KR20120070603A - High-toughness abrasion-resistant steel and manufacturing method therefor - Google Patents

Abstract

(Problem) Providing highly tough wear-resistant steel whose workability is good and a characteristic does not depend on manufacturing conditions.
(Measures) In mass%, C: 0.15-0.25%, Si: 0.1-1.0%, Mn: 0.4-1.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.2-0.9%, Nb: 0.005% to 0.03%, Ti: 0.005% to 0.03%, B: 0.0003% to 0.004%, Al: 0.005% to 0.08%, and N: 0.005% or less, and are made of residual Fe and unavoidable impurities. And (2) satisfying the above formula, and having a surface hardness of Brinell hardness, HBW400 to 500, and a high toughness wear-resistant steel and a method of manufacturing the same. Moreover, you may contain 1 type, or 2 or more types of elements of Cu, Ni, Mo, and V.
DI / t = 0.5-15.0... (1)
Ms≤430... (2)
Where t is the steel plate thickness (mm), DI is the hardenability index, and Ms is the martensite transformation start temperature.

Description

High toughness wear-resistant steel and manufacturing method thereof {HIGH-TOUGHNESS ABRASION-RESISTANT STEEL AND MANUFACTURING METHOD THEREFOR}

The present invention relates to a high toughness wear resistant steel suitable for use as a structural member of a machine that requires wear resistance, such as, for example, civil engineering, mining construction machinery or large industrial machinery.

Since the wear resistance of the constituent members of the machine is strongly controlled by the surface hardness thereof, high hardness steel is applied to the constituent members of machines requiring wear resistance, such as, for example, civil engineering, mining construction machinery, and large industrial machinery. . In recent years, the development of mines in cold districts has been active, and the demand for construction machinery used in cold districts has increased accordingly. In consideration of use in such cold regions, low temperature toughness is required for wear resistant steels. In addition, the demand of such wear-resistant steel which is equipped with the outstanding workability is also increasing.

For example, in order to solve the problem of high toughness, Patent Document 1 proposes a method of achieving both high hardness and high toughness by optimizing a component system, hot rolling, and heat treatment.

In addition, Patent Document 2 proposes a form control of austenite grains through the reduction of unrecrystallized regions, and high toughening by direct quenching.

Japanese Patent Laid-Open No. 09-118950 Japanese Patent Publication No. 2002-80930

However, the method proposed in Patent Literature 1 does not put the use in the cold district into the field of view, and when the use in the cold district is considered, the toughness which can be said to be sufficient cannot be secured.

Moreover, in the method proposed by patent document 2, it is necessary to make large the amount of reduction in a non-recrystallization area | region, and is severely restrict | limited by manufacturing conditions. Moreover, it is not suitable for manufacture of a thick material which is hard to penetrate under pressure.

In addition, all of these methods are not considered about the improvement of the workability of wear-resistant steel.

SUMMARY OF THE INVENTION In view of such a situation, an object of the present invention is to provide a highly tough wear-resistant steel having a toughness that can be used even in cold regions, having good workability, and in which characteristics are hardly influenced by manufacturing conditions, and a method for producing the same.

MEANS TO SOLVE THE PROBLEM The present inventors earned the knowledge of the following (a)-(h) as a result of earnestly researching in order to solve the said subject.

(a) Generally, the higher the hardness, the more the toughness tends to decrease. In the case of wear-resistant steel, a certain hardness is required in order to secure wear resistance. For this reason, as a result of examining various wear resistance, toughness, and workability, it was found that there exists a hardness range in which wear resistance, toughness, and workability can be parallel.

(b) And in order to control hardness, what is necessary is just to control C amount. However, in order to obtain more stable toughness, hardness control alone is not enough, and hardenability must also be controlled. In other words, when trying to manufacture abrasion-resistant steel at low cost, it is common to use martensitic structure, but when the upper bainite structure is formed due to lack of hardenability, toughness deteriorates greatly, and thus it is necessary to have a certain hardenability. do. Therefore, when the plate thickness increases, hardening becomes difficult, and not only increases a certain hardenability but also requires hardenability according to plate thickness.

(c) Thus, in order to obtain hardness and desired structure, it was found that by providing the steel material with hardenability according to the plate thickness, wear resistance, low temperature toughness and workability can be combined.

Specifically, while defining the steel composition including the amount of C, the surface hardness is defined in a predetermined range, the ratio with the quenchable sheet thickness, and the martensite transformation start temperature.

In addition, the ratio with the quenchable sheet thickness is an abrasion resistant steel, and it is prescribed | regulated so that it may become a range which is necessary in order to ensure the appropriate quenchability according to plate thickness, The part which hardenability of the center part of a plate thickness falls when plate | board thickness t becomes large. It is because there exists a possibility that the weldability and workability of the thing which can maintain hardenability will increase by increasing content of the alloy component in steel.

In addition, defining the martensite transformation start temperature can lower the temperature generated by the martensite as the martensite transformation start temperature is lower, and when the bayite structure is formed as a structure other than martensite, This is because tissues are easily generated, and high toughness is easily obtained.

(d) Specific steel composition is mass%, C: 0.15-0.25%, Si: 0.1-1.0%, Mn: 0.4-1.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.2-0.9 %, Nb: 0.005 to 0.03%, Ti: 0.005 to 0.03%, B: 0.0003 to 0.004%, Al: 0.005 to 0.08%, and N: 0.005% or less, and are made of residual Fe and unavoidable impurities. Moreover, you may contain 1 type (s) or 2 or more types of elements of Cu: 0.5% or less, Ni: 0.5% or less, Mo: 0.5% or less, V: 0.08% or less as an arbitrary addition component.

(e) The surface hardness of the steel is a hardness that can be easily machined and can be used as an abrasion resistant steel. Specifically, it is necessary to set HBW400 to 500 for Brinell hardness.

(f) The ratio of the quenchable sheet thickness and the martensite transformation start temperature is such that the ratio DI / t of the quenchability index DI to the plate thickness t (mm) satisfies the following Equation (1) and the martensite transformation: It is necessary for start temperature Ms to satisfy following formula (2).

DI / t = 0.5-15.0... (1)

Ms≤430... (2)

Where t is the steel plate thickness (mm), DI is the hardenability index, and Ms is the martensite transformation start temperature.

In addition, hardenability index DI depends on the chemical composition of steel, and can be computed by following formula (3). Originally, it means an abnormal critical diameter, and when quenching by cooling which is ideal for a round bar is performed, 50% of the center part of a round bar becomes a martensite structure. Therefore, it can be converted into a hardenability index.

(1＋0.64Si)(1＋4.1Mn)(1＋0.27Cu)(1＋0.5Ni)(1＋2.33Cr)(1＋3.14Mo)…(3)식DI = 9.238

(1 + 0.64Si) (1 + 4.1Mn) (1 + 0.27Cu) (1 + 0.5Ni) (1 + 2.33Cr) (1 + 3.14Mo)... (3) Expression

Here, the element symbol in a formula shows content (mass%) of each element in steel.

In addition, martensite transformation start temperature Ms is martensite transformation start temperature (degreeC) at the time of quenching cooling, and this also can be calculated by following formula (4) depending on chemical composition of steel.

Ms = 521-353xC-22xSi-24xMn-27xNi-18xCr-8xCu-16xMo. (4) expression

Here, the element symbol in a formula shows content (mass%) of each element in steel.

(g) Next, in order to obtain excellent toughness, it is preferable to set it as the structure mainly having martensite, specifically, the structure which makes a martensite ratio 70% or more.

However, martensite structure causes a decrease in workability. Moreover, carbon content in steel also becomes a cause which reduces workability. Therefore, in order to set it as the high toughness wear-resistant steel which has the outstanding workability, it is preferable to make the product of martensite ratio M and carbon content 23 or less.

(h) Steel having hardenability according to such hardness and microstructure and sheet thickness can be produced by any of the following methods (i) or (ii) from slabs having the above-described steel composition.

(i) 900? heating up to 1200 ℃, and still subjected to hot rolling, subjected to rolling at a temperature not higher than 1000 ℃, Ar ₃ and above -100 ℃ also cooled after the completion of the rolling at a temperature not higher than Ar ₃ + 150 ℃ , and then after re-heating to a temperature not higher than the Ac ₃ point or higher ℃ 950 also, the method according to "reheating quenching" to the water-cooled.

(ii) 900? heating up to 1200 ℃, and still subjected to hot rolling, subjected to rolling at a temperature not higher than 1000 ℃, Ar ₃ point or more and after the completion of the rolling at a temperature not higher than Ar ₃ + 150 ℃, Ar ₃ point The method by the "direct quenching" which cools to 200 degrees C or less by the surface temperature of a steel plate by cooling rate 3.0 degrees C / sec or more at the above temperature.

This invention is made | formed based on the said knowledge, The part made into the summary is as showing to following (1)-(5).

(1) In mass%, C: 0.15-0.25%, Si: 0.1-1.0%, Mn: 0.4-1.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.2-0.9%, Nb: 0.005 ? 0.03%, Ti: 0.005? 0.03%, B: 0.0003? 0.004%, Al: 0.005? 0.08%, and N: 0.005% or less, and are composed of the balance Fe and unavoidable impurities, and are represented by the following formula (1) and The high toughness wear-resistant steel which satisfy | fills Formula (2), and surface hardness is Brinell hardness HBW400-500.

DI / t = 0.5-15.0... (1)

Ms≤430... (2)

Here, t is the plate thickness (mm) of steel, DI is a hardenability index, Ms is martensite transformation start temperature, DI and Ms are computed based on following formula (3) and (4), respectively. In addition, the element symbol in a formula means content (mass%) of each element in steel.

(1＋0.64Si)(1＋4.1Mn)(1＋0.27Cu)(1＋0.5Ni)(1＋2.33Cr)(1＋3.14Mo)…(3)식DI = 9.238

(1 + 0.64Si) (1 + 4.1Mn) (1 + 0.27Cu) (1 + 0.5Ni) (1 + 2.33Cr) (1 + 3.14Mo)... (3) Expression

Ms = 521-353xC-22xSi-24xMn-27xNi-18xCr-8xCu-16xMo. (4) expression

(2) The high toughness wear-resistant steel of said (1) characterized by the martensite ratio M in a microstructure being 70% or more, and satisfy | filling following formula (5).

M x C≤23... (5)

Here, M represents the martensite ratio (%), and C represents the content (mass%) of carbon in the steel.

(3) In addition, in mass%, one or two or more of Cu: 0.5% or less, Ni: 0.5% or less, Mo: 0.5% or less, and V: 0.08% or less are contained. High toughness wear resistant steel of 1) or (2).

4 (1)? (3) of heating a slab having the chemical composition according to any one at a temperature of 900? 1200 ℃, and subjected to rolling at a temperature not higher than 1000 ℃, Ar ₃ and above -100 ℃ also Ar ₃ + cooling after the completion of the rolling at a temperature not higher than 150 ℃, and then Ac ₃ point or more and then reheated to a temperature below 950 ℃, said method characterized in that a water-cooled production toughness river wear resistance.

5 (1)? (3) of the slab having the chemical composition as described in any one, and heated to a temperature of 900? 1200 ℃, subjected to rolling at a temperature not higher than 1000 ℃, Ar ₃ point or more and Ar ₃ + 150 ℃ After finishing rolling at the following temperature, it cools to 200 degrees C or less at the surface temperature of a steel plate with cooling rate 3.0 degreeC / sec or more at the temperature of ₃ or more Ar, The manufacturing method of the high toughness wear-resistant steel characterized by the above-mentioned.

According to the present invention, a high toughness wear-resistant steel having toughness which can be used even in cold regions, good workability and in which characteristics are hardly influenced by manufacturing conditions is obtained.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

1. Chemical composition of high toughness wear resistant steel according to the present invention

First, the reason for defining the chemical composition of the high toughness wear-resistant steel which concerns on this invention as mentioned above is demonstrated in detail. In addition, "%" which shows content of each element means the "mass%."

C: 0.15-0.25%

C is the most effective element for improving the surface hardness and is inexpensive. However, when C content is less than 0.15%, since it is necessary to increase content of another alloying element and to supplement hardness, cost will increase. On the other hand, when C content exceeds 0.25%, hardness will become high too much, and toughness will deteriorate. Therefore, C content is made into 0.15 to 0.25%. The lower limit of the C content is preferably 0.17%. In addition, the upper limit of the C content is preferably 0.22%.

Si: 0.1 to 1.0%

Si is an element which contributes to the improvement of surface hardness. However, when the Si content is 0.1% or less, the effect of improving the surface hardness is insufficient. On the other hand, when the Si content exceeds 1.0%, the toughness deteriorates. Therefore, Si content is made into 0.1 to 1.0%. The lower limit of the Si content is preferably 0.2%. In addition, the upper limit of the Si content is preferably 0.8%.

Mn ： 0.4? 1.3%

Mn is an element which improves surface hardness through hardenability improvement. However, when Mn content is less than 0.4%, since it is necessary to increase content of another alloying element and to supplement hardness, cost will increase. On the other hand, when Mn content exceeds 1.3%, toughness will be remarkably impaired. Therefore, Mn content is made into 0.4 to 1.3%. The lower limit of the Mn content is preferably 0.6%. Moreover, the upper limit of Mn content becomes like this. Preferably it is 1.2%.

P: 0.015% or less

P is an element present in steel as an impurity, and segregates at grain boundaries to degrade the delayed fracture resistance and toughness of the steel, so that the P content is preferably as low as possible. In particular, when the P content is more than 0.015%, this adverse effect becomes remarkable, so the P content is limited to 0.015% or less.

S ： 0.005% or less

S is an element present in steel as an impurity, and deteriorates the ductility and toughness of the steel, so the S content is preferably as low as possible. In particular, when S content exceeds 0.005%, since this bad influence becomes remarkable, S content is limited to 0.005% or less.

Cr: 0.2-0.9%

Cr is an effective element with the improvement of hardness and toughness through the function which raises hardenability. However, when Cr content is less than 0.2%, such an effect is not enough. On the other hand, when Cr content exceeds 0.9%, toughness will deteriorate remarkably. Therefore, Cr content is made into 0.2 to 0.9%. The lower limit of the Cr content is preferably 0.3%. In addition, the upper limit of the Cr content is preferably 0.8%.

Nb ： 0.005? 0.03%

Since Nb is an element which suppresses coarsening of crystal grains not only at the time of slab heating but also at the time of quenching, Nb is an element effective for manufacture of the fine steel materials of a wavefront unit. However, when Nb content is less than 0.005%, such an effect is not enough. On the other hand, when the Nb content exceeds 0.03%, the effect is not only saturated but also significantly impairs the weldability. Therefore, Nb content is made into 0.005 to 0.03%. The lower limit of the Nb content is preferably 0.010%. The upper limit of the Nb content is preferably 0.025%.

Ti ： 0.005? 0.03%

Ti is an effective element for deoxidation and an element effective for refining grains upon heating through the formation of nitrides. In order to acquire this effect, it is necessary to make the total content of Ti in steel into 0.005% or more. However, when Ti is contained exceeding 0.03%, toughness deterioration by the carbide which Ti forms becomes remarkable. Therefore, Ti content is made into 0.005 to 0.03%. The lower limit of the Ti content is preferably 0.008%. The upper limit of the Ti content is preferably 0.025%.

B ： 0.0003? 0.004%

B is a very important element which remarkably improves hardenability. However, when the B content is less than 0.0003%, the effect of improving hardenability is not sufficient. On the other hand, when B content exceeds 0.002%, toughness will deteriorate remarkably. Therefore, B content is made into 0.0003 to 0.002%. The lower limit of the B content is preferably 0.0005%. The upper limit of the B content is preferably 0.003%.

Al ： 0.005? 0.08%

Al is an element capable of effectively suppressing overgrowth of initial austenite grains by generating AlN during slab heating. However, when Al is less than 0.005%, this effect is not enough. On the other hand, when Al content exceeds 0.08%, toughness will deteriorate remarkably. Therefore, Al content is made into 0.005 to 0.08%. The lower limit of the Al content is preferably 0.010%. The upper limit of the Al content is preferably 0.07%.

N: 0.005% or less

Since N is an element present in steel as an impurity and causes toughness to deteriorate, N content is preferably as low as possible. In particular, when the N content is more than 0.005%, the adverse effect on toughness becomes remarkable, so the N content is limited to 0.005% or less.

The high toughness wear resistant steel which concerns on this invention contains Fe and an impurity other than the component shown above. In addition, an impurity is a component mixed by various factors of a manufacturing process, such as raw materials, such as an ore or a scrap, when steel is manufactured industrially, and is what is acceptable in the range which does not adversely affect this invention. Point.

The high toughness wear-resistant steel which concerns on this invention may also contain 1 type (s) or 2 or more types of elements shown below as an arbitrary additional element.

Cu: 0.5% or less

Cu is an optional additional element and can be contained as necessary. Inclusion of Cu has the effect of further improving strength and corrosion resistance. However, even if it contains Cu exceeding 0.5%, the improvement of the performance suitable for a cost increase is not seen. Therefore, the upper limit in the case of containing Cu is made into 0.5%. Moreover, in order to reliably obtain the improvement effect of the strength and corrosion resistance by Cu, it is preferable to contain Cu 0.2% or more.

Ni: 0.5% or less

Ni is an optional addition element and can be contained as needed. When Ni is contained, the toughness of the matrix (dough) of the steel is improved in the solid solution state. However, even if it contains Ni exceeding 0.5%, the improvement of performance suitable for a cost increase is not seen. Therefore, the upper limit in the case of containing Ni is made into 0.5%. In addition, when it wants to acquire the improvement effect of toughness by Ni reliably, it is preferable to contain Ni 0.2% or more.

Mo: 0.5% or less

Mo is an optional addition element and can be contained as needed. When it contains Mo, there exists an effect of improving the strength and toughness of a base material. However, when Mo is contained in excess of 0.5%, in particular, the hardness of the HAZ is increased to impair toughness and weldability. Therefore, the upper limit in the case of containing Mo is made into 0.5%. In addition, when it is desired to reliably obtain the effect of improving the strength and toughness of the base metal by Mo, it is preferable to contain Mo by 0.1% or more.

V: 0.08% or less

V is an optional addition element and can be contained as needed. When V is contained, there is an effect of improving the strength of the base material mainly by precipitation of carbonitride during tempering. However, containing V exceeding 0.08% not only saturates the performance improvement effect of the base material but also causes toughness deterioration. Therefore, the upper limit in the case of containing V is made into 0.08%. Moreover, in order to reliably obtain the improvement effect of the strength of the base material by V, it is preferable to contain V 0.01% or more.

2. About microstructure of highly tough wear-resistant steel according to the present invention

In order to exhibit the excellent toughness of the high toughness wear-resistant steel which concerns on this invention, it is necessary to set it as the micro structure which mainly uses martensite to the sheet thickness center part of steel materials.

First, in order to obtain the microstructure mainly composed of martensite up to the sheet thickness center of the steel, it is necessary to control the ratio DI / t to the sheet thickness (mm) of the steel of the hardenability index DI to 0.5 to 15. If DI / t is less than 0.5, toughness deteriorates due to insufficient martensite ratio, while if DI / t is more than 15.0, it is necessary to add an alloying element in a large amount, resulting in not only excellent alloy cost but also toughness. This is because it greatly deteriorates.

Next, in order to obtain excellent toughness in the quenched state, it is necessary to suppress as much as possible the formation of the upper bainite, which is inferior in toughness, as a microstructure produced in addition to martensite. For this purpose, the martensite transformation start temperature Ms is set to 430 or less, whereby the formation of the upper bainite structure having poor toughness is suppressed. As microstructures generated in addition to martensite, the lower bainite structure excellent in toughness tends to be formed. Therefore, by setting the martensite transformation start temperature Ms to 430 or less, excellent toughness can be obtained in the quenched state.

Although the high toughness wear-resistant steel which concerns on this invention needs to be a microstructure mainly a martensite, it may contain other microstructures. In addition to the lower bainite structure described above, for example, residual austenite may be included. However, since retained austenite causes the base metal toughness to deteriorate, it is preferable to make it less than 5%.

3. Regarding the workability of the high toughness wear resistant steel according to the present invention

When the high toughness wear-resistant steel which concerns on this invention is used for the shovel of a shovel car, for example, it is necessary to process the steel itself in a shovel shape. In order to be excellent in the machinability by turning, drilling, etc., surface hardness becomes important.

Therefore, it is necessary to make the surface hardness of steel into HBW400-500 by Brinell hardness. It is because when it is less than HBW400, it is soft and it is difficult to use it as abrasion resistant steel. On the other hand, when it exceeds HBW500, it is too hard and machining becomes difficult. The preferable range of surface hardness is HBW410-470.

Next, in order to obtain excellent toughness, it is preferable to set it as the structure mainly having martensite, specifically, the structure which makes a martensite ratio 70% or more.

However, martensite structure causes a decrease in workability. Moreover, carbon content in steel also becomes a cause which reduces workability. Therefore, when both martensite ratio M and carbon content become too high, and when these products exceed 23, workability will fall remarkably.

Therefore, in order to set it as the high toughness wear-resistant steel which has the outstanding workability, it is preferable to satisfy following formula (5).

M x C≤23... (5)

Here, M represents the martensite ratio (%), and C represents the content (mass%) of carbon in the steel.

4. About the manufacturing method of high toughness wear-resistant steel which concerns on this invention

The steel of this invention can be manufactured from the slab which has the above-mentioned steel composition by the method in any one of following (i) or (ii).

(i) 900? heating up to 1200 ℃, and subjected to rolling at a temperature not higher than 1000 ℃, Ar ₃ and above -100 ℃ also cooled after the completion of the rolling at a temperature not higher than Ar ₃ + 150 ℃, then Ac ₃ The method by "reheating quenching" which water-cools after reheating to more than a point and the temperature below 950 degreeC.

(ii) heating to a temperature of 900-1200 ° C., rolling at a temperature of 1000 ° C. or less, and completing rolling at a temperature of at least ₃ Ar and at a temperature of at most Ar ₃ + 150 ° C., followed by a cooling rate of 3.0 at a temperature of at least ₃ Ar. The method by "direct quenching" which cools to 200 degrees C or less by the surface temperature of a steel plate in degrees C / sec or more.

Hereinafter, each process of the manufacturing method of high toughness abrasion resistant steel is demonstrated. In addition, the common process is demonstrated collectively.

(1) About the heating process

In both the above (i) reheating quenching method (RD) and (ii) direct quenching method (DQ), the slab having the above-described composition is heated to a temperature of 900 to 1200 ° C. The slab itself does not matter in particular the manufacturing method. It can manufacture by the manufacturing method normally performed, for example, a continuous casting method.

The heating of the slab to 900 ° C. or higher is intended to form a uniform structure by transforming austenite. As the slab heating temperature increases, the slab softens and the deformation resistance decreases, so that rolling in the rolling step, which is the next step, is facilitated. However, when the heating temperature is high, the energy consumption in the heating furnace becomes large, which is not preferable for the production cost or the natural environment. Therefore, the upper limit of the heating temperature is set to 1200 ° C. The upper limit with preferable heating temperature of a slab is 1150 degreeC or less, and a preferable minimum is 1000 degreeC.

Moreover, in order to make temperature uniform to the center part of a slab, it is preferable to make heating time in the said temperature range into 2 hours or more.

(2) About hot rolling process

The slab heated under the above conditions is subjected to hot working and finished to a desired shape. The hot working at this time is rolled at a temperature of 1000 ° C or lower. Rolling at 1000 degrees C or less is for promoting the refinement of the crystal grains by recrystallization. When slab heating temperature is high, rolling will start after slab temperature falls to 1000 degrees C or less.

And in the case of performing the re-heating quenching of, (i), thereby completing the rolling at the Ar ₃ point or higher -100 ℃ also a temperature not higher than Ar ₃ + 150 ℃. When the rolling completion temperature is low, that is, when the rolling completion temperature is below the Ar ₃ point, even if water cooling is continued, hardening does not occur and sufficient martensite structure cannot be obtained. In this case, martensite structure can be obtained by cooling once and then reheating and quenching. In this way, even if the rolling completion temperature falls below the Ar ₃ point, the martensite structure can be obtained by cooling once, reheating, and quenching. However, a too low rolling completion temperature surface since the deformation resistance of the slab is increased rolling is difficult, the lower limit of the rolling finished temperature was set to -100 ℃ Ar ₃ point. The preferred lower limit of the rolling completion temperature is the Ar ₃ point.

On the other hand, since the rolling completion temperature to perform a direct quenching is greater than or equal to the Ar ₃ point, (ii), it is not necessary to re-heating by the cooling purpose. However, the reheated side tends to be quenched, thus making it easier to obtain martensite structure. Therefore, the upper limit of the rolling completion temperature in the case of reheating hardening was made into Ar ₃ point +150 degreeC. In addition, when the rolling completing temperature of Ar ₃ point or more, then the performed in the direct quenching in (ii), the viewpoint can be omitted, re-heating is, the preferred upper limit of the rolling completion temperature is Ar ₃ point.

In addition, when a direct quenching of, (ii), to complete the Ar ₃ point or more and the rolling at a temperature not higher than Ar ₃ + 150 ℃. To from the water-cooling step shown in to the water-cooling start temperature to the Ar ₃ point or more, the lower limit of the rolling completion temperature is the Ar ₃ point. There is some lag time between the time of rolling completion and water cooling, and steel temperature may fall in the meantime. Therefore, the minimum with preferable rolling completion temperature shall be Ar ₃ point +50 degreeC. On the other hand, in order to make crystal grain into fine grain and to improve toughness, the upper limit of rolling completion temperature shall be Ar ₃ point +150 degreeC.

(3) About the cooling process

When performing the re-heating of the quenching (i) has, after the rolling has been completed, it was cooled, and then water-cooled and then reheated to a temperature not higher than the Ac ₃ point or higher also 950 ℃,. Cooling after rolling completion is irrespective of the method, and it is sufficient to cool in air. In addition, the to-be-rolled material does not need to cool to room temperature by the cooling after rolling, It is enough to cool to about 400 degreeC. After cooling, it is cooled by water after reheating to Ac ₃ or more and 950 degrees C or less. The reheating temperature is set to Ac ₃ or more, because the water cooling start temperature is set to Ac ₃ or more, and if it is not cooled from the austenite single-phase region, a sufficient martensite structure fraction is not obtained and hardness and toughness also decrease. . Considering the lag time from the reheating furnace to the water cooling, it is preferable that the lower limit of the reheating temperature is Ac ₃ point + 50 ° C. On the other hand, the upper limit of reheating temperature shall be 950 degreeC from a viewpoint of the cost of energy consumed by heating, or a reduction of time. In addition, water cooling does not need to cool a to-be-rolled material to room temperature, but it is enough to cool to about 200 degreeC.

In the case of performing direct quenching of (ii), water is cooled to 200 ° C. or less at the surface temperature of the steel sheet at a cooling rate of 3.0 ° C./sec or more at a temperature of at least ₃ Ar. Also in this case, cooling at a temperature of Ar ₃ or more is to secure sufficient martensite structure by cooling from the austenite single-phase region similarly to the case of reheating hardening of (i). It is preferable that a cooling rate is fast from a viewpoint of quenching, and it is preferable to cool at 5.0 degreeC / sec or more. There is no particular upper limit of the cooling rate, but considering the maximum cooling rate of the present cooling device, the maximum is about 60 ° C / sec. Moreover, a cooling system does not have a restriction | limiting in particular, For example, water cooling, mist cooling, etc. are mentioned. Cooling is said to be up to 200 ° C. over a wide range of surface temperatures, to obtain sufficient quenched tissue.

As mentioned above, although the manufacturing method of the steel of this invention was shown, you may perform processes, such as descale, distortion correction, temperature uniform heating, between each process or during each process. The steel of the present invention can be used as a wear resistant steel without tempering after being produced by the above-described manufacturing method.

Moreover, the wear resistant steel excellent in the workability and low-temperature toughness which concerns on this invention, and its manufacturing method are demonstrated more concretely using an Example. However, the present invention is not limited.

(Example)

A slab having a chemical composition and properties shown in Table 1 was subjected to heating and cracking, hot rolling, cooling to room temperature, reheating and quenching under the test conditions shown in Table 2, and then having a plate thickness of 12 to 50 mm ( No. 1 to 32) were obtained. In addition, no tempering treatment was performed on any of the samples.

About these samples, the Brinell surface hardness test was carried out, and the Charpy impact test was done at -40 degreeC at the quarter plate thickness part, ie, plate | board thickness (1/4) t position, from the surface of a steel plate. In Charpy impact test, _v were judged excellent in low-temperature toughness that represents the absorbed energy at least 27J at _-40 E. Moreover, the bending test was done and workability was evaluated. In the bending test, the JIS No. 1 test piece was sampled in parallel with the rolling direction, and it was judged as pass (○) that no crack occurred at a bending radius of 3t (t is plate thickness). Moreover, after etching with a nital, microstructure was observed 500 times and the martensite fraction was measured. The test result was combined with Table 2 and shown.

As a result, it turns out that all the samples No.1-24 exist in the scope of the present invention, and both hardness, toughness, and workability are excellent.

On the other hand, Sample No. 25 is a comparative example, and since the amount of C exceeds the scope of the present invention, it can be seen that the hardness is too high and workability and toughness deteriorate.

Sample No. 26 and 27 are comparative examples, and it turns out that Si and Mn are outside the range of this invention, respectively, and toughness deteriorated.

Sample No. 28 is a comparative example, and since Cr is outside the scope of the present invention and the direct quenching (DQ) start temperature is also lower than the Ar ₃ point, it can be seen that the toughness deteriorates.

Sample No. 29 is a comparative example. Since Ms is high and DI / t is low, the martensite fraction is low, and as a result, the toughness deteriorates.

Sample No. 30 is a comparative example, and it is understood that Ti is all outside the scope of the present invention and the toughness is deteriorated.

Sample No. 31 is a comparative example. Since the direct quenching (DQ) onset temperature is lower than the Ar ₃ point, it is understood that sufficient martensite fraction was not obtained and the hardness and toughness deteriorated.

Sample No. 32 is a comparative example. Since the reheating temperature at the time of reheat quenching is low, it is understood that sufficient martensite fraction was not obtained and the hardness and toughness deteriorated.

[Industrial Availability]

By this invention, the high toughness wear-resistant steel which has toughness which can be used also in a cold region, is good in workability, and a characteristic does not depend on manufacturing conditions is obtained. The steel of this invention can be used as a structural member of the machine which requires abrasion resistance, such as a construction machine for civil engineering, a mine, and a large industrial machine, for example.

Claims (5)

In mass%, C: 0.15 to 0.25%, Si: 0.1 to 1.0%, Mn: 0.4 to 1.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.2 to 0.9%, Nb: 0.005 to 0.03% , Ti: 0.005 to 0.03%, B: 0.0003 to 0.004%, Al: 0.005 to 0.08%, and N: 0.005% or less, and include residual Fe and inevitable impurities, and are represented by the following formulas (1) and (2) The high toughness wear-resistant steel which satisfy | fills an formula, and surface hardness is HBW400-500 as Brinell hardness.
DI / t = 0.5-15.0... (1)
Ms≤430... (2)
Here, t is the plate thickness (mm) of steel, DI is a hardenability index, Ms is martensite transformation start temperature, DI and Ms are computed based on following formula (3) and (4), respectively. In addition, the element symbol in a formula means content (mass%) of each element in steel.
DI = 9.238

(1 + 0.64Si) (1 + 4.1Mn) (1 + 0.27Cu) (1 + 0.5Ni) (1 + 2.33Cr) (1 + 3.14Mo)... (3) Expression
Ms = 521-353xC-22xSi-24xMn-27xNi-18xCr-8xCu-16xMo. (4) expression The method according to claim 1,
The martensitic ratio M in a microstructure is 70% or more, and satisfy | fills following formula (5), The high toughness wear-resistant steel characterized by the above-mentioned.
M x C≤23... (5)
Here, M represents the martensite ratio (%), and C represents the content (mass%) of carbon in the steel. The method according to claim 1 or 2,
Moreover, the high toughness wear-resistant steel characterized by containing 1 type (s) or 2 or more types of elements of Cu: 0.5% or less, Ni: 0.5% or less, Mo: 0.5% or less, V: 0.08% or less. . Claim 1 to Claim 3 of heating a slab having the chemical composition according to any one of a temperature of 900? 1200 ℃, and subjected to rolling at a temperature not higher than 1000 ℃, Ar ₃ and above -100 ℃ also less than Ar ₃ + 150 ℃ Cooling after completion of rolling at a temperature, and then reheating to Ac ₃ or more and a temperature of 950 ° C. or less, followed by water cooling. Heating the chemical composition having a slab according to any one of claims 1 to 3 at a temperature of 900? 1200 ℃, and subjected to rolling at a temperature not higher than 1000 ℃, Ar ₃ point or more and the rolling at a temperature not higher than Ar ₃ + 150 ℃ After completion of the above, the method for producing a high toughness wear resistant steel, characterized by cooling to a surface temperature of the steel plate at a cooling rate of 3.0 ° C / sec or more at a temperature of at least ₃ Ar to 200 ° C or less.