US3303061A - Bainitic iron alloys - Google Patents

Description

United States Patent Metal Climax, Inc., New York, N.Y., a corporation of.

New York No Drawing. Filed Feb. 24, 1965, Ser. No. 435,064 4,Claims. (Cl. 14812.3)

This application is a continuation-in-part of my copending application, Serial No. 365,815, filed May 7, 1964, now abandoned.

The present invention resides in a novel and useful structural steel composition characterized by its high strength and low cost and in a method of treating the same to obtain optimum properties.

So-called structural steel, which is hot rolled in the form of I-beams, channels, angle irons, plates, rods, etc., constitutes a major portion of all steel used. It is a low carbon steel of minimum cost and has limited yield strength compared with the more expensive alloy steels which are of greater strength because of their capacity to be hardened by heating and quenching. It has been recognized for many years that the designs of steel structures such as buildings, bridges and the like could be improved and a saving in material realized if stronger materials were available. However, the added cost of all prior alloy steels has more than otfset their advantage from a strength standpoint when used in place of conventional structural steel, except in unusual situations.

Accordingly, it is an object of this invention to provide structural steel members which are aseasily welded or otherwise fabricated as conventional structural steel members but which have significantly higher yield strength, and which are sufliciently economical to have a wide field of utiltiy in applications in which conventional structural steels have been used.

Another object is to provide a structural steel alloy which is capable of being age hardened at relatively low temperatures and which will, without any necessity of quenching or any other drastic heat treatment, develop a hardness and strength substantially in excess of that of conventional structural steel.

Another object of the invention is to provide a method of producing an economical structural steel member of greater strength than those made of conventional structural steels.

A further object of the invention is to provide a method of producing an economical structural steel member having unusually high toughness at very low temperatures.

Other objects and advantages of the invention will become apparent from the following specification and appended claims.

In accordance with the present invention, the foregoing objects are achieved with a low carbon steel alloy containing as essential ingredients molybdenum, copper, boron and aluminum. Carbon is not essential and is preferably no more than 0.10 percent but may be present in amounts up to 0.15 percent. Silicon and manganese may be employed in the usual fashion for deoxidization and retained amounts of 0.40 to 0.70 percent manganese and from 0.10 to 0.30 percent silicon are permissible but not essential.

The boron, molybdenum and aluminum are essential to produce an alloy which will form a bainitic structure when air cooled in the usual manner from conventional hot working temperatures. For this purpose, only minute amounts of boron are necessary, i.e., less than 0.008 percent; but in order to insure that this small quantity of boron is not tied up with oxygen, it is necessary to incorporate aluminum in an amount in excess of that required -for deoxidization. The aluminum which combines 3,303,061 Patented Feb. 7, 1967" with the oxygen is acid insoluble, and any excess is acid soluble aluminum. The alloy must have at least 0.01 percent acid soluble aluminum and preferably has a residual acid soluble aluminum content in the range of 0.20 to 0.40 percent. When acid soluble aluminum is present, then it is only necessary to have that very small amount of boron which is required to produce the bainitic structure. This minimum amount is believed to be in the order of .002 percent. The molybdenum content may range between a minimum of 0.35 and approximately 0.75 percent. Larger quantities of molybdenum are not objectionable except to the extent that they unnecessarily increase the cost of the resulting alloy.

The normal amounts of sulphur and phosphorus present in conventional steels as impurities may also be present. For this purpose a maximum of 0.04 percent each is tolerated.

A further essential element of the composition is copper which may be present in amounts ranging from 0.60 percent to 2.0 percent. To facilitate hot working, the

alloy preferably contains nickel ranging upwardly from 0 to 1.0 percent as the copper content increases from 0.60 percent to 2.00 percent.

Suitable alloys of the type mentioned comprise the following:

Constituent Example 1 Example 2 Example 3 Percent Percent Percent Carbon 0.07 O. 10 0.05 Mangane 0. 60 0. 70 0. 50 Silicon"... 0. 10 0. 20 0. 15 Molybdenum... 0. 52 0. 40 0. 60 ickel 0. 68 0. 20 0. Copper 1.16 0.80 l. 50 Boron 0. 0023 0. 003 0. 004 Aluminum (acid soluble). 0. 23 0. 07 0 30 Aluminum (acid insoluble)- 0. 04 0. 04 Sulphur 0. 02 0. 02 0.015 Phosphorus 0. 01 0. 015 Iron Balance Balance Hot worked alloys of the above type Wlll, on normal cooling, form a bainitic structure and will have a tensile yield strength (0.20 percent offset) in the order of 80,000 pounds per square inch in the hot worked condition. The yield strength of the hot worked structural members may be materially increased by age hardening from one to four hours at temperatures in the order of 900 to 1100 F.

Conventional hot rolling procedures may be employed and will result in a structural member having a low temperature ductility and toughness about equivalent to that of conventional structural steel. However, if the rolling or other working procedures are completed at a temperature lower than is customary, a remarkable increase in low temperature toughness is realized.

The bainitic structure of the alloys of the present invention results from their cooling from an austenitic condition following conventional hot working operations. Accordingly, the alloy should be heated prior to working to a temperature high enough to convert the alloy to austenite. The minimum temperature for this purpose will depend upon the carbon content, but will ordinarily be around 1750 F. Initial working may be conducted at or above this temperature or may be delayed until the alloy has partially cooled, but to obtain the maximum toughness at low temperatures, it is necessary to complete the hot working operation at a temperature of approximately 1200" F. Thus, for example, hammer forging may start at temperatures of 2000 F. or more, but, to realize maximum toughness, should be continued as the piece cools so that final working is efiected at a temperature in the range of 1100 F.-1300 F. and preferably at about 1200 F. Similarly, the alloys may be hot rolled by the procedures conventionally employed in forming structural steel members such as I-beams, channels, angle irons, plates, etc. In this case the initial rolling operations may be conducted at conventional temperatures such as 2050 F., but again maximum toughness is obtained if the final rolling pass is carried out at a temperature in the range of 1100 F. to 1300 F. and preferably about 1200 F. i

Low temperature toughness is also enhanced by continuing the age hardening operation beyond the point of maximum yield strength. This is shown by the following table setting forth data obtained in Charpy V- Notch impact tests on specimens cut from a hot worked 1% inch square bar having the composition of Example 1. This bar had been hammer forged in two stages, the first starting at a temperature of about 2050 F. reduced thebar from 3% inches square to 2% inches square. The second stage, which started at about 1700 F. and continued until the temperature was about 1200 F. reduced the bar to 1% inches square.

CHARPY V-NOTCH IMPACT PROPERTIES Fracture Test Impact Lateral Appear- Condition Temp., Strength, Expanance,

F. ft.-1b. sion, in. Percent Fibrous Fracture Hot-Worked -25 4 0. 003 0 5 0.005 3 72 14 0. 016 13 125 36 0.018 42 Hot-Worked and Aged -25 3 0.001 0 4 hr., 900 F. 0 4 0. 002 0 72 '37 0. 027 17 125 59 0. 045 40 Hot-Worked and Aged 25 0. 011 11 4 hr., 1,000 F. V 0 83 0.060 60 75 111 0. 083 100 125 115 0. 082 100 Hot-Worked and Aged. -100 63 "0. 050 42 4 hr., 1,050 F. -75 76 0. 057 77 50 109 0.077 82 -25 117 0.083 100 0 120 0. 087 100 75 112 0.083 100 Hot-Worked and Aged -50 18 0. 019 27 4.hr., 1,100 F. --25 99 O. 064 72 0 116 0.083 89 75 126 0.090 100 Average of three tests.

It will be observed from the above that while the material has a relatively high transition temperature and low impact strength in the hot worked condition, aging 'four hours at 1050" F. and above sharply lowered the transition temperature and increased the impact strength. Aging treatments for four hours at lower temperatures, such as 900 or 1000 F., or for shorter periods of time at higher temperatures, develop maximum yield strength but less impact strength. The material aged for four hours at 1050 F. is far superior to conventional plain carbon structural steels whichrhave a standard Charpy V-Notch impact strength ranging between about 0 and 10 ft. lbs. at 2S F. and a yield strength of about 35,000 p.s.1.

While in the above tests a four-hour aging treatment was employed, advantageous results may be obtained at substantially shorter periods of time. The time of treatment is a matter of choice, bearing in 'mind that within limits the longer the treatments and the higher the temperatures of treatment, the greater the impact strength and the lower the transition temperature with some sacrifice in yield strength. Thus, for example, a specimen having the composition of Example 1 had the following yield "strengths in the conditions indicated:

Lbs./sq. in. Hot-Worked 79,600 Aged 4 hrs. at 900 F. 98,400 Aged 4 hrs. at 1000" F 94,200 Aged 4 hrs. at 1050 F. 87,300 Aged 4 hrs. at 1100' F. 84,900

In view of the above, it is apparent that if maximum impact strength is required, the best results are obtained with long aging treatments and that for that purpose the optimum aging temperature for a four-hour treatment is about 1050 F. As will be understood by those skilled in the art, longer treatments at lower temperatures or shorter treatments at higher temperatures will produce similar results. If maximum yield strength'is desired and some sacrifice of impact strength can be tolerated, shorter times of treatment may be employed. Thus, treatments for as short a time as fifteen minutes at 1100 F. or one hour at 900 F. will result in a marked increase in yield strength.

The effect of the final rolling temperature on low temperature toughness is illustrated by the following table of Charpy V-Notch impact properties of rolled bars at very low temperatures. These bars, which had the composition of Example 2, where first rolled from 1% inch square bars to inch thick strips at 1750 F. and then rolled to /2 inch thick by 1% inch wide strips at the final rolling temperature indicated below. After rolling the bars were aged four hours at 1050 F. and then subject to the impact test at -50 F. and -125 F.

Impact Strength, Final Rolling tt.-lbs.

Temp, F.

50 F. l25 F.

While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the following claims. For example, minor amounts of unspecified elements may be present so long as they do not impair the beneficial eifects of the essential elements or change the essential character of the alloy.

What is claimed is:

1. Abainitic structural steel having from 0 to 0.15 percent carbon, from 0.35 to 0.75 percent molybdenum, from 0 to 1.0 percent nickel, from 0.60 to 2.00 percent copper, from 0.002 to 0.008 percent boron, from 0.01 to 0.40 percent acid soluble aluminum, and the balance essentially iron.

2. An age hardened structural steel member having a bainitic metallurgical structure and a finely divided copper-containing precipitate dispersed through the crystal matrix and having a composition containing from 0 to 0.15 percent carbon, from 0.35 to 0.75 percent molybdenum, from 0 to 1.0 percent nickel, from 0.60 to 2.00 percent copper, from 0.002 to 0.008 percent boron, from 0.01 to 0.40 percent acid soluble aluminum, and the balance essentially iron.

3. An age hardened bainitic structural steel member having a bainitic metallurgical structure and a coppercontaining precipitate dispersed through the crystal matrix .60 to 2.00 percent copper, from about .002 'to about .008

percent boron, from .20 to .40 percent acid soluble aluminum, and the balance essentially iron.

4. The method of making a structural steel member which consists in Working a steel alloy containing from 0 to 0.15 percent carbon, from 0.35 to 0.75 percent molybdenum, from 0 to 1.0 percent nickel, from 0.60 to 2.00 percent copper, from 0.002 to 0.008 percent boron, from 0.01 to 0.40 percent acid soluble aluminum, and the balance essentially iron, at least a portion of said working being conducted after the alloy has been heated to a temperature suflicient to convert it to an austenite structure and the final working to form the structural member being conducted while the alloy is at a temperature in the range of 1100 'F. to 1300 F., and age hardening References Cited by the Examiner UNITED STATES PATENTS 1,972,241 9/1934 Lorig 148-142 X 1,972,248 '9/193'4 Smith 148142 X 10 3,132,025 5/1964 Hurley 75-124 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner.

Claims (1)

1. 4. THE METHOD OF MAKING A STRUCTURAL STEEL MEMBER WHICH CONSISTS IN WORKING A STEEL ALLOY CONTAINING FROM 0 TO 0.15 PERCENT CARBON, FROM 0.35 TO 0.75 PERCENT MOLYBDENUM, FROM 0 TO 1.0 PERCENT NICKEL, FROM 0.60 TO 2.00 PERCENT COPPER, FROM 0.002 TO 0.008 PERCENT BORON, FROM 0.01 TO 0.40 PERCENT ACID SOLUBLE ALUMINUM, AND THE BALANCE ESSENTIALLY IRON, AT LEAST A PORTION OF SAID WORKING BEING CONDUCTED AFTER THE ALLOY A PORTION OF SAID WORKING BEING CONDUCTED AFTER THE ALLOY HAS BEEN HEATED TO A TEMPERATURE SUFFICIENT TO CONVERT IT TO AN AUSTENITE STRUCTURE AND THE FINAL WORKING TO FORM THE STRUCTURAL MEMBER BEING CONDUCTED WHILE THE ALLOY IS AT A TEMPERATURE IN THE RANGE OF 1100*F. TO 1300*F., AND AGE HARDENING THE STRUCTURAL MEMBER AT A TEMPERATURE IN THE RANGE OF 900*F. TO 1100*F. FOR A PERIOD OF TIME SUFFICIENT TO LOWER ITS TRANSITION TEMPERATURE AND INCREASE ITS CHARPY V-NOTCH IMPACT STRENGTH.