JPH0551691A - Wear resistant steel sheet excellent in delayed fracture resistance and its production - Google Patents

Abstract

PURPOSE:To produce a high hardness steel sheet excellent in wear resistance and delayed fracture resistance. CONSTITUTION:The steel sheet is a plate of steel having a composition which contains, by weight, 0.15-0.45% C, <0.05% Si, 0.30-0.60% Mn, 0.01-1.00% Cr, 0.03-0.70% Mo, 0.010-0.O80% Sol.Al, and 0.005-0.15% Nb and further contains proper amounts of one or more elements among Cu, V, Ti, B, and Ca and where the contents of P and S are limited to <=0.010% and <=0.005%, respectively. The above steel sheet can be produced by subjecting a steel as a stock to heating up to 1000-1200 deg.C, to hot rolling at 750-950 deg.C finishing temp., and immediately to quenching from the temp. not lower than the Ar3 transformation point to perform hardening, or, tempering can be done at <=500 deg.C after hardening. This steel sheet can be used for civil engineering machinery and mining machinery and remarkably prolong the service lives of these machineries.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high hardness wear resistance used for civil engineering, buckets of mining machinery, cargo trucks of large dump trucks, excavator plates of bulldozers, etc., where abrasion due to ore or sediment causes problems It relates to a steel plate.

[0002]

2. Description of the Related Art Generally, the wear of a portion of steel used in civil engineering and mining machinery that comes into direct contact with ore or earth and sand is much higher than expected. It is said that the number of years will be decided.

It is known that the wear of steel materials due to such ores and earth and sand is governed by the hardness of the steel surface, and the harder the material, the smaller the amount of wear. Therefore, increasing the hardness of the steel material is an effective means for reducing the amount of wear of the steel material. However, when the hardness of steel is increased, as described in Japanese Patent Publication No. 1-21846, the susceptibility to delayed fracture due to hydrogen increases, and fracture occurs during the production of civil engineering, mining machinery, etc. or during its use. The difficult problem of becoming easier occurs. This delayed fracture phenomenon is a heat-affected zone (hardened zone) that occurs when the steel sheet is gas-fused
However, it is difficult to use steel plates with high wear resistance and high surface hardness for civil engineering and mining machinery that require welding.

The invention steel disclosed in Japanese Patent Publication No. 1-21846 is
By reducing the Mn content in steel, the problem of delayed fracture in high hardness steel is solved. This is a finding that if the Mn content in steel is reduced from 0.6 to 1.2% of ordinary steel (hereinafter,% representing the composition ratio is% by weight) to less than 0.3%, the delayed fracture time is significantly extended. It is due to. However, like the steel of the present invention, in addition to being relatively inexpensive, by improving the hardenability and suppressing Mn, which is a convenient element for ensuring strength, a relatively expensive element such as Cr or Mo is obtained. It is necessary to add a large amount of to compensate for the decrease in strength due to the reduction of the Mn content, which increases the manufacturing cost.

For these reasons, it has been desired to develop a wear-resistant steel sheet which is inexpensive and has excellent delayed fracture resistance.

[0006]

DISCLOSURE OF THE INVENTION An object of the present invention is to meet the above-mentioned demands, and it is inexpensive and excellent in delayed fracture resistance, and can be used for civil engineering, buckets of mining machinery, cargo beds of large dump trucks, and bulldozers. Abrasion resistant steel plate used as an earth dumping plate, and more specifically, an inexpensive thick steel plate having a Brinell hardness of about 360 to 600 and low susceptibility to delayed fracture, and a practical manufacturing method thereof. To provide.

[0007]

Means for Solving the Problems The present inventors have obtained the following findings as a result of research to achieve the above objects.

[0008] In a steel having a chemical composition that achieves a steel hardness required for civil engineering and mining machinery, if the Mn content is reduced, delayed fracture resistance is greatly improved.
Even if it is not reduced to less than less than that, the improvement effect can be obtained by controlling it in the range of 0.60 to 0.30%.

If a so-called direct quenching process in which an appropriate amount of Nb is further added and hot rolling is performed immediately followed by quenching and quenching is adopted as a manufacturing method, delayed fracture resistance is further improved.

Since delayed fracture is essentially grain boundary fracture, it is preferable to avoid mixing of impurity elements such as P and S that cause grain boundary embrittlement as much as possible.

Even if the content of P is suppressed to a low level, the effect of improving delayed fracture resistance is reduced in the presence of an alloying element such as Si that pushes P from the grain to the grain boundary.
It is preferable to keep the content of these elements as low as possible.

The present invention has been made on the basis of such findings, and has as its gist the following wear-resistant steel plate and its manufacturing method.

(I) C: 0.15 to 0.45% by weight, Si:
Less than 0.05%, Mn: 0.30 to 0.60%, Cr: 0.01 to 1.00%, M
o: 0.03 to 0.70%, Sol.Al: 0.010 to 0.080%, Nb: 0.0
05 to 0.15%, further Cu: 0.05 to 0.75%, Ni:
0.05-1.5%, V: 0.005-0.15%, Ti: 0.005-
0.25%, B: 0.00015-0.0025%, Ca: 0.0005-0.
It is characterized by containing at least one of 080%, the balance being Fe and unavoidable impurities, and the contents of P and S in the impurities being P: 0.010% or less and S: 0.005% or less, respectively. A wear-resistant steel plate with excellent delayed fracture resistance.

(II) The steel having the chemical composition described in (I) above is heated to 1000 ° C to 1200 ° C, and then the finishing temperature is 750 ° C.
After a predetermined thickness by hot rolling to a temperature range of ~ 950 ° C., immediately delayed wear resistance with excellent fracture resistance, characterized by hardening by quenching from Ar ₃ transformation point or more temperature range Steel plate manufacturing method.

(III) The method for producing a wear-resistant steel sheet having excellent delayed fracture resistance as described in (II) above, which is characterized by further performing tempering treatment in a temperature range of 500 ° C. or lower after quenching.

[0016]

In the present invention, the reason why the chemical composition of steel and the manufacturing conditions are limited as described above will be explained.

A) Chemical composition of steel C: C has the function of improving the hardness of the steel sheet, and a content of 0.15% or more is necessary to secure a surface hardness of 360 or more in terms of Brinell hardness, Even if the content exceeds 0.45%, the retained austenite remains untransformed and further improvement in hardness cannot be obtained, so the content is set to 0.15 to 0.45.
%.

Si: Si exhibits a negative interaction with P. That is,
Si has the property of easily existing in the grain, and pushes P, which increases the susceptibility to delayed fracture, to the grain boundary to promote grain boundary embrittlement. In order to suppress such grain boundary embrittlement, it is desirable to keep the Si content as low as possible, so the content was made less than 0.05%.

Mn: Mn is an element that greatly affects the delayed fracture resistance of steel, and it is very important to adjust the content thereof. The lower the content, the lower the susceptibility to delayed fracture, but even if it is not kept below 0.30% as in the steel sheet described in Japanese Examined Patent Publication No. 1-21846, it will be at least 0.60% resistant. Since the effect of improving delayed fracture can be obtained, 0.60% was made the upper limit. On the other hand, even if the content is lower than 0.30%, the effect of improving delayed fracture resistance is saturated, and conversely, the decrease in hardenability due to the reduction of the Mn content is caused by the relatively expensive elements such as Mo and Cr. Therefore, the lower limit is set to 0.30% because only the manufacturing cost is increased.

Cr: Cr is an element added to compensate for the lack of hardenability due to the suppressed Mn content. To this end, it is necessary to contain 0.01% or more, but if the content exceeds 1.0%, the toughness and weldability of steel deteriorate, so the content was made 0.01 to 1.00%.

Mo: Mo is also an element effective for improving the hardenability like Cr, and it is necessary to contain Mo by 0.03% or more in order to make up for the insufficient hardenability by suppressing the Mn content. But,
Even if the content exceeds 0.70%, the hardening effect is saturated,
Not only will this lead to an increase in manufacturing costs, but on the contrary, weldability will be impaired, so the content was made 0.03 to 0.70%.

Sol.Al: Al is a component used as a deoxidizer during steelmaking, and has the effect of refining the crystal grains to improve toughness. For that purpose, it is necessary to contain more than 0.010% in Sol.Al. However, if it exceeds 0.080%, coarsening may occur and the toughness may be deteriorated. Therefore, the content of Sol.Al is 0.010 to 0.080. %.

Nb: Nb is an important element in the present invention, and its addition significantly improves delayed fracture resistance. For that purpose, it is necessary to contain 0.005% or more, but if the content exceeds 0.15%, on the contrary, the toughness becomes remarkably deteriorated, so the content was made 0.005 to 0.15%.

Cu: Cu is an element effective for increasing the strength without impairing the weldability. For that purpose, it is necessary to contain 0.05% or more, but even if the content exceeds 0.75%, the effect is saturated and only the cost increases, so the content was made 0.05 to 0.75%.

Ni: Ni is a very effective element for improving the hardenability because it enhances the hardenability of steel and does not impair the toughness. Therefore, it is necessary to contain 0.05% or more. However, since Ni is a very expensive element, its content was set to 0.05 to 1.5% in consideration of economic efficiency.

V: V is an element that remarkably enhances the softening resistance during tempering, and is an extremely effective element for removing residual stress by tempering and maintaining high strength. Therefore, it is necessary to contain 0.005% or more, but 0.15%
If the content exceeds 0.005%, further increase in strength cannot be expected, and on the contrary, not only the toughness is impaired but also the manufacturing cost increases, so the content was made 0.005 to 0.15%.

Ti: Ti is also an element effective for improving strength like V and Nb. For that purpose, it is necessary to contain 0.005% or more, but even if the content exceeds 0.25%, further strength increase cannot be expected and only the manufacturing cost will increase, so the content should be 0.005-0.25%. And

B: B is an element that does not impair the weldability and significantly increases the strength of steel by the addition of a trace amount. For that purpose, it is necessary to contain 0.00015% or more, but 0.00
If the content exceeds 25%, a boron compound precipitates during quenching, which rather reduces the hardenability, so the content was made 0.00015 to 0.0025%.

Ca: Ca has an effect of desulfurization and is an element effective for controlling the morphology of inclusions. In order to secure these effects, it is necessary to contain 0.0005% or more, but if the content exceeds 0.0080%, the cleanliness of steel will deteriorate, so the content was made 0.0005 to 0.0080%.

The above Cu or less components may be used alone, or two or more may be added in combination.

P: P is a harmful element that deteriorates the toughness of steel and increases the susceptibility to delayed fracture, so it is desirable to keep its content as low as possible. The content was set to 0.010% or less in consideration of economical efficiency and obtained characteristics.

S: When the S content increases, not only the anisotropy of toughness and ductility increases, but also the toughness and ductility values themselves decrease, so the content was made 0.005% or less.

B) Manufacturing conditions a) Heating temperature during hot rolling: If the heating temperature prior to hot rolling exceeds 1200 ° C., it will be out of the range of normal operating conditions, resulting in unsteady operation and an increase in manufacturing cost. In addition to this, there is a possibility of promoting cracking of the steel ingot due to overheating, so 1200 ° C was made the upper limit. On the other hand, in order to form a solid solution of elements such as Nb, V and Ti, it is necessary to heat at a temperature of 1000 ° C. or higher, so 1000 ° C. was made the lower limit.

B) Finishing temperature in hot rolling: In order to manufacture a high hardness steel plate by a process of quenching immediately after hot rolling, the quenching must be performed at least in a temperature range of Ar ₃ transformation point or higher. .. Therefore, unless the rolling finishing temperature is set to 750 ° C or higher, the quenching temperature above the Ar ₃ transformation point cannot be secured, so the lower limit was set to 750 ° C. On the other hand, if finishing at temperatures above 950 ° C, the susceptibility to delayed fracture cannot be kept low, so 950 ° C was set as the upper limit.

C) Quenching temperature In order to secure a high hardness of 360 or more in Brinell hardness,
The structure at the time of quenching must be a structure mainly composed of martensite. For that purpose, it is necessary to start quenching in a temperature range of Ar ₃ transformation point or higher, so that temperature is set to Ar ₃ transformation point or higher.

D) Tempering temperature: The steel sheet obtained by the quenching treatment may be used as it is, but it may be warmed by a leveler to reduce residual stress and secure flatness. As described above, a reheating treatment corresponding to tempering may be performed. At this time, if tempering is performed at a temperature higher than 500 ° C, the hardness decreases and the wear resistance deteriorates. Therefore, the tempering temperature was set to 500 ° C or lower. It should be noted that it is desirable to set the lower limit temperature to 200 ° C., because the desired effect such as reduction of residual stress cannot be obtained even if tempered at an excessively low temperature.

[0037]

[Examples] Steels having the chemical compositions shown in Table 1 were melted, heated, and then hot-rolled into thick steel plates, which were immediately water-quenched. After quenching, tempering was performed except for some thick steel plates.

Test pieces were cut out from each of the thick steel plates thus obtained, and the mechanical properties, surface hardness and delayed fracture occurrence time were measured. The results of these measurements are shown in Table 2 together with the production conditions.

The delayed fracture occurrence time was measured by a constant displacement delayed fracture test in 55 ° C. hot water.

[0040]

[Table 1]

[0041]

[Table 2]

From Table 2, all of the thick steel plates (Invention Examples Nos. 1 to 10) obtained by the production method of the present invention using steel having the chemical composition specified in the present invention have the target surface hardness ( 36 in HBS
0 to 600) and the delayed fracture occurrence time is 2500 hours or more. However, even if the steel having the chemical composition defined in the present invention such as Nos. 11 to 13 is used, one or both of the surface hardness and the delayed fracture resistance are bad if the manufacturing method is wrong. Further, as in Nos. 14 to 17, the manufacturing conditions are within the range specified by the present invention, but when the chemical composition of steel deviates from the range specified by the present invention, both surface hardness and delayed fracture resistance are poor.

[0043]

As described above, according to the present invention,
Has high hardness with good wear resistance to ores, earth and sand, etc.
Moreover, it is possible to supply an inexpensive steel plate having excellent delayed fracture resistance and weldability. If this steel plate is used for civil engineering and mining machinery, the service life of these machinery can be greatly extended.

Claims (3)

[Claims] 1. By weight%, C: 0.15 to 0.45%, Si: less than 0.05%, Mn: 0.30 to 0.60.
%, Cr: 0.01 to 1.00%, Mo: 0.03 to 0.70%, Sol.Al: 0.
010 to 0.080%, Nb: 0.005 to 0.15%, and
Cu: 0.05 to 0.75%, Ni: 0.05 to 1.5%, V: 0.005 to 0.
15%, Ti: 0.005-0.25%, B: 0.00015-0.0025%, C
a: containing at least one of 0.0005 to 0.0080%, the balance consisting of Fe and unavoidable impurities, and the contents of P and S in the impurities are P: 0.010% or less and S: 0.005, respectively.
% Or less, a wear-resistant steel sheet excellent in delayed fracture resistance. 2. The steel having the chemical composition according to claim 1, 1000
℃ ~ 1200 ℃, finish temperature 750 ℃ ~ 950 ℃
After a predetermined thickness by hot rolling to a temperature range of, immediately production of wear-resistant steel sheet excellent in delayed fracture resistance, which was quenched from Ar ₃ transformation point or more temperature range, characterized in that quenching Method. 3. The method for producing a wear-resistant steel sheet having excellent delayed fracture resistance according to claim 2, further comprising tempering in a temperature range of 500 ° C. or lower after quenching.