Abrasion resistant steel plate with excellent impact properties and method for producing said steel plate

Description

[0001] The present invention relates to an abrasion resistant steel plate having excellent impact toughness and also relates to a method for manufacturing such an abrasion resistant steel plate having excellent impact toughness.

[0002] In construction machineries used for excavations within mines and earthworks, many components require frequent regular replacements due to ongoing abrasion and impact loading. Because frequent member exchange results in a deterioration in the equipment operating efficiency, there is considerable demand for a steel material (an abrasion resistant steel) that exhibits superior abrasion resistance in combination with excellent impact properties.

[0003] Tata Steel have been producing and marketing such steels as ABRAZO^® for many years.

[0004] On the other hand, in order to enable application to various shaped sites, or significantly reduce the number of welded sections, favourable workability of the steel plate is often very important for an abrasion resistant steel.

[0005] Increasing of the hardness is effective in improving the abrasion resistance. However, when a steel plate having high hardness is subjected to bending, and particularly bending with a small bend radius, the steel plate tends to be prone to breaking or cracking and have low impact properties. Generally speaking, having a high degree of hardness for a steel plate is disadvantageous for achieving favourable impact properties and workability. In other words, the abrasion resistance and impact properties are generally mutually opposing properties. For example, an HB500 class abrasion resistant steel plate (with a Brinell hardness at room temperature of approximately 450 to 550) exhibits excellent abrasion resistance, but has relatively poor impact properties. A steel having a lower degree of hardness such as an HB400 class abrasion resistant steel plate (with a Brinell hardness at room temperature of approximately 360 to 440) can be subjected to bending work comparatively easily, and can therefore be applied to all manner of members that require favourable workability, but cannot exhibit totally satisfactory impact properties

[0006] Accordingly, imparting a abrasion resistant steel having an HB400 class room temperature hardness with good impact properties properties could be said to be one effective method of achieving a combination of favourable bending workability and superior abrasion resistance at high temperatures.

[0007] An abrasion resistant steel plate does not generally require a particularly high toughness value, but must have a certain level of toughness to ensure that the steel does not crack even when the thickness of the steel plate decreases during use.

[0008] It is an object of the invention to provide an abrasion resistant steel plate having excellent impact toughness.

[0009] It is also an object of the invention to provide an abrasion resistant steel plate having a Charpy absorption energy at -40°C of at least 27J.

[0010] It is also an object of the invention to provide an abrasion resistant steel plate having a hardness value HB value (Brinell hardness) at 25°C of at least 360 HB.

[0011] It is also an object of the invention to provide a process to provide such an abrasion resistant steel plate.

[0012] One or more objects of the invention are reached by an abrasion resistant steel plate having excellent impact toughness, comprising in wt.%,

• C: not less than 0.10% and not more than 0.35;
• Si: not less than 0.10% but less than 0.50;
• Mn: not less than 0.8% and not more than 1.6%;
• Cr: not less than 0.1% and not more than 1.0%;
• Mo: not less than 0.05% and not more than 0.70%;
• P: not more than 0.020%;
• S: not more than 0.010%;
• Nb: not more than 0.08%;
• Ti: not more than 0.10%;
• B: not less than 0.0005% and not more than 0.0030%
• Al: not less than 0.01% and not more than 0.20%;
• Ca: not less than 0.0005% and not more than 0.0030%;
• N: not more than 0.010%;
• wherein S + P + N + Al + Nb + Ca is not more than 0.09%;
• with a remainder being Fe and unavoidable impurities as claimed in claim 1. Said abrasion resistant steel plate is obtainable by a process comprising the steps of:
  a. Producing a steel slab by ingot casting, thick slab continuous casting or thin slab continuous casting having a composition comprising in wt.%,

  • C: not less than 0.10% and not more than 0.35;
  • Si: not less than 0.10% but less than 0.50;
  • Mn: not less than 0.8% and not more than 1.6%;
  • Cr: not less than 0.1% and not more than 1.0%;
  • Mo: not less than 0.05% and not more than 0.70%;
  • P: not more than 0.020%;
  • S: not more than 0.010%;
  • Nb: not more than 0.08%;
  • Ti: more than 0.03% but not more than 0.10%;
  • B: not less than 0.0005% and not more than 0.0030%
  • Al: not less than 0.01% and not more than 0.20%;
  • Ca: not less than 0.0005% and not more than 0.0030%;
  • N: not more than 0.010%;
  • wherein S + P + N + Al + Nb + Ca is not more than 0.09%;
  • with a remainder being Fe and unavoidable impurities

  b. heating said to a temperature of at least 1150°C;

  c. hot rolling said slab to a hot rolled plate and finishing said hot rolling at a temperature of not less than 900°C;

  d. slow cooling said hot rolled plate to a temperature of 200°C or lower;

  e. reheating said cooled plate to a temperature above the Ac3 transformation temperature

  f. quenching said reheated plate to form the desired microstructure. Preferable embodiments are disclosed in the dependent claims.

[0013] In order to enhance the abrasion resistance it is important to produce a microstructure having sufficient hardness at the temperatures of use. The most economical way of achieving sufficiently high room temperature hardness is to employ a martensite structure. However, a steel plate having a martensite structure undergoes a large reduction in hardness as the temperature is increased leading to increased abrasion due to frictional heat generated during use of the steel in e.g. ground moving.

[0014] A more detailed description of the present invention is presented below. First is a description of the reasons for restricting each of the steel components within the abrasion resistant steel plate of the present invention.

[0015] C is an important element in determining the hardness of the martensite. In the present invention, in order to ensure that the room temperature HB value within the plate thickness center portion of a plate having a thickness of up to 50 mm is sufficient, the C content is set to not less than 0.10% and not more than 0.35%.

[0016] Si is a particularly effective element for improving the abrasion resistance, particularly at somewhat elevated temperatures, and is also an inexpensive alloy element. However, when a large amount of Si is added, reductions in the toughness and the workability are caused. For these reasons, the added amount of Si is set to not less than 0.10% but less than 0.5%.

[0017] Mn, by forming MnS, is essential for preventing a reduction in the toughness and a deterioration in the bending workability caused by grain boundary segregation of S, and is added in an amount of not less than 0.8%. Since Mn enhances the hardenability, it is preferable to add Mn in a large amount for the purpose of ensuring more favourable room temperature hardness within the plate thickness center portion of a plate having a thickness of up to 50 mm. In terms of enhancing the impact properties, the upper limit for the Mn content is 1.8%.

[0018] P is a harmful element that causes deterioration in the bending workability and the toughness, and is present in the steel as an unavoidable impurity. Accordingly, the P content is suppressed to not more than 0.020%, This amount is preferably 0.010% or lower. The amount of P is preferably as low as possible in terms of the bending workability and the toughness. However, since unavoidable increases in the refining costs are required in order to reduce the P content to less than 0.0005%, there is no necessity to limit the P content to this type of extremely low level.

[0019] S is also a harmful element that causes deterioration in the bending workability and the toughness, and is incorporated as an unavoidable imparity. Accordingly, the S content is suppressed to not more than 0.010%. This amount is preferably 0.005% or lower. The amount of S is preferably as low as possible in terms of the bending workability and the toughness. However, since unavoidable increases in the refining costs are required in order to reduce the S content to less than 0.0005%, there is no necessity to limit the S content to this type of extremely low level.

[0020] Cr is effective in improving the hardenability and improving the abrasion resistance, and is therefore added in an amount of at least 0.1%. However, excessive addition of Cr can cause a reduction in the toughness, and therefore the Cr content is limited to not more than 1.0%.

[0021] Mo improves the abrasion resistance, and adding a small amount in the presence of Nb produces a large improvement in the hardenability. For this reason, at least 0.05% of Mo must be added. However, excessive addition of Mo can cause a reduction in the toughness, and therefore the added amount of Mo has an upper limit of 0.70%.

[0022] Al is added in an amount of not less than 0.01% as a deoxidizing element or element for morphology control of inclusions. Further, Al is also added in an amount of not less than 0.05% for the purpose of fixing N in order to ensure the necessary amount of free B required to improve the hardenability. In either case, excessive addition of Al can cause a reduction in the toughness, and therefore the upper limit for the Al content is 0.20%, and preferably 0.10%.

[0023] B is an essential element that is extremely effective in improving the hardenability. In order to ensure satisfactory manifestation of this effect, at least 0.0005% of B is necessary. However, if B is added in an amount exceeding 0.0030%, then the weldability and the toughness of the steel may deteriorate, and therefore the B content is set to not less than 0.0005% and not more than 0.0030%.

[0024] If N is added in excess, N causes a reduction in the toughness, and also forms BN; thereby, the effect of improving hardenability that is provided by B is inhibited. As a result, the N content is suppressed to not more than 0.010%. The N content is preferably 0.006% or less. In terms of preventing any deterioration in the toughness and avoiding BN formation, the amount of N is preferably as low as possible. However, since unavoidable increases in the refining costs are required in order to reduce the N content to less than 0.001%, there is no necessity to limit the N content to this type of extremely low level.

[0025] Ti may be added to fix N as TiN; thereby, the formation of BN is prevented. As a result, the necessary amount of free B required to improve the hardenability is ensured. An amount of 0.003% or more of Ti may be added for this purpose. However, addition of Ti tends to cause a deterioration in the abrasion resistance at the higher temperatures of above 300°C. Accordingly, the added amount of Ti is limited to not more than 0.030%. The stoichiometric relation of Ti to N is such that all N can be bound to Ti if the ratio of Ti to N (in wt%) is 47.9/14.0 (= 3.42). For AIN the ratio is 27.0/14.0 (1.93). By combining the nitride forming effect of these two elements, titanium being the stronger nitride former, it can be deduced that no free nitrogen should be present in the steel when ((Ti/3.42)+(Al/1.92))>N and that therefore the formation of BN is prevented. To account for losses of Ti and Al during the steelmaking process (e.g. due to formation of TiC, titania or alumina) normally an excess of Ti and Al is used. A value of Al:N of 8 or more has proven a safe value to use.

[0026] The above elements represent the basic components within the steel of the present invention; however, one or more of the elements Nb, Cu, Ni and V may also be added in addition to the elements described above.

[0027] Nb is an optional element in the steel according to the invention. Nb, due to its existence in a solid solution state within the steel plate, can be effective in improving the abrasion resistance. The amount of Nb required to ensure a satisfactory amount of solid solution Nb is an amount of greater than 0.03%, and the amount is preferably 0.04% or greater. In the present invention, because 0.10% or greater of C is included to ensure a Brinell hardness at room temperature of not less than 360, if the amount of Nb is too large, then Nb(CN) may not be dissolve completely during heating. This type of insoluble Nb does not contribute to an improvement in the high-temperature hardness, and may actually cause a reduction in the toughness. For this reason, the added amount of Nb is not more than 0.10%, and is preferably 0.08% or lower.

[0028] Cu is an element that is capable of improving the hardness without reducing the toughness, and 0.05% or more of Cu may be added for this purpose. However, if Cu is added in excess, then the toughness may actually decrease, and therefore the added amount of Cu is not more than 1.5%.

[0029] Ni is an element that is effective in improving the toughness, and 0.05% or more of Ni may be added for this purpose. However, because Ni is an expensive element, the amount added is limited to not more than 1.0%.

[0030] V is an element that is effective in improving the abrasion resistance. An amount of 0.01% or more of V may be added for this purpose. However, V is also an expensive element and may cause a deterioration in the toughness if added in excess, and therefore if added, the amount is limited to not more than 0.20%, preferably to not more than 0.15%. A suitable minimum vanadium content is 0.015%. A suitable maximum content is 0.10% or even 0.08%.

[0031] In addition to the restrictions on the component ranges outlined above, as mentioned above, the element composition of the present invention is also restricted so that the value of S + P + N + Al + Nb + Ca is not more than 0.09%;

[0032] Next is a description of a method for manufacturing the abrasion resistant steel plate of the present invention.

[0033] First, a slab having the steel component composition described above is heated and subjected to hot rolling. In the present invention, there are no particular restrictions on the method used for manufacturing the slab prior to the hot rolling. In other words, after melting in a blast furnace, converter furnace or electric furnace or the like, a component adjustment process can be conducted using any of the various secondary refining techniques to achieve the targeted amount of each element, and casting may then be conducted using a typical continuous casting method, casting by an ingot method, or casting by another method such as thin slab casting. Scrap metal may be used as a raw material. In the case of a slab obtained by continuous casting, the high-temperature cast slab may be fed directly to the hot rolling apparatus, or may be cooled to room temperature and then reheated in a furnace before undergoing hot rolling. The components within the slab are the same as the components within the abrasion resistant steel plate of the present invention described above. Prior to being hot rolled, the slab may also be subjected to a dehydrogenation treatment, e.g. by allowing the slab to (very) slowly cool under insulated hoods after casting. The cooling process may take days or even weeks.

[0034] In order to ensure satisfactory solid solubilisation of the elements in the steel, the heating temperature for the slab is 1150°C or higher. However, if a heating temperature is too high, coarsening of the austenite structures occurs and a deterioration in the toughness is the likely result. Therefore, the heating temperature for the slab is preferably not more than 1350°C.

[0035] Further, in order to suppress unnecessary precipitation of precipitates like carbonitrides and maximize the amount of solid solution during hot rolling, the hot rolling is preferably finished at a temperature of not less than 900°C. Furthermore, the hot rolling finishing temperature preferably is not higher than 960°C.

[0036] After the hot rolling, slow cooling to ambient temperature, or at least below 200°C is conducted. The slow cooling may consist of natural air cooling. The cooling rate may also be reduced to reduce the potentially deleterious effects of hydrogen by allowing the steel to dehydrogenate. Usually this reduced cooling treatment entails a slow cooling from 550 to 350°C, e.g. by stacking a number of plates to cool down simultaneously thereby reducing the cooling rate. In some cases the plate may be reheated to a temperature of about 600°C to achieve the same effect.

[0037] By reheating the rolled steel after the slow cooling down ambient temperatures to a temperature of above the Ac3 transformation point, and then subjected the plate to accelerated cooling to a temperature of 200°C or lower the desired microstructure can be produced. Preferably the cooling rate within the center of the plate thickness is at least 5°C/s. During the accelerated cooling conducted after reheating in the case of reheating and quenching, the cooling rate increases as the thickness of the steel plate decreases. In the present invention, the target plate thickness is typically assumed to be approximately within a range from 4.5 mm to 50 mm. The cooling rate for a plate having a thickness of 4.5 mm may be extremely high; however, there are no particular problems associated with such a high rate, and no upper limit is specified for the cooling rate.

[0038] By ending the quenching at a temperature above ambient temperatures, there is a certain degree of self tempering which can be effective in relieving some of the stresses caused by the quenching. A further tempering heat treatment may therefore not be particularly necessary. Nevertheless a heat treatment at a temperature of not more than 250°C and preferably not more than 200°C does not cause the properties of the steel plate to depart from the scope of the present invention and may be effective in relieving some of the stresses caused by the quenching, particularly when the quenching was executed to temperatures near ambient temperatures.

[0039] The invention will now be explained by means of the following, non-limitative examples (see table 1 for chemical composition). The thickness of the plates was 25 mm. The slab thickness was 225 mm. The reheating temperature prior to hot rolling was 1200°C and the finish hot rolling temperature was 950°C. The heat treatment was performed at 930°C for 45 minutes and the cooling rate after the heat treatment was about 20°C/s in the centre of the plate. Depending on the local cooling rate in the plate the microstructure consists of martensite or a mixture of martensite and bainite and the hardness values were measured between 388 and 415 HB (Brinell Hardness). The Charpy Impact values at -40°C were al above 27J ranging. Values of 50J are easily achieved, whereas values of 100J are also achieveable. Tensile strength was between 1200 and 1300 MPa, and the yield strength between 1040 and 1100 MPa after heat treatment. Elongation values are above 10%. Table 1 - Chemical composition (in weight percent).

	C	Si	Mn	P	S	Cr	Mo	Nb	Ti	B	V	Altot	Ca	N	Ni	Cu	Σ
F77488	0.16	0.34	1.23	0.012	0.002	0.50	0.15	0.002	0.004	0.0018	0.027	0.067	0.001	0.0044	0.019	0.012	0.088
F77490	0.14	0.36	1.25	0.009	0.002	0.50	0.15	0.001	0.003	0.0019	0.020	0.054	0.001	0.0047	0.020	0.019	0.072
F77490*	0.14	0.36	1.25	0.009	0.002	0.50	0.15	0.038	0.003	0.0019	0.020	0.034	0.001	0.0047	0.020	0.019	0.089

Σ = S + P + N + Al + Nb + Ca

Claims

1. An abrasion resistant steel plate having excellent impact toughness, comprising in wt%,

• C: not less than 0.10% and not more than 0.35;

• Si: not less than 0.10% but less than 0.50;

• Mn: not less than 0.8% and not more than .6%;

• Cr: not less than 0.1% and not more than 1.0%;

• Mo: not less than 0.05% and not more than 0.70%;

• P: not more than 0.020%;

• S: not more than 0.010%;

• Nb: not more than 0.08%;

• Ti: not more than 0.10%;

• V: not more than 0.20%

• B: not less than 0.0005% and not more than 0.0030%

• Al: not less than 0.01% and not more than 0.20%;

• Ca: not less than 0.0005% and not more than 0.0030%;

• N: not more than 0.010%;

• wherein S + P + N + Al + Nb + Ca is not more than 0.09%;

• with a remainder being Fe and unavoidable impurities

2. An abrasion resistant steel plate according to claim 1 wherein (Ti/3.42) + (Al/1.92) > N to prevent the formation of boron nitride.

3. An abrasion resistant steel plate according to claim 1 or 2 wherein the microstructure is predominantly bainite, martensite or tempered martensite.

4. An abrasion resistant steel plate according to any one of claims 1 to 3 wherein the microstructure is predominantly a mixture of two or more of bainite, martensite and tempered martensite.

5. An abrasion resistant steel plate according to any one of claims 1 to 4 wherein the Charpy Impact Toughness is at least 27 J at -40°C when measured in the longitudinal direction.

6. An abrasion resistant steel plate according to any one of claims 1 to 5 wherein the HB value (Brinell hardness) at 25°C is not less than 360.

7. An abrasion resistant steel plate according to any one of claims 1 to 6 wherein the HB value at 25°C is not more than 440HB.

8. An abrasion resistant steel plate according to any one of claims 1 to 7 wherein the Nb value is between 0.03 and 0.07%.

9. Process for producing the abrasion resistant steel plate comprising the steps of:

a. Producing a steel slab by ingot casting, thick slab continuous casting or thin slab continuous casting having a composition comprising in wt%,

• C: not less than 0.10% and not more than 0.35;

• Si: not less than 0.10% but less than 0.50;

• Mn: not less than 0.8% and not more than 1.6%;

• Cr: not less than 0.1% and not more than 1.0%;

• Mo: not less than 0.05% and not more than 0.70%;

• P: not more than 0.020%;

• S: not more than 0.010%;

• Nb: not more than 0.08%;

• Ti: more than 0.03% but not more than 0.10%;

• V: not more than 0.20%

• B: not less than 0.0005% and not more than 0.0030%

• Al: not less than 0.01% and not more than 0.20%;

• Ca: not less than 0.0005% and not more than 0.0030%;

• N: not more than 0.010%;

• wherein S + P + N + Al + Nb + Ca is not more than 0.09%;

• with a remainder being Fe and unavoidable impurities

b. heating said to a temperature of at least 1150°C;

c. hot rolling said slab to a hot rolled plate and finishing said hot rolling at a temperature of not less than 900°C;

d. slow cooling said hot rolled plate to a temperature of 200°C or lower;

e. reheating said cooled plate to a temperature above the Ac3 transformation temperature

f. quenching said reheated plate to form the desired microstructure.

10. Process according to claim 9 wherein the quenching rate is at least 5°C/s in the centre of the plate.

11. Process according to claim 9 or 10 wherein (Ti/3.42) + (Al/1.92) > N to prevent the formation of boron nitride prior to quenching.

12. Process according to any one of claim 9 to 11 wherein the desired microstructure is predominantly bainite, martensite or tempered martensite.

13. Process according to any one of claim 9 to 12 wherein the desired microstructure is predominantly a mixture of two or more of bainite, martensite and tempered martensite.

14. Process according to any one of claim 9 to 13 wherein the Charpy Impact Toughness of the quenched plate is at least 27 J at -40°C when measured in the longitudinal direction.

15. Process according to any one of claim 9 to 14 wherein the HB value at 25°C is not less than 360 and/or not more than 440HB.