Patent Application
20100139820
The present invention relates to a wear-resistant steel plate having excellent wear resistance at high temperatures and excellent bending workability that can be used in construction machinery and industrial machinery, and also relates to a method for manufacturing such a wear-resistant steel plate.
The present application claims priority on Japanese Patent Application No. 2008-000301, filed on Jan. 7, 2008, and Japanese Patent Application No. 2008-268253, filed on Oct. 17, 2008, the contents of which are incorporated herein by reference.
In construction machineries used for excavations within mines and earthworks, many components require frequent regular replacements due to ongoing wear. Among these types of components, for steel materials, usage under conditions of high temperature represents the most severe operating conditions. Because the hardness of wear-resistant steel decreases with increasing temperature, the wear of the steel tends to accelerate rapidly at a temperature of not less than a certain value. This wear is particularly marked for members such as bulldozer buckets in which frictional heat is generated as a result of strong impacts, and hoppers for sintered coke which are exposed to impacts with high-temperature bodies. In these types of members, the temperature of the surface of the steel plate that constitutes the member may temporarily reach temperatures of approximately 300° C. to 400° C. Because frequent member exchange results in a deterioration in the equipment operating efficiency, there is considerable demand for a steel material (a wear-resistant steel) that exhibits superior wear resistance even under these types of conditions.
On the other hand, in order to enable application to various shaped sites, or significantly reduce the number of welded sections, favorable bending workability of the steel plate is often very important for a wear-resistant steel.
Increasing of the hardness is effective in improving the wear 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. Moreover, if consideration is also given to factors such as the value of the deformation resistance to bending and the degree of spring-back, then having a high degree of hardness for a steel plate is disadvantageous for achieving favorable bending workability. In other words, the wear resistance and the bending workability are generally mutually opposing properties. For example, an HB500 class wear-resistant steel plate (with a Brinell hardness at room temperature of approximately 450 to 550) exhibits excellent wear resistance, but has relatively poor bending workability. A steel having a lower degree of hardness such as an HB400 class wear-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 favorable workability, but cannot exhibit totally satisfactory wear resistance, particularly in terms of the wear resistance under high-temperature conditions.
Accordingly, imparting a wear-resistant steel having an HB400 class room temperature hardness with favorable high-temperature wear resistance properties could be said to be one effective method of achieving a combination of favorable bending workability and superior wear resistance at high temperatures.
A wear-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. In consideration of use within cold regions, it is generally considered that the Charpy absorption energy at −40° C. should be not less than 27 J.
The inventors of the present invention have previously disclosed, in Patent Document 1, a wear-resistant steel for high-temperature applications having a Brinell hardness in the order of HB500 class. The invention disclosed in this document was designed with the high-temperature wear resistance as the overriding priority, with no particular measures taken to improve the bending workability, and therefore the steel is limited to applications in which the bend radius is comparatively large.
Patent Document 2 relates to a wear-resistant steel for intermediate and moderate temperatures that can be used in regions of 300° C. to 400° C. This document gives no consideration to toughness or workability, and no disclosure is made regarding these properties; however, because the steel includes an extremely high level of Si, it is thought that neither the toughness nor the workability would be particularly favorable.
Patent Document 3 relates to an HB400 class wear-resistant steel having excellent bending workability, but absolutely no consideration is given to the wear resistance under high-temperature conditions.
In this manner, up until this point there have been no suitable examples of HB400 class wear-resistant steels that exhibit favorable bending workability as well as a high degree of wear resistance under high-temperature conditions of 300° C. to 400° C.
Moreover, because a wear-resistant steel plate is a consumable item, economy is also an important factor, and it is desirable that the amount of expensive alloy elements added to the steel is kept to a minimum.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2001-49387
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H03-243743
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2005-240135
The present invention aims to provide a wear-resistant steel having a room temperature hardness in the order of HB400 class that indicates favorable bending workability, has a high degree of wear resistance even under high-temperature conditions of 300° C. to 400° C., and is very economical.
It is thought that in order to enhance the wear resistance at high temperatures of 300° C. to 400° C., it is important to maintain the hardness of the steel at these high temperatures. On the other hand, the most economical way of achieving a room temperature hardness of approximately HB400 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. Accordingly, with regard to steels containing martensite structures (martensite steels) and having a room temperature hardness of approximately HB400, methods of improving the high-temperature wear resistance were investigated, from the viewpoint of attempting to maintain the high-temperature hardness at a level as high as possible.
The present invention has been developed on the assumption of high-temperature conditions of 300° C. to 400° C., and a temperature of 350° C. was used as a representative temperature for evaluating the properties of the steel. The wear resistance at 350° C. (350° C. wear resistance) was investigated for martensite steels having a variety of different chemical compositions. These wear resistance evaluations were conducted in the manner outlined below. Namely, the temperature of the sample was controlled within a pin-on-disk wear testing apparatus prescribed in ASTM G99-05, and wear testing was conducted while the sample temperature was set to 350° C.; thereby, the amounts of wear for the test sample and for a standard sample (SS400) were measured. The result for the SS400 as a standard was used, and a 350° C. wear resistance ratio was defined as [amount of wear of SS400/amount of wear of test sample]. Thereby, the 350° C. wear resistance ratio was determined for the sample. The larger the value for this wear resistance ratio becomes, the more favorable the 350° C. wear resistance is.
The inventors of the present invention conducted detailed investigations of the relationship between the steel alloy elements and the 350° C. wear resistance for a multitude of martensite steels having an HB value at 25° C. within a range from 360 to 440. As a result, they derived a formula (I) below for predicting the 350° C. wear resistance ratio from the chemical composition:
HI=[C]+0.59[Si]−0.58[Mn]+0.29[Cr]+0.39[Mo]+2.11([Nb]−0.02)−0.72[Ti]+0.56[V] (1)
wherein [C], [Si], [Mn], [Cr], [Mo], [Nb], [Ti] and [V] represent the amounts (mass %) of C, Si, Mn, Cr, Mo, Nb, Ti and V, respectively. In formula (I), the reason for subtracting 0.02 from the Nb amount is to account for the amount of Nb that precipitates during rolling.
In the present invention, the target value for the high-temperature wear resistance is set as a 350° C. wear resistance ratio of not less than 3.0, that is, an amount of frictional wear that is ⅓ or less than that of SS400. From the relationship illustrated in
The formula (I) indicates that besides Nb, increasing the added amounts of Si, Cr, Mo and V is also effective in improving the 350° C. wear resistance for a martensite steel.
Of these elements, both of Mo and V are elements that have conventionally been added in large amounts to high-temperature steels; however, because recent costs for these elements are extremely high, the added amounts are preferably kept as small as possible from the viewpoint of economic viability.
In contrast, Si and Cr are comparatively low-cost elements, and are therefore advantageous in terms of improving the 350° C. wear resistance. On the other hand, reducing the amount of Mn is actually also advantageous in terms of achieving a favorable 350° C. wear resistance.
In order to ensure that martensite structures exist right through to the center of the plate thickness, it is necessary to ensure that the steel has satisfactory hardenability. Most wear-resistant steel plate has a plate thickness of not more than 50 mm. If the value of Ceq in the following formula exceeds 0.50, sufficient hardenability can be achieved to ensure that martensite structures exist right through to the center of a steel plate having a thickness of 50 mm.
Ceq=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14
wherein [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] represent the amounts (mass %) of C, Si, Mn, Ni, Cr, Mo and V, respectively.
In terms of toughness, appropriate upper limits must be specified for the amounts of Si, P, S, Cr, Mo, Al, B and N in order to ensure a Charpy absorption energy at −40° C. of not less than 27 J.
The present invention has been developed in light of the above findings, and provides the aspects described below.
(1) A wear-resistant steel plate of the present invention having excellent wear resistance at high temperatures and excellent bending workability includes, in mass % values, C: not less than 0.13% and not more than 0.18%, Si: not less than 0.5% but less than 1.0%, Mn: not less than 0.2% and not more than 0.8%, P: not more than 0.020%, S: not more than 0.010%, Cr: not less than 0.5% and not more than 2.0%, Mo: not less than 0.03% and not more than 0.30%, Nb: more than 0.03% but not more than 0.10%, Al: not less than 0.01% and not more than 0.20%, B: not less than 0.0005% and not more than 0.0030%, and N: not more than 0.010%, with the remainder being Fe and unavoidable impurities, wherein an element composition is such that HI defined below is 0.7 or greater and Ceq exceeds 0.50, and an HB value (Brinell hardness) at 25° C. is not less than 360 and not more than 440.
HI=[C]+0.59[Si]−0.58[Mn]+0.29[Cr]+0.39[Mo]+2.11([Nb]-0.02)−0.72[Ti]+0.56[V]
Ceq=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14
wherein [C], [Si], [Mn], [Ni], [Cr], [Mo], [Nb], [Ti] and [V] represent the amounts (mass %) of C, Si, Mn, Ni, Cr, Mo, Nb, Ti and V, respectively.
(2) The wear-resistant steel plate having excellent wear resistance at high temperatures and excellent bending workability according to the aspect of the present invention disclosed in (1) above may further include, in mass % values, one or more selected from the group consisting of Cu: not less than 0.05% and not more than 1.5%, Ni: not less than 0.05% and not more than 1.0%, Ti: not less than 0.003% and not more than 0.03%, and V: not less than 0.01% and not more than 0.20%.
(3) A method for manufacturing a wear-resistant steel plate having excellent wear resistance at high temperatures and excellent bending workability according to the present invention includes: heating a slab having the composition disclosed in (1) or (2) above to a temperature of at least 1,200° C., conducting hot rolling with a cumulative reduction ratio of not less than 30% and not more than 65% at a temperature of not more than 960° C. and not less than 900° C., finishing the hot rolling at a temperature of not less than 900° C.; and after completion of the hot rolling, either immediately performing accelerated cooling to a temperature of 200° C. or lower such that a cooling rate within the center of the plate thickness is at least 5° C./s, or conducting cooling to a temperature of 200° C. or lower, subsequently reheating to a temperature of not less than an Ac3 transformation point, and then performing accelerated cooling to a temperature of 200° C. or lower such that a cooling rate within the center of the plate thickness is at least 5° C./s.
According to the present invention, a wear-resistant steel plate having a room temperature hardness in the order of HB400 class that indicates favorable bending workability, has a high degree of wear resistance even under high-temperature conditions of 300° C. to 400° C., and is very economical can be manufactured relatively easily.
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 wear-resistant steel plate of the present invention.
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 not less than 360 and not more than 440, the C content is set to not less than 0.13% and not more than 0.18%.
Si is a particularly effective element for improving the 350° C. wear resistance, 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 0.50% but less than 1.0%. If particular emphasis is placed on the workability, then the added amount of Si is preferably less than 0.8%.
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 0.2%. Since Mn enhances the hardenability, it is preferable to add Mn in a large amount for the purpose of ensuring more favorable room temperature hardness within the plate thickness center portion of a plate having a thickness of up to 50 mm. However, on the other hand, Mn causes a reduction in the high-temperature strength, and actually causes a decrease in the 350° C. wear resistance. For this reason, the added amount of Mn is preferably less than 0.5%. Even in terms of enhancing the hardenability, the upper limit for the Mn content is 0.8%. Accordingly, the added amount of Mn is set to not less than 0.2% and not more than 0.8%, and is preferably not less than 0.2% but less than 0.5%.
P is a harmful element that causes deterioration in the bending workability and the toughness, and is incorporated 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.
S is also a harmful element that causes deterioration in the bending workability and the toughness, and is incorporated as an unavoidable impurity. 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.
Cr is effective in improving the hardenability and improving the 350° C. wear resistance, and is therefore added in an amount of at least 0.5%. In order to obtain satisfactory hardenability within the plate thickness center portion of a plate having a thickness of up to 50 mm, the added amount of Cr is preferably 1.0% or greater. However, excessive addition of Cr can cause a reduction in the toughness, and therefore the Cr content is limited to not more than 2.0%.
Mo improves the 350° C. wear resistance, and adding a small amount in the presence of Nb produces a large improvement in the hardenability. For this reason, at least 0.03% 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 0.30%. Further, Mo has been extremely expensive in recent years, and in terms of suppressing the alloy cost, the added amount of Mo is preferably less than 0.10%.
Nb, due to its existence in a solid solution state within the steel plate, is extremely effective in improving the 350° C. wear 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 0.13% 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 solid-solubilized 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.
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%.
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%.
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.
The above elements represent the basic components within the steel of the present invention; however, one or more of the elements Cu, Ni, V and Ti may also be added in addition to the elements described above.
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%.
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%.
V is an element that is effective in improving the 350° C. wear 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%.
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 350° C. wear resistance. Accordingly, the added amount of Ti is limited to not more than 0.030%.
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 HI in formula (1) is not less than 0.7, and the value of Ceq is greater than 0.50. However, if the values of HI and Ceq are increased too much, then the toughness may deteriorate, and therefore HI is preferably not more than 1.2 and Ceq is preferably not more than 0.70.
Next is a description of a method for manufacturing the wear-resistant steel plate of the present invention.
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 wear-resistant steel plate of the present invention described above.
In order to ensure satisfactory solid solubilization of Nb, the heating temperature for the slab is 1,200° C. or higher. However, if a heating temperature is too high, coarsening of the austenite structures occurs; thereby, a microstructure after hot rolling does not become sufficiently fine and a deterioration in the toughness is caused. Therefore, the heating temperature for the slab is preferably not more than 1,350° C.
During hot rolling, the cumulative reduction ratio is set to not less than 30% and not more than 65% at a temperature of not more than 960° C. and not less than 900° C. The temperature and the reduction ratio are restricted to these ranges so as to reduce the amount of Nb carbonitrides precipitated during rolling to a requisite minimum which is necessary for ensuring favorable grain refinement.
Further, in order to suppress unnecessary precipitation of Nb carbonitrides and maximize the amount of solid solution Nb, the hot rolling is preferably finished at a temperature of not less than 900° C. Furthermore, the hot rolling finishing temperature must be not more than 960° C.
After the hot rolling, accelerated cooling is conducted to obtain martensite structures, either by performing direct quenching or by reheating the rolled steel and then performing quenching.
In the case of direct quenching, after completion of the hot rolling, the rolled plate is immediately subjected to accelerated cooling to a temperature of 200° C. or lower at a cooling rate of at least 5° C./s (the cooling rate within the center of the plate thickness).
In the case of reheating and quenching, after completion of the hot rolling, the rolled plate is cooled once to a temperature of 200° C. or lower (the cooling rate is arbitrary), subsequently reheated to a temperature of not less than the Ac3 transformation point, and then subjected to accelerated cooling to a temperature of 200° C. or lower such that the cooling rate within the center of the plate thickness is at least 5° C./s.
During the accelerated cooling conducted immediately after completion of the hot rolling in the case of direct quenching, or 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.
A tempering heat treatment is not particularly necessary; however, a heat treatment at a temperature of not more than 300° C. does not cause the properties of the steel plate to depart from the scope of the present invention.
Steels A to AI having the compositions shown in Tables 1 and 2 were melted to obtain slabs. The obtained slabs were heated to a temperature of at least 1,230° C., and then were subjected to processes under the manufacturing conditions shown in Tables 3 and 4 to manufacture steel plates having plate thicknesses ranging from 6 to 45 mm (each of the Steels No. 1 to 17 represents an example of the present invention, whereas each of the Steels No. 18 to 44 represents a comparative example).
0.111
0.48
0.217
0.35
0.61
1.37
0.08
0.91
0.63
0.037
0.015
1.90
0.42
0.41
2.50
0.01
0.56
0.23
0.013
0.135
0.060
0.240
0.0001
0.0055
0.0125
0.66
0.47
Each of these steel plates was evaluated for room temperature hardness, wear resistance at 350° C., bending workability, and toughness.
The room temperature hardness was evaluated by using a Brinell hardness test method (JIS Z 2243) to measure the hardness at 25° C. The target value for the room temperature hardness was a value of not less than HB360 and not more than HB440.
As described above, the wear resistance was evaluated by conducting wear testing using a pin-on-disk wear testing apparatus prescribed in ASTM G99-05 with the temperature of the sample held at 350° C., and then determining a wear resistance ratio relative to a SS400 standard sample (amount of wear of SS400/amount of wear of test sample). The target value for the wear resistance was a wear resistance ratio of 3.0 or greater.
Evaluation of the bending workability was conducted in the following manner. Namely, using the method prescribed in JIS Z 2248, a JIS No. 1 test piece was subjected to a bend test to 180° in the C-direction at a bend radius of four times the plate thickness (4t), and after the bend test, the external appearance of the curved portion of the test piece was examined. The steel plate was deemed to have passed if no cracking or other defects were observed on the outside of the curved portion.
Evaluation of the toughness was conducted in the manner described below. Namely, a No. 4 Charpy test piece prescribed in JIS Z 2201 was sampled from the center of the plate thickness in a direction orthogonal to the rolling direction, an impact test was performed at −40° C., and the absorption energy was measured. Three test pieces were subjected to impact tests at −40° C., and the average value for the absorption energy was determined. The target value for the toughness was an average value of not less than 27 J.
The results obtained are tabled in Tables 5 and 6.
In Tables 1 to 6, underlined numerical values represent component values outside the ranges specified by the present invention, or unsatisfactory temperature conditions or properties.
2.62
16
18
22
347
2.74
18
2.89
20
18
2.80
351
21
19
315
1.79
19
15
2.80
349
2.62
20
2.53
312
1.64
In Steel No. 1 to 17 that represent examples of the present invention in Table 5, all of the values for the above-mentioned room temperature hardness, 350° C. wear resistance, bending workability, and toughness satisfied the respective target values. In contrast, in Steel No. 18 to 40 of the comparative examples, in which the steel composition departed from the chemical composition range specified in the present invention, even though manufacture of the steel was conducted using the method of the present invention, at least one of the room temperature hardness, the 350° C. wear resistance, the bending workability or the toughness did not satisfy the target value. Moreover, in Steel No. 41 to 44, in which the steel composition satisfied the range specified in the present invention, but the manufacturing method departed from the method prescribed in the present invention, at least one of the room temperature hardness, the 350° C. wear resistance, the bending workability or the toughness failed to satisfy the target value.
According to the present invention, a wear-resistant steel plate having an HB400 class room temperature hardness, that indicates favorable bending workability, has a high degree of wear resistance even under high-temperature conditions of 300° C. to 400° C., and is very economical can be manufactured relatively easily. As a result, the present invention can be used favorably for construction machinery and industrial machinery members that require superior wear resistance under high-temperature conditions, such as bulldozer buckets in which frictional heat is generated as a result of strong impacts, and hoppers for sintered coke which are exposed to impacts with high-temperature bodies.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-000301 | Jan 2008 | JP | national |
| 2008-268253 | Oct 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/050024 | 1/6/2009 | WO | 00 | 9/18/2009 |
