ES2208030A1 - Forged steel composition used in the production of crankshafts for ships includes silicon, manganese, chromium and molybdenum - Google Patents

Abstract

Forged steel composition contains (in wt.%): 0.3-0.5 carbon, 0.1-0.4 silicon, 0.7-1.5 manganese, 1.2-3.5 chromium, 0.1-0.6 molybdenum, not more than 0.7 nickel, 0.03-0.35 V, Nb and/or Ta, 30-250 ppm nitrogen, and a balance of iron. The steel forms a microstructure consisting of bainite and martensite and contains nitrogen in solid solution with V, Nb and Ta in such that (total wt.% of V, Nb and Ta) + 0.001 x (N (ppm) in solid solution) \- 0.068. Preferred Features: The steel contains (in wt.%): 0.36-0.45 carbon, 0.15-0.4 silicon, 0.8-1.2 manganese, 1.5-3.0 chromium, 0.15-0.35 molybdenum, 0.17-0.35 V. The steel further contains 0.001-0.10 aluminum and not more than 0.006 wt.% sulfur.

Description

High strength steel for forging and crankshaft made from it.

Technical field to which the invention belongs

The present invention relates to a steel of high strength forging (hereinafter referred to as large steel resistance for forging), and more specifically to a large steel forging resistance characterized by low price (because of its low content of expensive alloy elements, especially nickel), as well as great resistance. Said forging steel will be used As raw material of large crankshafts to transmit power on a ship or the like. Therefore, said crankshafts made of high strength steel forging also fall inside of the scope of the present invention.

Related prior art

\hbox{32CrMoI2}

. De estos aceros, el primero contiene una cantidad comparativamente grande de C y una cantidad comparativamente pequeña de Cr y Mo, lo que da lugar a una resistencia ligeramente pobre y precio relativamente bajo, y por lo tanto tiene uso en áreas donde la carga no es muy alta. El segundo contiene una gran cantidad de níquel como un elemento de aleación, lo que hace que sea el mejor de los tres en resistencia y tenacidad, y por lo tanto se usa en áreas donde la carga es alta. El tercero es intermedio en términos de precio y resistencia entre los dos anteriores, y por lo tanto se usa en áreas donde la tenacidad es importante.Crankshafts to transmit power on ships are usually made of chrome-molybdenum steel typified by ISO 42CrMo4, ISO 36CrNiMo6 and DIN

 \ hbox {32CrMoI2} 

. Of these steels, the first contains a comparatively large amount of C and a comparatively small amount of Cr and Mo, which results in a slightly poor strength and relatively low price, and therefore has use in areas where the load is not very high. The second contains a large amount of nickel as an alloy element, which makes it the best of the three in strength and toughness, and therefore is used in areas where the load is high. The third is intermediate in terms of price and resistance between the previous two, and therefore is used in areas where toughness is important.

Recently it is common to make large crankshafts for boats from ISO 36CrNiMo6, which is expensive, but  It has great resistance, to avoid marine hazards.

Description of the invention

Said ISO 36CrNiMo6, which is a nickel steel- chrome-molybdenum, has excellent strength and toughness, but it is more expensive than other steels chrome-molybdenum for forging because of its high expensive nickel content as an alloying element resistance. This is an obstacle to its general acceptance.

The present invention has been realized having in  Count the above. An object of the present invention is provide a high strength forging steel that is less expensive and of comparable resistance, toughness and hardenability or even higher (which is a demand of recent users) to ISO 36CrNiMo6 as forging steel of Ni-Cr-Mo high strength in the practical use. Another object of the present invention is to provide a crankshaft made of said forging steel characterized by Low price, great strength and toughness, and good hardenability

The essence of the present invention consists in a high strength forging steel that includes C (0.3-0.5%), Si (0.1-0.4%), Mn (0.7-1.5%), Cr (1.2-3.5%), Mo (0.1-0.6%), Ni (no more than 0.7%), at least one item selected from the group that It consists of V, Nb and Ta (0.03-0.35% in total), and N (30-250 ppm), the rest being Fe and impurities inevitable, forming a microstructure composed mainly of bainite and martensite and containing N in solid solution and V, Nb and Ta so that the following expression (1) is fulfilled.

[Total (%) of V, Nb and Ta] + 0.001 x [N (ppm) in solid solution] ≥ 0.068 ... (1)

(% means% by mass. The same will apply more ahead).

The high strength steel forging the The present invention is characterized by the limited content of nickel as specified above, nitrogen added intentionally (N) and at least one element selected from the group consisting of V, Nb and Ta, fulfilling the amount of N in solution solid and the total amount of V, Nb and Ta the expression (1) above, and the microstructure composed mainly of bainite and martensite It has high tensile strength and toughness and good hardenability and yet it is comparatively cheap.

The content of each item has been set on the basis explained below. Great steel forging strength of the present invention exhibits its characteristics regardless of its basic composition to condition that the nickel content, the content of V, Nb and Ta, the total nitrogen content, and the content of N is Solid solution meet those requirements. Great steel forging strength of the present invention should have the following basic composition so that it produces completely its effect as mentioned above.

       \ dotable {\ tabskip6pt # \ hfil \ + # \ hfil \ tabskip0ptplus1fil \ dddarstrut \ cr} {
 C: \ + 0.3-0.5%, preferably
0.36 - 0.45% \ cr If: \ + 0.1 - 0.4%,
preferably 0.15-0.4% \ cr Mn: \ +
0.7-1.5%, preferably
0.8-1.2% Cr: \ + 1.2-3.5%,
preferably 1.5-2.5% Mo: \ +
0.1-0.6%, preferably
0.15-0.35%
    

the rest being substantially Faith.

The forging steel that has the composition Basic above may contain a small amount of Al as deoxidizing element Al produces its effect when its content is greater than 0.001%, but does not produce additional effect when its content exceeds 0.040%. Therefore, Al content should stay below 0.040%. Another harmful element that could entering the forging steel is S, which adversely affects the toughness and fatigue resistance. The content of S should preferably kept below 0.006%.

In addition, high strength steel forging of the present invention is characterized by the DI value not less than 30 mm. (The DI value is a critical diameter of a product of steel whose central part acquires 50% martensitic hardness at rapid cooling with water). Such characteristic properties they make the forging steel extremely suitable for large crankshafts for boats. The present invention It also covers crankshafts, particularly large ones, made by forging from the forging steel indicated above.

Brief description of the drawings

Figure 1 is a graph showing the relationship  between the nickel content and the tensile strength of experimental forging steels.

Figure 2 is a graph showing the relationship  between the nitrogen content and the tensile strength of experimental forging steels.

Figure 3 is a graph showing the values DI in Examples and Comparative Examples.

Figure 4 is a graph that shows how it looks affected the tensile strength by the content of V in the Steel and N content in solid solution.

Figure 5 is a diagram showing the procedure to determine the content of N in solution solid.

Description of a special embodiment

With the above problem in mind, the authors of the present invention conducted extensive research  to develop a new forging steel that is less expensive and of resistance and toughness comparable to known `` ISO 36CrNiMo6 '' as large Ni-Cr-Mo steel forging resistance. The new forging steel contains a smaller amount of nickel as an alloy element to reduce the cost and has good good hardenability in forging products Large with great resistance. As a result of the investigations it was found that said Cr-Mo steel for forging not containing more than 0.7% nickel as a reinforcing element has high strength and toughness that compensate for the lack of resistance due to the lower nickel content if at least one element selected from the group consisting of V, Nb and Ta and a very small amount of nitrogen such that the amount total of V, Nb and Ta and the amount of nitrogen dissolved in solution solid comply with the expression (1) indicated above. Steel forging It also exhibits very good temperability. This finding resulted to the present invention.

Cr-Mo steels used in general forging require nickel to have great strength and Tenacity and better hardenability. Unfortunately, nickel is so expensive that raises the cost of steel above price that can Pay users if their content is excessive. The present invention aims to develop a new strength steel and hardenability comparable to those of steel Ni-Cr-Mo for conventional forging a Despite its minimum nickel content. For the reduction of cost, the nickel content should be less than 0.7%, preferably less than 0.5%, more preferably less than 0.3%

The reduction of nickel content decreases the  resistance, toughness and hardenability attributable to nickel, and therefore the resulting steel does not meet the requirements of the users. The authors of the present invention investigated widely how to compensate for insufficient benefits due to reduced nickel content by incorporation of other elements, therefore developing a new steel for forge that meets the requirements of users in terms of Price as well as benefits. It turned out that this object is achieved if incorporated into a Cr-Mo steel forging with reduced nickel content at least one element selected from V, Nb and Ta and nitrogen (which until now has been considered a harmful trace element) such that the amount of V, Nb and Ta and the amount of nitrogen dissolved in solid solution meets the expression (1) indicated above.

For the forging steel to produce its effect Practical, it is essential that the nickel content should not be greater than 0.7%, the content of at least one selected item of V, Nb and Ta should be 0.03%, preferably 0.045%, at 0.35%, preferably 0.15%, and the nitrogen content should be 30 ppm, preferably 40 ppm, at 250 ppm, preferably 100 ppm. In addition, it is also essential that the total content of V, Nb and Ta and the nitrogen content dissolved in solid solution meet the expression (1) indicated above.

By the way, if the total content of V, Nb and Ta and the nitrogen content are out of the bands before specified and the requirements of the expression (1) are not met, the resulting forging steel will not have great strength and tenacity and good hardenability as intended herein invention.

Despite the research of the authors of the present invention, the reason why the steel for low nickel forging greatly improves strength and tenacity when the total content of V, Nb and Ta and the content of nitrogen dissolved in solid solution meet the expression (1) before indicated. One probable reason is that nitrogen combines with V, Nb and Ta forming nitrides and properly dissolved nitrogen said produces a good resistance improvement effect. Given the such reinforcing elements such as V, Nb and Ta are very expensive, it is economically advantageous to increase the resistance by adding them in small amounts in combination with cheap nitrogen dissolved in solid solution

The effect of increasing resistance occurs only when the reinforcing elements and nitrogen are add to a Cr-Mo steel for forging containing no more than 0.7% of Ni. This effect does not occur when added to a high quality forging steel containing more than 0.7% Ni.

The forging steel according to the present invention is based on a Cr-Mo forging steel containing no more than 0.7% Ni. After the incorporation of one or more reinforcing elements (V, Nb, Ta) and nitrogen in such a way that the total content of V, Nb and Ta and the total content of dissolved and undissolved nitrogen in solid solution and nitrogen in total complying with the expression indicated above, the desired forging steel is obtained which has a strength, toughness and hardenability comparable to Cr-Mo steels for comparatively cheap high quality forging with high nickel content.

As indicated above, the steel for forging according to the present invention is characterized by containing Nickel in a limited quantity. It is also characterized by contain at least one species of V, Nb and Ta and nitrogen (and nitrogen dissolved in solid solution) in specific amounts. The steel for forging is not specifically restricted in its composition basic A Cr-Mo steel forging with the Following basic composition is used to make the most of the characteristics of the present invention, as resistance, toughness and hardenability, which are required in crankshafts and analogues

       \ dotable {\ tabskip6pt # \ hfil \ + # \ hfil \ tabskip0ptplus1fil \ dddarstrut \ cr} {
 C: \ + 0.3-0.5%, preferably
0.36 - 0.45% \ cr If: \ + 0.1 - 0.4%,
preferably 0.15-0.4% \ cr Mn: \ +
0.7-1.5%, preferably
0.8-1.2% Cr: \ + 1.2-3.5%,
preferably 1.5-2.5% Mo: \ +
0.1-0.6%, preferably
0.15-0.35%. \ Cr}
    

The content of each item indicated above is has established for the reasons explained below.

C: 0.3-0.5%, preferably 0.36-0.45%

Carbon contributes to hardenability as well as to the resistance For maximum effect, carbon must contained in an amount of 0.3% or more, preferably 0.36% or more, and more preferably 0.38% or more. However, the carbon in an excessive amount has an adverse effect on the toughness and promotes inverted V-shaped segregation. Therefore the upper limit of carbon content should be 0.5% or less, preferably 0.45% or less, and more preferably 0.42% or less.

Yes: 0.1-0.4%, preferably 0.15-0.4%

Silicon contributes to resistance. For what its effect is maximum, the silicon must be contained in an amount 0.1% or more, preferably 0.15% or more, and more preferably 0.20% or more. However, silicon in an excessive amount promotes inverted V-shaped segregation, leading to Poor cleaning ingots. Therefore, the upper limit of the silicon content should be 0.4% or less, preferably 0.3% or less.

Mn: 0.7-1.5%, preferably 0.8-1.2%

Manganese contributes to hardenability as well As to the resistance. For maximum effect, manganese should be contained in an amount of 0.7% or more, preferably 0.8% or more, and more preferably 0.9% or more. However the Manganese in an excessive amount promotes segregation in the form of inverted V. Therefore, the upper content limit of manganese should be 1.5% or less, preferably 1.2% or less, and more preferably 1.1% or less.

Cr: 1.2-3.5%, preferably 1.5-2.5%

Chromium contributes to toughness as well as to resistance. For maximum effect, chrome should contained in an amount of 1.2% or more, preferably 1.5% or more, and more preferably 1.75% or more. However, the chrome in an excessive amount promotes V-shaped segregation inverted, resulting in poor cleaning ingots. Thus, the upper limit of chromium content should be 3.5% or less, preferably 2.5% or less.

Mo: 0.1-0.6%, preferably 0.15-0.35%

Molybdenum contributes to resistance, toughness and hardenability. For maximum effect, the Molybdenum should be contained in an amount of 0.1% or more, preferably 0.15% or more, and more preferably 0.20% or more. Without However, molybdenum is easily subjected to microsegregation (normal segregation) because of its small coefficient of partition. Therefore, the upper limit of the content of molybdenum should be 0.6% or less, preferably 0.35% or less, and more preferably 0.30% or less.

Forging steel according to the present invention It has the following basic composition, the balance being substantially Faith. It is not necessary to state that it may contain a trace amount of unavoidable impurities, or you can intentionally incorporate other elements that are harmless for the purpose of the present invention. Examples of such Additional elements include B (which contributes to the hardenability), Ti (which contributes to deoxidation), and Ca, Mg, Ce, Zr and Te (which control the form of MnS). They can be used alone or in combination with each other. Your total amount must be less than approximately 0.03%.

In addition to the previous basic composition, the forging steel according to the present invention contains aluminum as One of the inevitable impurities. Aluminum (as agent deoxidant) is added to reduce the oxygen content in the steel forging. To produce a good deoxidizing effect, aluminum should be contained in an amount of approximately 0.001% or more. However, aluminum in an excessive amount fixes the nitrogen forming AlN (making it difficult to reinforcement by incorporating N and V), and combined with many other elements to form nonmetallic inclusions and a intermetallic compound (thereby negatively affecting the tenacity). Therefore, the maximum aluminum content should be 0.10% or less, preferably 0.04% or less. The prevention of appearance of AlN is important from the point of view of reinforcement by incorporation of V, etc, and nitrogen. If he Aluminum content is very low, namely about 0.001%, the amount of AlN formed is small and therefore the reinforcing effect even if the nitrogen content is of approximately 30 ppm However, if the aluminum content is high, namely approximately 0.03%, AlN is formed in large quantity, and therefore the nitrogen content must be increased to approximately 250 ppm or increase the content of V, etc., of so that the reinforcement effect occurs, as wish.

Another element of impurity is sulfur, which proceeds of sulfides contained in the coke for iron manufacturing. The sulfur in steel forms sulfides (such as MnS) that deteriorate fatigue characteristics To avoid such problems, you have to keep the sulfur content below 0.006%, preferably below 0.005%.

Forging steel according to the present invention it is characterized because it has the following basic composition and it also has the microstructure composed mainly of Bainite and martensite. It does not have the high resistance expected in the present invention if the fraction of ferrite and perlite area exceeds 10%. The expression `` composed mainly of bainite and martensite '' means that the sectional structure that can be distinguish by observation with an optical microscope is in its mostly bainite and martensite and the fraction of bainite area or Martensite is less than about 10%. By the way, there is no consolidated way of quantitatively determining the fraction of Bainite and martensite area. However, it is possible to differentiate empirically the microstructure composed mainly of bainite and martensite from photographs of the structure in section.

The microstructure composed mainly of bainite and martensite can be obtained from a steel that have the chemical composition indicated above if the steel is coated at an average cooling rate of approximately 0.5-100 ° C / min in the 870 ° C to 500 ° C band.

Forging steel according to the present invention It has the chemical composition indicated above. Despite its limited nickel content (less than 0.7%), has high resistance without very expensive alloy elements due to at least a kind of V, Nb and Ta and nitrogen that are added in quantities specific. It has excellent hardenability as shown in the Examples set forth below. This good hardenability is indicated for the high DI value greater than 30 mm (even 32 mm) in the test Jominy (The DI value is a critical diameter for the center have the hardness of 50% martensite after cooling with Water). For this reason, forging steel is very useful for crankshafts, especially large crankshafts for boats, They experience rapid cooling after forging.

Because of its remarkable hardenability indicated by The high DI value, the forging steel of the present invention is extremely suitable for forges with great mass effect, such as 150-1000mm diameter crankshafts, which need  high strength in both the center and the surface after fast cooling

There are no specific restrictions in the process of produce the forging steel of the present invention. It can produce by casting after mixing with the components desired in a high frequency induction oven, oven Electric or converter. The mixture can be followed by vacuum degassing. The laundry can be done by ingot casting for large forges or by continuous casting For small forges. By the way, according to the present invention, there are that strictly control the nitrogen content in the steel Forging A desirable way to achieve this object is to add a nitrogen-containing raw material, such as manganese nitride and chromium nitride, to molten steel or blowing nitrogen gas to steel molten. It is possible to reduce the nitrogen content by vacuum degassing under appropriate conditions.

The forging steel of the present invention is can transform into crankshafts, etc., by any process That is not specifically limited. A typical process consists of The following steps.

* Prepare a steel of the desired composition in an electric oven or the like.

* Remove impurity elements (such as sulfur) and gaseous components (such as oxygen) by vacuum refining.

* Form ingots.

* Heat the ingot and forge the ingot heated.

* Intermediate inspection, overheating and crankshaft forging.

* Homogenizing heat treatment and tempering by rapid cooling.

* Finishing machining.

The crankshaft forge can be made by free forging or forging R-R (or T-R). The free forge forms the crank arms and stumps in a single block, which later forms the crankshaft by gas cutting and machining. The forge R-R (or T-R) is carried out in such so that the axis of the ingot matches the main axis of the crankshaft. The advantage of this method of forging is that the part central that easily deteriorates by segregation is the axis crankshaft main. Therefore, the resulting crankshaft has a clean surface layer and therefore has excellent resistance and fatigue characteristics.

By virtue of its high strength and low price, the forging steel of the present invention can be used to any other forging other than crankshafts, such as intermediate shafts and propeller shafts for ships, elbows and armed crankshafts, and objects gaps without welding.

Examples

The invention will be described in more detail with reference to the following examples, which are not intended to limit their scope. You can make several changes and modifications in the invention without departing from its spirit and scope.

Examples 1 to 19 and Comparative Examples 1 to 22

In each example a steel forging was prepared  a chemical composition shown in Tables 1 and 2 using a high frequency induction oven. The forging steel slipped to an ingot measuring 158-132 (diameter) x 323 mm (height) and weighed 50 kg. The nitrogen content dissolved in the steel was adjusted by controlling the amount of manganese nitride added to the atmospheric gas composition. In addition, it was incorporated to steel V, Nb or Ta in a predetermined amount.

With the cutter part cut, the ingot The resulting was heated at 1230 ° C for 5-10 hours. The heated ingot was compressed until its weight was reduced to half and then it lengthened (with its central line turned 90 °) until which measured 90 x 90 x 450 mm. The forging continued cooling with air. The resulting sample was heated using a simulation oven Small for austenitizing treatment. This step consisted of heat the sample to 870 ° C at a heating rate of  40 ° C / h, kept at this temperature for 1 hour, cooled to an average cooling rate of 20 ° C / min (in the band of 870 ° C to 500 ° C), kept for annealing at 610 ° C for 13 hours, and cooled in the oven.

By the way, the amount of nitrogen dissolved in the sample denotes the sum of nitrogen dissolved in solid solution and nitrogen forming compounds. Total nitrogen is determined by the inert gas fusion method. The nitrogen forming compounds were determined by Indophenol absorptiometry that was performed after having separated the precipitates from the steel by extraction electrolytic The amount of nitrogen dissolved in solid solution It was calculated by subtracting the amount of compounds that form nitrogen of the amount of total nitrogen.

The microstructure of the sample that had been heat treated with austenitizing and annealing treatment, it examined as follows. The cross section of the sample underwent an attack with nital and then was observed under a microscope optical (x100). More than one field was photographed and the area belonging to ferrite and perlite in the photographs resulting. It was found that all samples of the Examples and Comparative Examples have the composite microstructure mainly of bainite and martensite, being substantially nil  the area belonging to ferrite and perlite.

The mechanical properties and hardenability of Each steel sample obtained as mentioned above,  They were examined as follows. The results are shown in Tables 3 and 4 and figures 1 and 2.

[Mechanical properties at room temperature]

Tensile strength was measured according to ISO 7892 (using a test piece in a form defined by L_ {0} = 5.65 \ surdS_ {0}). Charpy impact resistance was measured according to ISO 148 (using a test piece with a slot of 2 mm).

[Templability]

The hardenability was evaluated with the Jominy method according to ISO 642. This test method uses a test piece with flange and consists of heating at 870 ° C and cooling with Water. The hardenability is classified in terms of the diameter Critical (DI) of the central part with 50% martensite.

6

7

TABLE 3

       \ nobreak \ vskip.5 \ baselineskip \ centering \ small \ begin {tabular} {| l | c | c | c | c | c | c |} \ hline
  \ + Limit of \ + Resist. a \ + Elonga- \ + Training \ + Energy
\ + DI value \\ \ + elastici- \ + traction \ + tion (%) \ + of
neck \ + of impac- \ + (mm) \\ \ + dad 0.2% \ + (MPa) \ + \ + (%)
 \ + to (J) \ + \\ \ + (MPa) \ + \ + \ + \ + \ + \\\ hline Comparative Ex.
1 \ + 803.6 \ + 947.7 \ + 18.6 \ + 65.7 \ + 72.9 \ + 27 \\\ hline
Example 1 \ + 764.4 \ + 915.3 \ + 18.7 \ + 67.2 \ + 64.2 \ + 30
\\\ hline Example 2 \ + 806.5 \ + 963.3 \ + 18.4 \ + 67.4 \ + 58.3
\ + 32 \\\ hline Example 3 \ + 769.3 \ + 917.3 \ + 20.0 \ + 70.2 \ +
60.2 \ + 34 \\\ hline Example 4 \ + 762.4 \ + 919.2 \ + 17.7 \ +
67.6 \ + 48.7 \ + 38 \\\ hline Example 5 \ + 760.5 \ + 909.4 \ +
20.0 \ + 71.2 \ + 55.1 \ + 41 \\\ hline Example 6 \ + 772.2 \ +
934.9 \ + 19.4 \ + 69.5 \ + 48.1 \ + 44 \\\ hline Comparative Example 2
\ + 720.3 \ + 885.9 \ + 20.1 \ + 70.6 \ + 106.4 \ + 27 \\\ hline Ex.
Comparative 3 \ + 720.3 \ + 885.9 \ + 20.1 \ + 70.6 \ + 75.6 \ + 25
\\\ hline Ex. Comparative 4 \ + 699.7 \ + 872.2 \ + 17.8 \ + 69.4
\ + 42.7 \ + 23 \\\ hline Ex. Comparative 5 \ + 705.6 \ + 887.9 \ +
18.7 \ + 66.7 \ + 42.2 \ + 22 \\\ hline Ex. Comparative 6 \ + 723.2
\ + 898.7 \ + 19.9 \ + 68.2 \ + 67.7 \ + 27 \\\ hline Comparative Ex.
7 \ + 711.5 \ + 896.7 \ + 19.3 \ + 69.8 \ + 54.4 \ + 27 \\\ hline
Ex. Comparative 8 \ + 685.0 \ + 874.0 \ + 20.3 \ + 68.7 \ + 79.0
\ + 29 \\\ hline Ex. Comparative 9 \ + 705.0 \ + 898.0 \ + 19.5 \ +
66.2 \ + 73.0 \ + 30 \\\ hline Ex. Comparative 10 \ + 677.0 \ +
873.0 \ + 20.3 \ + 67.5 \ + 81.3 \ + 31 \\\ hline Comparative Ex. 11
 \ + 702.6 \ + 888.9 \ + 19.3 \ + 63.9 \ + 39.2 \ + 24 \\\ hline
Example 7 \ + 775.0 \ + 935.0 \ + 19.6 \ + 66.2 \ + 53.9 \ + 32
\\\ hline Example 8 \ + 825.0 \ + 989.0 \ + 12.7 \ + 64.6 \ + 46.3
\ + 32 \\\ hline Example 9 \ + 819.0 \ + 979.0 \ + 18.6 \ + 65.2 \ +
36.5 \ + 33
\\\ hline \ end {tabular} \ par \ vskip.5 \ baselineskip
    

TABLE 4

       \ nobreak \ vskip.5 \ baselineskip \ centering \ small \ begin {tabular} {| l | c | c | c | c | c | c |} \ hline
  \ + Limit of \ + Resist. a \ + Elonga- \ + Training \ + Energy
\ + DI value \\ \ + elastici- \ + traction \ + tion (%) \ + of
neck \ + of impac- \ + (mm) \\ \ + dad 0.2% \ + (MPa) \ + \ + (%)
 \ + to (J) \ + \\ \ + (MPa) \ + \ + \ + \ + \ + \\\ hline Comparative Ex.
12 \ + 774 \ + 935 \ + 18.9 \ + 68.4 \ + 72.0 \ + - - \\\ hline
 Comparative Ex. 13 \ + 770 \ + 930 \ + 19.1 \ + 68.6 \ + 74.0 \ +
- - \\\ hline Comparative Ex. 14 \ + 800 \ + 942 \ + 19.0 \ +
66.0 \ + 71.0 \ + 28 \\\ hline Ex. Comparative 15 \ + 788 \ + 930
\ + 18.8 \ + 65.8 \ + 77.0 \ + 29 \\\ hline Ex. Comparative 16 \ +
740 \ + 901 \ + 18.6 \ + 67.7 \ + 55.3 \ + 31 \\\ hline Ex.
Comparative 17 \ + 728 \ + 878 \ + 2.0 \ + 68.9 \ + 56.3 \ +
- - \\\ hline Example 10 \ + 791 \ + 946.5 \ + 18.7 \ + 67.9
\ + 60.5 \ + - - \\\ hline Example 11 \ + 801 \ + 952 \ + 18.6
\ + 66.2 \ + 49.6 \ + - - \\\ hline Comparative Ex. 18 \ + 779
\ + 928 \ + 18.1 \ + 65.0 \ + 47.3 \ + 25 \\\ hline Comparative Ex.
19 \ + 722 \ + 900 \ + 19.7 \ + 68.5 \ + 94.0 \ + - - \\\ hline
 Ex. Comparative 20 \ + 724 \ + 904 \ + 19.5 \ + 67.2 \ + 75.0 \ +
- - \\\ hline Example 12 \ + 775 \ + 935 \ + 19.6 \ + 66.2 \ +
54.0 \ + 32 \\\ hline Example 13 \ + 805 \ + 959 \ + 18.9 \ + 66.8
\ + 87.5 \ + 33 \\\ hline Example 14 \ + 803 \ + 966 \ + 17.7 \ +
66.0 \ + 64.0 \ + 33 \\\ hline Example 15 \ + 790 \ + 945 \ + 21.4
\ + 68.2 \ + 103.5 \ + 32 \\\ hline Comparative Ex. 21 \ + 718 \ +
902 \ + 20.5 \ + 67.6 \ + 115.8 \ + 30 \\\ hline Example 16 \ + 775
\ + 924 \ + 19.1 \ + 65.2 \ + 83.0 \ + 33 \\\ hline Example 17 \ + 84
 \ + 992 \ + 16.3 \ + 62.7 \ + 37.7 \ + 32 \\\ hline Comparative Ex.
22 \ + 809 \ + 951 \ + 18.1 \ + 62.8 \ + 82.7 \ + 29 \\\ hline
Example 18 \ + 783 \ + 930 \ + 17.2 \ + 62.1 \ + 45.0 \ + 32
\\\ hline Example 19 \ + 806 \ + 954 \ + 17.6 \ + 64.1 \ + 42.0 \ +
32
\\\ hline \ end {tabular} \ par \ vskip.5 \ baselineskip
    

Observations: - -: do not measured.

Figures 1 and 2 are graphs that show, respectively, the relationship between the Ni content and the tensile strength and the relationship between the content of nitrogen and tensile strength. These graphics are elaborated from the data in Tables 1 and 2. It is noted by figure 1 that the maximum effect of improving resistance by incorporating nitrogen and some of V, Nb and Ta se Produces when steel contains 0.7% Ni or less. I also know notes by figure 2 that nitrogen produces its effect when its content is 30 ppm or more, preferably 40 ppm or more, and very desirably 50 ppm or more. It is also noted that the Nitrogen effect almost levels off when its quantity exceeds 60-70 ppm. This suggests that excessive nitrogen (in particular greater than 100 ppm) has no effect on improvement of resistance, but increases nitrides that have an effect adverse in tenacity. Therefore, the nitrogen content should be less than 100 ppm, preferably less than 80 ppm.

Figure 3 is a graph that compares the values DI of the Examples and Comparative Examples. It is evident from the Figure 3 that the forging steels of the Examples have values DI higher and therefore better hardenability than those of Comparative Examples

Figure 4 is a graph that shows how the tensile strength is affected by the content of V and the amount of nitrogen dissolved in solid solution. It is noted that samples containing V and nitrogen as defined in the Expression (1) above have high resistance. It is evident from the Figure 4 that if the total amount of V, Nb and Ta is greater than 0.068%, forging steel has sufficient strength although the amount of nitrogen dissolved in solid solution substantially null.

Industrial applicability

As mentioned above, this invention refers to a new steel of Ni-Cr-Mo for forging that is cheap and, nevertheless, it exhibits high performance. The forging steel is characterized by its low content of controlled nickel (for cost reduction) and its high resistance due to an amount of trace of V, Nb and Ta added. Forging steel also has excellent hardenability that makes it suitable for use as a piece for large forges, especially large crankshafts for boats

Claims (9)

1. A high strength steel forging that It includes:

         \ dotable {\ tabskip6pt # \ hfil \ + # \ hfil \ tabskip0ptplus1fil \ dddarstrut \ cr} {
 C: \ + 0.3 - 0.5% by mass, \ cr Si: \ +
0.1 - 0.4% by mass, \ cr Mn: \ +
0.7 - 1.5% by mass, \ cr Cr: \ +
1.2 - 3.5% by mass, \ cr Mo: \ +
0.1 - 0.6% by mass, \ cr Ni: \ + no more than 0.7% in
mass, cr
      

at least one item selected from the group that It consists of V, Nb and Ta: 0.03-0.35% by mass in total, Y N: 30-250 ppm, the rest being unavoidable Faith and impurities, said steel forming to forge a composite microstructure mainly of bainite and martensite and containing N in solution solid and V, Nb and Ta in such a way that the expression (1) is fulfilled next: [Total (% by mass) of V, Nb and Ta] + 0.001 x [N (ppm) in solid solution] ≥ 0.068 ... (1). 2. High strength steel forging defined in claim 1, wherein the amount of each component is specified as follows:

         \ dotable {\ tabskip6pt # \ hfil \ + # \ hfil \ tabskip0ptplus1fil \ dddarstrut \ cr} {
 C: \ + 0.36 - 0.45% by mass, \ cr Si: \ +
0.15 - 0.4% by mass, \ cr Mn: \ +
0.8 - 1.2% by mass, \ cr Cr: \ +
1.5 - 3.0% by mass, \ cr Mo: \ +
0.15 - 0.35% by mass, and \ cr V: \ +
0.35 - 0.17% in
mass. \ cr}
      

3. High strength steel forging defined in claim 1, further including: At: 0.001-0.10% in mass. 4. High strength steel forging defined in claim 1, which does not contain more than 0.006% in sulfur mass 5. High strength steel forging defined in claim 1, which has a DI value not lower at 30 mm, where the DI value is a critical diameter of a product of steel whose central part acquires a hardness of 50% martensite to rapid cooling with water. 6. High strength steel forging defined in any one of claims 1 to 5, which is used for the production of large crankshafts. 7. High strength steel forging defined in claim 6, which is used for the production of large crankshafts for boats. 8. A crankshaft produced by forging from high strength steel for forging defined in any of the claims 1 to 5. 9. The crankshaft defined in claim 8, which is a large crankshaft for boats.