TW201912811A - Wear-resistant steel plate with excellent roughness and manufacturing method thereof - Google Patents

Abstract

An objective of this invention is to satisfy both of "wear resistance" and "toughness" in high levels in non-refined materials. A steel plate of this invention has a chemical composition consisting of, in mass %, S: 0.60 to 1.25% C, Si: 0.50% or less, Mn: 0.30 to 1.20%, P: 0.030% or less, S: 0 to 30%, Cr: 0.30 to 1.50%, Nb: 0.10 to 0.50%, Ti: 0 to 0.50%, Mo: 0 to 0.50%, V: 0 to 0. 50%, Ni: 0 to 2.00%, and the balance of Fe and inevitable impurities, and has a metallic structure having cementite particles and particles of a carbide containing at least one of Nb and Ti (hereinafter referred to as "Nb/Ti-based carbide") in a metallic basis material of ferrite phase, wherein in a cross-section parallel to a rolling direction and a thickness direction (L section), the number density of the Nb/Ti based carbide having circle equivalent diameters of 0.5 [mu]m or more is 3000 to 9000 pieces/mm2, and the number density of voids having circle equivalent diameters of 1.0 [mu]m or more is 1250 pieces/mm2 or less.

Description

 Wear-resistant steel sheet with excellent toughness and manufacturing method  

The present invention relates to a steel sheet which is particularly resistant to toughness in an abrasion resistant steel sheet in which a hard Nb/Ti-based carbide is dispersed, and a method for producing the same.

For automotive parts, chain parts for industrial machinery, power transmission members such as gears, and cutting tools such as circular saws and band saws for wood cutting and mowing, wear resistance is required. In general, the wear resistance of steel is increased by increasing the hardness. Therefore, a member which emphasizes wear resistance is often a steel material which is hardened by heat treatment such as calcination or a steel material having a high content of alloy elements such as carbon. That is, the hardness of the steel is closely related to the wear resistance. In the past, the method of imparting wear resistance to the steel generally adopts a method of increasing the hardness.

On the other hand, it is extremely important that the cutter member such as a circular saw that rotates at a high speed does not break during use. To prevent breakage, the toughness of the steel must be ensured. The hardening of the improvement in wear resistance is the reason for the reduction of toughness. Therefore, in general, "wear resistance" and "toughness" have a trade-off relationship.

In some tools such as circular saws that harvest agricultural products such as fruits, cereals, and cotton flowers, the wear is relatively mild, so the "toughness" that is advantageous in preventing breakage is more important than hardness. In such a tool use, it is not a "tempering material" that has been hardened by quenching and tempering heat treatment such as calcination, but rather a "non-regulating material" of the ferrite phase + spheroidized stellite carbon iron structure. Be applicable. However, there is a strong demand for long life of the product, and even if the wear is relatively mild, the demand for improvement in wear resistance continues to increase. It is hoped that the technology of "wear resistance" and "toughness" will be constructed at a high level in non-tempered materials.

In Patent Documents 1 and 2, it is described that in the hot forging steel, the hardening of the ferroalloy phase by promoting wear and the reduction in the area ratio are improved, and the wear resistance of the steel is improved. However, the steel in which these documents are targeted is a ferrite-pearlite structure, and its toughness is inferior to that of the ferrite-spheroidized stellite structure.

Patent Document 3 discloses a technique for directly cutting a heat-expanded steel material having high strength and high toughness by refining crystal grains to obtain a mechanical structural component that can be used without heat treatment. However, in applications requiring wear resistance, high-frequency calcination-tempering treatment is required.

Patent Document 4 discloses a steel sheet for a circular saw which is obtained by cladding a core material of a high carbon steel and a core material of a low carbon steel to provide wear resistance and toughness. However, there must be a step of cladding.

Patent Documents 5 and 6 disclose a technique for improving wear resistance by dispersion of hard Nb/Ti-based carbide. These technologies are targeted at quenched materials that are hardened by calcining and tempering. Although high wear resistance is obtained, it is expected to be further improved on the tough surface.

 [Previous Technical Literature]    [Patent Literature]  

[Patent Document 1] Japanese Patent Laid-Open No. Hei 10-137888

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2003-201536

[Patent Document 3] Japanese Laid-Open Patent Publication No. 2011-195858

[Patent Document 4] Japanese Laid-Open Patent Publication No. 60-82647

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2010-138453

[Patent Document 6] Japanese Laid-Open Patent Publication No. 2013-136820

The object of the present invention is to provide both "wear resistance" and "toughness" in a non-tempered material at a high level.

According to the investigation by the inventors, the steel sheet (non-tempered material) before the quenching and heat treatment such as calcination is applied to the technique of imparting wear resistance by the hard Nb/Ti-based carbide disclosed in Patent Documents 5 and 6. It is also relatively soft, and it is not necessary to present good toughness at this stage. As a result of the detailed investigation, it was found that the cold rolling delay caused pores in the vicinity of the hard carbide particles, which became a cause of hindrance of toughness. Therefore, the inventors have continued to study for the production conditions in which pores are not easily generated. As a result, first, it was confirmed that if the cold rolling rate is lowered, pores tend to be less likely to occur. As a result of further investigation, it is understood that when cold rolling and annealing are repeatedly performed, the toughness is remarkably deteriorated when the intermediate cold rolling ratio exceeds 35%. It has been found that by performing intermediate cold rolling with a light rolling rate of 35% or less, applying an intermediate annealing, and then applying a final final cold rolling step, a steel sheet having less coarse pores can be obtained, which can be stabilized and high. Resilience. At this time, it was confirmed that the final cold rolling elongation was allowed to be about 60%. The present invention is based on such discoveries.

The above object can be attained by a steel sheet having C: 0.60 to 1.25%, Si: 0.50% or less, Mn: 0.30 to 1.20%, P: 0.030% or less, and S: 0.030% or less, Cr: 0.30 to 1.50%, Nb: 0.10 to 0.50%, Ti: 0 to 0.50%, Mo: 0 to 0.50%, V: 0 to 0.50%, Ni: 0 to 2.00%, and the residual portion is Fe And a chemical composition composed of an unavoidable impurity, which has a cementite particle dispersed in a metal material of the ferroalloy phase and a carbide containing one or more of Nb and Ti (hereinafter referred to as "Nb/ The metal structure of the particles of the Ti-based carbide") has a number density of Nb/Ti-based carbide particles having a circular equivalent diameter of 0.5 μm or more in a cross section (L-section) parallel to the rolling direction and the thickness direction. To 9000 pieces/mm ² , the number density of the pores having a circular equivalent diameter of 1.0 μm or more is 1,250 pieces/mm ² or less. Here, Ti, Mo, V, and Ni are arbitrary addition elements. The thickness of the steel sheet is, for example, 0.2 to 4.0 mm.

Here, the "Nb/Ti-based carbide" is one type or two types of Nb and Ti which are hard carbides which constitute a metal element of a carbide. The type of the Nb/Ti-based carbide may be, for example, a type mainly composed of NbC, a type mainly composed of TiC, and a type mainly composed of (Nb, Ti) C. In the present invention, since steel containing a predetermined amount of Nb is used as the object, when Ti is not contained in the steel component, a hard carbide mainly composed of NbC is formed. The Nb-containing hard carbide which does not contain such a Ti type is also referred to as "Nb/Ti-based carbide" in the present specification. When Ti is contained in the steel component, it is considered that a type having (Nb, Ti) C as a main component is formed, and depending on the Ti content, a type mainly composed of TiC or a type having NbC as a main component may be mixed. Spheroidized stellite carbon (Fe ₃ C) particles are also present in the steel material. Whether or not a certain carbide is a Nb/Ti-based carbide can be confirmed by an analytical method such as EDX (energy dispersive fluorescent X-ray analysis).

The pore system exists in the space between the surface of the Nb/Ti-based carbide particles and the steel material (matrix). The number density of the pores having a circular equivalent diameter of 1.0 μm or more can be obtained in the following manner.

 [Method for obtaining the number density of pores]  

The observation surface parallel to the rolling direction and the thickness direction of the cross section (L section) is observed by a confocal laser microscope, and the observation image is adjacent to the pores existing in the Nb/Ti-based carbide. The number of pores having a circle equivalent diameter of 1.0 μm or more was calculated, and the total number of counts was divided by the total area of observation (mm ² ) as the number density of pores (number/mm ² ) of a circle equivalent diameter of 1.0 μm or more. However, the observation area was set to 90 μm × 60 μm × 20 fields of view. The pores which are exuded from a part of the observation field are counted when the equivalent circle diameter of the portion appearing in the observation field is 1.0 μm or more. Here, the circle equivalent diameter of a certain aperture is the diameter of a circle equal to the area of the aperture on the observed image. The area of the pores can be determined by processing the observed image with an image processing software.

In the above steel sheet, the number density of the Nb/Ti-based carbide particles having an equivalent circle diameter of 0.5 μm or more is more preferably from 3,000 to 9,000 pieces/mm ² . The number density of the Nb-containing carbide particles can be determined as follows.

 [Method for Calculating the Number Density of Nb/Ti Carbide Particles]  

The cross section parallel to the rolling direction and the thickness direction (L section) is polished and then etched, and observed by a confocal laser microscope. The equivalent diameter of the circle is calculated to be 0.5 μm or more on the observed image. The number of Nb/Ti-based carbide particles, the total number of counts divided by the total area of observation (mm ² ) as the number density of Nb/Ti-based carbide particles having a circle equivalent diameter of 0.5 μm or more (pieces/mm ^{2 )} ). However, the observation area was set to 90 μm × 60 μm × 20 fields of view. The Nb/Ti-based carbide particles which are exuded from a part of the observation field of view have a partial equivalent diameter of 0.5 μm or more in the observation field, and are counted. Here, the circle equivalent diameter of a certain Nb/Ti-based carbide particle is a diameter of a circle equal to the area of the Nb/Ti-based carbide particle on the observed image. The area of the Nb/Ti-based carbide particles can be measured by processing the observed image with an image processing software.

The steel sheet can be produced by, for example, a method having the following procedure.

The step of cooling the molten steel from the liquidus temperature to the solidus temperature is controlled at 5 to 20 ° C / min to produce a cast piece (casting step); heating the cast piece to 1200 to 1350 ° C to maintain 0.5 to 4 hours step (casting step); application of hot rolling step (heating step); if necessary, the temperature of the hot-rolled steel sheet obtained in the heat-expanding step is less than 500 ° C and less than Ac ₁ point Annealing step of cooling after 10 to 50 hours (hot plate annealing step)

The administration and rolling rate of 35% and then cold cast above 500 ℃ temperature of less than _1:00 Ac order to maintain the cooling after 10-50 hours once or more steps of (cold rolling the intermediate annealing step); administered A cold rolling step of 60% or less (final cooling step).

A step of annealing at 300 to 500 ° C for 1 to 5 hours (de-deformation annealing step) is applied as needed.

The rolling rate is determined by the following formula (1).

Rolling rate (%) = (h ₀ -h ₁ )/h ₀ ×100...(1)

Where h ₀ is the thickness (mm) before rolling and h ₁ is the thickness (mm) after rolling.

According to the present invention, the non-tempered material of the Nb-containing steel can improve the toughness. This steel material combines excellent wear resistance and toughness. For cutting tool parts such as fruits, cereals, and cotton flowers, which have been used for non-tempered materials, the life extension effect due to improved wear resistance can be obtained. Moreover, it is possible to suppress the deterioration of the toughness which is conventionally improved in abrasion resistance and the selectivity.

 [chemical components]  

In this specification, the "%" of the constituent elements of steel means "% by mass" unless otherwise stated.

The C system is an element required to secure the strength of the steel sheet. Here, steel having a C content of 0.60% or more is targeted. When the C content is increased, coarse carbides are increased, which causes a decrease in toughness. The C content is limited to less than 1.25%.

Si is sometimes added as an oxygen scavenger, but when it is contained in a large amount, the toughness is deteriorated. The Si content is limited to 0.50% or less. Generally, it is only required to be adjusted in the range of 0.01 to 0.50%.

Mn is effective in increasing the strength of the steel sheet, ensuring a content of 0.30% or more. The inclusion of a large amount of Mn causes hardening of the hot-rolled steel sheet and causes a decrease in manufacturability. The Mn content is limited to 1.20% or less, and can be controlled to less than 1.00%.

Since P and S have adverse effects on toughness, it is preferable to use less. P is limited to 0.030% or less, and S is limited to 0.030% or less. In general, P is preferably 0.001% or more, and S is adjusted in a range of 0.0005% or more.

Cr is effective in increasing the strength of the steel sheet, ensuring a content of 0.30% or more. Containing a large amount of Cr can be a cause of reduced toughness. The Cr content is limited to 1.50% or less.

Nb forms very hard Nb/Ti-based carbide particles in the steel during the cooling process after casting, which contributes to the wear resistance, especially the wear resistance of the wear-resistant material. In order to fully exert the above effects, the Nb content of 0.10% or more is ensured. However, when Nb is added in a large amount, the amount of formation of Nb/Ti-based carbide particles becomes excessive, which is a cause of damage to the toughness. As a result of various studies, the Nb content must be limited to 0.50% or less. Can be controlled to less than 0.45%.

Like the Nb, the Ti system forms very hard Nb/Ti-based carbide particles in the steel during the cooling process after casting, which contributes to the improvement of wear resistance. Therefore, Ti can be added as needed. At this time, it is more effective to set the Ti content to 0.01% or more. However, if Ti is added in a large amount, it is a cause of damage to the toughness. As a result of various studies, when Ti is added, it must be carried out in a content range of 0.50% or less. It can be controlled to a Ti content of 0.30% or less.

Any of Mo, V, and Ni is an effective element in the improvement of toughness. Therefore, one or more of these may be added as needed. In this case, it is more effective to set Mo in an amount of 0.10% or more, V of 0.10% or more, and Ni of 0.10% or more. Even if these elements are added in excess, the toughness improvement effect of cost cannot be expected. It is preferable to suppress the content range of Mo to 0.50% or less, V of 0.50% or less, and Ni of 2.00% or less.

 [Metal organization]  

In the present invention, the structural adjustment (so-called quenching and tempering heat treatment) represented by calcination tempering or austenitic isothermal quenching is not carried out, and the wear resistance and toughness in the non-tempered material are intended. coexist. Therefore, the steel plate-based metal material (matrix) according to the present invention is a ferric iron phase. The spheroidized stellite carbon particles and the Nb/Ti-based carbide particles are dispersed in the metal material.

This steel sheet has a small amount of pores formed in the vicinity of the Nb/Ti-based carbide particles in the cold rolling step. Specifically, the number density of the pores having an equivalent circle diameter of 1.0 μm or more in the cross section (L section) parallel to the rolling direction and the thickness direction is 1,250 pieces/mm ² or less, and more preferably suppressed to 1000 pieces/mm ^{2 .} the following. It is known that the pores having a circle equivalent diameter of 1.0 μm or more in such pores are the main reason for lowering the toughness of the steel sheet of the non-tempered material. When the Nb content and the Ti content are suppressed to the above-described appropriate ranges, the number density of pores having a circular equivalent diameter of 1.0 μm or more is limited to 1,250 pieces/mm ² or less, whereby a remarkable improvement in toughness can be obtained. The number density of pores having a circular equivalent diameter of 1.0 μm or more is more preferably 1000/mm ² or less. The less the pore formation, the better the improvement of the toughness, but the excessive restriction of the pores is the reason for the limitation of the steps in addition to obtaining the cold rolled product of the appropriate thickness. In general, the number density of pores having a circular equivalent diameter of 1.0 μm or more may be set to a range of 300 pieces/mm ² or more. The reduction in the number density of the pores can be achieved, for example, by a production method (described later) in which an intermediate cold rolling step of a relatively light rolling ratio is inserted.

Nb/Ti-based carbide particles function to improve wear resistance. In particular, it is more effective to adjust the number density of Nb/Ti-based carbide particles having a circular equivalent diameter of 0.5 μm or more to 3000 to 9000 pieces/mm ² in the L section. The number density of the Nb/Ti-based carbide particles can be controlled by a known method (for example, the technique disclosed in Patent Document 5) for optimizing the cooling rate at the time of casting or the slab heating temperature before the hot rolling.

 [Production method]  

The wear resistant steel sheet according to the present invention can be produced, for example, by the following steps.

Casting→casting sheet heating→hot rolling delay→(hot-rolled sheet annealing)→intermediate cold rolling delay→intermediate annealing→final cold rolling delay→(deformation annealing)

At this time, the step of the "intermediate cold rolling extension → intermediate annealing" portion may be performed once or plural times. In the present specification, the step of performing "intermediate cold rolling and intermediate annealing" once or plural times is referred to as "intermediate cooling annealing step". Further, a descaling step such as pickling is inserted as needed. Hereinafter, each step described above will be described.

 [casting/casting heating]  

In the casting step, Nb/Ti-based carbides are formed during the cooling process. The formation size of the Nb/Ti-based carbide can be controlled by the cooling rate of the cast piece and the heating temperature of the cast piece. For example, the cooling rate of the molten steel from the liquidus temperature to the solidus temperature is controlled at 5 to 20 ° C / min, ensuring that the residence time in the temperature range from 1500 ° C to 900 ° C is more than 30 minutes, which will be obtained. The method of heating the cast piece to 1200 to 1350 ° C for about 0.5 to 4.0 hours is effective. The heat treatment of the cast piece can be carried out by heating the cast piece before the hot rolling.

 [Hot rolling extension / (hot plate annealing)]  

The hot rolling condition can be, for example, a final rolling temperature of 800 to 900 ° C and a coiling temperature of 750 ° C or less. Hot-deck annealing can be performed as needed. When the hot-deck annealing is performed, it is possible to maintain the temperature in the temperature region of 500 ° C or more and less than the Ac ₁ point for, for example, 10 to 50 hours. The number density of Nb/Ti-based carbide particles having a circular equivalent diameter of 0.5 μm or more in the cross section of the steel sheet L can be set to 3,000 to 9,000 pieces/mm ^{2 in} accordance with the above-described casting/casting heating conditions and hot rolling conditions. The number density of the Nb/Ti-based carbide particles in this stage is roughly reflected in the steel sheet after the final cold rolling.

 [intermediate cold rolling delay]  

The intermediate product sheet is subjected to a mild cold rolling of a rolling reduction of 35% or less. This cold rolling is performed before the final final cold rolling, and is referred to as "intermediate cold rolling" in this specification. It can be seen that when the intermediate cold rolling elongation is 35% or less, it is difficult to generate pore growth at the final cold rolling delay. The reaction mechanism has not been fully explained, but it is considered as follows. That is, the Nb/Ti-based carbide particles are very hard and not plastically deformed, so that voids are generated around the Nb/Ti-based carbide particles during the cold rolling, but the fine pores are eliminated during the annealing, so that the pores are generated. Very small, the toughness does not change. However, if the intermediate cold rolling rate exceeds 35%, coarse pores which are not destroyed during annealing are generated, and the pores grow in the final cold rolling, and the number density of pores having a circular equivalent diameter of 1.0 μm or more is increased and the toughness is changed. Poor situation. Further, as the intermediate cold rolling elongation becomes larger, the influence becomes large. In particular, if the intermediate cold rolling elongation exceeds 45%, the deterioration of toughness is remarkable. Moreover, even if the intermediate cold rolling elongation is in the range of more than 35% and 45% or less, and the intermediate cold rolling and the intermediate annealing are repeated several times, it is confirmed that the pores remaining due to the non-destruction in the intermediate annealing and the pores of the cold rolling delay Growth is repeated and the toughness is significantly degraded. Therefore, the intermediate cold rolling is preferably carried out in such a manner that the pores generated around the Nb/Ti-based carbide particles are sufficiently eliminated during annealing, and the rolling ratio is 35% or less. However, in the intermediate cold rolling, for example, it is effective to ensure that the rolling rate of 10% or more is effective, and the rolling rate of 15% or more can be controlled, but if it is too low, the effect of providing this step cannot be fully enjoyed.

 [intermediate annealing]  

The steel sheet subjected to the above intermediate cold rolling is subjected to annealing. This annealing is performed earlier than the final cold rolling, and is referred to as "intermediate annealing" in the present specification. The heating and holding temperature of the intermediate annealing was set to 500 ° C or more and did not reach Ac ₁ point. By maintaining at this temperature, the elimination of the voids generated in the middle cold rolling is sufficiently performed. In addition, spheroidization of Xueming carbon iron is also carried out. When the temperature is less than 500 ° C, the disappearance of the pores is insufficient. Further, the spheroidization of the stellite carbon is sometimes insufficient. On the other hand, when the temperature is raised to the Ac ₁ point or more, the Worthite iron phase is generated, and the metal material is not obtained as the ferrite phase. The heating maintenance time of the intermediate annealing (the time when the material temperature is 500 ° C or more and less than the Ac ₁ point range) is preferably 10 to 50 hours.

Further, the steps of "intermediate cold rolling and intermediate annealing" may be carried out as many times as necessary. At this time, the rolling rate of each intermediate cold rolling was set to 35% or less, and the heating maintenance temperature and the heating maintenance time of each intermediate annealing were also as described above.

 [Final cold rolling delay]  

The steel sheet after the intermediate annealing is subjected to cold rolling. This cold rolling is a step of reducing the thickness to the final target, and is referred to as "final cold rolling" in this specification. The final cold rolling rate must be set to 60% or less. If the rolling ratio is larger than this, it is easy to excessively form pores even if the above-described intermediate cold rolling and intermediate annealing are carried out under appropriate conditions. That is, it is difficult to stabilize and improve the toughness of the steel sheet. On the other hand, the final cold rolling is also effective for improving the final shape (flatness) of the steel sheet. Therefore, it is preferable to ensure a rolling rate of, for example, 10% or more. The final sheet thickness can be set, for example, in the range of 0.2 to 4.0 mm.

 [Deformation Annealing]  

De-deformation annealing can be performed as needed after the final cold rolling. The heating temperature and the holding time are controlled in accordance with the chemical composition and the final cold rolling rate, whereby the strength level can be adjusted. The heating temperature for the de-annealing annealing is set in the range of 300 to 500 °C. The heating maintenance time of the intermediate annealing (the time when the material temperature is in the range of 300 ° C or more and 500 or less) is preferably 1 to 5 hours.

 [Examples]  

The steel of the chemical composition shown in Table 1 was melted, and the steel sheet of the test material was prepared by the steps of casting, casting, heating, hot rolling, intermediate cold rolling, intermediate annealing, final cold rolling, and de-annealing.

The cast piece was obtained by controlling the cooling rate between the liquidus temperature and the solidus temperature at the time of casting to be controlled at 5 to 20 ° C / min. The cast piece was heated and heated at 1,250 to 1,350 ° C for 1 hour, and taken out, and hot rolled. The hot rolling condition was set to a final rolling temperature (rolling temperature of the final pass of the hot rolling) of 850 ° C and a coiling temperature of 590 ° C to obtain a hot-rolled steel sheet having a thickness of 7.0 mm. In order to make the thickness of the test piece obtained when the final cold rolling rate is changed in the subsequent step, the hot-rolled steel sheet is subjected to grinding processing, and the prepared plate thickness is set to 3.1 mm (for 40% rolling), 4.2 mm. Intermediate product sheet (for 55% rolling) or 6.3 mm (for 70% rolling).

An intermediate cold rolling was applied to each of the intermediate product sheets at a rolling rate of 20%, and an intermediate annealing of 550 ° C × 17 hours was applied. The plate after the intermediate annealing was subjected to final cold rolling at a rolling ratio as shown in Table 2 to obtain a cold-rolled steel sheet having a thickness of 1.5 mm. Thereafter, de-deformation annealing was carried out at a temperature of 300 to 450 ° C for 3 hours in accordance with the composition and the final cold rolling elongation at a hardness of 32 ± 2 HRC, as a test material.

For each of the test materials, a metal structure parallel to the cross section (L profile) in the rolling direction and the thickness direction was observed. As a result, any of the metal materials is a ferrite phase, and the metal material has a metal structure in which spheroidized stellite carbon particles and Nb/Ti-based carbide particles are dispersed.

Further, the L-profile of each test material was observed by a confocal laser microscope (OLY3000); and the number density of Nb/Ti-based carbide particles having a circle equivalent diameter of 0.5 μm or more and the circle equivalent were measured. The number density of pores having a diameter of 1.0 μm or more. These measurements are based on the "Method for Determining the Number Density of Nb/Ti Carbide Particles" and "Method for Calculating the Number Density of Pores" of the upper layer. Further, for each of the test materials, the abrasion resistance test and the punching test were carried out in the following manner.

 [Abrasion resistance test]  

A test piece which was cut out from the test piece into a circular shape having a diameter of 10 mm was tested by a Pin-on-Disc type abrasion tester. The abrasion material is prepared from WA (alumina) abrasive grains having a particle size of #3000 as defined in JIS R6001. This abrasive grain was mixed with 300 mL of water per 50 g to prepare a polishing liquid. The test piece was fixed to the sample support frame, and the surface of the test piece was subjected to a test load F=5 N while supplying a sufficient amount of the polishing liquid to the flat surface of the rotating body on which the surface of the steel disk was attached with a polishing cloth. The pressing test was carried out under the conditions of a friction speed of 0.4 m/s and a frictional distance of L = 750 m. The volume of the material that disappeared due to abrasion was calculated from the difference in the thickness of the test piece before and after the test, and this was taken as the wear loss W (mm ³ ). Then, the specific wear amount C (mm ³ /(Nm)) is obtained by the following formula (2).

Specific wear amount C = wear reduction W / (test load F × friction distance L)... (2)

The hardness of the above abrasive particles is about 1600 HV. This abrasion test simulates the frictional wear caused by the incorporation of fine sand. When it is adjusted to a steel having a hardness of 32 ± 2 HRC, if the specific abrasion amount C obtained by this test is 5.0 × 10 ^-4 mm ³ /Nm or less, it can be judged that the wear resistance is excellent. Therefore, the specific wear amount C was 5.0 × 10 ^-4 mm ³ /(Nm) or less, and it was judged to be acceptable (wear resistance; good).

 [rushing test]  

A test piece of 2 mm U notch was prepared from each test piece (test piece length: 55 mm, test piece height: 10 mm, test piece width: plate thickness = 1.5 mm, punching direction: rolling direction), according to JIS Z 2242:2005 The method measures the Xiao's value at room temperature (23 ° C). Here, the number of tests is set to n=5, and among these, the lowest value (the value of the score difference) is used as the punch value of the test material. When it is considered to use a material for a high-speed rotary cutter (a circular saw for agricultural product cutting, etc.) which is applicable to a non-adjustable material, it is desirable that the punching value obtained by this test is 50 J/cm ² or more. Therefore, the value of the punching value of 50 J/cm ² or more was judged to be acceptable (toughness; good).

The examples of the present invention have less porosity and excellent toughness. Excellent wear resistance. That is, in the non-tempered material, a non-tune material having excellent wear resistance and toughness can be realized.

On the other hand, in No. 5, 10, and 13 which are comparative examples, since the final cold rolling rate is high, the number of pores having a circular equivalent diameter of 1.0 μm or more is increased, and the toughness is poor. In Comparative Examples 14 to 16, the steel containing no Nb was used, so that no hard Nb/Ti-based carbide was formed, and the abrasion resistance was poor. In Comparative Examples 17 to 19, since steel having a small C content was used, formation of hard Nb/Ti-based carbide was insufficient, and improvement in wear resistance was insufficient. No. 20 and 21 use steel with excessive Ti content, and No. 22 and 23 use steel with excessive Nb content. Therefore, the amount of Nb/Ti-based carbide is large, and the equivalent diameter of the circle formed is 1.0 μm or more. The pores become more numerous. As a result, the toughness cannot be improved.

Claims (4)

A steel sheet having C: 0.60 to 1.25%, Si: 0.50% or less, Mn: 0.30 to 1.20%, P: 0.030% or less, S: 0.030% or less, Cr: 0.30 to 1.50%, and Nb by mass%. : 0.10 to 0.50%, Ti: 0 to 0.50%, Mo: 0 to 0.50%, V: 0 to 0.50%, Ni: 0 to 2.00%, the residual portion is Fe and the unavoidable impurities constitute a chemical composition having In the metal material of the ferrite phase, a cementite particle and a metal structure containing particles of one or more kinds of carbides of Nb and Ti (hereinafter referred to as "Nb/Ti-based carbide") are dispersed. In the cross section parallel to the rolling direction and the thickness direction, that is, the L section, the number density of Nb/Ti-based carbide particles having a circular equivalent diameter of 0.5 μm or more is 3000 to 9000 pieces/mm ² , and the circle equivalent diameter is 1.0. The number density of the pores of μm or more is 1,250 pieces/mm ² or less. A method for producing a steel sheet according to claim 1, which has a cooling rate of cooling the molten steel from a liquidus temperature to a solidus temperature to 5 to 20 ° C/min to manufacture a casting. The step of the sheet is also a casting step; the step of heating the cast piece to 1200 to 1350 ° C for 0.5 to 4 hours, that is, the step of heating the cast piece; the step of applying the hot rolling step, that is, the step of heat stretching; the rolling rate is applied 35% or less of the cold rolling is then maintained at a temperature of 500 ° C or more and less than the temperature of Ac ₁ for 10 to 50 hours, followed by cooling, and one or more steps, that is, an intermediate cooling annealing step; The step of cold rolling below % is also the final cold rolling step. The method for producing a steel sheet according to claim 2, wherein between the heat-expanding step and the intermediate cooling annealing step, the hot-rolled steel sheet obtained in the heat-expanding step is subjected to 500 ° C or more and less than Ac ₁ The step of annealing the temperature after the temperature of the point is maintained for 10 to 50 hours is also a step of annealing the hot plate.  The method for producing a steel sheet according to claim 2, wherein after the final cooling step, the step of annealing at 300 to 500 ° C for 1 to 5 hours, that is, the de-annealing annealing step.