2 After melting the steel with the chemical composition shown in the table, it is made into an ingot, and then this steel ingot is bloomed and rolled, and then 20 mm square x 50 pieces are made from the rolled material.
A rectangular parallelepiped sample with a length of mm was cut out. Note that a portion of the corner of each sample was machined into a pin angle following the shape of the slab formed by continuous casting.

Next, each sample was held in a heat treatment furnace heated and held at 1000°C for 5 hours, and then cooled. After cooling, the depth of the decarburized layer 2 formed on each sample l was measured at the side ( depth a
) and a part of the corner (depth b).

FIG. 4 takes 5UP6 (No. 1) as a comparative steel, and a steel (No. 4) whose three components of C, Si, and Mn are relatively similar to the above-mentioned 5UP6 as an inventive steel.

5, 6, and 7), the depth a of the decarburized layer at the side and the depth b of the decarburized layer at a part of the corner after heating and holding were measured, and the vertical axis in Fig. 4 shows the depth of the decarburized layer at the part of the corner. It is shown based on the degree of decarburization layer depth (b/a). As shown in Fig. 4, in the comparative steel, the decarburized layer depth b in the corner part is larger than the decarburized layer depth a in the side part, whereas
In the steel of the present invention, the depth b of the decarburized layer in a part of the corner is smaller than the depth a of the decarburized layer in the side part, and it is clear that decarburization in the part of the corner is suppressed more than in the conventional steel. It is. This indicates that the occurrence of local residual decarburized layers could be effectively prevented even at the corner pin corners of slabs manufactured by continuous casting.

FIG. 5 shows 5UP7 (No. 2) as a comparison steel, and a steel (No. 8) whose three components of C25i and Mn are relatively similar to the above-mentioned 5UP7 as an inventive steel.

9, to, 11, 12, 13, 14.15), measure the depth a of the decarburized layer at the side and the depth b of the decarburized layer at a part of the corner after heating and holding, and plot them on the vertical axis. The figure shows the depth (b/a) of the decarburized layer in a part of the corner. In the case of FIG. 5 as well, as in the case of FIG. 4, it is clear that the acceleration of decarburization in a part of the corner is sufficiently suppressed in all of the steels of the present invention.

Furthermore, Fig. 6 shows SAE 9254 as a comparative steel.
(No. 3) and the above SAE as the invention steel.
Taking steel (No. 16.17, 18.19) whose three components of C, Si, and Mn are relatively similar to 9254,
Similarly to the above, the depth of the decarburized layer (b/a) at a portion of the corner is plotted on the vertical axis. In the case of FIG. 6 as well, as in the case of FIGS. 4 and 5, it is clear that the acceleration of decarburization in a part of the corner is sufficiently suppressed in all of the steels of the present invention.

- As explained above, the spring steel according to the present invention is one of the high-Si spring steel numbers V, Nb, Mo, W, Ta, Zr, and Ti whose basic components are C, Si, Mn, and Cr. By containing the above, it is possible to slow down the diffusion of C to the outside during heating, and it has an excellent effect of preventing local decarburization. Even when manufacturing spring steel, deep decarburized layers can be prevented from forming at the pin corners of the bloom.
Conventionally, even after hot scarfing, the decarburized layer in some corners cannot be completely removed and remains in spots.
This is an extremely advantageous feature that can completely eliminate problems such as significantly impairing the quality of springs and decreasing yield due to an increase in the amount of hot scarfing, and allows production of spring steel to be carried out with high efficiency through continuous casting. It has a great effect.

[Brief explanation of drawings]

1 to 3 are graphs showing the results of investigating the relationship between the heating time of each sample and the total decarburization layer depth in Example 1 of the present invention, and Figures 4 to 6 are graphs showing the results of investigating the relationship between the heating time of each sample and the total decarburization layer depth in Example 1 of the present invention. Figure 7 shows the depth of the decarburized layer at the edges measured when examining the depth of the decarburized layer in a part of the corner with the pin angle of each sample. FIG. 3 is an explanatory diagram showing a procedure for measuring the depth of a coal seam and a part of a corner. Patent Applicant Daido Steel Co., Ltd. Representative Patent Attorney Yutaka Oshio
Figure 3: Calculation time (hr, 1ooo'c) Figure 4: 1455'7 (JIS 5llP6) Sample No. Figure 5 No. 6 3 /θ /7/8'IQ (5AEQ254) -2'

Claims (1)

[Claims] (1) C20, 40-0.75%, Si: 1 in weight%
．． 0-2.5%, Mn: 0.5-1.0%, Cr: 0.
1-1.0%, and V: 0.03-0.3%, Nb:
0.01-0.3%, M. :0.01~0.3%, W:0.01~1.5%, Ta
:0.01~1.5%, Zr:0.01~0.3%, T
A low decarburization spring steel for continuous casting, which has a strong effect of preventing local decarburization, and is characterized by containing one or more of i:0.01 to 0.3%, with the remainder consisting of Fe and impurities.