JPH04191348A - High toughness non-thermal steel - Google Patents

Abstract

PURPOSE:To produce a non-heattreated steel excellent in toughness by regulating respective contents of C, Si, and V and the total content of Cr and Mn in a medium-carbon low-alloy steel to values in specific ranges, respectively. CONSTITUTION:This non-heattreated steel is a steel having a composition which contains, by weight, 0.30-0.55% C, <0.80% Si, 0.80-2.0% Mn, 0.20-0.8O% Cr, 0.05-0.35% V, 0.010-0.050% Al, 0.010-0.050% Nb, and 0.015-0.03% N or further contains either or both of <2.00% Ni and <0.5% Mo or one or >=2 kinds among <0.10% S, <0.35% Pb, <0.010% Ca, <0.30% Be, <0.30% Te, and <0.020% B independently or in combination with the above Ni, Mo, etc., and in which the total content of Mn and Cr is regulated to 1.2-1.O%. By this method, the non-heattreated steel excellent in machinability as well as in toughness can be obtained.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to non-tempered steel with excellent toughness.

(Background of the Invention) In recent years, the application of non-thermal steel, which can omit quenching and tempering treatment (thermal treatment), has been expanded as parts for automobiles, construction machinery, etc.

Normally, steel ingots made in the steelmaking process are bloomed and rolled, then cut into predetermined dimensions and subjected to processing such as forging, and then quenched and tempered (tempered). However, in the case of non-tempered steel that is used without being quenched and tempered, its crystal grains are coarse and its toughness is lower than that of quenched and tempered materials. However, it has the disadvantage of having a low toughness, which makes it currently difficult to apply to parts that require toughness, and the range of use is limited.

(Means for Solving the Problems) The present invention was made to provide a non-tempered steel with superior toughness compared to conventional materials, and its gist is that on a weight basis, Co
o, 30-0.55%, 5ilo.

80% or less, Mn: 0. 80-2. 00%,
CrO, 20-0.80% and Mn+c:r=1.2
~1.9%, V: o, 05~0.35%, 'A l
:0.010~0.050%, Nb:0,010~0
．． 050%, N: 0.015 to 0.03%, and the balance was adjusted to consist essentially of Fe.

The present inventor focused on improving the toughness of the matrix and refining the initial austenite grains during hot working in order to improve the properties of non-tempered steel. As a result of repeated research, we have found that in order to improve the toughness of the matrix, it is best to adjust the amounts of C, Si, and V within the above range, and especially add and contain Mn and Cr within the above range. Also, in refining initial austenite grains, 'A1
.

The present invention was completed based on the discovery that it is optimal to contain Nb and N in the above ranges. Here, SA1 means soluble A1.

The latter refinement of the initial austenite grains due to the inclusion of AI, Nb, and N results in 'Al, Nb' in the steel.

(2) By including N, Al and Nb combine with N to form nitrides, which is achieved by preventing movement of crystal grain boundaries. Here, if the N content is as low as in conventional non-thermal treated steel (in the case of conventional non-thermal treated steel, the N content is generally suppressed to 0.01% or less), a sufficient amount of It is not possible to remove nitrides. Therefore, in the present invention, the N content is increased to 0.015% or more.

The non-tempered steel of the present invention has improved toughness compared to conventional non-tempered steel, and therefore can be used for parts to which conventional non-tempered steel cannot be applied.

In the present invention, N1゜Mo
Ni: 2.00% or less, M
o:015% or less, and in this case, the strength and toughness of the non-tempered steel can be further improved.

The present invention uses one or more of S, Pb, Ca, Be, Te, and B in the following amounts, that is, S:O, 10% or less, Pb:
0.35% or less, Ca: 0.010% or less, Be: 0.
30% or less, Te: 0.30% or less, Boo, 020%
It can be contained in the following range, and in this case, both good machinability and toughness can be imparted.

Next, the reason for limiting the content of each component element in the non-tempered steel of the present invention will be explained in detail.

C: 0.30-0.55% C is a main element that determines the hardness after processing such as forging, and 0.30% to 0.55%.
It is necessary to contain 30% or more.

However, if it is added in excess, the hardness will become too high and the toughness will decrease. Therefore, it is necessary to set the upper limit to 0.55%.

Si: 0.80% or less Si is added to deoxidize and ensure strength during steel manufacturing, but if the amount exceeds 0.80%, toughness decreases, so the upper limit is set to 0.80%. .

Mn: 0380-2.00% Mn is extremely effective in improving the toughness of the ferrite-pearlite structure. An increase in Mn lowers the transformation point and reduces the lamellar spacing of pearlite. This increases the strength of the pearlite portion and makes it difficult for cracks to occur. However, if it is less than 0.80%, no significant improvement in toughness will be observed, and if it exceeds 2.00%, upper bainite will occur and the toughness will decrease.

Cr: 0.20 to 0.80% Cr is effective in improving the toughness of the ferrite/pearlite structure. The addition of Cr also reduces the lamellar spacing of pearlite and increases the crack initiation energy. However, 0.
If it is less than 20%, no significant improvement in toughness is observed, while if it exceeds 0.80%, upper bainite tends to occur and the toughness decreases. Therefore, the addition amount range is 0.2
0 to 0.80%.

Mn+Cr: L, 20-1.90% Mn and Cr need to be further adjusted to a total of 1.20-190%.

This is because if it is less than 1.20%, pearlite spacing becomes large, and if it exceeds 1.90%, upper bainite appears, both of which impair the toughness of the steel.

V: 0.05-0, 35% (1) dissolves in solid solution during hot working and heating, precipitates as carbonitrides during cooling, and increases strength. However, if it is less than 0.05%, sufficient strength will not be obtained, and if it is more than 0.35%, the toughness will actually decrease. Therefore, the addition amount range is 0.05
~0.35%.

'Al: 0.010-0.050%' Al is not only effective as a deoxidizing agent, but also combines with N in steel to
IN is precipitated to refine initial austenite grains during hot working heating. However, if the amount is less than 0.010%, the effect will be small, and if it exceeds 0.050%, the effect will be saturated.

Nb: 0,010 to 0.050% Nb precipitates as carbonitride, prevents coarsening of initial austenite grains during hot working, and keeps them fine.

If the amount is less than 0.01%, the effect is not sufficient;
．． If the amount is more than 0.05%, the effect will be saturated.

N: 0.015% to 0.603% N forms nitrides with Nb and A1 and is important for refining initial austenite grains. If the amount of N is small, the added A1 cannot be fully utilized.

In addition, in the case of Nb carbonitride, Nb is enriched with N than NbC.
b (CN) is coarser and difficult to form a solid solution even when heated at high temperatures. Therefore, a larger amount of N is more effective. In order to obtain such an effect, at least 0.015% N
It is necessary to contain.

Ni: 2.00% or less Ni is effective in improving strength and toughness. However, since adding a large amount of Ni causes a decrease in machinability and an increase in cost, the upper limit is set at 2.00%.

Mo: 0.5% or less Mo is effective in improving strength and toughness. However, since adding a large amount of MO causes an increase in cost, the upper limit needs to be 0.5%.

S: O, 10% or less Pb: 0,35% or less Ca: 0.010% or less Be: 0.30% or less Te: 0.30% or less B: 0.020% or less These elements affect machinability. Effective for improvement. However, if any of the elements is added in a large amount above a certain level, the toughness will decrease. Therefore, in the present invention, the upper limit values of these components are defined as the above values.

(Example) Next, in order to further clarify the characteristics of the present invention, examples thereof will be described in detail below.

Example (1) As shown in Table 1, ingots with various compositions with different amounts of Mn and Cr were melted in a vacuum melting furnace, and this was melted into a 50 mm piece.
Hot forged corners. Furthermore, after heating the forged product to 1100℃,
A round bar with a diameter of 22 mm was hot-worked and allowed to cool to obtain a test material. When a Charpy impact test was conducted on the sample material, the first
The results shown in the table were obtained.

(The following is a blank space) FIG. 1 shows the steel of the present invention and the comparative steel, with hardness plotted on the horizontal axis and Charpy impact value plotted on the vertical axis. As is clear from this figure and the results in Table 1, the steel of the present invention has improved impact resistance compared to the comparative steel.

Example (2) An ingot (50 kg) having the composition shown in Table 2 was melted in a vacuum melting furnace and hot forged into a 50 mm square piece. This material was heated to 1100° C. for 30 minutes and then cooled with water, and the particle size of the initial austenite grains before subsequent hot forging was measured. In addition, a Charpy impact test (JIS No. 3) was conducted on the material heated to each temperature, hot forged into a 22 mmφ round bar, and then allowed to cool, and the austenite crystal grain size was measured by a slow cooling method. The results are also shown in Table 2.

(Left below) From these results, it can be seen that in the case of the steel of the present invention, the grain size of the initial austenite grains before forging is small, and therefore the grain size after forging is also smaller than that of the comparative steel, and the impact resistance is also good. I understand that.

Although the embodiments of the present invention have been described in detail above, this is merely a specific example of the present invention, and the present invention can be carried out with various modifications based on the knowledge of those skilled in the art without departing from the spirit thereof. It is possible.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between hardness and Charpy impact value of the steel of the present invention in comparison with comparative steel. Patent applicant: Daido Steel Co., Ltd.

Claims (3)

[Claims] (1) Based on weight, C: 0.30-0.55%, Si:
0.80% or less, Mn: 0.80-2.00%, Cr:
0.20-0.80% and Mn+Cr=1.2-1.
9%, V: 0.05-0.35%, ^9Al: 0.01
0 to 0.050%, Nb: 0.010 to 0.050%,
A high-toughness non-thermal steel characterized by comprising N: 0.015 to 0.03%, and the remainder substantially consisting of Fe. (2) In the high toughness non-thermal steel of claim (1), further N
Ni: 2.00 based on weight of one or two of i and Mo
% or less, Mo: 0.5% or less. (3) In the high-toughness non-tempered steel according to claim (1) or (2), one or more of S, Pb, Ca, Be, Te, and B are further added to each S: 0.10 on a weight basis. % or less, P
b: 0.35% or less Ca: 0.010% or less, Be: 0
．． 30% or less, Te: 0.30% or less, B: 0.020
High toughness non-tempered steel characterized by containing less than %.