Patent Application
20100276038
The present invention relates to steelmaking, more precisely martensitic stainless steels which are intended, for example, for the production of moulds for producing plastics materials by means of injection.
In order to produce moulds for material injection, the industry uses stainless steels of the family AISI 420 which have a chromium content of from 12 to 15% (in percentages by weight, as is the case for all the contents indicated in the remainder of the text), a silicon content of less than 1%, a manganese content of less than 1%, a carbon content of from 0.16 to 0.45% and a nitrogen content which is the one which results naturally from the production operation and which is generally up to 0.03%. Generally, the content of vanadium does not exceed 0.1% and is the result of simple fusion of the raw materials. In the same manner, the content of molybdenum is the result of the fusion of the raw materials and does not exceed 0.2%, unless from 0.2 to 1.0% is added in order to improve the corrosion resistance. More specifically, the steel with the reference X40Cr14 which is capable, owing to its carbon content of from 0.36 to 0.45%, of exceeding a hardness of 50 HRC, provides significant abrasion resistance.
Taking into account the application envisaged, the effectiveness of the material must be evaluated by obtaining a good compromise between the following properties:
After machining a blank to approximate dimensions of the final shape, the moulds are subjected to the following thermal processing operation in an oven under a controlled atmosphere:
Two temperature ranges are generally proposed for tempering:
Theoretically, the two successive tempering operations are both carried out in the same range.
Careful consideration must be given as to the precise selection of the processing parameters.
For the quenching, it is metallurgically recommended to seek high quenching temperatures in order to achieve a favourable martensitic microstructure. However, high quenching temperatures promote deformation and produce residual stresses which are capable of leading to fractures. In practice, the gas pressures are limited to values of from 2 to 4 bar.
When quenching stops, before continuing with the tempering operations, fractures are possible if the cooling continues down to ambient temperature. However, the usual selection of stopping the cooling at approximately 80° C. involves the risk of retaining residual austenite, in particular if the subsequent tempering operations are fixed below 500° C., and consequently not being able to obtain the desired nominal hardness.
For the tempering operations, the selection of low temperatures only allows the constraints to be partially overcome, and if the composition of the steel and the quenching cycle have allowed residual austenite to remain, with the tempering operation not decomposing it, the desired hardness is not achieved. High-temperature tempering operations decompose the austenite and relax the residual stresses, but reduce the strength and the corrosion resistance.
There is also the problem of the cost of these steels, owing to the high contents of alloy elements which they require and which it would be necessary to be able to minimise without impairing the desired properties.
The object of the invention is to define an economic steel composition for applications involving moulds for the production of articles of plastics materials which have, with respect to the references AISI 420 and X40Cr14, the following properties:
To this end, the invention relates to a martensitic stainless steel, characterised in that it comprises, in percentages by weight:
The invention also relates to a method for producing a component of martensitic stainless steel, characterised in that:
The tempering operations can each be carried out at a temperature of from 200 to 400° C., preferably from 300 to 380° C. for a minimum of 2 hours whilst ensuring that the nominal temperature is maintained in the core for at least 1 hour, in order to obtain a hardness of from 49 to 55 HRC.
The tempering operations can each be carried out at a temperature of from 530 to 560° C. for a minimum of 2 hours whilst ensuring that the nominal temperature is maintained in the core for at least 1 hour, in order to obtain a hardness of between 42 and 50 HRC.
The invention also relates to a component of martensitic stainless steel, characterised in that the element produced using the method is produced in accordance with the preceding method.
This may be a mould element which is intended for the production of articles of plastics materials.
As will be appreciated, the invention is based on a steel composition whose contents of carbon and chromium are simultaneously at the lower end of the ranges generally required, or even sometimes below in the case of the chromium content, with the imposition of precise conditions on other elements which are present or which must be limited or avoided. A production method is associated with this composition.
The inventors concentrated on actually taking into consideration the properties of the steel following the production operation, and in particular the industrial processing as described above, and not in accordance with laboratory conditions. The research was carried out with the intent of optimising the action of the alloy elements in order to limit the quantity thereof introduced.
The main considerations which have led to the invention are as follows.
The polishability and the surface quality of the polished state of the steel are degraded by:
Generally, the strength, which is moderate for this family of steels, becomes lower for a given hardness, as the chromium content increases. It could be improved by balancing the composition, in particular with additions of nickel and manganese which allow a residue of austenite to be retained during the quenching operation. This solution which further no longer has an effect if the tempering operations are carried out above 500° C. is, however, found to be unstable and impedes the production of the hardness. It has not been retained, particularly since it was not compatible with the desired reduction of the content of alloy elements.
In order to achieve the set objectives, it has been decided:
The invention will be better understood from the following description, given with reference to the following appended Figures:
Table 1 sets out the compositions of the samples examined. The sample “reference” corresponds to a steel of the conventional type X40Cr14. The samples Exp. 1 to Exp. 7 are not in accordance with the invention but allow the disadvantages to be overcome which are involved in not complying with all the conditions required by the invention. The samples Inv. 1 and Inv. 2 are in accordance with the invention.
An object of the invention is therefore to provide an optimised steel which is intended to be processed in accordance with the range of industrial quenching rates, preferably with a subsequent dual tempering operation at low temperature (<400° C.) for a hardness of 52 HRC with a strength and a corrosion resistance which are equal to or greater than those of the reference steel AISI 420 or X40Cr14 in its usual application.
Furthermore, an object of the invention is to limit to the greatest possible extent the additions of alloy elements, in particular metal elements, in order to reduce the production cost, prevent the presence of residual austenite after the quenching operation and reduce the extent of interdendritic segregation which is detrimental to the strength and the quality of the polishing.
To this end, the inventors have arrived at the following results relating to the definition of the composition of the steels of the invention.
The nitrogen content must be between 0.05% and 0.15% and preferably between 0.08% and 0.12%. This element is therefore systematically present with a high content since it is indispensable in order to form carbonitrides of the type V(C,N) which are capable of preventing grain enlargement during austenisation after the chromium carbides have dissolved. However, an excessive content would be detrimental if exceeding the solubility limit in the solid state and would be a source of metallurgical defects. The nitrogen associates with the carbon in order to confer the hardness and is involved in the corrosion resistance. The content of nitrogen can be adjusted by insufflation of gaseous nitrogen when the liquid steel is produced.
Carbon contributes mainly to conferring the hardness required, associated with the nitrogen. Taking into account the hardness required after tempering at low temperature, the percentage must be between 0.22% and 0.32%. Furthermore, the total C+N must be between 0.33% and 0.43% in order to allow the desired hardness to be achieved after tempering.
Chromium confers on the steel the corrosion resistance thereof. Taking into account the industrial quenching rates used, and the tempering range selected, and in accordance with the mechanisms set out above, the content thereof must be between 10 and 12.4% and preferably between 11.0 and 12.4%.
Vanadium must be present at a content of between 0.10% and 0.40% and preferably between 0.15% and 0.35%. The presence thereof is indispensable for forming with the carbon and the nitrogen a sufficient density of micro- and nano-precipitates which are capable of preventing the grain enlargement. An excessive content would be detrimental owing to the excessive fixing of carbon which would impair the hardening and owing to the formation, during solidification, of carbides which are isolated or in a cluster which are unfavourable for the strength and the polish state quality.
Molybdenum complements the action of the chromium for the corrosion resistance; it is present owing to the recycling operations or by intentional addition, at percentages of between 0.10 and 1.0%. A greater content would be detrimental owing to the increase in the extent of the interdendritic segregation and owing to the risk of forming delta ferrite.
Nickel may be present at contents of less than 1.0%, in particular owing to the contribution by the raw materials. No beneficial effect of an addition within this limit has been found in terms of the toughness. However, a greater content would be capable of maintaining the residual austenite in the processed state.
Manganese is an element which is naturally present in this family of steel owing to the production methods and the raw materials available. No beneficial effect has been found and it has been found to be necessary to limit the concentration thereof to 1.0% in order to prevent residual austenite after thermal processing.
Silicon is naturally present for the production and the deoxidation of the steel. The content thereof must be limited to 1.0% and preferably 0.5% since it acts on the process of solidification and the delta/gamma conversion and consequently can bring about the presence of delta ferrite or local segregations owing to the presence of this phase at the end of solidification before forging.
Tungsten may be present at contents of less than 1.0% without having any favourable or detrimental effect on the product. Nonetheless, owing to its individual action or synergy with molybdenum, it may promote the presence of delta ferrite in the state for use or local precipitations or segregations resulting from the presence of delta ferrite at any stage of the thermomechanical process. It will be preferable to comply with the condition 0.10%≦Mo+W/2≦1.20%.
Cobalt and copper have no beneficial effect which has been identified but may be present at contents of less than or equal to 1.0%: higher contents could promote the presence of residual austenite.
It is preferable for the total of the contents of Mn, Cu and Co to be ≦1.8% in order to limit the risks of the presence of residual austenite.
Titanium and niobium are very reactive elements which form very hard precipitates which are detrimental to the quality of polish state. The content thereof must be kept as low as possible: a maximum of 0.010%, preferably a maximum of 0.003% for Ti and a maximum of 0.050%, preferably a maximum of 0.010% for Nb.
The aluminium added for the deoxidation of the steel may remain present in oxide inclusions which are very detrimental to the polish state. The level of addition must be adapted to the production methods used. A maximum content of 0.050% is tolerable, on condition that it does not lead to the presence of inclusions of aluminium oxide or silico aluminates in large quantities which would lead to the acceptable content of O being exceeded (0.0040%, preferably 0.0015%).
Sulphur is preferably limited to a content of less than 0.003% in order to prevent the formation of sulphur inclusions. Optionally, however, it is possible to elect to carry out a voluntary addition in the range of from 0.003 to 0.020% associated preferably with another element (Se up to 0.010%, Ca up to 0.020%, La up to 0.040%, Ce up to 0.040%) promoting the formation of globular sulphides in order to improve the machinability, to the detriment to a certain extent of the quality of the polish state.
The maximum content of oxygen is 0.0040%, preferably 0.0015%. This element is an indicator of the inclusion density, which is detrimental to the polish state of the surface when it is too high. This content must be kept as low as possible, and the production method of the steel must be selected as a result. In practice, known methods allow values as low as O=5 ppm under economically acceptable conditions.
The content of phosphorus is limited to 0.03% which is a common content in this class of steels. No detrimental effect of P has been noted in this range.
Boron can be added in order to improve the quenchability, at a content which does not exceed 0.0050%.
The preferable contents indicated for some elements may be imposed alone and not necessarily in combination with the other preferred contents indicated.
The non-cited elements may be present at contents of the level of impurities resulting from the production which do not modify the properties which the invention seeks to optimise.
The products must be produced in accordance with the provisions of the prevailing art for special high-quality steels which are intended for the applications for moulding articles of plastics materials with the objective of limiting the content of inclusions and the segregation in order to obtain a high quality polish state. The production must comprise, after fusion, a step for deoxidation and elimination of the inclusions in a metallurgical reactor. Preferably, in particular for the production of large moulds and in order to obtain the highest qualities of polish state, a remelting operation using a consumable electrode under a slag will be carried out in order to improve the inclusion purity and distribute the alloy elements, and in particular nitrogen in a homogeneous manner in the entire mass.
A thermomechanical transformation by means of forging or rolling finishing with an annealing operation must follow in order to complement the homogeneity and the compactness of the microstructure.
After machining the component to the final shape and before operation, the products must, in accordance with the preferred operating method, be subjected to a thermal processing operation which comprises austenisation at approximately 1020° C. (from 990 to 1040° C., preferably 1000-1030° C.), a controlled quenching operation, for example, under neutral gas pressure, at a rate of between 10 and 40° C./minute adapted to the size of the component, then two tempering operations at a temperature of from 200 to 400° C., preferably between 300 and 380° C., in order to obtain a hardness of approximately 52 HRC±2 HRC and generally between 49 and 55 HRC.
Optionally, for applications which do not require a hardness greater than 50 HRC, the steel defined by the invention could be processed by means of dual tempering at 530° C. to 560° C. for hardnesses which are less than or equal to 50 HRC and greater than or equal to ±42 HRC, under which conditions the corrosion resistance is found to be adequate.
For the reference steel, the chromium carbides (M23C6) which exist in the delivery state are dissolved during the austenisation which precedes the quenching operation, and the temperature to be maintained is limited to 1020/1030° C. in order to prevent grain growth. However, at this dissolution temperature, a significant quantity of carbides which are distributed in a heterogeneous manner remains. By substituting from approximately 0.10 to 0.15% of the content of carbon with nitrogen, a reduction of approximately 2% of the content of chromium and a simultaneous introduction of vanadium, it is observed at the adequate quenching temperature that the grain which is fixed by means of nanometric precipitates of vanadium carbonitrides V(C,N) does not grow whilst the majority of the chromium carbides are dissolved.
For three of the compositions examined, the compared calculation of the equilibriums at 1030° by means of thermodynamic simulation using the software THERMOCALC (commonly used by metallurgists) illustrates this mutation (see Table 2).
The effective density of the micrometric carbides observed on industrial products and illustrated in
For the reference steel, the capacity for corrosion resistance is, theoretically in accordance with basic knowledge, above all linked to the content of chromium available in the matrix; the thermodynamic calculations show that the carbides which are not dissolved during austenisation fix approximately 0.9% of chromium. This quantity of chromium which is not available for the corrosion resistance becomes lower than 0.1% for the experimental grades which are alloyed with vanadium and nitrogen. In accordance with the following formula:
P.R.E. (Pitting Resistance Equivalent)=% Cr+3.3×% Mo+30×% N
which conventionally allows the compositions to be classified in accordance with their resistance to pitting and applied to the effective composition of the matrix, it is found according to Table 2 that the experimental compositions Inv 1 and 2 have a coefficient which is close to that of the reference.
In addition to the considerations set out above expressing a potential in the crude quenching state, it is advantageous to carry out measurements in the effective state of the metal at the stage of use. The electrochemical method carried out from the standard ASTM G 108 involves polarising the sample for 15 minutes in an aqueous solution of H2SO4 at 1% by weight, at a potential of −550 mV/ECS then carrying out a scanning operation back and forth at 60 mV/mn from −550 mV to +500 mV. The characteristic lines intensity/potential on return may have two peaks, one (peak 1) owing to the dissolution of the matrix, the second (peak 2), at a higher potential, linked to the dissolution in the region of the precipitates of chromium carbides. The steel becomes more sensitive to corrosion as the dissolution current becomes more intense. Characteristic lines are set out in
According to current practices for reference steels, whilst obtaining the desired hardness of approximately 52 HRC, two parameters of the thermal processing are found to be influential for the corrosion resistance: the tempering temperature and the quenching rate.
These effects have been set out by laboratory tests:
This tendency is confirmed for all the compositions tested, as illustrated in
For low tempering temperatures, it has further been verified that the corrosion resistance decreases slightly between 200° C. and 380° C. and degrades rapidly above 400° C.
So that the tempering operations have the intended effect, they must last at least 2 hours, and their nominal temperature must be maintained at the core of the component for at least 1 hour.
Unexpectedly, as illustrated in
These results are confirmed for the various compositions examined.
According to the invention, a quenching rate is selected which is compatible with the technical knowledge of the thermal processing operation and between 10 and 40° C./min in the temperature range from 800 to 400° C.
In conclusion, in the context of an industrial quenching operation the best corrosion resistance is obtained with the low-temperature tempering operations and, in this configuration, the variation of the chromium content in the range from 10.5 to 15% does not confirm the beneficial effect normally recognised for this alloy element.
The same unfavourable effects of reducing the quenching rate and increasing the tempering temperature are found with respect to the toughness. This property is commonly simply appreciated from the conventional mechanical characteristics of elongation and reduction of area during the traction and impact flexion energy test on non-notched bars having dimensions of 55×10×7 mm. For the tests concerned, a quenching operation of 16° C./min was carried out on all the samples then a dual tempering operation of 2 hours. The results set out in Table 3 illustrate:
The compositions of the invention allow the hardness of 52 HRC or above to be obtained after quenching under industrial conditions and dual tempering at 380° C., in spite of the softening occurring in this range for this family of steels from the crude quenching material, as illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 07 57451 | Sep 2007 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2008/051525 | 8/25/2008 | WO | 00 | 6/28/2010 |
