KR20140084932A - Hot rolled steel sheet having superior strength and ductility and manufacturing method thereof - Google Patents

Abstract

Provided are a hot rolled steel sheet with excellent strength and ductility which contains 0.15-0.55 wt% of carbon (C), 0.3-1.0 wt% of silicon (Si), 1.5 wt% or less of manganese (Mn), 0.03-0.3 wt% of chrome (Cr), 0.0008-0.0060 wt% of boron (B), 0.01-0.03 wt% of titanium (Ti), 0.02 wt% or less of phosphorous (P), 0.02 wt% or less of sulfur (S), and the remainder consisting of iron (Fe) and inevitable impurities and has a fine structure comprising a composite structure composed of retained austenite with an area fraction of 3-25 wt%, pro-eutectoid ferrite remainder, and a martensite, and a method for manufacturing the same. According to the present invention, the hot rolled steel sheet with excellent strength and ductility and the method for manufacturing the same can reduce material costs by using an inexpensive steel material and can manufacture a steel sheet with various properties and a hot rolled steel sheet with excellent strength and ductility with a tensile strength range of 900-1200 MPa even by using the conventional TRIP steel or Q&P steel.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet having excellent strength and ductility,

The present invention relates to a hot-rolled steel sheet excellent in strength and ductility and a method for producing the same.

Because of the inverse relationship between strength and ductility, several prior art techniques have been used to exploit the stress-induced martensitic transformation of residual austenite to produce a relatively steep combination of strength and ductility.

TRIP steel is a representative steel material. TRIP steel manufacturing technology is a method widely applied to automotive steel sheets because it improves the elongation of steel by transformation of unstable retained austenite at room temperature to stress-induced martensite by stress. However, currently produced TRIP steel is a low-temperature transformed structure that takes charge of strength and mainly uses a bainite phase or a very small amount of martensite phase and a high fraction of ferrite phase. Therefore, the elongation rate is very high but the strength is not sufficient. The cold rolled TRIP steel produced through the cold-rolling plating process has a disadvantage that it is very expensive due to a complicated process.

Q & P Steel (Quenching & Partitioning Steel), which has relatively high strength and low elongation as compared with general TRIP steel, is used in the case of so-called M-TRIP steel, similar to TRIP steel, but utilizes residual austenite. Strengthen the organization. The main structures affecting strength and elongation are martensite and retained austenite produced by quenching and partitioning, respectively. Since this steel contains a very strong martensite as a matrix and also contains residual austenite phase, excellent properties can be obtained in various strength ranges by controlling the amount of carbon.

However, in case of Q & P steel, in case of super high strength range of 1200 MPa or more, stable carbonaceous concentration at room temperature can be obtained by accumulating internal carbon concentration even with a short time distribution by using high carbon steel, There is a problem in that a relatively low carbon steel of about 0.2% is distributed for a long time in order to obtain a steel containing an appropriate amount of the retained austenite, and the carbon concentration must be accumulated inside the retained austenite. Therefore, there is a disadvantage in that the manufacturing cost is relatively high because of the long distribution time required to implement the method, and the configuration of the heat treatment equipment is also large and complicated.

In the case of Q & P steel, it is composed of two phases. Since the only phase capable of securing ductility is a residual austenite phase which is difficult to produce, there is a problem that it is difficult to secure various properties of a region with high ductility in the same composition.

In addition, in the Q & P steel, if carbides are generated in the distribution process, the distribution time must be kept longer to ensure sufficient carbon to stabilize the retained austenite at room temperature, thus slowing the rate of carbide formation for efficient distribution However, since expensive alloying elements or a large amount of Si must be used for this purpose, it is disadvantageous in terms of cost, and surface defects and brittleness may occur in the hot-rolled steel sheet.

One aspect of the present invention is to secure a high-strength, high-ductility hot-rolled steel sheet at a tensile strength of 900 to 1200 MPa, which is an area of strength that is difficult to obtain easily from TRIP steel and Q & P steel (M-TRIP steel), which utilize representative retained austenite. And a method of manufacturing the same.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In order to achieve the above object, according to one aspect of the present invention, there is provided a ferritic steel comprising 0.15 to 0.55% of C, 0.3 to 1.0% of Si, 1.5 to less than 1.5% of Mn, 0.03 to 0.3% of Cr, 0.002% of Ti, 0.01 to 0.03% of Ti, 0.02% or less of P, 0.02% or less of S, and the balance Fe and other unavoidable impurities. The microstructure is retained in an area fraction of 3 to 25% And a martensite composite structure, which is excellent in strength and ductility.

Another aspect of the present invention is to provide a steel sheet comprising 0.15 to 0.55% of C, 0.3 to 1.0% of Si, 1.5% or less of Mn, 0.03 to 0.3% of Cr, 0.0008 to 0.0060% of B, 0.01 to 0.03% , P: not more than 0.02%, S: not more than 0.02%, and the balance Fe and other unavoidable impurities to a temperature of A _f or more, followed by rough rolling; An intercritical annealing step of forming a pro-eutectoid ferrite by maintaining the steel material at a temperature higher than the A ₁ temperature and lower than the ferrite formation temperature (Ar ₃ ) for 20 to 200 seconds; Finishing the steel material; Quenching the steel material at a cooling rate of 80 ° C / sec or more; Winding the steel material at a temperature of M _f to M _s to produce martensite from the retained austenite; Maintaining the cooling rate of the steel at a rate of 1 DEG C / min or less to accumulate carbon in the final retained austenite; And cooling the steel material to a normal temperature. The present invention also provides a method of manufacturing a hot-rolled steel sheet excellent in strength and ductility.

According to the present invention, it is possible to manufacture a hot-rolled steel having both strength and ductility at a tensile strength range of 900 to 1200 MPa, which is difficult to manufacture in conventional TRIP steel or Q & P steel, It is possible to easily produce a steel having various physical properties.

1 is a diagram illustrating a process of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.
2 is a graph showing tensile strength and elongation according to the inventive and comparative examples of the present invention.

The inventors of the present invention have conducted intensive researches to derive steel materials having both strength and ductility at the strength regions that are difficult to obtain in conventional TRIP steels or quenching and partitioning steels using residual stress austenite martensite transformation of austenite. The present invention has been accomplished by confirming that a high strength and high ductility hot rolled steel sheet having a tensile strength of 900 to 1200 MPa can be manufactured by controlling the heat treatment conditions while using the high carbon steel to which the present invention is added.

The high carbon steel to which boron (B) is added is low in price and solvated by the solute dragging effect, the boron is adhered to the grain boundaries and the continuous cooling transformation (CCT) ) Pearlite area is pushed to the right in the curve. 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.

Hereinafter, a hot-rolled steel sheet excellent in strength and ductility according to the present invention and a method of manufacturing the same will be described in detail so that those skilled in the art can easily carry out the present invention.

The steel material for producing the hot-rolled steel sheet according to the present invention contains 0.15 to 0.55% of C, 0.3 to 1.0% of Si, 1.5% or less of Mn, 0.03 to 0.3% of Cr, 0.0008 to 0.0060% of B, 0.01 to 0.03%, P: not more than 0.02%, S: not more than 0.02%, and the balance Fe and other unavoidable impurities. In the case of expensive high carbon steels containing a large amount of alloying elements, if the hot rolling method according to the present invention is followed, a sufficient curing ability can be ensured, so that an effect similar to that of the present invention can be obtained.

The reason for limiting the numerical values of the above components will be described as follows. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

C: 0.15-0.55%

The steel containing a large amount of carbon is advantageous in securing the martensite structure by quenching, and the secured martensite structure contributes greatly to the strength. Therefore, the lower limit of C is set to 0.15% in order to secure the strength. In the case of the upper limit, the degree of enhancement of the hardenability by the addition of B, which is an important functional group of the present invention, is gradually saturated at the addition of 0.55% or more, so that the effect of increasing the hardenability of the steel by the addition of B is insignificant and limited to 0.15 to 0.55%.

Si : 0.3 to 1.0%

Si is the five major elements of steel, and a small amount is added naturally during the manufacturing process, contributing to the strength increase, suppressing the formation of carbide, and not carbon being formed as carbide in the holding step after winding, being distributed and retaining in the retained austenite The retained austenite is allowed to remain in the austenite phase at room temperature. However, if the added amount is too large, a surface defect such as a severe scale is caused in the high carbon hot-rolled steel and brittleness is caused in the steel, do. For this reason, the content was limited to 0.3 to 1.0%.

Mn : 1.5% or less

Mn has little help for increasing the hardenability, but its effect is not so large as compared with boron (B), so that the lower limit is not required, and if it is too high, segregation such as center segregation or micro segregation becomes severe. For this reason, its content should be limited to 1.5% or less.

Cr : 0.03 to 0.3%

Cr has an effect of preventing the decarburization of molten steel by forming a Cr layer on the surface of the molten steel, which is effective for improving the hardenability. However, since the cost of the alloy element is high and the hardenability can be ensured through the addition of B in the present invention, the upper limit and the lower limit are set as described above with the proper amount (0.1%) for use for the purpose of preventing decarburization of molten steel Respectively.

B: 0.0008 to 0.0060%

B is a ferrite or a pearlite (or bainite) by decreasing the grain boundary energy by segregating in grain boundaries, or by the effect that the fine precipitates of Fe ₂₃ (C, B) ₆ are segregated in grain boundaries and lowering the grain boundary area It is an element that has the effect of inhibiting transformation. As in the present invention, by controlling the cooling rate after hot rolling, it is an important alloying element in the case of producing martensite or martensite and a small amount of bainite as a main phase after being rolled in the hot rolling process. In the case of bonding with nitrogen (N), BN is generated and the curing ability is lowered. Therefore, it is usually added in the same manner as Ti. When boron (B) is added in an amount of less than 8 ppm, the effect of increasing the hardenability by addition of B is insignificant. It is expected that deterioration of toughness and degradability due to grain boundary precipitation of precipitates of B are expected to be 0.0008 to 0.0060% because of the tendency that the hardening ability increase effect increases sharply as solute boron increases and gradually decreases. Respectively.

Ti  0.01 to 0.03%

Ti is added in order to prevent the nitrogen (N) from binding to the solid solution B to precipitate as BN by fixing the nitrogen present in the steel to TiN to prevent the hardening ability of the B addition steel from deteriorating. Nitrogen in the steel is usually 100 ppm or less. When 0.01% or more of Ti is added, the lower limit is defined because it can be combined with most of the nitrogen present in the steel to form TiN. On the contrary, if the amount of Ti is too high, the toughness of the steel is remarkably lowered, so the upper limit is limited to 0.03% or less.

P: not more than 0.02%, S: not more than 0.02%

P, and S belong to the five elements of steel. They are elements that are always contained in impurities that are inevitably included, but they are not included in the steel if possible due to problems such as brittleness. In the case of specially required cleanliness, the upper limit is strictly limited. Otherwise, it is an element which is not good for the steel itself. Therefore, when the upper limit is usually restricted, it is defined as about 0.02% or less.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

The hot-rolled steel sheet is produced through a process as shown in Fig. 1 with a steel having the above-described components and contents.

First, the prepared steel is heated to a temperature _{equal to} or higher than A _f and then rough-rolled. A _f or more means a temperature that is reversely transformed into full austenite.

Usually, the slab is heated to at least 1050 DEG C in the furnace, so that the low cost added B added steel used in the present invention maintains the A _f temperature or more. Considering cooling in the subsequent process, it is advantageous to operate the furnace temperature close to the lower limit of the normal temperature. The slab exiting the heating furnace is rolled into a bar shape while passing through a roughing mill and simultaneously cooled.

After cooling through rough rolling and rough rolling, core steel is formed by intercritical annealing which maintains the steel at a temperature above the A ₁ temperature and below the ferrite forming temperature for 20 to 200 seconds.

In this case, the pro-eutectoid ferrite is further generated during the subsequent finishing rolling, so that the holding temperature and time are adjusted so as to secure the proper amount of pro-eutectoid ferrite. The steel is then finishing rolled, during which the bar is hot rolled to the desired thickness and is further cooled to produce additional pro-eutectoid ferrite in the steel.

Finish rolling after consisting foundation retained austenite in the ferrite and the high temperature state steel material will run-out table in a subsequent cooling process; of is quenched with 80 ℃ / sec or more cooling rate of the steel material by rapid cooling, fitted at _{(ROT Run Out Table) M f} ~ The martensite structure is produced from the austenite that remains at high temperature by being wound at a temperature of M _s .

Since the hot-rolled coils wound on the heat-conducting surface can be seen as bulk-rolled steel sheets instead of steel sheets of the corresponding thickness, they are very slow in the coil Speed cooling phenomenon occurs. As a result of investigating the cooling behavior in the yard and the winter in the winter by inserting the thermocouple at the midpoint of the coil according to the investigation of the present inventor, Cooling rate. In addition, when another hot-rolled coil is surrounded by two or more stages, it has a slower cooling rate, which is more advantageous for the movement of carbon. Therefore, during the cooling process of the hot-rolled coil to the yard, the cooling rate is naturally lower than 1 ° C / min, and the carbon from the martensite generated in the winding process moves to the final retained austenite, Retained Austenite). The coils are then slowly cooled to room temperature, where the final retained austenite with sufficient carbon concentration remains in the austenite phase after cooling.

The microstructure of the hot-rolled steel sheet produced by this method is composed of 3 to 25% of retained austenite and the remainder of eutectoid ferrite and martensite (or tempered martensite) composite structure at an ordinary temperature. In other words, unlike the conventional TRIP steel and Q & P steel, the present invention uses a three-phase composite microstructure composed of pro-eutectoid ferrite, martensite and retained austenite to produce a hot-rolled steel sheet having both strength and ductility in the intermediate strength region between TRIP steel and Q & Can be used.

Bainite can be partially produced in the cooling and winding process, but it is not preferable that pearlite is produced because carbon is not used for stabilization of retained austenite and is consumed for carbide production. Most of the steel according to the present invention does not have a pearlite phase, or only a very small amount is formed in the deep portion during the cooling process. Bainite, which can be produced in a small amount in the cooling and winding process, is hardly distinguishable from the martensite produced by the present invention on the microstructure and has similar physical properties, and therefore can be handled in the same way.

As described above, the fraction of pro-eutectoid ferrite is an intercritical annealing time which is maintained at a temperature of not less than the A ₁ temperature and the ferrite formation temperature for 20 to 200 seconds before the finishing mill (FM) after rough rolling, The final rolling austenite fraction is determined by the amount of martensite produced in the ROT cooling and coiling process and the transfer of carbon to retained austenite on martensite.

When the residual austenite fraction of the hot-rolled steel sheet produced by the present invention is less than 3%, it is difficult to expect an effect of improving the strength and elongation through stress-induced martensite transformation because there is not a sufficient amount of retained austenite. Further, even if most of the carbon is concentrated in the retained austenite in the carbon concentration range of the steel used in the present invention, it is difficult to secure more than 25% of the retained austenite stable at room temperature.

The present invention is similar to TRIP steel and Q & P steel in that it utilizes the final microstructure containing residual austenite, but with different levels of tensile strength sought, different final microstructures other than retained austenite, Roughing Mill The annealing time is adjusted by adjusting the intercritical annealing time to keep the annealing temperature above the A ₁ temperature and the ferrite formation temperature for 20 to 200 seconds before the finishing mill. The fraction can be adjusted, thereby making it possible to produce a variety of steels having relatively high ductility at the same tensile strength level.

The hot-rolled steel sheet produced by the present invention has a tensile strength of 900 to 1200 MPa and an elongation of 8% or more. The upper limit is not set because the higher the elongation is, the better.

The tensile strength of 900 to 1200 MPa is a strength region that is difficult to obtain easily from TRIP steel and Q & P Steel (M-TRIP steel), which are representative of conventional retained austenite (Retained Austenite). However, It has been confirmed that it can be sufficiently achieved while utilizing residual austenite.

As can be seen from FIG. 2, in the case of the inventive example, it is found that the combination of the tensile strength and the elongation is better by being positioned to the right of the trend line of the comparative example.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of the present invention.

[ Example ]

Steel slabs having the components shown in Table 1 were prepared.

That is, slabs of POS10B50 steel (specimen for the inventive and comparative examples) and boron (B) and S50C steel (specimen for comparative example), which are general high carbon steels containing no titanium (Ti) Were prepared.

The content of the component is in weight%.

division C Mn Si Cr P S B Ti Remainder POS10B50 0.49 0.63 0.31 0.05 0.013 0.004 0.0014 0.024 The balance Fe and other unavoidable impurities S50C 0.53 0.54 0.18 0.12 0.011 0.003 - -

In the furnace maintained at more than 1100 ℃, POS10B50 steel was maintained for 182 minutes and S50C steel was maintained for 175 minutes. The hot-rolled steel sheet was manufactured through the process conditions shown in Table 2 below.

division Used steels FM pre-idling ROT cooling rate
(° C / sec) Coiling temperature
(° C) Yard
Cooling rate Starting temperature (℃) Holding time (sec) Honor POS10B50 770 50 103 272 0.3 ℃ / min or less Comparative Example 1 POS10B50 770 50 32 255 0.3 ℃ / min or less Comparative Example 2 POS10B50 873 0 102 543 0.3 ℃ / min or less Comparative Example 3 S50C 770 50 99 267 0.3 ℃ / min or less Comparative Example 4 S50C 850 ℃ -20 minutes holding + 400 ℃ -30 minutes tempering

The martensite starting temperature (M _s ) of the POS 10B50 was 301 ° C, the martensite termination temperature (M _f ) was 192 ° C, and the S50C The M _s temperature was 278 ° C, and the M _f temperature was 182 ° C. The A ₁ temperature can be seen at 727 ° C in steel, and the ferrite formation temperature of each steel can be seen thermodynamically as A ₃ line. In this embodiment, the rolling in the subsequent finishing rolling process was maintained at 770 캜 in consideration of a temperature drop.

The present invention has been made in accordance with the hot rolling conditions for the B-added high carbon steel hot-rolled steel sheet excellent in strength and ductility as defined in the present invention.

Comparative Example 1 uses the same B-added high-carbon steel as in the present invention and is the same up to the initial holding (Idling) before finish rolling (FM) after rough rolling but after hot rolling without quenching in ROT after finishing rolling , And cooled to the coiling temperature at a normal cooling rate.

In Comparative Example 2, the same steel as in the present invention was used, and a state in which no holding (Idling) process was performed before finishing rolling was simulated. In Comparative Example 2, quenching was performed in the ROT process, but since the initial structure before the start of the ROT section was full austenite free from pro-eutectoid ferrite, there was a fear that the martensite phase was excessively generated by quenching and plate breakage occurred. Therefore, the coiling temperature can not be lowered to the M _s temperature or lower, and only the cooling to the bainite forming temperature is carried out and the coiling is carried out.

Comparative Example 3 is an experiment in which the same heat treatment condition as that of the inventive example is used, and the case where the steel specified in the present invention is not used is simulated.

Comparative Example 4 is a general QT heat treatment of S50C steel to secure a microstructure containing a large amount of martensite phase in a similar steel type.

Here, the temperature measurement except for Comparative Example 4 uses surface temperature facilities of the hot rolling mill, and the surface temperature is indicated. The cooling rate is, in the case of Comparative Example 1, the difference between the finish rolling finish temperature measured immediately after finish rolling and the coiling temperature ROT average passing time. In the case of Inventive Example, Comparative Example 2, and Comparative Example 3, the difference between the finish rolling finish temperature measured immediately after finish rolling and the temperature measured in the intermediate thermometer was calculated as an average passing time Respectively.

Tensile tests were conducted on the specimens of the inventive and comparative examples manufactured under the above-mentioned process conditions, and the results are shown in Table 3 and FIG.

division Tensile Strength (MPa) Yield strength (MPa) Elongation (%) Honor 1127 985 9.5 Comparative Example 1 532 462 20.7 Comparative Example 2 736 655 15.1 Comparative Example 3 828 730 11.3 Comparative Example 4 1152 1069 4.5

Referring to Table 3 and FIG. 2, in the case of the invention example, retained austenite is secured to exhibit a tensile strength of 900 to 1200 MPa, an elongation of 8% or more, and an excellent combination of strength and ductility. In the case of the inventive example, the microstructure showed an area fraction of protonic ferrite of 20 to 30%, martensite of about 50 to 60% and retained austenite of 10 to 20%.

In the case of Comparative Example 1, the formation of pro-eutectoid ferrite was normally performed, but the rate of cooling in the hot rolling ROT process was relatively slow, so that a large amount of pearlite was produced in the steel, so that carbon in the steel was consumed for forming carbide, thereby making it difficult to generate retained austenite. The combination of strength and elongation is small.

In the case of Comparative Example 2, pro-eutectoid ferrite was not produced because it was not held (Idling) before finish rolling, and a low-temperature transformation structure was formed by quenching. In the case of forming a large amount of martensite by quenching immediately in the state of full austenite without pro-eutectoid ferrite, the plate breakage due to the edge crack was a concern, and in the case of Comparative Example 2, it was wound at a bainite producing temperature. Most phases consist of bainite and pearlite produced in some cooling processes, and the strength range is beyond the scope of the present invention.

In the case of Comparative Example 3, a general S50C steel other than the B-added high carbon steel was used, which is a result of manufacturing under process conditions similar to those of the present invention. It can be said that the formation of pro-eutectoid ferrite is excessive as compared with that of the conventional art and the formation of pearlite in the ROT cooling process and the carbon is consumed as a carbide due to the low hardenability due to the characteristics of the steel itself.

In the case of Comparative Example 4, S50C was subjected to general QT heat treatment, which shows that the strength is higher than that of the present invention, but the elongation is very low.

While the present invention has been described in connection with certain exemplary embodiments and drawings, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be understood that various modifications and changes may be made without departing from the scope of the appended claims.

Claims (3)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.15 to 0.55% of C, 0.3 to 1.0% of Si, 1.5% or less of Mn, 0.03 to 0.3% of Cr, 0.0008 to 0.0060% of B, 0.01 to 0.03% 0.02% or less of S, the balance Fe and other unavoidable impurities, and the microstructure is composed of residual austenite having an area fraction of 3 to 25% and the remainder cornerstone ferrite and martensite composite structure. The method according to claim 1,
The hot-rolled steel sheet has a tensile strength of 900 to 1200 MPa and an elongation of 8% or more, and is excellent in strength and ductility. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.15 to 0.55% of C, 0.3 to 1.0% of Si, 1.5% or less of Mn, 0.03 to 0.3% of Cr, 0.0008 to 0.0060% of B, 0.01 to 0.03% S: not more than 0.02%, and the balance Fe and other unavoidable impurities to a temperature of A _f or more, followed by rough rolling;
An intercritical annealing step of forming a pro-eutectoid ferrite by maintaining the steel material at a temperature higher than the A ₁ temperature and lower than the ferrite formation temperature for 20 to 200 seconds;
Finishing the steel material;
Quenching the steel material at a cooling rate of 80 ° C / sec or more;
Winding the steel material at a temperature of M _f to M _s to produce martensite from the retained austenite;
Maintaining the cooling rate of the steel at a rate of 1 DEG C / min or less to accumulate carbon in the final retained austenite; And
And cooling the steel material to a normal temperature.

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