JPH02153042A - High strength and high toughness non-heat treated steel for hot forging - Google Patents

Abstract

PURPOSE:To obtain the title steel having the strength and toughness equal to or above those of a hardening-tempering treated material only by executing air cooling or controlled cooling after hot forging by adding specific alloy elements to the steel. CONSTITUTION:The title steel is the low alloy one to which, by weight, 0.15 to 0.35% C, 0.05 to 1.0% Si, 0.6 to 2.5% Mn, 0.035 to 0.12% S, 0.03 to 0.30% V, 0.003 to 0.015% Ti, 0.015 to 0.060% Al and 0.003 to 0.020% N or furthermore one or more kinds among <2.0% Cr, <1.0% Ni and <0.5% Mo or one or more kinds among <0.30% Pb, <0.20% Zr, <0.01% Ca, and <0.10% Te or one or more kinds among <0.30% Pb, <0.20% Zr, <0.01% Ca and <0.10% Te are incorporated independently or compoundedly. By the precipitation strengthening by the carbon nitride of V, by the increase of hardenability by Mn, by the increase of ferrite caused by the reduction of the content of C and by the refining of austenitic crystal grains by the compounded addition of S and Ti, the non-heat treated steel having high strength and excellent impact value at normal temp. and low temp. can be obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to high strength and high toughness non-thermal steel for hot forging,
More specifically, as a material used as parts for automobiles, construction machinery, and various industrial machines, it can achieve strength and toughness equal to or greater than quenched and tempered materials simply by allowing it to cool or adjusting cooling after hot forging. This relates to a high-strength, high-toughness non-temperature steel for hot forging that can be obtained.

[Prior art] In the past, various structural parts were often made from mechanical structural carbon steel or alloy steel, which was hot forged and then subjected to heat treatment such as quenching and tempering to achieve the intended purpose. It has been manufactured with added strength characteristics depending on the purpose.

However, in order to perform such processing, a large amount of thermal energy is required, and the cost of manufacturing parts increases due to an increase in the number of processing steps and an increase in work in progress. There's a problem.

Therefore, in recent years, in the manufacture of forged steel products, in order to simplify the manufacturing process, and in particular to omit the quenching and tempering process, carbon steel for machine structures specified in JIS G 4051 and carbon steel specified in JIS G 4106 are used. So-called non-thermal steel for hot forging, which avoids containing trace amounts of precipitation-hardening alloying elements such as V, Nb, and Ti, has been attracting attention in manganese steel for machine structures, and has been put into practical use in some cases. has been made into

However, non-tempered forged steel products manufactured from the non-tempered steel for hot forging mentioned above have the same strength as those that have been quenched and tempered, but have the disadvantage of lower toughness.
For example, when applied to a part that requires a tensile strength of 80 kg/mm" or more, the Charpy impact value of a 2 mm U notch at room temperature is 4.5 kgf・m/ctn.
6K, which is required when quenching and tempering carbon steel for machine structural use, which has the same strength.
gf-m/cm'' or more.

Furthermore, when applied to parts requiring tensile strength exceeding 90 kgf/mm'', the Charpy impact value is extremely reduced to 3 kgr-ffi/cl+lj or less.

This is because the structure of conventional non-thermal forged steel products is coarse ferrite/pearlite transformed from coarse austenite grains.

For this reason, some sea bream contain Ti, etc. to make the austenite crystal grains finer during heating, but depending on the forging conditions, stable properties cannot be obtained, and furthermore, the tensile strength is 90%.
kgf/nun', sufficient toughness cannot be obtained, and steels with a bainite or bainite-ferrite structure have been proposed, and attempts have been made to improve toughness, but fatigue strength may decrease or alloying Material costs increase due to the inclusion of large amounts of elements.

[Problems to be Solved by the Invention] As explained above, the present invention has been made in view of the various problems of conventional non-tempered steel, and as a result of intensive research by the present inventor, In order to provide even higher strength and superior toughness, we adjusted the contained elements and achieved a tensile strength of 7.
We have developed a high-strength, high-toughness non-thermal steel for hot forging that does not require quenching and tempering after hot forging, compared to steels with a strength of 0 to 100 kgf/mm.

[Means for Solving the Problems] The high-strength, high-toughness non-temperature steel for hot forging according to the present invention has the following characteristics: (1) C0.15-0.35wt%, Si 0.0
5-1. Owt%, Mn 0.6-2.5mt%, S
0.035-0.12wt%, vo, oa-0.30
wt%, Ti 0.003-0.015wt%, AI
0.015-0.060wt%, N 0.003-
The first is a high-strength, high-toughness non-thermal steel for hot forging, which contains 0.020 wt% and the remainder consists of Fe and unavoidable impurities.
(2) G 0.15-0.35wt%, Si0.05
~1.0wt%, Mn 0.6~2.5wt%, S0
．． 035-0.12wt%, V 0.03-0.3(m
%, Ti 0.003-0.015wt%, Al 0.
015-0.060wt%, N 0.003-0.02
0wt%, and further contains Cr 2.0wt% or less, Ni 1.0wt% or less, M
o A second high-strength, high-toughness non-thermal steel for hot forging characterized by containing one or more selected from 0.50 wt% or less, with the remainder consisting of Fe and unavoidable impurities.
(3) G 0. t5~0.35wt%, Si0.05
~1.0wt%, Mn 0.6~2.5wt%, S0
．． 035-0.12wt%, V 0.03-0.30w
t%, T i 0.003-0. Q15wt%, At
0.015-0.060wt%, N 0.003-0
．． 020 wt%, and further contains one or more selected from Pb 0.30 wt% or less, Zr 0.20 wt% or less, Ca 0.01 wt% or less, Te 0.10 wt% or less, The third high-strength, high-toughness non-thermal steel for hot forging is characterized by consisting of Fe and unavoidable impurities.
(4) C0.15-0.35wt%, S i 0.0
5-1. Owt%, Mn 0.6-2.5wt%, S
0.035-0.12wt%, V 0.03-0.30
wt%, Ti 0.003-0.015wt%, A
I 0. Q15~0.060wt%, N 0.003~
0.020wt%, and further contains Cr 2.0wt% or less, Ni 1.0wt% or less, M
o Contains one or more selected from 0.50wt% or less, and Pb 0.30wt% or less, Zr 0.20wt% or less, Ca 0.01wt% or less, Te 0.10wt% or less 4. A high-strength, high-toughness, non-heat-treated steel for hot forging which is characterized by containing one or more selected from the following, with the balance consisting of Fe and unavoidable impurities.
This invention consists of four inventions.

The high-strength, high-toughness non-temperature steel for hot forging according to the present invention will be explained in detail below.

That is, the high-strength, high-toughness non-tempered steel for hot forging according to the present invention has strength mainly achieved by precipitation strengthening due to carbonitrides of V, increasing hardenability due to Mn, etc., and solid solution strengthening. In order to improve toughness, the content of C is reduced and the amount of ferrite is increased, and the combined inclusion of S and Ti refines the austenite grains during heating, and a large number of ferrites are generated during cooling. By doing so, the structure after cooling becomes a fine ferrite/pearlite, thereby improving toughness.

First, the components and content ratios of the high-strength, high-toughness non-temperature steel for hot forging according to the present invention will be explained.

C is an essential element to increase the amount of pearlite and increase the strength of forged steel products, and to form carbides with V to exhibit its precipitation strengthening effect, and the content is 0.15 wt%.
Below 045w, such strengthening effect is small, and
If the content exceeds t%, the impact value will be lower than that of the quenched and tempered material, resulting in insufficient toughness. Therefore, the C content is 0
．． The content is 15 to 0.35 wt%.

In addition to deoxidizing, Si is an effective element for strengthening the ferrite structure after forging and cooling, and the content is 0.05 wt%.
Below 1.0 wt%, this effect is small (and below 1.0 wt%
If the content exceeds the above, toughness and machinability will deteriorate. Therefore, the Si content is 0.05 to 1. Let it be OwL%.

Mn is a deoxidizing and desulfurizing element, and is an essential element for increasing the strength and toughness of forged steel products. If the content is less than 0.6 wt%, such effects are small.
Furthermore, if the content exceeds 2.5 wt%, the toughness decreases and has a detrimental effect on machinability. Therefore, the Mn content is set to 0.6 to 2.5 wt%.

S exists in the form of MnS in steel and is an element that improves the machinability of steel during cutting. It exists in the analyzed state and suppresses the growth of austenite crystal grains during heating, and furthermore, coarse MnS becomes a nucleation site for ferrite during cooling, especially for austenite grains with a coarse structure such as in non-thermal forged steel products. In the case of ferrite/pearlite transformed from 0 to 0, the number of coarse MnS increases as the S content increases, and a large number of pro-eutectoid ferrites precipitate during cooling, resulting in a fine structure. If the content is less than 0.035 wt%, the above-mentioned effect on impact properties will be small, and if the content exceeds 0.12 wt%, the effect will be saturated. Therefore, the S content is 0.0
35 to 0.12 wt%.

■ is an element that forms carbonitrides with C and N during cooling after forging to provide strength, and the content is 0.03wt
If it is less than 0.30w, this effect is small, and if it is less than 0.30w
If the content exceeds t%, the toughness will deteriorate. Therefore, ■
The content is 0.03 to 0.30 wt%.

Ti is an element that suppresses the growth of austenite grains during heating, and when combined with S, it refines the structure and significantly improves toughness, and when the content is less than 0.003 wt%, it inhibits the growth of austenite grains. The inhibitory effect is small, and if the content exceeds 0.015 wt%, the inhibitory effect will be saturated. Therefore, the Ti content is between 0.003 and 0.
．． 015wt%.

At is an effective element for deoxidizing effect and refining grain size.If the content is less than 0.015wt%, this effect will be small, and if the content is more than 0.060wt%, the effect will be reduced. The increase is small, and it also has a detrimental effect on machinability. Therefore, the Al content is 0.
O12 to 0.060 wt%.

N is a useful element for combining with ■ and Al to form carbonitrides, refining crystal grains, and strengthening steel. If the content is less than 0.003 wt%, this effect is small, and
Even if the content exceeds 0,020 wt%, no increase in the effect can be expected and the toughness will deteriorate. Therefore, the N content is set to 0.003 to 0.020 wt%.

Or, Ni, and Mo are all elements that increase the strength of forged steel products without impairing toughness, and the Cr content h'2.
0wt%, Ni content is t, owt%, MO content is 0
．． If the content exceeds 50 wt%, bainite will be generated and yield strength and fatigue strength will decrease. Therefore, the Cr content is 2.0 wt% or less, and the Ni content is 1.0 wt% or less.
The MO content is 0.50 wt% or less.

Both PbSZr and Ca%Te are elements that improve machinability, and are included when machining the product after forging.
．． 20wt%, Ca content 0.01wt%, Te0.1
When each content exceeds 0 wt%, toughness decreases,
No significant effect on machinability was observed. Therefore,
PB content is 040wt% or less, Zr content is 0.20
The Ca content is 0.01 wt% or less, and the Te content is 0.10 wt% or less.

[Example] Next, an example of the high-strength, high-toughness non-thermal steel for hot forging according to the present invention will be described.

Example Steel having the components and content ratios shown in Table 1 was melted in a high frequency furnace, cast and then rolled to a length of 50 mm.
+aml was cut.

Furthermore, heat it to a temperature of 1250℃ and use a hammer to
It was forged into a 5a+m round bar. The finishing temperature during final molding was 1150° C., and then it was allowed to cool in the atmosphere.

The test steel No. 7 in Table 1 was 855C, and was heated and quenched and tempered after hot forging.

JIS No. 4 tensile test pieces and JIS Sa impact test pieces were taken from the center of each of these round bars in the longitudinal direction, and their tensile strengths and Charpy impact values at room temperature and -50°C were investigated.

In Table 1, No. 1 = No. 5 is a high-strength, high-toughness non-heat-refined tjR for hot forging according to the present invention (invention steel), and No. 66 to No. 19 are comparative steels.

Table 1 shows that the steel of the present invention has a strength equal to or greater than that of NO19 quenched and tempered material, and the impact value at room temperature and low temperature is also the same.

It can be seen that comparative steel No. 6 has a high C content, comparative steel No. 7 has a low S content, and comparative steel No. 8 contains no Ti, and all have low impact values at room temperature and low temperature.

[Effects of the Invention] As explained above, since the high-strength, high-toughness non-heat-treated steel for hot forging according to the present invention has the above-mentioned structure, it has a strength equal to or higher than that of conventional quenched and tempered materials. Furthermore, it has the effect of being superior in impact value at room temperature or low temperature.

Claims (4)

[Claims] (1) C0.15-0.35wt%, Si0.05-1
．． 0wt%, Mn0.6-2.5wt%, S0.035
~0.12wt%, V0.03~0.30wt%, Ti
0.003-0.015wt%, Al0.015-0.
A high-strength, high-toughness non-thermal steel for hot forging, characterized in that it contains 0.060 wt%, N0.003 to 0.020 wt%, and the balance consists of Fe and inevitable impurities. (2) C0.15-0.35wt%, Si0.05-1
．． 0wt%, Mn0.6-2.5wt%, S0.035
~0.12wt%, V0.03~0.30wt%, Ti
0.003-0.015wt%, Al0.015-0.
060wt%, N0.003~0.020wt%, and further contains Cr2.0wt% or less, Ni1.0wt% or less, Mo0
．． A high-strength, high-toughness non-thermal steel for hot forging, characterized in that it contains one or more selected from 50 wt% or less, with the remainder consisting of Fe and inevitable impurities. (3) C0.15-0.35wt%, Si0.05-1
．． 0wt%, Mn0.6-2.5wt%, S0.035
~0.12wt%, V0.03~0.30wt%, Ti
0.003-0.015wt%, Al0.015-0.
060wt%, N0.003-0.020wt%, and further contains Pb0.30wt% or less, Zr0.20wt% or less, C
A high-strength, high-toughness non-touching material for hot forging characterized by containing one or more selected from the following: a of 0.01 wt% or less and Te of 0.10 wt% or less, with the remainder consisting of Fe and unavoidable impurities. Quality steel. (4) C0.15-0.35wt%, Si0.05-1
．． 0wt%, Mn0.6-2.5wt%, S0.035
~0.12wt%, V0.03~0.30wt%, Ti
0.003-0.015wt%, Al0.015-0.
060wt%, N0.003~0.020wt%, and further contains Cr2.0wt% or less, Ni1.0wt% or less, Mo0
．． Contains one or more selected from 50 wt% or less, and also contains Pb 0.30 wt% or less, Zr 0.20 wt% or less, C
A high-strength, high-toughness non-touching material for hot forging characterized by containing one or more selected from the following: a of 0.01 wt% or less and Te of 0.10 wt% or less, with the remainder consisting of Fe and unavoidable impurities. Quality steel.