KR101977488B1 - Hot rolled steel sheet with excellent expandability and method for manufacturing thereof - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel used for a chassis component of an automobile, and more particularly, to a hot rolled steel sheet for an all-steel tubular steel pipe having excellent ductility and a manufacturing method thereof.

Description

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

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel used for a chassis component of an automobile, and more particularly, to a hot rolled steel sheet for an all-steel tubular steel pipe having excellent ductility and a manufacturing method thereof.

Recently, the automobile industry has been increasing the adoption of high-strength steels that can secure fuel economy and collision safety at a relatively low cost in order to regulate fuel consumption for global environmental protection and to ensure passenger collision stability. This movement to lighten the same is applied to chassis components as well as chassis components.

Generally, the physical properties required for the steel for the body include strength, elongation for molding, and spot weldability required for assembly.

On the other hand, steel components for chassis components are required to have fatigue properties in order to secure the arc weldability and the durability of parts to be applied when assembling parts, in addition to the strength and elongation required for the molding due to the characteristics of parts.

Particularly, in parts such as CTBA (Coupled Torsion Beam Axle) of chassis components, a hollow pipe is molded and used to secure both rigidity and weight, and the strength of the material is also being increased for additional weight saving.

Since the pipe material is generally manufactured by means of electrical resistance welding, the roll forming property of the material at the time of casting, and the cold forming property after the pipe forming are very important along with the electric resistance weldability. Therefore, it is very important to secure the integrity of the welded part in the electrical resistance welding as the physical properties that these materials should possess. The reason for this is that most of the fractures are concentrated in the welded portion or the welded heat affected portion compared to the base metal due to the deformation during the forming of the welded steel pipe (electric resistance welded steel pipe).

In order to improve the roll-forming property when the material is formed, it is advantageous that the yield ratio of the material is as low as possible. When the material is a high-strength steel, the yield strength is high, so that when the yield ratio is high, springback is increased during roll forming, There is a difficult problem.

In order to finally perform cold forming using a pipe, it is necessary to secure the elongation of the material. In order to satisfy this requirement, a steel material having a low resistance and an excellent elongation is required. As a material capable of satisfying such characteristics, there is a representative type of low-resistance-type hot-rolled steel sheet called dual phase steel (DP steel).

The conventional resistance-to-restrained hot-rolled steel sheet is usually an ideal composite steel of ferrite-martensite, and exhibits continuous yielding behavior and low yield strength due to the movable potential introduced at the time of martensitic transformation, and has excellent elongation properties.

In order to secure such physical properties, conventionally, in order to secure a ferrite fraction at the time of cooling after the hot rolling, a component system containing a large amount of Si in the steel was controlled. However, when a pipe is manufactured by the electric resistance welding method, a large amount of Si oxide is generated in the molten portion, causing a defect called a penetrator in the welded portion. After the ferrite transformation, the ferrite is quenched to a martensite transformation starting temperature (Ms) or less to obtain martensite. If the residual phase is composed only of pure martensite, there is a problem that the strength decreases due to heat during welding. Particularly, the hardness decrease (? Hv) of the weld heat affected zone exceeds 30.

On the other hand, as a method for reducing the above-mentioned hardness decrease phenomenon, after the ferrite transformation, quenching to below the bainite transformation starting temperature (Bs) to obtain a pure bainite phase can reduce the hardness drop. However, There is a problem of deterioration.

Japanese Patent Application Laid-Open No. 2000-063955

One aspect of the present invention is to provide a method of manufacturing a high-strength hot-rolled steel sheet, which has a reduced strength of a weld heat affected zone (HAZ) formed at the time of electric resistance welding, A steel sheet and a manufacturing method thereof are provided.

One aspect of the present invention is a method of manufacturing a semiconductor device, comprising: 0.05 to 0.14% carbon, 0.1 to 1.0% silicon, 0.8 to 1.8% manganese, 0.001 to 0.02% phosphorus, 0.001 to 0.01% of sulfur (S), 0.001 to 0.095% of aluminum (Al), 0.3 to 1.0% of chromium (Cr), 0.01 to 0.05% of titanium (Ti), 0.025% of niobium (Nb) 0.03% or less of vanadium (V), 0.001 to 0.01% of nitrogen (N), the balance of Fe and other unavoidable impurities, wherein Mn and Si satisfy the following relational expression 1,

Wherein the microstructure comprises a mixture of martensite and a hard phase composed of a bainite phase with a ferrite phase as a matrix, and the martensite phase and the bainite phase are contained in a single grain of the total fraction (area fraction) Wherein a fraction of crystal grains in which a nitride phase is mixed is 50% or more, and a carbon distribution in the crystal grains satisfies the following relational expression (2).

[Relation 1]

4 < Mn / Si < 12

(Where Mn and Si mean the weight content of each element).

[Relation 2]

1.3 ≤ P _CB / P _CC ≤ 1.9

Where P _CB is the EPMA intensity measurement of carbon at 70% of the distance from the center of the grains where the martensite and bainite phases are mixed to the boundary of the grains within the hard phase, and P _CC is the center of the same grains Means the EPMA intensity measurement of the carbon at the point.)

According to another aspect of the present invention, there is provided a method of manufacturing a steel slab, comprising the steps of: reheating a steel slab satisfying the alloy composition and the above-described formula 1 at a temperature range of 1180 to 1300 占 폚; Subjecting the reheated steel slab to finish hot rolling at a temperature equal to or higher than Ar3 to produce a hot-rolled steel sheet; Cooling the hot-rolled steel sheet to a temperature range of 550 to 750 ° C at a cooling rate of 20 ° C / s or more; A secondary cooling step of cooling the material at a cooling rate of 0.05 to 2.0 占 폚 / s within a range satisfying the following relational expression (3) after the primary cooling; After the secondary cooling, tertiary cooling at a cooling rate of 20 ° C / s or higher to a temperature range of room temperature to 400 ° C; And a step of winding after the third cooling step, thereby providing a method of manufacturing a hot rolled steel sheet having excellent ductility.

[Relation 3]

T-ta | ≤ 2

(T = 251 + (109C) + (10.5Mn) + (22.7Cr) - (6.1Si) - (0.87Temp) + (0.00068Temp ^ 2) Temp is the secondary cooling intermediate temperature, which means the temperature midway between the start point of the secondary cooling and the end point, and each alloy component means the weight content.

Another aspect of the present invention provides a seamless steel pipe having excellent ductility produced by electrical resistance welding of the hot-rolled steel sheet described above.

According to the present invention, it is possible to provide a hot-rolled steel sheet having a tensile strength of 590 MPa or more and a low resistance.

In addition, since such a hot-rolled steel sheet has excellent ductility at the time of electrical resistance welding, there is no decrease in the hardness of the weld heat affected portion and no crack occurs at the welded portion or welded heat affected portion at the time of pipe- .

FIG. 1 is a photograph (a) showing the shape of a tissue occupying 50% of the total hard phase inner area ratio of Example 1 according to an embodiment of the present invention using an EPMA (Electro Probe X-ray Micro Analyzer) (B) the distribution of measured carbon (C) content by tissue section.
FIG. 2 is a photograph (a) and FIG. 2 (b) showing the shape of a tissue occupying 50% of the total hard phase internal area ratio of Comparative Example 7 according to one embodiment of the present invention by using an EPMA (Electro Probe X-ray Micro Analyzer) (B) the distribution of measured carbon (C) content by tissue section.

The inventors of the present invention have found that when the yield ratio is controlled to less than 0.8, the roll forming can be easily performed, and the electrical resistance weldability is excellent, and the strength of the welded heat affected portion is small, In order to fabricate 590MPa hot rolled steel sheet which does not occur and has excellent cold forming property, it is studied deeply.

As a result, it has been confirmed that a hot-rolled steel sheet having high strength and excellent ductility can be provided by forming a microstructure favorable to the above-mentioned physical properties by optimizing the alloy composition and the manufacturing conditions of the steel, .

Hereinafter, the present invention will be described in detail.

According to an aspect of the present invention, a hot rolled steel sheet having excellent ductility is characterized by comprising 0.05 to 0.14% carbon (C), 0.1 to 1.0% silicon (Si), 0.8 to 1.8% manganese (Mn) 0.001 to 0.02% of phosphorus, 0.001 to 0.01% of sulfur, 0.001 to 0.095% of aluminum (Al), 0.3 to 1.0% of chromium (Cr), 0.01 to 0.05% of titanium (Ti) (N): 0.025% or less, vanadium (V): 0.035% or less, and nitrogen (N): 0.001 to 0.01%.

Hereinafter, the reason why the alloy composition of the hot-rolled steel sheet provided in the present invention is limited as described above will be described in detail. At this time, unless otherwise stated, the content of each element is% by weight.

C: 0.05 to 0.14%

Carbon (C) is the most economical and effective element for strengthening the steel. As the amount of carbon (C) increases, the fraction of low-temperature transformation phase such as bainite and martensite increases in a composite structure steel composed of ferrite, bainite and martensite, The strength is improved.

In the present invention, when the content of C is less than 0.05%, formation of a low-temperature transformation phase during cooling after hot rolling is not easy and the strength at the target level can not be secured. On the other hand, if the content exceeds 0.14%, the strength excessively increases, and weldability, formability and toughness are deteriorated.

Therefore, in the present invention, the content of C is preferably controlled to 0.05 to 0.14%, more preferably 0.07 to 0.13%.

Si: 0.1 to 1.0%

Silicon (Si) deoxidizes molten steel, has a solid solution strengthening effect, and has an effect of promoting ferrite transformation during cooling after hot rolling as a ferrite stabilizing element. That is, it is an element effective for increasing the ferrite fraction constituting the base of ferrite, bainite and martensite composite steel.

If the content of Si is less than 0.1%, the effect of stabilizing ferrite is small and it is difficult to form the base structure into a ferrite structure. On the other hand, if the content exceeds 1.0%, a red color scale due to Si is formed on the surface of the steel sheet during hot rolling, which not only deteriorates the surface quality of the steel sheet but also deteriorates ductility and electrical resistance weldability.

Therefore, in the present invention, the content of Si is preferably controlled to 0.1 to 1.0%, more preferably 0.2 to 0.8%.

Mn: 0.8 to 1.8%

Manganese (Mn) is an effective element for strengthening the steel in the same manner as Si, and increases the hardenability of the steel to facilitate formation of bainite or martensite phase during cooling after hot rolling.

However, if the content is less than 0.8%, the above-mentioned effect can not be sufficiently obtained. On the other hand, when the content exceeds 1.8%, the ferrite transformation is delayed excessively and it is difficult to secure a proper fraction of the ferrite phase. In the casting process, the segregation portion at the center of thickness is greatly developed during the casting process, There is a problem that the weldability is deteriorated.

Therefore, in the present invention, the content of Mn is preferably controlled to 0.8 to 1.8%, more preferably 1.0 to 1.75%.

P: 0.001 to 0.02%

Phosphorus (P) is an impurity present in the steel. When the content exceeds 0.02%, the softness degradation due to micro segregation and the impact characteristics of the steel are impaired. However, in order to produce the P content of less than 0.001%, it takes a long time to perform the steelmaking operation, which results in a problem that productivity is greatly reduced. Therefore, in the present invention, it is preferable to control the P content to 0.001 to 0.02%.

S: 0.001 to 0.01%

Sulfur (S) is an impurity present in the steel. When the content exceeds 0.01%, it forms a nonmetallic inclusion by binding with Mn or the like, thereby significantly lowering the toughness of the steel. However, in order to produce the S content of less than 0.001%, it takes a long time to perform the steelmaking operation, resulting in a problem of low productivity. Therefore, in the present invention, it is preferable to control the content of S to 0.001 to 0.01%.

Al: 0.001 to 0.095%

Aluminum (Al) is a component mainly added for deoxidation and is an element effective for forming a ferrite phase during cooling after hot rolling as a ferrite stabilizing element.

If the content of Al is less than 0.001%, the effect of addition is insufficient, the above-mentioned effect can not be sufficiently obtained, and a long time is required for steelmaking. On the other hand, when the content exceeds 0.095%, generation of an Al-based oxide (for example, Al ₂ O ₃ ) having a relatively high melting point in electrical resistance welding is facilitated, so that stress is locally concentrated around the inclusion, There is a possibility that it becomes a factor.

Therefore, in the present invention, the content of Al is preferably controlled to 0.001 to 0.095%, and more preferably 0.003 to 0.095%.

Cr: 0.3 to 1.0%

Chromium (Cr) strengthens the steel and enhances the formation of martensite by delaying the ferrite phase transformation during cooling as in the case of Mn.

If the content of Cr is less than 0.3%, the above-mentioned effect can not be sufficiently obtained. On the other hand, when the content exceeds 1.0%, the ferrite transformation is excessively retarded, and the elongation is drastically lowered by increasing the fraction of the low-temperature transformation phase such as bainite or martensite phase more than necessary.

Therefore, in the present invention, the content of Cr is preferably controlled to 0.3 to 1.0%, more preferably 0.3 to 0.8%.

Ti: 0.01 to 0.05%

Titanium (Ti) forms a coarse precipitate by binding with nitrogen (N) at the time of playing. When reheating for hot rolling, some of the titanium remains in the material without being reused. The non-reusable precipitate has a high melting point So that it plays a role of suppressing grain growth of the weld heat affected zone. In addition, the re-heated Ti is minutely precipitated during the phase transformation process during the cooling process after the hot rolling, thereby greatly improving the strength of the steel.

In order to sufficiently obtain the above-mentioned effect, it is preferable to contain Ti at 0.01% or more. However, when the content exceeds 0.05%, the yield ratio of the steel increases due to the precipitates precipitated fine, which makes roll forming difficult at the time of casting.

Therefore, in the present invention, the content of Ti is preferably controlled to 0.01 to 0.05%.

Nb: 0.025% or less (excluding 0%)

Niobium (Nb) is an element that enhances strength by forming precipitates in the form of carbonitrides. Particularly precipitates precipitated during the phase transformation during the cooling process after hot rolling greatly improve the strength of the steel.

If the content of Nb is more than 0.025%, the yield ratio of the steel is greatly increased, which makes roll forming difficult during casting, which is not preferable. Therefore, in the present invention, it is preferable to control the content of Nb to 0.025% or less, and 0% is excluded.

V: 0.035% or less (excluding 0%)

Vanadium (V) plays a role in improving the strength by forming precipitates in the form of carbonitrides. Particularly precipitates precipitated during the phase transformation during the cooling process after hot rolling greatly improve the strength of the steel.

If the content of V exceeds 0.035%, the yield ratio of the steel is increased so that it is difficult to form the roll during casting. Therefore, in the present invention, it is preferable to control the V content to 0.035% or less, and 0% is excluded.

N: 0.001 to 0.01%

Nitrogen (N) is a typical solid solution strengthening element together with C, and coarse precipitates are formed together with Ti, Al and the like.

In general, the solubility strengthening effect of N is superior to C, but the toughness is greatly reduced as the amount of N increases in steel. Therefore, in the present invention, the upper limit of N is preferably limited to 0.01%. However, in order to produce the N content of less than 0.001%, it takes a long time to perform the steelmaking and the productivity is low.

Therefore, in the present invention, it is preferable to control the N content to 0.001 to 0.01%.

In the present invention, manganese (Mn) and silicon (Si) controlled by the above-mentioned content preferably satisfy the following relational expression (1).

[Relation 1]

4 < Mn / Si < 12

(Where Mn and Si mean the weight content of each element).

When the value of the relational expression 1 is 4 or less or 12 or more, Si oxide or Mn oxide is excessively generated in the welded portion when manufactured into a seamless steel pipe, which increases the incidence of penetrator defects. That is, the melting point of the oxide generated in the molten part during manufacture by the electropolishing steel pipe is increased, and the probability of remaining in the welded part during the compression discharge process is increased.

Therefore, in the present invention, it is preferable to satisfy the above-described content range and satisfy the above-mentioned relational expression 1. [

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.

It is preferable that the hot-rolled steel sheet of the present invention satisfying the above-described alloy composition and the relationship (1) contains a composite of a hard phase composed of martensite and bainite with a microstructure having a ferrite phase as a matrix.

At this time, it is preferable that the ferrite phase is contained in an area fraction of 60 to 85%. If the phase fraction of the ferrite is less than 60%, the elongation of the steel may drop sharply, whereas when it exceeds 85%, the fraction of the hard phase (bainite, martensite) decreases and the desired strength can be secured I will not.

It is preferable that the present invention includes crystal grains in which martensite phase and bainite phase are mixed, that is, crystal grains in which M phase and B phase exist in old austenite grains. It is more preferable that such crystal grains contain at least 50% of the total hard phase fraction (area fraction). Except for crystal grains in which the M phase and the B phase are mixed in the hard phase, the remainder is a martensite single phase and / or a bainite single phase structure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph showing a texture of an inventive steel according to an embodiment of the present invention, specifically, a crystal grain of a structure occupying 50% or more of the total hard phase internal area ratio and a carbon content As a result, it can be confirmed that there is a difference between the carbon content around the grain boundary and the carbon content in the central region of the crystal grains. This means that in the single grain in which the martensite phase and the bainite phase are mixed, a bainite phase is present at the center of the grain boundary and a martensite phase around the grain boundary.

Fig. 2 is a photograph of the texture of a comparative steel having a normal DP steel structure, that is, as a result of measuring the martensitic grains occupying 50% or more of the hard phase internal area ratio and the carbon content of the crystal grains. In contrast to the present invention, It can be confirmed that the carbon distribution to the center portion of the crystal grains is relatively uniform.

As described above, in the hot-rolled steel sheet of the present invention, the bainite phase is sufficiently differentiated from the conventional DP steel and a sufficient movable potential is introduced at the boundary between the hard phase and the ferrite phase to minimize the hardness drop in the weld heat affected zone And it is effective to secure the excellent ductility of the steel pipe by realizing the low resistance.

More specifically, in the hot-rolled steel sheet of the present invention, it is preferable that a single grain contains crystal grains in which a martensite phase and a bainite phase are mixed in an amount of 50% or more of the total fraction (area fraction) of the hard phase, It is preferable that the carbon distribution in the carbon nanotube satisfies the following relational expression (2).

When the carbon distribution represented by the following formula 2 is less than 1.3, it is impossible to achieve the aim of the present invention because a crystal phase in which a martensite and a bainite phase are mixed is not realized in a hard phase and a single phase martensite structure is formed. On the other hand, when the value exceeds 1.9, there is a problem that martensite is formed in the vicinity of the grain boundaries of the crystal grains, and the ferrite phase other than bainite is formed in the center region, resulting in drastically decreasing the waviness.

[Relation 2]

1.3 ≤ P _CB / P _CC ≤ 1.9

Where P _CB is the EPMA intensity measurement of carbon at 70% of the distance from the center of the grains where the martensite and bainite phases are mixed to the boundary of the grains within the hard phase, and P _CC is the center of the same grains Means the EPMA intensity measurement of the carbon at the point.)

As described above, the hot-rolled steel sheet of the present invention, which satisfies both the alloy composition, the relational expression 1 and the microstructure, has a tensile strength of 590 MPa or more and can secure a yield ratio (YR = YS / TS) of 0.8 or less.

Further, when the hot-rolled steel sheet of the present invention is pipe-routed, it is possible to secure the expansion ratio of the pipe to 85% or more of the elongation of the hot-rolled steel sheet.

Hereinafter, a method of manufacturing a hot-rolled steel sheet having excellent ductility in accordance with another aspect of the present invention will be described in detail.

Briefly, the present invention can produce a target hot-rolled steel sheet through a process of [steel slab reheating-hot rolling-first cooling-second cooling-third cooling-coiling], and the conditions of each step are described in detail below do.

[Reheating step]

First, it is preferable to prepare a steel slab satisfying the above-described alloy composition and the relationship 1 and then reheat the steel slab in a temperature range of 1180 to 1300 ° C.

If the reheating temperature is less than 1,180 占 폚, the heat of the slabs is insufficient and it is difficult to secure the temperature during the subsequent hot rolling, and diffusion of segregation generated during playing becomes insufficient. Further, precipitates precipitated at the time of playing can not be sufficiently reused, and it is difficult to obtain precipitation strengthening effect in the process after hot rolling. On the other hand, if the temperature exceeds 1300 DEG C, the strength is lowered due to abnormal grain growth of the austenite grains, and there is a problem that the unevenness of the structure is promoted.

Therefore, in the present invention, it is preferable that the steel slab is heated at a temperature of 1180 to 1300 ° C during reheating.

[Hot rolling step]

It is preferable to prepare the hot-rolled steel sheet by hot-rolling the reheated steel slab. At this time, it is preferable that the finish hot rolling is Ar3 (ferrite phase transformation start temperature) or higher.

If the temperature is lower than Ar3 during the final hot rolling, it is difficult to secure the desired structure and physical properties by performing the rolling after the ferrite transformation. On the other hand, when the temperature exceeds 1000 deg. C and the finish rolling is performed, There is a problem.

Therefore, in the present invention, it is preferable that the annealing is performed in a temperature range satisfying Ar3 to 1000 deg. C during the final hot rolling.

[Primary cooling step]

It is preferable to cool the hot-rolled steel sheet obtained by the above-described hot rolling, and it is preferable to perform the cooling stepwise.

First, the hot-rolled steel sheet is preferably subjected to primary cooling at a cooling rate of 20 ° C / s or higher to a temperature range of 550 to 750 ° C.

If the temperature at which the primary cooling is terminated is less than 550 占 폚, the steel microstructure mainly contains bainite phase, so that the ferrite phase can not be obtained as a base structure, so that sufficient elongation and low resistance can not be secured. On the other hand, when the temperature exceeds 750 ° C, coarse ferrite and pearlite structure are formed, and desired physical properties can not be secured.

Further, when cooling is carried out at a cooling rate of less than 20 캜 / s during cooling to the above-mentioned temperature range, ferrite and pearlite phase transformation occur during cooling, and a hard phase can not be secured to a desired level. The upper limit of the cooling rate is not particularly limited and can be appropriately selected in consideration of the cooling facility.

[Second cooling step]

It is preferable to cool the hot-rolled steel sheet after the primary cooling is completed under a specific condition in an extreme cold-weather zone. More specifically, it is preferable to cool to a very low temperature at a cooling rate of 0.05 to 2.0 캜 / s within a range satisfying the following relational expression (3).

[Relation 3]

T-ta | ≤ 2

(Wherein ta = 251 + (109C) + (10.5Mn) + (22.7Cr) - (6.1Si) - (0.87Temp) + (0.00068Temp ^ 2) , wherein t is the holding time of the secondary cooling, ta is best Temp is the secondary cooling intermediate temperature, which means the temperature midway between the start point of the secondary cooling and the end point, and each alloy component means the weight content.

The above-mentioned relational expression 3 is for obtaining a target microstructure, specifically, a microstructure satisfying the above-mentioned relational expression 2. Specifically, by optimizing the intermediate temperature (Temp) in the extreme extreme cold extinction and the retention time in the extreme extreme cold extinction It is possible to obtain not only a structure in which not less than 50% of the total hard phase fraction has a martensite phase and a bainite phase but also a carbon distribution of the structure satisfies the above-mentioned relational expression (2).

More specifically, when the austenite to ferrite phase transformation takes place during the primary cooling or the extreme cold weather holding time (secondary cooling), the carbon is diffused into the residual austenite where the intermediate temperature (Temp ) And the holding time, it is possible to raise the carbon concentration only at a portion adjacent to the ferrite. When the rear end cooling is started in this state, a part of the bainite is transformed into martensite due to the difference in carbon concentration, and a structure satisfying the relationship (2) can be obtained.

If the above-mentioned second cooling control is not satisfied, it is impossible to realize a structure in which martensite and bainite phases are mixed in the hard phase, and a general DP steel structure is formed, so that the hardness There is a problem that the decline is widening and the dilatancy is weakened.

When the cooling rate exceeds 2.0 캜 / s during the secondary cooling control, it is impossible to secure a sufficient time to form a carbon distribution in the structure in which the martensite and bainite phases in the hard phase are mixed. On the other hand, / s, the ferrite content is excessively increased and the target structure and physical properties can not be secured.

[Third Cooling Step]

It is preferable to carry out a third cooling step at a cooling rate of 20 DEG C / s or higher to a temperature range of room temperature to 400 DEG C after completion of the second cooling in the pole cold spots. Here, the normal temperature means a range of about 15 to 35 占 폚.

When the temperature at which the tertiary cooling is terminated exceeds 400 ° C, since the Ms (martensitic transformation starting temperature) is higher than the above temperature, most of the remaining untransformed phase is transformed into a bainite phase to obtain a microstructure satisfying the relational expression 2 of the present invention I will not.

If the cooling rate during the third cooling is less than 20 캜 / s, the bainite phase is excessively formed and the target physical properties and microstructure of the present invention can not be obtained.

The upper limit of the cooling rate is not particularly limited and can be appropriately selected in consideration of the cooling facility.

[Winding step]

It is preferable to carry out the step of winding the hot-rolled steel sheet, which has been subjected to the third cooling, at the temperature.

In the meantime, the present invention can further include a step of naturally cooling the rolled hot-rolled steel sheet to a temperature range of room temperature to 200 ° C, pickling the coated hot-rolled steel sheet to remove the surface layer scale, and then raising. At this time, if the steel sheet temperature before the pickling treatment exceeds 200 ° C, there is a problem that the surface layer portion of the hot-rolled steel sheet is overheated and the surface layer roughness becomes worse.

The present invention provides a seamless steel pipe produced by electric resistance welding of the hot-rolled steel sheet manufactured according to the above, and the above-mentioned seamless steel pipe has an excellent effect of expandability.

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 )

Steel slabs having the component systems shown in the following Table 1 were prepared, and then each steel slab was heated to 1250 占 폚 and then subjected to finish hot rolling (finish hot rolling temperature in Table 2) to produce a hot-rolled steel sheet having a thickness of 3 mmt. Thereafter, primary cooling (indicated by the cooling end temperature in Table 2) at a cooling rate of 80 캜 / s to 550 캜 to 750 캜, control cooling (secondary cooling) at the intermediate cold temperature and holding time shown in Table 2 below, And then subjected to tertiary cooling to a room temperature at a cooling rate of 60 DEG C / s, followed by reeling.

Each of the hot-rolled steel sheets manufactured in the above-mentioned manner was subjected to SEM photographing at a magnification of 3,000 times and the area fraction of each phase (ferrite: F, martensite: M, bainite: B) was analyzed by an image analyzer ). The distribution behavior of carbon (C) was measured by using EPMA line scanning technique at a grain size of 7,000 times at intervals of 20 to 35 nm in a crystal grain structure occupying 50% or more of the total percentage of the hard phase (Acc V : 15.0 kV, Probe C: 1.009E-007A).

JIS No. 5 specimens were prepared for each hot-rolled steel sheet, and tensile tests were carried out at room temperature at a strain rate of 10 mm / min.

Each pipe was hot-rolled using a hot-rolled steel sheet to obtain a 101.6Φ-diameter pipe, and then subjected to an expansion test based on the KS standard B ISO 8493 (metal material-pipe-tube-expansion test) standard or the equivalent standard. At this time, the expansion ratio of the pipe is shown in comparison with the elongation of the hot-rolled steel plate.

The results thus obtained are shown in Table 3 below.

division Alloy composition (% by weight) relation
Equation 1 C Si Mn P S Cr Ti Nb V Al N invent
River 1 0.09 0.25 1.35 0.02 0.003 0.5 0.01 0.01 0.01 0.022 0.004 5.4 invent
River 2 0.11 0.25 1.02 0.02 0.003 0.7 0.01 0.01 0.01 0.021 0.004 4.1 invent
River 3 0.09 0.25 1.55 0.01 0.003 0.3 0.01 0.01 0.01 0.009 0.003 6.2 invent
River 4 0.08 0.12 1.13 0.01 0.004 0.5 0.01 0.01 0.01 0.015 0.003 9.4 invent
River 5 0.08 0.33 1.72 0.01 0.004 0.5 0.01 0.01 0.01 0.092 0.004 5.2 invent
River 6 0.12 0.35 1.67 0.02 0.003 0.3 0.01 0.01 0.01 0.074 0.003 4.8 invent
River 7 0.07 0.29 1.38 0.01 0.003 0.7 0.01 0.01 0.01 0.011 0.005 4.8 invent
River 8 0.09 0.25 1.21 0.02 0.004 0.7 0.01 0.02 0.01 0.003 0.003 4.8 invent
River 9 0.09 0.21 1.37 0.02 0.005 0.7 0.04 0.01 0.01 0.002 0.004 6.5 invent
River 10 0.13 0.22 1.11 0.01 0.003 0.4 0.01 0.01 0.03 0.028 0.008 5.0 compare
River 1 0.19 0.21 1.42 0.01 0.003 0.5 0.03 0.01 0.01 0.002 0.003 6.8 compare
River 2 0.01 0.21 1.11 0.02 0.004 0.5 0.03 0.01 0.01 0.015 0.004 5.3 compare
River 3 0.09 2.11 1.42 0.02 0.003 0.5 0.03 0.01 0.01 0.066 0.013 0.7 compare
River 4 0.10 0.01 1.47 0.02 0.003 0.6 0.03 0.01 0.01 0.092 0.013 147.0 compare
River 5 0.10 0.21 2.33 0.02 0.005 0.5 0.03 0.01 0.01 0.045 0.004 11.1 compare
River 6 0.12 0.21 0.62 0.02 0.004 0.5 0.03 0.01 0.01 0.011 0.003 3.0 compare
River 7 0.09 0.27 1.42 0.01 0.003 1.52 0.03 0.01 0.01 0.023 0.003 5.3 compare
River 8 0.09 0.27 1.42 0.01 0.003 0.1 0.03 0.01 0.01 0.051 0.003 5.3 compare
River 9 0.09 0.27 1.55 0.01 0.003 0.3 0.01 0.01 0.01 0.110 0.003 5.7 compare
River 10 0.09 0.27 1.55 0.01 0.003 0.3 0.01 0.01 0.01 0.210 0.003 5.7 invent
River 11 0.12 0.27 1.41 0.01 0.004 0.5 0.03 0.01 0.01 0.092 0.003 5.2 invent
River 12 0.12 0.27 1.39 0.01 0.004 0.5 0.03 0.01 0.01 0.074 0.003 5.1 invent
River 13 0.11 0.27 1.42 0.02 0.004 0.5 0.03 0.01 0.01 0.011 0.003 5.3 invent
River 14 0.11 0.25 1.33 0.02 0.003 0.5 0.03 0.01 0.01 0.003 0.004 5.3 invent
River 15 0.11 0.25 1.11 0.02 0.003 0.7 0.01 0.01 0.01 0.002 0.004 4.4

Steel grade
Wrap-up
Hot rolling
Temperature (℃) Primary cooling Secondary cooling conditions Relation 3 division Cooling shutdown
Temperature (℃) Intermediate temperature
(Temp) (占 폚) Retention time
(t) (sec) Cooling rate
(° C / s) ta │t-ta│ Inventive Steel 1 875 640 635 6 1.6 6.6 0.6 Inventory 1 Invention river 2 880 600 595 10 1.1 10.8 0.8 Inventory 2 Invention steel 3 878 640 635 6 1.1 4.1 1.9 Inventory 3 Inventive Steel 4 865 620 615 6 1.7 4.3 1.7 Honorable 4 Invention steel 5 875 640 635 9 1.1 8.9 0.1 Inventory 5 Invention steel 6 880 620 615 9 1.1 8.4 0.6 Inventory 6 Invention steel 7 865 600 595 9 1.4 8.0 1.0 Honorable 7 Inventive Steel 8 870 605 595 10 1.8 11.0 1.0 Honors 8 Invention river 9 875 640 635 10 1.2 11.5 1.5 Proposition 9 Invented Steel 10 880 600 595 9 1.2 7.7 1.3 Inventory 10 Comparative River 1 880 640 635 10 1.3 18.4 8.4 Comparative Example 1 Comparative River 2 875 640 635 6 1.7 -4.4 10.4 Comparative Example 2 Comparative Steel 3 877 640 635 6 1.7 -4.1 10.1 Comparative Example 3 Comparative Steel 4 880 640 635 6 1.7 12.6 6.6 Comparative Example 4 Comparative Steel 5 865 640 635 10 1.2 18.2 8.2 Comparative Example 5 Comparative Steel 6 880 640 635 0 1.7 2.4 2.4 Comparative Example 6 Comparative Steel 7 870 640 635 10 1.5 30.3 20.3 Comparative Example 7 Comparative Steel 8 875 600 595 6 1.7 -0.6 6.6 Comparative Example 8 Comparative Steel 9 865 600 595 6 1.7 5.3 0.7 Comparative Example 9 Comparative Steel 10 870 600 595 6 1.7 5.3 0.7 Comparative Example 10 Invention steel 11 883 645 635 15 1.3 10.3 4.7 Comparative Example 11 Invention steel 12 875 645 520 0 21.8 19.9 19.9 Comparative Example 12 Invention steel 13 875 785 780 8 1.3 22.7 14.7 Comparative Example 13 Invented Steel 14 880 525 520 8 1.3 18.3 10.3 Comparative Example 14 Invented Steel 15 875 615 595 10 4.1 12.1 2.1 Comparative Example 15

division Microstructure (fraction) relation
Equation 2
(P _CB / P _CC ) Mechanical properties pipe
Expansion ratio / Steel elongation F + P M + B P _CB P _CC YS
(MPa) TS
(MPa) YR Hand
(%) Inventory 1 69 31 2502 1511 1.66 593 824 0.72 19 0.91 Inventory 2 61 39 2633 1872 1.41 618 871 0.71 19 0.95 Inventory 3 60 40 2233 1626 1.37 590 855 0.69 18 0.92 Honorable 4 74 26 3067 1949 1.57 555 781 0.71 21 0.94 Inventory 5 65 35 2673 1585 1.69 553 813 0.68 20 0.91 Inventory 6 62 38 2801 1502 1.86 627 950 0.66 21 0.89 Honorable 7 80 20 2978 1833 1.62 469 651 0.72 24 0.99 Honors 8 65 35 3205 1742 1.84 630 863 0.73 19 0.91 Proposition 9 63 37 2816 1818 1.55 614 830 0.74 20 0.89 Inventory 10 63 37 2918 1929 1.51 623 865 0.72 19 0.88 Comparative Example 1 52 48 2436 2038 1.20 856 1172 0.73 14 0.79 Comparative Example 2 88 12 1935 1916 1.01 345 421 0.82 44 0.77 Comparative Example 3 81 19 2837 2734 1.04 633 782 0.81 20 0.66 Comparative Example 4 55 45 2658 2593 1.03 516 833 0.62 18 0.71 Comparative Example 5 57 43 3091 2754 1.12 574 869 0.66 20 0.73 Comparative Example 6 62 38 2768 2719 1.02 460 676 0.68 24 0.71 Comparative Example 7 40 60 2893 2659 1.09 580 841 0.69 20 0.75 Comparative Example 8 81 19 3133 2835 1.11 729 868 0.84 19 0.77 Comparative Example 9 84 16 3197 2842 1.12 740 871 0.85 20 0.78 Comparative Example 10 87 13 3263 2877 1.13 746 878 0.85 20 0.77 Comparative Example 11 78 22 3091 2988 1.03 680 839 0.81 19 0.72 Comparative Example 12 55 45 2763 2543 1.09 738 971 0.76 17 0.76 Comparative Example 13 59 41 2895 2785 1.04 596 903 0.66 20 0.72 Comparative Example 14 56 44 2548 2516 1.01 604 888 0.68 19 0.69 Comparative Example 15 61 39 2681 2095 1.28 613 852 0.72 20 0.67

(In the above Table 3, 'F' denotes a ferrite phase, 'P' denotes a pearlite phase, 'M' denotes a martensite phase and 'B' denotes a bainite phase, YS denotes yield strength, TS denotes tensile strength, YR is the yield ratio (yield strength / tensile strength), and El is elongation. Here, pearlite has an area fraction of 5% or less.

As shown in Tables 1 to 3, Inventive Examples 1 to 10, in which the alloy composition, component relationship, and manufacturing conditions all satisfy the requirements of the present invention, have an intended microstructure formed to obtain the desired physical properties, It can be confirmed that the post-expansion ratio is excellent at not less than 85% of the elongation of the base material (hot-rolled steel sheet).

On the other hand, Comparative Examples 1 to 10 are cases in which the alloy composition to be limited in the present invention is unsatisfactory.

Among them, in Comparative Example 1, the content of C was excessive, and in Comparative Example 7, the Cr content was excessive, and it was confirmed that ta values in the relational expression 3 were calculated to be 18.4 and 30.3, respectively. That is, the comparative examples 1 and 7 are excessively required to maintain the optimum phase fraction, which exceeds the controllable retention time range in the extreme cold zone (ROT zone) of the present embodiment.

In Comparative Example 2 and Comparative Example 8, the content of C and Cr was insufficient, Values of 10.4 and 6.6, respectively, it was impossible to secure the intended microstructure (the structure satisfying the relational expression 2) in the present invention.

On the other hand, in Comparative Examples 9 and 10, the Al content promoting ferrite transformation is excessive, and satisfies Relation 3 of the present invention. However, since oxidized inclusions having a high melting point such as Al ₂ O ₃ are formed in the welded portion, There is a problem that the stress locally concentrates around the inclusion, which causes crack initiation.

In Comparative Examples 3 and 4, the content of Si is out of the range, and in Comparative Examples 5 and 6, the content of Mn is out of the range. When the content of Si (corresponding to the formula 1) As the value exceeds 2, there is a high possibility of the occurrence of the penetrator defect in the welded portion during the electric resistance welding, and cracks are easily generated at the pipe joint and the welded portion at the time of spreading. In fact, the above comparisons were poorly distributed.

In the comparative examples 11 to 15, the alloy composition and the relational expression 1 correspond to the steel satisfying the present invention, but in the comparative examples 11 and 12, the holding time in the secondary cooling was controlled to be 15 seconds and 0 seconds, respectively, -ta | Value is greater than two. In Comparative Examples 13 and 14, the primary cooling termination temperature was out of the range of the present invention. In Comparative Example 15, the cooling rate in the secondary cooling exceeded 2.0 占 폚 / s. Value is greater than two.

As in the comparative examples 11 to 15, the carbon distribution in the crystal grains occupying 50% or more in the total hard phase inner area ratio does not satisfy the relation formula 2 of the present invention, and thus the ductility ratio after gypsum hardening does not reach 85% or more of the hot rolled steel sheet elongation can confirm.

On the other hand, the present invention does not disclose a comparative example in which the content of Al is less than 0.001%. However, in this case, productivity is remarkably lowered in terms of operability, and this can be known to a person skilled in the art.

Claims (7)

(P): 0.001 to 0.02%, sulfur (S): 0.001 to 0.15% by weight, carbon (C): 0.05 to 0.14%, silicon (Si): 0.1 to 1.0%, manganese (Ti): 0.01 to 0.05%, Nb: not more than 0.025% (excluding 0%), vanadium (Ti): 0.01 to 0.05%, aluminum (Al): 0.001 to 0.095%, chromium V): not more than 0.035% (excluding 0%), nitrogen (N): 0.001 to 0.01%, the balance Fe and other unavoidable impurities,
Mn and Si satisfy the following relational expression 1,
Wherein the microstructure comprises a mixture of a martensite phase and a hard phase composed of a bainite phase with a ferrite phase as a matrix,
Wherein the fraction of the crystal grains in which the martensite phase and the bainite phase are mixed is 50% or more in one grain of the total fraction (area fraction) of the hard phase, and the carbon distribution in the crystal grain satisfies the following relational expression 2 It is characterized by high ductility.

[Relation 1]
4 < Mn / Si < 12
(Where Mn and Si mean the weight content of each element).

[Relation 2]
1.3? P _CB / P _CC ? 1.9
Where P _CB is the EPMA intensity measurement of carbon at 70% of the distance from the center of the grains where the martensite and bainite phases are mixed to the boundary of the grains within the hard phase, and P _CC is the center of the same grains Means the EPMA intensity measurement of the carbon at the point.)
The method according to claim 1,
Wherein the ferrite phase contains an area fraction of 60 to 85%.
The method according to claim 1,
The hot-rolled steel sheet has a tensile strength (TS) of 580 MPa or more and a yield ratio (YR = YS / TS) of 0.8 or less.
The method according to claim 1,
Wherein the hot-rolled steel sheet has excellent ductility when the expansion ratio of the pipe is 85% or more of an elongation percentage of the hot-rolled steel sheet after being pipe-roughened.
(P): 0.001 to 0.02%, sulfur (S): 0.001 to 0.15% by weight, carbon (C): 0.05 to 0.14%, silicon (Si): 0.1 to 1.0%, manganese (Ti): 0.01 to 0.05%, Nb: not more than 0.025% (excluding 0%), vanadium (Ti): 0.01 to 0.05%, aluminum (Al): 0.001 to 0.095%, chromium V): not more than 0.035% (excluding 0%), nitrogen (N): 0.001 to 0.01%, the balance Fe and other unavoidable impurities,
Wherein the Mn and Si satisfy the following relational expression 1, and reheating the steel slab in a temperature range of 1180 to 1300 캜;
Subjecting the reheated steel slab to finish hot rolling at a temperature equal to or higher than Ar3 to produce a hot-rolled steel sheet;
Cooling the hot-rolled steel sheet to a temperature range of 550 to 750 ° C at a cooling rate of 20 ° C / s or more;
A secondary cooling step of cooling the material at a cooling rate of 0.05 to 2.0 占 폚 / s within a range satisfying the following relational expression (3) after the primary cooling;
After the secondary cooling, tertiary cooling at a cooling rate of 20 ° C / s or higher to a temperature range of room temperature to 400 ° C; And
The step of winding up after the third cooling
Wherein the hot rolled steel sheet has an excellent ductility.

[Relation 1]
4 < Mn / Si < 12
(Where Mn and Si mean the weight content of each element).

[Relation 3]
T-ta | ≤ 2
(T = 251 + (109C) + (10.5Mn) + (22.7Cr) - (6.1Si) - (0.87Temp) + (0.00068Temp ^ 2) Temp is the secondary cooling intermediate temperature, which means the temperature midway between the start point of the secondary cooling and the end point, and each alloy component means the weight content.
6. The method of claim 5,
Further comprising a step of pickling the rolled hot-rolled steel sheet and pickling the rolled hot-rolled steel sheet.
A seamless steel pipe produced by electric resistance welding of the hot-rolled steel sheet according to any one of claims 1 to 4 and having excellent ductility.

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