KR20180030109A - High-strength steel sheet and production method for same - Google Patents

Abstract

A high strength steel sheet having a high shear strength and a small anisotropy of tensile properties is obtained. Wherein the steel structure is composed of ferrites of 90% or more, a total of pearlite and cementite: 0 to 10%, a total of martensite and retained austenite: 0 to 2% Wherein the ferrite has an average crystal grain size of 15.0 占 퐉 or less and an average aspect ratio of 5.0 or less and contains Nb carbide, Ti carbide and / or V carbide, the Nb carbide, Ti carbide and / or V carbide having an average grain size of 5 to 50 Nb carbide, Ti carbide and V carbide in a total volume ratio of 0.005 to 0.050%.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel sheet,

TECHNICAL FIELD [0001] The present invention relates to a high strength steel sheet to be applied to automobile parts and the like, and a manufacturing method thereof.

As a material for automobile parts and the like, a high-strength steel sheet is preferably used from the viewpoint of reduction in parts weight due to thinning of the material. For example, in a skeletal part or an impact-resistant part, a high yield ratio, that is, a high yield ratio is required in order to secure the safety of the crew. On the other hand, a high-strength steel sheet having uniform tensile properties in the steel sheet, that is, having a small anisotropic tensile property is required for stable press forming without causing cracks. In response to this demand, various steel sheets and their manufacturing techniques have been disclosed.

Patent Document 1 discloses a high strength steel sheet which contains 0.01 mass% or more of Nb and Ti in total, and has a recrystallization ratio of 80% or more as a main phase of ferrite and a manufacturing method thereof.

In addition, Patent Document 2 discloses a high-strength steel sheet excellent in impact resistance including a non-recrystallized ferrite of 20 to 50 area% in a steel structure, and a manufacturing method thereof.

Patent Document 3 discloses a method in which at least one of V, Ti, and Nb is added to limit the precipitation amount of iron carbide in the grain boundaries to ferrite or bainite in the main phase, A hot-dip galvanized steel sheet whose particle size is controlled to 1 占 퐉 or less and a method for producing the same.

Japanese Patent No. 4740099 Japanese Patent No. 4995109 Japanese Patent Application Laid-Open No. 6-322479

However, in the technique of Patent Document 1, neither the holding time of 650 ° C after hot rolling nor the holding time of 500 to 400 ° C in cooling after cracking in the continuous annealing furnace is controlled, and Nb Carbide and Ti carbide and / or V carbide can not be controlled, it is impossible to obtain a high strength steel sheet having a high porosity and a small anisotropy of tensile properties.

In the technique of Patent Document 2, a large amount of Nb or Ti is added, and since the non-recrystallized ferrite is included as the steel structure in an area ratio of 20% or more, not only the desired yield strength and tensile strength are out of the range, It can be locally surrendered easily because it is dispersed unevenly in the structure, so that a high strength steel sheet having a low specific gravity and a small anisotropy of tensile properties can not be obtained.

In the technique of Patent Document 3, neither the holding time at 650 ° C after hot rolling nor the holding time at 500 to 400 ° C in cooling after cracking in the continuous annealing furnace is controlled, and Nb carbide and It is considered that the average grain size of the Ti carbide and / or V carbide can not be controlled. Therefore, a high strength steel sheet having a small anisotropy of tensile properties together with a high yielding beam can not be obtained.

The present invention has been made in view of such circumstances, and an object of the present invention is to obtain a high strength steel sheet having a low anisotropic tensile property together with a high yielding beam.

The present inventors have conducted intensive studies in order to solve the above problems. As a result, it was found that, in the steel structure mainly composed of ferrite, the mean crystal grain size of the ferrite was made finer to a certain value or less and the isobaric structure having an average aspect ratio of less than or equal to a certain value was obtained. Further, Nb carbide and Ti carbide and / It has been found that it is important to properly control the volume ratio and the particle diameter. In order to adjust to a desired structure or the like, it is effective to adjust the composition to a predetermined composition, and control the winding temperature after hot rolling, the retention time at a predetermined temperature range and the cracking temperature in an appropriate range at the time of increasing the temperature of the annealing I got it.

The present invention has been made based on the above findings, and its gist of the invention is as follows.

[1] A ferritic stainless steel comprising, by mass%, C: less than 0.02% to less than 0.10%, Si: less than 0.10%, Mn: less than 1.0%, P: , N: not more than 0.010%, Nb: 0.005 to 0.070%, Ti: not more than 0.100% (including 0%), V: not more than 0.100% (including 0%), 0.100% and the balance of Fe and inevitable impurities, and the steel structure has a ferrite content of 90% or more, an area ratio of 0 to 10% of pearlite and cementite, a ratio of martensite and retained austenite Wherein the ferrite has an average crystal grain size of 15.0 占 퐉 or less and an average aspect ratio of 5.0 or less and contains Nb carbide and Ti carbide and / or V carbide, the Nb carbide and Ti carbide and / Or V carbide has an average particle diameter of 5 to 50 nm, and Nb carbide, Ti carbide and V carbide Is 0.005 to 0.050% in terms of the volume ratio.

[2] The steel sheet according to any one of the above items [1] to [3], further comprising 0.3% or less of Cr, 0.3% or less of Mo, (1), wherein the high-strength steel sheet contains one or two or more of the following.

[3] A high strength steel sheet according to [1] or [2], wherein the steel sheet has a zinc plated layer on its surface.

[4] The high strength steel sheet according to [3], wherein the zinc plated layer is a hot-dip galvanized layer.

[5] The high strength steel sheet according to [4], wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

[6] The high strength steel sheet according to [3], wherein the zinc plated layer is an electrogalvanized layer.

[7] A method for producing a high strength steel sheet according to [1] or [2], wherein the steel is subjected to hot rolling, and after the hot rolling, a residence time in a temperature range of finish rolling temperature to 650 [ A cold rolling step of cold rolling the hot rolled steel sheet obtained by the hot rolling step at a rolling ratio of 75% or less; and a cold rolling step of cold rolling the hot rolled steel sheet obtained by the cold rolling step, In a continuous annealing furnace, the substrate is held at a temperature range of 650 to 750 ° C at the time of temperature rise to a retention time of 60 seconds or less and cracked under the conditions of a cracking temperature of 760 to 880 ° C and a cracking time of 120 seconds or less And an annealing step of cooling the steel sheet at a temperature in the range of 400 to 500 DEG C of 100 seconds or less.

[8] The method for producing a high strength steel sheet according to [7], further comprising a plating step of plating the cold rolled steel sheet after the annealing step.

[9] The method for producing a high-strength steel sheet according to [8], wherein the plating treatment is a hot-dip galvanizing treatment.

[10] The method for producing a high strength steel sheet according to [9], wherein the cold-rolled steel sheet after the plating step is subjected to an alloying process.

[11] The method for producing a high-strength steel sheet according to [8], wherein the plating treatment is an electro-galvanizing treatment.

In the present invention, the production conditions such as the composition of the components, the winding conditions after hot rolling, the residence time at the predetermined temperature range at the time of the temperature increase of the annealing, and the cracking temperature are appropriately controlled. With this control, a steel structure aimed at by the present invention is obtained, and as a result, it becomes possible to stably manufacture a high strength steel sheet having a high porosity ratio required for use in automobile parts and the like and having small anisotropy of tensile properties. With the high strength steel sheet of the present invention, it is possible to further reduce the weight of automobiles, and the utility value of the present invention in the automobile and steel industries is very high.

Hereinafter, an embodiment of the present invention will be described. The present invention is not limited to the following embodiments.

First, the outline of the high-strength steel sheet of the present invention will be described.

The high strength steel sheet of the present invention has a tensile strength of less than 330 MPa to less than 500 MPa and a yield ratio of 0.70 or more and has a yield strength in a tensile test in a direction in which the tensile direction is parallel to the rolling direction, The difference in yield strength in the tensile test performed in the vertical direction is 30 MPa or less. In the point that the yield ratio is not less than 0.70, the high strength steel sheet of the present invention has a high yield ratio. In addition, since the difference in yield strength is 30 MPa or less, the high-strength steel sheet of the present invention has a small anisotropic tensile property.

In the present invention, the total amount of Nb, Ti, and V is 0.005 to 0.005% in total, and the content of Nb is 0.005 to 0.070%, Ti is 0.100% or less (including 0%), V is 0.100% 0.100% or less.

The steel structure is composed of essential ferrite and optional pearlite and the like and the ferrite has an average crystal grain size of 15.0 占 퐉 or less and an average aspect ratio of 5.0 or less and Nb carbide and Ti carbide and And / or V carbide, wherein the average particle diameter of the Nb carbide, Ti carbide and / or V carbide is 5 to 50 nm, and the total amount of deposition of Nb carbide, Ti carbide and V carbide is 0.005 to 0.050% It is possible to obtain a high strength steel sheet having a high shear strength and a small anisotropy of tensile properties.

In the present invention, the Nb carbide, the Ti carbide and the V carbide also include Nb carbonitride, Ti carbonitride, V carbonitride, Nb, Ti composite carbonitride, Nb, V composite carbonitride, and Nb, Ti, . Further, the Nb and Ti composite carbonitrides may be taken as carbides of Nb or taken as carbides of Ti, and the average particle size and the total volume ratio may be taken into consideration. Nb, V composite carbonitrides and Nb, Ti, V composite carbonitrides.

As described above, in order for the average grain size, the average aspect ratio, and the average grain size and deposition amount of the carbides (Nb carbide, Ti carbide and / or V carbide) to satisfy the desired conditions, Do. Specifically, in the cooling after hot rolling, the retention time in the temperature range from the finish rolling temperature to 650 ° C is set to 10 seconds or less, and the coiling temperature is set to 500 to 700 ° C. Further, in the heating of the annealing, the residence time at a temperature range of 650 to 750 占 폚 is set to 60 seconds or less, and subsequently, the crack is cracked at a cracking temperature of 760 to 880 占 폚 for 120 seconds or less. The Nb carbide, the Ti carbide and / or the V carbide are uniformly and finely precipitated during the cooling after the winding, and the ferrite is recrystallized at a relatively low temperature by annealing after the cold rolling and the residence time at the recrystallization temperature is controlled to be not more than a predetermined value A steel structure intended for the invention is obtained.

The yield strength and tensile strength are determined by tensile test according to JIS Z 2241, taking the JIS No. 5 tensile test specimen so that the tensile direction is perpendicular to the rolling direction. The anisotropy of the tensile properties is determined from the difference in yield strength by taking tensile test specimens of JIS No. 5 from the direction in which the tensile direction is perpendicular and parallel to the rolling direction and performing a tensile test in accordance with JIS Z 2241.

The high-strength steel sheet of the present invention, which is completed on the basis of the above findings, is characterized by having a high shear strength ratio and a low anisotropic tensile property, which are required for materials such as automobile parts.

Next, the reason for limiting the composition of the present invention, the reason for limiting the steel structure, and the reason for limiting the manufacturing conditions will be described.

(1) Component composition

The high strength steel sheet according to the present invention is characterized by containing, by mass%, less than 0.02% to less than 0.10% of C, less than 0.10% of Si, less than 1.0% of Mn, less than 0.10% , N: not more than 0.010%, Nb: 0.005 to 0.070%, Ti: not more than 0.100% (including 0%), V: not more than 0.100% (including 0%), 0.100% or less.

The high-strength steel sheet according to the present invention may further comprise, as optional components, 0.3% or less of Cr, 0.3% or less of Mo, 0.005% or less of B, 0.3% or less of Cu, And Sb: 0.3% or less.

The remainder other than the above are Fe and inevitable impurities.

In the following description of the component composition, "% " means " mass% ".

C: less than 0.02% to less than 0.10%

C is an element effective for increasing yield strength and tensile strength in that it becomes Nb carbide, Ti carbide or V carbide, or increases pearlite or martensite. When the C content is less than 0.02%, the total precipitation amount of the carbides does not fall within the desired range, so that the intended tensile strength of the present invention can not be obtained. When the C content is 0.10% or more, pearlite or martensite is excessively produced, so the yield ratio is lowered and the anisotropy of tensile properties is increased. Therefore, the C content is set to be less than 0.02% to less than 0.10%. And preferably 0.02 to 0.06%.

Si: less than 0.10%

Si is generally effective in increasing the yield strength and tensile strength by strengthening the ferrite solid solution. However, when Si is added, an increase in the tensile strength is increased as compared with the yield strength due to remarkable improvement in the work hardenability, the yield ratio is lowered, and the surface property is deteriorated. Therefore, the Si content should be less than 0.10%. The lower limit of the Si content is not particularly limited, but the yield strength and the tensile strength are higher than those of Si. Therefore, in the present invention, the Si content is preferably as small as possible. Therefore, in the present invention, there is no need to add Si, but Si may inevitably be contained in an amount of 0.005% in some cases.

Mn: less than 1.0%

Mn is effective for increasing the yield strength and tensile strength by strengthening the ferrite. However, when the Mn content is 1.0% or more, the martensite fraction in the steel structure increases, so that the tensile strength excessively increases and the desired tensile strength of the present invention can not be obtained, and the yield ratio decreases. Therefore, the Mn content should be less than 1.0%. Mn is not required to be added, but when Mn is added, the Mn content is preferably at least 0.2% with respect to the lower limit. The preferable Mn content with respect to the upper limit is 0.8% or less.

P: not more than 0.10%

P is effective in increasing the yield strength and tensile strength by strengthening the solid solution of ferrite. Therefore, P can be suitably contained in the present invention. However, if the P content exceeds 0.10%, the yield of the ferrite locally occurs due to the segregation of the ferrite and the segregation of the ferrite, thereby increasing the anisotropy of the tensile properties. Therefore, the P content should be 0.10% or less. P is not required to be added, but when P is added, the preferable P content with respect to the lower limit is 0.01% or more. The preferable P content with respect to the upper limit is 0.04% or less.

S: not more than 0.020%

S is an element inevitably included as an impurity. The formability such as bending property and local elongation is lowered by the formation of inclusions such as MnS. Therefore, it is preferable to reduce the S content as much as possible. In the present invention, the S content is 0.020% or less. Preferably 0.015% or less. In addition, as described above, the lower the S content is, the better, and S may not be added in the present invention. However, there are cases in which the production S is 0.0003%.

Al: 0.01 to 0.10%

Al is added to deoxidize in the refining process and to fix solid N as AlN. In order to obtain a sufficient effect, the Al content needs to be 0.01% or more. On the other hand, when the Al content exceeds 0.10%, a large amount of AlN is precipitated to increase the average aspect ratio of the ferrite, resulting in an increase in anisotropy of tensile properties. Therefore, the Al content is set to 0.01 to 0.10%. It is preferably 0.01 to 0.07%. Further, it is more preferably 0.01 to 0.06%.

N: 0.010% or less

N is an element that is inevitably incorporated into the iron refining process. If the N content exceeds 0.010%, Nb carbide, Ti carbide or V carbide precipitates at the time of casting, and Nb carbide, Ti carbide or V carbide remains as coarse carbide after the precipitation and heating of the slab, . Therefore, the N content should be 0.010% or less. In the present invention, there is no need to add N, but there may be a case where the N content in the manufacturing process is 0.0005%.

Nb: 0.005 to 0.070%

Nb is an important element contributing to the miniaturization of the average grain size of ferrite and the increase of the yield ratio due to the precipitation of Nb carbide. If the Nb content is less than 0.005%, the effect of the present invention can not be obtained due to, for example, the deposit amount of the carbide becomes insufficient. If the Nb content exceeds 0.070%, Nb carbide precipitates excessively, and the non-recrystallized ferrite having poor ductility after annealing remains, or the average aspect ratio of ferrite exceeds 5.0, so that anisotropy of tensile properties increases. Therefore, the content of Nb is 0.005 to 0.070%. And preferably 0.005 to 0.040%.

Ti: 0.100% or less (including 0%)

V: 0.100% or less (including 0%)

Nb, Ti and V in a total amount of 0.005 to 0.100%

Ti and V are precipitated as Ti carbide and / or V carbide by addition of Nb and contributing to controlling the average aspect ratio of ferrite to 5.0 or less. When the total amount of Nb and Ti and V is less than 0.005%, the volume ratio of Nb carbide and Ti carbide and / or V carbide becomes insufficient. As a result, the deposition amount of carbide does not reach the desired range and the effect of the present invention can not be obtained. Further, when the total of Nb and Ti and V exceeds 0.100%, Ti carbide and / or V carbide are excessively precipitated, and the non-recrystallized ferrite remaining deficient in ductility after annealing remains, thereby increasing anisotropy of tensile properties. Accordingly, Ti and V are set so that the total amount of Ti is 0.100% or less (including 0%) and V is 0.100% or less (including 0%), and Nb, Ti and V are 0.005 to 0.100% in total. The preferable total amount is 0.007 to 0.040%.

The high-strength steel sheet of the present invention may contain the following components as optional components.

Cr: not more than 0.3%

Cr may be contained as a trace element which does not inhibit the action and effect of the present invention. When the Cr content exceeds 0.3%, martensite is excessively produced due to improvement in quenching property, which may result in lowering of the yield ratio. Therefore, when Cr is added, the Cr content should be 0.3% or less.

Mo: 0.3% or less

Mo may be contained as a trace element which does not inhibit the action and effect of the present invention. However, when the Mo content exceeds 0.3%, martensite is excessively produced due to improvement in quenching property, which may result in lowering of the yield ratio. Therefore, when Mo is added, the Mo content is set to 0.3% or less.

B: not more than 0.005%

B may be contained as a trace element which does not inhibit the action and effect of the present invention. However, when the B content exceeds 0.005%, martensite is excessively generated due to improvement in quenching property, which may result in lowering of the yield ratio. Therefore, when B is added, the B content should be 0.005% or less.

Cu: not more than 0.3%

Cu may be contained as a trace element which does not inhibit the action and effect of the present invention. However, when the Cu content exceeds 0.3%, martensite is excessively generated due to the improvement of the quenching property, resulting in a decrease in the yield ratio. Therefore, when Cu is added, the Cu content is set to 0.3% or less.

Ni: not more than 0.3%

Ni may be contained as a trace element which does not inhibit the action and effect of the present invention. However, when the Ni content exceeds 0.3%, martensite is excessively generated due to improvement in quenching property, which may result in lowering of the yield ratio. Therefore, when Ni is added, the Ni content is set to 0.3% or less.

Sb: not more than 0.3%

Sb may be contained as a trace element which does not inhibit the action and effect of the present invention. However, if the Sb content exceeds 0.3%, it results in embrittlement of the high-strength steel sheet. Therefore, when Sb is added, the Sb content should be 0.3% or less.

The remainder other than the above are Fe and inevitable impurities. In addition, in the present invention, elements such as Sn, Co, W, Ca, Na, Mg and the like may be contained as inevitable impurities in a very small amount which does not hinder the action and effect of the present invention. The term " trace amount " means a total of 0.01% or less of these elements.

(2) Steel structures

The steel structure of the high-strength steel sheet according to the present invention has an area ratio of ferrite of 90% or more, a total of pearlite and cementite: 0 to 10%, and a total of martensite and retained austenite: 0 to 3%. In the steel structure, the average crystal grain size of the ferrite is 15.0 占 퐉 or less, the average grain size of the Nb carbide, the Ti carbide and / or V carbide is 5 to 50 nm, and the Nb carbide, the Ti carbide and / Is 0.005 to 0.050% by volume.

Ferrite: 90% or more

The ferrite has good ductility and is contained in the steel structure as a columnar phase, and its content is 90% or more in area ratio. If the content of ferrite is less than 90% by area, a desired high yield ratio of the present invention can not be obtained, and also anisotropy of tensile properties is increased. Therefore, the content of ferrite is 90% or more in area ratio. Preferably 95% or more. The steel structure of the high strength steel sheet of the present invention may be a ferrite single phase (content of ferrite is 100% in area ratio).

Total of pearlite and cementite: 0 to 10%

Perlite and cementite are effective to obtain the desired yield strength and tensile strength. However, if the total area of pearlite and cementite exceeds 10% by area, the intended yield ratio of the present invention can not be obtained, and the anisotropy of the tensile properties is also increased. Therefore, the total area of pearlite and cementite is 0 to 10% in area ratio. Preferably 0 to 5%.

Total of martensite and retained austenite: 0 to 3%

The steel structure may contain a total of 0 to 3% of martensite and retained austenite in an area ratio. When the total amount of martensite and retained austenite exceeds 3%, a yield ratio of 0.70 or more can not be obtained. Therefore, the total amount of martensite and retained austenite is set to 0 to 3%.

When the average crystal grain size of the ferrite is 15.0 탆 or less

It is important to adjust the average crystal grain size of ferrite to a desired range in order to obtain a high porosity ratio of 0.70 or more for the purpose of the present invention. If the average crystal grain size of the ferrite exceeds 15.0 占 퐉, a yield ratio of 0.70 or more can not be obtained. Therefore, the average crystal grain size of the ferrite is set to 15.0 m or less. Preferably 10.0 占 퐉 or less. The lower limit of the mean grain size of ferrite is not particularly limited, but if it is less than 1.0 m, the tensile strength and the yield strength excessively increase, which may lead to bending property and elongation deterioration. desirable.

When the average aspect ratio of the ferrite is 5.0 or less

If the average aspect ratio of the ferrite exceeds 5.0, the non-recrystallized ferrite increases and anisotropy of tensile properties increases. In the present invention, the average aspect ratio of the ferrite is preferably 4.5 or less, more preferably 4.2 or less.

The Nb carbide and the Ti carbide and / or V carbide have an average particle diameter of 5 to 50 nm

The Nb carbide, Ti carbide or V carbide mainly precipitates in the ferrite grains, and the average grain size is important for achieving both the high porosity and the isotropy of the tensile properties of the present invention. When the particle diameter is less than 5 nm, not only the yield strength and tensile strength are excessively increased but also anisotropy of tensile properties is increased. If the particle diameter exceeds 50 nm, the increase in the yield strength becomes insufficient, and the desired high yield ratio of the present invention can not be obtained. Therefore, the particle diameters of Nb carbide and Ti carbide and / or V carbide are set to 5 to 50 nm. The preferable average particle diameter with respect to the lower limit is 10 nm or more. The preferable average particle diameter with respect to the upper limit is 40 nm or less. Further, in the present invention, the average particle diameter is measured without distinguishing between Nb carbide, Ti carbide, and V carbide.

The total amount of deposition of Nb carbide, Ti carbide and V carbide is 0.005 to 0.050% by volume,

It is important to adjust the precipitation amount of Nb carbide, Ti carbide and V carbide to a desired range in order to balance both the high porosity and the isotropy of the tensile properties of the present invention. If the total amount of precipitated Nb carbide, Ti carbide and V carbide is less than 0.005% by volume, the increase in the yield strength becomes insufficient, and the desired high yield ratio of the present invention can not be obtained. If the sum of the amounts of deposition of Nb carbide, Ti carbide and V carbide exceeds 0.050% by volume, the recrystallization of the ferrite is remarkably suppressed and the yield strength and tensile strength are excessively increased and the anisotropy of tensile properties is increased . Therefore, the total deposition amount of Nb carbide, Ti carbide and V carbide is 0.005 to 0.050% by volume. The preferable total volume ratio with respect to the lower limit is 0.010% or more. The preferable total volume ratio with respect to the upper limit is 0.040% or less. When Ti carbide is not contained, Ti carbide is considered to be 0, and when V carbide is not contained, V carbide is considered to be 0.

In addition, the area ratio of each structure was observed by SEM in the range of 1/8 to 3/8 of the plate thickness centering on the 1/4 position in the thickness direction from the steel sheet surface side perpendicular to the rolling width direction, and ASTM E 562-05, which is incorporated herein by reference. The average crystal grain size of ferrite is determined by observing a range of 1/8 to 3/8 of the plate thickness centered on the plate thickness 1/4 position with an SEM and calculating the circle equivalent diameter from the observation area and the number of crystal grains. The average aspect ratio of the ferrite was obtained by observing the 1/4 position from the surface of the steel sheet in the thickness direction in the cross section perpendicular to the rolling width direction with an optical microscope and measuring the average segment length per crystal grain described in Table 1 of JIS G 0551 (Average crystal grain length in the rolling direction) / (average crystal grain length in the plate thickness direction) by calculating the average crystal grain length in the rolling direction and the plate thickness direction. The particle size of Nb carbide, Ti carbide or V carbide is obtained by preparing a thin film sample from a high-strength steel sheet and calculating the circle-equivalent diameter from the TEM observation image (calculated from the observation area and the number of particles). The total volume ratio of Nb carbide, Ti carbide and V carbide is determined by the extraction residue method.

(3) Manufacturing conditions

The high-strength steel sheet of the present invention is produced by dissolving a steel having the above-mentioned composition and then producing a slab (steel strip) by casting, followed by hot rolling and cold rolling followed by annealing in a continuous annealing furnace. It may be pickled after hot rolling. Hereinafter, a manufacturing method of the present invention having a hot rolling step, a cold rolling step, and an annealing step will be described. In the following description, the temperature means the surface temperature.

The casting method is not particularly limited, and casting may be carried out by any of the rough casting method and the continuous casting method, unless significant compositional deterioration or unevenness of the structure occurs.

The hot rolling may be performed by rolling the cast slab at a high temperature as it is, or may be rolled after reheating the slab cooled to room temperature. In the case where there is a surface defect such as a crack at the viewpoint of the slab, the slab can be polished by a grinder or the like. When reheating the slab, it is preferable to heat the slab to a temperature of 1100 DEG C or higher in order to dissolve the Nb carbide and the Ti carbide and / or V carbide.

The hot rolling step is a step of hot rolling the steel and cooling the steel sheet under the condition that the residence time in the temperature range from the finish rolling temperature to 650 ° C is 10 seconds or less after the hot rolling and winding at 500 to 700 ° C.

In hot rolling, the slab is subjected to rough rolling and finish rolling. Thereafter, the hot-rolled steel sheet is wound to obtain a hot-rolled coil. The conditions of rough rolling and finish rolling in hot rolling are not particularly limited and may be determined according to a conventional method. If the finish rolling temperature is lower than the Ar3 point, coarse ferrite extending in the rolling direction is generated in the steel structure of the hot-rolled steel sheet, resulting in a decrease in ductility after annealing. For this reason, the finishing rolling temperature is preferably Ar3 point or more. The Ar3 point can be obtained by measuring the temperature at which the ferrite transformation starts when the austenite single phase temperature region is continuously cooled at a rate of 1 占 폚 / s using a transformation point measuring apparatus (for example, a forma master tester).

Finishing rolling temperature ~ 650 ° C Retention time in the temperature range: 10 seconds or less

By appropriately controlling the residence time in the temperature range of the finish rolling temperature to 650 DEG C in the cooling after the hot rolling, the coarsening of the average crystal grain size of the ferrite can be suppressed. Therefore, the cooling conditions are important in the present invention. If the retention time in the temperature range from the finish rolling temperature to 650 占 폚 in the cooling after the finish rolling exceeds 10 seconds, coarse Nb carbide, Ti carbide or V carbide will precipitate excessively after winding by hot rolling, The ferrite particles tend to become loose and the average grain size of the ferrite exceeds 15.0 占 퐉, so that the yield ratio decreases. Therefore, the residence time in the temperature range from the finish rolling temperature to 650 占 폚 in the cooling is set to 10 seconds or less. Although the lower limit of the residence time is not particularly limited, it is preferable to stagnate at least one second from the viewpoint of uniformly precipitating Nb carbide, Ti carbide or V carbide at the time of annealing to uniformize the ferrite crystal grain size. The lower limit of the temperature range in which the residence time is controlled is preferably set to 650 deg. C because the average particle diameter of Nb carbide or the like falls outside the scope of the present invention, or the total amount of precipitates such as Nb carbide falls outside the scope of the present invention. .

Coiling temperature: 500 ~ 700 ℃

The coiling temperature is important for controlling the ferrite mean grain size after annealing to 15.0 占 퐉 or less by adjusting the deposition amount of Nb carbide, Ti carbide or V carbide, and the average grain size thereof. When the coiling temperature is less than 500 캜 at the center in the width direction of the steel sheet, the carbide does not sufficiently precipitate during cooling after winding, coarse carbides are precipitated at the time of annealing and cracking, The yield ratio is not obtained, and the tensile strength is also reduced. If the coiling temperature exceeds 700 DEG C, coarse Nb carbide, Ti carbide or V carbide is precipitated during coiling after winding, and the ferrite grain size becomes coarse at the time of annealing, so that a high yield ratio is not obtained and the tensile strength becomes small . Therefore, the coiling temperature is 500 to 700 ° C. The preferred coiling temperature for the lower limit is 550 占 폚 or higher. The preferred coiling temperature for the upper limit is 650 占 폚 or less.

The cold rolling step is a step of cold rolling the hot rolled steel sheet obtained in the hot rolling step. The rolling rate of cold rolling is 75% or less. And preferably 30 to 75%. When the rolling rate exceeds 75%, the average particle size of the carbide becomes too large, and the target high YR can not be obtained. When the rolling rate is 30% or more, it is preferable that the ferrite is completely recrystallized at the time of annealing to obtain isotropic tensile properties.

The annealing is performed by using a continuous annealing furnace to raise the temperature up to the cracking temperature, followed by cooling. The annealing step in the present invention is a method in which the cold-rolled steel sheet obtained in the cold rolling step is allowed to stand in a continuous annealing furnace at a temperature range of 650 to 750 占 폚 at the time of temperature rise to a residence time of 60 seconds or less, A temperature of 760 to 880 deg. C, a cracking time of 120 seconds or less, and cooling after cooling to a condition that the residence time in the temperature range of 400 to 500 deg. C is 100 seconds or less.

Retention time at the temperature range of 650 to 750 ° C at the time of temperature rise: 60 seconds or less

The retention time at 650 to 750 ° C at the time of heating is an important production condition for controlling the average aspect ratio of the ferrite after annealing to 5.0 or less. If the residence time at 650 to 750 ° C at the time of the temperature increase exceeds 60 seconds, the ferrite easily grows in the rolling direction, and the average aspect ratio of the ferrite exceeds 5.0. Therefore, the residence time at 650 to 750 ° C at the time of temperature rise is set to 60 seconds or less. Preferably, the residence time at 650 to 750 ° C at the time of temperature elevation is set to 50 seconds or less. The lower limit of the residence time is not particularly limited, but if the residence time is too short, the recrystallization of the ferrite does not proceed sufficiently, so the residence time is preferably 5 seconds or more.

Cracking temperature: 760 ~ 880 ℃, Cracking time: 120 seconds or less

The cracking temperature and cracking time are important conditions for controlling the mean grain size of ferrite. When the cracking temperature is less than 760 DEG C, recrystallization of the ferrite becomes insufficient and the anisotropy of the tensile properties increases. If the crack temperature exceeds 880 DEG C, the average ferrite crystal grain size becomes coarse, and the desired yield ratio of the present invention can not be obtained, and the tensile strength also becomes small. For this reason, the crack temperature is set at 760 to 880 ° C. On the other hand, when the cracking time exceeds 120 seconds, since the ferrite average crystal grain size becomes coarse, the aimed tensile strength and high yield ratio of the present invention can not be obtained. Therefore, the cracking time should be 120 seconds or less. Preferably 60 seconds or less. The lower limit of the cracking time is not particularly limited, but from the viewpoint of reducing the anisotropy of the tensile properties, it is preferable to completely recrystallize the ferrite. Therefore, the cracking time is preferably 30 seconds or more.

The heating method at the time of heating and cracking is not particularly limited, and can be carried out by a radiant tube method, a direct heating method or the like.

The cooling condition in the cooling after the crack has a residence time in the temperature range of 400 to 500 DEG C of 100 seconds or less. The retention time of 100 seconds or less is necessary to reduce the average particle size of the carbide to 50 nm or less. Although the lower limit of the residence time is not particularly limited, if the temperature is shortened excessively, the solute C in the ferrite increases to deteriorate the aging resistance characteristic, or excessive investment in the cooling equipment is required, so that it is preferably 5 seconds or more. More preferably 10 seconds or more. Here, the " residence time in the temperature range of 400 to 500 deg. C " means the sum of the time during which the steel sheet during cooling is at a temperature of 400 to 500 deg. C, Lt; 0 > C. The stay at this temperature range corresponds to an overexposure treatment. The other cooling conditions are not particularly limited, and conditions include a cooling stop temperature of 400 to 500 DEG C and an average cooling rate of 30 DEG C / s or less.

The surface of the high-strength steel sheet thus obtained can be plated. The plating is preferably zinc plating, and a zinc plating layer is formed on the high-strength steel sheet by zinc plating the high-strength steel sheet of the present invention. Of zinc plating (electro-galvanizing, hot-dip galvanizing, etc.), hot-dip galvanizing is preferably performed by immersing in a hot-dip galvanizing bath.

A galvannealing layer is formed by subjecting a high-strength steel sheet to alloying treatment to a hot-dip galvanized layer formed by hot-dip galvanizing. When the alloying treatment is carried out, if the holding temperature is less than 450 캜, the alloying may not proceed sufficiently, and the adhesion of the plating and the corrosion resistance may be deteriorated. If the holding temperature is higher than 560 DEG C, alloying may proceed excessively, which may cause problems such as powder ring during pressing. Therefore, the holding temperature is preferably 450 to 560 占 폚. If the holding time is less than 5 seconds, the alloying may not proceed sufficiently and the plating adhesion and corrosion resistance may deteriorate. Therefore, the holding time is preferably 5 seconds or more.

Thereafter, temper rolling may be carried out with an elongation of 0.1 to 5.0%, if necessary.

Thus, a high strength steel sheet for the purpose of the present invention can be obtained. The high strength steel sheet of the present invention does not inhibit the object of the present invention even if surface treatment such as chemical conversion treatment or organic coating treatment or painting is carried out.

Example

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

The steel slabs having the composition shown in Table 1 were crushed at 1250 占 폚 for 1 hour, rolled under the conditions of finishing plate thickness of 3.2 mm and finishing rolling temperature of 900 占 폚 or higher at Ar3 point or higher, And wound up at the winding temperature shown in Table 2. The produced hot-rolled steel sheet was pickled, cold-rolled to a finish plate thickness of 1.4 mm to obtain a cold-rolled steel sheet, and annealed under the conditions shown in Table 2, 1 to 31 high strength steel sheets were produced. The cooling conditions for cooling in the annealing were as follows: the cooling stop temperature was 480 DEG C, the average cooling rate was 20 DEG C / s or less, the residence time in the temperature range of 400 to 500 DEG C (500 DEG C to the temperature of the cooling stop temperature) 30 seconds. The annealing was performed using CAL when plating was not performed. In the case of performing plating, hot dip galvanizing or galvannealed hot dip galvanizing was performed using CGL. When the plated layer was made of a galvannealed hot-dip galvanized layer, the galvannealing treatment was carried out at 510 DEG C for 10 seconds.

The obtained high-strength steel sheet was subjected to observation of a steel structure and a tensile test.

The area ratio of the steel structure is such that the area ratio of each structure is in the range of 1/8 to 3/8 of the plate thickness centering on the 1/4 position in the thickness direction from the steel plate surface side perpendicular to the rolling width direction to SEM And was obtained by the point count method described in ASTM E 562-05. The average crystal grain size of ferrite was determined by observing a range of 1/8 to 3/8 of the plate thickness centered on the plate thickness 1/4 position with an SEM and calculating the circle equivalent diameter from the observation area and the number of crystal grains . The average aspect ratio of the ferrite was obtained by observing the 1/4 position from the surface of the steel sheet in the thickness direction in the cross section perpendicular to the rolling width direction with an optical microscope and measuring the average segment length per crystal grain described in Table 1 of JIS G 0551 (Average crystal grain length in the rolling direction) / (average crystal grain length in the plate thickness direction) by calculating the average crystal grain length in the rolling direction and the plate thickness direction. The average particle diameter of carbide (Nb carbide, Ti carbide, V carbide) was observed by TEM, and the circle equivalent diameter was determined by image processing. The total volume ratio of Nb carbide, Ti carbide and V carbide was determined by the extraction residue method. All observations were performed at 10 fields of view, and the average was calculated. The results are shown in Table 2 (Table 2-1 and Table 2-2 are shown in Table 2 together). In Table 2,? Denotes ferrite, P denotes pearlite, M denotes martensite, and? Denotes cementite , the? particle diameter means the ferrite average crystal grain size, and the M (C, N) particle diameter means the sum of the average particle size of carbide and the volume ratio of M (C, N) of Nb carbide, Ti carbide and V carbide . Further, M in M (C, N) means Nb, Ti or V.

The tensile strength (TS) and yield ratio (YR) were determined by a tensile test according to JIS Z 2241 using a tensile test specimen of JIS No. 5 taken so that the tensile direction was perpendicular to the rolling direction. The anisotropy of the tensile properties was evaluated by the difference between the yield strength in the tensile test conducted in the tensile direction parallel to the rolling direction and the yield strength in the tensile test in the tensile direction perpendicular to the rolling direction, &Quot;, and " x ", respectively. The tensile strength of less than 330 MPa to less than 500 MPa and the yield ratio of 0.70 or more were evaluated as good.

[Table 2-1]

[Table 2-2]

Table 2 shows the tensile test results of the steel structure. No. 1 to 3, 6, 8, 9, 12 to 16, 18, 19, 22, 24, 25 and 28 satisfy all the requirements of the present invention, This small high strength steel plate is obtained. On the other hand, 4, 5, 7, 10, 11, 17, 20, 21, 23, 26, 27, 29, 30 and 31, A high strength steel sheet for the purpose of the invention was not obtained.

Industrial availability

The high-strength steel sheet of the present invention is preferable in fields where isotropy of tensile strength and tensile properties is required, especially in automotive plate parts and the like.

Claims (11)

Wherein the composition of C is 0.02 to less than 0.10%, Si is less than 0.10%, Mn is less than 1.0%, P is less than 0.10%, S is less than 0.020%, Al is from 0.01 to 0.10% (Including 0%), V: not more than 0.100% (including 0%), Nb, Ti and V in a total amount of 0.005 to 0.100% , The balance being Fe and inevitable impurities,
The steel structure has an area ratio of ferrite: 90% or more, a total of pearlite and cementite: 0 to 10%, a total of martensite and retained austenite: 0 to 3%
Wherein the ferrite has an average crystal grain size of 15.0 占 퐉 or less and an average aspect ratio of 5.0 or less,
Nb carbide and Ti carbide and / or V carbide, wherein the Nb carbide and Ti carbide and / or V carbide have an average particle diameter of 5 to 50 nm,
Nb carbide, Ti carbide, and V carbide is 0.005 to 0.050% in terms of the volume ratio. The method according to claim 1,
The composition of the above composition may further comprise at least one of the following elements: 1% or less of Cr, 0.3% or less of Mo, 0.3% or less of Mo, 0.005% or less of B, 0.3% or less of Cu, 0.3% or less of Ni, High strength steel sheet containing two or more species. 3. The method according to claim 1 or 2,
A high strength steel plate having a zinc plated layer on its surface. The method of claim 3,
Wherein the zinc plated layer is a hot-dip galvanized layer. 5. The method of claim 4,
Wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer. The method of claim 3,
Wherein the zinc plated layer is an electrogalvanized layer. A method for manufacturing a high strength steel sheet according to any one of claims 1 to 3,
A hot rolling step in which the steel is hot-rolled and the steel sheet is cooled under the condition that the residence time in the temperature range from the finish rolling temperature to 650 ° C is 10 seconds or less after the hot-rolling,
A cold rolling step of cold-rolling the hot-rolled steel sheet obtained by the hot-rolling step at a rolling rate of 75% or less,
The cold-rolled steel sheet obtained in the cold rolling step is allowed to stand in a continuous annealing furnace at a temperature in the range of 650 to 750 ° C at a temperature rising time of 60 seconds or less and to retain the crack at a temperature of 760 to 880 ° C, Time: 120 seconds or less, and cooling in a condition that the residence time in a temperature range of 400 to 500 DEG C is 100 seconds or less. 8. The method of claim 7,
And a plating step of plating the cold rolled steel sheet after the annealing step. 9. The method of claim 8,
Wherein the plating treatment is a hot-dip galvanizing treatment. 10. The method of claim 9,
And a galvannealing step of alloying the cold-rolled steel sheet after the plating step. 9. The method of claim 8,
Wherein the plating treatment is an electro-galvanizing treatment.