KR20150046927A - Ultrahigh strength hot rolled steel sheet having excellent impact resistant property and manufacturing mehtod the same - Google Patents

Abstract

The present invention relates to an ultrahigh strength hot rolled steel sheet having excellent impact resistant properties and a manufacturing method thereof which can secure high yield strength and excellent impact toughness at the same time by controlling an alloying element and a microstructure distribution, and by precisely controlling a cooling speed for a manufacturing process. The ultrahigh strength hot rolled steel sheet having excellent impact resistant property comprises: 0.12-0.16 wt% of C; less than or equal to 0.4 wt% of Si (except 0); 1.5-2.1 wt% of Mn; less than or equal to 0.5 wt% of Cr (except 0); less than or equal to 0.3 wt% of Mo (except 0); 0.01-0.05 wt% of Al; less than or equal to 0.003 wt% (except 0); 0.005-0.05 wt% of Ti; less than or equal to 0.01 wt% (except 0); less than or equal to 0.02 wt% (except 0); less than or equal to 0.005 wt% (except 0); and the rest of Fe and other unavoidable impurities. The microstructure comprises: greater than or equal to 95 surface integral% of bainite and martensite; and less than 50 surface integral% of the martensite, including the rest of proeutectoid ferrite and the remaining austenite.

Description

TECHNICAL FIELD [0001] The present invention relates to an ultra-high strength hot-rolled steel sheet having excellent impact properties and a method of manufacturing the same. [0002]

The present invention relates to an ultrahigh-strength hot-rolled steel sheet excellent in impact properties and a method of manufacturing the same.

Ultra-high strength hot-rolled steel sheet has been applied to a variety of uses including crane, concrete pump truck, and boom boom. The thickness of the steel sheet used for the above-mentioned purposes is generally in the range of 3 to 10 mm. In the case of super-high strength hot-rolled steel sheet as a backing material compared to a steel sheet for automobiles, in addition to high yield strength for supporting the design load, High impact properties are required. In particular, heavy equipment used in cold regions requires high impact characteristics even at low temperatures.

However, the conventional high-strength hot-rolled steel sheet has been studied mainly for the purpose of controlling the alloying elements in steel or controlling the manufacturing conditions to secure a high yield strength.

Japanese Patent Application Laid-Open No. 2003-089848 (entitled "Ultra-high tensile strength steel sheet excellent in workability, method for manufacturing the same, and processing method thereof") as one method for securing a high yield strength by controlling an alloy element (Patent Document 1) A method of securing a tensile strength of 950 MPa or more by dispersing and precipitating a precipitate containing Ti and Mo in a dispersion medium. However, this method not only increases the production cost by adding a high-priced alloy component but also fails to secure the impact resistance characteristics required for the after-hot-rolled steel sheet.

Japanese Patent Application Laid-Open No. 2003-321737 (entitled: High-tensile hot-rolled steel sheet excellent in workability, method for manufacturing the same, and method for processing the same) as another method for securing a high yield strength by controlling an alloy element (Patent Document 2) A method of manufacturing a high strength hot-rolled steel sheet using ferrite and martensite dual phase steel (DP) is disclosed. However, since the step-cooling technique is used, it is difficult to apply to the hot-rolled steel which is a post-product, and the manufacturing cost is also increased by adding an expensive alloy component. Further, since the yield ratio is low due to the characteristics of the composite structure steel, an excessively high tensile strength and a large amount of alloying elements are required to satisfy the desired yield strength.

On the other hand, Japanese Patent Application Laid-Open No. 2003-105446 (entitled "High Strength Hot-Rolled Steel Sheet and Method of Manufacturing") (Patent Document 3) discloses a method for securing a high yield strength by controlling the manufacturing conditions And controlling the cooling rate to 150 to 350 DEG C / sec. However, in the case of producing martensite by cooling at a too fast cooling rate, it is difficult to obtain a high yield strength due to a low yield ratio, and a high tensile strength is required to meet the yield strength standard, There is a problem.

As another method of securing a high yield strength by controlling the manufacturing conditions, a method of winding a high strength steel sheet having excellent stretch flangeability and a manufacturing method thereof (Japanese Patent Application Laid-Open No. 2000-109951) And the temperature is controlled at 300 to 550 占 폚. However, in the case of winding at a temperature of 300 DEG C or higher, the bainite structure is close to the low-aspect ratio isotropic crystal, which is advantageous in terms of formability but low in impact resistance and requires a large amount of alloy element addition as the coiling temperature is increased. .

In the super high strength hot-rolled steel sheet as described above, most conventional techniques (Patent Documents 1 to 4) are mainly used for solid solution strengthening by alloying elements such as C, Si, Mn, Cr, Mo and W and alloys such as alloys of Ti, High strength is achieved by strengthening precipitation strengthened by the component, but the problem of impact toughness dullness is not solved. Therefore, in the case of a high strength hot-rolled steel sheet used for a specific commercial vehicle sub-frame or a heavy equipment arm, high strength and excellent impact toughness are required. It is necessary to develop a new ultra high strength hot rolled steel sheet that meets the needs of this industry.

Japanese Laid-Open Patent Publication No. 2003-089848: Ultra-high tensile steel sheet having excellent workability, a method for manufacturing the same, and a processing method thereof (Applicant: NKK CORP), 2003. 03. 28. Open Japanese Laid-Open Patent Publication No. 2003-321737: High-strength hot-rolled steel sheet excellent in workability, method of manufacturing the same, and method of processing the same (Applicant: JFE STEEL KK) Japanese Patent Application Laid-Open No. 2003-105446: High Strength Hot-Rolled Steel Sheet and Method for Manufacturing the Same (Applicant: NKK CORP), 2003. 04. 09. Open Japanese Patent Application Laid-Open No. 2000-109951: High-strength hot-rolled steel sheet excellent in elongation flangeability and method of manufacturing the same (Applicant: KAWASAKI STEEL CORP), 2000. 04. 18. Disclosed

An aspect of the present invention provides an ultrahigh-strength hot-rolled steel sheet capable of simultaneously securing a high yield strength and an excellent impact toughness by controlling the distribution of alloying elements and microstructure and precisely controlling the cooling rate in the manufacturing process, and a method for manufacturing the same. There is a main purpose in this.

In order to achieve the above object, the present invention provides a super high strength hot-rolled steel sheet having excellent impact properties, comprising 0.12 to 0.16% of C, 0.4% or less of Si (excluding 0), 1.5 to 2.1% of Mn, : Not more than 0.5% (excluding 0), Mo: not more than 0.3% (excluding 0), Al: 0.01 to 0.05%, B: not more than 0.003% (Excluding 0), P: not more than 0.02% (excluding 0), S: not more than 0.005% (excluding 0), the remainder Fe and other unavoidable impurities, and the microstructure includes a bay having a lath structure The area fraction of the nite and martensite phase is 95% or more, and the area fraction of the martensite phase is less than 50%, and the remainder contains europaeic ferrite or retained austenite.

In order to further improve the impact characteristics of the present invention, it is preferable to further contain Ca in an amount of 0.0005 to 0.005% by weight, and the Ca / S ratio is preferably 2.0 or less.

The aspect ratio of the major axis to the minor axis of the bastite and the lance structure of martensite is preferably 5.0 or more.

Further, it is preferable that the lath structure of the bainite and the martensite is 0.2 to 1.2 per one square micrometer (탆 ² ).

The yield strength of the hot-rolled steel sheet is preferably 1100 to 1300 MPa, the yield ratio is 0.80 or more, and the Charpy impact absorption energy is preferably 50 J or more at a temperature of -40 캜.

In order to achieve the above object, the present invention provides a method of manufacturing an ultra-high strength hot-rolled steel sheet having excellent impact properties, comprising 0.12 to 0.16% of C, 0.4% or less of Si (excluding 0) (Excluding 0), Mo: not more than 0.3% (excluding 0), Al: 0.01 to 0.05%, B: not more than 0.003% (excluding 0), Ti: 0.005 to 0.05% , N: not more than 0.01% (excluding 0), P: not more than 0.02% (excluding 0), S: not more than 0.005% (excluding 0), the balance Fe and other unavoidable impurities; Subjecting the reheated slab to a hot finish rolling at a temperature of 800 to 1000 占 폚; Cooling the hot-rolled steel sheet subjected to the hot-rolling to a cooling rate satisfying the following equation (1); And winding the cooled hot-rolled steel sheet at a temperature of 200 DEG C or less.

[Equation 1]

Cond1? Cooling rate (占 폚 / sec) <100,

Cond1 = 450 + 280 x C - 500 x (C ^ 0.5) - 100 x Mn - 75 x Cr - 150 x Mo - 90 x Beff or 10.

However, Beff is 1 when 0.0008% <B <0.003%, 0 otherwise

C, Mn, Cr, and Mo are the weight percent of the alloy element, respectively.

In order to further improve the impact characteristics of the present invention, it is preferable to further contain Ca in an amount of 0.0005 to 0.005% by weight and control the Ca / S ratio to be 2.0 or less.

In the reheating step, the reheating temperature is preferably controlled to 1100 to 1300 ° C.

In addition, it is preferable that the winding temperature is controlled to 150 캜 or less.

According to the ultra-high strength hot-rolled steel sheet having excellent impact properties according to the present invention and the method of manufacturing the same, the alloy element and its content, the final microstructure, the process conditions and the like contained in the steel material are appropriately controlled, And provides a hot-rolled steel sheet having excellent strength and impact properties.

The use of an ultrahigh-strength hot-rolled steel sheet excellent in impact characteristics not only achieves weight reduction according to high strength of the material but also improves work stability even under harsh conditions.

Particularly, a high impact property at low temperature can be suitably used as a material for a crane used for cold regions, boom rocks for specially equipped vehicles such as concrete pump trucks, and the like.

1 is an optical microscope photograph of microstructure of Inventive Example 1 of the present invention.
2 is a SEM photograph of the microstructure of Inventive Example 2 of the present invention.
3 is an optical microscope photograph of the microstructure of Comparative Example 3. Fig.

Hereinafter, the technical construction according to the present invention will be described in more detail with reference to various embodiments and attached drawings.

In general, impact strength tends to deteriorate if attempts are made to achieve high strength of the steel through solid solution strengthening or precipitation strengthening. In order to solve this problem, the present inventors appropriately controlled the content of alloying elements, microstructure, manufacturing conditions and the like, and in particular by deriving a component relation formula for ensuring impact toughness, the inventors have found that even if a very high yield strength of 1100 MPa or more is secured, It is possible to manufacture a hot-rolled steel sheet having a high Charpy impact absorption energy of 50 J or more at a low temperature, leading to the present invention.

The composition of the ultra high strength hot-rolled steel sheet having excellent impact properties, which is one aspect of the present invention, is 0.12 to 0.16% of C, 0.4 to 2.0% of Si (excluding 0), 1.5 to 2.1% of Mn, : Not more than 0.5% (excluding 0), Mo: not more than 0.3% (excluding 0), Al: 0.01 to 0.05%, B: not more than 0.003% (Excluding 0), P: not more than 0.02% (excluding 0), S: not more than 0.005% (excluding 0), the balance Fe and other unavoidable impurities. Hereinafter, the critical significance of the characteristics and the composition range of each alloy element will be briefly described.

(1) Carbon (C): 0.12 to 0.16 wt%

C is an element necessary for securing the strength of the hot-rolled material. In order to obtain a yield strength of 1100 MPa or more in the bainite-martensite structure, it is necessary to add 0.12 wt% or more of carbon. On the other hand, if the content exceeds 0.16% by weight, the yield strength of 1300 MPa or more can be obtained while the fraction of martensite increases, but the impact absorption energy is lowered and the formability and weldability are deteriorated.

(2) Silicon (Si): 0.4% by weight or less (excluding 0% by weight)

Silicon (Si) is an element to be added together with Al for deoxidation. When Si is excessively added, a complete scale is generated and stabilization of ferrite may increase the material deviation. Therefore, the upper limit is limited to 0.4 wt% desirable.

(3) manganese (Mn): 1.5 to 2.1 wt%

Manganese (Mn) increases the hardenability and prevents the formation of weak ferrite structure during cooling. It also contributes to the strength through strengthening of harden. If the content of Mn is too low to less than 1.5% by weight, the cooling rate required for obtaining the desired bainite-martensite phase must be too fast, so that sufficient strength can not be obtained or the plate shape is lowered. On the other hand, when the content of manganese exceeds 2.1% by weight, the amount of segregation of Mn increases and the quality of the steel is deteriorated.

(4) Cr (Cr): 0.5% by weight or less (excluding 0% by weight)

Chromium (Cr) increases the hardenability and contributes to securing strength on bainite-martensite. Cr has similar properties to Mn but causes weld brittleness, so the upper limit of Cr is preferably limited to 0.5% by weight.

(5) Molybdenum (Mo): 0.3% by weight or less (excluding 0% by weight)

When Ti is added together with Ti, (TiMo) C is formed and contributes greatly to precipitation strengthening. In addition, when Mo remaining in the carbide is left in the steel, it is a useful component for enhancing the yield strength by strengthening the solid solution and improving the impact toughness by strengthening the grain boundaries. However, if it exceeds 0.3% by weight, the above effect is saturated, which not only raises the manufacturing cost but also lowers the weldability.

(6) Aluminum (Al): 0.01 to 0.05 wt%

Aluminum (Al) is an element added for deoxidation and acts as a deoxidizer in the steelmaking process. When Al is less than 0.01% by weight, the deoxidation effect becomes too small. On the other hand, if it exceeds 0.05% by weight, clogging of the nozzle may occur during playing, so it is preferable to limit the upper limit to 0.05%.

(7) Boron (B): 0.003 wt% or less (excluding 0),

Boron (B) is an element contributing greatly to securing the hardenability of steel. In order to exhibit the above-mentioned effect, it is preferable to contain 0.0008 wt% or more. However, when the necessary curing ability is secured by elements such as C, Mn, Cr, and Mo, Further, when boron is added in a large amount, boron carbide is formed in the grain boundaries to provide a nucleation site, which may deteriorate the hardenability. Therefore, it is preferable to limit the upper limit to 0.003% by weight.

(8) Titanium (Ti): 0.005 to 0.05 wt%

Titanium (Ti) is an element added for the so-called boron protection which inhibits the formation of BN by reacting with nitrogen (N) to form TiN. If the content of Ti is less than 0.005% by weight, nitrogen may not be effectively fixed in the steel. On the other hand, when the amount of Ti is excessively large, there is a possibility that the steel is weakened due to coarsening of the formed TiN. 0.05% by weight.

(9) Nitrogen (N): 0.01 wt% or less (excluding 0 wt%)

Nitrogen (N) is an element that contributes to the hardness of the steel but is difficult to control. If the content of N exceeds 0.01% by weight, the risk of brittleness is greatly increased. In addition, there is a possibility that excess N remaining after forming TiN may consume B, which should contribute to hardenability, in the form of BN. Is limited to 0.01% by weight.

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, since they can be known by any ordinary person skilled in the art. However, since phosphorus (P) and sulfur (S) are generally referred to as impurities, they will be briefly described as follows.

(10) phosphorus (P): 0.02% by weight or less (excluding 0% by weight)

Phosphorus (P) is an impurity which is inevitably contained. It is mainly segregated at the center of the steel sheet to decrease the toughness and significantly reduce the weldability. Therefore, it is preferable to control the phosphorus content as low as possible in order to ensure low temperature impact toughness at the center of the post material. In theory, it is advantageous to limit the content of P to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the P content is preferably limited to 0.02 wt%.

(11) Sulfur (S): 0.005% by weight or less (excluding 0% by weight)

Sulfur (S) is an impurity which is inevitably contained, and forms a nonmetallic inclusion by binding with Mn or the like, thereby greatly impairing ductility, impact toughness and weldability of steel. Therefore, it is desirable to suppress the content to the maximum. Theoretically, the content of S is limited to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the S content is preferably limited to 0.005% by weight.

In addition, when the steel material of the present invention is additionally added with the calcium (Ca) element described below, the effect of increasing the impact characteristics can be further improved.

(12) Calcium (Ca): 0.0005 to 0.005 wt%

When calcium (Ca) is added, CaS which retains the shape of a spherical shape is formed instead of MnS which is elongated in the rolling to deteriorate the quality of the steel, so that the molding property and the impact resistance property of the steel can be improved. In order to exhibit the above-mentioned effect, it is preferable that it contains 0.0005 wt% or more. If the content of Ca is excessive, the size and the fraction of CaS and CaO are increased to deteriorate the integrity of the steel. Therefore, the upper limit is limited to 0.005 wt%.

It is preferable that the content of calcium (Ca) and sulfur (S) is controlled such that the Ca / S ratio is 2.0 or less. When the Ca / S ratio exceeds 2.0, coarse precipitation of CaS or distribution of the CaS in close contact with each other results in deterioration of the cleanliness of the steel.

Next, in order to ensure excellent impact properties, the ultrahigh-strength hot-rolled steel sheet of the present invention needs to satisfy the following conditions in addition to the above alloy composition, the kind of microstructure and the phase fraction.

That is, the microstructure of the hot-rolled steel sheet according to the present invention has an areal percentage of bainite having a lath structure and a martensite phase fraction of 95% or more, wherein the area fraction of the martensite phase is less than 50% , The average aspect ratio of the lath structure is 5.0 or more, the number of lathes per square micrometer (占 퐉 ² ) in the microstructure is in the range of 0.2 to 1.2, and the remainder is pro-eutectoid ferrite or retained austenite structure.

More specifically, the bainite and the martensite structure of the present invention have a lath structure in the form of a needle bed as a substructure. However, the bainite of the present invention has a long rod-like cementite Cementite. That is, not a rounded cementite formed at the boundary of the lath by diffusion during tempering, but carbon is accumulated between the lattices of the bainitic ferrite during the phase transformation to form a long rod-shaped bainitic cementite Organization.

If the area fraction of bainite and martensite having lath structure in the entire microstructure is 95% or more, high strength can be obtained. This is because, when a pro-eutectoid ferrite structure is formed, the strength is significantly deteriorated, and when the retained austenite structure is formed, the yield strength is lowered. Therefore, the fraction of pro-eutectoid ferrite and the retained austenite structure is preferably controlled to be less than 5% in total.

When the transformation from the austenite phase is performed, the transformation ratio of the bainite or martensite having a lase structure is lowered at a lower temperature, the ratio of the major axis to the minor axis of the lase becomes larger. In the hot rolling process, It was observed that the ratio of the aspect ratio of the lattice structure was 5.0 or more when the coiling temperature was low or the coiling temperature was low, and both the yield strength and the impact characteristics were equal to or higher than 1100 MPa. On the other hand, when the cooling rate of the phase transformation is slow or the coiling temperature is high, microstructure is formed closer to the equiaxed crystal with aspect ratio of 1.0 and it is difficult to obtain a high yield strength of 1100 MPa or more when the aspect ratio of the lattice structure is lower than 5.0 .

By controlling the needle-like structure so that the aspect ratio of the bainite and the martensite lattice structure is 5.0 or more and the ratio of the structure having the average aspect ratio of the lath structure of 5.0 or more to be 95% or more, Or more of high yield strength. As the lath structure develops in the form of a needle-like shape, the number of lathes in the lath is increased in a packet, and the impact characteristic of the steel is increased by such a dense structure. Also, as the barrier of dislocation movement increases, the strength also increases.

However, if the ratio of the martensite phase exceeds 50%, the density of the moving dislocations becomes high and the yield ratio lowers to less than 0.8. In order to obtain the required yield strength of 1100 MPa or more, too much alloy is required and high tensile strength is required, do.

Further, it is preferable that the number of lasers per square micrometer (mu m &lt; ² &gt;) in the microstructure is in the range of 0.2 to 1.2. When the number of laths is less than 0.2, it is difficult to obtain a high strength in the case where the structure is coarsened. When the number of laths is larger than 1.2, the cooling rate is excessively fast within the range of the present invention. The dislocation density in the microstructure becomes higher, especially the density of the initial movable dislocation, so that the yield ratio is lowered to less than 0.8 and it becomes difficult to obtain a yield strength of 1100 MPa or more.

The hot-rolled steel sheet having the composition range of the alloy element and the microstructure described above has a high yield strength of 1100 MPa or more and less than 1300 MPa, and can have a high yield ratio of 0.80 or more. At the same time, since it has a high Charpy impact absorption energy of 50J or more at a low temperature of -40 ° C, excellent shock characteristics can be exhibited.

Hereinafter, a method for manufacturing an ultrahigh-strength hot-rolled steel sheet having an excellent impact characteristic, which is another aspect of the present invention, will be described in detail.

A method of manufacturing an ultra-high strength hot-rolled steel sheet excellent in impact properties according to the present invention is characterized by comprising 0.12 to 0.16% of C, 0.4 to 2% of Si (excluding 0), 1.5 to 2.1% of Mn, 0.5% (Excluding 0), Mo: not more than 0.3% (excluding 0), Al: 0.01 to 0.05%, B: 0.0005 to 0.005%, Ti: 0.005 to 0.05% P: not more than 0.02% (excluding 0), S: not more than 0.005% (excluding 0), the balance Fe and other unavoidable impurities; Subjecting the reheated slab to a hot finish rolling at a temperature of 800 to 1000 占 폚; Cooling the hot-rolled steel sheet subjected to the hot-rolling to a cooling rate satisfying the following equation (1); And winding the cooled hot-rolled steel sheet at a temperature of 200 DEG C or less.

[Equation 1]

Cond1? Cooling rate (占 폚 / sec) <100,

Cond1 = 450 + 280 x C - 500 x (C ^ 0.5) - 100 x Mn - 75 x Cr - 150 x Mo - 90 x Beff or 10.

However, Beff is 1 when 0.0008% <B <0.003%, 0 otherwise

C, Mn, Cr, and Mo are the weight percent of the alloy element, respectively.

(1) Reheating step

The step of reheating the slab satisfying the composition of the above-described alloying elements and the composition range thereof is a step of heating the steel in order to smoothly perform the subsequent rolling process and sufficiently obtain the physical properties of the target steel sheet. Therefore, the heating process should be performed within an appropriate temperature range in accordance with the purpose.

Accordingly, in the present invention, it is preferable to reheat the slab satisfying the above-mentioned component system at 1100 to 1300 占 폚. When the reheating temperature is less than 1100 ° C, the hot rolling load increases sharply. On the other hand, when the reheating temperature is higher than 1300 ° C, the amount of surface scale increases, leading to loss of material and heating cost. Considering this point, the reheating temperature of the slab is preferably limited to 1100 to 1300 ° C.

(2) Hot rolling step

As described above, the hot-rolled slab can be subjected to hot rolling. At this time, the hot finish rolling is preferably performed at 800 to 1000 占 폚. If the hot rolling temperature is less than 800 ° C, the rolling load increases greatly. On the other hand, if the temperature exceeds 1000 ° C, the structure of the steel sheet becomes coarse, the steel becomes weak, the scale becomes thick, The quality is deteriorated.

(3) Cooling step

The hot-rolled steel sheet is cooled from the finish hot rolling temperature in a ROT (Run Out Table) until the coiling temperature is reached.

At this time, it is preferable that the cooling rate of the hot-rolled steel sheet satisfies the following [Expression 1].

[Equation 1]

Cond1? Cooling rate (占 폚 / sec) <100,

Cond1 = 450 + 280 x C - 500 x (C ^ 0.5) - 100 x Mn - 75 x Cr - 150 x Mo - 90 x Beff or 10.

However, Beff is 1 when 0.0008% <B <0.003%, 0 otherwise

Here, C, Mn, Cr, and Mo represent weight percentages of the alloying elements, respectively.

As described in the above-mentioned [Expression 1], the cooling rate is preferably controlled to be in the range of less than 100 ° C / sec to Cond1 or more. When the cooling rate is less than Cond1, pro-eutectoid ferrite is formed at an upper limit value or more during cooling or a lattice structure having an aspect ratio of less than 5.0 is formed, so that the strength required by the present invention can not be obtained. On the other hand, when cooling is carried out at 100 ° C / sec or more, the plate shape is largely lowered, the fraction of retained austenite phase is increased, the moving dislocation density in martensite is increased, and the yield strength is lowered.

(4) Coiling step

It is preferable that the cooled steel sheet is rolled up into a rolled coil by passing through the water cooled stand (ROT). At this time, the coiling temperature is preferably controlled to 200 占 폚 or less. If the coiling temperature exceeds 200 DEG C, the material is deteriorated due to the tempering effect in the holding step after winding, and the possibility of forming a high-temperature-transformation bainite structure in the cooling process is increased. The lower limit of the coiling temperature may be regarded as being equal to the lower limit of the atmospheric temperature, and the coiling temperature is not particularly affected.

Further, in order to obtain a bainite and martensite structure having a glass structure required in the present invention by suppressing the change of the microstructure as much as possible in the winding step, it is more preferable to control the coiling temperature to 150 캜 or less.

(Example)

Hereinafter, the present invention will be described in more detail with reference to specific 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.

A steel satisfying the component system described in the following [Table 1] was continuously cast to prepare a slab. The prepared slab was reheated at 1200 DEG C for 1 hour or more. Thereafter, the reheated slab was subjected to finish hot rolling at 900 ° C to produce a hot-rolled steel sheet having a thickness of 4 to 6 mm.

After the hot finish rolling, the steel sheets were cooled from the water cooling stand (ROT) to the coiling temperature (CT) at a cooling rate determined by the following equation (1). Thereafter, winding was performed at each target winding temperature.

The microstructure of the final hot-rolled steel sheet obtained by completing the winding process was analyzed, and tensile and impact materials were measured and shown in Table 2 below. The sub-size V-notch Charpy impact, which is polished to 5 mm for a 6 mm thick thermal laminate, to 3.3 mm for a 5 mm thick thermal laminate and to 2.5 mm for a 4 mm thick thermal laminate, Is converted into a value corresponding to a standard Charpy impact specimen size of 10 mm and is shown in Table 2 below.

Steel grade C Si Mn Cr Mo Al B Ti N P S Ca Remarks A 0.142 0.226 1.802 0.005 0.098 0.029 0.0015 0.023 0.0040 0.0093 0.0024 - Invention river B 0.131 0.003 1.623 0.302 0.058 0.018 0.0022 0.017 0.0035 0.0113 0.0033 - Invention river C 0.155 0.312 1.956 0.003 0.251 0.036 0.0027 0.033 0.0044 0.0132 0.0022 - Invention river D 0.152 0.278 1.912 0.002 0.248 0.032 0.0018 0.019 0.0033 0.0112 0.0027 0.0013 Invention river E 0.105 0.207 1.701 0.201 0.056 0.033 0.0010 0.019 0.0039 0.0105 0.0032 - Comparative steel F 0.125 0.202 1.840 0.105 0.115 0.025 0.0003 0.017 0.0033 0.0104 0.0027 - Comparative steel G 0.173 0.178 1.883 0.350 0.005 0.021 0.0021 0.025 0.0038 0.0065 0.0017 - Comparative steel

River
Bell thickness
(mm) Cond1 Cooling
speed Coiling
Temperature brother
Prize
ratio Lath
density
(Number / 탆 ²⁾ Bainite
Fraction
(%) Martensite
Fraction
(%) Foundation stone
Ferrite
Fraction
(%) surrender
burglar
(MPa) Seal
burglar
(MPa) -40 ° C
Charpy
Shock absorption energy (J) Inventory 1 A 6 16 50 93 5.3 0.75 57 41 0 1121 1335 62 Inventory 2 B 5 22 80 85 5.5 0.93 62 36 0 1125 1293 86 Inventory 3 C 4 10 40 83 5.6 1.05 54 43 0 1156 1393 57 Honorable 4 D 4 10 40 78 5.4 0.97 56 42 0 1137 1374 70 Comparative Example 1 E 4 34 70 75 4.6 0.36 76 22 2 1047 1204 76 Comparative Example 2 B 5 22 60 305 4.3 0.17 81 12 7 1023 1189 77 Comparative Example 3 F 6 99 50 83 3.5 0.09 22 21 57 875 1057 58 Comparative Example 4 G 4 10 70 77 5.8 1.45 14 79 0 1083 1457 46

(Except for the fraction shown in [Table 2], the remainder is composed of retained austenite)

In the case of Comparative Examples 1 and 4 using the comparative steels E and G in which the carbon (C) content did not satisfy the composition range of the present invention as shown in [Table 1] and [Table 2] And the winding speed) satisfy the present invention, Comparative Example 1 having a low carbon content has a low aspect ratio and thus has a shortened yield strength. In Comparative Example 4 in which the amount of carbon is high, the fraction of martensite is increased to 50% or more and the yield ratio is low Thus showing low yield strength and impact characteristics.

In the case of Comparative Example 3 in which the content of boron (B) is lower than the lower limit of the present invention, in Comparative Example 3, even when a cooling rate corresponding to the process conditions of the present invention is added, a hardening ferrite phase is formed during cooling The aspect ratio and the yield strength.

On the other hand, in the case of Comparative Example 2 in which the inventive steel B satisfying the present invention was used but the coiling temperature condition did not satisfy the present invention, a microstructure having a low aspect ratio was formed due to a high coiling temperature, Respectively.

On the other hand, in each of Examples 1, 2 and 3 obtained from invention steels A, B, and C satisfying the composition ranges and process conditions provided by the present invention, it was confirmed that both the yield strength and the impact characteristics were compatible have. The photographs of microstructures of Inventive Example 1 shown in Figs. 1 and 2 reveal that they have an acicular shape. By obtaining the acicular bainite and martensite structure in this manner, it is possible to provide a hot-rolled steel sheet excellent in strength and impact properties. On the other hand, FIG. 3 shows the microstructure observed in Comparative Example 3, in which the proportion of pro-eutectoid ferrite phase was formed to be considerably high and the strength was low.

On the other hand, Inventive Example 4 in which invention steel D further containing calcium (Ca) within the composition range provided by Inventive Steel C according to the present invention was produced under the same process conditions as Inventive Example 3, And showed excellent impact properties. This is because calcium (Ca) bonds with sulfur (S) to form CaS that retains spherical shape, thereby improving the moldability and impact resistance of the steel.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (11)

0.1% to 0.16% of C, 0.4% or less of Si (excluding 0), 1.5 to 2.1% of Mn, 0.5% or less of Cr (excluding 0) , Ti: 0.005 to 0.05%, N: 0.01% or less (excluding 0), P: 0.02% or less (excluding 0), S: 0.01 to 0.05% 0.005% or less (excluding 0), the balance Fe and other unavoidable impurities,
The microstructure has an area fraction of the bainite and martensite phase having a lath structure of 95% or more, an area fraction of the martensite phase of less than 50%, and an impact characteristic including the remainder of eucalyptus ferrite or retained austenite Excellent super high strength hot rolled steel sheet.
The method according to claim 1,
By weight, Ca: 0.0005 to 0.005%, and the content of S and Ca has a Ca / S ratio of 2.0 or less.
The method according to claim 1,
Wherein the bainite and the lath structure of martensite have an aspect ratio of major axis to minor axis of 5.0 or more.
The method of claim 3,
Wherein the lath structure of the bainite and the martensite has a ratio of 0.2 to 1.2 per one square micrometer (mu m &lt; ² &gt;).
The method according to claim 1,
Wherein the yield strength of the hot-rolled steel sheet is 1100 to 1300 MPa.
The method according to claim 1,
Wherein the yield ratio of the hot-rolled steel sheet is 0.80 or more.
The method according to claim 1,
Characterized in that the Charpy impact absorption energy of the hot-rolled steel sheet is 50 J or more at a temperature of -40 캜.
0.1% to 0.16% of C, 0.4% or less of Si (excluding 0), 1.5 to 2.1% of Mn, 0.5% or less of Cr (excluding 0) , Ti: 0.005 to 0.05%, N: 0.01% or less (excluding 0), P: 0.02% or less (excluding 0), S: 0.01 to 0.05% Not more than 0.005% (excluding 0), the balance Fe and other unavoidable impurities;
Subjecting the reheated slab to a hot finish rolling at a temperature of 800 to 1000 占 폚;
Cooling the hot-rolled steel sheet subjected to the hot-rolling to a cooling rate satisfying the following equation (1); And
And winding the cooled hot-rolled steel sheet at a temperature of 200 ° C or less.
[Equation 1]
Cond1? Cooling rate (占 폚 / sec) <100,
Cond1 = 450 + 280 x C - 500 x (C ^ 0.5) - 100 x Mn - 75 x Cr - 150 x Mo - 90 x Beff or 10.
However, Beff is 1 when 0.0008% <B <0.003% and 0 otherwise.
Here, C, Mn, Cr, and Mo are weight percent of the alloy element, respectively.
The method of claim 8,
By weight, Ca: 0.0005 to 0.005%, and the content of S and Ca has a Ca / S ratio of 2.0 or less.
The method of claim 8,
Wherein the reheating step controls the reheating temperature to 1100 to 1300 占 폚.
The method of claim 8,
Wherein the coiling step is controlled at a coiling temperature of 150 占 폚 or less.

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