Powder lubricant composition and method for manufacturing seamless steel pipe

Abstract

A powder lubricant composition according to the present invention includes 65 parts by mass to 95 parts by mass of sodium borate, and 5 parts by mass to 35 parts by mass of cryolite. A method for manufacturing a seamless steel pipe according to the present invention includes adhering the above-described powder lubricant composition to a pipe inner surface of a work piece which is piercing rolled to have a tubular shape, and elongation rolling on the work piece after the adhering of the powder lubricant composition.

Claims

1. A method for manufacturing a seamless steel pipe, comprising: adhering a powder lubricant composition to a pipe inner surface of a work piece which is piercing rolled to have a tubular shape; and elongation rolling on the work piece after the adhering of the powder lubricant composition, wherein the powder lubricant composition comprises: sodium borate: 65 parts by mass to 95 parts by mass; cryolite: 5 parts by mass to 35 parts by mass; and at least one selected from the group consisting of a fatty acid sodium salt and a fatty acid calcium salt: 0 parts by mass to 25 parts by mass, wherein a total of sodium borate and cryolite is 100 parts by mass, and wherein a temperature of the work piece before initial rolling in the elongation rolling is 700 C. or higher and lower than 820 C.

2. The method for manufacturing a seamless steel pipe according to claim 1, wherein a total reduction in the elongation rolling is 50% to 80%.

3. The method for manufacturing a seamless steel pipe according to claim 2, wherein the elongation rolling is performed by a mandrel mill rolling.

4. The method for manufacturing a seamless steel pipe according to claim 1, wherein the elongation rolling is performed by a mandrel mill rolling.

5. The method for manufacturing a seamless steel pipe according to claim 1, wherein a concentration of the sodium borate in the powder lubricant composition is 70 parts by mass to 95 parts by mass, and a concentration of the cryolite in the powder lubricant composition is 5 parts by mass to 30 parts by mass.

6. The method for manufacturing a seamless steel pipe according to claim 1, wherein a concentration of the sodium borate in the powder lubricant composition is 70 parts by muss to 90 parts by mass, and a concentration of the cryolite in the powder lubricant composition is 10 parts by mass to 30 parts by mass.

7. The method for manufacturing a seamless steel pipe according to claim 1, wherein a concentration of the sodium borate in the powder lubricant composition is 70 parts by mass to 80 parts by mass, and a concentration of the cryolite in the powder lubricant composition is 20 parts by mass to 30 parts by mass.

8. The method for manufacturing a seamless steel pipe according to claim 1, wherein the total of sodium borate and cryolite is 80 mass % to 100 mass % of the powder lubricant composition.

9. The method for manufacturing a seamless steel pipe according to claim 1, wherein a concentration of the sodium borate in the powder lubricant composition is 65 parts by mass to 95 parts by mass, a concentration of the cryolite in the powder lubricant composition is 5 parts by mass to 35 parts by mass, and a concentration of the at least one selected from the group consisting of a fatty acid sodium salt and a fatty acid calcium salt is 5 parts by mass to 25 parts by mass.

10. The method for manufacturing a seamless steel pipe according to claim 1, wherein the elongation rolling starts when the work piece has a temperature of 700 C. or higher and lower than 820 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a flow chart illustrating one embodiment of a method for manufacturing a seamless steel pipe according to a second aspect of an embodiment.

(2) FIG. 2 is a schematic view illustrating a hot rolling testing apparatus used for evaluation in Examples and Comparative Examples as seen from a cross-sectional view.

(3) FIG. 3 is a photograph showing a relationship between an amount of cryolite added and a melting starting temperature of sodium borate.

EMBODIMENTS OF THE INVENTION

(4) The above-described actions and effects of the present invention will become apparent from embodiments for carrying out the present invention which will be described later. Hereinafter, the embodiments of the present invention will be described with reference to drawings. The embodiments described below are merely examples of the present invention and the present invention is not limited thereto. In addition, unless otherwise specified, regarding a numerical value range, the expression of A to B means A or more and B or less. In the expression, when a unit is attached only to the numerical value B, it is deemed that the unit is also applied to the numerical value A.

(5) <1. Powder Lubricant Composition>

(6) A powder lubricant composition according to a first aspect of the present invention will be described. The powder lubricant composition of the embodiment includes 65 parts by mass to 95 parts by mass of sodium borate, and 5 parts by mass to 35 parts by mass of cryolite.

(7) (Sodium Borate)

(8) Sodium borate has a function of securing a fluid lubrication capacity and a scale melting capacity. It is preferable that the sodium borate of the embodiment include one or more compounds selected from the group consisting of anhydrous sodium tetraborate Na.sub.2B.sub.4O.sub.7, sodium tetraborate pentahydrate Na.sub.2B.sub.4O.sub.5(OH).sub.4-3H.sub.2O, and sodium tetraborate decahydrate (borax) Na.sub.2B.sub.4O.sub.5(OH).sub.4-8H.sub.2O from the viewpoint of stability, ease of acquisition, and the like. It is particularly preferable that the powder lubricant composition include anhydrous sodium tetraborate Na.sub.2B.sub.4O.sub.7 not containing water of hydration out of the above-mentioned group. When the powder lubricant composition contains anhydrous sodium tetraborate Na.sub.2B.sub.4O.sub.7, the powder lubricant composition melts more quickly.

(9) (Cryolite)

(10) Cryolite has a function of sufficiently melting sodium borate at a rolling temperature of lower than 1000 C. which is considered a relatively low temperature in the technical field to exhibit a lubrication capacity by lowering the melting point of the lubricant composition including sodium borate. Accordingly, the lubricant composition according to the embodiment can reduce friction force at a rolling temperature of lower than 1000 C. through a fluid lubrication mechanism.

(11) From the experimental results which will be described later, the inventors have assumed that cryolite has a function of lowering the melting point of the lubricant composition and also lowering the viscosity of the molten material of the sodium borate and the scales so as to adjust the viscosity within a viscosity range suitable for fluid lubrication. As for the mechanism, it may be considered that when sodium borate is changed to a glassy form by melting, fluoride ions F supplied from the cryolite compete with negatively charged oxygen atoms (such as BO.sup.) and are coordinated with boron atoms, and thus, the molten sodium borate interferes with a process of forming a higher network.

(12) The above-mentioned results can be obtained by adding fluoride salts other than cryolite to the lubricant. However, when a fluoride salt having high hardness relative to other fluoride salts is used, the powder supply nozzle may be worn easily depending on the material of the powder supply nozzle. In order to prevent the wearing of the powder supply nozzle, it is necessary to use a fluoride salt having a Mohs hardness of 3 or lower. Examples of the fluoride salt satisfying the condition include NaF (having a Mohs hardness of 2 to 2.5), cryolite (having a Mohs hardness of 2.5 to 3), and the like. Among them, from the viewpoint of exhibiting a better friction reduction capacity in a low temperature range of lower than 1000 C., cryolite can be particularly preferably used. Contrarily, for example, fluorite CaF.sub.2 (having a Mohs hardness of 4) can function as a lubricant. However, since the powder supply nozzle is worn, the use of fluorite CaF.sub.2 is not preferable.

(13) (Amount of Sodium Borate and Cryolite)

(14) The lubricant composition of the embodiment includes 65 parts by mass to 95 parts by mass of sodium borate, and 5 parts by mass to 35 parts by mass of cryolite as described above. When a ratio between the amounts of the sodium borate and cryolite is within the above-described ranges, a good lubrication capacity can be stably exhibited in both the low temperature range of lower than 1000 C., in which sodium borate hardly melts alone, and the high temperature range of 1000 C. or higher, in which the amount of scales generated increases.

(15) When elongation rolling is performed at a temperature of lower than 1000 C. using the lubricant composition in which the amount of cryolite is lower than the above-mentioned range, sodium borate does not melt sufficiently and fluid lubrication is not achieved. When fluid lubrication is not achieved, as in a case in which a conventional lubricant is used, an increase in friction and flaw generation on the inner surface of the pipe cannot be suppressed. On the other hand, even when elongation rolling is performed at a temperature of lower than 1000 C. using the lubricant composition in which the amount of cryolite is higher than the above-mentioned range, sodium borate does not melt sufficiently and fluid lubrication is not achieved. The inventors have confirmed through the experiments that even when the amount of cryolite is above or below the above-mentioned appropriate range, sodium borate does not melt sufficiently.

(16) FIG. 3 is a photograph showing experimental results for investigating a relationship between an amount of cryolite added and a melting starting temperature of sodium borate. In this experiment, borax was used as the sodium borate. Sample 1 is a sample including 100 parts by mass of borax and 0 parts by mass of cryolite (that is, a sample including only borax), Sample 2 is a sample including 90 parts by mass of borax and 10 parts by mass of cryolite, Sample 3 is a sample including 80 parts by mass of borax and 20 parts by mass of cryolite, Sample 4 is a sample including 70 parts by mass of borax and 30 parts by mass of cryolite, and Sample 5 is a sample including 60 parts by mass of borax and 40 parts by mass of cryolite. Samples 1 and 5 are samples out of the above-described defined range. When these samples were heated, Sample 1 hardly melted at 700 C., started to melt at about 600 C. to 650 C., and completely melted at 750 C.

(17) Contrarily, Sample 2 started to melt at about 600 C. to 650 C., almost melted at 650 C., and completely melted at 700 C. Sample 3 started to melt at about 600 C. to 650 C., almost melted at 700 C., and completely melted at 750 C. Sample 4 started to melt at about 600 C. to 650 C., almost half melted at 700 C., and completely melted at 750 C. In this manner, Samples 2 to 4 in which the amount of cryolite added was within the defined range melted more clearly and easily compared to Sample 1 in which cryolite was not added.

(18) However, Sample 5 in which the amount of cryolite added was above the defined range did not melt completely even at 800 C. It is assumed that when cryolite is added excessively, cryolite does not melt and Sample 5 does not melt completely until the temperature reaches a high temperature.

(19) In the embodiment, it is more preferable that the amount of sodium borate be set to 75 parts by mass to 85 parts by mass and the amount of cryolite be set to 15 parts by mass to 25 parts by mass.

(20) (Other Components)

(21) In the lubricant composition of the embodiment, as a balance, components other than sodium borate and cryolite may be appropriately contained according to desired properties. Examples of such arbitrary components include a fatty acid sodium salt and/or a fatty acid calcium salt. When the fatty acid sodium salt or the fatty acid calcium salt are contained, the fluidity of the lubricant composition of the embodiment can be improved in a powdered state (that is, before melting). Preferable examples of the fatty acid sodium salt and the fatty acid calcium salt include saturated fatty acid salts such as stearic acid and palmitic acid, fatty acid salts obtained from natural vegetable fats and oils such as palm oil fatty acid, and fatty acid salts obtained from animal fats and oils such as tallow acid. The amount of the fatty acid sodium salt and/or the fatty acid calcium salt is preferably 5 parts by mass or more with respect to 100 parts by mass of the total amount of sodium borate and cryolite from the viewpoint of smooth and easy transfer through the pipe. In addition, from the viewpoint of obtaining economical efficiency and securing a relative amount of sodium borate and cryolite, the amount of the fatty acid sodium salt and/or the fatty acid calcium salt is preferably 25 parts by mass or less, more preferably 20 parts by mass or less, and still more preferably 18 parts by mass or less with respect to 100 parts by mass of the total amount of sodium borate and cryolite.

(22) From the viewpoint of furthering the friction reduction effect and flaw generation suppressing effect, the total amount of sodium borate and cryolite in the lubricant composition of the embodiment is preferably 80 mass % or more, more preferably 83 mass % or more, and still more preferably 85 mass % or more when the total amount of the lubricant composition of the embodiment is 100 mass %. The total amount of sodium borate and cryolite may be 100 mass %. However, from the viewpoint of easily exhibiting the effect resulting from other additive components (for example, the fatty acid salts shown as examples), the total amount thereof is preferably 95 mass % or less.

(23) <2. Method for Manufacturing Seamless Steel Pipe>

(24) A method for manufacturing a seamless steel pipe according to a second aspect of the present invention will be described. FIG. 1 is a flow chart illustrating one example of a method S10 for manufacturing a seamless steel pipe (hereinafter, abbreviated to a manufacturing method S10). As illustrated in FIG. 1, the manufacturing method S10 has sequential steps S1 to S6. Hereinafter, with reference to FIG. 1, the manufacturing method S10 will be described.

(25) (Piercing Rolling S1)

(26) In a piercing rolling S1, a hollow blank pipe is prepared by performing piercing rolling on a round billet, which is heated to a predetermined temperature, to have a tubular shape. When the S1 is performed, known piercing rolling methods such as the Mannesmann method can be adopted without being particularly limited. As described below, the hollow blank pipe may be heated to a temperature of 700 C. or higher and lower than 1000 C. in the piercing rolling S1.

(27) (Heating S2)

(28) In a heating S2, the hollow blank pipe prepared in the piercing rolling S1 is heated to a temperature of 700 C. or higher and lower than 1000 C. Typically, the billet may be heated in the piercing rolling S1 so that the temperature of the hollow blank pipe before initial rolling in the elongation rolling is 700 C. or higher and lower than 1000 C. In this case, the heating S2 is included in the piercing rolling S1. Further, when the hollow blank pipe which has undergone the piercing rolling S1 is cooled, the heating S2 can be performed by reheating the hollow blank pipe in a heating furnace.

(29) (Adhering Powder Lubricant Composition S3)

(30) In adhering a powder lubricant composition S3, the powder lubricant composition according to the first aspect of the embodiment is adhered to the inner surface of the hollow blank pipe whose temperature has been adjusted through heating in the piercing rolling S1 or the heating S2. As a method for adhering the powder lubricant composition to the inner surface of the hollow blank pipe, for example, a method of blowing the powder lubricant composition from one open end of the hollow blank pipe with a carrier gas such as nitrogen can be used. The powder lubricant composition adhered to the inner surface of the hollow blank pipe melts by absorbing heat from the hollow blank pipe and forms a lubricating film on the inner surface of the pipe. When the hollow blank pipe is reheated in the heating furnace in the heating S2, the adhering the powder lubricant composition S3 may be performed before the heating S2.

(31) (Elongation Rolling S4)

(32) In an elongation rolling S4, the hollow blank pipe, which has undergone the adhering the powder lubricant composition S3, is elongation rolled with a total reduction being 50% or more by mandrel mill rolling. The elongation rolling may be continuous elongation rolling in which elongation rolling is continuously performed.

(33) First, a mandrel bar in which a lubricant is applied to the surface is inserted into a hollow portion of the hollow blank pipe which has undergone the adhering the powder lubricant composition S3. As the lubricant to be applied to the surface of the mandrel bar, known lubricants such as a lubricant including graphite as a main component as described in Patent Document 1, a lubricant including mica as a main component as described in Patent Document 2, and the like may be appropriately adopted.

(34) Next, the hollow blank pipe into which the mandrel bar is inserted is elongation rolled by a mandrel mill. In the elongation rolling S4, elongation rolling is performed on the hollow blank pipe whose temperature before initial rolling in the mandrel mill is 700 C. or higher and lower than 1000 C. so as to have a total reduction of 50% or more. A plurality of stands (typically, 5 to 9) is generally provided in the mandrel mill. The specific distribution of the reduction in each stand can be appropriately set depending on the number of stands or the like. It is not necessary to particularly define the upper limit of the total reduction. However, in consideration of equipment capability, the upper limit of the total reduction is practically 80%.

(35) In this manner, it is possible to cause the grains of the pipe to be finer by performing significant plastic deformation at a relatively low temperature of lower than 1000 C. Since significant plastic deformation is performed at a relatively low temperature, the condition for sliding between the inner surface of the hollow blank pipe and the outer surface of the mandrel bar is a severe condition for lubrication. However, since the powder lubricant composition according to the first aspect of the present invention is supplied to the inner surface of the pipe in the adhering the powder lubricant composition S3, the friction on the inner surface of the pipe can be reduced and flaw generation on the inner surface of the pipe can be suppressed.

(36) (Drawing S5)

(37) In a drawing S5, the mandrel bar is drawn from the pipe which has undergone the elongation rolling S4. Since the powder lubricant composition according to the first aspect of the present invention is supplied to the inner surface of the pipe in the adhering the powder lubricant composition S3, there is no conventional problem in which the drawing of the mandrel is difficult in the drawing S5.

(38) (Post-Treating S6)

(39) In a post-treating S6, post-treating is performed on the pipe from which the mandrel bar has been drawn in the drawing S5. The content of the post-treatment in the post-treating S6 is the same as the content of a post-treatment normally performed after elongation rolling in manufacturing of a seamless steel pipe. As specific conditions of the post-treating, for example, cutting and removing of the end of the pipe, reheating, descaling, and the like are illustrated by an example.

(40) The manufacturing method S10 is completed through S1 to S6.

(41) In the description of the embodiment, the form of the method S10 for manufacturing a seamless steel pipe in which elongation rolling is performed on the hollow blank pipe (work piece), of which the temperature before initial rolling in the mandrel mill is 700 C. or higher and lower than 1000 C., and in which the total reduction is 50% or more, has been illustrated. However, the embodiment is not limited to such a form. A form in which the total reduction is less than 50% in the elongation rolling can be also employed.

(42) In the description of the embodiment, the form of the method S10 for manufacturing a seamless steel pipe in which the temperature of the hollow blank pipe before initial rolling in the elongation rolling is 700 C. or higher and lower than 1000 C. has been illustrated. However, the embodiment is not limited to such a form. A form in which the temperature of the hollow blank pipe before initial rolling in the elongation rolling is 1000 C. or higher can be also employed. In the form, the effects of reducing friction on the inner surface of the pipe and suppressing flaw generation on the inner surface of the pipe are exhibited.

(43) In the description of the embodiment, the form of the method S10 for manufacturing a seamless steel pipe in which the elongation rolling is performed by mandrel mill rolling in S4 has been illustrated. However, the embodiment is not limited to such a form. A form of the method for manufacturing a seamless steel pipe in which the elongation rolling is performed by other methods, for example, plug mill rolling, can be employed. In such a form, the effects of reducing friction on the inner surface of the pipe and suppressing flaw generation on the inner surface of the pipe are also exhibited.

EXAMPLES

(44) Hereinafter, the present invention will be described in more detail below based on Examples and Comparative Examples. However, the present invention is not limited to these examples.

Examples 1 to 5 and Comparative Examples 1 to 4

(45) The effects exhibited by the powder lubricant composition of the present invention will be described based on the evaluation by a hot rolling test.

Examples 1 to 5

(46) Each component shown in Table 1 was mixed by a powder mixer to have a content ratio shown in Table 1. Thus, the powder lubricant composition according to the first aspect of the present invention was prepared. The details of each component are as follows.

(47) Borax: sodium tetraborate decahydrate (manufactured by Kishida Chemical Co., Ltd., having a purity of 98%)

(48) Cryolite: manufactured by Kishida Chemical Co., Ltd., having a purity of 97%

Comparative Examples 1 to 4

(49) Each powder lubricant composition was prepared in the same manner as in Examples 1 to 5 except that the content ratio of each component was changed as shown in Table 1.

(50) [Table 1]

(51) (Hot Rolling Test)

(52) The lubrication properties of each of the prepared powder lubricant compositions were evaluated by a hot rolling test. FIG. 2 is a schematic view illustrating a hot rolling testing apparatus 10 used in the evaluation as seen from a cross-sectional view. The up-and-down direction of the drawing of FIG. 2 is a vertical direction and a direction from the left side of the drawing of FIG. 2 to the right side of the drawing is a rolling direction. The hot rolling testing apparatus 10 shown in FIG. 2 includes a roll 1 and a plate-like tool 2. In the test, a heated material to be rolled 3 (corresponding to the work piece) was interposed between the plate-like tool 2 to which a graphite-based lubricant was applied (the plate-like tool 2 being movable in the rolling direction at a predetermined rate) and the roll 1 and was rolled to simulate a rolling state with a single stand in the mandrel mill. Then, the thrust force acting on the plate-like tool 2 in the rolling direction was measured. In FIG. 2, the arrow A represents a rotation direction of the roll 1, the arrow B represents a moving direction of the plate-like tool 2, the arrow P represents a pressing force loaded on the roll 1, and the arrow F represents a force loaded to keep the moving rate of the plate-like tool 2 constant against the thrust force acting on the plate-like tool 2. The place in which the lubricant of each Example and the lubricant of each Comparative Example are present is shown as a hatched portion indicated by the numerical reference 4.

(53) Further, in addition to the measurement of the thrust force, after the test was completed, whether seizure occurred or not was confirmed by observing the surface of the plate-like tool 2 to evaluate the seizure properties of each lubricant.

(54) As the material to be rolled, a plate of carbon steel (S25C) having a thickness of 10 mm was applied. The material to be rolled was heated to a predetermined temperature in a heating furnace and then taken out of the furnace. The powder lubricant composition was scattered on the surface of the material to be rolled on the side in which the material to be rolled was in contact with a plate-like tool, and immediately after the scattering, the material to be rolled was fed for rolling. As the plate-like tool, SKD6 with Cr plating was applied. As the graphite-based lubricant to be applied to the plate-like tool, a graphite-vinyl acetate-based lubricant was applied, in which the lubricant was applied to the plate-like tool with a brush. The heating was performed until the temperature reached 830 C. to 1200 C. in a nitrogen atmosphere. The rolling was performed under the conditions of a roll peripheral rate of 78.5 mm/s, a moving rate of the plate-like tool of 30 mm/s, and a reduction of 30%. The results are shown in Table 2.

(55) The condition of a reduction of 30% with a single stand is a severe condition for lubrication. This is because a mill including 5 to 9 stands is used and a total reduction of 50% is achieved in typical mandrel mill rolling.

(56) [Table 2]

(57) In Table 2, the evaluation criteria for the hot rolling test results are as follows.

(58) A: When the thrust force is 0.14 tons or less

(59) B: When the thrust force is more than 0.14 tons and equal to 0.17 tons or less

(60) C: When the thrust force is more than 0.17 tons and equal to 0.20 tons or less

(61) D: When the thrust force is more than 0.20 tons

(62) The lubricants with the evaluations A to C were defined as acceptable products and the lubricants with the evaluation D were defined as unacceptable products.

(63) In Table 2, the evaluation criteria for the seizure resistance test results are as follows.

(64) a: When there is no seizure

(65) b: When fine seizure which can be confirmed by observing a 10-times enlarged sample surface is generated

(66) c: When seizure which can be confirmed by observing the sample surface with the naked eyes is generated

(67) The lubricants with evaluation a or b were defined as acceptable products and the lubricants with evaluation c were defined as unacceptable products.

(68) (Evaluation Results)

(69) As shown in Table 2, the powder lubricant compositions of Examples 1 to 5 in which the blending ratio between borax and cryolite was within a range of 95:5 to 65:35 exhibited good lubrication properties in a wide temperature range. Among the lubricant compositions, the powder lubricant composition of Example 3 in which the blending ratio between borax and cryolite was within a range of 85:15 to 75:25 was able to reduce the thrust force to 0.14 tons or less, did not cause seizure, and exhibited very good lubrication properties in a wide temperature range from a low temperature of 750 C. to a high temperature of higher than 1000 C.

(70) Contrarily, the powder lubricant composition of Comparative Example 1 containing only borax without cryolite exhibited poor lubrication properties in a low temperature range of lower than 1000 C. The powder lubricant composition of Comparative Example 4 containing a large amount of cryolite exhibited poor lubrication properties in a high temperature range of higher than 1000 C.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

(71) 10: HOT ROLLING TESTING APPARATUS 1: ROLL 2: PLATE-LIKE TOOL 3: MATERIAL TO BE ROLLED 4: LUBRICANT S1: PIERCING ROLLING S2: HEATING S3: ADHERING POWDER LUBRICANT COMPOSITION S4: ELONGATION ROLLING S5: DRAWING S6: POST-TREATING S10: MANUFACTURING METHOD

(72) TABLE-US-00001 TABLE 1 COMPONENT EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 BORAX 95 90 80 70 65 CRYOLITE 5 10 20 30 35 COMPARATIVE COMPARATIVE COMPARATIVE COMPARATIVE COMPONENT EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 BORAX 100 97 63 60 CRYOLITE 3 37 40 UNIT: PARTS BY MASS

(73) TABLE-US-00002 TABLE 2 TEMPERATURE BEFORE INITIAL ROLLING ( C.) EVALUATION EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 750 THRUST 0.136 0.099 0.107 0.117 0.122 FORCE A A A A A SEIZURE a a a a a RESISTANCE 820 THRUST 0.173 0.153 0.134 0.14 0.143 FORCE C B A A B SEIZURE a a a a a RESISTANCE 1080 THRUST 0.166 0.163 0.134 0.147 0.185 FORCE B B A B C SEIZURE a a a a a RESISTANCE TEMPERATURE BEFORE INITIAL ROLLING COMPARATIVE COMPARATIVE COMPARATIVE COMPARATIVE ( C.) EVALUATION EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 750 THRUST 0.208 0.171 0.125 0.129 FORCE D C A A SEIZURE c b a a RESISTANCE 820 THRUST 0.262 0.206 0.145 0.151 FORCE D D B B SEIZURE c c a a RESISTANCE 1080 THRUST 0.184 0.188 0.201 0.202 FORCE C C D D SEIZURE b b c c RESISTANCE UNIT OF THRUST FORCE: ton