JP5145805B2 - Wear-resistant steel plate with excellent gas cut surface properties and low-temperature tempering embrittlement cracking resistance - Google Patents

Description

  The present invention is a wear-resistant steel sheet that is excellent in preventing cracks generated in a region reheated to a low temperature temper embrittlement temperature range in a heat affected zone after welding or a heat affected zone after thermal fusing such as gas cutting or plasma cutting. In particular, the present invention relates to an industrial machine, a transport device, or the like that is suitable for a member manufactured by being subjected to processing such as welding or fusing.

  The wear resistance of steel materials is improved by increasing the hardness, and wear resistant steel plates that have been subjected to quenching heat treatment or quenching-low temperature tempering heat treatment are used for members that require wear resistance. Has been.

  However, when hardened steel materials such as wear-resistant steel produced by quenching heat treatment or quenching-low temperature tempering heat treatment are reheated to a low temperature temper embrittlement temperature range of about 300 to 400 ° C, delayed fracture occurs after cooling to room temperature. May occur. If there is a portion reheated in the low temperature temper embrittlement temperature region by thermal fusing such as welding, gas cutting or plasma cutting, delayed fracture is likely to occur after cooling to room temperature.

  When using wear-resistant steel, processing such as welding and gas cutting is often performed, and it is reheated to the low temperature temper embrittlement temperature range in the vicinity of the gas cutting surface and in the heat affected zone. It is impossible to avoid this, and there is a demand for wear-resistant steel that is excellent in preventing the occurrence of the above-described delayed cracking.

The wear-resistant steel excellent in delayed fracture resistance proposed in Patent Documents 1 to 6 is an as-manufactured steel sheet that improves delayed fracture resistance and is reheated to a low temperature temper embrittlement temperature range. It does not improve the delayed fracture characteristics.
Japanese Patent Application Laid-Open No. 2002-115024 JP 2002-80930 A Japanese Patent Laid-Open No. 05-51691 Japanese Patent Laid-Open No. 01-255622 Japanese Unexamined Patent Publication No. Sho 63-317623 JP 2003-171730 A

  Therefore, the present invention provides a wear-resistant steel sheet having excellent low-temperature tempering embrittlement cracking resistance in which delayed fracture after cooling to room temperature is suppressed in a region heated to a low-temperature tempering embrittlement temperature region due to the thermal effect of welding or fusing. The purpose is to provide.

  In order to achieve the above-mentioned object, the inventors diligently studied various factors affecting the low-temperature tempering embrittlement cracking property in wear-resistant steel, adding 0.20 to 0.30% of C, Mn 1.2% or less, P is 0.010% or less, and the prior austenite grain size is 30 μm or less, thereby significantly improving the low temperature temper embrittlement cracking resistance property while ensuring excellent wear resistance. Found that is possible.

  Moreover, it discovered that gas cut surface property improved by making Mo and W less than 0.05%.

Low temperature tempering embrittlement cracking resistance is Mass%, 0.23% C-0.25% Si-0.2-1.5% Mn-0.003-0.030% P-0.02% Nb A steel slab containing −0.01% Ti−0.001% B, Cu, Ni, Cr added as appropriate and Ceq * constant at 0.50% was used as a test steel, rolled to 32 mmt, and then air-cooled Then, after reheating to 900 ° C., quenching was performed, and the obtained steel sheet was subjected to a T-shaped fillet weld cracking test shown in FIG. The welding method was covered arc welding and heat input was 17 kJ / cm, and three-layer six-pass welding was performed.

FIG. 2 shows the results of arranging the T-shaped fillet weld cracking test results by Mn amount and P amount. If the Mn content exceeds 1.2% or the P content exceeds 0.010%, cracking occurs. The prior austenite grain sizes of these steel sheets were all less than 30 μm, and the form of cracking was a grain boundary fracture surface.

The present invention has been completed by further investigation based on the obtained knowledge. That is, the gist of the present invention is as follows.
1. In mass%, C: 0.20 to 0.30%, Si: 0.05 to 0.40 %, Mn: 0.45 to 1.2%, Cr: 0.1 to 1.0% , Nb: 0.005 to 0.024%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0030%, Al: 0.1% or less, P: 0.010% or less, S: 0 0.005% or less, Mo: less than 0.05%, W: less than 0.05%, Cu: 0.1 to 1.0%, Ni: 0.1 to 2.0%, Cr: 0.00. 1 to 1.0%, V: 0.005 to 0.10%, or one or more of Ceq * represented by the formula (1) is 0.55% or less, and the formula (2) in shown by DI * is 50 or more, having a composition the balance being Fe and unavoidable impurities, and, to a base phase microstructure, the austenite grain size less martensite 30μm Excellent abrasion steel plate gas cutting surface properties and low-temperature tempering embrittlement cracking properties.
Ceq * = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + W / 10
(1)
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) (2)
The composition described in 2.1 further contains one or two of Ca: 0.0005 to 0.0050% and REM: 0.0005 to 0.0050%, and Ceq * represented by the formula (1) Is 0.55% or less, DI * represented by the formula (2) is 50 or more, has a composition composed of the balance Fe and inevitable impurities, and has a microstructure with a prior austenite grain size of 30 μm or less. A wear-resistant steel sheet with excellent gas-cut surface properties and low-temperature tempering embrittlement cracking resistance.
Ceq * = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + W / 10
(1)
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) (2)
3. 3. A wear-resistant steel sheet excellent in gas cut surface properties and low-temperature temper embrittlement cracking resistance according to 1 or 2, wherein the surface hardness is Brinell hardness of 400 HBW 10/3000 or more.

  According to the present invention, it is possible to produce a wear-resistant steel sheet that is excellent in gas cut surface properties and excellent in suppressing delayed fracture in a region that has been tempered at a low temperature due to the thermal effect of welding or gas cutting. There is an effect.

  First, the reason for defining the composition of the steel sheet of the present invention will be described. In addition, all the following% display is represented by the mass%.

C: 0.20 to 0.30%
C is an element that increases the matrix hardness and improves the wear resistance. In order to remarkably improve the wear resistance, addition of 0.20% or more is necessary. On the other hand, if added over 0.30%, weldability deteriorates. Therefore, it was set to 0.20 to 0.30%. In addition, Preferably, it is 0.20 to 0.28%.

Si: 0.05-1.0%
Si is an effective element as a deoxidizing element, and in order to obtain such an effect, the content of 0.05% or more is required. Si is an effective element that contributes to high hardness by solid solution strengthening by solid solution in steel. However, if the content exceeds 1.0%, ductility and toughness are reduced, and the amount of inclusions is further increased. . For this reason, Si is limited to the range of 0.05 to 1.0%. In addition, More preferably, it is 0.05 to 0.40%.

Mn: 0.45 to 1.2%
Mn promotes the grain boundary segregation of P and makes delayed fracture easier to occur. When the prior austenite grain size of the microstructure is 30 μm or less, the occurrence of intergranular fracture can be suppressed by setting it to 1.2% or less.

  On the other hand, in order to ensure hardenability, it is desirable to add a certain amount, and also from the viewpoint of alloy cost reduction, addition of Mn is desirable, and the amount of Mn is limited to a range of 0.45 to 1.2%. .

Al: 0.1% or less Al is added as a deoxidizer, and the effect is recognized with a content of 0.0020% or more, but a large content exceeding 0.1% lowers the cleanliness of steel. Let For this reason, Al is made 0.1% or less.

Nb: 0.005 to 0.024%
Nb precipitates as carbonitrides or carbides, refines the structure, and has the effect of suppressing the occurrence of delayed fracture. In order to obtain the effect, 0.005% or more is necessary. If added over 0.024%, coarse carbonitride precipitates and may become the starting point of fracture, so the content is made 0.024% or less.

Ti: 0.005% to 0.05%
Ti has the effect of suppressing BN precipitation and promoting the effect of improving the hardenability of B by fixing N. In order to obtain the effect, addition of 0.005% or more is necessary. On the other hand, if added over 0.05%, TiC is precipitated and the base metal toughness is deteriorated, so the content is made 0.005 to 0.05%.

B: 0.0003% to 0.0030%
B significantly improves the hardenability by adding a small amount. In order to obtain the effect, 0.0003% or more is necessary. On the other hand, if adding over 0.0030%, weldability deteriorates, so 0.0003% to 0.0030%.

P: 0.010% or less P is segregated at the grain boundary and becomes the starting point of delayed fracture occurrence, so is 0.010% or less.

S: 0.005% or less Since S forms MnS and serves as a starting point of fracture, it is made 0.005% or less.

Mo: less than 0.05%, W: less than 0.05% Mo and W form a high melting point oxide during gas cutting and deteriorate the fluidity of the molten iron, so the gas cutting surface properties deteriorate. Therefore, it is desirable not to add, and each is limited to less than 0.05%. It is preferable that neither Mo nor W is contained.

Cu: 0.1 to 1.0%, Ni: 0.1 to 2.0%, Cr: 0.1 to 1.0%, V: 0.005 to 0.10%, 1 type or 2 More than seed Cu: 0.1-1.0%
Cu is an element that improves hardenability by solid solution, and needs to contain 0.1% or more in order to obtain this effect. On the other hand, the content exceeding 1.0% decreases the hot workability. For this reason, when adding Cu, it is preferable to limit to 0.1 to 1.0% of range. In addition, More preferably, it is 0.1 to 0.5%.

Ni: 0.1 to 2.0%
Ni is an element that improves hardenability by solid solution, and such an effect becomes remarkable when the content is 0.1% or more. On the other hand, the content exceeding 2.0% significantly increases the material cost. For this reason, when adding Ni, it is preferable to limit to 0.1 to 2.0% of range. In addition, More preferably, it is 0.1 to 1.0%.

Cr: 0.1 to 1.0%
Cr has an effect of improving hardenability, and in order to obtain such an effect, the content of 0.1% or more is required. However, the content exceeding 1.0 % lowers the weldability. . Therefore, when adding Cr, it is preferable to limit to 0.1 to 1.0% of range. In addition, More preferably, it is 0.1 to 0.80%.

V: 0.005-0.10%
V is an element that improves hardenability. In order to obtain such an effect, 0.005% or more is necessary, but if it exceeds 0.10%, the weldability is lowered. Therefore, V is preferably limited to a range of 0.005 to 0.10%.

Ca, REM: 0.0005 to 0.0050%
Ca and REM bind to S and suppress MnS generation. In order to obtain this effect, 0.0005% or more is necessary. However, if it exceeds 0.0050%, the cleanliness of the steel is deteriorated. Therefore, Ca and REM are 0.0005 to 0.0050%.

Ceq * (= C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + W / 10): 0.55% or less When Ceq * exceeds 0.55%, weldability deteriorates. Therefore, it was made into 0.55% or less.

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)): 45 or more When DI * 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. Therefore, DI * is 45 or more.

When the surface hardness is Brinell hardness of 400 HBW 10/3000 or more, when the surface hardness is Brinell hardness of less than 400 HBW 10/3000, the life of the wear resistant steel is shortened. Therefore, it is desirable that the surface hardness is 400HBW10 / 3000 or more in terms of Brinell hardness. Next, a manufacturing method will be described.
[Production method]
The wear-resistant steel sheet according to the present invention is made by melting the molten steel having the above composition by a known melting method, and using a continuous casting method or an ingot-bundling rolling method as a steel material such as a slab having a predetermined size. Is preferred.

  Subsequently, the obtained steel material is reheated to 950 to 1250 ° C. immediately after cooling or after cooling, and then hot-rolled to obtain a steel plate having a desired plate thickness (wall thickness). Quenching is performed immediately after hot rolling or by reheating. Tempering at 300 ° C. or lower is performed as necessary.

  Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace to obtain a small steel ingot (150 kg) (steel material). These steel materials are heated to 1050 to 1250 ° C. and then hot-rolled to a thickness of 6 to 32 mm. Some steel plates are quenched immediately after rolling (DQ), and other steel plates are rolled. After air cooling, quenching (RQ) was performed after reheating to 900 ° C.

About the obtained steel plate, the old austenite particle size measurement, the surface hardness measurement, and the T-shaped fillet weld cracking test were implemented as follows.
[Old austenite particle size measurement]
The particle diameter in the 1/4 * t (plate thickness) part of the obtained steel plate was measured. Observation was performed with an optical microscope at a magnification of 400 times, the equivalent circular particle diameter of each of the 100 prior austenite grains was measured, and the average value thereof was defined as the prior austenite grain diameter.
[Surface hardness measurement]
In accordance with JIS standard Z2243 (1998), the surface hardness under the surface layer was measured. The measurement is
A 10MM tungsten hard ball was used, and the load was 3000 kgf.
[T-shaped fillet weld cracking test]
It implemented as FIG. The welding method is covering arc welding, heat input 17 kJ / cm, 3
Layer 6 passes are welded and the test weld length is 200 mm.
[Gas cutting surface property survey]
The roughness of the cut surface was evaluated with a grade based on WES2801. The gas cutting conditions were as follows.

Cutting oxygen pressure: 5 kgf / mm ² , propane pressure: 4 kgf / mm ² , cutting speed: 350 mm / min. The obtained results are shown in Table 2.

  The inventive examples (steel plates Nos. 1, 2, 3, and 4) have a surface hardness of 400 HBW 10/3000 or more, do not generate cracks in the T-shaped fillet weld crack test, and have a gas cut surface grade. Has excellent gas cutting surface properties of 1 grade.

  On the other hand, in the comparative examples (steel plates No. 5, 6, 7, 8, 9), cracks occurred in the T-shaped fillet weld cracking test, or the gas cut surface grade was 2 grade, compared with the present invention example. Inferior to gas cut surface properties.

In the display of the T-shaped fillet weld cracking test in Table 2, “No” indicates that no crack was observed, and “Yes” indicates that crack was observed.

The figure explaining a T-shaped fillet weld cracking test. The figure which shows the influence of the amount of Mn and the amount of P which have on the T type fillet weld crack test result.

Claims (3)

In mass%, C: 0.20 to 0.30%, Si: 0.05 to 0.40 %, Mn: 0.45 to 1.2%, Cr: 0.1 to 1.0% , Nb: 0.005 to 0.024%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0030%, Al: 0.1% or less, P: 0.010% or less, S: 0 0.005% or less, Mo: less than 0.05%, W: less than 0.05%, Cu: 0.1 to 1.0%, Ni: 0.1 to 2.0%, V: 0.0. 005 to 0.10%, or one or more of Ceq * represented by formula (1) is 0.55% or less, DI * represented by formula (2) is 50 or more, and the balance Fe In addition, the gas-cut surface properties and resistance to martensite having a composition composed of inevitable impurities and having a microstructure of martensite having a prior austenite grain size of 30 μm or less as a base phase Excellent abrasion steel sheet temperature temper embrittlement cracking properties.
Ceq * = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + W / 10
(1)
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) (2) The composition according to claim 1, further comprising Ca: 0.0005 to 0.0050%, REM: 0.00.
0005 to 0.0050% of 1 type or 2 types, Ceq * represented by the formula (1) is 0
. 55% or less, DI * expressed by the formula (2) is 50 or more, the composition is composed of the balance Fe and inevitable impurities, and the microstructure is martensite whose prior austenite grain size is 30 μm or less. A wear-resistant steel sheet with excellent gas-cut surface properties and low-temperature tempering embrittlement cracking resistance.
Ceq * = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + W / 10
(1)
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) (2)   The wear-resistant steel sheet having excellent gas-cut surface properties and low-temperature tempering embrittlement cracking properties according to claim 1 or 2, wherein the surface hardness is 400HBW10 / 3000 or more in terms of Brinell hardness.

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