JP6946887B2 - Abrasion resistant steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to an wear-resistant steel plate and a method for manufacturing the same, and specifically, as a component of an industrial machine that requires wear resistance, such as a large excavator used for excavation of a mine and a large dump truck for transporting minerals. The present invention relates to a wear-resistant steel plate having excellent low-temperature toughness and HAZ toughness, which is suitable for use, and a method for producing the same.

Since the wear resistance of a member is strongly controlled by its surface hardness, a wear-resistant steel plate having a surface with a high hardness is applied to a component member of a machine that requires wear resistance. For example, wear-resistant steel sheets having a surface hardness of HB450 or higher have been widely used so far.

In recent years, the size of industrial machines used in mines and the like has been increasing, and the wear-resistant steel sheets used for them have become harder and thicker. Therefore, wear-resistant steel sheets are required to have low-temperature toughness and HAZ toughness in addition to surface hardness and delayed fracture resistance.

As a technique for improving the delayed fracture resistance, for example, Patent Document 1 states that Mn, which is an element that increases the delayed fracture resistance, is 0.30 to 0.60% (“%” regarding the chemical composition or concentration in the present specification. Discloses an invention that reduces the hardness to "% by mass" unless otherwise specified) and compensates for the decrease in hardness due to Mn reduction by adding Cr, Mo, or the like.

Further, in Patent Document 2, the Mn content is reduced to 0.50 to 0.80%, and the delay fracture resistance is improved by adding 0.005 to 0.025% of Ti, and the hardness due to the reduction of Mn is obtained. An invention is disclosed in which the decrease is compensated by the addition of Nb.

However, in the inventions disclosed in Patent Documents 1 and 2, since quenching is directly performed after hot rolling, the toughness of the wear-resistant steel sheet deteriorates.
Regarding ensuring toughness, Patent Document 3 discloses an invention for enhancing the toughness of a obtained steel sheet by specifying each of the steel composition and manufacturing conditions, and Patent Document 4 discloses delay fracture resistance and excellent toughness resistance. A worn steel sheet and a method for manufacturing the same are disclosed.

Japanese Unexamined Patent Publication No. 5-51691 Japanese Unexamined Patent Publication No. 63-317623 Japanese Unexamined Patent Publication No. 1-172514 Japanese Unexamined Patent Publication No. 11-71631

However, in the inventions disclosed in Patent Documents 3 and 4, the toughness at low temperature has not been evaluated, and the securing of HAZ toughness due to the thickening of the steel sheet has not been examined at all.
The present invention relates to a wear-resistant steel sheet used as a component of a large industrial machine such as for mining, and provides a wear-resistant steel sheet having excellent surface hardness and delayed fracture resistance, as well as low-temperature toughness and HAZ toughness, and a method for manufacturing the same. The purpose is to provide.

In order to solve the above technical problems, the present inventors examined the C, Nb, Ceq and CNB balance described later, and further controlled the number density and morphology of inclusions contained in the steel to control the surface surface. It has been found that a wear-resistant steel sheet having excellent hardness and delayed fracture resistance and also excellent low-temperature toughness and HAZ toughness can be obtained.

The wear-resistant steel sheet is hardened in order to obtain hardness, and the quenching hardness is substantially determined by the C content. Therefore, the wear-resistant steel sheet contains C in order to obtain the required hardness.

Nb expands the unrecrystallized region, introduces high-density dislocations by rolling in that region, and can achieve microstructure miniaturization by increasing the number of metamorphic nucleation sites, thus improving surface hardness and low-temperature toughness. Can be done. Further, Nb forms a carbonitride, whereby good low temperature toughness and HAZ toughness can be obtained by suppressing coarsening of austenite grains by a pinning effect during slab heating and welding.

On the other hand, since C and Nb forming the carbonitride do not contribute to the above-mentioned quenching hardness and expansion of the unrecrystallized region, it is important to secure the necessary solid solution C amount and solid solution Nb amount. The optimum CNB balance was found by examining the amount of carbonitride required to secure them and obtain the pinning effect.

Furthermore, by controlling the number density and morphology of inclusions contained in steel, the HAZ toughness in welding of thick wear-resistant steel sheets is also improved by miniaturizing the structure by promoting intragranular transformation, and the structure in the grains is miniaturized. It was found that by doing so, the stress concentration zone is dispersed and the delayed fracture resistance is also ensured.

That is, in order to effectively grow the intragranular ferrite in the old austenite grains during welding, it is essential to control the inclusions that are the nucleating nuclei of the intragranular ferrite. In particular, in a thick steel sheet with a thick plate, it is difficult to control the composition and number of inclusions in the plate thickness direction due to the difference in cooling rate between the surface and the inside. Need to be controlled. Therefore, as a result of investigating the growth mechanism of intragranular ferrite, the following (i) to (iii) were found.

(I) If MnS is precipitated around the Ti-based oxide in the steelmaking stage to generate a composite inclusion containing the Ti-based oxide and MnS, Mn is deficient at the matrix interface between MnS and the base metal. A region is formed. In this Mn-deficient region (hereinafter, referred to as “initial Mn-deficient region”), the ferrite growth start temperature rises significantly. Therefore, when the base metal is welded, intragranular ferrite grows preferentially from this Mn-deficient region in the cooling process.

(Ii) When the base metal is welded, Mn in the matrix of the base metal existing in the vicinity of the Ti-based oxide is diffused and absorbed by the atomic vacancies existing inside the Ti-based oxide. As a result, a Mn-deficient region is formed at the interface between the HAZ of the base metal and the Ti-based oxide that has undergone thermal history by welding. This Mn-deficient region (hereinafter, referred to as “welded Mn-deficient region”) also serves as a starting point for preferential growth of intragranular ferrite.

(Iii) Since the amount of ferrite in HAZ can be secured by both actions (i) and (ii) above, the required low temperature toughness of HAZ can be obtained.

Based on the above mechanism, the present inventors have found that intragranular ferrite can be effectively precipitated by controlling the amount of MnS compounded with inclusions and the number density. Furthermore, the present inventors have found that inclusions in steel must satisfy the requirements [1] to [3] listed below in order to obtain the grain refinement effect.

[1] A composite inclusion having MnS around a Ti oxide in steel, and the area ratio of MnS in the cross section of the composite inclusion is 10 to 50%.
[2] The ratio of MnS to the circumference of the composite inclusion is 10% or more.
[3] The number density of the composite inclusions having a particle size of 0.5 to 5.0 μm is 10 to 40 pieces / mm ² .

The present invention is based on these findings and is as follows.
(1) The chemical composition is C: 0.15 to 0.28%, Si: 0.10 to 0.8%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.001 to 0.004%, Nb: 0.005 to 0.035%, Al: 0.003% or less, Ti: 0.005 to 0.03%, B: 0.0005 to 0.0025%, N: 0.0020 to 0.0050%, O: 0.0005 to 0.0050%, respectively, Cu: 0.05 to 0.5%, Cr: 0.05 to 1.30%, Ni : 0.1 to 1.0%, Mo: 0.05 to 1.0% and V: 0.02 to 0.10%, and the balance is Fe and impurities.
The carbon equivalent Ceq defined by the following formula (1) is 0.40 to 0.75.
The CNB defined by the formula (2) is 6 to 32, and the steel contains a composite inclusion in which MnS is present around the Ti oxide, and the area ratio of the MnS in the cross section of the composite inclusion is 10. The ratio of MnS to the peripheral length of the composite inclusions is 10% or more, and the number density of the composite inclusions having a particle size of 0.5 to 5.0 um is 10 to 40 pieces / mm. ^{2. A} wear-resistant steel sheet having excellent low-temperature toughness and HAZ toughness.
Ceq = [C%] + [Si%] / 24+ [Mn%] / 6+ [Ni%] / 40+ [Cr%] / 5+ [Mo%] / 4+ [V%] / 14 ・ ・・ (1)
CNB = [C%] / [Nb%] ・ ・ ・ (2)
However, the elements with [] in the formulas (1) and (2) indicate the content of each element.

(2) Further, Ca: 0.008% or less (preferably 0.001 to 0.008%), Mg: 0.005% or less (preferably 0.001 to 0.005%), and REM: 0. Item 2. The wear-resistant steel plate having excellent low-temperature toughness and HAZ toughness, which contains at least one of 010% or less (preferably 0.001 to 0.010%).

(3) Before the RH vacuum degassing treatment, the oxygen potential in the molten steel is set to 10 to 30 ppm, the chemical composition is adjusted in the RH vacuum degassing treatment to produce the molten steel, and the slab is produced by the continuous casting method using the molten steel. The low temperature toughness and low temperature toughness according to item 1 or 2, wherein the slab is manufactured, heated to 1000 to 1200 ° C. and soothed, then hot rolled to cool to room temperature, and then heated to be _{baked from 3 or more points of Ac.} A method for manufacturing a wear-resistant steel plate having excellent HAZ toughness.

(4) Further, the method for producing a wear-resistant steel sheet having excellent low-temperature toughness and HAZ toughness according to Item 3, which is tempered at a temperature of _{1 Ac or less.}

According to the wear-resistant steel sheet according to the present invention and the manufacturing method thereof, it is possible to provide a wear-resistant steel sheet having excellent low-temperature toughness and HAZ toughness even if it is a wear-resistant steel sheet used as a component of a large industrial machine such as for mining. Become.

As a result, the hardness and thickness of the wear-resistant steel sheet can be increased, and the replacement cycle of the wear-resistant steel sheet can be lengthened. Therefore, the repair cost and the like of the large industrial machine can be significantly reduced, and the industrial effect of the present invention is extremely large.

FIG. 1 is an explanatory diagram showing a procedure for collecting test pieces for evaluating HAZ toughness in Examples.

1 Wear-resistant steel plate 2 Back plate 3 Welded bead 4 2 mm V notch Charpy test piece

Hereinafter, the present invention will be described in detail.
1. 1. Chemical Composition First, as described above, the reason for limiting the chemical composition of the wear-resistant steel sheet having excellent low-temperature toughness and HAZ toughness according to the present invention will be described. First, the essential elements will be described.

(1-1) C: 0.15-0.28%
C is most effective in improving the surface hardness of the wear-resistant steel sheet. If the C content is less than 0.15%, it is difficult to secure the required surface hardness, and it becomes necessary to contain another alloying element to compensate for the decrease in hardness, which increases the manufacturing cost. Therefore, the C content is 0.15% or more, preferably 0.16% or more, and more preferably 0.17% or more.

On the other hand, if the C content is too high, the low temperature toughness and HAZ toughness deteriorate and the susceptibility to delayed fracture increases. Therefore, the C content is 0.28% or less, preferably 0.27% or less, and more preferably 0.26% or less.

(1-2) Si: 0.10 to 0.8%
Si is an element effective for pre-deoxidation of molten steel and also contributes to improvement of the surface hardness of wear-resistant steel sheets. If the Si content is less than 0.10%, these effects are insufficient. Therefore, the Si content is 0.10% or more, preferably 0.14% or more, and more preferably 0.15% or more.

On the other hand, when the Si content exceeds 0.8%, the toughness of the wear-resistant steel sheet is significantly deteriorated. Therefore, the Si content is 0.8% or less, preferably 0.79% or less, and more preferably 0.63% or less. In order to stably secure the toughness of the wear-resistant steel sheet, the Si content is preferably 0.7% or less.

(1-3) Mn: 0.8 to 1.5%
Mn is necessary to improve the surface hardness of the wear-resistant steel sheet through the improvement of hardenability and to obtain the form of inclusions suitable for improving the HAZ toughness. In order to obtain these effects, the Mn content is 0.8% or more, preferably 0.82% or more, and more preferably 0.84% or more.

On the other hand, if the Mn content exceeds 1.5%, the hardenability of the wear-resistant steel sheet becomes excessive and the toughness deteriorates. Therefore, the Mn content is 1.5% or less, preferably 1.48% or less, and more preferably 1.47% or less.

(1-4) P: 0.015% or less Since P segregates at the grain boundaries and deteriorates the toughness of the wear-resistant steel sheet, it is desirable that the P content is as low as possible. When the P content exceeds 0.015%, the toughness of the wear-resistant steel sheet is significantly deteriorated. Therefore, the P content is 0.015% or less, preferably 0.012% or less, and more preferably 0. It is 008% or less.

(1-5) S: 0.001 to 0.004%
S precipitates MnS on the surface of the oxide and forms a Mn-deficient region at the matrix interface between MnS and the base material. Therefore, when the base metal is welded, the intragranular ferrite grows preferentially from this Mn-deficient region, so that the low temperature toughness of HAZ can be ensured. Therefore, the S content is 0.001% or more.

However, if the S content exceeds 0.004%, it causes deterioration of ductility and toughness of the wear-resistant steel sheet. Therefore, the S content is 0.004% or less, preferably 0.003% or less, and more preferably 0.002% or less.

(1-6) Nb: 0.005 to 0.035%
Nb expands the unrecrystallized region and introduces high-density dislocations by rolling in that region, increasing the number of metamorphic nucleation sites. As a result, the structure can be miniaturized, and the surface hardness and low temperature toughness of the wear-resistant steel sheet can be improved. Therefore, the Nb content is 0.005% or more, preferably 0.008% or more, and more preferably 0.010% or more.

On the other hand, when the Nb content exceeds 0.035%, not only the above effect is saturated and the cost is increased, but also the delayed fracture resistance of the wear-resistant steel sheet is deteriorated. Therefore, the Nb content is 0.035% or less, preferably 0.028% or less, and more preferably 0.020% or less.

(1-7) Al: 0.003% or less Al is contained in order to obtain the cleanliness of molten steel. Since Al forms an oxide preferentially over other elements, it becomes impossible to obtain a Ti-based oxide that contributes to improvement of low temperature toughness and HAZ toughness. Therefore, the Al content is 0.003% or less, preferably 0.002% or less, and more preferably 0.001% or less.

(1-8) Ti: 0.005 to 0.03%
Ti is necessary for forming nitrides to suppress coarsening of crystal grains and for forming composite inclusions to be intragranular transformation nuclei. If the Ti content is less than 0.005%, this effect is not exhibited. Therefore, the Ti content is 0.005% or more, preferably 0.010%, and even more preferably 0.011%.

On the other hand, if the Ti content exceeds 0.03%, Ti carbides are excessively precipitated, which adversely affects the toughness of the base material and the toughness of the welded portion of the wear-resistant steel sheet. Therefore, the Ti content is 0.03% or less, preferably 0.025% or less, and more preferably 0.020% or less.

(1-9) B: 0.0005 to 0.0025%
B is an element that remarkably improves hardenability and is necessary for ensuring the hardness of the wear-resistant steel sheet. Therefore, the B content is 0.0005% or more, preferably 0.0007% or more, and more preferably 0.0008% or more.

On the other hand, when the B content exceeds 0.0025%, the toughness of the wear-resistant steel sheet is significantly deteriorated. Therefore, the B content is 0.0025% or less, preferably 0.0024% or less, and more preferably 0.0015% or less.

(1-10) N: 0.0020 to 0.0050%
By forming a nitride, N suppresses the coarsening of the structure during heating and contributes to the improvement of the toughness of the wear-resistant steel sheet. Therefore, the N content is 0.0020% or more, preferably 0.0021% or more, and more preferably 0.0025% or more.

On the other hand, when the N content exceeds 0.0050%, the toughness of the wear-resistant steel sheet deteriorates due to the coarsening of the nitride. Therefore, the N content is 0.0050% or less, preferably 0.0049% or less, and more preferably 0.0040% or less.

(1-11) O: 0.0005 to 0.0050%
O (oxygen) is essential for the formation of complex oxides that serve as intragranular transformation nuclei. Therefore, the O content is 0.0005% or more, preferably 0.0007% or more, and more preferably 0.0008% or more.

On the other hand, if O is contained in a large amount, the cleanliness is significantly deteriorated, so that practical toughness cannot be ensured in any of the base material of the wear-resistant steel sheet, the weld metal portion and the HAZ. Therefore, the O content is 0.0050% or less, preferably 0.0049% or less, and more preferably 0.0035% or less.

(1-12) Cu: 0.05 to 0.5%, Cr: 0.05 to 1.30%, Ni: 0.1 to 1.0%, Mo: 0.05 to 1.0% and V : One or more of 0.02 to 0.10% The wear-resistant steel plate according to the present invention is one or more of Cu, Cr, Ni, Mo, and V depending on the required surface hardness and toughness. Contains. These elements will be described.

(1-12-1) Cu: 0.05 to 0.5%
By containing 0.05% or more of Cu, the hardenability of the wear-resistant steel sheet can be enhanced and the hardness can be improved. Therefore, the Cu content is 0.05% or more, preferably 0.10% or more, and more preferably 0.15% or more.

On the other hand, if the Cu content exceeds 0.5%, the toughness of the wear-resistant steel sheet deteriorates. Therefore, the Cu content is 0.5% or less, preferably 0.49% or less, and more preferably 0.30% or less.

(1-12-2) Cr: 0.05 to 1.30%
Cr can improve the hardenability of the wear-resistant steel sheet and improve the hardness. Therefore, the Cr content is 0.05% or more, preferably 0.22% or more, and more preferably 0.24% or more.

On the other hand, when the Cr content exceeds 1.30%, the toughness of the wear-resistant steel sheet is significantly deteriorated. Therefore, the Cr content is 1.30% or less, preferably 1.28% or less, and more preferably 1.02% or less.

(1-12-3) Ni: 0.1 to 1.0%
Ni improves both hardenability and toughness of wear-resistant steel sheets. Therefore, the Ni content is 0.1% or more, preferably 0.14% or more, and more preferably 0.15% or more.

On the other hand, if the Ni content exceeds 1.0%, the cost will increase. Therefore, the Ni content is 1.0% or less, preferably 0.97% or less, and more preferably 0.64% or less.

(1-12-4) Mo: 0.05 to 1.0%
Mo enhances the hardenability of the wear-resistant steel sheet and contributes to the improvement of hardness. Therefore, the Mo content is 0.05% or more, preferably 0.12% or more, and more preferably 0.15% or more.

On the other hand, if the Mo content exceeds 1.0%, the toughness of the wear-resistant steel sheet deteriorates. Therefore, the Mo content is 1.0% or less, preferably 0.99% or less, and more preferably 0.57% or less.

(1-12-5) V: 0.02 to 0.10%
V enhances the hardenability of the wear-resistant steel sheet and contributes to the improvement of hardness. Therefore, the V content is 0.02% or more, preferably 0.03% or more, and more preferably 0.04% or more.

On the other hand, if the V content exceeds 0.10%, the toughness of the wear-resistant steel sheet deteriorates. Therefore, the V content is 0.10% or less, preferably 0.09% or less.

(1-13) One or more of Ca: 0.008% or less, Mg: 0.005% or less, and REM: 0.01% or less Next, an arbitrary element will be described. In addition to the above-mentioned essential elements, the wear-resistant steel sheet according to the present invention may contain one or more of Ca, Mg, and REM as optional elements in order to improve the cleanliness of the steel sheet.

(1-13-1) Ca: 0.008% or less Ca improves the toughness of a wear-resistant steel sheet by controlling the morphology of sulfide-based non-metal inclusions in addition to reducing oxygen. However, when the Ca content exceeds 0.008%, coarse oxides are formed and Ti-based composite inclusions are reduced, so that the toughness deteriorates. Therefore, the Ca content is 0.008% or less.

In order to surely obtain the above effect, the Ca content is preferably 0.001% or more, and more preferably 0.002% or more.

(1-13-2) Mg: 0.005% or less Mg improves the toughness of the wear-resistant steel sheet by reducing oxygen. However, if the Mg content exceeds 0.005%, coarse oxides are formed and the toughness of the wear-resistant steel sheet deteriorates. Therefore, the Mg content is 0.005% or less.

In order to surely obtain the above effect, the Mg content is preferably 0.002% or more, and more preferably 0.003% or more.

(1-13-3) REM: 0.010% or less REM reduces oxygen and improves the toughness of wear-resistant steel sheets. However, if the REM content exceeds 0.010%, the toughness of the wear-resistant steel sheet deteriorates. Therefore, the REM content is 0.010% or less, preferably 0.003% or less.

In order to surely obtain the above effect, the REM content is 0.001% or more, preferably 0.002% or more.

(1-14) Remaining Remaining other than the above is Fe and impurities. Examples of impurities include those contained in raw materials such as ore and scrap, and those contained in the manufacturing process.

(1-15) Carbon equivalent Ceq defined by Eq. (1): 0.40 to 0.75
Since the wear-resistant steel sheet according to the present invention has the required surface hardness, the carbon equivalent Ceq, which is an index of quenching hardness defined by the Japan Welding Engineering Society standard (WES), is set to 0.40 as shown in the following formula (1). It is set to ~ 0.75%. The elements with [] in the formula (1) indicate the content of each element.
Ceq = [C%] + [Si%] / 24+ [Mn%] / 6+ [Ni%] / 40+ [Cr%] / 5+ [Mo%] / 4+ [V%] / 14 ・ ・・ (1)

The carbon equivalent Ceq defined by the formula (1) is an index showing the hardenability of the wear-resistant steel sheet. In order to secure the surface hardness of the wear-resistant steel sheet, the C, Si, Mn, Ni, Cr, Mo, and V contents contained in the wear-resistant steel sheet are set to 0. It is necessary to adjust so that it becomes 40 or more. If the carbon equivalent Ceq is less than 0.40%, sufficient surface hardness cannot be obtained due to insufficient quenching hardness, and wear resistance cannot be ensured. Therefore, the carbon equivalent Ceq is 0.40% or more, preferably 0.41% or more, and more preferably 0.44% or more.

On the other hand, the larger the carbon equivalent Ceq, the higher the surface hardness, but when the carbon equivalent Ceq exceeds 0.75%, the toughness of the wear-resistant steel sheet deteriorates. Therefore, the carbon equivalent Ceq is 0.75% or less, preferably 0.71% or less, and more preferably 0.65% or less.

CNB defined by Eq. (1-16) (2): 6 to 32
In order to secure the required low temperature toughness and HAZ toughness in the wear resistant steel sheet according to the present invention, the CNB defined by the formula (2) is set to 6 to 32. However, the element with [] in the formula (2) indicates the content of each element.
CNB = [C%] / [Nb%] ・ ・ ・ (2)

The solid solution Nb expands the unrecrystallized region and rolls in that region to introduce high-density dislocations and increase the number of metamorphic nucleation sites. As a result, the structure can be miniaturized, so that the surface hardness and low temperature toughness of the wear-resistant steel sheet can be improved.

Further, during slab heating and welding, the Nb carbonitride exerts a pinning effect, thereby suppressing coarsening of austenite grains and contributing to improvement of HAZ toughness. In order to obtain such an effect of Nb, it is necessary to set CNB to 6 or more. If the CNB is less than 6, the hardenability is insufficient due to the insufficient amount of solid solution C that does not form Nb carbonitride due to the lack of C with respect to Nb, and sufficient surface hardness cannot be obtained. Abrasion resistance cannot be ensured. Therefore, the CNB is 6 or more, preferably 6.1 or more, and preferably 7.4 or more.

On the other hand, when CNB exceeds 32, the solid solution Nb is insufficient, the effect of widening the unrecrystallized region is insufficient, and it becomes difficult to secure low temperature toughness by microstructuring. Therefore, the CNB is 32 or less, preferably 31.7 or less, and more preferably 31.4.

2. Composite inclusions The present invention includes composite inclusions in which MnS is present around the Ti oxide in the steel as the composite inclusions that contribute to the miniaturization of the HAZ structure. Then, the area ratio of MnS in the cross section of the composite inclusions, the ratio of MnS at the interface, the particle size and the number density of the inclusions are limited to the following ranges.

(2-1) Area ratio of MnS in the cross section of the composite inclusions: 10 to 50%
In the present invention, the amount of MnS in the composite inclusions is defined by analyzing the composite inclusions appearing on an arbitrary cut surface and measuring the area ratio of MnS in the cross-sectional area of the composite inclusions.

When the area ratio of MnS in the cross section of the composite inclusion is less than 10%, the amount of MnS in the composite inclusion is small, and the initial Mn-deficient layer is not sufficiently formed at the interface between MnS and the matrix. As a result, it becomes difficult to generate intragranular ferrite when welding.

On the other hand, when the proportion of MnS in the cross section of the composite inclusions exceeds 50%, the composite inclusions are mainly MnS. In this case, the amount of Mn absorbed by the atomic vacancies in the Ti-based oxide is small, the welded Mn-deficient layer is not formed, and it becomes difficult to generate intragranular ferrite.
Therefore, the area ratio of MnS in the cross section of the composite inclusion is 10 to 50%.

(2-2) Ratio of MnS to the peripheral length of the composite inclusions: 10% or more MnS in the composite inclusions is formed around the Ti-based oxide. When the ratio of MnS to the circumference of the composite inclusions is less than 10%, the initial Mn-deficient region formed at the interface between MnS and the matrix is small. Therefore, even if welding is performed, the amount of intragranular ferrite formed is not sufficient, so that good low-temperature HAZ toughness cannot be obtained. Therefore, the ratio of MnS to the circumference of the composite inclusion matrix is 10% or more.

The larger the proportion of MnS, the larger the initial Mn-deficient layer, and the easier it is for intragranular ferrite to be formed. Therefore, the upper limit of this ratio is not set, but it is usually 80% or less.

(2-3) Number density of composite inclusions (number density of composite inclusions having a particle size of 0.5 to 5.0 μm): 10 to 40 pieces / mm ²

The number density of composite inclusions means the number of composite inclusions having a specified particle size per unit area. When the particle size of the composite inclusions is less than 0.5 μm, the amount of Mn that can be absorbed from the periphery of the composite inclusions is small, and as a result, the amount of intragranular ferrite produced decreases. On the other hand, when the particle size of the composite inclusions is larger than 5.0 μm, the composite inclusions become the starting point of fracture. Therefore, in the present invention, the particle size of the target composite inclusion is set to 0.5 to 5.0 μm.

The number density of the composite inclusions having a particle size of 0.5 to 5.0 μm is related to the amount of Mn absorbed. In order to generate stable intragranular ferrite, it is necessary that at least one of each composite inclusion is contained in the austenite. Therefore, the number density of the composite inclusions is set to 10 pieces / mm ² or more.

On the other hand, if the amount of composite inclusions is excessive, the absorbed energy of ductile fracture decreases. Therefore, the number density of the composite inclusions is set to 40 pieces / mm ² or less.

3. 3. Manufacturing Method Next, even if the wear-resistant steel sheet according to the present invention has the above-mentioned chemical composition, it has the above-mentioned composite inclusions to ensure the required HAZ toughness. Must be appropriate.

(3-1) Step I
In the production of slabs, which are the material of the wear-resistant steel sheet according to the present invention, Ar gas is blown into the molten steel from above before the RH (Ruhrstahl-Hausen) vacuum degassing treatment in order to control inclusions in the steel. The surface slag reacts with the molten steel. Thereby, the total amount of Fe in the slag is adjusted, and the oxygen potential in the molten steel is controlled to 10 to 30 ppm.

Here, the flow rate of Ar gas is adjusted between 100 and 200 L / min, and the blowing time is adjusted between 5 and 15 min. After that, each element is added by RH vacuum degassing treatment to adjust the composition, and a slab having a thickness of about 300 mm is cast by continuous casting.

(3-2) Steps II to IV
Next, the following steps II to IV are sequentially performed on the slab.
Step II: Heating and soaking to a temperature range of 1000 to 1200 ° C. Step III: Hot rolling to the desired plate thickness Step IV: After cooling to room temperature, _{heat to 3} or more Ac points and quench.

If the heating temperature in step II is less than 1000 ° C., the solid solution Nb is insufficient, and the effect of widening the unrecrystallized region is insufficient. Therefore, high-density dislocations cannot be introduced, and metamorphic nucleation sites are insufficient. As a result, the structure cannot be miniaturized, and the surface hardness and low temperature toughness of the wear-resistant steel sheet cannot be ensured. On the other hand, if the heating temperature exceeds 1200 ° C., the amount of scale adhered increases, which may cause flaws during hot rolling. Therefore, the heating temperature of the slab in step II is set to 1000 to 1200 ° C.

The hot rolling in the step III is not particularly limited, and the hot rolling may be appropriately performed to a desired plate thickness according to the conventional hot rolling conditions.
In step IV, the steel sheet after hot rolling is cooled to a room temperature equivalent, then _{heated to 3} or more points of Ac in order to be austenitized, and quenched from that temperature range to obtain a desired hardness.

(3-3) Step V
Further, the toughness of the wear-resistant steel sheet can be further improved by performing the step (V) after the step (IV) as necessary and _{tempering at 1 point or less of Ac.}
In this way, the wear-resistant steel sheet having excellent low-temperature toughness and HAZ toughness according to the present invention can be produced.

Further, the wear-resistant steel sheet according to the present invention and a method for producing the same will be specifically described with reference to Examples. This is an example of the present invention, and the present invention is not limited thereto.
In the present invention, a 300 mm thick slab having the chemical composition shown in Tables 1 and 2 (the balance is Fe and impurities) was prepared by a continuous casting method by melting in a converter.

Here, from the viewpoint of controlling the composite inclusions, the oxygen potential in the molten steel before the RH vacuum degassing treatment was adjusted to the amounts shown in Tables 3 and 4 in the converter, and then Ti and the like were added to adjust the components. After that, in the continuous casting process, the temperature of the molten steel is not excessively raised, and the difference between the solidification temperature determined by the chemical composition of the molten steel is controlled to be within 50 ° C. Was reduced to obtain a slab having a thickness of 300 mm.

The slabs are heated and soothed under the manufacturing conditions shown in Tables 3 and 4, hot-rolled to a plate thickness of 40 mm, cooled to room temperature, then heated and quenched, and some samples are further tempered. A steel plate was obtained by performing the above.

For the calculation of the MnS area ratio and the MnS peripheral length ratio in the cross section of the Ti-based composite inclusions, a test piece for analyzing the composite inclusions collected from a 1/4 t portion of the steel plate was used. For the composite inclusions, the MnS area ratio and the MnS peripheral length ratio at the interface of the composite inclusions were measured from the mapping image obtained by surface-analyzing the composite inclusions using an electron probe microanalyzer (EPMA). The MnS area ratio and the MnS peripheral length ratio at the interface of the composite inclusions were determined by performing an EPMA analysis of 20 samples for each sample and calculating an average value.

Further, the number density of Ti-based composite inclusions is a composite having a particle size in the range of 0.5 to 5.0 μm from the shape measurement data of the composite inclusions obtained from the automatic inclusion analyzer combined with SEM-EDX. The number density was calculated by calculating the number of inclusions. The calculated results are shown in Tables 3 and 4.

The surface of these samples was cut by about 0.7 mm to remove the decarburized layer, and then the Brinell surface hardness test was performed. The low temperature toughness was subjected to a Charpy impact test (L direction) at a position of 1 / 4t (a position of 1/4 in the plate thickness direction), and the absorbed energy at −40 ° C. was evaluated. For delayed fracture resistance, a round bar tensile test piece is _{immersed in a 1% NH 4} SCN aqueous solution for 24 hours without a load to charge hydrogen, and then Zn plating is performed on the test piece to perform a low strain rate tensile test (SSRT). Evaluation was performed at a crosshead speed of 1 × 10 ^{-3 mm / min.}

FIG. 1 is an explanatory diagram showing a procedure for collecting test pieces for evaluating HAZ toughness in Examples. Reference numeral 1 indicates a wear-resistant steel plate, reference numeral 2 indicates a back plate, reference numeral 3 indicates a weld bead, and reference numeral 4 indicates a 2 mm V notch Charpy test piece.

As shown in FIG. 1, the HAZ toughness is determined by using a solid wire for gas shielded arc welding: trade name YM-26 (manufactured by Nippon Steel & Sumitomo Metal Welding Industry Co., Ltd.) after abutting the wear-resistant steel plate 1 with a drilled groove. _{Then, CO 2} gas or a mixed gas of Ar and CO ₂ is used as the shield gas, and the welding bead 3 is piled up in multiple layers so that the height of the welding bead 3 is 12 to 20 mm at a welding heat input of 15.0 to 20.0 kJ / cm. Welding is performed, and then the weld is cut perpendicular to the longitudinal direction of welding, and the back surface of the wear-resistant steel plate 1 is notched in the Z direction from the point where the groove is deepest in the steel plate direction with respect to the fusion line on the straight side in the cut cross section. A 1 mm to 2 mm V notch Charpy test piece 4 was sampled and subjected to a Charpy impact test at −40 ° C. to evaluate the absorbed energy.

The evaluation criteria for the characteristics are as follows.
-Brinell surface hardness: 400 or more was accepted.
-40 ° C absorption energy: 27J or more was accepted.
-Low strain tensile stress: Those that did not break at 1000 MPa or more were regarded as acceptable.
-HAZ part -40 ° C absorption energy: 27J or more was accepted.

The obtained test results are shown in Tables 3 and 4.
Symbols A01 to A35 in Tables 1 and 3 are examples of the present invention that satisfy all the provisions of the present invention, and symbols B01 to B31 in Tables 2 and 4 are comparative examples that do not satisfy the provisions of the present invention.

Symbols A01 to A35 have mechanical properties such as Brinell surface hardness: 400 or more, -40 ° C absorption energy: 27J or more, low strain tensile stress: 1000MPa or more and no breakage, and HAZ part -40 ° C absorption energy: 27J or more. It is a wear steel plate and has high wear resistance, good low temperature toughness and HAZ toughness. Therefore, it is suitable for use as a component of industrial machines such as large excavators used for excavation of mines and large dump trucks for transporting minerals.

On the other hand, the symbol B01 had a insufficient surface hardness because the C content was below the lower limit of the range of the present invention.
In symbol B02, the toughness of the steel sheet was insufficient because the C content exceeded the upper limit of the range of the present invention.

The symbol B03 has insufficient delayed fracture resistance because the Si content is below the lower limit of the range of the present invention.
In symbol B04, the toughness of the steel sheet was insufficient because the Si content exceeded the upper limit of the range of the present invention.

The symbol B05 lacked HAZ toughness because the Mn content was below the lower limit of the range of the present invention.
In symbol B06, the toughness of the steel sheet was insufficient because the Mn content exceeded the upper limit of the range of the present invention.
The symbol B07 has insufficient surface hardness because the Nb content is below the lower limit of the range of the present invention.

The symbol B08 lacked the delayed fracture resistance because the Nb content was below the upper limit of the range of the present invention.
The symbol B09 lacked HAZ toughness because the Al content exceeded the upper limit of the range of the present invention.

The symbol B10 lacked HAZ toughness because the Ti content was below the lower limit of the range of the present invention.
The symbol B11 has insufficient toughness of the steel sheet because the Ti content exceeds the upper limit of the range of the present invention.
The symbol B12 has insufficient surface hardness because the B content is below the lower limit of the range of the present invention.

The symbol B13 has insufficient toughness of the steel sheet because the B content exceeds the upper limit of the range of the present invention.
The symbol B14 has insufficient toughness of the steel sheet because the N content is below the lower limit of the range of the present invention.
The symbol B15 lacked the toughness of the steel sheet because the N content exceeded the upper limit of the range of the present invention.

The symbol B16 lacked HAZ toughness because the oxygen potential in the molten steel before the RH vacuum degassing treatment was below the lower limit of the range of the present invention and the O content was below the lower limit of the range of the present invention.

The symbol B17 lacked HAZ toughness because the oxygen potential in the molten steel before the RH vacuum degassing treatment exceeded the upper limit of the range of the present invention and the O content exceeded the upper limit of the range of the present invention.

The symbol B18 has insufficient surface hardness because the carbon equivalent Ceq is below the lower limit of the range of the present invention.
The symbol B19 has insufficient toughness of the steel sheet because the carbon equivalent Ceq exceeds the upper limit of the range of the present invention.

The symbol B20 has insufficient surface hardness because CNB is below the lower limit of the range of the present invention.
The symbol B21 lacks the toughness of the steel sheet because CNB exceeds the upper limit of the range of the present invention.
The symbol B22 lacks the toughness of the steel sheet because Cu exceeds the upper limit of the range of the present invention.

The symbol B23 lacks the toughness of the steel sheet because the Cr content exceeds the upper limit of the range of the present invention.
The symbol B24 has insufficient toughness of the steel sheet because the Mo content exceeds the upper limit of the range of the present invention.
The symbol B25 has insufficient toughness of the steel sheet because the V content exceeds the upper limit of the range of the present invention.

Symbol B26 lacked HAZ toughness because the Ca content exceeded the upper limit of the range of the present invention.
The symbol B27 lacked HAZ toughness because the Mg content exceeded the upper limit of the range of the present invention.
The symbol B28 lacked the toughness of the steel sheet because the REM content exceeded the upper limit of the range of the present invention.

The symbol B29 has HAZ toughness because the oxygen potential in the molten steel before the RH vacuum degassing treatment was below the lower limit of the range of the present invention and the number density of the composite inclusions was reduced.

The symbol B30 has HAZ toughness because the oxygen potential in the molten steel before the RH vacuum degassing treatment exceeds the upper limit of the range of the present invention and the number density of the composite inclusions is increased.
Further, since the S content of the symbol B31 is below the lower limit of the range of the present invention, the MnS area ratio and the MnS peripheral length ratio are insufficient, and the HAZ toughness is insufficient.

Claims (4)

The chemical composition is mass%,
C: 0.15-0.28%,
Si: 0.10 to 0.8%,
Mn: 0.8-1.5%,
P: 0.015% or less,
S: 0.001 to 0.004%,
Nb: 0.005 to 0.035%,
Al: 0.003% or less,
Ti: 0.005 to 0.03%,
B: 0.0005 to 0.0025%,
N: 0.0020 to 0.0050%,
O: Contains 0.0005 to 0.0050%, respectively, and further
Cu: 0.05-0.5%,
Cr: 0.05 to 1.30%,
Ni: 0.1 to 1.0%,
Contains one or more of Mo: 0.05 to 1.0% and V: 0.02 to 0.10%.
The rest is Fe and impurities,
The carbon equivalent Ceq defined by the following formula (1) is 0.40 to 0.75.
The CNB defined by the formula (2) is 6 to 32, and further contains a composite inclusion in which MnS is present around the Ti oxide in the steel.
The area ratio of the MnS in the cross section of the composite inclusion is 10 to 50%.
The ratio of MnS to the circumference of the composite inclusion is 10% or more, and
A wear-resistant steel sheet having a density of 10 to 40 pieces / mm ² of the composite inclusions having a particle size of 0.5 to 5.0 μm.
Ceq = [C%] + [Si%] / 24+ [Mn%] / 6+ [Ni%] / 40+ [Cr%] / 5+ [Mo%] / 4+ [V%] / 14 ・ ・・ (1)
CNB = [C%] / [Nb%] ・ ・ ・ (2)
However, the elements with [] in the formulas (1) and (2) indicate the content of each element. In addition, in% by mass,
Ca: 0.008% or less,
The wear-resistant steel sheet according to claim 1, which contains one or more of Mg: 0.005% or less and REM: 0.010% or less. Before the RH vacuum degassing treatment, the oxygen potential in the molten steel was set to 10 to 30 ppm.
In RH vacuum degassing treatment, the chemical composition is adjusted to produce molten steel,
Manufacture slabs by continuous casting using molten steel
After heating and soaking the slab to 1000-1200 ° C, hot rolling is performed to cool it to room temperature.
The method for producing a wear-resistant steel sheet according to claim 1 or 2, wherein the steel sheet is then heated and _{baked from three or more points of Ac.} The method for producing a wear-resistant steel sheet according to claim 3, wherein the wear-resistant steel sheet is baked at a temperature of _{1 Ac or less.}

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