US3336168A - Weldable tough steel essentially composed of chromium and manganese and method of manufacturing the same - Google Patents

Description

United States Patent 3,336,168 WELDABLE TOUGH STEEL ESSENTIALLY COM- POSED OF CHROMIUlVI AND MANGANESE AND METHOD OF MANUFACTURING THE SAME Sadayoshi Morita, Susumu Goda. Makoto Sato, and Yoshinori Yamaguchi, all of Kitakyushu, Fukuoka Prefecture, Japan, assignors to Yawata Iron & Steel Co., Ltd'., Tokyo, Japan No Drawing. Continuation of application Ser. No. 319,504, Oct. 28, 1963. This application Dec. 30, 1966, Ser. No. 606,447 Claims priority, application Japan, Nov. 1, 1962, 37/49,242 1 Claim. (Cl. 148-38) This is a continuation of application Ser. No. 319,504, filed Oct. 28, 1963, now abandoned.

This invention relates to a tough steel of good weldability, and more particularly relates to a super high tension steel essentially composed of chromium and manganese, in which nickel is contained in much smaller amount than that in any conventional high tension steel.

As a conventional super high tension steel there may be enumerated such kinds of steels as SAE 4340 type steel, hot-rolled die-steel, cold-rolled AISO type 301 series steel or PH stainless steel. Among them PH stainless steel is most frequently used because of its superior qualities in weldability, workability and anticorrosiveness. However, it has a defect in relatively high production cost, because it contains a relatively high percentage of nickel.

As a result of investigations the inventors have discovered that a tough steel of novel composition series, in which nickel is reduced to an amount less than 3.5 wt. percent and chromium and manganese are the essential constituents in place thereof, renders such a superior weld as never found in the conventional high tension steels.

The main object of the present invention is to provide a novel tough steel characterized by a good weldability even with a lower nickel content than that in any conventional tough steel.

Another object of the present invention is to provide a novel weldable steel of superior qualities at much lower production cost than that of any conventional tough steel.

Further objects of the present invention is to provide a weldable tough steel of a novel composition series, in

which nickel is contained in an amount less than 3.5

Wt. percent C Less than 0.20 Si Less than 1.5 Cr 3.0 to 17.0

Wt. percent Ni Less than 3.5 Mn 2.0 to 10.0 Cu 0.2 to 5.0

Balance accompanying impurities and iron.

(2) The weldable tough steel of the second series is characterized by the following composition:

Balance accompanying impurities and iron.

Besides the aforesaid alloying elements this series of the weldable tough steel may contain the following elements according to the necessities:

One or more than two kinds of elements selected from a group consisting of V, Nb, Ta and Ti 0 to 10 times so much as C content in total. N 0 to 0.2 wt. percent. Be 0 to 0.25 wt. percent.

in which 0 designates no addition.

The last second series tough steel difiiers from the first series in that besides C, Si, Cr, Ni, Mn and Cu there are added Mo, Al, N, Be and one or more than two kinds of elements selected from a group consisting of V, Nb, Ta and Ti.

(3) The third series of weldable tough steel is characterized by the following composition:

C Less than 0.20 wt. percent. Si Less than 1.5 wt. percent. Cr 3.0 to 17.0 wt. percent.

7 Ni Less than 3.5 wt. percent.

Mn 2.0 to 10.0 wt. percent. Cu 0.2 to 5.0 wt. percent.

M0 or/ and W 0.4 to 3.0 wt. percent in total. N Less than 0.2 wt. percent.

One or more than two kinds of elements selected from a group consisting of V, Nb, Ta Ti and Al Less than 1.5 wt. percent in total.

The difference between the second and third series resides in that in the latter W is added an place of or together with Mo.

By elucidating the oritieality of each element in the compositions of the aforesaid weldable tough steels the features of the present invention will be made clear.

In general, the carbon content of steel is desirable to be low to improve the weldability thereof and the percentage of less than 0.20 was found to be adequate to maintain the weldability and notch toughness. Si serves to remarkably improve the acid-resistance of steel when it coexists with Cr. By adding Si up to 1.5 wt. percent to steel a favorable result could be obtained. If more than 1.5 wt. percent, the toughness of steel was sacrificed. Cr improves the anticorrosiveness, lowers the Ms temperature and is necessary to obtain the desired structure of steel. By adding Cr in the range of 3 to 17.0 wt. percent to steel the expected results were obtained. Less than 3 wt. percent, no effect of the addition of this element was demonstrated and more than 17 wt. percent, the 6 ferrite was precipitated, thereby the strength of steel was damaged. As the present invention is to improve the weldability by leaving some quantity of the austenite in the weld, it is necessary to confine the amount of 5 ferrite to be produced by the addition of Cr to a proper amount. For this purpose the addition of the austenite-forming elements such as C, Ni, Mn and N in proper amounts are required to counterbalance the effects of the ferrite-forming elements such as Cr, Si, Mo and the like. As Ni is limited to a relatively low amount in the steel prepared by the present invention, the content of Mn is to be regulated to obtain the desired structure of steel. The addition of Mn in an amount of 2.0 to 10.0 wt. percent, preferably 3.0 to 8.0 wt. percent, was found to be adequate. A higher amount than 10.0 wt. percent produces stable austenite, thereby the hardening is hampered. Further, the amount of Ni should be reduced in correspondence with Cr in order to prevent the formation of 6 ferrite. However, in view of the necessity of preserving the anticor-rosiveness of steel the lower limit of the addition of Ni should be such as to balance with 10 wt. percent of Cr to be added.

Cu serves to stabilize the austenite and provide steel with the precipitation hardenability by itself or the compounds made by the reaction with afterwards mentioned precipitation-hardening elements. Cu was proved to be a cheap and effective strength-increasing element. For this reason Cu was added up to about 5 wt. percent in the steel prepared by the present invention. However, a higher percentage than that causes remarkable red shortness. The lower limit thereof was 0.2 wt. percent. Below this lower limit, there appeared no precipitation effect. But, when it coexists with V, Nb and Ti and the like, the precipitation hardenability increases by the synergistical effect, thereby the required addition of Cu may be reduced.

Besides the aforesaid C, Si, Cr, Mn and Cu, there were further added 0.003 to 1.0 wt. percent of Al, 0 to 10 times so much as C content of one or more than two kinds of elements selected from the group consisting of V, Nb, Ta and Ti, and 0 to 0.2 wt. percent of N and 0 to 0.25 wt. percent of Be in the tough steel of the second series prepared by the present invention. V, Nb, Ta and Ti function as precipitation-hardening elements. These elements produce intermetallic compounds in combination with Mn, Cu and Ni, which compounds effect the precipitationhardening in the martensitic structure formed by the tempering, thereby the yield strength of steel is enhanced. That is, by the addition of these elements a steel superior in hot-workability and weldability may be obtained due to a favorable precipitationhardenability given by them. Al and Be belong also to the precipitation-hardening elements. Further, A1 is required to effect the grain refining. The addition of Al up to 1.0 wt. percent was effective, but below 0.003 wt. percent no effect could be perceived. Be could be added up to 0.25 Wt. percent. The addition of N up to 0.2 wt. percent was possible when no nitrideforming element such as V, Nb, Ta and Ti was added, but when the nitride-forming element was added, the addition of N of below 0.12 wt. percent was adequate, particularly below 0.05 wt. percent.

As already mentioned, the weldable tough steel of the third series is distinguished from that of the second se ries by the addition of W in place of or together with Mo.

W and Mo have the effects of preventing the temper brittleness, improving the low-temperature toughness and increasing the temper resistance by the secondary hardening effected at the time of tempering. In view of the economic conditions the addition of both elements or of one of them up to 3 wt. percent was feasible, but no effect could be perceived with an addition of below 0.4 wt. percent.

Summarily, the compositions of the aforesaid tough steels prepared by the present invention may be characterized by the following three features: Ni is substituted by Cr and Mn to economize the former, various kinds of precipitation-hardening elements, particularly those of the group consisting of V, Nb, Ta and Ti, are used to improve the weldability and hot-workability, and W is used as an equivalent of M0 for preventing the temper brittleness and improving the temper resistance.

The method of manufacturing the weldable tough steel according to the present invention has somewhat different steps according to the series of compositions, though in principle the same. In the case of the second series of composition, after the specified elements have been added to a steel containing less than 0.20 wt. percent of C to prepare an alloyed steel, said alloyed steel is hot-rolled, then the hot-rolled steel is subjected to a solution treatment, the steel is held at a temperature of Ac to 1,100 C. for 1.3 hours and then subjected to an air-cooling. The solution treatment may be carried out more than one time. The solution treatment is immediately followed by the tempering treatment at a temperature of below Ac to effect the precipitation-hardening. It is also possible to carry out between the solution treatment and tempering treatment a sub-zero treatment, in which the steel is held at a temperature below zero degree for 2 to 3 hours to develop the transformation from austenite to martensite, or a martensitic transformation from austenite to martensite by reheating the material up to the temperature above A0 transformation point. That is, the tempering process may be carried out after the martensitic transformation has been effected.

Also in the case of the third series of the composition, after the specified elements have been added to a steel containing less than 0.20 wt. percent of C to prepare an alloyed steel, said alloyed steel is subjected to the same heat treatments as above mentioned. However, in this case, the heat treatment comprises only the steps of the solution treatment and tempering treatment, when the sum of Ni content and /2 Mn content amounts to about 3 to 5 wt. percent, and when the sum of Ni content and /2 Mn content exceeds about 5 wt. percent, the step of the martensitic transformation treatment or cold-rolling or sub-zero treatment is interposed between the solution treatment and the tempering treatment, because in the former case, that is, in the case of the sum of Ni and /2 Mn amounting to about 3 to 5 wt. percent only a small amount of austenite remains in the martensitic structure after the solution treatment has been carried out and hence the desired product may be obtained by immediately carrying out the sequent precipitation-hardening treatment, whereas as in the latter case, that is, in the case of the sum of both elements exceeding about 5 wt. percent a large amount of austenite still remains in the martensitic structure after the solution treatment it is necessary to carry out the intermediate treatment such as the martensitic transformation treatment or cold-rolling or sub-zero treatment to produce the martensitic structure before the precipitation-hardening treatment is carried out.

Example 1 Tests have been carried out on four specimens of the second series of the composition as shown in Table 1.

TABLE L-CHEMICAL COMPOSITION IN WEIGHT PERCENT C Si Mn N i G! M 011 T1 Nb V Al N After the specified alloying elements have been added 400 Hv (Vickers hardness), indicating that the welding to a steel containing less than 0.20 wt. percent of C, the hardening occurred in a small degree.

TABLE 2.-MECHANICAL PROPERTIES Heat treatment 0.2% elonga- 0 C. V Hardness of weld Steel Thickness tion proof Tensile Elongation Oharpy imaccord to thermospecimen of steel Primary tr. Secondary Precipitation stress strength GL 50 mm. pact value cycle reproduction (mm.) (solution treatment hardening (kg/mm?) (kgJmmfi) (percent) (kg.-m./cm. method (Hv) treatment) 0.) treatment Transformation treatment.

alloyed steels were rolled to 20 and 25 mm. in thickness Example 2 and then Tests have been earned out on five speclmens of the (a) The specimen A of 20 mm. 1n thickness was normauled at temperatures of 1 C and C third series of the composltron, as shown in Table 3. The

I a lution treatment) and then tempered at a temperature of hot-rolled Specimens were sublected to the followmg heat 450 C. (precipitation treatment). treatments:

TABLE 3.-OHEMICAL COMPOSITION IN WEIGHT PERCENT Steel 0 Si Mn P S Ni Or Mo W Cu Ti Nb V Ta. N 0 Al specimen (b) The specimen B of 20 mm.-in thickness was nor- (a) The specimens 1, 2 and 3 of 12 mm. in thickness malized at temperatures of 1,050 C. and 950 C. and 0 Were subjected to the solution treatment at a temperathen tempered at a temperature of 400 C., ture of 1,050 C. and then tempered at a temperature (c) The specimen C of 20 mm. in thickness was norof 460 C. (precipitation treat-ment) (martensite type malized at temperatures of 1,050 C. and 950 C. and steel plate), then tempered at a temperature of 400 C., (b) The specimens 4 and 5 of 12 mm. in thickness (d) The specimen D of 25 mm. in thickness was nor- Were subjected to the solution treatment at a temperature malized at a temperature of 1,050 C. and then subjected of 950 C. and then to the transformation treatment at a to transformation treatment at a temperature of 750 C. emperature of 750 C. and subsequently to the precipitaand subsequently was tempered at a temperature of tion treatment at a temperature of 460 C. (semi-austen- 500 C. ite type steel plate).

Another sample of D was only tempered at a tempera- Another sample of 4 and 5 of 3 mm. in thickness ture of 500 C, without carrying out the normalizing Were subjected to the solution treatment at a temperaprocess (solution treatment). ture of 1,050 C. and then to the cold-rolling and the Th results obtai d b th e rocesses are Shown i cold-rolled materials were subjected to the precipitation Table 2. As evidently seen from Table 2, in all specimens treatment at a temperature of a tensile strength above 120 kg./mm. could be achieved. Further samplesof 4 and 5 0f 12 in n ss W re Th V Charpy impact t t t 0 (1 h d 44 t 7 9 subjected to the solution treatment at a temperature of kg.-m./cm. demonstrating the sufficient toughness rethen to the Sub-Zero treatment at a p quired for the high tension steel of this kind. Further, ture of -70 to 80 C. with a holding time of 2 to 3 the test of the highest hardness of weld according to the hours and then subsequently to the precipitation treatwelding thermocycle reproduction method showed around ment at a temperature of 460 C The results obtained by these processes are shown in Table 4. Also in these cases, a yield strength of above 120 kg./mm. and an elongation (gauge length 50 mm.) of 8 to were obtained, indicating the strength and ductility SllfllClCl'ltlY comparable with Ni-Cr type precipitation-hardened stainless steel.

TABLE 4.-ME CHANICAL PROPERTIES Thickn ss Heat treatment i 0.2% E1 Steel of steel elongation an tion GL specimen (mnr) proof stress (kg/.mm?) mm.

Primary treatment Secondary treatment Tlllld treatment (kg/mm!) (percent) 12 Solution treatment at Precipitation hardening 127 139 10 1,050 0 treatment at 400 C 12 d0 do 143 11 12 do do 128 142 10 12 Solution treatment at Transformation ti'eat- Precipitation hardening 145 9 950 ment at 750 0 treatment at 460 C 4 3 Solutiogareetment at Gold rolling d0 145 161 3 1,050 12 solutignctreatment at Sub-zero treatment do 126 147 12 950 12 Solution treatment at Transformation treat- .do 128 14s 8 950 C ment at 750 C 5 3 solutiOglcllfeatment at Cold-rolling ..do 148 1 5 9 1 050 12 sollltgog treatment at Sub-zero treatment .-do 130 143 11 What is claimed is: A weldable tough steel obtained by subjecting a steel consisting of less than 0.2 wt. percent C, less than 1.5

10 times so much as the C content and the rest being lIOIl and impurities, to the precipitation-hardening treatment.

References Cited UNITED STATES PATENTS DAVID L. RECK, Primary Examiner.

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