JPH02156044A - High-carbon sheet metal with high toughness - Google Patents

Abstract

PURPOSE:To manufacture a high-toughness high-carbon sheet metal excellent in strength and hydrogen embrittlement resistance by incorporating specified percentages of C, Si, Mn, P, Cr, Mo, Nb, solAl, and N to Fe. CONSTITUTION:A steel which has a composition consisting of, by weight ratio, 0.30-0.60% C, <=0.70% Si, 0.05-1.00% Mn, <=0.030% P, 0.50-2.00% Cr, 0.10-0.50% Mo, 0.005-0.100% Nb, <=0.08% solAl, <=0.006% N, and the balance essentially Fe with inevitable impurities and further containing, if necessary, 0.05-0.50% Cu is refined and formed into a high-toughness high-carbon sheet metal of desired sheet thickness by means of hot rolling and cold rolling. By this method, the sheet metal excellent in impact resistance and wear resistance and having superior hydrogen embrittlement resistance can be obtained at a low cost.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention has excellent impact resistance and abrasion resistance after heat treatment, as well as an effect of preventing cracking due to hydrogen embrittlement during use, and has good manufacturability and processability. Specifically, the present invention relates to a high-toughness, high-carbon thin steel sheet suitable for use in chain parts, gear parts, clutch parts, seat belt buckles, washers, and the like.

(Prior art) -a. Chain parts, gear parts, clutch parts, seat belt buckles, washer parts, etc. are "polished special steel strips" such as 535CM or S45CM specified in JIS G3311, or specified in JIS G4105. There are 3
It is customary to use 0M435 to SCM445 chromium molybdenum steel sheets as raw materials, mold them, and then harden them through heat treatment such as quenching and tempering to make products. The steel sheets used for each of the above products are: - soft and easy to process before forming, - required strength only achieved through heat treatment after forming.
■Products are required to exhibit sufficient impact resistance and abrasion resistance during use. Therefore, materials with high carbon content are selected as per JIS, and
- Generally, thin steel sheets shipped from steel manufacturers are subjected to spheroidizing annealing to make them soft. And after shipping material N
The ttiJ temporary is molded into a desired shape by the user and heat treated by quenching and tempering to obtain the required performance of each product. In this case, since the tempering temperature particularly affects the impact resistance and wear resistance of the product, the tempering temperature should be carefully selected within the range of "as-quenched" to "up to 650°C" depending on the product type and usage conditions. be done. (Usually 180-450°C).

(Problem to be solved by the invention) However, the quenching specified in the JIS in the previous section,
Tempered high carbon thin steel sheets may have insufficient impact resistance despite careful selection of heat treatment conditions. A typical example is the case of motorcycle chains, where it has been difficult to completely prevent brittle fractures caused by contact impact with chain covers, etc., or cracks caused by hydrogen absorption from the contact areas.

Therefore, as a measure to prevent the occurrence of brittle fracture and hydrogen embrittlement, chain manufacturers (users from the perspective of the steel industry) are using austempering treatment, which involves isothermal transformation of appropriately cooled austenite, for SCM435, etc., to improve toughness. I'm trying. However, this austempering process is more complicated than quenching and tempering processes, and the heat treatment equipment is also larger, so an increase in cost is unavoidable.

This cost increase is a major problem since motorcycle chains are mass-produced products and motorcycle manufacturers are expected to demand cost reductions.

The general object of the present invention is to provide a thinner material that is cheaper than the conventional method by combining material costs and manufacturing costs for parts that require high impact resistance and wear resistance, such as the aforementioned chain parts and gear parts. Our goal is to provide steel plates.

In addition, the specific purpose of the present invention is to satisfy the required high impact resistance and wear resistance, and to achieve it by simple quenching and tempering treatment, or by austempering treatment, without resorting to special heat treatment. It is an object of the present invention to provide an 11 m plate which can obtain a certain degree of impact resistance improvement effect and has a large effect of preventing cracking due to hydrogen absorption.

Furthermore, to give a specific example, it has enough wear resistance and impact resistance as a material for motorcycle chains, etc., and has good workability, so it can be used in the rolling process and the forming process into final products.
Furthermore, it is an object of the present invention to provide a thin steel plate that does not cause problems such as cracking even if it is subjected to impact, friction, etc. while running as a finished product.

(Means for Solving the Problems) Therefore, in order to achieve the above-mentioned objectives, the present inventors conducted research and found the following methods for improving impact resistance and preventing cracking due to hydrogen absorption. The following findings were obtained (hereinafter, the proportions of components are expressed as weight %).

Above: (a) Complete prevention of hydrogen embrittlement, which tends to occur in steel types with high material strength, was previously thought to be impossible, but the ingredients have been strictly adjusted for such steel types. When a specific amount of Nb (0.005 to 0.100%) is added, austenite grains are effectively refined and cracking due to hydrogen embrittlement is significantly suppressed.

(bl Furthermore, the addition of an appropriate amount of Cu forms a sulfide film on the surface of moving gears, chains, etc., and has the property of suppressing the penetration of hydrogen from the surface, which is effective in preventing the occurrence of cracks due to hydrogen embrittlement. There is.

Supervised improvement: When the P content in Ic11i is reduced to a specific value or less, the amount of P segregated at the austenite grain boundaries is reduced, grain boundary embrittlement, which causes brittle fracture, is suppressed and toughness is improved.

(Reducing d1Mn content also contributes to improving rice grain quality by suppressing MnS formation.The decrease in hardenability that is expected due to the reduction of Mn for the purpose of cost reduction is also due to the high hardenability due to the thin plate of the product.) On the other hand, Cr,
The effect of adding Mo can sufficiently guarantee strength while maintaining toughness.

In general, in order to increase the toughness of high-carbon steel sheets, it has been unavoidable to reduce the formability and punchability before quenching and tempering, but when a certain amount of Mo is added as a steel component, the above-mentioned formability and punchability are impaired. The toughness after quenching and tempering is improved, and an effect called "low-temperature tempering toughness" appears in particular.

The present inventors have previously filed a patent application for steel with additions of Nb, Cu, Tf, and B (hereinafter referred to as Hikari WR), but although this steel type has excellent toughness, the addition of Ti and B is required, so an increase in the cost of steel plates is unavoidable.
Therefore, if there is no need to add Ti or B, a low-cost steel with high practical value can be obtained, as long as the strength and hydrogen embrittlement resistance are sufficiently satisfied, even if the toughness is slightly lower than that of the steel of the prior application.

The present invention was completed based on the above findings, and its gist is as follows: C: 0.30-0.60%, Si: 0.70% in weight proportions
Below, Mn: 0.05”1.00%, P: 0
．． 030% or less, Cr: 0.50-2.00%,
Mo: 0.10~0.50%, Wb: 0.005~
0.100%, sol. Al: 0.08% or less,
N: 0.006% or less, Cu: 0.05 to 0.50% is added as necessary, and the balance is substantially Fe and inevitable impurities.This is a high toughness, high carbon thin steel plate.

The thin steel sheet according to the present invention exhibits excellent wear resistance, toughness (impact resistance), and hydrogen embrittlement resistance, and in particular, the impact resistance is improved by reducing P and Mn and adding Mo.
This is an yl steel sheet characterized by an improved effect of preventing cracking caused by hydrogen embrittlement due to the addition of Cu.

Although not particularly limited to, "thin steel plate"
is generally a steel plate with a thickness of 0.2 to 6.0 ms, or in other words, unlike a so-called thick steel plate, the occurrence of temper brittleness is not remarkable, and the above-mentioned steel composition can sufficiently eliminate this problem. It can be said to be a thick steel plate.

(Function) Here, the reason why the component composition of the present invention is limited as described above will be explained.

C: The C content was determined to be 0.30 to 0.6091i in order to give the steel plate the required hardness and wear resistance.

Si: Active addition is not particularly necessary, but if the content exceeds 0.70%, the steel plate tends to become hard and brittle, so the St content was set at 0.70% or less.

Mn: The main uses of the steel of the present invention to which Cr and Mo are added are motorcycle gears, chains, etc., and unlike general wear-resistant steel plates, it is necessary to reduce Mn in order to improve toughness. In particular, in the steel sheet of the present invention, if the Mn content exceeds 1.00%, it will easily harden during heat treatment and become too hard, resulting in a decrease in toughness.On the other hand, if the Mn content is less than 0.05%,
The solid solution S increases, causing embrittlement during hot working and impairing the manufacturability (rolling) of the steel sheet. The former poses a problem for users, while the latter poses a problem for steel manufacturers for more than 0 reasons, so the Mn content is set at 0.05 to 1.00%, preferably limited to 0.80% or less.

P: For steel sheets containing MO, a normal level may be sufficient, but it goes without saying that the lower the P content, the better in terms of toughness.The P content is set at 0.030% or less, but preferably 0.020%. It is best to limit it to % or less.

Mo: Nb has the effect of improving the toughness of steel by refining austenite grains, and this effect is also very effective in preventing fracture due to hydrogen embrittlement. Therefore, Nb is added for the purpose of preventing these cracks (fractures) from occurring, but
If the Nb content is less than o, oos%, the desired effects will not be exhibited, while if the content exceeds 0.100%, these effects will reach a saturated state.
It was set as oos~0.100%. Preferably 0.005
~0.050%.

Cr: Cr is a component added mainly for the purpose of improving hardenability, but if the content exceeds 2.00%, the steel will harden and become brittle, so the Cr content should be 0.00%. 50~
It was set at 2.00%.

MO = Mo is an important component, and adding 0.10% or more of Mo has the effect of maintaining high toughness after heat treatment without deteriorating the workability of the steel plate before heat treatment (quenching/tempering).

Generally, when steel is tempered at a temperature of around 300° C. after quenching, it undergoes so-called "low temperature tempering embrittlement" and becomes extremely brittle.

By the way, there are cases where tempering at the above-mentioned temperature is absolutely necessary, such as when desired hardness is desired. In fact, the above-mentioned "low-temperature tempering embrittlement" is noticeable in thick plates and tends to be reduced in thin plates, so tempering at this temperature is sometimes used for thin plates.

However, in that case, depending on the usage conditions, a decrease in toughness still becomes a problem. M also against such embrittlement.

The addition of is very effective. However, since adding more than 0.50% of Mo has the property of offsetting the hydrogen absorption suppressing effect of adding Cu, the upper limit is set at 0.50%.

The content was determined to be 0.10 to 0.50%.

sol. Al: AQ is a component added as necessary as a deoxidizing agent for steel, but sol. If the Al content exceeds 0.08%, the steel sheet will be hardened and the cost will increase, with no benefit. Al content 0°08%
It was determined as follows.

N: The inclusion of N is effective in improving the hardness and tensile strength of steel, but
Excessive content increases hardness after annealing more than necessary and impairs workability. To prevent this, the N content was reduced to 0.006
% or less.

Cu: Cu is added as desired. Although the effect on hardenability is not so great, it forms a sulfide film on the surface,
The effect of suppressing hydrogen intrusion is remarkable. This effect has been confirmed at 0.05% or more, but since this effect is saturated at more than 0.50%, the range of addition amount has been changed to 0.05% or more.
It was set at ~0.50%.

Others: Even in ordinary steel, it is better to have a low S content, but especially in high-strength steel plates such as those involved in the present invention, the presence of MnS has a significant effect on toughness deterioration. It is preferable to suppress the content to 0.0040% or less.

Rolling and heat treatment: The thin steel sheet according to the present invention is produced by melting steel containing the above components and the remainder substantially consisting of Fe and unavoidable impurities, and by hot rolling and cold rolling, for example, 0.2~ 6
．． It is manufactured by being stretched to a desired thickness of approximately 0 mm.

During this time, annealing may be performed for softening if necessary.

In any case, annealing is ultimately performed to improve toughness, and it is appropriate to perform this final annealing at a temperature of 650 to 720°C.

The thickness of the thin m-plate according to the present invention is not particularly limited, but when manufacturing chain parts, gear parts, etc., it is generally 1.
．． Advantageously, the plate thickness is approximately 0 to 3.0 mm. In addition, since punching, bending, etc. are performed, the characteristics of such thin steel sheets are such that the hardness is II in the annealed state.
I? It is preferable to satisfy mechanical properties such as 880 or less, yield point of 45 kgf/am'' or less, and elongation within a specific range.

The thin TLL material produced as described above is usually molded into a desired shape by the user, and then heat treated to produce a product with desired hardness and performance.

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 First, steel having the respective compositions shown in Table 1 was melted, then hot rolled into a 4 mm thick hot rolled plate, then softened by annealing, and then rolled into a 2.5 mm thick sheet. Cold rolled to

In steel A-B, Mn was reduced to 0.41%, and
．． 04-0.05% Nb was added. Further, in Steel C, the amount of Mn added was set to 0.76% to increase hardness. Furthermore, 29 to 0.31% of CuO was added to Steels B and C.

On the other hand, in Comparative 1iE and F, there is no addition of Nb, and in Comparative Steel F, there is no addition of Cu, and in the case of rope, the amount of Mn5P added exceeds the range of the present invention, and even in Steel F, the amount of P added is within the range of the present invention. It is beyond the scope of the invention.

Table 1 Test steel (wt%) (Note) Chapter: Outside the scope of the present invention.

Next, these cold-rolled steel plates were subjected to batch annealing at 680°C for 16 hours.Next, samples were taken from these annealed plates, heated to 850°C, oil-quenched, and further subjected to various treatments. It was tempered at a temperature of .

Note that the tempering time was 30 minutes. Hardness was measured for each sample after the above heat treatment, and the hardness at that time was measured as the first
This is shown graphically in the figure.

From this result, the steel plate according to the present invention is considered to have a desirable hardness)ll? It can be seen that a tempering temperature of 250° C. or lower is appropriate to satisfy the condition of c > 45.

Example 2 Samples of cold rolled sheets of each steel type in Example 1 2
．． A JIS No. 4 Notch Charpy test piece with a thickness of 5 m + m was prepared and an impact test was conducted at 0°C.
The energy harvested was investigated. The correlation between sample hardness and absorbed energy obtained from these results is shown graphically in Figure 2.
In the figure, the symbols are the same as in FIG. 1.

The following can be seen from the results shown in Figure 2.

Regarding the C content level of the sample steel sheet, it is said that it is desirable to have a hardness of HRC45 or higher when used in chains and gears, but it is difficult to ensure an absorbed energy of 1.4 kgf-m while satisfying this condition. is rope A tempered at 200℃
~C and steel F.

Example 3 Samples after each heat treatment in Example 1 whose hardness HRC is at a level of 48.1 to 50.6 are 2°5
A test piece (50X 11■■) was prepared with an opening 12 with a notch lO in the material whose shape and dimensions are shown in Fig. 3 (5) to Fig. 3 (cl) with a plate thickness of mm. Push the maraging preparation wedge 16 through the narrow notched opening 12 as shown in Figure 3 (see arrow C1) to widen the opening 12 to 80kgf/l1l.
Approximately 1" stress is applied. Figure 3 (C1 is an enlarged partial view of the opening with a narrow notch. The fine notch 10 is held in hot water at 50°C in a state where it is expanded in this way. The occurrence of cracks 8 caused by hydrogen absorption from the tip of 10 was observed.The numbers in Fig. 3 fC1 are the dimensions (am) of each location, and the results are summarized in Table 2.

In the steel plate according to the present invention, cracking does not occur for more than 20 hours even when the hardness is at the IIRc>48 level, but in the comparative steel, cracking occurs within 11 hours, and some steel types were observed to break within 20 hours after propagation. For F, it is considered that there is a high risk of hydrogen cracking in the as-quenched material or in the material after low-temperature tempering.

As mentioned above, from the results of Examples 1 to 3, it was confirmed that only the steel plate according to the present invention satisfies hardness, absorbed energy, and durability against cracking due to hydrogen absorption for 8 hours or more.

Table 2 (Note) Q: oil quenching, T: tempering Example 4 Next, the 68 types of thin steel plates shown in Table 1 (plate thickness 2°511
After punching out the gear plate from 11), 860℃ x 2
After heating to 0+win, quenching in oil, and then tempering, the rotating gear (thickness: 2.Smra, diameter:
It was assembled into a 250m crane, tooth length: 10++m). The rotating gear 30 thus obtained was rotated at a speed of 1200 rpm as shown in FIG. While spraying water intermittently on the surface of this rotating chain, the mild steel plate 3
A rotation test was conducted in which two (10'X10'X50'l, HRB-70) were rubbed together.

As a result, as shown in Table 3, in the case of 1iA-C, 80
No damage to the gears was observed up to 70 minutes, but all of cables D-F were broken by 70 minutes, and all of their fracture surfaces exhibited intergranular fracture surfaces. The results are shown in FIG. 5 as a graph of hardness and durability time until cracking occurs.

From the above results, it was confirmed that the steel of the present invention is very effective in preventing cracks in chains, gears, etc. while ensuring high hardness.

Example 5 In this example, a steel having the composition shown in Table 4 was cold rolled to 2.5+wm in the same manner as in Example 1. The obtained steel sheets of the present invention were subjected to various heat treatments. The hardness and absorbed energy at that time, as well as the time until cracking occurred in the test shown in FIG. 3, were measured and summarized in Table 4.

As described above, it was confirmed that the steel sheet according to the present invention is excellent in preventing cracking in an atmosphere that easily absorbs hydrogen, and is also a steel type with extremely high strength.

Table 3 (Effects of the Invention) As detailed above, according to the present invention, the following excellent effects can be obtained.

(1) Although the heat treatment conditions have been limited to austemperature, it is now possible to perform quenching and tempering treatments, simplifying the heat treatment equipment required by the user.

(2) This also makes it possible to improve the hardness level, which has been suppressed until now to prevent cracking due to hydrogen embrittlement, by low-temperature tempering.

(3) It can be used even in high temperature and humid atmospheres, expanding the range of applications of the product.

[Brief explanation of the drawing]

Figure 1 shows the steel types listed in Table 1 after oil quenching at 850°C.
A graph showing the relationship between tempering temperature (time is 30 minutes) and hardness (HIIC); Figure 2 is a graph showing the relationship between hardness (HRC) and absorbed energy for the steel types in Table 1; Figure 3 ( al to Figure 3 fcl are explanatory diagrams of the method for testing the occurrence and growth of cracks using non-chips and wedges in plate test pieces;
The figure is an explanatory diagram of the gear tooth impact crack test method; and Figure 5 is the hardness (HRC) of the material in the test of Figure 4.
FIG. 2 is a graph showing the relationship between cracking occurrence and durability time until rupture. 1 Old thin notch 12: Opening 14: Test piece 16: Resistance 18: Crack

Claims (2)

[Claims] (1) Weight percentage: C: 0.30-0.60%, Si: 0.70% or less, M
n: 0.05-1.00%, P: 0.030% or less, C
r: 0.50-2.00%, Mo: 0.10-0.50
%, Nb: 0.005-0.100%, sol. Al:
A high-toughness, high-carbon thin steel plate comprising: 0.08% or less, N: 0.006% or less, and the remainder substantially consisting of Fe and inevitable impurities. (2) Weight percentage: C: 0.30-0.60%, Si: 0.70% or less, M
n: 0.05-1.00%, P: 0.030% or less, C
r: 0.50-2.00%, Mo: 0.10-0.50
%, Nb: 0.005-0.100%, sol. Al:
0.08% or less, N: 0.006% or less, Cu: 0.0
5 to 0.50%, and the balance substantially consists of Fe and inevitable impurities.