KR101253813B1 - Steel without spheroidizing heat treatment and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel material and a method for manufacturing the same, which can omit the spheroidizing heat treatment process in order to prevent a decrease in the life of the die during cold forming for the final product and to facilitate the manufacture of the material shape.

To this end, the present invention is by weight, C: 0.6-1.4%, Si: 1.0-3.5%, Mn: 0.3-1.5%, Cr: 0.8-1.5%, Ni: 0.01-1.0%, B: 0.001-0.02 %, O: 0.0015% or less, Al: 0.001-0.1%, P: 0.02% or less, S: 0.02% or less, N: 0.001-0.02%, V: 0.005-0.5%, and Ti: 0.005-0.5% It includes one or two of the remaining, the remainder comprises Fe and other unavoidable impurities, and the microstructure provides a spheroidized heat treatment omitted steel material containing ferrite as a base structure and a spheroidized cementite and a method for producing the same.

3-phase region, nodular heat treatment, ferrite, cementite

Description

Spheroidal heat treatment omitted steel and manufacturing method {STEEL WITHOUT SPHEROIDIZING HEAT TREATMENT AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a spheroidized heat treatment omitted steels, and more particularly to a steel and a method for manufacturing the spheroidized heat treatment process can be omitted so as to prevent the die life of the die during cold forming for the final product manufacturing and to easily manufacture the material shape. It is about.

And in recent years the demand for such process is omitted and simplified process for the steel product increased to environmental pollution are increasingly serious become thereto according problems are suppressed efficiently it as soybeans, reducing CO ₂ in each of steel in order to meet these needs We are racing hard for our efforts.

Meanwhile, in the process of manufacturing wire rod products, wire rods manufactured in the form of primary rings are cold-formed during the production of cold rolling steel wire (CHQ) and bearings, and if the strength of the material is high, die life is reduced and dies are broken. Such problems lead to lower productivity and higher manufacturing costs.

In order to alleviate this problem, heat treatment is performed. This heat treatment is referred to as spheroidizing softening heat treatment. The microstructure of the steel produced primarily has a full pearlite or pearlite and cementite cementite structure according to the amount of carbon in the material, and the cementite in the plate-shaped pearlite structure is segmented and spherical during the spheroidizing softening heat treatment so that the decrease in material strength is achieved. Will occur. For this reason, when the spheroidizing heat treatment is performed, there is an advantage that the cold forming is smooth due to the decrease in the strength of the material.

However, this spheroidizing heat treatment has a problem that it is disadvantageous in that the heat treatment cost is high and the production time is extended because it must be treated for a long time, generally 20 hours or more.

One aspect of the present invention is to provide a steel material and a method for manufacturing the same, which have equivalent physical properties to existing materials, and which can significantly reduce cost and production time by omitting the spheroidizing heat treatment process.

In the present invention, by weight%, C: 0.6-1.4%, Si: 1.0-3.5%, Mn: 0.3-1.5%, Cr: 0.8-1.5%, Ni: 0.01-1.0%, B: 0.001-0.02%, O : 0.0015% or less, Al: 0.001-0.1%, P: 0.02% or less, S: 0.02% or less, N: 0.001-0.02%, V: 0.005-0.5% and Ti: 0.005-0.5% Or two, the remainder comprising Fe and other unavoidable impurities, and the microstructure provides a spheroidized heat treatment omitted steel comprising ferrite as the base structure and including spheroidized cementite.

In addition, the present invention comprises the steps of hot rolling a steel piece satisfying the composition;

Winding through a laying head after the hot rolling; And

It provides a method for producing a spheroidized heat treatment omitted steel material comprising the step of cooling after the winding.

Since the steel of the present invention does not need a separate spheroidizing softening heat treatment during cold forming, it can prevent the die from decreasing the life of the die and easily manufacture the shape of the material, thereby reducing the cost and reducing the production time.

Hereinafter, the present invention will be described in detail.

Hereinafter, the composition range of the present invention will be described in detail (hereinafter,% by weight).

Carbon (C) is an essential element added to secure the strength of the material. If the content of C is less than 0.6%, it is impossible to secure the minimum strength required for steel. In addition, when the C content is more than 1.4%, the suppression of the cornerstone cementite is impossible, so the ductility of the material is significantly reduced, the content is preferably 0.6 to 1.4%.

The content of silicon (Si) is preferably 1.0 to 3.5%. Si increases the ferrite (α), cementite (θ), and austenite (γ) three-phase regions directly under Ac1 as its amount is increased. That is, as shown in Figure 2, when comparing the case of the Si content of 1.0% and the case of 2.0%, it can be seen that the three-phase region is widely formed at 2.0%.

The present invention serves to provide a spheroidized carbide seed for omission of spheroidization by segmenting plate-like cementite during finishing rolling in the three-phase region. If the content of Si is less than 1.0%, the three-phase zone is formed too narrow, and the finishing rolling temperature is lowered to 750 ° C. or less, resulting in high rolling load. In addition, when the amount of Si added as a solid solution strengthening element exceeds 3.5%, the rolling load is increased due to the strengthening of the material, so the content of Si is preferably 1.0 to 3.5%.

The content of manganese (Mn) is preferably 0.3 to 1.5%. Mn is an element that is beneficial to secure strength by improving the hardenability of steel when present in steel. Therefore, if the Mn content is less than 0.3%, it is difficult to obtain the required strength and quenchability, whereas if the Mn content exceeds 1.5%, toughness is lowered, so the Mn content is preferably 0.3 to 1.5%.

The content of chromium (Cr) is preferably 0.8 to 1.5%. Cr is an element useful for preventing surface decarburization, oxidation resistance, tempering softening and hardening. However, when the Cr content is less than 0.8%, it is difficult to secure sufficient oxidation resistance, temper softening, surface decarburization and quenching effects. In addition, when the content exceeds 1.5%, the deformation resistance may be lowered, which may lead to a decrease in strength.

The content of nickel (Ni) is preferably 0.01 to 1.0%. Ni can improve the toughness of steel materials efficiently. Therefore, when the Ni content is less than 0.01%, the effect is not sufficient, and when the Ni content is more than 1.0%, the residual austenite content is increased to reduce the fatigue life, and due to the expensive Ni property, a sharp rise in manufacturing cost is caused. Therefore, the addition amount needs to be limited to 0.01 to 1.0%.

The content of boron (B) is preferably 0.001 to 0.02%. The B may combine with nitrogen and carbon to form boron-based carbon / nitride at a high temperature, thereby promoting spheroidization by acting as a seed of pearlite generation during filamentous rolling and spheroidizing generated particles during cooling. At this time, when the content is less than 0.001%, it is difficult to produce sufficient carbon / nitride, and when it exceeds 0.02%, carbon / nitride-based precipitates are coarsened, and the role of spheroidization promoting particles disappears, but rather adversely affects fatigue characteristics.

The content of oxygen (O) is preferably 0.0015% or less. The content of O is limited to 0.0015% by weight or less, because when the content exceeds 0.0015% by weight coarse oxide-based non-metallic inclusions are formed to rapidly reduce the fatigue life.

The content of aluminum (Al) is preferably 0.001 to 0.1%. The addition of Al refines the grain size and improves material toughness. When the Al content is less than 0.001%, the grain refining effect disappears, and when the Al content exceeds 0.1%, the amount of oxide-based precipitates is increased and its size is coarsened to adversely affect the fatigue characteristics.

The content of phosphorus (P) and sulfur (S) is preferably 0.02% or less, respectively.

The content of P and S is limited to 0.02% or less. Since P segregates at grain boundaries to reduce toughness, the upper limit thereof is limited to 0.02%, and S segregates with low melting elements to reduce toughness and form an emulsion. It is preferable to limit the upper limit to 0.02% by weight since it has a detrimental effect on the spring characteristics.

The content of nitrogen (N) is preferably 0.001 to 0.02%. N is an element that reacts with boron to form nitrides such as BN to serve as seeds during pearlite generation and spheroidization particle formation to promote spheroidization. If the content of N is less than 0.001%, it is difficult to form nitride, and if it exceeds 0.02%, the nitride-based precipitate is coarsened, and the role of spheroidization facilitating particles disappears, but rather adversely affects the fatigue characteristics.

Vanadium (V) 0.005 to 0.5% and titanium (Ti) 0.005 to 0.5%. V and Ti are carbon / nitride forming elements alone or in combination, and their content is preferably 0.005 to 0.5%. When the content is small, precipitation of V and Ti-based carbon / nitride is reduced, so that the effect of improving the grain size control is insufficient. If the content is high, the manufacturing cost rises sharply, and the improvement effect by the precipitate is saturated, and the coarse alloy carbide which is not dissolved in the base material increases during the austenitic heat treatment, which acts like a non-metallic inclusion. The strengthening effect is lowered.

The remainder is composed of Fe and unavoidable impurities.

Hereinafter, the production method of the present invention will be described in detail. Meanwhile, the manufacturing process of the present invention is briefly shown in the graph of FIG. 3 in relation to the heat treatment time and the heat treatment temperature.

The present invention is hot rolled after heating the steel strip satisfying the above composition. Heating and hot rolling in this invention are based on the conventional wire rod rolling method.

However, the finishing milling during the hot rolling is limited as follows.

As shown in FIG. 3, the hot milling in the present invention is performed in a temperature range in which ferrite, cementite and austenite three-phase regions exist. In more detail, it is preferable to carry out at 750 degreeC or more and Ac1 temperature or less.

Since the three-phase zone is composed of a pearlite structure composed of ferrite and cementite and austenite, which is a high temperature structure, during the finishing rolling in the three phase region, the pearlite fraction is increased by promoting the formation of the pearlite structure with a high amount of deformation, and the pearlite structure is increased. A segment of my cementite is possible. This segmented cementite helps the austenite phase in the three phase zone to grow into globular cementite during the cooling process.

Moreover, it is preferable to carry out so that the temperature variation at the time of finishing rolling may be 20 degrees C or less. When the temperature deviation exceeds 20 ° C., the variation of the fraction of the pearlite structure in the three-phase zone becomes large, and it is difficult to obtain a homogeneous fraction of cementite, which can cause a large physical property deviation in the tissue.

The hot rolled steel is wound by passing through a laying head. At this time, it is preferable to perform the temperature which winds through a laying head at 800-900 degreeC.

The laying head temperature is related to the austenite phase cementite which precipitates from the spheroidized cementite which is segmented in finishing rolling. If the temperature exceeds 900 ° C., the segmental cementite is redissolved, causing the seed for spheroidization to disappear, and at too low temperature below 800 ° C., the driving force of phase transformation from austenite to pearlite tissue is too high. There is a problem that can be transformed into a plate-like cementite tissue.

After winding through the said laying head, it cools to 3 degrees C / s or less of average cooling rates. The average cooling rate is controlled so that the segmented cementite particles do not dissolve and serve as seeds of the spheroidized particles through slow cooling.

Hereinafter, the steel of the present invention will be described in detail.

The steel produced by the composition and production method has a ferrite as a matrix and includes a spheroidized cementite structure.

In addition, the steel material of the present invention preferably satisfies the hardness difference of 220 or less in Vickers hardness (Hv). More preferably, Vickers hardness satisfies 140-220.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following examples.

(Example)

After casting steels having a composition as shown in Table 1 (wt%, but N and O are ppm) and fabricating a steel sheet, various finishing rolling, laying head temperature and cooling rate shown in Table 2 were changed. Steel was prepared while going.

The microstructure of the material was observed by selecting an arbitrary part from the steel wire manufactured under such conditions, and the change of the material hardness according to the degree of spheroidization was measured using a Vickers hardness tester and the results are shown in Table 2. At this time, the average value obtained by taking five samples for accurate hardness measurement was determined as the average hardness value.

division C Si Mn Ni Cr V Ti B P S Al N O Comparative Example 1 0.80 1.3 0.70 0.5 1.2 0.30 0.010 - 0.009 0.012 0.010 50 16 Comparative Example 2 0.90 1.7 0.80 0.3 1.1 0.20 0.020 0.003 0.008 0.010 0.008 52 17 Comparative Example 3 1.00 1.8 0.85 0.6 1.2 0.25 0.017 0.002 0.007 0.019 0.004 53 18 Inventory 1 0.70 2.0 0.70 0.5 1.0 0.20 0.018 0.002 0.008 0.009 0.008 49 18 Inventive Example 2 0.80 2.5 0.70 0.4 1.2 0.10 0.015 0.003 0.007 0.008 0.009 51 19 Inventory 3 1.00 1.5 0.50 0.5 1.0 0.10 0.019 0.003 0.009 0.005 0.009 53 17

division Filament rolling temperature
(℃) Laying head temperature
(℃) Average cooling speed
(° C / s) Material hardness
/ Hv Comparative Example 1 780 805 0.5 254 Comparative Example 2 820 860 0.5 340 Comparative Example 3 785 805 4 350 Inventory 1 780 805 0.7 163 Inventive Example 2 785 810 0.5 185 Inventory 3 775 805 0.5 202

As can be seen from the results of Table 2, in the case of Comparative Example 1 is Hv 254, this result is directly under Ac1 with low Si addition amount, despite the fact that the segment of plate-like cementite in pearlite occurs under effective rolling and cooling conditions. The temperature range in which the three phases of ferrite, cementite and austenite can coexist at the same time is not wide enough, which may be due to the fact that the plate-like cementite segment was not easy during rolling in this section. In addition, the spheroidization rate was considerably reduced due to the absence of the B nodular boron-based carbon / nitride.

In addition, Comparative Example 2 is Hv 340, and this result is due to the high filament rolling temperature and the laying head temperature of Ac1 ~ Acm and the formation of spheroidized seed due to the absence of pearlite, that is, no plate-like cementite formation. It seems to be due to something impossible.

Finally, in the case of Comparative Example 3, as shown in Figure 2, despite the chemical composition that can produce a stable three-phase region and the plate-like cementite segment has easy rolling conditions, the seed (seed) due to the high cooling rate The growth of cementite is not controlled slowly, and the high hardness value of Hv 350 appears to be due to the promotion of the formation of the platelet pearlite.

On the contrary, as shown in Inventive Examples 1 to 3, there are three phase sections of a sufficient temperature section with sufficient Si addition, and spheroidized seed generation and growth are easy at the appropriate rolling conditions and cooling rates. The average hardness tended to decrease significantly to 50% compared to the comparative example.

On the other hand, Figure 3 (a) is a tissue photograph of Comparative Example 3, Figure 3 (b) is a tissue photograph of Inventive Example 2 it can be confirmed that the invention example 2 is superior in spheroidization than Comparative Example 3.

1 is a graph showing the concept of the present invention as a relation between heat treatment time and heat treatment temperature.

2 is a graph showing a change in the Fe-C state diagram according to the Si content.

3 (a) is a comparative example 3, Figure 3 (b) shows a tissue photograph of the invention example 2.

Claims (7)

By weight%, C: 0.6-1.4%, Si: 1.0-3.5%, Mn: 0.3-1.5%, Cr: 0.8-1.5%, Ni: 0.01-1.0%, B: 0.001-0.02%, O: 0.0015% Or less: Al: 0.001-0.1%, P: 0.02% or less, S: 0.02% or less, N: 0.001-0.02%, V: 0.005-0.5% and Ti: 0.005-0.5% Including the remainder containing Fe and other unavoidable impurities, Microstructure is a spherical heat treatment omitted type steel containing ferrite as a base structure and containing spheroidized cementite. The method according to claim 1, The steel is a spherical heat treatment omitted type steel material having a beaker hardness (Hv) of 140 ~ 220. By weight%, C: 0.6-1.4%, Si: 1.0-3.5%, Mn: 0.3-1.5%, Cr: 0.8-1.5%, Ni: 0.01-1.0%, B: 0.001-0.02%, O: 0.0015% Or less: Al: 0.001-0.1%, P: 0.02% or less, S: 0.02% or less, N: 0.001-0.02%, V: 0.005-0.5% and Ti: 0.005-0.5% Including, the remainder being hot rolled a steel strip containing Fe and other unavoidable impurities; Winding through a laying head after the hot rolling; And Cooling after the winding Method for producing a spheroidized heat treatment omitted type steel comprising a. The method of claim 3, Finish rolling during hot rolling (finish milling) is a method for producing a spheroidized heat treatment omitted steel, including performing in the three-phase inverse temperature range of ferrite, cementite and austenite. The method according to claim 3 or 4, Finish rolling during hot rolling is carried out at a temperature range of 750 ° C to Ac1, and the temperature width is 20 ° C or less. The method of claim 3, The winding method for producing a spheroidized heat treatment omitted steel material comprising performing in the temperature range of 800 ~ 900 ℃. The method of claim 3, The cooling method of the spherical heat treatment omitted steels, including cooling to the average cooling rate of less than 3 ℃ / s.

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