JPS61253347A - Low carbon steel having superior cold workability - Google Patents

Abstract

PURPOSE:To obtain a steel having superior cold workability by extremely reducing the amounts of P, S and O as impurities in a low carbon steel and adding specified amounts of Cr, Ti and B as well as the irreducible minimum amounts of Si and Al so that wear resistance and toughness are provided by carburization, hardening and tempering. CONSTITUTION:This low carbon steel contains, by weight, 0.10-0.30% C, <0.3% Si, 0.10-0.40% Mn, <0.60% Cr, 0.01-0.05% Al, <0.010% P, <0.010% S, <50ppm O, 0.01-0.05% Ti and 0.0005-0.005% B. Since the steel contains Ti and B, the hardness is increased by carburization, hardening and tempering to provide superior wear resistance. The steel has superior suitability to cold working such as cold drawing or cold forging.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a low carbon steel that has excellent cold workability, and more specifically, to a low carbon steel that has excellent cold workability such as cold drawing, wire drawing, and cold forging. The present invention relates to a low carbon steel having excellent cold workability and capable of imparting wear resistance and toughness through carburizing, quenching and tempering. .

In general, cold-open headed products such as bolts and screws are manufactured by descaling hot rolled wire rods and steel bars, treating them with a lubricating coating, and then cold drawing them to predetermined diameters and dimensions.

However, when the cold working rate is high, the mechanical properties and tensile strength increase due to work hardening of the steel, and the elongation and reduction of area gradually decrease. It is necessary to perform softening annealing or spheroidizing annealing in an intermediate process because this may reduce the life of the tool during pressing or forging, or increase the defective rate due to cracking of the forged product. In addition, cold raw wire rods and steel bars such as hot rolled materials and intermediate annealed materials are required to have low deformation resistance, high ductility, and good cold heading properties without heading defects such as cracks.

Therefore, by reducing the chemical composition of steel materials, especially impurities such as P1S%O, and by improving the rolling method, materials with good cold heading properties are being manufactured. Small carburized heat treated products are C0.
After cold-rolling low carbon steel of about 10 to 0.30 wt%, carburizing or carbonitriding is performed to impart necessary wear resistance, followed by quenching and tempering.

After carburizing, this type of surface carburized steel is heated above the transformation point to form a uniform austenitic structure, and then quenched in water or oil at a temperature of around 880°C to form a martensitic structure in the surface carburized parts and core. After curing for 30 minutes
It is tempered at a temperature of 0 to 350°C to impart toughness to the surface carburized part and core part.

Therefore, the hardness of the surface hardened layer and the toughness of the core of a carburized heat-treated material have contradictory properties, and are closely related to the tempering treatment described above and are controlled by the temperature thereof.

In other words, in order to ensure the hardness of the hardened surface layer, the tempering temperature must be kept low; if the tempering temperature is low, the toughness of the core cannot be recovered sufficiently, and the surface is hard but the core is not tough. It becomes a deteriorated version.

In addition, in order to improve the toughness of the core, the tempering temperature must be increased, and if the tempering temperature is too high, the hardened surface layer will soften, and although the core has high toughness, The surface is soft and the carburizing effect is lost. Furthermore, although it is related to the effective depth of carburization, the balance between the hardness or toughness of the surface and core is also an issue.

If the surface hardness or core toughness is unbalanced, it may cause problems such as the driving load, time, or head jump during installation of tuffing screws, drywall, etc. In the case of heat-treated materials, delayed fracture characteristics are often a problem.

The main cause of this delayed fracture is mostly hydrogen that has penetrated into the steel.1. There are two possible timings when hydrogen permeates into steel:

(a) Penetration occurs during processes such as pickling of wire rods and steel bars or electroplating of products.

(b) Infiltration by the external environment after the product is used.

In the case of (b), hydrogen generated by a corrosion reaction infiltrates the steel, and generally, the more corrosive the environment, the more likely it is that delayed fracture will occur.
The hardness and strength of the material also greatly affect delayed fracture resistance, especially
Delayed fracture characteristics have become a problem in materials whose strength has been increased through quenching and tempering.

Therefore, in high-strength annealed steel, it is common practice to improve the delayed fracture characteristics by increasing the tempering temperature as high as possible, for example, at a temperature of 400° C. or higher.

However, since tapping screws are made of low carbon steel, if the tempering temperature exceeds 400°C, the surface hardness required for the product cannot be obtained. is the low-temperature tempering embrittlement range of high-strength tempered steels, especially low-alloy steels that have been quenched and tempered, and is also the temperature range in which delayed fracture properties deteriorate; low-temperature tempering embrittlement is not noticeable in carbon steel. Although it is difficult to appear,
Tempering treatment in the same temperature range should be good for toughness (although the delayed fracture properties are noticeably steep, similar to low alloy steel.

[The secret theory that the invention will solve] In view of the various problems of low carbon steel in the prior art explained above, the present invention is based on the research conducted by the inventor, and has been made to solve the problems of deformation resistance and deformation of wire rods and steel bars. In order to improve cold heading properties, impurity elements such as P, S, and 0 in the steel are reduced to purify the steel, and Si and AI are kept to the necessary minimum. It is a light solid solution strengthening element and reduces the Mn content, which increases work hardening, and exhibits a solid solution softening function with a relatively small amount of Cr, which has an extremely small amount of work hardening during cold-open drawing. By containing Ti and B, it is possible to soften the cold-drawn material and reduce its deformation resistance, and by containing Ti and B, it can be hardened sufficiently by either water or oil quenching, and by carburizing heat treatment, it can be made with a low Mn content. By reducing the hardening properties of the material itself while keeping the deformation resistance low, the core has low hardness and high toughness without reducing the hardness of the surface layer, and furthermore, tempering can be performed at a lower temperature than before. We have developed a low-carbon steel with excellent cold workability that can alleviate the material's low-temperature tempering brittleness and improve its delayed fracture resistance.

[Means for solving the problems] The characteristics of the low carbon steel with excellent cold workability according to the present invention are as follows: C0.10-0.30u+t%, 51003wt% or less, Mn 0.10-0 .40wt%, Or 0.60u
+t% or less, AIo, 01-0.05wt%, P 0.
010wt% or less, S 0.010iut% or less, 05
Contains 0 ppm or less, furthermore, Ti 0.01-0.05u+t%, B 000005
~0.005 wt%, with the remainder consisting of Fe and impurities.

The low carbon steel with excellent cold workability according to the present invention will be described in detail below.

First, the components and component ratios of the low carbon steel with excellent cold workability according to the present invention will be explained.

C is necessary to improve hardenability including the core and maintain strength after carburizing heat treatment, and the content is 0.10wt.
If it is less than 0.30w, this effect is small, and if it is less than 0.30w
If the content is too high, exceeding t%, the hardness will increase and the impact resistance will be impaired. Therefore, the C content is between 0.10 and 0.
．． 30u+t%. Incidentally, the content within this range is suitable for applications such as tapping screws.

Si is an element that improves temper softening resistance and hardenability and increases strength, but it is also an element that hardens the ferrite base and has a negative effect on cold workability. It is important to have excellent properties, so from that point of view as well, it is desirable that the Si content is low.
Moreover, Si also becomes a negative factor in carburizability. Therefore, Si
The content shall be OJu+t% or less.

Mn is an element that greatly contributes to improving hardenability, and is contained in large amounts in tempered steel, but it is also a ferrite solid solution strengthening element, promotes work hardening during cold working, and, like Si, increases the strength of steel. This is a major obstacle to reducing deformation resistance, and in the steel according to the present invention, a lower limit of 0.10 wt% is required as the lower limit content to fix S as MnS and render it harmless. If this happens, the above-mentioned adverse effects will occur, so the upper limit is set at 0.40u+t%.

Cr, like Si and Mn, is a ferrite solid solution strengthening element and is an element that promotes work hardening, but the degree of solid solution strengthening is the smallest, and when cold working, the amount of work hardening is M
smaller than n, conversely softens during processing and promotes the softening characteristics of steel, and furthermore combines with C1N in steel to form carbonitrides,
By relaxing the strain age hardening characteristics, it is possible to lower the tensile strength and yield point level when cold drawing hot rolled wire rods and steel bars, and reduce the deterioration of ductility such as elongation and reduction of area. The inclusion of an appropriate amount of Cr in steel that undergoes heat treatment such as carburizing, quenching, and tempering increases the effective carburizing depth and improves the steel properties, but a large amount of Cr increases the tensile strength and increases the deformation resistance during cold-open heading. Become. Therefore, the Cr content is set to 0.60 wt% or less. At this content, there is little adverse effect on work hardening during cold working.

AI is an effective element for deoxidizing, fixing N in steel, and adjusting grain size. If the content is less than 0.01 wt%, this effect is small, and the amount of A1□0 produced during deoxidation is ．． Since system inclusions can cause cracks during cold-open heading, it is not necessary to include them in large amounts exceeding 0.05 Illt%, and furthermore, a low Al content is important in the steel according to the present invention. be. Therefore, A1 content is 0
．． 01 to 0.05 wt%.

0 combines with A1 and Si to form an oxide, and since Ti oxide tends to become large when there is a large amount of O in steel, it is desirable to minimize the amount of Ti oxide. It is maintained at a low level by deoxidation. Therefore, the 0 content is set to 50 ppm or less.

The P content should be as low as possible to lower the strength level of the steel and the deformation resistance during forging.Also, when achieving strength and wear resistance through carburizing heat treatment, delayed fracture characteristics become a problem, so the P content is It is extremely effective to lower the However, there is a problem with P removal in the steel manufacturing process, and P
The content shall be 0.010 wt% or less.

S forms sulfides with iron in steel, causing cracks or flaws during rolling, and also worsens the deformability of steel. Therefore, the S content is o, oi.

1% or less.

Ti and B are effective elements for improving the hardenability of steel. Ti fixes N in steel, supplements the hardenability improvement of B, and also contributes to preventing crystal grain coarsening by turning into TiN. ,
There is a relationship with A1 content, but the lower limit of Ti content is 0.0
A small amount of about 1 wt% is sufficient, and if the content is too large, exceeding 0.05 wt%, the amount of carbonitrides produced increases and becomes coarse, which impairs the cold rollability of the steel. Go up, Ti
The content is 0.01 to 0.05 wt%.

B is an element that assists hardenability due to the low Si content and low Mn content, and is extremely effective in increasing the strength of the core by carburizing heat treatment. If it is less than 0.0005 wt%, this effect is small, and If the content exceeds 11 t% of Q,0O5, the effect will be saturated, and even with such a small amount of content, these effects will be exhibited. Therefore, the S content is 0.00
05 to 0.005 wt%. Furthermore, even if B is contained, it has no effect on the cold workability of the steel, and it is possible to lower the Mn content and, in some cases, the C content by the amount that improves the hardenability. This results in improved workability.

[Example] Next, an example of a low carbon steel having excellent cold workability according to the present invention will be described.

Example Low carbon steel having the components and ratios shown in Table 1 was produced by melting in a converter furnace.

F and G in Table 1 are steels specified as low carbon steels with excellent cold workability according to the present invention, and A to E are comparative steels.

Table 2 shows the mechanical properties of hot-rolled wire rods with a diameter of 5.5 mm obtained by rolling the steel shown in Table 1 into wire rods.

Figure 1 (a) and (b) show the work hardening properties (expressed in tensile strength) of the same hot-rolled wire rod as above, pickled and lime-coated, and then cold-drawn using a continuous wire drawing machine. ) and shows changes in aperture.

As can be seen from Fig. 1, the relationship between the cold drawing rate and the amount of work hardening is such that C, Si, and M
Among n, P, and Cr, the low carbon steel with excellent cold workability according to the present invention is particularly A with low Si content and low M content.
Since it is a 1-killed steel, if the C content is constant, Mn,
It depends on the Cr content.

The lower the Mn content, the less the amount of work hardening, the softer the material, and the lower the deformation resistance during cold working. Even if we do this, there will be no change in this trend. In addition, Ti, B
The cold workability of the steel containing Ti and B, such as cold drawing, wire drawing, and cold heading properties, is approximately the same as that of steel that does not contain Ti and B. Therefore,
The effect of improving tool life during cold heading can be obtained, and depending on the cold drawing process, there is a possibility of simplifying or omitting the intermediate annealing process. Note that the change in yield point strength or 0.2% proof stress shows the same tendency as the tensile strength and increases.

Figure 2 shows the low carbon steel with excellent cold workability according to the present invention and the comparative steel obtained by the Jominy single end quenching method (billet → forging → normalizing → EL test piece and scissors piece quenching → hardness measurement). This figure shows the hardenability measured.

Among low carbon steels with the same level of C content, the difference in Mn content, Ti, and B content clearly appears in the difference in hardenability.

Figure 3 (a) and (b) show the difference in hardenability in Figure 2,
In particular, the effect of containing Ti5B was applied to low Mn low carbon steel that undergoes carburizing, quenching and tempering treatment, and the results of investigation on carburizing and hardenability are shown.

Mn is one of the factors that lowers the deformation resistance of steel, but on the other hand, it is an element that improves hardenability, so Mn is 0.10 to 0.4011t%.
Although it depends on the quenching medium, in particular, in the case of oil quenching of a test piece with a wire diameter of 3.0Illal as shown in Figure 3(b) (8
If the material is hardened in oil by holding it at a temperature of 80°C for 45 minutes, then cooling in air after being held at a temperature of 180°C for 30 minutes, the hardness of the core may not be sufficient.

On the other hand, by containing appropriate amounts of Ti and B, the hardness of the core becomes sufficiently high, making it possible to avoid the low-temperature tempering embrittlement region and freely select the tempering temperature that improves toughness. .

In the case of water quenching (water quenching after holding at a temperature of 880"C for 45 minutes, air cooling after holding at a temperature of 180"C for 30 minutes),
Similarly, the test piece diameter is 3. Although it is Oma+, sufficient hardness can be obtained in both the surface layer and the core without the inclusion of Ti and B, but furthermore, by containing Ti and B, it is possible to achieve a larger diameter, and it is also possible to select the tempering temperature. This makes it possible to improve the properties of a steel with higher toughness.

Figure 4 is a tempering performance curve, showing Ti, B-containing low Mn,
In a comparison of high Mn carbon steels, the yield point, tensile strength, elongation, area of area, and impact value were investigated by water quenching test pieces to a predetermined temperature and changing the tempering temperature to a temperature range of 200 to 500°C. is shown.

As is clear from Fig. 4, as the tempering temperature increases, the yield point and tensile strength decrease, and the elongation, reduction of area, and impact value increase, but low Mn carbon steel reduces and the impact value has a remarkable improvement effect. It can be seen that it is.

Figure 5 shows the relationship between tensile strength and impact value.
It is a steel with a high impact value level and a good balance of toughness for the same tensile strength as steel.

This solves the problem of head skipping during wedge tensile tests or construction of carburized heat-treated steel such as tufting screws.The hardness of the back surface to obtain wear resistance depends on the carburizing heat treatment conditions, and It is not enough just to have high hardness (hardness distribution in the direction of the core and balance with toughness are more important). 300
Tempering is carried out in a temperature range of ~350°C, which is also effective in solving the problem of avoiding this temperature range6 Carbon steel has the above structure, so by carburizing, quenching and tempering, the surface layer of the steel can be hardened to achieve wear resistance, while the core has good toughness. For example, there is no problem of insufficient hardening even for small items with a diameter of 5 mm or less, such as tufted screws and dry walls.Furthermore, since the tempering temperature can be lowered, low-temperature tempering brittleness can be alleviated. It has the excellent effect of improving delayed fracture resistance.

[Brief explanation of the drawing]

Figure 1 shows the relationship between area reduction rate and tensile strength, and area reduction rate and area of area reduction, Figure 2 shows the relationship between hardness and distance from the specimen surface, and Figure 3 shows the relationship between water quenching and oil quenching. FIG. 4 is a diagram showing the relationship between hardness due to quenching and distance from the surface layer, FIG. 4 is a diagram showing a tempering performance curve, and FIG. 5 is a diagram showing the relationship between tensile strength and impact value. Sai 2 Zu Rinda) ￥ に @ & Kosare Shimatake (笥匍) Fang 311A (phantom surface total 1, Uno>Giant Yi) Spear 3 (b) 1 head of barley? Ro・Ukue gigantic electric power (lateral angle) 5 figure 3I 礒屹;

Claims (1)

[Claims] C 0.10-0.30wt%, Si 0.3wt% or less, Mn 0.10-0.40wt%, Cr 0.60
wt% or less, Al 0.01-0.05wt%, P 0
．． 0.010wt% or less, S 0.010wt% or less, O
Contains 50 ppm or less, and further contains Ti 0.01 to 0.05 wt%, B 0.0005 to
A low carbon steel with excellent cold workability, characterized by containing 0.005 wt% and the remainder consisting of Fe and impurities.