Claims
- 1. A method of continuously carburizing a metal strip comprising the steps of:
- (a) preheating the metal strip;
- (b) heating the metal strip in a heating zone following step (a), to a temperature of 700.about.950.degree. C.;
- (c) maintaining the metal strip heated in step (b) at the temperature of 700.about.950.degree. C. in a uniform heating zone to form a congregated structure having a (1,1,1) organization;
- (d) carburizing the metal strip in a carburizing heating zone at a furnace temperature of 700.about.950.degree. C., in an atmosphere having a carbon monoxide concentration of 0%<CO.ltoreq.22% and hydrogen concentration of 0%.ltoreq.H.sub.2 .ltoreq.30%;
- (e) rapidly cooling the metal strip in a first cooling zone to a temperature of 500.about.400.degree. C. at a cooling speed of approximately 5.degree. C./sec or higher; and
- (f) cooling the metal strip in a second cooling zone to a temperature of 250.about.200.degree. C.
- 2. A method according to claim 1, wherein in step (d), the H.sub.2 concentration in the atmosphere is selected to meet the expression:
- H.sub.2 concentration=.alpha..multidot.(CO concentration),
- where .alpha. is a constant in the range of 0 .ltoreq..alpha.<5, so that a carburizing reaction speed is based on a surface reaction speed.
- 3. A method of continuously carburizing a metal strip, within the carbon surface reaction rate governing basis, by using a host computer, comprising the steps of:
- (a) inputting carburizing conditions including a target carburizing quantity (.DELTA.C.sub.0), composition of atmospheric gas, a flow rate of supplied gas, a carburizing temperature and a strip-passing speed;
- (b) setting a carburizing quantity (.DELTA.C) for introduction to the strip;
- (c) calculating a carburizing quantity (.DELTA.C') for introduction to the strip on the basis of the said carburizing conditions input in step (a);
- (d) comparing the calculated carburizing quantity (.DELTA.C') with the set carburizing quantity (.DELTA.C);
- (e) outputting the composition of atmospheric gas, flow rate of supplied gas, carburizing temperature and the strip passing speed input in step (a) when the result of the comparison in step (d) indicates that the calculated carburizing quantity (.DELTA.C') is approximately equal to the set carburizing quantity (.DELTA.C);
- (f) controlling the carburizing conditions in a carburizing furnace of a continuous carburizing facility to correspond to the output composition of atmospheric gas, flow rate of supplied gas, carburizing temperature and the strip passing speed, if step (e) is performed;
- (g) correcting the set carburizing quantity (.DELTA.C) to be introduced to the strip when the result of the comparison in step (d) indicates that a difference between the calculated carburizing quantity (.DELTA.C') and the set carburizing quantity (.DELTA.C) is larger than a predetermined value;
- (h) comparing the set carburizing quantity (.DELTA.C) corrected in step (g) with the target carburizing quantity (.DELTA.C.sub.0), if step (g) is performed;
- (i) correcting at least one carburizing condition selected from the group consisting of composition of atmospheric gas, carburizing temperature, and strip passing speed when the result of the comparison in step (h) indicates that a difference between the set carburizing quantity (.DELTA.C) corrected in step (g) and the target carburizing quantity (.DELTA.C.sub.0) is larger than a predetermined value, if step (g) and (h) are performed;
- (j) outputting the carburizing conditions corrected in step (i), if steps (g) through (i) are performed; and
- (k) controlling the carburizing conditions in the carburizing furnace in accordance with the carburizing conditions output in step (j), if steps (g) through (j) are performed.
- 4. A method of continuously carburizing a steel strip in a carburizing furnace while being passed through other heating zone for obtaining a desired carburizing quantity and carburizing concentration from the surface of steel strip, comprising the steps of:
- (a) continuously passing the steel strip through a carburizing furnace;
- (b) using a computer, calculating an atmospheric gas composition and the carburizing furnace temperature at which sooting is not generated, said calculation being based on a surface reaction rate of carbon at a surface of the steel strip and on a carbon balance in which the quantity per unit time of carbon in atmospheric gas supplied to the carburizing furnace is equal to the sum of the quantity per unit time of carbon brought out by the steel strip due to carburization and the quantity per unit time of carbon in the atmospheric gas which exits from the carburizing furnace; and
- (c) controlling the atmospheric gas composition and the furnace temperature within the carburizing furnace based on the atmospheric gas composition and furnace temperature calculated in step (b) within the basis of the surface reaction rate governing of carbon.
- 5. A method of continuously carburizing a steel strip according to claim 4, wherein at least one of the atmospheric gas composition and furnace temperature are calculated to achieve a carbon concentration in the steel strip which is equal to or less than an equilibrium concentration with the carbon concentration in the atmospheric gas.
- 6. A method of continuously carburizing a steel strip according to claim 4, wherein the atmospheric gas composition and furnace temperature are calculated in step (b) based on thermodynamics formulae which minimize Gibbs-free energy in the furnace and thereby to obtain an equilibrium state in the furnace.
- 7. A method of continuously carburizing a steel strip according to claim 6, wherein the atmospheric gas composition comprises carbon, oxygen and nitrogen.
- 8. A method of continuously carburizing a steel strip according to claim 6, wherein the atmospheric gas composition comprises carbon, oxygen, hydrogen and nitrogen.
- 9. A method of continuously carburizing a steel strip according to claim 8, wherein:
- the atmospheric gas is calculated and controlled to have a carbon monoxide concentration of 0%<CO concentration.ltoreq.22% and a hydrogen concentration of 0%.ltoreq.H.sub.2 concentration.ltoreq.30%; and
- the furnace temperature is calculated and controlled to be within the range 700.degree. C. to 950.degree. C.
- 10. A method of continuously carburizing a steel strip within the carbon surface reaction rate governing basis, comprising the steps of:
- (a) providing at least one formula selected from the group consisting of a first carburizing surface reaction rate formula based on a steel strip temperature and a carbon monoxide partial pressure, a second carburizing surface reaction rate formula based on the steel strip temperature, the carbon monoxide partial pressure and a hydrogen partial pressure and a formula for predicting a carburizing quantity based on a carburizing time;
- (b) calculating steel strip temperature, atmospheric gas composition and carburizing time based on the at least one formula provided in step (a);
- (c) supplying a carburizing gas into a carburizing furnace and plate-passing the steel strip through the carburizing furnace; and
- (d) controlling the steel strip temperature, atmospheric gas composition and carburizing time to the values calculated in step (b) to achieve reaction conditions where the carbon concentration in the steel strip is equal to or less than an equilibrium concentration with the carbon concentration in an atmospheric gas, and where a carburizing rate into the steel strip is greater than a diffusion rate within the steel strip.
- 11. A method of continuously carburizing a steel strip according to claim 10,
- wherein at least one of the first and second carburizing reaction rate formulas is provided in step (a) and is based on at least one of carbon dioxide partial pressure and water partial pressure.
- 12. A method of continuously carburizing a steel strip according to claim 10, wherein the carburizing time is calculated and controlled to correspond with a plate-passing speed, the plate-passing speed being restricted by an operating conditions other than.
- 13. A method of continuously carburizing a steel strip within the carbon surface reaction rate governing basis, comprising the steps of:
- (a) providing a carbon diffusion model based on Fick's law and a surface reaction rate formula for calculating a desired carbon concentration in a thickness direction of the steel strip at at least one depth in the steel strip;
- (b) calculating a suitable steel strip temperature, a suitable atmospheric gas composition and a carburizing time required for obtaining the desired carbon concentration at the at least one depth in the steel strip based on the carbon diffusion model provided in step (a);
- (c) plate-passing the steel strip through a carburizing furnace supplied with a carburizing gas; and
- (d) controlling the steel strip temperature, atmospheric gas composition and carburizing time within the carburizing furnace based on the values calculated in step (b).
- 14. A method of continuously carburizing a steel strip according to claim 13, wherein a suitable carbon monoxide partial pressure and hydrogen partial pressure are calculated and controlled respectively in steps (b) and (d) when calculating and controlling the suitable atmospheric gas composition.
- 15. A method of continuously carburizing a steel strip according to claim 13, wherein a suitable carbon monoxide partial pressure, a suitable carbon dioxide partial pressure and water partial pressure are calculated and controlled respectively in steps (b) and (d) when calculating and controlling the suitable atmospheric gas composition.
- 16. A method of continuously carburizing a steel strip according to claim 13, wherein the desired carburizing concentration in step (a) is at at least one depth in a range of from 10 to 250 .mu.m.
- 17. A method of continuously carburizing a steel strip according to claim 13, further comprising the step of (e) controlling the temperature of the steel strip after carburizing to thereby control the carbon concentration distribution in the thickness direction of the steel strip.
- 18. A method of continuously carburizing a steel strip comprising the steps of:
- (a) calculating a total carburizing quantity (i) based on one of the following formula for determining a surface carburizing reaction rate (V) of carbon diffusing into a surface of the steel strip without reaching an equilibrium concentration with an atmospheric gas:
- V=k.sub.1 .multidot.f.sub.1 (PCO, PH.sub.2, .theta..sub.0).multidot..alpha..multidot.f.sub.3 (PCO, PCO.sub.2) and
- V=k.sub.1 .multidot.f.sub.1 (PCO, PH.sub.2, .theta..sub.0)-k.sub.2 .multidot.f.sub.2 (PCO.sub.2, PH.sub.2 O),
- where .alpha. is a constant, k.sub.1 and k.sub.2 are reaction rate constants, PCO, PH.sub.2 and PCO.sub.2 are respectively CO, H.sub.2 and CO.sub.2 partial pressures, and (ii) based on the following formula for in-steel carbon diffusion:
- dC/dt=D.multidot.d.sup.2 C/dX.sup.2
- where C is the carbon concentration in steel, t is time, D is a diffusion coefficient, and X is a diffusion distance;
- (b) obtaining suitable ranges for a carburizing temperature, concentrations of CO, H.sub.2, CO.sub.2 and H.sub.2 O in the atmospheric gas, and a carburizing time, for achieving the total carburizing quantity calculated in step (a);
- (c) controlling said carburizing temperature, said concentrations of CO, H.sub.2, CO.sub.2, and H.sub.2 O, and said carburizing time in a carburizing furnace; and
- (d) passing the steel strip through the carburizing furnace.
- 19. A method of continuously carburizing a steel strip according to claim 18, wherein
- the total carburizing quantity calculated in step (a) is also (iii) based on a carburizing time which is determined by a plate-passing speed, the plate-passing speed being restricted by operating conditions other than carburizing; and
- suitable ranges for the carburizing temperature and concentrations of CO, H.sub.2, CO.sub.2 and H.sub.2 O in the atmospheric gas are obtained in step (b) and controlled in step (c) with respect to the carburizing time which is determined by the plate-passing speed.
- 20. A method of continuously carburizing a steel strip according to claim 18 wherein the carburizing concentration is controlled to a concentration distribution in a range of depth of 10 to 250 .mu.m.
- 21. A method of continuously carburizing a steel strip according to claim 18, further comprising the step of (e) after step (d), controlling the temperature of the steel strip to thereby control the carburizing concentration distribution in a thickness direction of the steel strip.
- 22. A method of continuously carburizing a steel strip comprising the steps of:
- (a) calculating a total carburizing quantity (i) based on the following formula of a surface carburizing reaction rate (V) of carbon diffusing into a surface of the steel strip without reaching an equilibrium concentration with an atmospheric gas:
- V=k.sub.1 .multidot.f.sub.1 (PCO, PH.sub.2, .theta..sub.0).multidot..alpha..multidot.f.sub.3 (PCO, PCO.sub.2) and
- V=k.sub.1 .multidot.f.sub.1 (PCO, PH.sub.2, .theta..sub.0)-k.sub.2 .multidot.f.sub.2 (PCO.sub.2, PH.sub.2 O),
- where .alpha. is a constant, k.sub.1 and k.sub.2 are reaction rate constants, PCO, PH.sub.2 and PCO.sub.2 are respectively CO, H.sub.2 and CO.sub.2 partial pressures, and (ii) based on the following formula for in-steel carbon diffusion:
- dC/dt=D.multidot.d.sup.2 C/dX.sup.2
- where C is the carbon concentration in steel, t is time, D is a diffusion coefficient, and X is a diffusion distance;
- (b) obtaining suitable ranges for a carburizing temperature, concentrations of CO, H.sub.2, CO.sub.2 and H.sub.2 O in the atmospheric gas, and a carburizing time, for achieving the total carburizing quantity calculated in step (a);
- (c) obtaining a flow rate of atmospheric gas to be supplied to a carburizing furnace, the atmospheric gas having the suitable ranges obtained in step (b) for the concentrations of CO, H.sub.2, CO.sub.2 and H.sub.2 O, the flow rate and the concentrations of CO, H.sub.2, CO.sub.2 and H.sub.2 O
- (i) satisfying a carbon balance in the furnace expressed by W.sup.g.sub.I =W.sup.s.sub.c +W.sup.g.sub.o, and
- W.sup.s.sub.c =.xi. (V, t, w, LS)
- where W.sup.g.sub.I is the mass of carbon in the atmospheric gas entering the furnace, W.sup.s.sub.c is the mass of carbon diffused into the steel strip and exiting the furnace in the steel strip, W.sup.g.sub.o is the mass of carbon in the atmospheric gas exiting the furnace, V is the surface reaction rate used in step (a), t is the carburizing time, w is the width of the steel strip, and LS is the line speed of the steel strip,
- (ii) satisfying a requirement that free, condensed carbon is zero, and
- (iii) minimizing the Gibbs' free energy f(x) expressed by: ##EQU8## where n is the number of kinds of gases and p is the number of kinds of condensations;
- (d) in a carburizing furnace, controlling the carburizing temperature, and concentrations of CO, H.sub.2, CO.sub.2 and H.sub.2 O to the ranges obtained in step (b) and controlling the flow rate of atmospheric gas to the rate obtained in step (c); and
- (e) passing the steel strip through the carburizing furnace.
- 23. A method of continuously carburizing a steel strip, comprising the steps of:
- (a) inputting a target carburizing quantity (.DELTA.C.sub.O), a composition of atmospheric gas, a flow rate of supplied gas, a carburizing temperature, a plate-passing speed, and a size of the steel strip;
- (b) calculating a concentration of each component gas in the atmospheric gas, at which concentration sooting generation is prevented, Gibbs' total free energy (F(x)) is minimized and the quantity of carbon in the atmospheric gas supplied to the furnace is equal to the sum of the quantity of carbon brought out by the steel strip due to carburization and the quantity of carbon in the atmospheric gas which exists the carburizing furnace;
- (c) calculating a surface reaction rate (V) per unit area by one of the following formulae under the premise that the carbon concentration in a surface layer of the steel strip is below an equilibrium concentration with the carbon concentration in the atmospheric gas:
- V=k.sub.1 .multidot.f.sub.1 (PCO, PH.sub.2, .theta..sub.0).multidot..alpha..multidot.f.sub.3 (PCO, PCO.sub.2) and
- V=k.sub.1 .multidot.f.sub.1 (PCO, PH.sub.2, .theta..sub.0)-k.sub.2 .multidot.f.sub.2 (PCO.sub.2, PH.sub.2 O),
- where .alpha. is a constant, PCO, PH.sub.2, PCO.sub.2 and PH.sub.2 O are respectively CO, H.sub.2, CO.sub.2 and H.sub.2 O partial pressures, .theta..sub.0 is the coating rate of the absorbed oxygen and k.sub.1 and k.sub.2 are reaction rate constants determined by the following formula:
- k.sub.i =A.sub.i .multidot.exp(-E.sub.i /RT)
- where A.sub.i is a frequency factor, E.sub.i is the activation energy, R is the gas constant, and T is absolute temperature;
- (d) calculating a carburizing quantity (.DELTA.C') by integrating the surface reaction rate (V) per unit area with respect to a carburizing time and with respect to a total area of the steel strip;
- (e) comparing the carburizing quantity (.DELTA.C') calculated in step (d) with the target carburizing quantity (.DELTA.C.sub.0) input in step (a), and changing at least one of the carburizing temperature, the plate-passing speed and the atmospheric gas composition, and repeating steps (b)-(d) to recalculate the concentration of each component gas, the surface reaction rate (V) the carburizing quantity (.DELTA.C') when a difference between the calculated carburizing quantity (.DELTA.C') and the target carburizing quantity (.DELTA.C.sub.0) is greater than or equal to a predetermined value;
- (f) outputting the carburization temperature, the plate-passing speed and the concentration of each component gas of the atmospheric gas when the difference between the calculated carburizing quantity (.DELTA.C') and the target carburizing quantity (.DELTA.C.sub.0) is less than the predetermined value; and
- (g) on the basis of the output in step (f), controlling a carburizing furnace temperature to 700.about.950.degree. C., the carbon monoxide concentration to 0%<CO concentration.ltoreq.22%, and the hydrogen concentration to 0%.ltoreq.H.sub.2 concentration.ltoreq.30%; and
- (h) passing the steel strip through the carburizing furnace.
- 24. A method of continuously carburizing a steel strip, comprising the steps of:
- (a) inputting a target carburizing quantity (.DELTA.C.sub.0), a target carbon concentration (C.sub.1) at a designated depth (X.sub.1) from a surface of the steel strip, a composition of an atmospheric gas, a flow rate of the atmospheric gas, a carburizing temperature, a plate-passing speed, and a size of the steel strip plate;
- (b) calculating a concentration of each component gas in an atmospheric gas system, at which concentration sooting generation is prevented, Gibbs' total free energy (F(x)) is minimized and the quantity of carbon in the atmospheric gas supplied to the furnace is equal to the sum of the quantity of carbon brought out by the steel strip due to carburization and the quantity of carbon in atmospheric gas which exists the carburizing furnace;
- (c) calculating a diffusion rate per unit area (dC/dt) of solid carbon into the steel strip and obtaining a carbon diffusion quantity into the steel strip using the following formula:
- dC/dt=D.multidot.d.sup.2 C/dX.sup.2
- where C is the carbon concentration in the steel strip, t is time, D is a diffusion coefficient, and X is a diffusion distance;
- (d) calculating a carburizing quantity (.DELTA.C') by integrating the diffusion rate per unit area (dC/dt) with respect to a carburizing time and with respect to a total area of the steel strip;
- (e) comparing the carburizing quantity (.DELTA.C') calculated in step (d) with the target carburizing quantity (.DELTA.C.sub.0) input in step (a), and changing at least one of the carburizing temperature, the plate-passing speed and the atmospheric gas composition, and repeating steps (b)-(d) to recalculate the concentration of each component gas, the diffusion rate per unit area (dC/dt) of solid carbon, and the carburizing quantity (.DELTA.C') when a difference between the calculated carburizing quantity (.DELTA.C') and the target carburizing quantity (.DELTA.C.sub.0) is greater than or equal to a predetermined value;
- (f) calculating a carbon concentration (C'.sub.1) at the designated depth (X.sub.1) from the surface of the steel plate by the formula dC/dt=D.multidot.d.sup.2 C/dX.sup.2 used in step (c), when the difference between the calculated carburizing quantity (.DELTA.C') and the target carburizing quantity (.DELTA.C.sub.0) is smaller than the predetermined value;
- (g) comparing the carbon concentration (C'.sub.1) at the designated depth (X.sub.1) calculated in the step (f) with the target carbon concentration (C.sub.1) at the designated depth (X.sub.1) input in step (a), and changing at least one of the carburizing temperature, the plate-passing speed, and the atmospheric composition and repeating steps (b)-(f) when a difference between the calculated carbon concentration (C'.sub.1) and the target C concentration (C.sub.1) is greater than or equal to a predetermined value, and outputting the carburizing temperature, the plate-passing speed, the concentration of each component in the atmospheric gas, and the carburizing concentration to a depth of at least 10-250 .mu.m below the surface of the steel strip when the difference is less than the predetermined value;
- (h) on the basis of the output in step (g), controlling a carburizing furnace temperature to 700-950.degree. C., the carbon monoxide concentration to 0% to 22%, and the hydrogen concentration to 0% to 30% and controlling the carburizing concentration to the output carburizing concentration to a depth of at least 10-250 .mu.m; and
- (i) passing the steel strip through the carburizing furnace.
- 25. A method of continuously carburizing a steel strip according to claim 18, wherein the carburizing furnace is incorporated as a part of a continuous annealing furnace.
- 26. A method of continuously carburizing a steel strip according to claim 22, wherein the carburizing furnace is a continuous annealing furnace.
- 27. A method of continuously carburizing a steel strip according to claim 23, wherein the carburizing furnace is a continuous annealing furnace.
- 28. A method of continuously carburizing a steel strip according to claim 24, wherein the carburizing furnace is a continuous annealing furnace.
- 29. A method of continuously carburizing a steel strip according to claim 7, wherein:
- the atmospheric gas is calculated and controlled to have a carbon monoxide concentration of 0%<CO concentration.ltoreq.22%; and
- the furnace temperature is calculated and controlled to be within the range 700.degree. C. to 950.degree. C.
- 30. A method of continuously carburizing a steel strip according to claim 13, wherein a suitable carbon monoxide partial pressure is calculated and controlled respectively in steps (b) and (d) when calculating and controlling the suitable atmospheric gas composition.
- 31. A method of continuously carburizing a steel strip according to claim 13, wherein a suitable carbon monoxide partial pressure, suitable hydrogen partial pressure, suitable carbon dioxide partial pressure and water partial pressure are calculated and controlled respectively in steps (b) and (d) when calculating and controlling the suitable atmospheric gas composition.
- 32. A method of continuously carburizing a steel strip according to claim 12, wherein the plate-passing speed is restricted within a range by operating conditions other than carburizing, and the plate-passing speed is controlled within the range to optimize carburizing time.
- 33. A method of carburizing a steel strip, comprising the steps of:
- (a) passing the steel strip through a carburizing furnace;
- (b) determining a surface reaction rate of carbon at a surface of the steel strip;
- (c) using calculations governed by the surface reaction rate of carbon at the surface of the steel strip, determining an atmospheric gas composition and a furnace temperature at which sooting is not generated; and
- (d) controlling the atmospheric gas composition and the furnace temperature within the carburizing furnace based on the atmospheric gas composition and furnace temperature determined in step (c).
Parent Case Info
This application is a continuation, of application Ser. No. 08/638,868, filed Apr. 29, 1996, now abandoned which is a continuation of application Ser. No. 08/244,991, filed Jun. 15, 1994, now abandoned.
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Continuations (2)
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Number |
Date |
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Parent |
638868 |
Apr 1996 |
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Parent |
244991 |
Jun 1994 |
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