Patent Application
20080156403
1. Field of the Invention
The present invention relates to a steel for cold working to be made into machine parts, such as bolts and nuts, particularly automotive parts, said steel being particularly in the form of wire or rod suitable for cold working. The present invention also covers cold-worked parts made out of said steel for cold working.
2. Description of the Related Art
There has been an increasing demand for weight reduction of automotive parts from the view point of fuel economy and environmental protection. Weight reduction needs a high-strength steel, which is obtained usually by increasing the amount of alloying elements in iron.
Various kinds of parts are commonly produced by cold working (in an atmosphere at 200° C. or below) which surpasses hot and warm working in productivity, dimensional accuracy, and yields. High-speed cold working is prevailing for better productivity.
The situation mentioned above requires a steel for cold working which exhibits a low deformation resistance and keeps its deformability at the time of cold working. Any steel with a high deformation resistance leads to a reduced life of molds used for cold working, and any steel with poor deformability is liable to cracking at the time of cold working.
It is known that any steel decreases in deformation resistance and improves in deformability in proportion to the decreasing amounts of its alloying elements such as C, Si, and Mn. The decreased deformation resistance, which is achieved merely by reducing the amount of alloying elements, extends the mold life but adversely affects the strength of worked parts. The conventional way to address this problem, thereby assuring prescribed strength or hardness, was by heat treatment, such as annealing, that follows cold working to make steel into a desired shape.
Unfortunately, heat treatment following cold working brings about a dimensional change in worked parts, and this necessitates additional machining, such as cutting, to restore correct dimensions. It is desirable to ensure prescribed strength for worked parts without heat treatment and ensuing machining from the standpoint of improved productivity and energy saving.
Prior art technologies to address the foregoing problems include the following. Japanese Patent No. 3515923 discloses that a steel keeps a low deformation resistance during working if it has the metallographic structure such that ferrite grains contain fine nitride precipitates which function as nuclei for precipitation of C compounds such as cementite.
Japanese Patent Laid-open No. Sho-60-82618 discloses a steel exempt from age hardening due to dissolved C and dissolved N which contains N and dissolved Al, whose amounts are controlled such that N is fixed in the form of AlN, and which contains C compounds precipitating from C upon aging treatment.
The prior art technologies disclosed in the foregoing documents (Japanese Patent No. 3515923 and Japanese Patent Laid-open No. Sho-60-82618) involve the step of fixing dissolved N and dissolved C in the form of N compounds and C compounds in ferrite grains in order to suppress dynamic strain aging and keep deformation resistance low. The fixing of dissolved N requires incorporation with Al, which is an element to form N compounds. In the presence of 0.039-0.045% Al, as in Example, dissolved N is likely to be nearly absent even though the amount of N is 0.015%. Moreover, N compounds suppress precipitation hardening and prevent crystal grains from becoming coarse, and hence they are likely to increase deformation resistance by other factors than dynamic strain aging.
Japanese Patent Publication No. Sho-57-60416 discloses a method of decreasing deformation resistance at the time of cold working by incorporation with Cr (for solid-solution softening) and Al, thereby fixing dissolved N. This method, however, has the disadvantage that dissolved N is nearly absent (as in the case of technologies disclosed in Japanese Patent No. 3515923 and Japanese Patent Laid-open No. Sho-60-82618 mentioned above) because dissolved N is fixed in the form of N compounds by incorporation with Al. Moreover, N compounds suppress precipitation hardening and prevent crystal grains from becoming coarse, and hence they are likely to increase deformation resistance by other factors than dynamic strain aging, as in the case of the technologies disclosed in Japanese Patent No. 3515923 and Japanese Patent Laid-open No. Sho-60-82618 mentioned above.
Cold-worked parts usually undergo hardening heat treatment, such as quenching and tempering, so that they have prescribed strength. However, such heat treatment should preferably be omitted for improved productivity and energy saving.
For example, Japanese Patent Laid-open No. 2003-266144 discloses that the age hardening treatment (quenching and tempering) after cold working is not necessary if worked parts are cooled at a rate of 50-70° C./h from the temperature raised by cold working to the normal temperature.
Workpieces for cold working should meet contradictory requirements—cold workability (deformation resistance and deformability) and strength after cold working. Workpieces with prescribed strength reduce the mold life and undergo cracking during working. By contrast, workpieces with improved cold workability (for better mold life) lacks strength. So far, there is no steel for cold working which meets the foregoing two requirements. It is an object of the present invention to provide a steel for cold working, particularly a steel wire and rod for cold working, which excels in cold workability during working and exhibits good strength after working.
The first aspect of the present invention is directed to a steel for high-speed cold working which contains:
C: 0.03 to 0.6% (by mass),
P: no more than 0.05% (excluding 0%),
S: no more than 0.05% (excluding 0%), and
N: no more than 0.04% (excluding 0%).
with the remainder being iron and inevitable impurities and the amount of dissolved nitrogen in the steel being no less than 0.006%.
According to the present invention, the steel for cold working may contain C in an amount of 0.03 to 0.15% (in the first embodiment) or 0.15 to 0.6% (in the second embodiment) depending on application and performance required.
According to the basic embodiment and the first and second embodiments of the present invention, the content of N should preferably be no less than 0.007% so that the amount of dissolved N is no less than 0.006%.
According to the basic embodiment and the first and second embodiments of the present invention, the steel may additionally contain, when necessary, Al: no more than 0.1% (excluding 0%).
According to the basic embodiment and the first and second embodiments of the present invention, the steel may additionally contain, when necessary, at least one species selected from the group consisting of:
Zr: no more than 0.2% (excluding 0%),
Ti: no more than 0.1% (excluding 0%),
Nb: no more than 0.1% (excluding 0%),
V: no more than 0.5% (excluding 0%),
Ta: no more than 0.1% (excluding 0%), and
Hf: no more than 0.1% (excluding 0%).
According to the basic embodiment and the first and second embodiments of the present invention, the steel may additionally contain, when necessary, at least one species selected from the group consisting of:
B: no more than 0.0015% (excluding 0%) and/or
Cr: no more than 2% (excluding 0%).
The steel for high-speed cold working mentioned above should preferably satisfy the formula (1) below.
[N]−(14[Al]/27+14[Ti]/47.9+14[Nb]/92.9+14[V]/50.9+14[Zr]/91.2+14[B]/10.8+14[Ta]/180.9+14[Hf]/178.5)≧0.006 Formula (1)
where the square brackets [ ] represent the total amount (in mass %) of each element contained in the steel.
According to the third embodiment of the present invention, the inevitable impurities mentioned above may include:
Al: no more than 0.001% (including 0%),
Ti: no more than 0.002% (including 0%),
Nb: no more than 0.001% (including 0%),
V: no more than 0.001% (including 0%),
Zr: no more than 0.001% (including 0%),
B: no more than 0.0001% (including 0%),
Ta: no more than 0.0001% (including 0%), and
Hf: no more than 0.0001% (including 0%);
and also satisfy the formula (2) below.
14[Al]/27+14[Ti]/47.9+14[Nb]/92.9+14[V]/50.9+14[Zr]/91.2+14[B]/10.8+14[Ta]/180.9+14[Hf]/178.5≦0.002% Formula (2)
where the square brackets [ ] represent the total amount (in mass %) of each element contained in the steel.
According to the third embodiment of the present invention, the steel may additionally contain:
Cr: no more than 2% (excluding 0%).
According to the present invention, the steel for high-speed cold working may additionally contain, when necessary:
Cu: no more than 5% (excluding 0%).
According to the present invention, the steel for high-speed cold working may additionally contain, when necessary:
Ni: no more than 5% (excluding 0%) and/or
Co: no more than 5% (excluding 0%).
According to the present invention, the steel for high-speed cold working may additionally contain, when necessary:
Mo: no more than 2% (excluding 0%) and/or
W: no more than 2% (excluding 0%).
According to the present invention, the steel for high-speed cold working may additionally contain, when necessary:
at least one species selected from the group consisting of
Ca: no more than 0.05% (excluding 0%),
Rare earth elements (REM): no more than 0.05% (excluding 0%),
Mg: no more than 0.02% (excluding 0%),
Li: no more than 0.02% (excluding 0%),
Pb: no more than 0.1% (excluding 0%), and
Bi: no more than 0.1% (excluding 0.1%).
According to the present invention, the steel for high-speed cold working should desirably be used for high-speed cold working at a working temperature no higher than 200° C. and at a strain rate no lower than 100/s.
Incidentally, the strain rate is defined as a ratio of true strain to unit time.
The present invention is also directed to a method for producing a steel for high-speed cold working, said method comprising subjecting the steel stock having the above-mentioned composition consecutively to heating at a temperature above Ac3 point plus 30° C., hot rolling at a temperature above Ac3 point plus 30° C., and quenching to 500° C. or below at a cooling rate no smaller than 0.5° C./s.
The present invention is also directed to a method for producing a steel for high-speed cold working, said method comprising subjecting the steel stock having the above-mentioned composition to heating at a temperature above Ac3 point plus 30° C. and subsequent quenching to 500° C. or below at a cooling rate no smaller than 0.5° C./s.
The present invention is also directed to a part formed from the above-mentioned steel for high-speed cold working by high-speed cold working at a working temperature no higher than 200° C. and at a strain rate no lower than 100/s, wherein said part has a value of H and a value of DR which satisfy the formula (3) below
H≧(DR+1000)/6 formula (3)
where H denotes the strength of part (in terms of Hv) after high-speed cold working and DR denotes the maximum value of deformation resistance (in terms of MPa) during high-speed cold working.
According to the present invention, the steel for high-speed cold working offers the following advantages.
(a) Containing dissolved N in an amount more than a specific level, it imparts desired strength to cold-worked parts thereof despite the omission of heat treatment (such as quenching and tempering) that follows cold working.
(b) It is intended solely for high-speed cold working (preferably at a strain rate no smaller than 100/s).
(c) It has a specific chemical composition suitable for good cold working.
According to the present invention, the steel for high-speed cold working is characterized by containing dissolved N in an amount more than a specific level. Therefore, it is suitable for high-speed cold working and it imparts desired strength to cold-worked parts thereof. This fact is contrary to a common brief that a steel containing a large amount of dissolved N has a large deformation resistance, deteriorates the mold life, and causes cracking to cold-worked parts. The steel according to the present invention permits smooth cold working at high speeds. In other words, the steel according to the present invention is intended solely for high-speed cold working. The present invention is based on a new technical idea that the steel containing dissolved N in an amount more than a specific level provides cold-worked parts thereof having improved strength and permits smooth cold working without adverse effect of dissolved N when it undergoes high-speed cold working.
In addition, high-speed cold working contributes to parts productivity and energy saving.
According to the basic embodiment of the present invention, the steel for high-speed cold working is characterized by an adequate chemical composition for its good cold-workability. The following is concerned with the chemical composition of the steel and the amount of dissolved N in the steel.
C is an element essential for the steel to impart adequate strength to parts formed therefrom by high-speed cold working. The content of C is specified as being no less than 0.03%, preferably no less than 0.04%, more preferably no less than 0.05%. By contrast, an excess amount of C adversely affects machinability and cold-workability. Thus, the upper limit of the C content is specified as being 0.6%, preferably 0.5%, more preferably 0.4%.
Si is an element which is used as a deoxidizer in the steel-making process. Insufficient Si leads to incomplete deoxidation and causes the steel to give off gas during solidification, and the resulting steel is poor in deformability. The amount necessary for Si to fully exhibit its effect is no less than 0.005%, with a preferable lower limit being 0.008% and a more preferable lower limit being 0.01%. However, excess Si does not produce additional deoxidizing effect but deteriorates cold workability. The upper limit of Si content should be 0.6%, preferably 0.5%.
Mn is an element which is used for deoxidization and desulfurization in the steel-making process. Insufficient Mn leads to incomplete desulfurization which causes FeS film to separate out in the grain boundary, thereby greatly decreasing grain boundary strength and deteriorating deformability. The amount necessary for Mn to fully exhibit its effect is no less than 0.05%, with a preferable lower limit being 0.1%, and a more preferable lower limit being 1%. However, excess Mn adversely affects cold workability. The upper limit of Mn content should be 2%, preferably 1.5%, more preferably 1%.
(P: no more than 0.05% (excluding 0%))
P is an element in inevitable impurities. When contained in ferrite, P segregates in the ferrite grain boundary, thereby deteriorating cold workability. In addition, P subjects ferrite to solid-solution hardening, thereby increasing deformation resistance. Therefore, the P content should be as low as possible from the standpoint of cold working. However, reducing the P content to an extreme leads to an increased steel making cost. With cold workability and process performance taken into account, the upper limit of P content is specified as being 0.05%, preferably 0.03%. It is industrially difficult to completely eliminate P, however.
(S: no more than 0.05% (excluding 0%))
S is an element in inevitable impurities. S forms MnS as an inclusion, thereby deteriorating deformability. Therefore, the Mn content should be as low as possible from the standpoint of deformability. With deformability taken into account, the upper limit of S content is specified as being 0.05%, preferably 0.03%. On the other hand, S effectively improves machinability and hence it is sometimes added to steel intentionally. With machinability taken into account, the S content should preferably be no less than 0.002%, more preferably no less than 0.006%.
(N: no more than 0.04% (excluding 0%))
The description in this paragraph is concerned with the total amount of N in the steel. N dissolves in steel to form solid solution, thereby improving the strength of cold-worked parts. It plays an important role in the present invention. An excess total amount of N in the steel leads to an excess amount of dissolved N, which would cause cracking at the time of cold working. Moreover, excess dissolved N tends to cause ingot internal defects and slab cracking during continuous casting. The upper limit of the total N content in the steel is specified as being 0.04%, preferably 0.03%, from the standpoint of steel deformability, material stability, and good yields in continuous casting. According to the basic embodiment of the present invention, the lower limit of total N content is specified; however, the total N content should preferably be no less than 0.007%, more preferably no less than 0.008%, and most desirably no less than 0.009%, so that the amount of dissolved N is more than the lower limit mentioned later.
(Dissolved N: No Less than 0.006%)
Dissolved N contributes to the strength of parts produced by high-speed cold working. For its maximum effect, dissolved N should be contained in an amount more than 0.006%, preferably 0.007%, more preferably 0.008%. On the other hand, excess dissolved N deteriorates deformability. Therefore, the amount of dissolved N should preferably be no more than 0.035%, more preferably 0.030%, most desirably 0.025%. Incidentally, the amount of dissolved N does not exceed the total amount of N in the steel as a matter of course. The value of “the amount of dissolved N” in the steel can be calculated by subtracting the total amount of N compounds from the total amount of N in the steel, according to JIS G1228.
(a) The total amount of N in the steel is determined by the inert gas fusion method combined with the heat conduction method. To be concrete, a specimen cut out of steel is placed in a crucible, which is subsequently heated in an inert gas stream so that N is liberated from the molten sample. The liberated N is transferred to a heat conduction cell for measurement of change in heat conduction.
(b) The total amount of N compounds in the steel is determined by indophenol blue absorptiometry that follows ammonia distillation separation. To be concrete, a specimen cut out of steel is electrolyzed with constant current in a 10% AA electrolyte, which is a non-aqueous electrolytic solution incapable of forming passive film on the steel surface (consisting of 10% acetylacetone and 10% tetramethylammonium chloride, with the remainder being methanol). Electrolysis is carried out to such an extent that about 0.5 g of specimen is dissolved. Insoluble residues (N compounds) are filtered out through a polycarbonate filter with a pore size of 0.1 μm. The separated insoluble residues are decomposed in hot sulfuric acid containing potassium sulfate and pure copper chips. The decomposition solution is combined with the filtrate. The resulting solution is rendered alkaline with sodium hydroxide. The alkaline solution undergoes steam distillation and distilled ammonia is absorbed in dilute sulfuric acid. The dilute sulfuric acid which has absorbed ammonia is given phenol, sodium hypochlorite, and sodium pentacyanonitrosylferrate (III) which form a blue complex. The amount of the blue complex is determined by absorptiometry with a photometer.
The amount of total N and the amount of total N compounds, which have been determined as mentioned above, give the amount of dissolved N in the steel from difference between them.
The steel according to the basic embodiment of the present invention has the fundamental chemical composition as mentioned above, with the remainder being essentially iron. It may contain inevitable impurities resulting from raw materials, subsidiary materials, and manufacturing equipment, as a matter of course. Also, it may optionally contain any of the following elements.
(Al: no more than 0.1% (excluding 0%))
Al is an element that essentially functions as a deoxidizer in the steel-making process. It is also effective for steel cracking resistance. The Al content, which varies according to need, should preferably be no less than 0.001%, more preferably no less than 0.005%. According to the basic embodiment of the present invention, the upper limit of the Al content is specified as being 0.1% because Al with a strong affinity with N forms AlN to decrease the amount of dissolved N in the steel. The Al content should preferably be no more than 0.05%, more preferably no more than 0.03%.
(At least one species selected from the group consisting of Zr: no more than 0.2% (excluding 0%), Ti: no more than 0.1% (excluding 0%), Nb: no more than 0.1% (excluding 0%), V: no more than 0.5% (excluding 0%), Ta: no more than 0.1% (excluding 0%), and Hf: no more than 0.1% (excluding 0%))
Any of Zr, Ti, Nb, V, Ta, and Hf combines with N to form N compounds, thereby making crystal grains fine and contributing to toughness of parts obtained by cold working. The steel according to the basic embodiment of the present invention may optionally contain these elements in an amount specified below.
Zr: no less than 0.002%, preferably no less than 0.004%,
Ti: no less than 0.001%, preferably no less than 0.002%,
Nb: no less than 0.001%, preferably no less than 0.002%,
V: no less than 0.001%, preferably no less than 0.002%,
Ta: no less than 0.003%, preferably no less than 0.006%, and
Hf: no less than 0.002%, preferably no less than 0.004%.
These elements also have such a strong affinity that they form N compounds, thereby decreasing the total amount of dissolved N. Consequently, the upper limit of their content is specified as follows according to the basic embodiment of the present invention.
Zr: 0.2%, preferably 0.1%, more preferably 0.05%, most desirably 0.03%;
Ti: 0.1%, preferably 0.05%, more preferably 0.03%;
Nb: 0.1%, preferably 0.06%, more preferably 0.04%;
V: 0.5%, preferably 0.2%, more preferably 0.1%, most desirably 0.05%, particularly 0.03%;
Ta: 0.1%, preferably 0.05%, more preferably 0.03%;
Hf: 0.1%, preferably 0.05%, more preferably 0.03%.
(B: no more than 0.0015% (excluding 0%))
B is an element that enhances the strength of grain boundaries, thereby improving the deformability of steel. Therefore, no less than 0.0001% B, preferably no less than 0.0002% B may optionally be contained in the steel according to the basic embodiment of the present invention. Unfortunately, B has a strong affinity with N and hence forms BN to decrease the amount of dissolved N, and excess B deteriorates cold workability. Therefore, the desirable B content according to the basic embodiment of the present invention is no more than 0.0015%, preferably no more than 0.001%, and more preferably no more than 0.0008%.
(Cr: no more than 2% (excluding 0%))
Cr, like B, improves the deformability of steel. Thus, no less than 0.1% Cr, preferably no less than 0.2% Cr, may optionally be contained in the steel. Unfortunately, excess Cr deteriorates deformability and deteriorates cold workability. Therefore, the Cr content should be no more than 2%, preferably no more than 1.5%, and more preferably no more than 1%.
(Cu: no more than 5% (excluding 0%))
Cu hardens steel through strain aging and hence improves the strength of parts after working. Therefore, no less than 0.1% Cu, preferably no less than 0.5% Cu, may optionally be contained in the steel. However, excess Cu is wasted without any additional effect and adversely affects cold workability and parts' surface state. The Cu content should be limited to 5%, preferably 4%, more preferably 3%, and most desirably 2%.
(Ni: no more than 5% (excluding 0%) and/or Co: no more than 5% (excluding 0%))
Ni effectively improves the deformability of ferrite-pearlite steel. It also effectively eliminates the surface defects of steel containing Cu. The Ni content should be no less than 0.1%, preferably no less than 0.5%. In other words, it should be equal to or more than 70% of the Cu content. Excess Ni (exceeding 5%) is wasted without any additional effect and detrimental to cold workability. The Ni content is limited to 5%, preferably 4%, more preferably 3%, and most desirably 2%.
Co, like Ni, effectively improves the deformability of ferrite-pearlite steel. The Co content should be no less than 0.1%, preferably no less than 0.5%. However, excess Co (exceeding 5%) deteriorates the grain boundary strength in the manufacturing steps, such as casting and rolling, thereby causing cracking. The Co content is limited to 5%, preferably 4%, more preferably 3%, and most desirably 2%.
(Mo: no more than 2% (excluding 0%) and/or W no more than 2% (excluding 0%))
Mo increases hardness after working and enhances deformability. The Mo content should be no less than 0.04%, preferably no less than 0.08%, and more preferably no less than 0.1%. However, Mo in excess of 2% deteriorates cold workability. Thus, the Mo content should be limited to 2%, preferably 1.5%, and more preferably 1%.
W, like Mo, increases hardness after working and enhances deformability. The W content should be no less than 0.04%, preferably no less than 0.08%, and more preferably no less than 0.1%. However, W in excess of 2% deteriorates cold workability. Thus, the W content should be limited to 2%, preferably 1.5%, more preferably 1%, and most desirably 0.5%.
(at least one species selected from the group consisting of Ca: no more than 0.05%, REM: no more than 0.05%, Mg: no more than 0.02%, Li: no more than 0.02%, Pb: no more than 0.1%, and Bi: no more than 0.1%)
Ca, REM, Mg, Li, Pb, and Bi contribute to the machinability of steel. Especially, Li lowers the melting point of Al oxides, thereby making Al oxides harmless, and hence improves machinability. In addition, Ca, REM, Mg, and Li also make sulfide inclusions (such as MnS) spherical, thereby enhancing the toughness and deformability of steel. Each of them should be contained in an amount no less than specified below.
Ca: 0.005%, preferably 0.01%,
REM: 0.005%, preferably 0.01%,
Mg: 0.002%, preferably 0.005%, more preferably 0.008%,
Li: 0.001%, preferably 0.002%, more preferably 0.005%,
Pb: 0.005%, preferably 0.01%, more preferably 0.02%, and
Bi: 0.005%, preferably 0.01%, more preferably 0.02%.
These elements added in excess amounts do not produce additional effects. Excess Pb causes rolling defects. The amount of these elements is limited as specified as above. Their desirable upper limits are as follows.
Ca: 0.04%, preferably 0.03%, more preferably 0.02%,
REM: 0.04%, preferably 0.03%, more preferably 0.02%, most desirably 0.01%,
Mg: 0.018%, preferably 0.015%, more preferably 0.01%,
Li: 0.018%, preferably 0.015%, more preferably 0.01%,
Pb: 0.09%, preferably 0.08%, more preferably 0.06%, and
Bi: 0.09%, preferably 0.08%.
The following is concerned with the method for producing the steel for cold working according to the present invention. The steel according to the present invention is characterized in containing no less than 0.006% dissolved N. The desired amount of dissolved N is effectively ensured by (i) increasing the total amount of N in the steel and decreasing the amount of elements having a strong affinity with N in the steel and (ii) heating the steel above a prescribed temperature and then quenching the steel at a cooling rate greater than a prescribed value, thereby increasing the amount of dissolved N.
(i) Method for Increasing the Total Amount of Dissolved N in the Steel and Decreasing the Amount of Elements Having a Strong Affinity with N in the Steel.
Nitrogen in a steel containing Al (which has a strong affinity with N) combines with it to form N compounds, thereby decreasing the amount of dissolved N. However, a steel containing more nitrogen than consumed by Al that forms N compounds will permit dissolved N to remain sufficiently in it. To be concrete, a steel will eventually contain no less than 0.006% dissolved N if it initially contains N in an amount that satisfies the formula (1) below.
[N]−(14[Al]/27+14[Ti]/47.9+14[Nb]/92.9+14[V]/50.9+14[Zr]/91.2+14[B]/10.8+14[Ta]/180.9+14[Hf]/178.5)≧0.006 Formula (1)
where the square brackets [ ] represent the total amount (in mass %) of each element contained in the steel.
(ii) Method for Heating the Steel Above a Prescribed Temperature and then Quenching the Steel at a Cooling Rate Greater than a Prescribed Value, Thereby Increasing the Amount of Dissolved N.
A steel not having the chemical composition that satisfies the formula (1) permits N compounds of Al etc. to be formed in large amounts, and this leads to an insufficient content of dissolved N. This trouble is circumvented by heating and keeping the steel at a temperature at which N compounds resulting from hot rolling dissolves to form solid solution and then quenching the steel. This solid-solution heat treatment to prevent N compounds from precipitation increases the amount of dissolved N. To be concrete, the object is achieved by heating the steel at a temperature above Ac3 point plus 30° C. and then quenching it to 500° C. or below at a cooling rate no smaller than 0.5° C./s.
For the steel to contain a sufficient amount of dissolved N, the heating temperature should be no lower than Ac3 point plus 30° C., preferably Ac3 point plus 40° C., and more preferably Ac3 point plus 50° C. The duration of heating should be no shorter than 10 minutes, preferably no shorter than 30 minutes. The heating temperature determined from the standpoint of production cost is below Ac3 point plus 500° C., preferably Ac3 point plus 450° C., more preferably Ac3 point plus 400° C., and most desirably Ac3 point plus 300° C. The duration of heating should be no longer than 2.5 hours, preferably no longer than 1.5 hours.
The heating step may optionally include hot working such as drawing, rolling, and pressing. Heating should be followed by quenching at a cooling rate no smaller than 0.5° C./s, preferably 1° C./s, more preferably 5° C./s. Quenching down to 500° C. or below, preferably 450° C. or below, for dissolved N to exist stably is desirable. Thus the resulting steel contains sufficient dissolved N without N compounds precipitating.
One feature of the present invention resides in a method of high-speed cold working for a steel having the above-mentioned chemical composition and containing dissolved N. For the steel according to the present invention to exhibit good cold workability despite its comparatively high content of dissolved N, it should undergo cold working at a strain rate which is no lower than 100/s, preferably 120/s, more preferably 140/s, particularly 150/s, and most desirably 200/s. However, the strain rate should be within a certain limit so that the steel is protected from cracking due to adiabatic temperature rise. The upper limit of the strain rate is 500/s, preferably 450/s, more preferably 400/s, particularly 300/s, most desirably 280/s, and 260/s most of all.
The working temperature which affects cold workability should be limited to 200° C., preferably 180° C., more preferably 160° C. An excessively high working temperature leads to the dynamic strain aging during deformation which increases deformation resistance and deteriorates the mold life. On the other hand, cold working is usually carried out at room temperature. Cold working at 0° C. or below suffers an increased deformation resistance due to temperature dependency. Therefore, the lower limit of the cold working temperature is 0° C. Incidentally, the working temperature is defined as the ambient temperature at the time of working.
The steel stock (such as wire and rod) produced as mentioned above is made into machine parts (such as bolts and nuts) by high-speed cold working under the foregoing conditions. The cold working includes cold forging, cold pressing, cold rolling, cold drawing, and cold extrusion. The thus produced machine parts may optionally undergo drawing and rolling.
The machine parts produced by high-speed cold working should have adequate strength and adequate deformation resistance which are balanced with each other. High-speed cold working at 200° C. or below at a strain rate of 100/s or above should give machine parts having the hardness (H) which depends on the maximum deformation resistance (DR) experienced during high-speed cold working as defined by the formula (3) below.
H≧(DR+1000)/6 (3)
where, H denotes the strength (Hv) of parts and DR denotes the deformation resistance (MPa).
The basic embodiment of the present invention specifies the steel according to its composition for fundamental components and optional components as mentioned above. It falls under three categories as follows according to applications and desired performance.
According to the first embodiment of the present invention, the steel for high-speed cold working contains 0.03-0.15% C, 0.005-0.6% Si, 0.05-2% Mn, no more than 0.05% P (excluding 0%), no more than 0.05% S (excluding 0%), and no more than 0.04% N (excluding 0%), with the remainder being iron and inevitable impurities. It is characterized by containing no less than 0.006% dissolved nitrogen.
The first embodiment of the present invention specifies that the steel should contain no more than 0.15% C for its good machinability and cold workability. The preferable upper limit of C content is 0.12%, and the lower limit of C content is 0.03%, preferably 0.04%, for good steel strength.
Except for C content, the first embodiment of the present invention is identical with the basic embodiment of the present invention in the content of fundamental components and optional components, the method for steel production, and the balance between parts strength and deformation resistance during high-speed cold working (which has been mentioned with reference to the formula (3) above).
According to the second embodiment of the present invention, the steel for high-speed cold working contains:
C: more than 0.15% up to 0.6%,
P: no more than 0.05% (excluding 0%),
S: no more than 0.05% (excluding 0%), and
N: no more than 0.04% (excluding 0%),
with the remainder being iron and inevitable impurities. It is characterized by containing no less than 0.006% dissolved N.
(C: More than 0.15% Up to 0.6%)
The second embodiment of the present invention specifies that the steel should contain more than 0.15% C for its good parts strength. The preferable C content is no less than 0.16%, preferably no less than 0.17%. The upper limit of C content should be 0.6%, preferably 0.5%, and more preferably 0.4%, because excess C deteriorates machinability and cold workability.
Except for C content, the second embodiment of the present invention is identical with the basic embodiment of the present invention in the content of fundamental components and optional components, the method for steel production, and the balance between parts strength and deformation resistance during high-speed cold working (which has been mentioned with reference to the formula (3) above).
The third embodiment of the present invention specifies the amount of such elements as Al, Ti, Nb, V, Zr, G, Ta, and Hf which reduce the amount of dissolved N in the steel. With the content of these elements kept low, the steel contains sufficient dissolved N, which leads to good cold workability and high parts strength.
According to the third embodiment of the present invention, the steel for high-speed cold working contains:
P: no more than 0.05% (excluding 0%),
S: no more than 0.05% (excluding 0%), and
with the remainder being iron and inevitable impurities. The inevitable impurities include the following elements.
Al: no more than 0.001% (including 0%),
Ti: no more than 0.002% (including 0%),
Nb: no more than 0.001% (including 0%),
V: no more than 0.001% (including 0%),
Zr: no more than 0.001% (including 0%),
B: no more than 0.0001% (including 0%),
Ta: no more than 0.0001% (including 0%), and
Hf: no more than 0.0001% (including 0%).
The amounts of these elements satisfy the formula (2) below.
14[Al]/27+14[Ti]/47.9+14[Nb]/92.9+14[V]/50.9+14[Zr]/91.2+14[B]/10.8+14[Ta]/180.9+14[Hf]/178.5≦0.002% Formula (2)
where the square brackets [ ] represent the total amount (in mass %) of each element contained in the steel.
The third embodiment of the present invention specifies that the lower limit of the total N content should be 0.008%, preferably 0.009%, so that the steel contains a prescribed amount of dissolved N. On the other hand, it also specifies that the upper limit of the total N content should be 0.04%, preferably 0.03%, from the standpoint of steel deformability, steel stability, and yields in continuous casting.
The inevitable impurities mentioned above may include any of Al, T, Nb, V, Zr, B, Ta, and Hf. These elements readily combine with dissolved N to decrease the amount of dissolved N in the steel. Consequently, the third embodiment of the present invention specifies the content of these elements as follows.
Al: no more than 0.001% (including 0%),
Ti: no more than 0.002% (including 0%),
Nb: no more than 0.001% (including 0%),
V: no more than 0.001% (including 0%),
Zr: no more than 0.001% (including 0%),
B: no more than 0.0001% (including 0%),
Ta: no more than 0.0001% (including 0%), and
Hf: no more than 0.0001% (including 0%).
Such elements as Al, Ti, Nb, V, Zr, B, Ta, and Hf readily combine with dissolved N to form nitrides (such as AlN, TiN, NbN, VN, ZrN, BN, TaN, and HfN), thereby decreasing the amount of dissolved N in the steel. These nitrides enhance precipitation strengthening and prevent crystal grains from becoming coarse, thereby increasing deformation resistance. For the steel to contain dissolved N sufficiently and to have adequate deformation resistance, the amounts of these elements should be as small as possible.
Thus, the amounts of these elements are specified as follows in the third embodiment of the present invention.
Al: no more than 0.001%, preferably no more than 0.0005%,
Ti: no more than 0.002%, preferably no more than 0.001%,
Nb: no more than 0.001%, preferably no more than 0.0005%,
V: no more than 0.001%, preferably no more than 0.0005%,
Zr: no more than 0.001%, preferably no more than 0.0005%,
B: no more than 0.0001%, preferably no more than 0.00005%,
Ta: no more than 0.0001%, preferably no more than 0.00005%,
Hf: no more than 0.0001%, preferably no more than 0.00005%.
The most desirable content of these elements is 0%.
The content of Al, Ti, Nb, V, Zr, B, Ta, and Hf should be lower than specified above, but, at the same time, it should satisfy the formula (2) below.
14[Al]/27+14[Ti]/47.9+14[Nb]/92.9+14[V]/50.9+14[Zr]/91.2+14[B]/10.8+14[Ta]/180.9+14[Hf]/178.5≦0.002% Formula (2)
The term (14[Al]/27) denotes the amount of nitrogen in the form of AlN existing in the steel. The entire left side of the formula (2) represents the total amount of nitrogen combined with any of Al, Ti, Nb, V, Zr, B, Ta, and Hf (or the total amount of N compounds in the steel). For the steel to have an adequate amount of dissolved N, it is desirable that the amount of N compounds should be small. Therefore, the sum of the left side should be smaller than 0.002%, preferably smaller than 0.0018%, more preferably smaller than 0.0016%.
Except for the description of the amount of N, Al, Ti, Nb, V, Zr, B, Ta and Hf, the third embodiment of the present invention is identical with the basic embodiment of the present invention in the content of fundamental components and optional components, the method for steel production, and the balance between parts strength and deformation resistance during high-speed cold working (which has been mentioned with reference to the formula (3) above). Incidentally, although the steel for high-speed cold working in the third embodiment of the present invention may be produced by the above-mentioned manufacturing method, it will contain an adequate amount of dissolved N irrespective of manufacturing method so used long as the formula (2) is satisfied.
The invention will be described in more detail with reference the following examples, which are not intended to restrict the scope thereof but may be modified within the scope thereof.
Steel samples (in the form of ingot) each having the chemical composition shown in Tables 1 to 3 were prepared by continuous casting from a converter. Each ingot was rolled into a wire, 12 mm in diameter.
The thus obtained wire underwent heat treatment under the conditions shown in Table 4. The heat-treated wire should preferably be kept at the heating temperature for at least 10 minutes, desirably at least 30 minutes. Then, a test specimen measuring 4 mm in diameter and 6 mm long was cut out of the central part of the heat-treated wire. Whether or not the test specimen satisfies the formula (1) is indicated respectively by symbols “◯” and “X”. In Tables 1 to 3, “dissolved N” represents the amount of dissolved N and “N” represents the total amount of N.
Each test specimen shown in Tables 1 to 3 was forged into parts by using a servo hydraulic type testing machine (with a capacity of 200 kN) under the following conditions.
Strain rate: 0.001 to 240/s
Working temperature: 20 to 400° C.
Compression ratio: 20 to 80%
The strain rate is an average of values measured during working (elastic deformation). The resulting parts were examined for their surface under a stereomicroscope with a magnification of ×20 to see if there is cracking. Tables 5 to 7 show the test results including working conditions, cracking, and deformation resistance.
Each part was also tested for Vickers hardness (Hv) by using a Vickers hardness tester under the following conditions.
Position of measurement: D/4 off the center of the cross section of the part (D=diameter of part)
Number of repetitions of measurement: 5
The results of measurement are shown in Tables 5 to 7.
Steels in this example are rated as good in cold workability if they give crack-free parts and exhibit low deformation resistance relative to parts hardness (or satisfy the formula (3)).
Those parts having a Vickers hardness (Hv) larger than 240 are rated as good in strength. Incidentally, Tables 5 to 7 show whether or not each specimen satisfies the formula (3) by symbols “◯” and “X” respectively.
It is noted from Tables 5 to 7 that the steel containing chemical components and dissolved nitrogen as specified in the first embodiment of the present invention excels in cold workability and gives parts having high strength when it is worked under desirable conditions (such as strain rate and working temperature). By contrast, the steel failing to meet the requirements specified in the first embodiment of the present invention lacks good cold workability or gives parts poor in strength as mentioned below.
Part No. I-1 (Steel No. I-1A) is low in strength because of insufficient carbon content (which is responsible for its hardness (Hv) lower than 240).
Part No. I-6 (Steel No. I-1F) suffers cracking due to excess carbon content.
Part No. I-7 (Steel No. I-1G) suffers cracking due to low Si content.
Part No. I-14 (Steel No. I-1N) suffers cracking due to excess Si content.
Part No. I-15 (Steel No. I-1O) suffers cracking due to low Mn content.
Part No. I-24 (Steel No. I-1X) suffers cracking due to excess Mn content.
Part Nos. I-25 and I-26 (Steel Nos. I-1Y and I-1Z) suffer cracking due to excess P content.
Steel samples, numbered II-1A to II-4A, (in the form of ingot) each having the chemical composition shown in Tables 8 to 10 were prepared by continuous casting from a converter. Each ingot was rolled into a wire, 12 mm in diameter. The thus obtained wire underwent heat treatment under the conditions shown in Table 11. The heat-treated wire should preferably be kept at the heating temperature for at least 10 minutes, desirably at least 30 minutes.
Then, a test specimen measuring 4 mm in diameter and 6 mm long was cut out of the central part of the heat-treated wire. Whether or not the test specimen satisfies the formula (1) is indicated respectively by symbols “◯” and “X”. In Tables 8 to 10, “dissolved N” represents the amount of dissolved N and “N” represents the total amount of N.
Each test specimen shown in Tables 8 to 10 was forged into parts by using a servo hydraulic type testing machine (with a capacity of 200 kN) under the following conditions.
Strain rate: 0.001 to 240/s
Working temperature: 20 to 400° C.
Compression ratio: 20 to 80%
The strain rate is an average of values measured during working (elastic deformation).
The resulting parts were examined for their surface under a stereomicroscope with a magnification of ×20 to see if there is cracking. Tables 12 to 14 show the test results including working conditions, cracking, and deformation resistance.
Each part was also tested for Vickers hardness (Hv) by using a Vickers hardness tester under the following conditions.
Position of measurement: D/4 off the center of the cross section of the part (D=diameter of part)
Number of repetitions of measurement: 5
The results of measurement are shown in Tables 12 to 14.
Steels in this example are rated as good in cold workability if they give crack-free parts and exhibit low deformation resistance relative to parts hardness (or satisfy the formula (3)). Those parts having a Vickers hardness (Hv) larger than 240 are rated as good in strength.
Incidentally, Tables 12 to 14 show whether or not each specimen satisfies the formula (3) by symbols “◯” and “X” respectively.
It is noted from Tables 12 to 14 that the steel containing chemical components and dissolved nitrogen as specified in the second embodiment of the present invention excels in cold workability and gives parts having high strength when it is worked under desirable conditions (such as strain rate and working temperature).
By contrast, the steel failing to meet the requirements specified in the second embodiment of the present invention is vulnerable to cracking or poor in balance between cold workability and parts hardness, with the formula (3) not satisfied, as mentioned below.
Part No. II-1 (formed from steel No. II-1A with a C content lower than specified in the second embodiment of the present invention) has a lower hardness than specified.
Part No. II-6 (formed from steel No. II-1F with a C content higher than specified in the second embodiment of the present invention) suffers cracking.
Part No. II-7 (formed from steel No. II-1G with a Si content lower than specified in the second embodiment of the present invention) suffers cracking.
Part No. II-14 (formed from steel No. II-1N with a Si content higher than specified in the second embodiment of the present invention) suffers cracking.
Part No. II-15 (formed from steel No. II-1O with a Mn content lower than specified in the second embodiment of the present invention) suffers cracking.
Part No. II-24 (formed from steel No. II-1X with a Mn content higher than specified in the second embodiment of the present invention) suffers cracking.
Parts No. II-25 and II-26 (formed from steels No. II-1Y and II-1Z respectively with a P content higher than specified in the second embodiment of the present invention) suffer cracking.
Parts No. II-27 and II-28 (formed from steels No. II-2A and II-2B respectively with a S content higher than specified in the second embodiment of the present invention) suffer cracking.
Part No. II-29 (formed from steel No. II-2C with a lower content of dissolved N than specified (no less than 0.007%) in the second embodiment of the present invention) does not satisfy the formula (2) and hence is poor in balance between cold workability and hardness.
Part No. II-42 (formed from steel No. II-2K with a higher content of dissolved N than specified in the second embodiment of the present invention) suffers cracking.
Parts No. II-31 to II-34 (formed from steel No. II-2E having the chemical composition as specified in the second embodiment of the present invention) suffer cracking due to dynamic strain ageing which results from a low strain rate at the time of high-speed cold working.
Parts No. II-37 and II-38 (formed from steels No. II-2F and II-2G respectively having the chemical composition as specified in the second embodiment of the present invention) suffer cracking due to dynamic strain ageing which results from a high temperature at the time of high-speed cold working.
Part No. II-50 (formed from steel II-2S containing a less amount of dissolved N than specified, with the formula (1) not satisfied, as shown in Table 9) is poor in balance between cold workability and hardness, with the formula (3) not satisfied. Those steels which do not satisfy the formula (1) may contain an adequate amount of dissolved N as specified in the second embodiment of the present invention if they undergo heat treatment adequately. This is true with material Nos. II-73, 74, 75, 78, 79, 81, 82, 83, 86, 87, 90, 91, 93, 94, 95, 98, 99, 101, 102, and 103, shown in Table 10.
Parts Nos. II-77, 81, 82, 85, 89, 90, 93, 94, 97, 101, 102, and 105 (formed from steels Nos. II-2S and II-3T to II-4A with the chemical composition specified in the second embodiment of the present invention) do not contain the prescribed amount of dissolved N because they are not produced according to the heat treatment patterns II-a to II-j shown in Table 11 and hence they do not satisfy the formula (3). In other words, they are poor in balance between cold workability and parts hardness.
Steel samples, numbered III-LA to III-3S, (in the form of ingot) each having the chemical composition shown in Tables 15 and 16, were prepared by continuous casting from a converter. (Steel numbers are accompanied by parts numbers for the sake of convenience.) Each ingot was rolled into a wire, 12 mm in diameter. The thus obtained wire underwent heat treatment consisting of steps of heating, (hot working), and quenching. This heat treatment conforms to the patterns III-a to III-j shown in Table 17. The heat-treated wire should preferably be kept at the heating temperature for at least 10 minutes, desirably at least 30 minutes.
Then, a test specimen measuring 4 mm in diameter and 6 mm long was cut out of the central part of the heat-treated wire.
In Tables 15 to 16, “N” represents the total amount of N (in mass %), “dissolved N” represents the amount of dissolved N (in mass %), and “N compounds” represents the amount of N compounds (in mass %). The amount of dissolved N is a difference between the total amount of N and the amount of N compounds in the steel calculated according to JIS G1228.
Each test specimen shown in Tables 15 and 16 was forged into parts by using a servo hydraulic type testing machine (with a capacity of 200 kN) under the following conditions.
Strain rate: 0.001 to 240/s
Working temperature: 20 to 400° C.
Compression ratio: 20 to 80%
The strain rate is an average of values measured during working (elastic deformation).
The resulting parts were examined for their surface under a stereomicroscope with a magnification of ×20 to see if there is cracking. Tables 18 and 19 show the test results including working conditions, cracking, and deformation resistance.
Each part was also tested for Vickers hardness (Hv) by using a micro-Vickers hardness tester under the following conditions.
Position of measurement: D/4 off the center of the cross section of the part (D=diameter of part)
Number of repetitions of measurement: 5
The results of measurement are shown in Tables 18 and 19.
Steels in this example are rated as good in cold workability if they give crack-free parts and exhibit low deformation resistance relative to parts hardness (or satisfy the formula (3)). Those parts which are rated as good in strength have a Vickers hardness (Hv) larger than 240.
Incidentally, Tables 18 and 19 show whether or not each specimen satisfies the formula (3) by symbols “◯” and “X” respectively.
The results shown in Tables 18 and 19 suggest the following.
All the parts (wires and rods) numbered as follows, which are formed by high-speed cold working as recommended in the present invention from the steels meeting the requirements of the third embodiment of the present invention, exhibit good balance between cold workability and strength (hardness) as wells as good cracking resistance.
By contrast, those parts failing to meet the requirements specified in the third embodiment of the present invention are vulnerable to cracking during high-speed cold working or poor in balance between cold workability and strength (hardness), with the formula (3) not satisfied, as mentioned below.
Part No. III-1 (formed from steel No. III-1A with a low C content) suffers cracking after working.
Part No. III-6 (formed from steel No. III-1F with a high C content) suffers cracking.
Part No. III-7 (formed from steel No. III-1G with a low Si content) suffers cracking.
Part No. III-14 (formed from steel No. III-1N with a high Si content) suffers cracking.
Part No. III-15 (formed from steel No. III-1O with a low Mn content) suffers cracking.
Part No. III-24 (formed from steel No. III-1X with a high Mn content) suffers cracking.
Parts Nos. III-25 and III-26 (formed from steels Nos. III-1Y and III-1Z respectively with a high P content) suffer cracking.
Parts Nos. III-27 and III-28 (formed from steels Nos. III-2A and III-2B respectively with a high S content) suffer cracking.
Part No. III-29 (formed from steel No. III-2C with a low N content and hence with a low content of dissolved N) is poor in balance between cold workability and hardness.
Part No. III-42 (formed from steel No. III-2K with a high N content) suffers cracking.
Parts No. III-31 to III-34 (formed from steel No. III-2E having the chemical composition as specified in the third embodiment of the present invention) suffer cracking due to dynamic strain ageing which results from a low strain rate at the time of high-speed cold working.
Parts No. III-37 and III-38 (formed from steels No. III-2F and III-2G respectively having the chemical composition as specified in the third embodiment of the present invention) suffer cracking due to dynamic strain ageing which results from a high temperature at the time of high-speed cold working.
Part No. III-50 (formed from steel No. III-2S with a high Al content, with the formula (2) not satisfied, is poor in balance between cold workability and hardness.
Part No. III-52 (formed from steel No. III-2U with a high B content, with the formula (2) not satisfied, is poor in balance between cold workability and hardness.
Part No. III-53 (formed from steel No. III-2V with a high Ti content, with the formula (2) not satisfied, is poor in balance between cold workability and hardness.
Part No. III-54 (formed from steel No. III-2W with a high V content, with the formula (2) not satisfied, is poor in balance between cold workability and hardness.
Part No. III-57 (formed from steel No. III-2Z with a high content of Ti, V, and B, with the formula (2) not satisfied, is poor in balance between cold workability and hardness.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-355497 | Dec 2006 | JP | national |
| 2007-097457 | Apr 2007 | JP | national |
| 2007-103027 | Apr 2007 | JP | national |
