GB2191506A

GB2191506A - Resulfurized and rephosphorized steel bars

Info

Publication number: GB2191506A
Application number: GB08713199A
Authority: GB
Inventors: Stephen P Birman; Richard B Smith; Richard L Thompson
Original assignee: Stanadyne LLC
Current assignee: Stanadyne LLC
Priority date: 1986-06-10
Filing date: 1987-06-05
Publication date: 1987-12-16
Anticipated expiration: 2007-06-05
Also published as: CA1301489C; FR2601697A1; FR2601697B1; KR880000615A; GB8713199D0; BR8702911A; DE3718772A1; JPS63441A; IT8748036A0; JPH062928B2; IT1206025B; DE3718772C2; MX168196B; GB2191506B

Description

GB2191506A 1

SPECIFICATION

Cold drawn free-machining resulfurized and rephosphorized steel bars having controlled mechanical properties and controlled machinability The present invention relates to a free-machining resulfurized and rephosphorized cold drawn steel bar and, more particularly, to such a steel bar having a controlled chemistry and whose yield strength is determined not only by the controlled chemical composition, but also by the size of the bar after hot rolling and the amount of draft used in reducing the bar.

A primary purpose of the invention is a cold drawn free-machining steel bar as described in 10 which mechanical properties of the bar, particularly yield strength, can be determined on the basis of the chemical composition of the bar and the size, cross section, and applicable draft.

Another purpose is a free-machining cold drawn steel bar as described in which the amount of carbon is reduced accompanied by controlled amounts of columbium, which when accompanied by size-related optimization of the ratios of the ingredients in the chemical composition, provide 15 a means for targeting yield strengths for desired bar application.

Another purpose is a cold drawn free-machining steel bar as described in which the necessary characteristics for increased machinability are optimized by the chemical composition and by the mechanical configuration of the bar.

Another purpose is a resulfurized and rephosphorized free-machining steel bar as described 20 having excellent machinability characteristics which are provided by optimizing the relationship between steel chemistry, steelmaking, cold drawing practice and machining conditions.

Another purpose is a process of providing a cold drawn free-machining steel bar utilizing the chemical characteristics of the bar, the size and steelmaking procedures used in the bar, which has application to carbon steel, manganese steel, resulfurized and rephosphorized steel, microalloyed steel and high-strength steel.

Another purpose is a cold drawn free-machining steel bar as described in which the amounts and ratios of manganese, sulfur and columbium and the amount and pattern of deformation in cold drawing are controlled to provide optimum machinability.

Other purposes will appear in the ensuing specification and claims. 30

It is well-known that certain elements, such as sulfur, lead, bismuth, tellurium and selenium, are useful for improving machinability of steel. Machinability is a complex and not a fully understood property. The problem is one in which the effect of the alloy composition, plastic flow of the metal workpiece and cutting dynamics are not readily recognized from the manner in which the alloy is machined by cutting tools in such operations as single point turning, forming, 35 drilling, reaming, boring, shaving and threading. There is a gap in the available knowledge of material behavior between test results obtained from conventional, nonsteady-state tension experiments and results obtained from cutting force data derived from in- process machining.

Metallurgists have long sought to improve the machinability of freemachining steel bars by modifying the chemical composition, optimizing size, shape, distribution and chemical compo- 40 sition of inclusions to enhance the brittleness of the chip and increase lubrication at the tool/chip interface. Further, it is desired to prevent formation of the abrasive particles and microconstitu ents which are in the steel bar. For example, for the purpose of improving machinability varying amounts of one or more such elements as bismuth and tellurium (U.S. Patent No. 4,236,939); lead, bismuth and tellurium and/or sulfur (U.S. Patent No. 4,244,737); tellurium and sulfur (U.S. 45 Patent No. 4,279,646) have been included in resulfurized and rephosphorized free-machining steels. Such products, however, have not completely satisfied the need for increased machinability of free-machining steel. Prior efforts at increasing machinability have been more specifically directed to the chemical composition rather than attempting to optimize the chemical compo- sition, the draft or percentage size reduction in cold forming and the size and cross section of 50 the bar. The present invention is specifically directed to increasing machinability by combining the optimum ratio of chemical ingredients, particularly, manganese, sulfur and columbium with optimum size and cross sections for the bar, as well as the amount of cold working. Thus, in the present invention, the chemical composition, draft, size and cross section of a bar are tailored to meet particular machinability applications. 55 The cold drawn free-machining resulfurized and rephosphorized steel bar of the present inven tion has a chemical composition, by weight, consisting of C up to 0.08%; Mn 0.6% to 1.4%; Si up to 0.1%; P at least 0.3%; S 0.25% to 0.50%; Cb 0.01% to 0.10%; V up to 0.1%; the sum of Ni, Cr, Mo, and Cu up to 0.15% with the balance being iron. The ratio of manganese, sulfur and columbium is particularly important in providing a steel bar of the appropriate chemical 60 characteristics and in predicting the yield strength of a particular bar. Thus, the ratio of %Mn %S is from 1.6 to 4.0 and the ratio of (%Mn-1.62x %S) / %Cb is from 2 to 50.

The yield strength of the bar and, hence, its machinability is determined by the raw material, size and the draft. The raw material may be, considering the available types of products from the mill, hot rolled coil, hot rolled bars up to two inches in diameter and hot rolled bars havIng a 65 2 GB2191506A 2 diameter of at least two inches. Such stock, after hot rolling to the specified size, and being cut off in appropriate lengths, will then be cold drawn and the draft or the size reduction in cold drawing is also extremely important in determining yield strength of the finished bar.

Carbon up to 0/08% 5 Considering the roles of the different chemical elements in the composition and their influence on machinability and performance of the steel, a reduced carbon content is essential for assuring low work hardening and strain hardening of a steel subject to cold drawing and machining. A low carbon content providing low strength in a resulfurized and rephosphorized steel, when combined with the sum of residual elements, such as nickel, chromium, molybdenum and copper 10 not exceeding 0.15%, provides a product of relatively low ductility and increased breakability of the chip formed at the tool-workpiece interface. If the residual elements are increased above the level specified, with the reduced carbon content specified, the product has increased ductility and decreased breakability of the chip, which are disadvantageous in a free-machining product.

Moreover, an increase in the carbon content above 0.08% increases the formation of abrasive 15 particles, creates a likelihood of increased fracture stress and an increase of surface hardness, particularly in cold drawn hexagonal bars.

Manganese 0.6% to 1.4% The specified amount of manganese is important for the formation of manganese sulfide (MnS) 20 based inclusions which exert influence on tool life. Manganese promotes hardenability and increases the strength of cold drawn bars. The The actual specification of manganese in a particular bar is dependent upon the diameter of the hot rolled steel, the required mechanical properties for the bar and the machining designation. The manganese content is increased with an increase in the size of bar and an increase in the target level of yield strength. 25 Silicon up to 0. 1 % The silicon content should be limited to 0.1% as an increased amount of silicon substantially increases the amount of abrasive silicates in the finished product.

30 Sulfur 0.25% to 0.50% Sulfur is also necessary for the formation of MnS inclusions, and it is for this reason that the sulfur content should be at least 0.25%. The particular specification of sulfur in a particular bar depends on the size of the bar and the manganese content. The minimal sulfur content is utilized with a hot rolled coil as the raw material, whereas the maximum suifur content is required for 35 large diameter cold drawn bars with increased yield strength up to approximately 80 ksi. An excess amount of sulfur causes hot shortness and low ductility and therefore 0.5% is the upper limit of the sulfur content of the product disclosed herein.

Phosphorus at least 0.03% 40 Phosphorus is necessary for improving the smoothness of the surface finish. However, phos phorus can increase the work hardening and the hardness of the chip formed in machining.

Thus, the amount of phosphorus must be reduced from what is customary in bars of this type (usually up to 0.09%) to permit high speed machining operations in cold drawn steel bars with increased strength. 45 Columbium 0. 0 1 % to 0. 10 % Columbium is essential in the present steel to increase strength, control the mechanical properties through the thickness of the bar and to reduce toughness of the chip. The specifica tion of columbium is different for different levels of yield strength and bar size. Columbium 50 promotes hardenability and increases work hardening of the core in large diameter cold drawn bars. Columbium-bearing steel may be cold drawn with reduced draft in order to obtain minimal strengthening of the surface and substantial strengthening of the core. At a higher content than that specified, however, the effect of the columbium is to excessively increase strength and thereby reduce tool life. 55 Vanadium up to 0. 1 % Vanadium provides for an increase of the surface-to-center mechanical properties of the steel bar and, in particular, for large diameter cold drawn bars. As the vanadium content is increased above that specified, the machinability characteristics of the bar deteriorate. 60 Residual elements up to 0. 15% The residual elements of nickel, chromium, molybdenum and copper are generally harmful for machinability because they increase strength and ductility and promote the formation of abrasive particles, all of which detract from the machinability characteristics of a steel. Thus, the residual 65 3 GB2191506A 3 elements must be kept within the range specified.

The ratio of %Mn / %S should be from 1.6 to 4.0 and this ratio defines the amount of manganese in solid solution and the amount of FeS inclusions.

The relationship between manganese, sulfur and columbium specified as (%Mn-1.62X%S) %Cb defines the relative contributions of manganese and columbium in strengthening the pro- 5 duct. Manganese affects strength through changing kinetics of austenite decomposition, whereas columbium decreases grain size and promotes precipitation hardening. The ratios specified will vary depending on the size of the hot rolled product, the amount of draft involved in reducing to a cold drawn product and the desired tensile strength in the ultimate application of the steel bar.

In addition to the elements described, machinability is improved by the addition of one or 10 more of the following: lead in an amount, by weight, of 0.03% to 0.35%; zirconium in an amount, by weight, of 0.005% to 0.05%; bismuth in an amount, by weight, of 0.05% to 0.25%; lead in an amount, by weight, of 0. 03% to 0.15% and bismuth in an amount, by weight, of 0.05% to 0.15%; nitrogen in an amount, by weight, of 0.006% to 0.012%; bismuth in an amount, by weight, of 0.05% to 0.25% and tellurium in an amount, by weight, of 0.005% 15 to 0.05%. Zirconium maximizes machinability by promoting globular-shaped MnS inclusions, whereas nitrogen promotes the breakability of chips which facilitates drilling operations.

The following tables illustrate the relationship between the amounts of manganese, sulfur and columbium, and the hot rolled raw material, i.e. whether it be hot rolled coil, hot rolled bars up to two inches in diameter or a hot rolled bar having a diameter at least two inches. The tables 20 provide an indication of the yield strength for particular products resulting from particular combi nations of elements. Table 1 illustrates the relationship between yield strength, percent mangan ese, percent columbium, percent sulfur and the ratio of these three elements as applied to hot rolled coil designed to be formed into cold drawn bars with a range of yield strength from 60 to 80 ksi. Table 2 relates the specification of manganese, sulfur and columbium for hot rolled bars 25 up to two inches and includes the effect on yield strength of the percent reduction in bar area in cold drawing. Table 3 is similar to table 2 and provides the same information for hot rolled bars having a diameter at least two inches.

1 4 GB2191506A 4 TABLE 1 Specification of Mn, S and Cb for Hot Rolled Coil Designated to Cold Drawn Bars with a Range of Yield Strength of 60 to 80 ksi

5 Yield Manganese Columbium Sulfur % Mn-1,622%S Strength, ksi % % % % Cb 0.010 13 to 25 10 0.65 0.25 to 0.32 0.020 6.5 to 12.5 0.010 23 to 35 60 0.75 0.25 to 0.32 15 0.020 12.5 to 17.5 0.020 5 to 12.5 0.70 0.28 to 0.35 0.045 2 to 5 20 0.020 15 to 22.5 0.90 0.28 to 0.35 0.045 6.5 to 10 25 0.035 3.5 to 7 0.75 0.30 to 0.38 0.055 2.5 to 4.5 0.035 10 to 14.5 30 1.00 0.30 to 0.38 0.055 7 to 9 0.060 6.5 to 8.5 80 1.05 0.32 to 0.40 35 0.080 5 to 6 0.060 8 to 10 1.15 0.32 to 0.40 0.080 6 to 8.5 40 GB2191506A 5 TABLE 2 Specification of Mn, S and Cb for Hot Rolled Bars up to X' Designated to Cold Drawn Bars with a Range of Yield Strength of 60 to 80 ksi

5 Yield Manganese Columbium Sulfur %Mn-1.62% S Reduction Strength, % % % in ksi %Cb Cold Drawing, % 0.010 13.3 to 25 10 0.70 0.28 to 0.35 5 to 9 0.020 6.5 to 12.5 0.010 33 to 45 15 0.90 0.28 to 0.35 4 to 8 0.020 16.5 to 22.5 0.010 18 to 31 65 0.8 0.30 to 0.38 5 to 10 20 0.020 9 to 15.5 0.010 38 to 51 1.0 0.30 to 0.38 4 to 9 0.020 19 to 25.5 25 0.040 6.2 to 9.5 0.9 0.32 to 0.40 5 to 9 0.060 4.2 to 6.3 30 0.040 11.3 to 14.5 1.1 0.32 to 0.40 4 to 8 0.060 7.5 to 9.7 0.070 5.6 to 7.4 35 1.1 0.36 to 0.44 5 to 10 0.090 4.3 to 5.7 0.070 8.4 to 10.3 80 1.3 0.36 to 0.44 4 to 8 40 0.090 6.6 to 8.0 6 GB2191506A 6 TABLE 3 Specification of Mn, S and Cb for Hot Rolled Bars at Least Z' Designated to Cold Drawn Bars with a Range of Yield Strength of 60 to 80 ksi

5 Yield Manganese, Columbium, Sulfur, %Mn-1.62%S Reduction Strength, % % % in Cold ksi %Cb Drawing, % 0.020 9 to 15.7 10 0.8 0.30 to 0.38 5 to 6 0.040 4.5 to 7.8 0.020 19 to 25.7 60 1.0 0.30 to 0.38 - 4 to 5 15 0.040 8.5 to 12.

0.030 8.3 to 12.7 0.9 0.32 to 0.40 5 to 6 0.050 5 to 7.6 20 0.030 15 to 19.3 1.1 0.32 to 0.40 4 to 5 0.050 9 to 11.6 25 0.040 11.2 to 14.5 1.1 0.32 to 0.40 5 to 6 0.060 7.5 to 9.7 0.040 16.3 to 19.5 30 1.3 0.32 to 0.40 4 to 5 0.060 10.8 to 13 0.080 6. 1 to 7.8 80 1.2 0.36 to 0.44 5 to 6 35 0.10 4.9 to 6.2 0.080 8.6 to 10.3 1.4 0.36 to 0.44 4 to 5 0.10 6.9 to 8.2 40 Further specific examples of products formed in accordance with the present invention are as follows.

Example 1: A hot rolled coil steel comprising: 45 C up to 0.06% Mn 0.6% to 1.5% Si up to 0.1% P 0.03% to 0.06% S 0.25% to 0.40% 50 Cb 0.01% to 0.07% The sum of Ni, Cr, Mo, and Cu up to 0.15%, with the balance iron. The ratio of (%Mn-1.62x %S) / %Cb is from 2.0 to 22.5 and the ratio percent %Mn / %S is from 2.0 to 3.5. The percent sulfur is further determined by multiplying the desired yield strength in ksi by (0.0042-0.0054). 55 Example 2: A hot rolled bar having a diameter up to two inches, comprising:

C up to 0.08% Mn 0.7% to 1.30% Si up to 0.1% P 0.03% to 0.09% 60 S 0.28% to 0.50% Cb 0.02% to 0.08% V up to 0.1% The sum of Ni, Cr, Mo, and Cu up to 0.15%, with the balance iron. The ratio of (%Mn-1.62x %S) / %Cb is from 4 to 51 and the ratio of %Mn / %S is from 2. 0 to 3.2. The 65 7 GB2191506A 7 percent sulfur is further determined by multiplying the desired yield strength in ksi by (0.0045-0.0058).

Example 3: A hot rolled bar having a diameter at least two inches comprising:

C up to 0.08% Mn 0.8% to 1.4% 5 Si up to 0.1% P 0.03% to 0.9% S 0.3% to 0.5% Cb 0.02% to 0.10% V up to 0.1% 10 The sum of Ni, Cr, Mo, and Cu up to 0.15%, with the balance iron. The ratio of (%Mn-1.62x %S) / %Cb being from 5 to 25 and the ratio of %Mn / %S is from 2.0 to 3.5.

The percent sulfur is further determined by multiplying the desired yield strength in ksi by (0.0045-0.0063).

The percentages of manganese, sulfur and columbium increase, along with an increase in the 15 desired yield strength of the product. The contribution of columbium into strengthening the steel bar also increases with an increase in the target yield strength of the product. Excellent machina bility in high speed drilling and forming is achieved using cold drawn bars with a yield strength of 60 to 65 ksi. Improved surface finish is achieved using cold drawn bars with a yield strength of 65 to 70 ksi. 20 An important aspect of the present invention resides in the fact that bars with different machining designations, capability and application may be selected from the range of the combi nation of mechanical and chemical properties disclosed herein. The percent reduction in cold drawing similarly has a substantial affect on yield strength. Yield strength directly relates to the percentage reduction in cold drawing, the type of hot rolled material used to form the bar, and 25 the percentage of manganese, columbium and sulfur.

Whereas the preferred form of the invention has been shown and described herein, it should be realized that there may be many modifications, alterations and substitutions thereto.

Claims

CLAIMS 30

1. A cold drawn resulfurized and rephosphorized free-machining steel bar having the compo sition, by weight:

C up to 0.08% Mn 0.6% to 1.4% Si up to 0.1% 35 P at least 0.03% S 0.25% to 0.50% Cb 0.01% to 0.10% V up to 0.1% The sum of Ni, Cr, Mo, and Cu up to 0.15%, 40 Balance iron, The ratio of %Mn / %S is from 1.6 to 4.0, and The ratio (%Mn-1.62 x %S) / %Cb is from 2 to 50.

2. The cold drawn steel bar of claim 1 further including Pb in an amount, by weight, of 0.03% to 0.35%. 45

3. The cold drawn steel bar of claim 1 further including Zr in an amount, by weight, of from 0.005% to 0.05%.

4. The cold drawn steel bar of claim 1 further including Bi in an amount, by weight, of from 0.05% to 0.25%.

5. The cold drawn steel bar of claim 1 further including, by weight, Pb in an amount of from 50 0.03% to 0.15% and Bi in an amount from 0.05% to 0.15%.

6. The cold drawn steel bar of claim 1 further including N in an amount, by weight, of from 0.006% to 0.012%.

7. The cold drawn steel bar of claim 1 further including, by weight, Bi in an amount from 0.05% to 0.25% and Te in an amount from 0.005% to 0.05%. 55

8. The cold drawn steel bar of claim 1 further characterized in that the amount of Cb is from 0.01% to 0.04% and the bar has a yield strength of approximately 60 ksi.

9. The cold drawn steel bar of claim 1 further characterized in that the amount of Cb is from 0.02% to 0.06% and the bar has a yield strength of approximately 65 ksi.

10. The cold drawn steel bar of claim 1 further characterized in that the amount of Cb is 60 from 0.02% to 0.07% and the bar has a yield strength of approximately 70 ksi.

11. The cold drawn steel bar of claim 1 further characterized in that the amount of Cb is from 0.06% to 0.1% and the bar has a yield strength of approximately 80 ksi.

12. A cold drawn rephosphorized and resulfurized free-machining steel bar formed from a hot rolled steel coil having the cc.rnposition, by weight: 65 8 GB2191506A 8 C up to 0.06% Mn 0.6% to 1.15% Si up to 0.1% P 0.03% to 0.06% S 0.25% to 0.40% 5 Cb 0.01% to 0.07% The sum of Ni, Cr, Mo, and Cu up to 0. 15%, Balance iron, The ratio of %Mn / %S is from 2.0 to 3.5, The ratio (%Mn-1.62 x %S) / %Cb is from 2 to 50, 10 and %S=(0.00420.0054) x the desired yield strength is ksi.

13. The cold drawn steel bar of claim 1 further characterized in that the reduction in area in cold drawing is from 15% to 30% of the hot rolled bar, with the bar having a yield strength of from 60 to 80 ksi. 15

14. A cold drawn rephosphorized and resulfurized free-machining steel bar formed by hot rolling a bar up to two inches in cross section, the steel bar having the composition, by weight:

C up to 0.08% Mn 0.7% to 1.30% Si up to 0.1% 20 P 0.03% to 0.09% S 0.28% to 0.50% Cb 0.02% to 0.08% V up to 0.1% The sum of Ni, Cr, Mo, and Cu up to 0.15%, 25 Balance iron, The ratio of %Mn / %S is from 2.0 to 3.2, The ratio (%Mn-1.62x %S) / %Cb is from 4 to 51, and %S= (0.0045-0.0058) x the desired yield strength in ksi. 30

15. A cold drawn rephosphorized and resulfurized free-machining steel bar formed of a hot rolled bar at least 2 inches in cross section, the steel having the composition, by weight:

C up to 0.08% Mn 0.8% to 1.4% Si up to 0.1% 35 P 0.03% to 0.9% S 0.03% to 0.50% Cb 0.02% to 0. 10% V up to 0.1% The sum of Ni, Cr, Mo, and Cu up to 0.15%, 40 Balance iron, The ratio %Mn / %S is from 2.0 to 3.5, The ratio (%Mn-1.62x%S) / %Cb is from 5 to 25, and %S= (0.0045-0.0063) x the desired yield strength in ksi. 45

16. A cold drawn resulfurized and rephosphorized free-machining steel bar according to claim 1 and as described in any one of the preceding Examples 1-3.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987.

Published at The Patent Office, 25 Southampton Buildings, London, WC2A 'I AY, from which copies may be obtained.