Method for production and forming of cast pieces of spheroidal graphite with improved mechanical properties

Abstract

The invention relates to a method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics. The method comprises the following steps: a) preparing a mixture in the liquid state comprising by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, 0.025% to 0.080% Mg, the balance being iron and impurities resulting from preparation; b) casting the mixture in the liquid state at a temperature of from 1350° C. to 1550° C. into a mold c) extracting a blank of the part from the mold at a temperature between solidus and AR3; d) forming the blank of the part by hot plastic deformation; the forming taking place at a temperature of from 1050° C. and AR3; e) quenching said part directly in the heat of forming at a temperature in the bainitic range, and maintaining said temperature; and f) cooling said part to ambient temperature.

1 - 20 . (canceled)  
   
   
       21 . A method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics, the method comprising the following steps: 
 a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%;    b) casting the mixture in the liquid state at a temperature of from 1350° C. to 1550° C. into a mold enabling a shape approaching that of the workpiece, which is referred to as the blank of the part;    c) extracting said blank of the part from the mold at a temperature Ts ranging from the solidus to AR3, wherein the solidus and AR3 representing the temperatures defining the austenitic region of said composition;    d) forming a blank of the part by hot plastic deformation directly in the heat of casting, in order to obtain the part with its final shape and dimensions, the forming taking place at a temperature Tf of from 1050° C. to AR3;    e) quenching said part directly in the heat of forming at a temperature Tb lying in the bainitic range, and maintaining the part at said temperature Tb for a duration tb; and    f) cooling said part to ambient temperature.    
   
   
       22 . A method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics, the method comprising the following steps: 
 a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%;    b) casting the mixture in the liquid state at a temperature of from 1350° C. to 1550° C. into a mold enabling a shape approaching that of the workpiece, which is referred to as the blank of the part;    c) extracting said blank of the part from the mold at a temperature Ts ranging from the solidus and AR3, the solidus and AR3 representing the temperatures defining the austenitic region of said composition;    c′) maintaining the blank at a temperature Tm that is 20° C. to 50° C. higher than a forging temperature, for a duration of from 10 min to 60 min, so as to ensure that the temperature inside the blank is homogeneous;    d) forming a blank of the part by hot plastic deformation, in order to obtain the part with its final shape and dimensions, the forming taking place at a temperature Tfoffrom 1050° C. to AR3;    e) quenching said part directly in the heat of forming at a temperature Tb lying in the bainitic range, and maintaining the part at said temperature Tb for a duration tb; and    f) cooling said part to ambient temperature.    
   
   
       23 . A method according to  claim 22 , wherein when the temperature Ts at which the blank is extracted from the mold is lower than the temperature Tf desired for forging, the blank is heated and maintained at the temperature Tm that is 20° C. to 50° C. greater than the forging temperature Tf for a duration of from 10 min to 60 min during step c′).  
   
   
       24 . A method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics, the method comprising the following steps: 
 a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%;    b) casting the mixture in the liquid state at a temperature of from 1350° C. to 1550° C. into a mold enabling a shape approaching that of the workpiece, which is referred to as the blank of the part;    c) extracting said blank of the part from the mold at a temperature Ts ranging from the solidus and AR3, the solidus and AR3 representing the temperatures defining the austenitic region of said composition;    d) forming a blank of the part by hot plastic deformation directly in the heat of casting, in order to obtain the part with its final shape and dimensions, the forming taking place at a temperature Tf of from 1050° C. to AR3;    e) cooling the part in the intercritical region at a temperature T ir  of from AR1+20° C. to AR3, the part being maintained at said temperature T ir  for a duration of from 15 min to 60 min to allow its structure to become homogeneous, and performing bainitic staged quenching at a temperature Tb situated in the bainitic region, and maintaining the part at the temperature Tb for a duration tb, AR1, AR3, AC1, and AC3 representing the limits of the critical interval or of the intercritical region of said composition as measured respectively on cooling (index R) and on heating (index C); and    f) cooling said part to ambient temperature.    
   
   
       25 . A method according to  claim 24 , wherein after the part has been formed by hot plastic deformation in step d), said part being allowed to cool to below the temperature AR1, and then heating the part and maintaining the part at a temperature T ic  of from AC1+20° C. to AC3, the intercritical region of the composition of the cast iron, for a duration of from 30 min to 180 min in order to allow the part's structure to become homogeneous, and bainitic staged quenching is performed at a temperature Tb situated in the bainitic region and the part is maintained at the temperature Tb for a duration tb, AR1, AR3, AC1, and AC3 representing the limits of the critical interval of said composition as measured respectively on cooling (index R) and on heating (index C); and 
 said part is cooled to ambient temperature.    
   
   
       26 . A method according to  claim 21 , wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.  
   
   
       27 . A method according to  claim 21 , wherein an additional step of cold calibration of the part is added after the step of cooling the part to ambient temperature, the additional step being performed between at least two matrices having the shape of the finished part in order to improve its dimensional precision and to increase the mechanical characteristics and the fatigue strength of the material of the part by work-hardening its surface.  
   
   
       28 . A method according to  claim 27 , wherein prior to the operation of cold calibration, a shot-blasting operation is performed for the purpose of removing scale and delivering compression stresses to the surface enabling the work-hardening effect due to calibration to be reinforced.  
   
   
       29 . A method according to  claim 21 , wherein the mold used for casting the blank of the part is a permanent mold constituted by at least two metal half-portions coated in a release agent.  
   
   
       30 . A method according to  claim 21 , wherein the rate of cooling corresponding to bainitic staged quenching lies in the range 15° C./sec to 150° C./sec.  
   
   
       31 . A method according to  claim 21 , wherein the temperature T ic  or T ir  at which the part is maintained in the intercritical region or critical interval lies in the range 740° C. to 850° C.  
   
   
       32 . A method according to  claim 21 , wherein the hot plastic deformation performed at a temperature greater than AC3 or AR3 for said cast iron composition comprises a stamping operation, an operation of forging, rolling, or hydroforming.  
   
   
       33 . A method according to  claim 21 , wherein the hot plastic deformation is deformation at a ratio lying overall in the range 2% to 60%.  
   
   
       34 . A method according to  claim 21 , wherein the cast blank of the part is of a shape approaching that of the part and possessing at least one dimension that is greater than that of the part so as to make said hot plastic deformation possible.  
   
   
       35 . A method according to  claim 21 , wherein the temperature Tb of the bainitic staged quenching lies in the range 260° C. to 420° C.  
   
   
       36 . A method according to  claim 21 , wherein the duration tb for which the temperature Tb of bainitic staged quenching is maintained is from 60 min to 180 min.  
   
   
       37 . Spheroidal graphite cast iron having high-grade mechanical characteristics, and having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%, characterized in that its structure is essentially bainitic.  
   
   
       38 . Spheroidal graphite cast iron having high-grade mechanical characteristics, and having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%, characterized in that its structure is made up of two varieties of ferrite and austenite.  
   
   
       39 . A cast iron part prepared from the spheroidal graphite cast iron according to  claim 37 .  
   
   
       40 . A method according to  claim 21 , wherein all of the successive steps needed for making the cast iron part from the casting step to the step of cooling the part to ambient temperature are performed without intermediate heating between said steps.  
   
   
       41 . A method according to  claim 22 , wherein an additional step of cold calibration of the part is added after the step of cooling the part to ambient temperature, the additional step being performed between at least two matrices having the shape of the finished part in order to improve its dimensional precision and to increase the mechanical characteristics and the fatigue strength of the material of the part by work-hardening its surface.  
   
   
       42 . A method according to  claim 41 , wherein prior to the operation of cold calibration, a shot-blasting operation is performed for the purpose of removing scale and delivering compression stresses to the surface enabling the work-hardening effect due to calibration to be reinforced.  
   
   
       43 . A method according to  claim 22 , wherein the mold used for casting the blank of the part is a permanent mold constituted by at least two metal half-portions coated in a release agent.  
   
   
       44 . A method according to  claim 22 , wherein the rate of cooling corresponding to bainitic staged quenching lies in the range 15° C./sec to 150° C./sec.  
   
   
       45 . A method according to  claim 22 , wherein the temperature Tic or T ir  at which the part is maintained in the intercritical region or critical interval lies in the range 740° C. to 850° C.  
   
   
       46 . A method according to  claim 22 , wherein the hot plastic deformation performed at a temperature greater than AC3 or AR3 for said cast iron composition comprises a stamping operation, an operation of forging, rolling, or hydroforming.  
   
   
       47 . A method according to  claim 22 , wherein the hot plastic deformation is deformation at a ratio lying overall in the range 2% to 60%.  
   
   
       48 . A method according to  claim 22 , wherein the cast blank of the part is of a shape approaching that of the part and possessing at least one dimension that is greater than that of the part so as to make said hot plastic deformation possible.  
   
   
       49 . A method according to  claim 22 , wherein the temperature Tb of the bainitic staged quenching lies in the range 260° C. to 420° C.  
   
   
       50 . A method according to  claim 22 , wherein the duration tb for which the temperature Tb of bainitic staged quenching is maintained is from 60 min to 180 min.  
   
   
       51 . A method according to  claim 24 , wherein an additional step of cold calibration of the part is added after the step of cooling the part to ambient temperature, the additional step being performed between at least two matrices having the shape of the finished part in order to improve its dimensional precision and to increase the mechanical characteristics and the fatigue strength of the material of the part by work-hardening its surface.  
   
   
       52 . A method according to  claim 51 , wherein prior to the operation of cold calibration, a shot-blasting operation is performed for the purpose of removing scale and delivering compression stresses to the surface enabling the work-hardening effect due to calibration to be reinforced.  
   
   
       53 . A method according to  claim 24 , wherein the mold used for casting the blank of the part is a permanent mold constituted by at least two metal half-portions coated in a release agent.  
   
   
       54 . A method according to  claim 24 , wherein the rate of cooling corresponding to bainitic staged quenching lies in the range 15° C./sec to 150° C./sec.  
   
   
       55 . A method according to  claim 24 , wherein the temperature T ic  or T ir  at which the part is maintained in the intercritical region or critical interval lies in the range 740° C. to 850° C.  
   
   
       56 . A method according to  claim 24 , wherein the hot plastic deformation performed at a temperature greater than AC3 or AR3 for said cast iron composition comprises a stamping operation, an operation of forging, rolling, or hydroforming.  
   
   
       57 . A method according to  claim 24 , wherein the hot plastic deformation is deformation at a ratio lying overall in the range 2% to 60%.  
   
   
       58 . A method according to  claim 24 , wherein the cast blank of the part is of a shape approaching that of the part and possessing at least one dimension that is greater than that of the part so as to make said hot plastic deformation possible.  
   
   
       59 . A method according to  claim 24 , wherein the temperature Tb of the bainitic staged quenching lies in the range 260° C. to 420° C.  
   
   
       60 . A method according to  claim 24 , wherein the duration tb for which the temperature Tb of bainitic staged quenching is maintained is from 60 min to 180 min.  
   
   
       61 . A method according to  claim 22 , wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.  
   
   
       62 . A method according to  claim 23 , wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.  
   
   
       63 . A method according to  claim 24 , wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.  
   
   
       64 . A method according to  claim 25 , wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.  
   
   
       65 . A cast iron part prepared from the spheroidal graphite cast iron according to  claim 38 .  
   
   
       66 . A method according to  claim 22 , wherein all of the successive steps needed for making the cast iron part from the casting step to the step of cooling the part to ambient temperature are performed without intermediate heating between said steps.