JP2006328512A - Wear resistant steel with excellent low-temperature toughness, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide wear resistant steel having excellent low-temperature toughness and used for industrial machinery, conveying equipment, etc., and also to provide its manufacturing method. <P>SOLUTION: The steel has a composition which consists of, by mass, 0.23 to 0.35% C, 0.05 to 1.0% Si, 0.1 to 2.0% Mn, ≤0.020% P, 0.001 to 0.005% S, 0.005 to 0.03% Nb, 0.005 to 0.1% Ti, 0.0003 to 0.002% B, 0.001 to 0.015% REM, further one or more elements among Cu, Ni, Cr, Mo and V, and the balance Fe with inevitable impurities and in which component index value Ha represented by Ha=C×(1+3×Mn)×(1+0.5×Cu)×(1+2×Ni)×(1+3×Cr)×(1+2×Mo)×(1+V)×(1+300×B) is ≥2.5 and also has a microstructure in which as-quenched martensite of ≤25μm grain size is made to ≥90%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to wear-resistant steel used for industrial machines, transportation equipment, and the like, and a method for producing the same, and particularly relates to one having excellent low-temperature toughness.

  Industrial machinery, parts, transport equipment, etc. used in the fields of construction, civil engineering, mining, etc. (for example, excavators, bulldozers, hoppers, buckets, etc.) have excellent wear resistance to ensure their life. Steel is used.

  In order to improve the wear resistance, it is necessary to increase the surface hardness by making the steel surface into a quenched structure. In general, the quenching hardness of steel can be ensured by increasing the amount of C. On the other hand, when the hardness increases, the material becomes brittle and the low temperature toughness deteriorates.

  Considering work in a low temperature range of around −40 ° C., if the wear resistance is good but the low temperature toughness is low, brittle fracture occurs and the work is seriously hindered. For this reason, there has been a demand for a wear-resistant steel having wear resistance and excellent low-temperature toughness.

  Several methods have been examined for such a requirement. For example, Patent Document 1, Patent Document 2, Patent Document 3, and the like disclose a technique for improving the toughness of a wear-resistant steel sheet by adding a large amount of an alloy element such as Cr or Mo.

In these techniques, Cr is added for the purpose of improving hardenability, and Mo is added for the purpose of improving hardenability and at the same time improving the grain boundary strength. On the other hand, there is Patent Literature 4 as a technique that devises the manufacturing process, and it is disclosed that the old γ grains are expanded and the toughness is improved by using ausfoam in the hot rolling process.
JP-A-8-41535 JP-A-2-179842 JP-A 61-166554 JP 2002-20837 A

  However, as in Patent Document 1, Patent Document 2, Patent Document 3, and the like, when the grain boundary strength is enhanced by adding a large amount of alloy elements to improve toughness, there is a problem that the alloy element addition cost increases. Moreover, when utilizing an ausfoam in a hot rolling process like patent document 4, a device is needed for stable manufacture and it is not necessarily an easy process on actual manufacture.

  In this way, it is extremely difficult to produce wear-resistant steel with excellent low-temperature toughness at low cost with the conventional technology, and after hot rolling, it is manufactured by offline reheating quenching and tempering. I'm sorry.

  The present invention solves these problems and stably secures strength and wear resistance, and further provides wear-resistant steel having excellent low-temperature toughness, preferably wear-resistant steel having a surface hardness of 450 or more in Brinell hardness. It aims at providing hot rolling without performing off-line heat processing.

  In the present invention, when REM (O, S) is generated in the steel, the non-metallic inclusions are heated during hot rolling before hot rolling, in order to provide wear resistant steel excellent in low temperature toughness at low cost. It has been found that the austenite grain growth is significantly suppressed and is extremely useful for improving toughness after hot rolling and accelerated cooling.

The invention has been made based on further studies based on the obtained knowledge.
1. In mass%, C: 0.23-0.35%, Si: 0.05-1.0%, Mn: 0.1-2.0%, P: 0.020% or less, S: 0.001 -0.005%, Nb: 0.005-0.03%, Ti: 0.005-0.1%, B: 0.0003-0.002%, REM: 0.001-0.015%, Furthermore, Cu: 0.1-2.0%, Ni: 0.03-2.0%, Cr: 0.03-2.0%, Mo: 0.03-1.0%, V: 0.00. It contains one or two or more of 01 to 0.1%, the component index value Ha represented by the formula (1) is 2.5 or more, the balance is made of Fe and inevitable impurities, and the particle size is 25 μm or less A wear-resistant steel excellent in low-temperature toughness, characterized in that the as-quenched martensite has a microstructure of 90% or more.
Ha = C × (1 + 3 × Mn) × (1 + 0.5 × Cu) × (1 + 2 × Ni) × (1 + 3 × Cr) × (1 + 2 × Mo) × (1 + V) × (1 + 300 × B) ----- ---------- (1)
2. In mass%, C: 0.23-0.35%, Si: 0.05-1.0%, Mn: 0.1-2.0%, P: 0.020% or less, S: 0.001 -0.005%, Nb: 0.005-0.03%, Ti: 0.005-0.1%, B: 0.0003-0.002%, REM: 0.001-0.015%, Furthermore, Cu: 0.1-2.0%, Ni: 0.03-2.0%, Cr: 0.03-2.0%, Mo: 0.03-1.0%, V: 0.00. A steel containing 01 to 0.1% of one or more, the component index value Ha represented by the formula (1) is 2.5 or more, and the balance is Fe and inevitable impurities,
Heating to a temperature of 1100 to 1300 ° C., hot rolling at a rolling end temperature of Ar ₃ temperature or higher, and then performing accelerated cooling to 300 ° C. or lower at a cooling rate of 10 ° C./S or higher. An excellent method for producing wear-resistant steel.
Ha = C × (1 + 3 × Mn) × (1 + 0.5 × Cu) × (1 + 2 × Ni) × (1 + 3 × Cr) × (1 + 2 × Mo) × (1 + V) × (1 + 300 × B) ----- ---------- (1)

  According to the present invention, it is possible to easily manufacture and provide a wear-resistant steel excellent in low-temperature toughness at a low price while ensuring hardness and wear resistance stably, which is very significant industrially.

Hereinafter, the reasons for limitation of the chemical component, microscopic structure and production method of the present invention will be described.
[Chemical composition]
C: 0.23-0.35%
Since it is an important element for increasing the hardness of steel, 0.23% or more is necessary. However, when a large amount of C is added, the weldability, toughness, and workability are deteriorated, so the upper limit is made 0.35%.

Si: 0.05-1.0%
It is an effective element as a deoxidizing element, and therefore 0.05% or more must be added. Moreover, although it is an element effective also for solid solution strengthening, when the addition amount exceeds 1.0%, there are problems such as a decrease in ductility and an increase in inclusions. For this reason, it is defined as 0.05 to 1.0%.

Mn: 0.1 to 2.0%
Mn is an effective element from the viewpoint of ensuring hardenability, and it is necessary to add 0.1% or more. On the other hand, when it exceeds 2.0%, weldability deteriorates. For this reason, it is specified as 0.1 to 2.0%.

P: 0.020% or less If contained in a large amount, toughness deterioration is caused, so the upper limit is made 0.020%.

S: 0.001 to 0.005% or less Precipitation as MnS in steel becomes a fracture starting point of high-strength steel as inclusions and causes toughness deterioration, so the upper limit is made 0.005%. However, if S is reduced to less than 0.001%, generation of REM (O, S) becomes insufficient, austenite grains in the heating stage become coarse, and low temperature toughness deteriorates, so the content is made 0.001% or more.

Nb: 0.005 to 0.03%
Nb is an element useful for improving hardenability, but if less than 0.005%, the effect is poor, and if added over 0.03%, coarse Nb (C, N) is formed, and the origin of brittle fracture And toughness is deteriorated, so the upper limit is made 0.03%.

Ti: 0.005 to 0.1%
While fixing solid solution N harmful to toughness as TiN, it has the effect of improving toughness and ensuring solid solution B effective in improving hardenability, but this effect is less than 0.005% Since the toughness deteriorates when added over 0.1%, it is made 0.005 to 0.1%.

B: 0.0003 to 0.002%
Although it is an element that enhances hardenability by adding a small amount, if less than 0.0003%, this effect cannot be exhibited, and if added over 0.002%, toughness deteriorates, so 0.0003-0.002% To do.

REM: 0.001 to 0.015%
By forming REM (O, S) in the steel and suppressing austenite grain growth in the heating stage, the structure as it is hot-rolled is refined and the toughness is improved, so it is an essential element in the present invention. If less than 0.001%, the effect is poor, and if added over 0.015%, coarse oxide-based non-metallic inclusions are formed, which becomes the starting point of brittle fracture and the toughness is reduced. Limited to .015%.

Furthermore, in the present invention, one or more of Cu, Ni, Cr, Mo, and V are added.
Cu: 0.1 to 2.0%
Although it is an element that enhances hardenability, if it is less than 0.1%, this effect cannot be exhibited, and if it exceeds 2.0%, hot workability decreases and the cost also increases. In the case, it is 0.1 to 2.0%.

Ni: 0.03-2.0%
Although it is an element that enhances hardenability and improves low temperature toughness, if less than 0.03%, this effect cannot be exhibited, and if it exceeds 2.0%, the cost increases, so when adding 0.03 to 2.0%.

Cr: 0.03-2.0%
Although it is an element that enhances hardenability, if less than 0.03%, this effect cannot be exerted, and if it exceeds 2.0%, weldability deteriorates and costs increase. Is 0.03 to 2.0%.

Mo: 0.03-1.0%
Although it is an element that enhances hardenability, if less than 0.03%, this effect cannot be exhibited, and if it exceeds 1.0%, weldability deteriorates and costs increase, so when adding Is 0.03 to 1.0%.

V: 0.01 to 0.1%
It is an element effective for precipitation strengthening and has the effect of increasing the hardness of steel. However, if it is less than 0.01%, this effect cannot be exhibited. When it is added, the content is made 0.01 to 0.1%.

Component index value Ha
Component index value Ha = C × (1 + 3 × Mn) × (1 + 0.5 × Cu) × (1 + 2 × Ni) × (1 + 3 × Cr) × (1 + 2 × Mo) × (1 + V) × (1 + 300 × B) − ------- (1)
However, the content of each element (mass%) is related to the structure after quenching, and as a result, it greatly affects the hardness of the steel. When the component index value Ha is less than 2.5, even if the structure itself is not a completely quenched structure, or even if the surface structure is a completely quenched structure, it is completely quenched from the surface layer to the center of the plate thickness. The component index value Ha is defined as 2.5 or more because hardness does not become a structure and decreases.

[Microscopic organization]
In order to obtain excellent low-temperature toughness, the wear-resistant steel according to the present invention is defined as a microstructure with an as-quenched martensite having a particle size of 25 μm or less and 90% or more.

[Production conditions]
The heating temperature before hot rolling shall be 1100-1300 degreeC. When the temperature is lower than 1100 ° C., Nb exists in the form of Nb (C, N), and it is difficult to secure solid solution Nb effective for hardenability. Further, heating above 1300 ° C. generates surface flaws on the steel sheet surface and reduces the effect of suppressing austenite grain growth by REM (O, S), and the target martensite structure of 25 μm or less cannot be obtained. Is defined as 1300 ° C.

In hot rolling, the rolling end temperature is set to Ar ₃ temperature or higher. If Ar is less than ₃ points, soft ferrite is generated, and the martensite main structure targeted by the steel structure cannot be obtained. After hot rolling, it is cooled to 300 ° C. or lower at a cooling rate of 10 ° C./s or higher. If it is less than 10 ° C / s, a martensite structure of 90% or more cannot be obtained, and if cooling is stopped at a temperature exceeding 300 ° C, a martensite structure cannot be obtained.

  Steel having the chemical composition (mass%) shown in Table 1 was used. 1 to 30 steel pieces (thickness 100 mm) were produced and used as test materials. Steels A to C satisfy the requirements of the present invention in chemical composition. Steel D is a B-free system, and Ha is below the provisions of the present invention.

  Steel E does not contain REM. Furthermore, S in steel is lower than the prescription | regulation of this invention, and as for steel G, REM is added exceeding the prescription | regulation of this invention. These steels were hot-rolled and accelerated cooled to a sheet thickness of 50 mm under the production conditions shown in Table 2, and the obtained steel sheets were subjected to structure observation to measure the martensite fraction and particle size. In addition, surface hardness and low temperature toughness were examined as characteristic values.

  The hardness was based on JIS standard Z2243, and an average value of 5 points randomly selected on the steel sheet surface was used. The low temperature toughness was measured in accordance with JIS standard Z2242, and the fracture surface transition temperature was measured. The obtained hardness (HB) and low temperature toughness (vTs: [° C.]) are also shown in Table 2.

  Example No. 1-5, 9-13 and 17-21 are cases where the hot rolling-cooling conditions do not meet the requirements of the present invention, and either or both of the surface hardness and the fracture surface transition temperature are significantly degraded. .

  Example No. Examples 6 to 8, 14 to 16, and 22 to 24 are examples suitable for the present invention. Both surface hardness and fracture surface transition temperature are excellent. Particularly, the fracture surface transition temperature is an excellent value of −40 ° C. or less. Yes.

Example No. In Nos. 30 to 32, the hot rolling-cooling conditions are within the scope of the present invention, but the hardness is significantly reduced due to the low Ha.
Example No. Nos. 33 to 35 are cases where the amount of REM falls below the range of the present invention, and since the amount of REM (O, S) produced is small, even when the rolling-cooling condition is within the range of the present invention, the fracture surface transition temperature is Extremely expensive.
Example No. 36 to 38 are cases in which the amount of S is less than the range of the present invention, and since the generated REM (O, S) is reduced, the fracture surface transition temperature is not limited even if the rolling-cooling condition is within the range of the present invention. Extremely expensive.

  Example No. 39 is a case where the amount of REM exceeds the range of the present invention, and as a result of the increase in coarse oxide-based nonmetallic inclusions other than REM (O, S), the rolling-cooling conditions are within the range of the present invention, Although the diameter is within the range of the present invention, the fracture surface transition temperature is extremely high.

Claims (2)

In mass%, C: 0.23-0.35%, Si: 0.05-1.0%, Mn: 0.1-2.0%, P: 0.020% or less, S: 0.001 -0.005%, Nb: 0.005-0.03%, Ti: 0.005-0.1%, B: 0.0003-0.002%, REM: 0.001-0.015%, Furthermore, Cu: 0.1-2.0%, Ni: 0.03-2.0%, Cr: 0.03-2.0%, Mo: 0.03-1.0%, V: 0.00. It contains one or two or more of 01 to 0.1%, the component index value Ha represented by the formula (1) is 2.5 or more, the balance is made of Fe and inevitable impurities, and the particle size is 25 μm or less A wear-resistant steel excellent in low-temperature toughness, characterized in that the as-quenched martensite has a microstructure of 90% or more.
Ha = C × (1 + 3 × Mn) × (1 + 0.5 × Cu) × (1 + 2 × Ni) × (1 + 3 × Cr) × (1 + 2 × Mo) × (1 + V) × (1 + 300 × B) ----- ---------- (1) In mass%, C: 0.23-0.35%, Si: 0.05-1.0%, Mn: 0.1-2.0%, P: 0.020% or less, S: 0.001 -0.005%, Nb: 0.005-0.03%, Ti: 0.005-0.1%, B: 0.0003-0.002%, REM: 0.001-0.015%, Furthermore, Cu: 0.1-2.0%, Ni: 0.03-2.0%, Cr: 0.03-2.0%, Mo: 0.03-1.0%, V: 0.00. A steel containing 01 to 0.1% of one or more, the component index value Ha represented by the formula (1) is 2.5 or more, and the balance is Fe and inevitable impurities,
Heating to a temperature of 1100 to 1300 ° C., hot rolling at a rolling end temperature of Ar ₃ temperature or higher, and then performing accelerated cooling to 300 ° C. or lower at a cooling rate of 10 ° C./S or higher. An excellent method for producing wear-resistant steel.
Ha = C × (1 + 3 × Mn) × (1 + 0.5 × Cu) × (1 + 2 × Ni) × (1 + 3 × Cr) × (1 + 2 × Mo) × (1 + V) × (1 + 300 × B) ----- ---------- (1)