JPS61147849A - Unnormalized tough hardening steel - Google Patents

Abstract

PURPOSE:To improve productivity and to reduce energy consumption by giving high strength and high toughness to the steel to be subjected to controlled hot forging, without subjecting the steel to usual refining treatment such as hardening and tempering. CONSTITUTION:The steel to be subjected to controlled hot forging consists of, by weight, 0.35-0.6% C, 0.3-2% Mn, 0.01-0.2% Nb, each as principal component, and the balance Fe with inevitable impurities. The steel is forged in the temp. range of Ac3 point - Ac3 point + 100 deg.C, the austenite unrecrystallization zone, or of Ac3 point + 100 deg.C - Ar3 point - 50 deg.C, i.e. from the austenite unrecrystallization zone to the two-phase area of austenite and ferrite, which is hardened to form the structure composed of martensite of fine lath or of martensite of fine lath and fine ferrite.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention relates to strong steel used as a material for various mechanical structural parts, and particularly relates to strong steel that is used as a material for various mechanical structural parts, and particularly relates to strong steel that is used as a material for various mechanical structural parts. The present invention relates to semi-hot forged non-temperature toughened steel having high strength and high toughness.

(Prior Art) Conventionally, there have been methods such as cutting, casting, and forging to manufacture mechanical structural parts, and cold forging and hot forging are also used in forging. On the other hand, there is also semi-hot forging as an intermediate between these. This semi-hot forging has been developed in recent years as a forging method that combines the advantages of hot forging with low deformation stress and the advantages of cold forging with a high processing yield of 9. It is increasingly being adopted for forming raw materials.

By the way, steel subjected to semi-hot forging has an Acl transformation point of -
It is forged at 50° C. to the lower limit of the hot forging temperature range, and after cooling is usually subjected to quenching and tempering treatments (hereinafter referred to as "refining") in order to impart desired strength and toughness.

Furthermore, for parts that require surface hardening treatment, induction hardening is increasingly performed after thermal refining and machining.

(Problems to be Solved by the Invention) However, for steels subjected to such conventional semi-hot forging, temperatures of 810°C to 890°C after forging are required for tempering.
heated to about ℃, then water quenched, and further heated to 550℃~
This required reheating twice, first to about 650 degrees Celsius, and then to rapid cooling, which hindered productivity and required a huge amount of heat energy, especially for parts produced in large quantities, such as automobile parts. The problem was that it was necessary.

This invention was made by focusing on these conventional problems, and it has high strength and high toughness without the conventional heat treatment such as quenching and tempering, and the production efficiency associated with the heat treatment is improved. The object of the present invention is to provide a semi-hot forged, non-heat treated strong steel that can avoid a decrease in properties and an increase in energy consumption.

[Structure of the Invention] (Means for Solving the Problems) The semi-hot forged non-thermal toughened steel according to the present invention solves the above problems, and has a C: 0.35 at 1% by weight. ~
0.6%, Mn: 0.3-2%.

Nb: 0.01 to 0.2% as a basic component, the balance consisting of Fe and impurities, in the austenite non-recrystallized region of Ac 3 points to Ac 3 points + 100°C, or Ac 3 points + 100
℃#A rs point -50℃, forged in the temperature range from the austenite non-recrystallized region to the austenite-ferrite dual phase region, and quenched to become fine lath martensite or fine lath martensite + fine ferrite structure. It is characterized by its existence.

The semi-hot forged non-thermal toughened steel according to the present invention has a weight %-1
', C: 0.35-0.6%, Mn: 0.3-2%, N
b: 0.01 to 0.2% as a basic component, and if necessary, as a hardenability improving element, Ni, 296 or less, Cr: 0
．． 5% or less, Mo: 1 or 2 of 0.5% or less
As seeds and grain refining elements, AA: 0.1% or less, ■=0.3% or less, Ti: 0.3% or less, N: 0.
03% or less contains Pb as an element imparting free machinability, and the remainder is substantially composed of Fe, and for steels with such compositions, Ac3 points to A
Forging is carried out in the temperature range from the austenite unrecrystallized region of c3 point + ioo °C, or the austenite unrecrystallized region of Acg point +100 °C to Ar3 point -50 °C, to the austenite-ferrite dual phase region, more preferably in water or hot water. By quenching it, it is characterized by having the structure shown below.

■Martensitic structure with fine lath, or.

^mm2i^Misaki 1. Biso 發 ih + Sakaki embroidery 7. Light m-woven Φ Next, the reason for limiting the component range (weight %) of the semi-hot forged non-thermal toughened steel according to the present invention will be explained.

C: 0.35 #0.6% C is effective as a mechanical structural steel to ensure strength-1, especially quenching hardness, and the lower limit is 0.35% to enable surface hardening by induction hardening. shall be. However, if the amount added exceeds 0.6%, the toughness decreases, so the upper limit is 0.6%.
It was set at 6%.

Mn: 0.3% to 2% Mn is effective for deoxidizing and desulfurizing, and at the same time is effective for ensuring hardening hardness. In order to obtain sufficient desulfurization and desulfurization effects and to improve hardenability, 0.3%
It is necessary to add more than 2%, but if it exceeds 2%, workability and machinability deteriorate, so the upper limit was set at 2%.

Nb: 0', O2N2.2% Nb increases the austenite recrystallization temperature and increases the Acs point +5
It is necessary to add at least 0.01% to maintain the temperature above 0°C, but if it exceeds 0.2%, the Nb carbide becomes coarse and the toughness decreases, so 0.2% is set as the upper limit.

Ni: 2% or less, Cr: 0.5% or less, Mo: 0.5%
One or more of the following Ni, Cr, and Mo are effective in further improving hardenability and strengthening the matrix, so it is recommended to add them as necessary.
Note that the toughness decreases when the Ni content exceeds 2%, the Cr content exceeds 0.5%, and the MO content exceeds 0.5%, Ni: 2%, Cr: 0.5%, M.

It is preferable to set the upper limit to 20.5%.

AfL: 0.1% or less, V: 0.3% or less, Ti: 0
．． 3% or less, N: 1 or 2 of 0.03% or less
Since V, Ti, and N are effective elements for refining crystal grains and improving toughness through the formation of carbonitrides,
If necessary, one or more of these may be added, but if the AIL content exceeds 0.1%, the ■ content exceeds 0.3%, and the Ti content exceeds 0.1%. If it exceeds 3%, the carbonitrides will become coarse, the grain refining effect will be reduced, and the toughness will be lowered. Therefore, when adding carbonitrides, it is best to keep them within the above ranges. 0.
If it exceeds 0.03%, the integrity of the steel ingot or slab will be damaged due to N blowholes, so the N content should be 0.03%.
% or less.

b Since Pb is dispersed in the matrix as inclusion particles and is an effective element for improving machinability, it can be added as necessary.

If it is added in an amount exceeding 0.5%, the dispersed particles will become coarse and the machinability will deteriorate, so when adding it, it is preferable that the upper limit is 0.5%.

Next, the reason for limiting the forging temperature range will be explained.

Non-austenite recrystallization region from Ac3 point to AcB point +100°C: In the non-thermal tough steel according to the present invention, the reaustenite recrystallization temperature becomes Ac3 point +50°C or higher due to the addition of Nb.
At temperatures exceeding Ac3 point +100°C, dislocations introduced by forging disappear due to recrystallization, making it impossible to achieve lath refinement and high dislocation density of martensite produced by quenching following forging, and pro-eutectoid ferrite. It is also difficult to make the steel finer and tougher, so the upper limit temperature for forging was set to Ac3 point +100°C.

Austenite non-recrystallized region from Ac3 point +100℃ to Ar3 point -50℃: In this case, heating is performed to the austenite non-recrystallized region from Ac3 point to Ac3 point +100℃, and the subsequent forging is performed from Ar3 point to
It may be in the austenite-ferrite dual phase temperature range of Ar3 point -50℃, but if the forging end temperature is Ar
When the temperature is lower than -50°C at point 3, the amount of ferrite increases at the end of forging and the strength decreases, so the lower limit temperature for forging was set at -50°C at point Ar3.

The steel whose composition has been adjusted as described above is forged in the above temperature range,
By quenching, it becomes a fine lath martensite or a fine lath martensite + fine ferrite structure, giving it high strength and toughness, making it suitable for axle parts such as outer races, housing shafts, rear spindles, and steering parts such as knuckle arms. , it becomes possible to omit one layer after forging of driving parts such as a drive shaft.

(Example 1) Steels with 15 types of chemical compositions shown in Table 1 (steel type No.

1 to 15), then ingots and forged into diameter 3
A 5-layer round bar was made, heated to 850°C, held for 60 minutes, and then air-cooled for normalization, then heated and soaked under the same conditions shown in Table 1 and Figure 1. (holding time: 30 minutes), then immediately forged in a 2000 mm press from diameter 35 mm to thickness 20 mm, quenched in 80℃ hot water, and then JIS
A minimum size tensile test piece and JIS No. 3 (
U-notch) impact test pieces were manufactured. The above heating and soaking conditions, forging completion temperature, tensile strength and Charpy impact value are also shown in Table 1. In this case, the holding temperature was set to AC3+20 to 30°C for each steel type.

(Comparative Example 1) Eight types of steel (
Test pieces were prepared for steel types No. 16 to 23) in the same manner as in the above examples, and the holding temperature, forging completion temperature, tensile strength, and Charpy impact value were measured and are also shown in Table 1.

(Comparative example 2) For comparison, 540C and 545C.

The tensile strength and Charpy impact value of 550C and 555C tempered materials were investigated. The results are also shown in Table 1, and the tempering conditions are as shown in Table 2.
Table The steel types in the composition range according to the present invention have a fine lath martensitic structure under the forging conditions shown in Table 1, and are 540C and 545C.

The tensile strength is higher than that of 550C and 555C tempered materials.
For steel types with C content of 0.5% by weight or less, the impact value is 10kgf.
・It has a m/c value of 2 or more, and has excellent strength and toughness. In addition, sample material No. 14.
No. 15 has a low impact value, but this is because the C content exceeds 0.5%. Even in tempered materials, the impact value decreases as the C content increases, and the same is true for sample material No. 14.15. Although the impact value was only slightly lower than that of tempered materials containing a certain amount of C, it was sufficient to withstand use. It can be particularly preferably used when induction hardening is performed to partially increase the hardness.

On the other hand, it is clear that materials outside the composition range of the present invention have significantly lower impact values than tempered materials with the same amount of C.

Next, machinability tests were conducted on the sample materials shown in Table 1, 2,3.11-13°17, under the conditions shown in Table 3.
The results are shown in Table 14.

As shown in Tables 3 and 4, it is clear that sample material No. 12 containing an appropriate amount of PB has considerably excellent machinability.

(Example 2) The three steel types according to the present invention shown in Example 1 (test materials ~0.
2.11 was subjected to two-step forging and quenching as shown in FIG. 2, and the same tests as in Example 1 were conducted to measure the tensile strength and Charpy impact value.

The results are shown in Table 5. The holding temperature was slightly higher than the Ac3 point.

In the forging and quenching shown in Table 5 in this Example 2, the forging end temperature is in the austenite + ferrite dual phase region, so the three steel types of this example are fine lath martensite + ferrite.
It becomes a fine ferrite structure, and in this case also the comparative 540
C, 545C.

It showed better tensile strength than 550C and 555G tempered materials.

[Effects of the Invention] As explained above, the semi-hot forged non-thermal toughened steel according to the present invention contains 1% by weight, C: 0.35-0.6%, M
The basic components are n: 0.3 to 2%, Nb: 0.01 to 0.2%, and if necessary, N is added as a hardenability improving element.
i: 2% or less.

One or more of Cr: 0.5% or less, Mo: 0.5% or less, and Pb: 0.5% as an element that imparts free machining properties.
The following, and grain refining element and LcAJl: 0.
1% or less, V: 0.3% or less.

Contains one or more of Ti: 0.3% or less, N: 0.03% or less, and the remainder is substantially Fe, and for steel with such a composition, Ac 3 points~
By forging and quenching in the temperature range from the austenite unrecrystallized region of Ac3 point +100℃ to the austenite unrecrystallized region of Ac3 point +100℃ to Ar3 point -50℃ to the austenite-ferrite two-phase region, fine Since it has a lath martensite structure, or a fine lath martensite + fine ferrite structure, it is possible to significantly improve strength and toughness without undergoing heat treatment such as quenching or tempering. Machine structural parts such as steering parts such as knuckle arms, drive parts such as axle shafts and drive shafts, outer races, rear spindles, etc. can be produced without heat treatment after forging, which requires a huge amount of heat energy and manufacturing process. A very excellent effect is obtained in that it is possible to manufacture axle parts of

[Brief explanation of the drawing]

FIG. 1 and FIG. 2 are explanatory diagrams of heating and forging procedures adopted in Example 1 and Example 2 of the present invention, respectively.宸しん（加℃）

Claims (3)

[Claims] (1) In weight%, C: 0.35-0.6%, Mn: 0.
3 to 2%, Nb: 0.01 to 0.2% as basic components,
The balance consists of Fe and impurities, Ac_3 points ~ Ac_3
Austenite unrecrystallized region at point +100℃, or A
It is forged in the temperature range from c_3 point +100℃ to Ar_3 point -50℃ from the austenite non-recrystallized region to the austenite-ferrite two-phase region and quenched to form fine lath martensite or fine lath martensite + fine ferrite structure. Semi-hot forged non-thermal tempered strong steel. (2) The balance Fe is 2% or less by weight, Ni: 2% or less, Cr:
0.5% or less, and Mo: 0.5% or less. (3) Remaining Fe is % by weight, Al: 0.1% or less, V
: 0.3% or less, Ti: 0.3% or less, N: 0.03%
The semi-hot forged non-temperature toughened steel according to claim 1 or 2, which contains one or more of the following: