JP5866820B2 - Wear-resistant steel plate with excellent weld toughness and delayed fracture resistance - Google Patents

Description

  The present invention relates to a wear-resistant steel plate having a thickness of 4 mm or more suitable for use in construction machinery, industrial machinery, shipbuilding, steel pipes, civil engineering, construction, etc., and in particular, excellent in welded portion toughness and delayed fracture resistance. About.

  When hot-rolled steel sheets are used in steel structures, machines, devices, etc., such as construction machines, industrial machines, shipbuilding, steel pipes, civil engineering, and construction, the wear characteristics of the steel sheets may be required. Conventionally, in order to retain excellent wear resistance as a steel material, it is common to increase the hardness, and it is possible to dramatically increase the martensite single phase structure. In order to increase the hardness of the martensite structure itself, it is effective to increase the amount of solid solution C.

  For this reason, wear-resistant steel plates are generally highly susceptible to cold cracking and have poor weld toughness, and when used in welded steel structures, they are used as a liner on the surface of steel members that come into contact with rocks or earth and sand. It was common. For example, a dump vessel may be used by attaching a wear-resistant steel plate only to the surface of the vessel in contact with earth and sand after being assembled by welding using mild steel.

  However, in the manufacturing method in which the welded steel sheets are bonded together after assembling the welded structure, the labor and cost of manufacturing increase. Therefore, there is no wear-resistant steel sheet that can be applied as a strength member of the welded structure. It was desired.

  Patent Document 1 relates to a wear-resistant steel sheet excellent in delayed fracture resistance and a method for producing the same, and in order to improve delayed fracture resistance, a low Si-low P-low S-Cr, Mo, Nb composition is used. It describes that steel containing one or more of Cu, V, Ti, B and Ca is directly quenched and tempered as necessary.

  Patent Document 2 relates to a method for manufacturing steel and steel products with high wear resistance, and satisfies the parameter formulas of the contents of these elements in the 0.24 to 0.3 C-Ni, Cr, Mo, and B systems. A steel having a martensite or martensite bainite structure containing 5 to 15% by volume residual austenite and improved wear resistance is described, and the steel of the component is between austenitizing temperature and 450 ° C. Is cooled at a cooling rate of 1 ° C./second or more.

  Patent Document 3 relates to a wear-resistant steel material excellent in toughness and delayed fracture resistance, and a method for producing the same, and a component composition in which Cr, Ti, and B are essential, a surface layer is tempered martensite, and an inner part is tempered martensite. In the tempered lower bainite structure, a steel material in which the ratio of the prior austenite grain size in the thickness direction and the rolling direction is specified, and the steel of the component composition are directly quenched and tempered after hot rolling at 900 ° C. or less and a cumulative reduction of 50% or more It is described.

  Patent Document 4 relates to a wear-resistant steel material excellent in toughness and delayed fracture resistance, and a method for producing the same, and a component composition in which Cr, Ti, and B are essential, a surface layer is martensite, and an internal part is martensite and lower bainite. A steel material that defines the prior austenite grain elongation expressed by the ratio of the prior austenite grain size in the rolling direction to the former austenite grain size in the central part of the sheet thickness in the mixed structure of the structure or the lower bainite single phase structure, and the components It describes that a steel having a composition is directly quenched after hot rolling at 900 ° C. or less and a cumulative reduction of 50% or more.

  Patent Document 5 relates to a wear-resistant steel excellent in weldability, wear resistance and corrosion resistance of a welded portion, and a method for producing the same, and uses 4 to 9 mass% of Cr as an essential element, and includes one or two of Cu and Ni. And steel satisfying the parameter formula consisting of the content of the specific component, and steel having the component composition after hot rolling at a cumulative reduction ratio of 30% or higher at 950 ° C. or lower and reheating and quenching. ing.

JP-A-5-51691 JP-A-8-295990 Japanese Patent Application Laid-Open No. 2002-115024 JP 2002-80930 A Japanese Patent Laid-Open No. 2004-162120

  By the way, when steel materials are welded, it is the toughness deterioration at the bond part of the weld heat-affected zone that causes the most toughness degradation, but in the wear-resistant steel having an as-quenched martensite structure, Even in a welding heat-affected zone that is reheated to around 300 ° C. away from the boundary line, toughness deterioration called low temperature temper embrittlement becomes a problem. Low temperature temper embrittlement is thought to be due to a synergistic effect of the change in the morphology of carbides in martensite and the segregation of grain boundaries such as impurity elements.

  In the region reheated to the low temperature temper embrittlement temperature, hydrogen that penetrates into the weld from shield gas during welding and the residual stress generated by the welding heat overlap, resulting in delayed fracture (this occurs at the weld). Such cracks are generally referred to as cold cracks), and delayed fracture is likely to occur particularly in high-strength wear-resistant steel.

  Therefore, when the wear-resistant steel plate is applied to a strength member of a welded structure, the toughness of the weld heat affected zone reheated to around 300 ° C. away from the bond zone and the melting boundary line of the weld heat affected zone is improved. However, conventional wear-resistant steel sheets are highly susceptible to cold cracking at the welds, and in order to prevent cold cracking, treatments such as preheating and afterheating are performed before and after welding, And the residual stress had to be reduced.

  Patent Documents 1 and 2 do not describe improving the toughness of the weld heat-affected zone in wear-resistant steel, and Patent Documents 3 and 4 also define the microstructure for the purpose of improving the toughness of the base material. is there. Patent Document 5 has examined weldability and wear resistance of welds, but is not intended to improve weld toughness. However, it has not reached to improve both the toughness and delayed fracture resistance of the weld heat affected zone.

  Accordingly, an object of the present invention is to provide a wear-resistant steel sheet that is excellent in weld toughness and delayed fracture resistance without causing a decrease in productivity and an increase in manufacturing cost. In the present invention, the weld zone toughness means the toughness of the weld heat-affected zone, and the excellent weld zone toughness means that the weld zone has particularly excellent toughness in the bond heat zone and low temperature temper embrittlement temperature range. .

  In order to achieve the above-mentioned problems, the inventors of the present invention are concerned with various factors that determine the chemical composition, manufacturing method, and microstructure of a steel sheet in order to ensure weld toughness and delayed fracture resistance for wear-resistant steel sheets. We conducted intensive research and obtained the following knowledge.

  1. In order to ensure excellent wear resistance, it is essential that the base structure of the steel sheet is martensite. For this purpose, it is important to strictly control the chemical composition of the steel sheet and ensure hardenability.

  2. In order to achieve excellent weld zone toughness, it is necessary to suppress the coarsening of crystal grains in the bond zone of the weld heat affected zone. For this purpose, fine precipitates are dispersed in the steel sheet and pinning is performed. It is effective to utilize the effect.

  3. In order to ensure excellent toughness in the low temperature temper embrittlement temperature range of the weld heat affected zone and suppress delayed fracture, it is important to properly manage the amount of alloying elements such as C, Mn, Cr, P, etc. .

The present invention has been made by further studying the obtained knowledge, that is, the present invention
1. In mass%, C: 0.20 to 0.30%, Si: 0.05 to 1.0%, Mn: 0.40 to 1.2%, P: 0.010% or less, S: 0.005 %: Cr: 0.40-1.5%, Nb: 0.005-0.025%, Ti: 0.005-0.03%, Al: 0.1% or less, N: 0.01% It contains the following, DI * represented by the formula (1) is 45 or more, has a composition consisting of the balance Fe and inevitable impurities, and has a microstructure with martensite as a base phase, weld toughness and delayed fracture resistance Excellent wear-resistant steel plate.
DI * = 33.85 × (0.1 × C) ^0.5 × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2.16 × Cr + 1)
× (3 × Mo + 1) × (1.75 × V + 1) × (1.5 × W + 1) (1)
In the formula (1), each element symbol is a content (mass%).
2. The steel composition further contains one or more of Mo: 0.05 to 1.0%, W: 0.05 to 1.0%, B: 0.0003 to 0.0030% in mass%. 2. A wear-resistant steel sheet excellent in weld toughness and delayed fracture resistance according to 1.
3. 1 or 2 characterized in that the steel composition further contains one or more of Cu: 1.5% or less, Ni: 2.0% or less, and V: 0.1% or less in mass%. Abrasion resistant steel plate with excellent weld toughness and delayed fracture resistance.
4). 1 to 3 characterized in that the steel composition further contains one or more of REM: 0.008% or less, Ca: 0.005% or less, Mg: 0.005% or less in mass%. The wear-resistant steel sheet having excellent weld toughness and delayed fracture resistance according to any one of the above.
5. The wear-resistant steel sheet having excellent weld toughness and delayed fracture resistance according to any one of 1 to 4, wherein the surface hardness is Brinell hardness of 400 HBW 10/3000 or more.
A wear-resistant steel plate having excellent weldability toughness and delayed fracture resistance with a hardenability index DI * of 180 or less, according to any one of 6.1 to 5.
A steel plate according to any one of 7.1 to 6, a wear-resistant steel plate excellent in weld toughness and delayed fracture resistance satisfying the formula (2).
C + Mn / 4-Cr / 3 + 10P ≦ 0.47 (2)
In the formula (2), each element symbol is a content (mass%).

  According to the present invention, a wear-resistant steel sheet having excellent weld heat-affected zone toughness and delayed fracture resistance can be obtained, which greatly contributes to the improvement of manufacturing efficiency and safety during the production of steel structures. The effect of.

Diagram explaining the T-shaped fillet weld cracking test Sampling position of Charpy impact test piece in weld zone

In the present invention, the component composition and the microstructure are defined.
[Component Composition] In the following description, “%” is “mass%”.

C: 0.20 to 0.30%
C is an important element for increasing the hardness of martensite and ensuring excellent wear resistance, so that its effect is required. On the other hand, if the content exceeds 0.30%, not only the weldability deteriorates, but also the toughness in the bond heat affected zone and the low temperature tempering region deteriorates. For this reason, it limits to 0.20 to 0.30% of range. Preferably, it is 0.20 to 0.28%.

Si: 0.05-1.0%
Si acts as a deoxidizer and is not only necessary for steelmaking, but also has the effect of increasing the hardness of the steel sheet by solid solution and solid solution strengthening. Furthermore, it has the effect of suppressing toughness deterioration in the temper embrittlement region of the weld heat affected zone. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 1.0%, the toughness of the weld heat affected zone is remarkably deteriorated, so the content is limited to 0.05 to 1.0%. Preferably, it is 0.07 to 0.5%.

Mn: 0.40 to 1.2%
Mn has the effect of increasing the hardenability of steel, and 0.40% or more is necessary to ensure the hardness of the base material. On the other hand, if the content exceeds 1.2%, not only the toughness, ductility and weldability of the base material deteriorate, but also the grain boundary segregation of P is promoted and the occurrence of delayed fracture is promoted. For this reason, it limits to 0.40 to 1.2% of range. Preferably, it is 0.40 to 1.1%.

P: 0.010% or less When P exceeds 0.010%, it segregates at the grain boundary, becomes the starting point of delayed fracture, and deteriorates the toughness of the heat affected zone. For this reason, it is desirable to make 0.010% an upper limit and to reduce as much as possible. In addition, since excessive P reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.002% or more.

S: 0.005% or less Since S deteriorates the low temperature toughness and ductility of the base material, it is desirable to reduce the upper limit to 0.005%.

Cr: 0.40 to 1.5%
Cr is an important alloying element in the present invention, and has the effect of increasing the hardenability of steel and the effect of suppressing toughness deterioration in the temper embrittlement region of the weld heat affected zone. This is because the diffusion of C in the steel sheet is delayed due to the Cr content, and when reheated to a temperature range where low temperature temper embrittlement occurs, the change in the morphology of carbides in the martensite is suppressed. . In order to have such an effect, the content of 0.40% or more is necessary. On the other hand, when it contains exceeding 1.5%, an effect will be saturated and it will become economically disadvantageous, and weldability will fall. For this reason, it limits to 0.40 to 1.5% of range. Preferably, it is 0.40 to 1.2%.

Nb: 0.005 to 0.025%
Nb precipitates as carbonitride, refines the microstructure of the base metal and the weld heat affected zone, fixes solid solution N, improves the toughness of the weld heat affected zone, and suppresses the occurrence of delayed fracture Is an important element. In order to acquire such an effect, 0.005% or more needs to be contained. On the other hand, if the content exceeds 0.025%, coarse carbonitrides may precipitate, which may be the starting point of fracture. For this reason, it limits to 0.005 to 0.025% of range. Preferably, it is 0.007 to 0.023%.

Ti: 0.005 to 0.03%
Ti has the effect of suppressing the coarsening of crystal grains in the bond part of the weld heat affected zone by fixing solid solution N to form TiN, and toughness deterioration in the low temperature tempering temperature region due to reduction of solid solution N And has the effect of suppressing the occurrence of delayed fracture. In order to acquire these effects, 0.005% or more needs to be contained. On the other hand, if the content exceeds 0.03%, TiC is precipitated and the base material toughness is deteriorated. For this reason, it limits to 0.005 to 0.03% of range. Preferably, it is 0.007 to 0.025%.

Al: 0.1% or less Al acts as a deoxidizer, and is most commonly used in the molten steel deoxidation process of steel sheets. Moreover, by fixing solid solution N in steel and forming AlN, it has the effect of suppressing the coarsening of crystal grains in the bond part of the weld heat affected zone, and in the low temperature tempering temperature range by reducing solid solution N It has the effect of suppressing toughness degradation and delayed fracture. On the other hand, when it contains exceeding 0.1%, it mixes with a weld metal part at the time of welding and deteriorates the toughness of the weld metal, so it is limited to 0.1% or less. Preferably, it is 0.01 to 0.07%.

N: 0.01% or less N forms nitrides with Nb and Ti, and has the effect of suppressing crystal grain coarsening in the weld heat affected zone. However, if the content exceeds 0.01%, the base metal and weld toughness are remarkably lowered, so the content is limited to 0.01% or less. Preferably, it is 0.0010 to 0.0070%. The balance is Fe and inevitable impurities.
In the present invention, in order to further improve the characteristics, one or more of Mo, W, B, Cu, Ni, V, REM, Ca, and Mg can be contained in addition to the basic component system.

Mo: 0.05-1.0%
Mo is an element that significantly increases the hardenability and is effective in increasing the hardness of the base material. In order to obtain such an effect, the content is preferably 0.05% or more. However, if it exceeds 1.0%, the base material toughness, ductility and weld crack resistance are adversely affected. The following.

W: 0.05-1.0%
W is an element that significantly increases the hardenability and is effective in increasing the hardness of the base material. In order to obtain such an effect, the content is preferably 0.05% or more. However, if it exceeds 1.0%, the base material toughness, ductility and weld crack resistance are adversely affected. The following.

B: 0.0003 to 0.0030%
B is an element that significantly increases the hardenability by adding a small amount and is effective in increasing the hardness of the base material. In order to obtain such an effect, the content is preferably 0.0003% or more. However, if it exceeds 0.0030%, the base material toughness, ductility and weld crack resistance are adversely affected. The following.

  Cu, Ni, and V are all elements that contribute to improving the strength of steel and can be appropriately contained depending on the desired strength.

Cu: 1.5% or less Cu is an element that increases hardenability and is effective in increasing the hardness of the base material. In order to obtain such an effect, the content is preferably set to 0.1% or more. However, when the content exceeds 1.5%, the effect is saturated, and hot brittleness is caused to deteriorate the surface properties of the steel sheet. .5% or less.

Ni: 2.0% or less Ni is an element that increases hardenability and is effective in increasing the hardness of the base material. In order to acquire such an effect, it is preferable to set it as 0.1% or more, However, if it exceeds 2.0%, since an effect will be saturated and it becomes economically disadvantageous, it shall be 2.0% or less.

V: 0.1% or less V is an element that increases the hardenability and is effective in increasing the hardness of the base material. In order to acquire such an effect, it is preferable to set it as 0.01% or more, However, If it exceeds 0.1%, in order to deteriorate a base material toughness and ductility, it is set as 0.1% or less.

  REM, Ca, and Mg all contribute to the improvement of toughness, and are selected and added according to desired characteristics. When adding REM, it is preferable to set it as 0.002% or more, but even if it exceeds 0.008%, the effect is saturated, so 0.008% is made the upper limit.

  When adding Ca, it is preferable to make it 0.0005% or more, but since the effect is saturated even if it exceeds 0.005%, the upper limit is made 0.005%.

  When adding Mg, it is preferable to set it as 0.001% or more, but since an effect will be saturated even if it exceeds 0.005%, 0.005% is made an upper limit.

DI * = 33.85 × (0.1 × C) ^0.5 × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2.16 × Cr + 1) × (3 × Mo + 1) × (1.75 × V + 1) × (1.5 × W + 1) (1)
Each element symbol is a content (mass%).

  This parameter: DI * (hardenability index) is specified in order to have excellent wear resistance with the matrix structure of the base material being martensite within the range of the component composition described above. 45 or more. When it is less than 45, the quenching depth from the plate thickness surface layer is less than 10 mm, and the life as a wear-resistant steel is shortened.

  When the value of this parameter exceeds 180, the matrix structure of the base material is martensite and wear resistance is good, but the low temperature toughness of the welded portion deteriorates, so it is preferable to set it to 180 or less. . More preferably, it is in the range of 50 to 160.

C + Mn / 4-Cr / 3 + 10P ≦ 0.47 (2)
Each element symbol is a content (mass%).

  When the base structure of the base material is martensite, and the component composition has excellent toughness in both the weld heat-affected zone bond and the low-temperature temper embrittlement zone when welding is performed, the components described above Within the range of the composition, the value of this parameter: C + Mn / 4-Cr / 3 + 10P is set to 0.47 or less. Even if it exceeds 0.47, the matrix structure of the base material is martensite and the wear resistance is good, but the toughness of the weld heat affected zone is significantly deteriorated. Preferably, it is 0.45 or less.

[Microstructure]
In the present invention, in order to improve the wear resistance, the matrix phase of the microstructure of the steel sheet is defined as martensite. It is preferable not to mix as much as possible the structure such as bainite and ferrite other than martensite because the wear resistance decreases. However, if the area fraction is less than 10%, the influence can be ignored. Further, when the surface hardness of the steel plate is less than 400 HBW 10/3000 in terms of Brinell hardness, the life as a wear-resistant steel is shortened. Therefore, it is desirable that the surface hardness is 400HBW10 / 3000 or more in terms of Brinell hardness.

  The microstructure of the bond part of the weld heat affected zone is a mixed structure of martensite and bainite. It is preferable not to mix as much as possible the structure of ferrite other than martensite and bainite because the wear resistance decreases. However, if the area fraction is less than 20%, the influence can be ignored.

Furthermore, in order to secure the toughness of the bond part of the weld heat affected zone, Nb and Ti carbonitrides with an average grain size of 1 μm or less are present at 1000 pieces / mm ² or more, and the average grain size of the prior austenite It is preferable that the average crystal grain size of the substructure surrounded by the large-angle grain boundaries having a diameter of less than 200 μm and an inclination angle of 15 ° or more is less than 70 μm.

  The wear resistant steel according to the present invention can be manufactured under the following manufacturing conditions. In the description, the “° C.” display relating to the temperature means a temperature at a half position of the plate thickness. It is preferable that the molten steel having the above composition is melted by a known melting method and used as a steel material such as a slab having a predetermined size by a continuous casting method or an ingot-bundling rolling method.

  Next, the obtained steel material is heated to 950 to 1250 ° C. immediately after cooling or after cooling, and then hot-rolled to obtain a steel plate having a desired thickness. Immediately after hot rolling, it is cooled with water or reheated for quenching. Then, tempering at 300 degrees C or less is implemented as needed.

  Steel slabs prepared in various compositions shown in Table 1 by the converter-ladder refining-continuous casting method were heated to 1000 to 1250 ° C. and then hot-rolled. The other steel plates were quenched (DQ), air cooled after rolling, and quenched after reheating (RQ).

  About the obtained steel sheet, surface hardness measurement, base metal toughness measurement, wear resistance evaluation, T-shaped fillet weld cracking test (delayed fracture resistance evaluation), welded part reproduction thermal cycle test, welded part toughness test of actual joint Was carried out as follows.

[Surface hardness 1]
The surface hardness was measured according to JIS Z2243 (1998), and the surface hardness under the surface layer (the surface hardness measured after removing the scale of the surface layer) was measured. The measurement used a tungsten hard sphere having a diameter of 10 mm, and the load was 3000 kgf.

[Base material toughness 1]
From the direction perpendicular to the rolling direction at a thickness of 1/4 of each steel plate, V-notch specimens were collected in accordance with JIS Z 2202 (1998), and conformed to JIS Z 2242 (1998). Then, each steel plate was subjected to a Charpy impact test at three temperatures, the absorbed energy at a test temperature of 0 ° C. was determined, and the base material toughness was evaluated. The test temperature of 0 ° C. was selected in consideration of use in a warm area.

The average value of the three absorbed energy at the test temperature of 0 ° C. (sometimes referred to as vE ₀ ) was 30 J or more, which was excellent in the base material toughness (within the scope of the present invention).

[Abrasion resistance 1]
The abrasion resistance was in accordance with ASTM G65, and a rubber wheel test was performed. The test piece is 10 mmt (t: plate thickness) x 75 mmw (w: width) x 20 mmL (L: length) (if the plate thickness is less than 10 mmt, t (plate thickness) x 75 mmw x 20 mmL). Performed using 100% SiO ₂ wear sand.

  The specimen weight before and after the test was measured, and the amount of wear was measured. The test results were evaluated based on the wear resistance ratio: (abrasion amount of mild steel plate) / (abrasion amount of each steel plate) with the wear amount of the mild steel plate (SS400) as a reference (1.0). The larger the wear resistance ratio, the better the wear resistance. In the scope of the present invention, the wear resistance ratio of 4.0 or more is excellent in wear resistance.

[Delayed destruction 1]
The T-shaped weld cracking test was conducted after subjecting a test body assembled into a T-shape as shown in FIG. 1 to be subjected to restraint welding by covering arc welding, and then preheating to room temperature (25 ° C. × 60% humidity) or 100 ° C. Test welding was performed.

  The welding method was covered arc welding (welding material: LB52UL (4.0 mmΦ)), heat input was 17 kJ / cm, and three-layer six-pass welding was performed. After welding, the sample was left at room temperature for 48 hours, and then a sample of the cross section of the welded portion of the test plate (bead length of 200 mm was divided into five equal parts) was sampled. Investigated with a microscope. With no preheating and 100 ° C. preheating, in each of the five cross-section samples taken, those having no cracking at the weld heat affected zone were evaluated as having excellent delayed fracture resistance.

[Weld toughness 1-1]
The welded part reproduction heat cycle test simulated each of the bond part and the low temperature temper embrittlement region of the welded heat affected zone when single layer carbon dioxide arc welding with a welding heat input of 17 kJ / cm was performed. The bond portion was simulated at 1400 ° C. for 1 second, and a cooling rate of 800 to 200 ° C. was set to 30 ° C./s. Moreover, simulation of the low temperature temper embrittlement area | region was hold | maintained at 300 degreeC for 1 second, and 300-100 degreeC was cooled at 5 degreeC / s.

  After giving the above-mentioned thermal cycle with a high frequency induction heating device to a square bar specimen taken from the rolling direction, a V-notch Charpy impact test was conducted according to JISZ2242 (1998). The V-notch Charpy impact test was performed with three test pieces for each steel plate at a test temperature of 0 ° C.

The average value of the three absorbed energy (vE ₀ ) of the bond portion of the weld heat affected zone and the low temperature temper embrittlement region was determined to be 30 J or more (within the range of the present invention).

[Weld toughness 1-2]
Furthermore, in order to confirm the toughness of the actual joint, bead-on-plate welding was performed on the steel plate by covered arc welding (heat input 17 kJ / cm, preheating 150 ° C., welding material: LB52UL (4.0 mmΦ)). A Charpy impact piece was taken from a position 1 mm below the surface, and a V-notch Charpy impact test was conducted according to JISZ2242 (1998) using the notch position as a bond. FIG. 2 shows the sampling position and notch position of the Charpy impact piece.

The V-notch Charpy impact test of the actual joint was performed with three test pieces at each test temperature at a test temperature of 0 ° C. The average value of the three absorbed energy (vE ₀ ) values of 30 J or more was determined to be excellent in the bond portion toughness of the weld heat affected zone (within the scope of the present invention).

  Table 2 shows the production conditions of the test steel sheets, and Table 3 shows the results of the above tests. Examples of the present invention (steel Nos. 1 to 5) have a surface hardness of 400 HBW 10/3000 or more, excellent wear resistance, a base metal toughness of 0 ° C. of 30 J or more, and a T-shaped fillet It was confirmed that no cracks were generated in the weld cracking test, and that excellent toughness was exhibited in the welded part thermal cycle test and the actual welded part, and the welded part toughness was excellent.

  On the other hand, comparative examples (steel Nos. 6 to 14) whose component composition is outside the scope of the present invention are surface hardness, wear resistance, T-shaped weld crack test, base metal toughness, reproducible thermal cycle Charpy impact test, actual joint Charpy. It was confirmed that one or more of the impact tests could not satisfy the target performance.

  A steel slab prepared in various compositions shown in Table 4 by a converter-ladder refining-continuous casting method was heated to 1000 to 1250 ° C. and then hot-rolled under the production conditions shown in Table 5. The steel sheets in the part were quenched (DQ) immediately after rolling, and the other steel sheets were air-cooled after rolling and quenched after reheating (RQ).

  About the obtained steel sheet, surface hardness measurement, base metal toughness measurement, wear resistance evaluation, T-shaped fillet weld cracking test (delayed fracture resistance evaluation), welded part reproduction thermal cycle test, welded part toughness test of actual joint Was carried out as follows.

[Surface hardness 2]
The surface hardness was measured according to JIS Z2243 (1998), and the surface hardness under the surface layer (the surface hardness measured after removing the scale of the surface layer) was measured. The measurement used a tungsten hard sphere having a diameter of 10 mm, and the load was 3000 kgf.

[Base material toughness 2]
From the direction perpendicular to the rolling direction at a thickness of 1/4 of each steel plate, V-notch specimens were collected in accordance with JIS Z 2202 (1998), and conformed to JIS Z 2242 (1998). Each steel plate was subjected to a Charpy impact test at three temperatures, the absorbed energy at test temperatures of 0 ° C. and −40 ° C. was determined, and the base material toughness was evaluated. A test temperature of 0 ° C. was selected in consideration of use in a warm region, and a test temperature of −40 ° C. was selected in consideration of use in a cold region.

The average value of the three absorbed energy at the test temperature of 0 ° C. (sometimes referred to as vE ₀ ) is 30 J or more, and the absorbed energy at the test temperature of −40 ° C. (sometimes referred to as vE- ₄₀ ). The average value of these three was determined to be 27 J or more with excellent base material toughness (within the scope of the present invention). For steel plates with a thickness of less than 10 mm, sub-size (5 mm × 10 mm) V-notch Charpy test pieces were collected, Charpy impact tests were performed, and the average value of three absorbed energy (vE ₀ ) was 15 J The average value of the three absorbed energy (vE- ₄₀ ) is 13J or more as described above, and the base material toughness is excellent (within the scope of the present invention).

[Abrasion resistance 2]
The abrasion resistance was in accordance with ASTM G65, and a rubber wheel test was performed. The test piece is 10 mmt (t: plate thickness) x 75 mmw (w: width) x 20 mmL (L: length) (if the plate thickness is less than 10 mmt, t (plate thickness) x 75 mmw x 20 mmL). Performed using 100% SiO ₂ wear sand.

  The specimen weight before and after the test was measured, and the amount of wear was measured. The test results were evaluated based on the wear resistance ratio: (abrasion amount of mild steel plate) / (abrasion amount of each steel plate) with the wear amount of the mild steel plate (SS400) as a reference (1.0). The larger the wear resistance ratio, the better the wear resistance. In the scope of the present invention, the wear resistance ratio of 4.0 or more is excellent in wear resistance.

[Delayed destruction 2]
The T-shaped weld cracking test was conducted after subjecting a test body assembled into a T-shape as shown in FIG. 1 to be subjected to restraint welding by covering arc welding, and then preheating to room temperature (25 ° C. × 60% humidity) or 100 ° C. Test welding was performed.

  The welding method was covered arc welding (welding material: LB52UL (4.0 mmΦ)), heat input was 17 kJ / cm, and three-layer six-pass welding was performed. After the test, the sample was left at room temperature for 48 hours, and then five samples of the welded plate cross-section observation sample (bead length 200 was divided into five equal parts) were collected. Investigated with a microscope. With no preheating and 100 ° C. preheating, in each of the five cross-section samples taken, those having no cracking at the weld heat affected zone were evaluated as having excellent delayed fracture resistance.

[Weld toughness 2-1]
The welded part reproduction heat cycle test simulated each of the bond part and the low temperature temper embrittlement region of the welded heat affected zone when single layer carbon dioxide arc welding with a welding heat input of 17 kJ / cm was performed. The bond portion was simulated at 1400 ° C. for 1 second, and a cooling rate of 800 to 200 ° C. was set to 30 ° C./s. Moreover, simulation of the low temperature temper embrittlement area | region was hold | maintained at 300 degreeC for 1 second, and 300-100 degreeC was cooled at 5 degreeC / s.

  After giving the above-mentioned thermal cycle with a high frequency induction heating device to a square bar specimen taken from the rolling direction, a V-notch Charpy impact test was conducted according to JISZ2242 (1998). The V-notch Charpy impact test was conducted with test pieces at three temperatures for each steel sheet at test temperatures of 0 ° C. and −40 ° C.

Welding the average value of the three absorbed energy (vE ₀ ) of the bond portion of the weld heat affected zone and the low temperature temper embrittlement region is 30 J or more and the average value of the three absorbed energy (vE ₋₄₀ ) is 27 J or more. The material was excellent in toughness (within the scope of the present invention).

For steel plates with a thickness of less than 10 mm, sub-size (5 mm × 10 mm) V-notch Charpy test pieces were collected, Charpy impact tests were conducted, and the weld heat affected zone bond and low temperature temper embrittlement regions The average value of the three absorbed energy (vE ₀ ) was 15 J or more and the average value of the three absorbed energy (vE ₋₄₀ ) was 13 J or more was determined to be excellent in weld zone toughness (within the scope of the present invention).

[Weld zone toughness 2-2]
Furthermore, in order to confirm the toughness of the actual joint, bead-on-plate welding was performed on the steel plate by covered arc welding (heat input 17 kJ / cm, preheating 150 ° C., welding material: LB52UL (4.0 mmΦ)). A Charpy impact piece was taken from a position 1 mm below the surface, and a V-notch Charpy impact test was conducted according to JISZ2242 (1998) using the notch position as a bond. FIG. 2 shows the sampling position and notch position of the Charpy impact piece.

The V-notch Charpy impact test of the actual joint was performed with three test pieces at each test temperature with the test temperature being 0 ° C. and −40 ° C. The average value of the three absorbed energy (vE ₀ ) is 30 J or more and the average value of the three absorbed energy (vE ₋₄₀ ) is 27 J or more, which is excellent in the bond part toughness of the weld heat affected zone (range of the present invention) Inside).

For steel sheets with a thickness of less than 10 mm, sub-size (5 mm × 10 mm) V-notch Charpy test pieces were collected, Charpy impact tests were performed, and the average value of three absorbed energy (vE ₀ ) was 15 J The average value of the three absorbed energy (vE _-40 ) values of 13 J or more was determined to be excellent in the bond portion toughness of the weld heat affected zone (within the scope of the present invention).

  Table 5 shows the production conditions of the test steel sheets, and Table 6 shows the results of the above tests. Examples of the present invention (steel Nos. 15 to 17 (No. 17 is a plate thickness of 8 mm)) have a surface hardness of 400 HBW 10/3000 or more, excellent wear resistance, and a base metal toughness of 0 ° C. of 30 J or more. In addition, the toughness of the base metal at −40 ° C. is 27 J or more, and further, no crack is generated in the T-shaped fillet weld cracking test, and it is excellent in the welded part thermal cycle test and the actual welded part. It was confirmed that the toughness was excellent and the weld zone toughness was excellent.

  On the other hand, the component composition is within the scope of the present invention, but the steel No. In the case of No. 18, the surface hardness, wear resistance, base metal toughness, and T-shaped weld cracking test are good, but the target performance is the reproducible thermal cycle Charpy impact test and actual joint Charpy impact test corresponding to the low temperature temper embrittlement region. It was confirmed that the low temperature toughness of the welded part was inferior to that of the other invention examples.

  Steel No. No. 19, among the component compositions, since Si is outside the scope of the present invention, the surface hardness, wear resistance, and base metal toughness are good, but the toughness in the temper embrittlement region of the weld heat affected zone deteriorates, and T The shape weld crack test, the reproducible thermal cycle Charpy impact test corresponding to the low temperature temper embrittlement region, and the actual joint Charpy impact test did not satisfy the target performance.

Steel No. Although the composition of the component 20 is within the range of the present invention, the equation (2) exceeds 0.47. Therefore, vE- ₄₀ is the lower limit of the target performance of the present invention in both the reproducible thermal cycle Charpy impact test and the actual joint Charpy impact test. It was confirmed that it was inferior to other invention examples. In Tables 4, 5, and 6, steel No. 18 and 20 are comparative examples because the component composition is within the scope of the present invention of claim 3, but the value of DI and the formula (2) is outside the scope of the present invention of claims 6 and 7.

Claims (5)

In mass%, C: 0.20 to 0.30%, Si: 0.05 to 0.5%, Mn: 0.40 to 1.2%, P: 0.010% or less, S: 0.005 %: Cr: 0.40-1.5%, Nb: 0.005-0.025%, Ti: 0.005-0.03%, Al: 0.1% or less, N: 0.01% The DI * in the formula (1) is 45 or more and 180 or less , the formula (2) is satisfied, the composition is composed of the remaining Fe and inevitable impurities, and the microstructure is based on martensite. A wear-resistant steel sheet with excellent weld toughness and delayed fracture resistance.
DI * = 33.85 × (0.1 × C) ^0.5 × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2.16 × Cr + 1) × (3 × Mo + 1) × (1.75 × V + 1) × (1.5 × W + 1) (1)
In the formula (1), each element symbol is a content (mass%).
C + Mn / 4-Cr / 3 + 10P ≦ 0.47 (2)
In the formula (2), each element symbol is a content (mass%). The steel composition contains, in mass%, C: 0.20 to 0.283%, S: 0.0025% or less , Cr: 0.49 to 1.5% , and Mo: 0.05 to 1 The weld toughness and resistance according to claim 1, characterized by containing one or more of 0.0%, W: 0.05 to 1.0%, and B: 0.0003 to 0.0030%. Wear-resistant steel plate with excellent delayed fracture characteristics.   The steel composition contains, in mass%, C: 0.20 to 0.283%, S: 0.0025% or less, Cu: 1.5% or less, Ni: 2.0% or less, V: The wear-resistant steel sheet having excellent weld toughness and delayed fracture resistance according to claim 1 or 2, characterized by containing one or more of 0.1% or less.   The steel composition contains, in mass%, C: 0.20 to 0.283%, S: 0.0025% or less, REM: 0.008% or less, Ca: 0.005% or less, Mg: The wear-resistant steel sheet having excellent weld toughness and delayed fracture resistance according to any one of claims 1 to 3, characterized by containing one or more of 0.005% or less.   The wear-resistant steel plate having excellent weld toughness and delayed fracture resistance according to any one of claims 1 to 4, wherein the surface hardness is Brinell hardness of 400HBW10 / 3000 or more.