JP5423806B2 - High toughness wear resistant steel and method for producing the same - Google Patents

Description

  The present invention relates to a high-toughness wear-resistant steel suitable for use as a structural member of machines that require wear resistance, such as civil engineering, mining construction machines, and large industrial machines, and a method for producing the same.

  Since the wear resistance of machine components is strongly governed by their surface hardness, for example, civil engineering, mining construction machines, and large industrial machines, such as machine components that require wear resistance, Hard steel is applied. In recent years, the development of mines in cold regions has become active, and accordingly, the demand for construction machinery used in cold regions has increased. Considering use in such cold regions, wear resistant steels are also required to have low temperature toughness. In addition, there is a growing need for such wear resistant steels with excellent workability.

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

  Further, Patent Document 2 proposes to increase the toughness by using a form control of austenite grains through reduction of an unrecrystallized region and direct quenching.

JP 09-118950 A JP 2002-80930 A

  However, the method proposed in Patent Document 1 is not intended for use in cold regions, and when considering use in cold regions, sufficient toughness can not be secured. .

  Moreover, in the method proposed in Patent Document 2, it is necessary to increase the amount of reduction in the non-recrystallized region, which is greatly restricted by manufacturing conditions. Moreover, it is unsuitable for manufacturing a thick material that does not easily penetrate under pressure.

  Furthermore, none of these methods are considered for improving the workability of the wear-resistant steel.

  In view of such circumstances, an object of the present invention is to provide a high-toughness wear-resistant steel that has toughness that can be used even in cold regions, has good workability, and is hardly affected by manufacturing conditions, and a method for manufacturing the same. It is to provide.

  As a result of intensive studies to solve the above problems, the present inventors have obtained the following findings (a) to (h).

  (a) Generally, as the hardness increases, the toughness tends to decrease, but in the case of wear-resistant steel, a certain level of hardness is required to ensure wear resistance. Therefore, as a result of various examinations of wear resistance, toughness, and workability, it has been found that there is a hardness range in which wear resistance, toughness, and workability can be aligned.

  (b) For the hardness control, the C amount may be controlled. However, in order to obtain more stable toughness, not only the hardness control is sufficient, but also the hardenability must be controlled. In other words, when trying to produce wear-resistant steel at low cost, it is common to use a martensite structure, but if the hardenability is insufficient and the upper bainite structure is generated, the toughness is greatly deteriorated, so that a certain level is required. It must have the above hardenability. Here, if the plate thickness increases, it becomes difficult to quench, so that not only a certain hardenability is increased but also a hardenability corresponding to the plate thickness is required.

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

  Specifically, the steel composition is defined starting with the C content, the surface hardness is defined within a predetermined range, the ratio to the hardenability plate thickness, and the martensitic transformation start temperature are defined.

  The ratio of the hardenability to the plate thickness is specified as a wear-resistant steel so as to be within a required range in order to ensure proper hardenability according to the plate thickness. This is because the hardenability at the central portion of the plate thickness decreases as the plate thickness t increases, but the hardenability can be maintained by increasing the content of the alloy component in the steel, but the weldability and workability. This is because there is a risk of damaging the sex.

  In addition, the martensite transformation start temperature is specified when the martensite transformation start temperature is lower, the temperature at which martensite is generated can be lowered, and when the bainite structure is generated as a structure other than martensite. This is because the lower bainite structure is likely to be formed, so that high toughness is easily obtained.

  (d) The specific steel composition is 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: 0.00. 0003 to 0.004%, Al: 0.005 to 0.08% and N: 0.005% or less, and the balance is Fe and inevitable impurities. Furthermore, as an optional additive component, by mass%, Cu: 0.5% or less, Ni: 0.5% or less, Mo: 0.5% or less, V: 0.08% or less You may contain 2 or more types.

  (e) About the surface hardness of steel, it is necessary to make it HBW400-500 by Brinell hardness as hardness which is easy to machine and can be used as wear-resistant steel.

(f) Regarding the ratio of the hardenability to the plate thickness and the martensite transformation start temperature, the ratio DI / t of the hardenability index DI to the plate thickness t (mm) satisfies the following formula (1). The martensitic transformation start temperature Ms (° C.) must satisfy the following formula (2).

DI / t = 0.5 to 15.0 (1) Formula Ms ≦ 430 (2) Formula where t is the steel thickness (mm), DI is the hardenability index, Ms is a martensitic transformation start temperature (° C.) .

  The hardenability index DI depends on the chemical composition of the steel and can be calculated by the following equation (3). Originally, it means the ideal critical diameter, and when the round bar is quenched by ideal cooling, 50% of the center part of the round bar has a martensite structure. Therefore, it can be diverted as a hardenability index.

DI = 9.238√C (1 + 0.64Si) (1 + 4.1Mn) (1 + 0.27Cu) (1 + 0.5Ni) (1 + 2.33Cr) (1 + 3.14Mo) ... (3) Formula here The element symbol in the formula represents the content (% by mass) of each element in the steel.

  Further, the martensite transformation start temperature Ms is the martensite transformation start temperature (° C.) during quenching cooling, which also depends on the chemical composition of the steel and can be calculated by the following equation (4).

Ms = 521-353xC-22xSi-24xMn-27xNi-18xCr-8xCu-16xMo ... (4) Formula Here, the element symbol in a formula represents content (mass%) of each element in steel.

  (g) Next, in order to obtain excellent toughness, it is preferable to use a structure mainly composed of martensite, specifically a structure having a martensite ratio of 70% or more.

  However, the martensite structure causes a decrease in workability. Moreover, the carbon content in the steel also causes a decrease in workability. Therefore, in order to obtain a high toughness wear-resistant steel having excellent workability, the product of the martensite ratio M and the carbon content is preferably 23 or less.

  (h) Steel having such hardness, microstructure, and hardenability according to sheet thickness is manufactured from a slab having the above steel composition by the following method (i) or (ii): be able to.

(i) 900 to 1200 heated to a temperature of ° C., subsequently subjected to hot rolling, carried out rolling at 1000 ° C. or less of the temperature, after completion of the rolling at Ar ₃ point -100 ° C. or higher and _Ar 3 + 0.99 ° C. below the temperature A method by “reheating and quenching” in which the sample is cooled, then reheated to a temperature of Ac ₃ points or higher and 950 ° C. or lower and then cooled with water.

(ii) After heating at a temperature of 900 to 1200 ° C., followed by hot rolling, rolling at a temperature of 1000 ° C. or lower, and completion of rolling at a temperature of Ar ₃ points or higher and Ar ₃ + 150 ° C. or lower, Ar ₃ A method by “direct quenching” in which the steel sheet is cooled at a cooling rate of 3.0 ° C./sec or higher to a temperature of 200 ° C. or lower at a cooling rate of 3.0 ° C./sec.

  The present invention has been made based on the above findings, and the gist thereof is as shown in the following (1) to (5).

(1) By 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: 0.2-0.9%, Nb: 0.005-0.03%, Ti: 0.005-0.03%, B: 0.0003-0.004%, Al: 0.005 to 0.08% and N: 0.005% or less, consisting of remaining Fe and inevitable impurities, satisfying the following formulas (1) and (2) , martensite in the microstructure A high tough wear-resistant steel having a ratio M of 70% or more, satisfying the following formula (5), and a surface hardness of Brinell hardness of HBW 400 to 500.
DI / t = 0.5 to 15.0 ... (1) formula
Ms ≦ 430 ・ ・ ・ ・ (2) Formula
M × C ≦ 23 (5)
Here, t is the steel plate thickness (mm), DI is the hardenability index, Ms is the martensite transformation start temperature, M is the martensite ratio (%), and C is the carbon content in the steel ( Mass%), and DI and Ms are calculated based on the following equations (3) and (4), respectively. In addition, the element symbol in a formula means content (mass%) of each element in steel.
DI = 9.238√C (1 + 0.64Si) (1 + 4.1Mn) (1 + 0.27Cu) (1 + 0.5Ni) (1 + 2.33Cr) (1 + 3.14Mo) ・ ・ ・ ・ (3)
Ms = 521-353xC-22xSi-24xMn-27xNi-18xCr-8xCu-16xMo (4)

(2) Further, by mass%, Cu: 0.5% or less, Ni: 0.5% or less, Mo: 0.5% or less, V: 0.08% or less The high-toughness wear-resistant steel according to (1) above , comprising the above .

(3) A slab having the chemical composition described in (1) or (2 ) above is heated to a temperature of 900 to 1200 ° C. and rolled at a temperature of 1000 ° C. or lower, and Ar ₃ points to −100 ° C. or higher and Ar _{3 +} 0.99 ° C. rolling at temperatures below was complete after cooling, then after reheating the Ac ₃ point or more and 950 ° C. temperature below above, wherein the water-cooling (1) or high toughness abrasion of (2) Steel manufacturing method.

(4) A slab having the chemical composition described in (1) or (2 ) above is heated to a temperature of 900 to 1200 ° C. and rolled at a temperature of 1000 ° C. or lower, and Ar ₃ points or higher and Ar ₃ + 150 ° C. above, wherein the cooling to 200 ° C. or less in the following after completing the rolling at a temperature, the surface temperature of the steel sheet at a cooling rate of 3.0 ° C. / sec or more from the Ar ₃ point or more temperature (1) or (2) Manufacturing method of high toughness wear-resistant steel.

  According to the present invention, a high-toughness wear-resistant steel having toughness that can be used even in cold regions, good workability, and hardly affected by production conditions is obtained.

  Hereinafter, the present invention will be described in detail.

1. Regarding the chemical composition of the 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 according to the present invention as described above will be described in detail. “%” Indicating the content of each element means “mass%”.

C: 0.15-0.25%
C is the most effective element for improving the surface hardness and is inexpensive. However, if the C content is less than 0.15%, it is necessary to increase the content of other alloy elements to supplement the hardness, resulting in an increase in cost. On the other hand, if the C content exceeds 0.25%, the hardness becomes too high, and the toughness deteriorates. Therefore, the C content is 0.15 to 0.25%. The lower limit of the C content is preferably 0.17%. Further, the upper limit of the C content is preferably 0.22%.

Si: 0.1 to 1.0%
Si is an element that contributes to an improvement in surface hardness. However, if the Si content is 0.1% or less, the effect of improving the surface hardness is insufficient, while if the Si content exceeds 1.0%, the toughness deteriorates. Therefore, the Si content is set to 0.1 to 1.0%. The lower limit of the Si content is preferably 0.2%. Further, the upper limit of the Si content is preferably 0.8%.

Mn: 0.4 to 1.3%
Mn is an element that improves surface hardness through improved hardenability. However, if the Mn content is less than 0.4%, it is necessary to increase the content of other alloy elements to supplement the hardness, resulting in an increase in cost. On the other hand, if the Mn content exceeds 1.3%, the toughness is significantly impaired. Therefore, the Mn content is set to 0.4 to 1.3%. The lower limit of the Mn content is preferably 0.6%. Further, the upper limit of the Mn content is preferably 1.2%.

P: 0.015% or less P is an element present in the steel as an impurity, and segregates at the grain boundaries to deteriorate the delayed fracture resistance and toughness of the steel. Therefore, the P content is desirably as low as possible. . In particular, when the P content exceeds 0.015%, such adverse effects become 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 it is desirable that the S content be as low as possible in order to degrade the ductility and toughness of the steel. In particular, when the S content exceeds 0.005%, such adverse effects become remarkable, so the S content is limited to 0.005% or less.

Cr: 0.2-0.9%
Cr is an element that is both effective in improving hardness and toughness through the work of improving hardenability. However, this effect is not sufficient when the Cr content is less than 0.2%. On the other hand, if the Cr content exceeds 0.9%, the toughness is remarkably deteriorated. Therefore, the Cr content is set to 0.2 to 0.9%. The lower limit of the Cr content is preferably 0.3%. Further, the upper limit of the Cr content is preferably 0.8%.

Nb: 0.005 to 0.03%
Since Nb is an element that suppresses the coarsening of crystal grains not only during slab heating but also during quenching, it is an effective element for the production of fine steel materials in units of fracture surfaces. However, such an effect is not sufficient when the Nb content is less than 0.005%. On the other hand, when the Nb content exceeds 0.03%, not only the effect is saturated, but also weldability is significantly impaired. Therefore, the Nb content is set to 0.005 to 0.03%. The lower limit of the Nb content is preferably 0.010%. Moreover, the upper limit of Nb content is preferably 0.025%.

Ti: 0.005 to 0.03%
In addition to being effective as a deoxidizing element, Ti is an element effective for refining crystal grains during heating through the formation of nitrides. In order to acquire this effect, it is necessary to make the total content of Ti in steel 0.005% or more. However, when Ti is contained exceeding 0.03%, toughness deterioration due to carbide formed by Ti becomes remarkable. Therefore, the Ti content is set to 0.005 to 0.03%. The lower limit of the Ti content is preferably 0.008%. Further, the upper limit of the Ti content is preferably 0.025%.

B: 0.0003 to 0.004%
B is an extremely important element that remarkably improves hardenability. However, if the B content is less than 0.0003%, the effect of improving hardenability is not sufficient. On the other hand, if the B content exceeds 0.004% , the toughness is remarkably deteriorated. Therefore, the B content is set to 0.0003 to 0.004% . The lower limit of the B content is preferably 0.0005%. Further, the upper limit of the B content is preferably 0.003%.

Al: 0.005 to 0.08%
Al is an element that can effectively suppress overgrowth of initial austenite grains by generating AlN during slab heating. However, this effect is not sufficient when Al is less than 0.005%. On the other hand, if the Al content exceeds 0.08%, the toughness is remarkably deteriorated. Therefore, the Al content is set to 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 N is an element present in steel as an impurity, and causes deterioration of toughness. Therefore, the N content is desirably as low as possible. In particular, if the N content exceeds 0.005%, the adverse effect on toughness becomes significant, so the N content is limited to 0.005% or less.

The high tough wear-resistant steel according to the present invention contains Fe and impurities in addition to the components shown above. The impurity is a component that is mixed due to various factors in the manufacturing process including raw materials such as ore or scrap when industrially manufacturing steel, and does not adversely affect the present invention. Indicates what is allowed.

  The high toughness wear resistant steel according to the present invention may further contain one or more of the following elements as an optional additive element.

Cu: 0.5% or less Cu is an optional additive element and can be contained as necessary. When Cu is contained, it has an effect of further improving the strength and corrosion resistance. However, even if Cu is contained in excess of 0.5%, the performance improvement commensurate with the cost increase is not observed. Therefore, the upper limit when Cu is contained is 0.5%. In addition, when it is desired to surely obtain the effect of improving the strength and corrosion resistance by Cu, it is preferable to contain Cu by 0.2% or more.

Ni: 0.5% or less Ni is an optional additive element and can be contained as necessary. When Ni is contained, there is an effect of increasing the toughness of the steel matrix (material) in the solid solution state. However, even if Ni is contained in excess of 0.5%, the performance improvement commensurate with the cost increase is not observed. Therefore, the upper limit when Ni is contained is 0.5%. In addition, when it is desired to reliably obtain the toughness improving effect by Ni, it is preferable to contain Ni by 0.2% or more.

Mo: 0.5% or less Mo is an optional additive element and can be contained as necessary. Inclusion of Mo has an effect of improving the strength and toughness of the base material. However, when Mo is contained in excess of 0.5%, the hardness of HAZ is particularly increased and the toughness and weldability are impaired. Therefore, the upper limit when Mo is contained is set to 0.5%. In addition, when it is desired to surely improve the strength and toughness of the base material by Mo, it is preferable to contain 0.1% or more of Mo.

V: 0.08% or less V is an optional additive element, and can be contained as necessary. Inclusion of V has an effect of improving the strength of the base material mainly due to carbonitride precipitation during tempering. However, when V is contained exceeding 0.08%, not only the performance improvement effect of the base material is saturated, but also the toughness is deteriorated. Therefore, the upper limit when V is contained is set to 0.08%. In addition, when it is desired to surely obtain the effect of improving the strength of the base material by V, it is preferable to contain V by 0.01% or more.

2. About the microstructure of the high toughness wear-resistant steel according to the present invention In order for the high toughness wear-resistant steel according to the present invention to exhibit excellent high toughness, a microstructure mainly composed of martensite up to the center of the plate thickness of the steel material and It becomes necessary to do.

  First, in order to obtain a microstructure mainly composed of martensite up to the center of the plate thickness of the steel material, the ratio DI / t of the hardenability index DI to the plate thickness (mm) of steel is 0.5 to 15.0. Need to control. When DI / t is less than 0.5, a sufficient martensite ratio cannot be obtained and toughness deteriorates. On the other hand, when DI / t exceeds 15.0, it is necessary to add a large amount of alloy elements. This is because not only the cost increases, but also the toughness greatly deteriorates.

Next, in order to obtain excellent toughness as-quenched, it is necessary to suppress as much as possible the formation of upper bainite having inferior toughness as a microstructure generated in addition to martensite. For that purpose, the production | generation of the upper bainite structure inferior to toughness is suppressed by making martensitic transformation start temperature Ms ( degreeC ) into 430 degrees C or less. As a microstructure generated in addition to martensite, a lower bainite structure excellent in toughness is easily generated. Therefore, by setting the martensitic transformation start temperature Ms (° C.) to 430 ° C. or less, excellent toughness can be obtained as it is quenched.

The high toughness wear-resistant steel according to the present invention needs to have a microstructure mainly composed of martensite, but may contain other microstructures. In addition to the above-described lower bainite structure, for example, residual austenite may be included. However, since retained austenite causes the base material toughness to deteriorate, the content is preferably less than 5%.

3. About the workability of the high toughness wear-resistant steel according to the present invention When the high toughness wear-resistant steel according to the present invention is used in, for example, an excavator of a shovel car, it is necessary to process the steel itself into a shovel shape. In order to be excellent in machinability by turning or drilling, the surface hardness is important.

  Therefore, it is necessary that the surface hardness of the steel be HBW 400 to 500 in terms of Brinell hardness. If it is less than HBW400, it is soft and difficult to use as wear-resistant steel, while if it exceeds HBW500, it is too hard and machining becomes difficult. A preferable range of the surface hardness is HBW410-470.

  Next, in order to obtain excellent toughness, a structure mainly composed of martensite, specifically, a structure having a martensite ratio of 70% or more is preferable.

  However, the martensite structure causes a decrease in workability. Moreover, the carbon content in the steel also causes a decrease in workability. Therefore, when both the martensite ratio M and the carbon content are too high and their product exceeds 23, the workability is significantly reduced.

  Therefore, in order to obtain a high toughness wear-resistant steel having excellent workability, it is preferable to satisfy the following formula (5).

M × C ≦ 23 (5) where M represents the martensite ratio (%) and C represents the carbon content (mass%) in the steel. .

4). About the manufacturing method of the high toughness abrasion-resistant steel which concerns on this invention Steel of this invention can be manufactured from the slab which has the above-mentioned steel composition by either of the following methods (i) or (ii).

(i) Heat to a temperature of 900-1200 ° C., perform rolling at a temperature of 1000 ° C. or less, cool after completion of rolling at a temperature of Ar ₃ points −100 ° C. or more and Ar ₃ + 150 ° C. or less, and then Ac ₃ points A method by “reheating quenching” in which water is cooled after reheating to a temperature of 950 ° C. or lower.

(ii) Heat to a temperature of 900 to 1200 ° C., perform rolling at a temperature of 1000 ° C. or less, complete the rolling at a temperature of Ar ₃ points or higher and Ar ₃ + 150 ° C. or lower, and then cool from the temperature of Ar ₃ points or higher. A method by “direct quenching” in which the steel sheet is cooled to 200 ° C. or less at a surface temperature of 3.0 ° C./sec or more.

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

(1) About a heating process In any of said (i) reheating quenching method (RD) and (ii) direct quenching method (DQ), the slab which has the above-mentioned composition is heated to the temperature of 900-1200 degreeC. The manufacturing method of the slab itself is not particularly limited. It can be produced by a usual production method, for example, a continuous casting method.

  The reason why the slab is heated to 900 ° C. or more is to transform it into an austenite to form a uniform structure. As the slab heating temperature increases, the slab softens and the deformation resistance decreases, and rolling in the next rolling process becomes easier. However, if the heating temperature is high, energy consumption in the heating furnace increases, which is not preferable for the manufacturing 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 degrees C or less, and a preferable minimum is 1000 degreeC.

  In addition, in order to make temperature uniform to the center part of a slab, it is preferable that the heating time in the said temperature range shall be 2 hours or more.

(2) About hot rolling process Although the slab heated on said conditions is hot-processed and finished to a desired shape, the hot processing in that case performs rolling at the temperature of 1000 degrees C or less. The reason why the rolling is performed at 1000 ° C. or less is to promote the refinement of crystal grains by recrystallization. When the slab heating temperature is high, rolling is started after the slab temperature is lowered to 1000 ° C. or lower.

Then, completing the rolling at reheating when performing hardening, Ar ₃ point -100 ° C. or higher and _Ar 3 + 0.99 ° C. below the temperature of (i). When the rolling completion temperature is low, that is, when the rolling completion temperature is lower than the Ar ₃ point, even if water cooling is subsequently performed, no quenching occurs and a sufficient martensite structure cannot be obtained. In this case, a martensite structure can be obtained by cooling once and then reheating and quenching. By doing so, even if the rolling completion temperature is lower than the Ar ₃ point, a martensitic structure can be obtained if it is once cooled and reheated and quenched. However, if the rolling completion temperature is too low, the deformation resistance of the slab becomes large and rolling becomes difficult, so the lower limit of the rolling completion temperature is Ar ₃ points-100 ° C. A preferable lower limit of the rolling completion temperature is Ar ₃ points.

On the other hand, when the rolling completion temperature is Ar ₃ or higher, the direct quenching of (ii) can be performed, so there is no need to bother cooling and reheating. However, reheating tends to cause firing, and thus a martensite structure is easily obtained. Therefore, the upper limit of the rolling completion temperature when reheating and quenching is set to Ar ₃ points + 150 ° C. When the rolling completion temperature is Ar ₃ point or higher, the direct quenching of (ii) may be performed, and from the viewpoint that reheating can be omitted, the preferable upper limit of the rolling completion temperature is Ar ₃ point.
Moreover, when performing the direct quenching of (ii), rolling is completed at the temperature of Ar ₃ point or more and Ar ₃ + 150 ° C. or less. In order to set the water cooling start temperature to Ar ₃ points or higher in the water cooling step shown below, the lower limit of the rolling completion temperature is set to Ar ₃ points. There is some lag time between the completion of rolling and water cooling, during which the temperature of the steel can drop. Therefore, the preferable lower limit of the rolling completion temperature is set to Ar ₃ point + 50 ° C.. On the other hand, in order to improve the toughness by making the crystal grains fine, the upper limit of the rolling completion temperature is Ar ₃ point + 150 ° C.

(3) Cooling process
In the case of performing the reheating and quenching of (i), the rolling is completed and then cooled, and then reheated to a temperature of Ac ₃ point or higher and 950 ° C. or lower and then water cooled. The method for cooling after the completion of rolling is not particularly limited, and it is sufficient to cool in the air. In addition, it is not necessary to cool a to-be-rolled material to room temperature by cooling after rolling, and it is enough if it cools to about 400 degreeC. After cooling, it is reheated to a temperature of Ac ₃ points or more and 950 ° C. or less, and then cooled with water. Is to the Ac ₃ point or higher reheating temperatures, and in order to the water cooling initiation temperature Ac ₃ point or more, not without cooling from the austenite single-phase region a sufficient martensitic structure fraction is obtained, hardness, toughness This is because both decrease. Considering the lag time from reheating to water cooling, the lower limit of the reheating temperature is preferably Ac ₃ points + 50 ° C. On the other hand, the upper limit of the reheating temperature is 950 ° C. from the viewpoint of reducing the cost of energy consumed for heating and time. In addition, water cooling does not need to cool a to-be-rolled material to room temperature, and it is enough if it cools to about 200 degreeC.

Moreover, water cooling when directly performing quenching until 200 ° C. or less at a surface temperature of the steel sheet at a cooling rate of 3.0 ° C. / sec or more from the Ar ₃ point or more temperatures (ii). Also in this case, the reason for cooling from the temperature of ₃ or more points of Ar is to secure a sufficient martensite structure by cooling from the austenite single phase region as in the case of reheating and quenching in (i). The cooling rate is preferably fast from the viewpoint of baking, and cooling is preferably performed at 5.0 ° C./sec or more. There is no particular upper limit for the cooling rate, but considering the current maximum cooling rate of the cooling device, the maximum is about 60 ° C./sec. Moreover, there is no restriction | limiting in particular in a cooling system, For example, water cooling, mist cooling, etc. are mentioned. The cooling is performed up to 200 ° C. or less at the surface temperature of the steel sheet in order to obtain a sufficiently quenched structure.

  As mentioned above, although the manufacturing method of this invention steel was shown, you may perform processes, such as descaling, distortion correction, and temperature equalization heating, during each process or during each process. In addition, the steel of the present invention can be used as a wear-resistant steel without being tempered after being manufactured by the above-described manufacturing method.

  Furthermore, the wear-resistant steel excellent in workability and low-temperature toughness according to the present invention and a method for producing the same will be described more specifically with reference to examples. However, the present invention is not limited.

  A slab having the chemical composition and characteristics shown in Table 1 is subjected to heating and soaking, hot rolling, cooling to room temperature, reheating and quenching under the test conditions shown in Table 2, and a sheet thickness of 12 to 50 mm. Samples (No. 1 to 32) were obtained. In addition, the tempering process is not performed about any sample.

These samples were subjected to a Brinell surface hardness test and a Charpy impact test at −40 ° C. at a thickness portion of 1/4 from the surface of the steel plate, that is, at a thickness (1/4) t position. In Charpy impact test, a shows the absorbed energy than 27J in _v E _-40 was determined that the low temperature toughness is good. Furthermore, a bending test was performed to evaluate workability. In the bending test, a JIS No. 1 test piece was taken in parallel with the rolling direction, and a test piece that was not cracked at a bending radius of 3 t (t is the plate thickness) was judged as acceptable (◯). Further, after etching with nital, the microstructure was observed 500 times, and the martensite fraction was measured. The test results are also shown in Table 2.

  As a result, it can be seen that Sample Nos. 1 to 24 are all within the scope of the present invention and are excellent in hardness, toughness, and workability.

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

  Samples Nos. 26 and 27 are comparative examples, and it can be seen that Si and Mn are outside the scope of the present invention and the toughness is deteriorated.

Sample No. 28 is a comparative example, in which Cr is out of the scope of the present invention, and the direct quenching (DQ) start temperature is lower than the Ar ₃ point, so it can be seen that the toughness is deteriorated.

  Sample No. 29 is a comparative example, and since Ms is high and DI / t is low, it can be seen that the martensite fraction is low, and as a result, the toughness is deteriorated.

Sample No.30 is a Comparative Example, Ti is outside the scope of the present invention, it can be seen that the toughness is degraded.

Sample No. 31 is a comparative example, and since the direct quenching (DQ) start temperature is lower than the Ar ₃ point, it can be seen that a sufficient martensite fraction cannot be obtained and the hardness and toughness are deteriorated.

  Sample No. 32 is a comparative example, and since the reheating temperature at the time of reheating and quenching is low, it is understood that a sufficient martensite fraction cannot be obtained, and the hardness and toughness are deteriorated.

  According to the present invention, a high-toughness wear-resistant steel having toughness that can be used even in cold regions, good workability, and hardly affected by production conditions is obtained. The steel of the present invention can be used as a structural member of machines that require wear resistance, such as civil engineering, mining construction machines, and large industrial machines.

Claims (4)

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, the balance being Fe and inevitable impurities, satisfying the following formulas (1) and (2) , the martensite ratio M in the microstructure is A high-toughness wear-resistant steel characterized by being 70% or more and satisfying the following formula (5) and having a surface hardness of Brinell hardness of HBW 400-500.
DI / t = 0.5 to 15.0 ··· (1) equation Ms ≤ 430 ··· (2) equation
M × C ≦ 23 (5) where t is the steel thickness (mm), DI is the hardenability index, and Ms is the start of martensitic transformation. Temperature , M is the martensite ratio (%), C is the carbon content (% by mass) in the steel , and DI and Ms are based on the following formulas (3) and (4), respectively. Calculated. In addition, the element symbol in a formula means content (mass%) of each element in steel.
DI = 9.238√C (1 + 0.64Si) (1 + 4.1Mn) (1 + 0.27Cu) (1 + 0.5Ni) (1 + 2.33Cr) (1 + 3.14Mo) ・ ・ ・ ・ (3) Formula Ms = 521−353xC−22xSi−24xMn−27xNi−18xCr−8xCu−16xMo (4) Further, in mass%, Cu: 0.5% or less, Ni: 0.5% or less, Mo: 0.5% or less, V: 0.08% or less, one or more elements included The high tough wear-resistant steel according to claim 1, wherein A slab having the chemical composition according to claim 1 or 2 is heated to a temperature of 900 to 1200 ° C, rolled at a temperature of 1000 ° C or lower, and a temperature of Ar ₃ points to 100 ° C or higher and Ar ₃ + 150 ° C or lower. _{3. The} method for producing high-toughness wear-resistant steel according to claim 1 , wherein after cooling is completed, the steel is cooled, and then re-heated to a temperature of Ac ₃ points or higher and 950 ° C. or lower and then water-cooled. The slab having the chemical composition described in claim 1 or 2 is heated to a temperature of 900 to 1200 ° C, rolled at a temperature of 1000 ° C or less, and rolled at a temperature of Ar ₃ points or more and Ar ₃ + 150 ° C or less. After completion, the high toughness wear-resistant steel according to claim 1 or 2 is cooled from a temperature of ₃ or more points of Ar to a surface temperature of the steel plate of 200 ° C or less at a cooling rate of 3.0 ° C / sec or more. Production method.