JPH02179842A - High-toughness wear-resistant steel sheet - Google Patents

Abstract

PURPOSE:To improve the toughness and delayed fracture resistance after water quenching and temperature treatment in the steel sheet by reducing the amt. of Mn, increasing suitable amt. of Mo and specifying the contents of Cr, B, Al, N, etc. CONSTITUTION:The compsn. of a wear-resistant steel is formed with, by weight, 0.18 to 0.25% C, 0.10 to 0.30% Si, 0.03 to 0.10% Mn, <=0.008% P, <=0.002% S, 1.00 to 2.00% Cr, >0.50 to 0.08% Mo, 0.007 to 0.020% Nb, 0.0003 to 0.0012% B, 0.020 to 0.080% sol Al, 0.0010 to 0.0050% N and the balance Fe with inevitable impurities. The steel sheet has excellent surface hardness and is preferably suitable as a stock for a construction equipment, a crushing press, a cutting machine for a steel frame, etc.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a material with excellent toughness and delayed fracture resistance after water quenching and tempering treatment, which is suitable as a material for construction machinery, crushers, steel frame cutting machines, etc. This relates to wear-resistant steel plates.

<Prior art and its problems> For example, steel materials used in parts such as the packets of construction power shovels that are subject to severe wear due to earth, sand, iron scrap, etc. are usually
Measures are being taken to increase its hardness. In order to make steel materials highly hard, it has been necessary to increase the C content, add alloying elements, and then perform quenching and tempering treatments.

However, it has been pointed out that such wear-resistant steels have problems such as poor toughness, high delayed fracture susceptibility, and poor workability and weldability. In other words, machines and equipment using steel materials with low toughness are likely to break due to impact during operation, and steel materials with high susceptibility to delayed fracture are likely to crack when used underwater, leading to damage. On the other hand, poor workability and weldability lead to inconveniences such as increased manufacturing costs for machines and equipment.

Therefore, a wear-resistant steel plate was proposed in which the amounts of alloying elements such as Cr and Mo were reduced as much as possible, the contents of C and Mn were increased, and a small amount of ■ was added to improve weldability and workability.
3-18036), but even in this proposal, priority was given to ensuring hardness and no consideration was given to toughness and delayed fracture susceptibility, so it still did not solve the problems of impact fracture and delayed fracture. .

Therefore, the purpose of the present invention is to not only provide wear resistance (that is, "hardness") equal to or higher than that of conventional wear-resistant steel plates, but also to have sufficiently satisfactory toughness and delayed fracture resistance. The aim was to provide a wear-resistant steel plate with a long service life and high reliability.

Means for Solving the Problem> In the process of conducting research from various viewpoints to achieve the above object, the present inventors discovered that Nb was added to clean steel with low P and S contents.
rc:o, 18 to 0.35%, was proposed based on the knowledge that the addition is effective in improving the toughness of high-strength steel (hereinafter, % representing the component ratio is expressed as weight %).

Si: 0.5% or less, Mn: 0.5% or less, P: 0
．． 01% or less.

S: 0.01% or less, Cr: 0.01-5%
, Nb: 0.005 to 0.1%, and further contains V
: 0.3% or less, B: 0.005% or less, Ti
: 0.05% or less and Mo: 0.5% or more, and the balance is Fe and unavoidable impurities.
As a result of repeated research on steels with high wear resistance, paying attention to the excellent delayed fracture resistance of steel (No. I came to know something.

(a) High-strength steel of the above composition system (Japanese Unexamined Patent Publication No. 61-174
When the Mn content is reduced to the very low region of 0.10% or less in the steel proposed as No. 326), the steel exhibits significantly improved toughness and delayed fracture resistance.

(b) However, reducing the Mn content in this way also reduces the hardness of the steel, making it impossible to secure the level required for wear-resistant steel. It is extremely effective to increase the amount of Mo.
Cr and B at the same time as measures to increase content.

If you also adjust the content of sol, kfl, N, etc.,
It is possible to ensure sufficient hardness (wear resistance) as a wear-resistant steel.

(C) Furthermore, increasing the amount of Mo in an appropriate amount is very effective not only for increasing hardness (but also for improving toughness and delayed fracture resistance).

The present invention has been made based on the above-mentioned findings, etc. [A wear-resistant steel plate, C: 0.18 to 0.25%, St: 0.10]
”0.30%.

Mn: 0.03-0.10%, P: 0.0
08% or less.

S: 0002% or less, Cr: 1.00
~2.00%.

Mo: more than 0.50 to 0.80%, Nb: 0
．． 007-0.020%.

B: 0.0003 to 0.0012%.

sol, Al: 0.020=0.080%.

By configuring the composition to contain N: 0.0010 to 0.0050% and the remainder consisting of Pe and unavoidable impurities, it has excellent wear resistance (hardness), toughness, and delay resistance after water quenching and tempering treatment. It is characterized by the fact that it has both destructive properties.

Next, the reason why the content ratio of each component in the wear-resistant steel plate according to the present invention is limited as described above will be explained in detail together with the effect of each component.

Ensure sufficient hardenability to achieve the hardness required for wear-resistant steel plates (
In order to achieve a Rockwell C hardness (HRC) of 40 or higher, a C content of 0.18% or higher is required;
．． Since containing C in excess of 25% causes deterioration of toughness, the C content was set at 0.18 to 0.25%.

In addition to deoxidizing the steel, Si St has the effect of preventing a decrease in hardness during tempering, but if its content is less than 0.10%, the desired effect due to the above effect cannot be obtained; 0.3
If the Si content exceeds 0%, the toughness and delayed fracture susceptibility deteriorate, so the Si content was set at 0.10 to 0.30%.

Mn Mn is the most important element to be noted in the present invention. That is, it is only by reducing the Mn content to a specific low range of 0.10% or less that it becomes possible to ensure sufficiently satisfactory toughness and delayed fracture susceptibility. This is due to the "effect of reducing the amount of segregation of impurity elements (P, Sn, etc.) to grain boundaries" brought about by reducing the Mn content. However, in order to sufficiently deoxidize the steel, it is necessary to contain 0.03% or more, so the amount of Mn content is 0.03% or more.
It was set at ~0.10%.

Since P segregates at grain boundaries and deteriorates the toughness and delayed fracture resistance of steel, it is desirable that its content be as low as possible. However, in the steel sheet of the present invention in which the Mn content is 0.10% or less, the P content is 0.008%.
Since the effect of reducing the amount of P decreases in the following range, the P content was determined to be 0,00% or less.

S is an impurity element that deteriorates the ductility and toughness of steel, and if its content exceeds 0.002%, the adverse effects become noticeable, so the S content was determined to be 0.002% or less.

Cr Cr has the effect of increasing the hardenability and ensuring a predetermined hardness of the steel plate, but if its content is less than 1.00%, the desired effect cannot be obtained by the above effect. 00%
If Cr is contained in an amount exceeding 1.00 to
It was set at 2.00%.

O As mentioned above, Mo not only has the effect of increasing the hardenability of the steel sheet to effectively ensure the specified hardness, but also improves delayed fracture resistance by suppressing the segregation of P at grain boundaries. Improvement effects have also been observed.

In order to obtain the hardness necessary for a wear-resistant steel sheet in the steel sheet of the present invention with a reduced Mn content, Mo must be contained in an amount exceeding 0.50%. However, if the Mo content exceeds 0.80%, the delayed fracture resistance will deteriorate and the hardenability improvement effect will not increase. % or less.

Nb Nb precipitates in steel as Nb (CN) and has the effect of refining the microstructure and improving toughness, as well as increasing hardness by increasing temper softening resistance.
．． If the Nb content is less than 0.007%, the desired effect due to the above action cannot be obtained, and on the other hand, if the Nb content is more than 0.020%, toughness decreases due to excessive precipitation hardening.
．． It was set at 0.007% to 0.020%.

B is an effective element that improves the hardenability of steel and increases its hardness, but if its content is less than 0.0003%, the desired hardness cannot be secured; on the other hand, if its content exceeds 0.0012%, Since containing B causes a decrease in toughness,
The S content was determined to be 0.0003 to 0.0012%.

sol, AJI AI has the effect of fixing N in steel as AIN, preventing the formation of BN, and ensuring the hardenability improvement effect of B, but its content is 0.020% for sol, AII. If it is less than o, o
If the content exceeds so%, it will cause a decrease in toughness.
soZ, Af content was determined to be 0.020 to 0.080%.

In order to ensure the hardenability improving effect of B, it is necessary to keep the N content in the steel at 0.0050% or less. However, in order to ensure the amount of Nb (CN) precipitates and exhibit the effect of Nb addition, the N content must be reduced to 0.001.
Since it needs to be 0% or more, the N content is 0.001
It was set at 0 to 0.0050%.

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

<Example> First, steel having the chemical composition shown in Table 1 was melted and hot rolled into a steel plate with a thickness of 50 mm.
It is heated to 80℃, water quenched, and then tempered (450℃).
℃ and then left to cool).

Next, the toughness of the obtained steel plate at the center of the plate thickness.

The surface hardness and delayed fracture characteristics were investigated, and the results are also shown in Table 1.

The toughness was measured using a Charpy impact test piece (JIS No. d Charpy impact test piece) from the center of the plate thickness in the rolling direction.

A 2 m V-notch) was collected and investigated. Further, the surface hardness was determined by removing the decarburized layer on the plate surface by machining and then measuring the surface with a Rockwell C hardness meter.

Furthermore, the delayed fracture characteristics were measured by the "wedge insertion type delayed fracture test method" shown in FIG.

That is, in this test, a test piece with the shape and dimensions shown in Fig. 1 (al) was taken, and a "wedge" shown in Fig. 1 (C) ”, apply a static load, and place it in warm water maintained at 55℃.
A method is used to investigate the time until cracking occurs. In addition, in this test, 5000 hours was used as one of the criteria for determining delayed fracture resistance, but this was because it was assumed that 3 months was a period for regular maintenance or inspection of equipment, and the test was conducted in hot water at 55°C. The conditions correspond to the most severe actual use environment.

Now, as is clear from the results shown in Table 1, the steel sheets A to G of the present invention are all sufficiently excellent in surface hardness, toughness, and delayed fracture resistance as wear-resistant steel sheets. It can be seen that the comparative steel sheets are inferior in any of the above properties.

For example, because the comparative steel sheet J has a low Mo content, and because the comparative steel sheet J has a low B content, both have low surface hardness.

Since comparative steel sheet H has a high C content, comparative steel sheet ■
Because the steel sheet has a high Mn content, the comparative steel sheet has a low Nb content, and the comparative steel sheet has a low B content, all of them have low toughness.

Furthermore, because the comparative steel sheet H has a high C content, and the comparative steel sheet ■ has a high Mn content, delayed fracture occurred in both cases in a short time. Moreover, it can be confirmed that the negative influence of Mn in particular is large.

<Summary of Effects> As explained above, according to the present invention, it has sufficiently satisfactory hardness (wear resistance), good toughness and delayed fracture resistance, and has a long service life and reliability. Industrially useful effects are brought about, such as making it possible to stably provide wear-resistant steel plates with excellent properties.

[Brief explanation of the drawing]

Figures 1 fa) to (C) are explanatory diagrams of the delayed fracture test method employed in the example.

Claims (1)

[Claims] Weight percentage: C: 0.18-0.25%, Si: 0.10-0.30
%, Mn: 0.03-0.10%, P: 0.008% or less, S: 0.002% or less, Cr: 1.00-2.00
%, Mo: more than 0.50 to 0.80%, Nb: 0.007
~0.020%, B: 0.0003~0.0012%, sol. Al: 0.020-0.080%, N: 0.0
A wear-resistant steel plate having excellent toughness and delayed fracture resistance after water quenching and tempering treatment, characterized by containing 0.010 to 0.0050% and the remainder consisting of Fe and unavoidable impurities.