KR101461729B1 - Boron-added high carbon steel having superior strength and ductility and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a method for producing boron (B) -added high-carbon steel excellent in strength and ductility and high-carbon steel added with boron (B).
For this purpose, in the present invention, a high strength and high ductility steel having a tensile strength of 900 to 1200 MPa can be manufactured by using the high carbon steel to which boron (B) is added while controlling the heat treatment conditions.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a boron-added high carbon steel having excellent strength and ductility and a method for producing the boron-

The present invention relates to a method for producing boron (B) -added high-carbon steel excellent in strength and ductility and high-carbon steel added with boron (B).

Since the strength and ductility are in inverse proportion, a technique using stress-induced martensite transformation of the retained austenite is applied to produce a steel material having a relatively good combination of strength and ductility.

TRIP (Transformation Induced Plasticity) steel is a typical steel to which stress austenite martensite transformation technique of residual austenite is applied. TRIP steel mainly has a carbon content of 0.1-0.3 wt%, and ferrite, bainite, It has a microstructure consisting of retained austenite. Its elongation is relatively high compared to other steels with retained austenite, and is used mainly in the strength range of 900 MPa or less. However, in the case of conventional TRIP steel, high ductility can be ensured, but it is difficult to secure ultra high strength.

There is Q & P Steel (so-called "M-TRIP Steel") which passes the process of securing retained austenite by quenching and partitioning instead of TRIP steel which is difficult to secure ultra high strength. Since martensite has a retained austenite phase as a matrix structure, relatively high mechanical strength of 1200 MPa or more can be secured relatively easily. By using the Q & P process, it is possible to control the amount of carbon in the steel to secure excellent physical properties in various strength ranges.

In the case of manufacturing steel using the Q & P process, it is possible to obtain a stable retained austenite at room temperature by increasing the internal carbon concentration even after a short time of partitioning by producing a steel having a high carbon content in order to obtain an ultra- However, in order to obtain a steel containing an appropriate amount of retained austenite in an intensity range of 900 to 1200 MPa, it is necessary to accumulate the carbon concentration in the retained austenite by partitioning the low carbon steel having a low carbon content of about 0.2 wt% for a long time . Therefore, there is a disadvantage in that the manufacturing cost is relatively high because the heat treatment time is long to realize this, and the configuration of the heat treatment equipment is also large and complicated.

In the case of Q & P steel composed of a two-phase composite structure, since the only phase capable of securing ductility is only the retained austenite phase which is difficult to manufacture, the properties of regions with high ductility can be varied There is a problem that is difficult to secure. In addition, in the Q & P process, when carbides are produced in the partitioning step, the partitioning time must be kept longer to secure sufficient carbon for stabilizing the retained austenite at room temperature, It is necessary to slow down the rate of carbide formation for partitioning and to use a large amount of Si which can cause expensive alloying elements or brittleness.

An aspect of the present invention is to provide a boron-added high-carbon steel excellent in both strength and ductility by controlling the heat treatment method using boron-added high-carbon steel in order to obtain a steel excellent in both strength and ductility.

An aspect of the present invention is a method for producing a carbon material comprising, by weight%, 0.15-0.55% carbon (C) and 8-60 ppm boron (B), the balance being Fe and unavoidable impurities,

The microstructure provides boron-added high-carbon steels having an excellent strength and ductility, containing 3 to 25% of retained austenite in an area fraction and a residual pre-ferrite and martensite composite.

Another aspect of the invention is, by weight%, carbon (C): 0.15 ~ 0.55% and boron (B): containing the 8 ~ 60ppm, and maintaining a steel material which is the composition with the balance Fe and unavoidable impurities in the above A _f temperature Thereby transforming austenite into austenite;

An intercritical annealing step of cooling the steel material to A ₁ to A _{r 3} and maintaining the steel material in the temperature range to form pro-eutectoid ferrite;

A quenching step of rapidly cooling the steel material to M _f to M _s to form martensite;

A partitioning step of maintaining the steel at a temperature of M _f to M _s to concentrate carbon inside the retained austenite; And

And a cooling step of cooling the steel material to an ordinary temperature. The present invention also provides a method for producing boron-added high carbon steel excellent in strength and ductility.

According to the present invention, it is possible to produce a steel material having excellent ductility at a tensile strength of 900 to 1200 MPa, which is difficult to obtain in conventional TRIP steel or Q & P steel, through heat treatment in a relatively short time. That is, by accelerating the carbon concentration in the retained austenite through the initial intercritical annealing step, it is possible to provide a relatively soft steel at the same tensile strength level.

FIG. 1 schematically illustrates a heat treatment process (from an intercritical annealing step to a cooling step) which is one aspect of the present invention.
Fig. 2 shows changes in the TTT curve when B or Mo is added to general high carbon steels and the general high carbon steels.
Fig. 3 shows the result of observing the microstructure of Inventive Example 1 with an optical microscope.

The inventors of the present invention have conducted intensive studies to derive an inexpensive steel material having excellent strength and ductility at the same time in a strength region difficult to obtain in conventional TRIP steel or Q & P (Quenching and Partitioning) steels utilizing residual stress austenite martensite transformation of austenite. It is possible to produce a high strength and high ductile steel having a tensile strength of 900 to 1200 MPa by controlling the heat treatment conditions while using the high carbon steel to which (B) is added.

The high carbon steel with boron (B) added to the grain boundary of solute boron due to the solute dragging effect of B, while the price is low, the pearlite region is pushed to the right side in the CCT curve as compared with general high carbon steel. Therefore, it is possible to prevent the austenite remaining after the primary pro-eutectoid ferrite formation from being transformed into another phase such as perlite during the heat treatment to consume the carbon necessary for the formation of the residual austenite in the form of a carbide such as cementite.

Therefore, since most of the carbon remaining after the pro-eutectoid ferrite formation remains in the austenite, it is possible to shorten the partitioning time for producing 'carbon-enriched retained austenite' which can be maintained at a relatively high temperature by austenite There will be.

The change of the TTT curve in the boron (B) -added high carbon steel (0.35% C + B) can be confirmed through FIG. For comparison, the TTT curves of high carbon steel (0.35% C + Mo) with boron (B) -free general high carbon steel (0.35% C) and molybdenum (Mo)

2, a Pearlite curve of molybdenum (Mo) -containing high carbon steel (0.35% C + Mo) compared to general high carbon steel (0.35% C) without boron (B) Bainite curves are relatively less pushed, but in the case of boron (B) added high carbon steel (0.35% C + B), both pearlite and bainite curves are pushed to the right . From this, it can be seen that general high carbon steel (0.35% C) without boron (B) is easy to produce pearlite during quenching heat treatment, which makes it difficult to efficiently concentrate carbon.

Therefore, in order to obtain the effect of the residual austenite stress induced organic martensite transformation, a high carbon steel (0.35% C + Mo) containing boron (B) added high carbon steel (0.35% C + B) or molybdenum (Mo) But it is more preferable to use high carbon steel containing B in comparison with high carbon steel containing high-priced molybdenum, considering economical aspects.

The ferrite formation temperature of each steel material is thermodynamically A ₃ line but varies depending on the holding time since it is actually influenced by kinetics. Therefore, the ferrite critical generation temperature of each steel material can be confirmed in Fig.

Hereinafter, a method for producing boron (B) -added high carbon steel excellent in strength and ductility, which is one aspect of the present invention, and boron (B) -added high carbon steel excellent in strength and ductility produced by the method will be described in detail.

A method for producing boron-added high carbon steels excellent in strength and ductility according to the present invention comprises 0.15 to 0.55% of carbon (C) and 8 to 60 ppm of boron (B), and the balance Fe and other unavoidable impurities To austenite at a temperature of Ar or higher; Anintercritical annealing step of cooling the steel material from A ₁ to a ferrite producing temperature and then maintaining the temperature within the temperature range to form pro-eutectoid ferrite; A quenching step of rapidly cooling the steel material to M _f to M _s to form martensite; A partitioning step of maintaining the steel at a temperature of M _f to M _s to concentrate carbon inside the retained austenite; And a cooling step of cooling the steel material to room temperature.

The steps from the intercritical annealing step to the cooling step are schematically shown in FIG.

The boron-added high carbon steel used in the present invention contains 0.15 to 0.55% of carbon (C) and 8 to 60 ppm of boron (B) in weight percent, and the addition of Mn, Si, Ti, Element and indispensable elements such as P and S, and the remainder Fe, and the content of these additional elements and the like is not limited as long as it can be contained in ordinary steels. More specifically, these elements may be contained in an amount of 1.5% or less of Mn, 0.3 to 1.0% of Si, 0.03 to 0.30% of Cr, and 0.01 to 0.03% of Ti by weight%.

The boron-added high-carbon steel is subjected to an intercritical annealing step in which it is maintained at a temperature _{equal to} or higher than A _f and transformed into austenite, cooled, and maintained in the range of A ₁ to A _{r 3} to secure pre-ferritic ferrite. In this case, it is preferable that the annealing is performed for 100 to 10000 seconds. If the holding time in the range of A1 to the ferrite forming temperature is less than 100 seconds, it is difficult to secure enough pro cornerstone ferrite. On the other hand, Formation is sufficient, but it is disadvantageous from an economical viewpoint.

More preferably, since the holding time for forming sufficient pro-eutectoid ferrite can be varied depending on the holding temperature, it is preferable that the time is not more than 50 hours, .

On the other hand, the pro-eutectoid ferrite formed from the above step plays a role of releasing its own carbon, which can be easily stabilized at room temperature as the carbon is concentrated in the retained austenite. Second, it plays a role in increasing the ductility in the multiphase complex with the retained austenite.

Since the percentage of pro-eutectoid ferrite varies depending on the carbon content of the base material, it is not particularly limited. However, for example, in the case of high carbon steel, the area percentage is 20 to 40% and in the case of low carbon steel, it is 40 to 60% In order to obtain the above-mentioned effects, it is preferable to secure the pro-eutectoid ferrite to a minimum of 20% in the present invention.

Thereafter, a quenching step is performed in which the steel material subjected to the abnormal zone annealing is quenched to M _f to M _s to form martensite. At this time, by forming an appropriate amount of martensite in the quenching step, desired strength can be secured.

Thereafter, the quenched steel is maintained at a temperature of M _f to M _s to carry out a partitioning step of concentrating the carbon inside the retained austenite.

At this time, the time for carrying out the distribution step may be varied depending on the concentration from the above-mentioned pro-eutectoid ferrite. That is, when carbon enrichment from pro-eutectoid ferrite occurs, it is possible to achieve sufficient carbon enrichment even in a short time in the subsequent distribution step, whereas when carbon enrichment from pro-eutectoid ferrite occurs at a low rate, . That is, the longer the time in the dispensing step, the more carbon enrichment will take place. However, in this case, the manufacturing cost rises sharply, resulting in a problem of increasing the cost of the product.

However, in the present invention, because of the formation of sufficient pro-eutectoid ferrite from intercritical annealing, sufficient carbon concentration can be achieved with a less time-consuming distribution process than with a conventional distribution process, preferably carried out at 600 seconds or less . If the dispensing time exceeds 600 seconds, there is a problem that the manufacturing cost increases as described above, which is not preferable.

The steel material finished up to the above-mentioned distribution step is cooled to room temperature by ordinary cooling method to produce boron (B) -added high carbon steel having excellent final strength and ductility.

The steel material that has been completed to the cooling process is a microstructure, which is composed of three phases of precious iron ferrite, martensite (or tempered martensite) and retained austenite. In this case, a small amount Of bainite. ≪ RTI ID = 0.0 > Since a small amount of bainite is difficult to distinguish from the martensite formed by the present invention on the microstructure and has similar physical properties, it can be handled in the same manner.

As described above, since the pro-eutectoid ferrite is determined by the retention time of the intercritical annealing, it is possible to obtain the desired fraction of the pro-eutectoid ferrite by adjusting the retention time. The fraction of retained austenite is inversely proportional to the fraction of martensite generated in the quenching process. In the present invention, the retained austenite is preferably formed in an area fraction of 3 to 25%.

If the fraction of retained austenite is less than 3%, since there is not a sufficient amount of retained austenite, it is difficult to expect the effect of improving the strength and elongation through stress induced transformation of martensite. On the other hand, even if most of the carbon is concentrated in the residual austenite by using the high carbon steel, it is difficult to form the retained austenite stable at room temperature in excess of 25%, so it is preferable to limit the upper limit of the retained austenite fraction to 25% Do.

The boron (B) -added high carbon steel of the present invention having such a microstructure constitution has a tensile strength of 900 to 1200 MPa and an elongation of 10 to 25%, whereby the combination of strength and ductility is excellent.

The most problematic part of the general Q & P steel manufacturing process is the process time in the partitioning process to concentrate (concentrate) carbon in the retained austenite. Since the time in the heat treatment process means directly the process cost, even if an excellent steel can be made, the effect is halved if it takes a long time.

However, according to the present invention, a steel material excellent in both strength and ductility combined with pro-eutectoid ferrite, martensite and retained austenite can be produced in a short time, so that even if the same properties as those of existing Q & P steel are realized, Technology.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

(B) and titanium (Ti) to the same carbon composition as the 0.38% C grade boron (B) added steel, which was developed for the purpose of replacing expensive high alloy high carbon steels commonly used in heat treatment processes. S35C steel, which is a general high carbon steel which does not contain carbon black, was prepared. The composition (% by weight) of each steel material is as shown in Table 1 below.

The steels having respective compositions as shown in the following Table 1 were made into hot rolled steel sheets through ordinary hot rolling and then subjected to normal spheroidizing annealing and cold rolling to prepare JIS13B specimens for tensile test.

The A ₁ temperature of each steel material shown in the following Table 1 can be seen at 727 ° C in the steel material and the ferrite formation temperature of each steel material is thermodynamically A ₃ line but actually varies depending on the holding time since it depends on kinetics . Therefore, the critical temperature for ferrite formation according to the holding time of the POS10B35 steel, which is a boron (B) added steel, and the S35C steel, which is a common high carbon steel, can be seen in FIG.

Each of the specimens was subjected to intercritical annealing under the conditions shown in Table 2, followed by heat treatment for cooling and holding. At this time, the cooling temperature and the holding time were different according to the conditions shown in Table 2 below.

division C Mn Si Cr P S B Ti Remainder POS10B35 0.35 0.49 0.19 0.19 0.008 0.002 0.0021 0.002 The balance Fe and
An unavoidable impurity S35C 0.37 0.49 0.17 0.20 0.010 0.003 - -

Steel Abnormal region annealing Quenching Distribution division
Temperature (℃) Holding Time (sec) Temperature (℃) Holding Time (sec) POS10B35 760 600 280 300 Inventory 1 POS10B35 735 1000 320 300 Inventory 2 POS10B35 680 300 320 300 Comparative Example 1 POS10B35 760 600 200 300 Comparative Example 2 S35C 760 600 280 300 Comparative Example 3 S35C 735 1000 320 300 Comparative Example 4

As shown in Table 2, Inventive Examples 1 and 2 show cases in which the heat treatment process proposed in the present invention is applied using boron (B) -added high carbon steel, wherein Inventive Example 1 has a ferrite fraction relatively , And the retention time was set to 600 seconds in the abnormal region annealing for the purpose of forming a microstructure having a high martensite fraction.

In Comparative Examples 1 and 2, the same steels as the inventive examples were used. However, the annealing temperature was low and the quenching temperature was low, so that the conditions in the heat treatment step proposed in the present invention were not satisfied. 3 and 4, the heat treatment process satisfies the present invention, but the applied steel material is a general high carbon steel to which boron (B) is not added.

The tensile strength and elongation of each of the above steels were measured, and the microstructures thereof were observed. The results are shown in Table 3 below.

division Tensile Strength (MPa) Elongation (%) Microstructure Inventory 1 1123 12.8 Ferrite + martensite + retained austenite Inventory 2 985 15.2 Ferrite + martensite + retained austenite Comparative Example 1 935 8.7 Ferrite + pearlite + martensite Comparative Example 2 872 10.1 Ferrite + martensite Comparative Example 3 886 9.3 Ferrite + pearlite + martensite Comparative Example 4 726 16.3 Ferrite + martensite + retained austenite

As shown in Table 3, it can be confirmed that Inventive Examples 1 and 2 produced according to the present invention have sufficient retained austenite and excellent combination of strength and ductility. Particularly, Inventive Example 1 is a case in which the holding time is set to 600 seconds in the abnormal region annealing, and Inventive Example 2 is a case in which the holding time is set to 1000 seconds in the abnormal region annealing, and each tensile strength is measured to be 1123 MPa and 985 MPa. From these results, it is possible to easily control the strength level of the steel material produced by controlling the fraction of ferrite and martensite formed by controlling the conditions at the time of the annealing in the ideal zone.

On the other hand, in Comparative Example 1, the temperature at the time of the abnormal zone annealing was too low. As a result, pearlite was generated and carbon in the steel was consumed to form carbide, so that the retained austenite was not produced. The combination was small.

Comparative Example 2 is a case in which the quenching after excess region annealing was excessive and the distribution process could not be performed in the range of Mr to Ms. The formation of ferrite was sufficient, but after all the austenite was transformed into martensite by excessive quenching, No formation was made. As a result, both strength and ductility were low.

Comparative Example 3 is a case where a general high carbon steel other than boron (B) -added high carbon steel was used. The formation of ferrite by the abnormal region annealing according to the present invention was sufficient, but the hardening ability was small due to the characteristics of the base material itself, , Pearlite was formed and carbon in the steel was consumed in the formation of carbide. As a result, the formation of retained austenite was not achieved, resulting in both low strength and ductility. The low hardenability of general high carbon steels can be confirmed based on the nose of the TTT curve in FIG. 2. As shown in FIG. 2, the curing ability of general high carbon steels is about 1/10 Level hardening ability.

Comparative Example 4 In the case of using general high carbon steel other than boron (B) added high carbon steel, the conditions of the ideal region annealing satisfied the present invention, but the ferrite was excessively produced due to the characteristics of the base material itself. As a result, the ductility showed excellent results, but sufficient martensite and retained austenite were not formed and the strength was measured as low as 750 MPa or less.

Further, the microstructure of Inventive Example 1 was observed with an optical microscope, and the result is shown in Fig. As shown in Fig. 3, it can be confirmed that ferrite, martensite, and retained austenite phase are constituted. However, the martensite was slightly tempered in the quenching and distributing process, and a small amount of tempering martensite was present.

As a result of the dilatometer measurement of the phase transformation temperature of each steel used, the M _s temperature and the M _f temperature of boron (B) -containing high carbon steel (POS10B35) were 357 ° C and 217 ° C, respectively, The M _f temperature of carbon steel (S35C) was measured at 358 ° C and the M _s temperature at 222 ° C.

As mentioned above, the A ₁ temperature can be seen at 727 ° C in steel, and the ferrite formation temperature of each steel is thermodynamically A ₃ , but it depends on the holding time because it is actually controlled by kinetics. Therefore, the ferrite critical generation temperature according to the holding time of the inventive example and comparative example can be confirmed in Fig. In FIG. 2, the ferrite formation temperature region of boron (B) -added high carbon steel (POS10B35) is about 700 ° C or higher, and a pro-eutectoid ferrite phase can be generated at this temperature without producing any other phase such as pearlite. However, it can be seen that the formation kinetics is very slow at a temperature of 775 ° C or higher, and therefore, it must be maintained at a lower temperature for the generation of a proper fraction of pro-eutectoid ferrite. On the other hand, in the case of general high carbon steel (S35C), ferrite and pearlite-forming kinetics are very fast, so it is possible to produce a proper amount of pre-ferrite in the initial stage of intercritical annealing. However, The cooling rate can not be obtained and the pearlite is formed in the steel, so that the carbon is not concentrated in the retained austenite but is likely to be consumed as the carbide.

Claims (7)

0.1 to 0.55% of carbon (C), 8 to 60 ppm of boron (B), and other components of Mn of 1.5% or less, Si of 0.3 to 1.0%, Cr of 0.03 to 0.30% , Ti: 0.01 to 0.03%, the balance being Fe and inevitable impurities,
Microstructure is boron (B) -doped high carbon steel with excellent strength and ductility, including 3-25% retained austenite in an area fraction and remainder cormorite ferrite and martensite composite.
The method according to claim 1,
The boron-added high carbon steel has a tensile strength of 900 to 1200 MPa and an elongation of 10 to 25%, and is excellent in strength and ductility.
0.1 to 0.55% of carbon (C), 8 to 60 ppm of boron (B), and other components of Mn of 1.5% or less, Si of 0.3 to 1.0%, Cr of 0.03 to 0.30% , Ti: 0.01 to 0.03%, and the remainder Fe and unavoidable impurities are maintained at a temperature _{equal to} or higher than A _f and transformed into austenite;
An intercritical annealing step of cooling the steel material to A ₁ to A _{r 3} and maintaining the steel material in the temperature range to form pro-eutectoid ferrite;
A quenching step of rapidly cooling the steel material to M _f to M _s to form martensite;
A partitioning step of maintaining the steel at a temperature of M _f to M _s to concentrate carbon inside the retained austenite; And
And a cooling step of cooling the steel material to a normal temperature, wherein the boron (B) added carbon steel is excellent in strength and ductility.
The method of claim 3,
The method of claim _1, wherein the intercritical annealing step is performed by maintaining at least one of A ₁ to A _{r 3} for 100 to 10000 seconds.
The method of claim 3,
The above zone annealing (Intercritical annealing) step is the A ₁ ~ 100 seconds in the A _r3, and is excellent boron strength and ductility carried below the time that the pro-eutectoid ferrite in the A ₁ ~ A _r3 generated up to 50% (B ) ≪ / RTI >
The method of claim 3,
The distribution (Partitioning) step is the production of the M _f ~ M _s temperature was added in a superior strength and ductility of boron carried to remain below 600 seconds (B) high-carbon steel.
The method of claim 3,
Wherein the residual austenite is contained in an amount of 3 to 25% in an area fraction.

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