JP2012031511A - Wear-resistant steel sheet having excellent toughness of multi-layer-welded part and lagging destruction resistance properties - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an wear-resistant steel sheet excellent in multilayer weld zone toughness and delayed fracture resistance.SOLUTION: The steel sheet contains in terms of mass%, 0.20-0.30% C, 0.05-1.0% Si, 0.40-1.2% Mn, P, S, 0.40-1.5% Cr, 0.005-0.025% Nb, 0.05-1.0% Mo, 0.005-0.03% Ti, ≤0.1% Al, ≤0.01% N and 0.0003-0.0020% B, and contains as the need arises one or two or more kinds of W, Cu, Ni, V, REM, Ca and Mg, wherein, in a microstructure, martensite is used as a base phase, under the condition of DI*(=33.85×(0.1×C)×(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)):45-180, with a relation, C+Mn/4-Cr/3+10P≤0.47.

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, in multi-layer weld weld toughness and delayed fracture resistance. Concerning excellent ones.

  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 sheets are generally sensitive to cold cracking and have poor weld toughness, so 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 to be. 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, after assembling the welded structure, the manufacturing method in which the wear-resistant steel plates are bonded together increases manufacturing effort and manufacturing cost, so that it is excellent in weldability and weld part toughness applicable as a strength member of the welded structure. For example, the wear-resistant steel sheets of Patent Documents 1 to 5 have been proposed.

  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 inner part is martensite. A steel material that defines the elongation of prior austenite grains expressed by the ratio of the prior austenite grain size in the rolling direction to the prior austenite grain size in the central part of the sheet thickness in a mixed structure of lower bainite or lower bainite single phase structure, and It is described that a steel having a component composition is directly quenched after hot rolling at 900 ° C. or less and a cumulative reduction ratio 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 has 4 to 9 mass% of Cr as an essential element, and one or two of Cu and Ni. And a steel satisfying a parameter formula composed of a specific component, and a steel of the component composition is subjected to a reheating quenching treatment after hot rolling at a cumulative reduction of 30% or more at 950 ° C. or less. .

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, in the case of a welded joint made of a steel plate having a plate thickness of 4 mm or more, multi-layer welding is often performed, but in the welded portion, the bond portion due to the preceding welding pass is reheated by subsequent welding, and the toughness is remarkable. In particular, in a wear-resistant steel sheet, when the bond part of the first layer is reheated to around 300 ° C. by subsequent welding, the toughness is significantly deteriorated by low temperature temper embrittlement. To do.

  Low temperature temper embrittlement is thought to be due to the synergistic effect of carbide morphology change in martensite and grain boundary segregation of impurity elements, etc., in the bond part where the crystal grains are coarse and contain a large amount of solute N Becomes prominent. It has also been pointed out that delayed fracture tends to occur in the region reheated to such a low temperature temper embrittlement temperature.

  Patent Documents 1 and 2 do not describe improving the toughness of the welded portion 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. 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 multi-layer welds.

  Therefore, an object of the present invention is to provide a wear-resistant steel plate that is excellent in toughness and delayed fracture resistance of a multi-layer weld without causing a decrease in productivity and an increase in manufacturing cost.

  In order to achieve the above-mentioned problems, the inventors of the present invention determine the chemical composition, manufacturing method and microstructure of a steel sheet in order to ensure the toughness and delayed fracture resistance of a multi-layer welded part for wear-resistant steel sheets. We conducted earnest research on various factors and obtained the following findings.

  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 multilayer weld weld toughness, it is necessary to suppress the coarsening of crystal grains in the weld heat affected zone. For this purpose, fine precipitates are dispersed in the steel sheet, and the pinning effect It is effective to utilize For this purpose, management of Ti and N is important.

  3. Reducing the solid solution N in the bond portion of the first layer is effective in suppressing low temperature temper embrittlement due to subsequent welding. For this purpose, it is important to strictly manage B in order to fix solute N as BN.

  4). In order to secure 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 alloy elements such as C, Mn, Cr, Mo, P It is.

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 to 1.5%, Mo: 0.05 to 1.0%, Nb: 0.005 to 0.025%, Ti: 0.005 to 0.03%, Al: 0 0.1% or less, N: 0.0015 to 0.0060%, B: 0.0003 to 0.0020%, DI * represented by the formula (1) is 45 or more, the remainder Fe and inevitable impurities A wear-resistant steel sheet having a composition comprising: a multi-layer welded toughness with a microstructure of martensite as a base phase, and excellent 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%).
2. 2. The wear-resistant steel sheet having excellent multilayer toughness and delayed fracture resistance according to 1, wherein the steel composition further contains W: 0.05 to 1.0% in mass%.
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 sheet with excellent multilayer weld 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%. A wear-resistant steel sheet having excellent multilayer weld weld toughness and delayed fracture resistance according to any one of the above.
5. The wear-resistant steel sheet having excellent multi-layer welded portion toughness and delayed fracture resistance as described in any one of 1 to 4 having a surface hardness of 400 HBW 10/3000 or more in terms of Brinell hardness.
6. A steel sheet according to any one of items 6.1 to 5, wherein the hardenability index DI * is 180 or less, and the multi-layer weld weld toughness and delayed fracture resistance are excellent.
A steel plate according to any one of 7.1 to 6, a wear-resistant steel plate excellent in multi-layer welded portion toughness and delayed fracture resistance satisfying 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 toughness and delayed fracture resistance in multi-layer welds can be obtained, which greatly contributes to the improvement of manufacturing efficiency and safety at the time of manufacturing a steel structure. 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, when the content exceeds 0.30%, not only the weldability deteriorates, but also the toughness deteriorates due to the low temperature tempering of the bond portion in the multi-layer welded portion. 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 due to low-temperature tempering of the bond part in the multilayer welded part. 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 multi-layer welded portion is remarkably deteriorated, so the content is limited to the range of 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 multilayer weld. 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 the steel, and also has the effect of suppressing toughness deterioration due to low-temperature tempering of the bond portion in the multilayer weld 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%.

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. Furthermore, it has the effect of suppressing toughness deterioration due to low-temperature tempering of the bond part in the multilayer welded part. In order to obtain such an effect, the content is made 0.05% or more. However, if it exceeds 1.0%, the base material toughness, ductility and weld crack resistance are adversely affected, so 1.0% or less. For this reason, it limits to 0.05 to 1.0% of range. Preferably, it is 0.1 to 0.8%.

Nb: 0.005 to 0.025%
Nb precipitates as carbonitride, refines the microstructure of the base metal and the multi-layer weld, fixes solid solution N, and improves the toughness of the multi-layer weld 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 portion of the multi-layer weld by fixing the solid solution N to form TiN, and toughness deterioration in the low temperature tempering temperature region due to the reduction of the 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 portion of the multi-layer welded portion, 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: 0.0015 to 0.0060%
N combines with Ti and precipitates as TiN, which suppresses the coarsening of austenite grains in the HAZ and contributes to high toughness. In order to secure the necessary amount of TiN having such an effect, it is necessary to contain 0.0015% or more of N. On the other hand, if the content exceeds 0.0060%, in the region heated to a temperature at which TiN dissolves during welding, the amount of solute N increases, and the toughness deterioration in the low temperature tempering temperature range becomes significant. For this reason, N is limited to 0.0015 to 0.0060%. Preferably, it is 0.0020 to 0.0055%.

B: 0.0003 to 0.0020%
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. Furthermore, in the region heated to a temperature at which TiN dissolves at the time of welding, solid solution N is fixed as BN and has an effect of suppressing toughness deterioration in a low temperature tempering temperature region due to subsequent welding. In order to obtain such an effect, the content is preferably 0.0003% or more. However, if it exceeds 0.0020%, the base material toughness, ductility, and weld crack resistance are adversely affected. The following. Preferably, it is 0.0005 to 0.0018%. The balance is Fe and inevitable impurities.

  In the present invention, in order to further improve the characteristics, one or more of W, Cu, Ni, V, REM, Ca and Mg can be contained in addition to the basic component system.

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.

  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 bond part and the low temperature temper embrittlement region when welding is performed, within the range of the component composition described above. 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 has good wear resistance, but the toughness of the welded portion 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.

  In the developed steel according to the present invention, the microstructure of the bond portion 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 the developed steel according to the present invention, in order to ensure the toughness of the bond portion, Nb and Ti carbonitrides having an average particle size of 1 μm or less are 1000 pieces / mm ² or more, and the former austenite It is preferable that the average crystal grain size is less than 200 μm, and the average crystal grain size of the substructure surrounded by the large-angle grain boundaries having 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, if necessary, tempering at 300 ° C or lower is performed.

  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 layers and six passes were welded. After the test, the test plate was left for 48 hours at room temperature. Weld section cross-sectional observation samples (bead length of 200 mm divided into 5 equal parts) were collected and examined for occurrence of cracks in the weld heat affected zone with a projector and an optical microscope. In each of the collected five cross-sectional samples, those having no cracks in the weld heat affected zone were evaluated as having excellent delayed fracture resistance.

[Weld toughness 1-1]
The welding reproduction heat cycle test simulated low temperature tempering of the bond portion of the weld heat affected zone when two-layer carbon dioxide arc welding with a welding heat input of 17 kJ / cm was performed. The bond part of 1 layer welding (first layer) is held at 1400 ° C. for 1 second, the cooling rate of 800 to 200 ° C. is set to 30 ° C./s, and then low temperature tempering by 2 layer welding (subsequent welding) is performed at 300 ° C. Was held for 1 second, and a heat cycle was performed in which 300 to 100 ° C was cooled at 5 ° C / 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 Charpy impact test was performed with three test pieces at each temperature for each steel plate at a test temperature of 0 ° C.

An average value of three absorbed energy (vE ₀ ) of 30 J or more was determined to be excellent in multi-layer welded portion toughness (within the scope of the present invention).

[Weld toughness 1-2]
Furthermore, in order to confirm the toughness of the actual joint, a multi-layer welded joint (lamination groove) is used by covering arc welding (heat input 17 kJ / cm, preheating 150 ° C., interpass temperature 150 ° C., welding material: LB52UL (4.0 mmΦ)) ) Was produced.

  From the welded joint, a Charpy impact test piece was taken from a position 1 mm below the surface. The notch position was a bond on the groove side perpendicular to the steel sheet surface at the groove. Using the test piece thus collected, a V-notch Charpy impact test was conducted according to JISZ2242 (1998). FIG. 2 shows the sampling position and notch position of the Charpy impact test piece.

The V-notch Charpy impact test of the actual joint was performed with three test pieces at a test temperature of 0 ° C. An average value of three absorbed energy (vE ₀ ) of 30 J or more was determined to be excellent in multi-layer welded portion toughness (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 cracks did not occur in the weld cracking test, and that the toughness was excellent in the welded part thermal cycle test and the actual welded joint toughness, and was excellent in multi-layer welded part toughness.

  On the other hand, comparative examples (Nos. 6 to 19) whose component composition is outside the scope of the present invention are surface hardness, wear resistance, T-shaped weld crack test, base material toughness, reproducible thermal cycle Charpy impact test, actual joint Charpy impact. It was confirmed that one or more of the tests could not meet the target performance.

  Steel slabs prepared in various compositions shown in Table 4 by the converter-ladder refining-continuous casting method were heated to 1000 to 1250 ° C. and then hot-rolled, and some steel plates were immediately after rolling. 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 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 layers and six passes were welded. After the test, the test plate was left for 48 hours at room temperature. Weld section cross-sectional observation samples (bead length of 200 mm divided into 5 equal parts) were collected and examined for occurrence of cracks in the weld heat affected zone with a projector and an optical microscope. In each of the collected five cross-sectional samples, those having no cracks in the weld heat affected zone were evaluated as having excellent delayed fracture resistance.

[Weld toughness 2-1]
The welding reproduction heat cycle test simulated low temperature tempering of the bond portion of the weld heat affected zone when two-layer carbon dioxide arc welding with a welding heat input of 17 kJ / cm was performed. The bond part of 1 layer welding (first layer) is held at 1400 ° C. for 1 second, the cooling rate of 800 to 200 ° C. is set to 30 ° C./s, and then low temperature tempering by 2 layer welding (subsequent welding) is performed at 300 ° C. Was held for 1 second, and a heat cycle was performed in which 300 to 100 ° C was cooled at 5 ° C / 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 Charpy impact test was performed with three test pieces at each temperature for each steel plate at test temperatures of 0 ° C and -40 ° C.

An average value of three absorbed energy (vE ₀ ) of 30 J or more and an average value of three absorbed energy (vE ₋₄₀ ) of 27 J or more are excellent in multi-layer welded portion toughness (within the scope of the present invention) did.

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 multi-layer welded portion toughness (within the scope of the present invention).

[Weld zone toughness 2-2]
Furthermore, in order to confirm the toughness of the actual joint, a multi-layer welded joint (lamination groove) is used by covering arc welding (heat input 17 kJ / cm, preheating 150 ° C., interpass temperature 150 ° C., welding material: LB52UL (4.0 mmΦ)) ) Was produced.

  From the welded joint, a Charpy impact test piece was taken from a position 1 mm below the surface. The notch position was a bond on the groove side perpendicular to the steel sheet surface at the groove. Using the test piece thus collected, a V-notch Charpy impact test was conducted according to JISZ2242 (1998). FIG. 2 shows the sampling position and notch position of the Charpy impact test 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. An average value of three absorbed energy (vE ₀ ) of 30 J or more and an average value of three absorbed energy (vE ₋₄₀ ) of 27 J or more are excellent in multi-layer welded portion toughness (within the scope of the present invention) did.

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 multi-layer welded portion toughness (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 No. 20 to 22 (where No. 22 is a plate thickness of 8 mm)) has a surface hardness of 400 HBW 10/3000 or more, excellent wear resistance, and a base material toughness of 0 ° C. is 30 J. In addition, the base metal toughness at −40 ° C. is 27 J or more, and further, no cracks are generated in the T-shaped fillet weld cracking test. It has also been confirmed that it has excellent toughness and is excellent in toughness of multi-layer welds.

  On the other hand, the component composition is within the scope of the present invention, but the steel No. 23, the surface hardness, wear resistance, base metal toughness, and T-shaped weld cracking test are good, but the welded portion reproducible thermal cycle test and actual weld joint toughness are close to the lower limit of the target performance, and other invention examples It was inferior compared with. Steel No. No. 24, among the component compositions, Si is outside the scope of the present invention, so the surface hardness, wear resistance, and base metal toughness are good, but the T-type weld cracking test, welded part reproduction thermal cycle test results and actual welding Joint toughness could not meet the target performance.

  Steel No. 25, the component composition is within the range of the present invention, but the value of the parameter on the left side of the formula (2): C + Mn / 4-Cr / 3 + 10P exceeds 0.47. The toughness was close to the lower limit of the target performance and was inferior compared to other invention examples. In Tables 4, 5, and 6, steel No. No. 23 is a comparative example because the component composition is within the range of the present invention of claim 3, but the value of DI is outside the range of the present invention of claim 6. Steel No. The component composition of No. 25 is within the scope of the present invention of claim 1, but does not satisfy the formula (2), and is outside the scope of the present invention of claim 7.

Claims (7)

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 to 1.5%, Mo: 0.05 to 1.0%, Nb: 0.005 to 0.025%, Ti: 0.005 to 0.03%, Al: 0 0.1% or less, N: 0.0015 to 0.0060%, B: 0.0003 to 0.0020%, DI * represented by the formula (1) is 45 or more, the remainder Fe and inevitable impurities A wear-resistant steel sheet having a composition comprising: a multi-layer welded toughness with a microstructure of martensite as a base phase, and excellent 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%).   The wear-resistant steel sheet having excellent multilayer toughness and delayed fracture resistance according to claim 1, wherein the steel composition further contains W: 0.05 to 1.0% in mass%.   The steel composition further includes one or more of Cu: 1.5% or less, Ni: 2.0% or less, and V: 0.1% or less in mass%. Or the wear-resistant steel plate excellent in the multilayer welded part toughness and delayed fracture resistance of 2 description.   The steel composition further includes one or more of REM: 0.008% or less, Ca: 0.005% or less, and Mg: 0.005% or less in mass%. 4. A wear-resistant steel sheet having excellent multi-layer welded toughness and delayed fracture resistance as described in any one of 1 to 3.   5. A wear-resistant steel sheet having excellent multilayer toughness and delayed fracture resistance according to any one of claims 1 to 4, having a surface hardness of Brinell hardness of 400 HBW 10/3000 or more.   A steel plate according to any one of claims 1 to 5, wherein the hardenability index DI * is 180 or less and the multi-layer welded portion toughness and delayed fracture resistance are excellent. A steel plate according to any one of claims 1 to 6, which is a wear-resistant steel plate excellent in multilayer weld 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%).