KR102084867B1 - High-strength steel sheet and production method for same - Google Patents

Abstract

A high strength steel sheet having a high yield ratio and small anisotropy of tensile properties is obtained. With a specific component composition, the steel structure consists of an area ratio of ferrite: 90% or more, a total of pearlite and cementite: 0 to 10%, a total of martensite and residual austenite: 0 to 2%, and The average grain size of ferrite is 15.0 µm or less, the average aspect ratio is 5.0 or less, and includes Nb carbide, Ti carbide and / or V carbide, and the average particle diameter of the Nb carbide and Ti carbide and / or V carbide is 5 to 50. It is set as the high strength steel plate which is nm and whose sum total of precipitation amount of Nb carbide, Ti carbide, and V carbide is 0.005 to 0.050% by volume ratio.

Description

High strength steel sheet and its manufacturing method {HIGH-STRENGTH STEEL SHEET AND PRODUCTION METHOD FOR SAME}

TECHNICAL FIELD This invention relates to a high strength steel plate applied to automobile parts, etc., and its manufacturing method.

As a raw material for automobile parts, a high strength steel sheet is preferably used from the viewpoint of weight reduction of parts due to the thinning of the raw material. For example, in skeletal parts, parts for collision resistance, etc., the thing which does not deform | transform well at the time of a collision in order to ensure safety of a crew, that is, high yield ratio is calculated | required. On the other hand, in order to press-form stably without cracking, a high-strength steel sheet having a uniform tensile property in the steel sheet, that is, a small anisotropy of the tensile property is required. With respect to such a request, various steel sheets and their manufacturing techniques have been disclosed so far.

Patent Literature 1 discloses a high-strength steel sheet containing 0.01% by mass or more of Nb and Ti in total and a ferrite having a recrystallization rate of 80% or more as a main phase and a method of manufacturing the same.

In addition, Patent Document 2 discloses a high-strength steel sheet having excellent collision resistance properties including 20 to 50 area% of unrecrystallized ferrite in a steel structure and a method of manufacturing the same.

In patent document 3, 1 type, or 2 or more types of V, Ti, and Nb are added, the main phase limits the amount of precipitation of iron carbide in the grain boundary to ferrite or bainite to a fixed value or less, and the maximum of the iron carbide A hot-dip high strength steel sheet and a method for producing the same are disclosed for controlling the particle size to 1 µm or less.

Japanese Patent No. 4740099 Japanese Patent No. 4995109 Japanese Unexamined Patent Publication No. Hei 6-322479

However, in the technique of Patent Literature 1, neither the holding time 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 is important in the present invention. Since it is thought that the average particle diameter of carbide and Ti carbide and / or V carbide cannot be controlled, a high strength steel sheet with high yield ratio and small anisotropy of tensile characteristics cannot be obtained.

In the technique of Patent Literature 2, since a large amount of Nb or Ti is added and the recrystallized ferrite is included as the steel structure in an area ratio of 20% or more, not only the desired yield strength and the tensile strength are out of the range, but also the recrystallized ferrite is a steel sheet. Since it is disperse | distributed uniformly in a structure, it is easy to yield locally, and the high strength steel plate with a high yield ratio and small anisotropy of tensile characteristics is not obtained.

In the technique of Patent Literature 3, none of the holding time after hot rolling and the holding time of 500 to 400 ° C in cooling after cracking in the continuous annealing furnace is controlled, and Nb carbide which is important in the present invention and Since it is thought that the average particle diameter of Ti carbide and / or V carbide cannot be controlled, a high strength steel sheet with high yield ratio and small anisotropy of tensile characteristics is not obtained.

This invention is made | formed in view of such a situation, The objective is to obtain a high strength steel plate with a high yield ratio and small anisotropy of tensile characteristics.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly researched in order to solve the said subject. As a result, in a steel structure mainly composed of ferrite, the average grain size of the ferrite is made to be smaller than or equal to a certain value, and the structure of the equiaxed structure whose average aspect ratio is equal to or less than that is constant. Further, Nb carbide and Ti carbide and / or V carbide It was found that it is important to properly control the volume ratio and the particle diameter. And in order to adjust to a desired structure | tissue etc., while adjusting to a predetermined component composition, it turns out that it is effective to control the winding temperature after hot rolling, and the residence time and the crack temperature in the predetermined temperature range at the time of the temperature increase of annealing to an appropriate range. Paid.

This invention is made | formed based on the above knowledge, The summary is as follows.

[1] The component composition is, in mass%, C: 0.02% to less than 0.10%, Si: less than 0.10%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01 to 0.10% , N: 0.010% or less, Nb: 0.005-0.070%, Ti: 0.100% or less (including 0%), V: 0.100% or less (including 0%) and Nb, Ti, and V in total from 0.005 to 0.100%, remainder consists of Fe and an unavoidable impurity, and a steel structure consists of an area ratio of ferrite: 90% or more, a total of pearlite and cementite: 0-10%, and martensite and residual austenite Total: 0-3%, the average grain size of the ferrite is 15.0 µm or less, the average aspect ratio is 5.0 or less, and includes Nb carbide and Ti carbide and / or V carbide, the Nb carbide and Ti carbide and And / or the average particle diameter of V carbide is 5 to 50 nm, and Nb carbide, Ti carbide and V carbide The high strength steel plate whose sum of precipitation amounts is 0.005 to 0.050% by volume ratio.

[2] The component composition is, in mass%, Cr: 0.3% or less, Mo: 0.3% or less, B: 0.005% or less, Cu: 0.3% or less, Ni: 0.3% or less, Sb: 0.3% or less The high strength steel plate as described in [1] containing any 1 type, or 2 or more types of.

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

[4] The high strength steel sheet according to [3], wherein the zinc plating layer is a hot dip galvanizing 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 galvanized layer is an electrogalvanized layer.

[7] The method for producing a high strength steel sheet according to [1] or [2], wherein the steel is hot rolled, and after the hot rolling, the residence time in the temperature range of the finish rolling temperature to 650 ° C is 10 seconds or less under the condition of 10 seconds or less. The cold rolling process obtained by cooling the above, the hot rolling process wound up at 500-700 degreeC, the cold rolling process which cold-rolls the hot rolled steel plate obtained by the said hot rolling process at a rolling rate of 75% or less, and the cold rolled steel plate obtained by the said cold rolling process, In the continuous annealing furnace, the residence time in the temperature range of 650 to 750 ° C. at the time of temperature increase stays at 60 seconds or less, and after the retention, cracks are carried out under the conditions of crack temperature: 760 to 880 ° C. and crack time: 120 seconds or less. And manufacturing method of high strength steel sheet which has an annealing process to cool on conditions whose residence time in the temperature range of 400-500 degreeC is 100 second or less.

[8] The method for producing a high strength steel sheet according to [7], which has 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 step.

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

In this invention, manufacturing conditions, such as a component composition, the coiling condition after hot rolling, the residence time in a predetermined | prescribed temperature range at the time of the temperature increase of annealing, a crack temperature, are controlled suitably. By this control, the steel structure which this invention aims at is obtained, and as a result, it becomes possible to stably manufacture the high strength steel plate with high yield ratio and small anisotropy of tensile characteristics required for the use of automobile parts etc. The high-strength steel sheet of the present invention makes it possible to further reduce the weight of automobiles, and the value of use in the automobile and steel industries of the present invention is very large.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described. In addition, this invention is not limited to the following embodiment.

First, the outline | summary of the high strength steel plate of this invention is demonstrated.

The high strength steel sheet of the present invention has a tensile strength of less than 330 MPa to 500 MPa, a yield ratio of 0.70 or more, and a yield strength in a tensile test performed in a direction in which the tensile direction is parallel to the rolling direction, and the tensile direction is determined by the rolling direction. The difference in yield strength in the tensile test conducted in the vertical direction is 30 MPa or less. Since the yield ratio is 0.70 or more, the high strength steel sheet of the present invention has a high yield ratio. Moreover, since the difference of the said yield strength is 30 Mpa or less, the high strength steel plate of this invention has small anisotropy of tensile characteristics.

In the present invention, Nb: 0.005% to 0.070%, Ti: 0.100% or less (including 0%), V: 0.100% or less (including 0%), and Nb, Ti and V in total, 0.005% to It is especially important to set it as the component composition containing 0.100% or less.

By adjusting the component composition and the production conditions, the steel structure is composed of the necessary ferrite and arbitrary pearlite, and the ferrite has an average grain size of 15.0 µm or less, an average aspect ratio of 5.0 or less, Nb carbide and Ti carbide and And / or V carbide, the average particle diameter of the Nb carbide and Ti carbide and / or V carbide is 5 to 50 nm, and the sum of precipitation amounts of Nb carbide, Ti carbide and V carbide is 0.005 to 0.050% by volume ratio. By adjusting so that it is possible to obtain a high strength steel sheet having a high yield ratio and small anisotropy of tensile characteristics.

In the present invention, Nb carbide, Ti carbide and V carbide include Nb carbonitride, Ti carbonitride, V carbonitride and Nb, Ti compound carbonitride, Nb, V compound carbonitride and Nb, Ti, V compound carbonitride . In addition, the Nb and Ti composite carbonitrides may be taken as carbides of Nb or taken as carbides of Ti, and the average particle diameter and the total volume fraction may be considered. The same applies to Nb, V complex carbonitrides and Nb, Ti, V complex carbonitrides.

As described above, in order to satisfy the desired conditions, the average grain size, average aspect ratio, and average grain size and precipitate amount of carbides (Nb carbide and Ti carbide and / or V carbide) of ferrite are important not only the composition but also the production conditions. Do. Specifically, in cooling after hot rolling, the residence time in the temperature range of finish rolling temperature-650 degreeC is made into 10 second or less, and winding temperature is made into 500-700 degreeC. Moreover, in heating of annealing, the residence time in the temperature range of 650-750 degreeC is 60 second or less, and it cracks 120 seconds or less at the cracking temperature of 760-880 degreeC continuously. During cooling after winding, Nb carbide, Ti carbide and / or V carbide are uniformly finely deposited, and after cold rolling, the ferrite is recrystallized at a relatively low temperature by annealing, and the residence time in the recrystallization temperature range is controlled to a predetermined value or less. The steel structure aimed at by this invention is obtained.

Yield strength and tensile strength collect | require the JIS No. 5 tensile test piece so that a tensile direction may become perpendicular | vertical to a rolling direction, and is calculated | required by the tensile test based on JISZ22241. The anisotropy of the tensile properties is obtained from the difference in yield strength by collecting a JIS No. 5 tensile test piece from a direction in which the tensile direction is perpendicular to and parallel to the rolling direction, and performing a tensile test according to JIS Z 2241.

The high strength steel sheet of this invention completed based on the above knowledge has the characteristics that it is a high yield ratio and the anisotropy of tensile characteristics is required for raw materials, such as an automotive component.

Next, the reason for limitation of the component composition of this invention, the reason for limitation of steel structure, and the reason for limitation of manufacturing conditions are demonstrated.

(1) ingredient composition

The high strength steel plate of this invention is C: 0.02%-less than 0.10%, Si: less than 0.10%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01-0.10% , N: 0.010% or less, Nb: 0.005-0.070%, Ti: 0.100% or less (including 0%), V: 0.100% or less (including 0%), and Nb, Ti, and V in total from 0.005% It contains 0.100% or less.

In addition, the high strength steel sheet of this invention is an arbitrary component, Furthermore, by mass%, Cr: 0.3% or less, Mo: 0.3% or less, B: 0.005% or less, Cu: 0.3% or less, Ni: 0.3% or less, Sb: You may contain any 1 type (s) or 2 or more types of 0.3% or less.

Remainder other than the above is Fe and an unavoidable impurity.

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

C: 0.02% to less than 0.10%

C becomes an Nb carbide, a Ti carbide, or a V carbide, and increases an pearlite and martensite, and is an element effective in increasing yield strength and tensile strength. When C content is less than 0.02%, since the total precipitation amount of carbide does not become a desired range, the tensile strength made into the objective of this invention cannot be obtained. When the C content is 0.10% or more, the pearlite or martensite is excessively produced, so the yield ratio is lowered, and the anisotropy of the tensile properties is increased. For this reason, C content is made into 0.02%-less than 0.10%. Preferably it is 0.02 to 0.06%.

Si: less than 0.10%

Si is generally effective for increasing yield strength and tensile strength by solid solution strengthening of ferrite. However, when Si is added, the amount of increase in tensile strength is greater than the yield strength due to the remarkable improvement in work hardenability, the yield ratio is lowered, and the surface properties deteriorate. For this reason, Si content is made into less than 0.10%. In addition, although the minimum of Si content is not specifically limited, Since yield strength and tensile strength also become high also in structures other than Si, in this invention, it is so preferable that there is little Si content. Therefore, although it is not necessary to add Si in this invention, in manufacturing, it may contain 0.005% inevitably.

Mn: less than 1.0%

Mn is effective for increasing yield strength and tensile strength by solid solution strengthening of 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, the tensile strength of the present invention is not obtained, and the yield ratio decreases. For this reason, Mn content is made into less than 1.0%. Although Mn does not need to be added, when adding Mn, preferable Mn content is 0.2% or more with respect to a minimum. Mn content with respect to an upper limit is 0.8% or less.

P: 0.10% or less

P is effective for increasing yield strength and tensile strength by solid solution strengthening of ferrite. For this reason, P can be contained suitably in this invention. However, when the P content exceeds 0.10%, the yield of ferrite locally occurs due to casting segregation or ferrite grain boundary segregation, so that the anisotropy of the tensile properties increases. For this reason, P content is made into 0.10% or less. Although P does not need to be added, when adding P, preferable P content is 0.01% or more with respect to a minimum. Preferable P content is 0.04% or less with respect to an upper limit.

S: 0.020% or less

S is an element that is inevitably included as an impurity. Since formability, such as bendability and local elongation, falls by formation of inclusions, such as MnS, it is preferable to reduce S content as much as possible. In this invention, S content is made into 0.020% or less. Preferably it is 0.015% or less. As described above, the lower the S content is, the better, and in the present invention, S may not be added. However, it may contain 0.0003% of manufacture S.

Al: 0.01% to 0.10%

Al is added for the deoxidation in the refining process and to fix the solid solution N as AlN. In order to acquire sufficient effect, it is necessary to make Al content 0.01% or more. Moreover, when Al content exceeds 0.10%, AlN will precipitate in a large amount, and the average aspect ratio of ferrite will increase and it will cause the anisotropy of tensile characteristics to increase. Therefore, Al content is made into 0.01 to 0.10%. Preferably, you may be 0.01 to 0.07%. Moreover, More preferably, you may be 0.01 to 0.06%.

N: 0.010% or less

N is an element that is inevitably incorporated until the refining process of molten iron. When the N content is more than 0.010%, Nb carbide, Ti carbide or V carbide precipitates during casting, and Nb carbide, Ti carbide or V carbide does not dissolve and remains as coarse carbide at the time of slab heating. Brings about coarsening. Therefore, N content is made into 0.010% or less. In addition, in this invention, although N does not need to be added, it may contain 0.0005% N on manufacture.

Nb: 0.005% to 0.070%

Nb is an important element that contributes to the refinement of the ferrite average grain and the increase in yield ratio due to precipitation of Nb carbides. If the Nb content is less than 0.005%, the amount of carbide may be insufficient, and thus the effect of the present invention cannot be obtained. Further, when the Nb content exceeds 0.070%, Nb carbides are excessively precipitated and unrecrystallized ferrite, which lacks ductility, remains after annealing, or the anisotropy of tensile properties increases because the average aspect ratio of ferrite exceeds 5.0. Therefore, Nb content is made into 0.005 to 0.070%. Preferably it is 0.005 to 0.040%.

Ti: 0.100% or less (including 0%)

V: 0.100% or less (including 0%)

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

Ti and V are complex-added with Nb to precipitate as Ti carbide and / or V carbide, and contribute to controlling the average aspect ratio of ferrite to 5.0 or less. If the total of Nb, Ti, and V is less than 0.005%, the volume ratio of Nb carbide and Ti carbide and / or V carbide becomes insufficient, and as a result, the precipitation amount of carbide does not become a desired range, and the effect of this invention is not obtained. In addition, when the total of Nb, Ti, and V exceeds 0.100%, Ti carbide and / or V carbide are excessively precipitated, so that unrefined uncrystallized ferrite remains after annealing, thereby increasing the anisotropy of tensile properties. Therefore, Ti and V are made into Ti: 0.100% or less (including 0%), V: 0.100% or less (containing 0%), and Nb, Ti, and V are 0.005-0.100% in total. Preferable total amount shall be 0.007 to 0.040%.

The high strength steel plate of this invention can contain the following components as an arbitrary component.

Cr: 0.3% or less

Cr may be contained as a trace element that does not impair the effects of the present invention. When Cr content exceeds 0.3%, martensite is produced | generated excessively by the improvement of hardenability, and it may cause the fall of yield ratio. Therefore, when adding Cr, Cr content shall be 0.3% or less.

Mo: 0.3% or less

You may contain Mo as a trace element which does not inhibit the effect of this invention. However, when Mo content exceeds 0.3%, martensite is produced | generated excessively by the improvement of hardenability, and it may cause the fall of yield ratio. Therefore, Mo content is made into 0.3% or less when Mo is added.

B: 0.005% or less

You may contain B as a trace element which does not inhibit the effect of this invention. However, when the B content is more than 0.005%, martensite is excessively generated due to the improvement in quenchability, which may cause a decrease in yield ratio. Therefore, when adding B, B content is made into 0.005% or less.

Cu: 0.3% or less

You may contain Cu as a trace element which does not inhibit the effect of this invention. However, when Cu content exceeds 0.3%, martensite is produced | generated excessively by the improvement of hardenability, and it may cause the yield ratio to fall. Therefore, when adding Cu, Cu content is made into 0.3% or less.

Ni: 0.3% or less

Ni may be contained as a trace element that does not impair the effects of the present invention. However, when the Ni content is more than 0.3%, martensite is excessively generated due to the improvement in quenchability, which may cause a decrease in yield ratio. Therefore, when Ni is added, Ni content is made into 0.3% or less.

Sb: 0.3% or less

You may contain Sb as a trace element which does not inhibit the effect of this invention. However, when the Sb content exceeds 0.3%, embrittlement of the high strength steel sheet is caused. Therefore, when adding Sb, Sb content is made into 0.3% or less.

Remainder other than the above is Fe and an unavoidable impurity. In the present invention, in addition to the above, elements such as Sn, Co, W, Ca, Na, and Mg may also be contained as unavoidable impurities within a small amount of range that does not impair the effect of the present invention. "A trace amount range" means 0.01% or less in total of these elements.

(2) steel organization

The steel structure of the high strength steel plate of this invention consists of an area ratio of ferrite: 90% or more, a total of pearlite and cementite: 0 to 10%, and a total of martensite and residual austenite: 0 to 3%. In this steel structure, the average grain size of the ferrite is 15.0 µm or less, and the average particle diameter of Nb carbide, Ti carbide and / or V carbide is 5 to 50 nm, and Nb carbide, Ti carbide and / or V carbide. The sum total of precipitation amount is 0.005 to 0.050% by volume ratio.

Ferrite: 90% or more

Ferrite has good ductility and is contained in steel structure as a columnar phase, and its content is 90% or more by area ratio. If the content of ferrite is less than 90% in area ratio, the high yield ratio intended for the present invention is not obtained, and the anisotropy of tensile properties also increases. Therefore, content of ferrite is made into 90% or more by area ratio. Preferably it is 95% or more. In addition, the steel structure of the high strength steel plate of this invention may be a ferrite single phase (content of ferrite is 100% by area ratio).

Total of pearlite and cementite: 0 to 10%

Pearlite and cementite are effective to obtain the desired yield strength and tensile strength. However, when the sum total of pearlite and cementite exceeds 10% by area ratio, the high yield ratio aimed at by this invention will not be obtained, and the anisotropy of tensile characteristics will also become large. For this reason, the sum total of pearlite and cementite shall be 0 to 10% by area ratio. Preferably, you may be 0 to 5%.

Total of martensite and residual austenite: 0 to 3%

The steel structure may contain 0 to 3% of martensite and residual austenite in total in an area ratio. If the total of martensite and residual austenite exceeds 3%, a yield ratio of 0.70 or more is not obtained. For this reason, the sum total of martensite and residual austenite shall be 0 to 3%.

Average grain size of ferrite is 15.0 µm or less

It is important to adjust the average grain size of ferrite to a desired range in order to obtain a high yield ratio of 0.70 or more, which is the object of the present invention. If the average crystal grain size of the ferrite exceeds 15.0 µm, a yield ratio of 0.70 or more is not obtained. Therefore, the average crystal grain size of ferrite is 15.0 µm or less. Preferably it is 10.0 micrometers or less. The lower limit of the ferrite average crystal grain size is not particularly limited, but if it is less than 1.0 µm, the tensile strength and the yield strength may increase excessively, leading to deterioration of bendability and elongation, so that the ferrite average grain size is 1.0 µm or more. desirable.

The average aspect ratio of ferrite is 5.0 or less

When the average aspect ratio of ferrite exceeds 5.0, unrecrystallized ferrite increases and the anisotropy of tensile characteristics increases. In this invention, 4.5 or less are preferable and, as for the average aspect ratio of ferrite, 4.2 or less are more preferable.

Average particle size of Nb carbide, Ti carbide and / or V carbide is 5-50 nm

Nb carbide, Ti carbide, and V carbide are mainly precipitated in the ferrite particles, and the average particle diameter is important for achieving both the high yield ratio and the isotropy of tensile properties aimed at by the present invention. When the particle diameter is less than 5 nm, not only the yield strength and the tensile strength excessively increase, but also the anisotropy of the tensile properties increases. When the particle diameter exceeds 50 nm, the increase in yield strength is insufficient, and the high yield ratio intended for the present invention is not obtained. Therefore, the particle diameters of Nb carbide, Ti carbide, and / or V carbide are 5-50 nm. A preferable average particle diameter is 10 nm or more with respect to a minimum. The average particle diameter is preferably 40 nm or less with respect to the upper limit. In addition, in this invention, an average particle diameter is measured, without distinguishing between Nb carbide, Ti carbide, and V carbide.

The total amount of precipitates of Nb carbide, Ti carbide, and V carbide is 0.005 to 0.050% in volume ratio.

Adjusting the amount of precipitation of Nb carbide, Ti carbide and V carbide to a desired range is important for achieving both high yield ratio and isotropy of tensile properties aimed at by the present invention. When the sum total of the amount of precipitation of Nb carbide, Ti carbide, and V carbide is less than 0.005% by volume ratio, the increase in yield strength is insufficient, and the high yield ratio intended for the present invention is not obtained. When the total amount of precipitates of Nb carbide, Ti carbide and V carbide exceeds 0.050% by volume ratio, recrystallization of ferrite is remarkably suppressed, yield strength and tensile strength excessively increase, and anisotropy of tensile characteristics increases. . Therefore, the sum total of the precipitation amount of Nb carbide, Ti carbide, and V carbide is 0.005 to 0.050% by volume ratio. Preferable total volume ratio with respect to a minimum is 0.010% or more. The total volume ratio is preferably 0.040% or less with respect to the upper limit. In addition, Ti carbide is considered 0 when Ti carbide is not included, and V carbide is 0 when V carbide is not included.

In addition, the area ratio of each structure observes the range of sheet thickness 1/8-3/8 centered on 1/4 position in plate | board thickness direction from the steel plate surface side of the cross section perpendicular | vertical to a rolling width direction, and SEM. It obtains by the point count method described in E 562-05. The average grain size of ferrite is determined by observing the range of sheet thickness 1/8 to 3/8 centered on the sheet thickness quarter position by SEM, and calculating the circle equivalent diameter from the observation area and the number of crystal grains. In the cross section perpendicular | vertical to a rolling width direction, the average aspect ratio of ferrite observes 1/4 position from the steel plate surface in the plate | board thickness direction with an optical microscope, and averages the line segment length per crystal grain of Table 1 of JIS G 0551. The average crystal grain length of a rolling direction and a plate | board thickness direction is computed by the method of calculating | required, and it can obtain | require by the (average crystal grain length of a rolling direction) / (average crystal grain length of a plate | board thickness direction). The particle diameters of Nb carbide, Ti carbide, and V carbide are calculated | required by making a thin film sample from a high strength steel plate, and calculating a circle equivalent diameter from a TEM observation image (calculated from observation area and number of particles). The total volume fraction of Nb carbide, Ti carbide and V carbide is obtained by the extraction residue method.

(3) manufacturing conditions

The high strength steel sheet of this invention is manufactured by making steel which has the said component composition, manufactures slab (steel piece) by casting, and after performing hot rolling and cold rolling, and annealing with a continuous annealing furnace. You may pickle after hot rolling. Hereinafter, the manufacturing method of this invention which has a hot rolling process, a cold rolling process, and an annealing process is demonstrated. In addition, in the following description, temperature means surface temperature.

A casting method is not specifically limited, As long as segregation of the outstanding component composition and the nonuniformity of a structure do not generate | occur | produce, you may cast by either of the ingot method and the continuous casting method.

Hot rolling may roll a hot cast slab as it is, or may roll after reheating the slab cooled to room temperature. In addition, when there is a surface defect such as a crack at the time of the slab, the slab can be cleaned by a grinder or the like. In the case of reheating the slab, it is preferable to heat it to 1100 ° C. or higher in order to dissolve Nb carbide and Ti carbide and / or V carbide.

A hot rolling process is a process which hot-rolls steel, cools a steel plate on the conditions of 10 second or less for the residence time of the temperature range of finishing rolling temperature-650 degreeC after the hot rolling, and winds it at 500-700 degreeC.

In hot rolling, rough rolling and finish rolling are performed to a slab. Thereafter, the steel sheet after hot rolling is wound to obtain a hot rolled coil. Rough rolling conditions and finish rolling conditions in hot rolling are not specifically limited, What is necessary is just to determine according to a conventional method. When the finish rolling temperature is less than the Ar3 point, coarse ferrite elongated in the rolling direction is generated in the steel structure of the hot rolled steel sheet, which may cause ductility deterioration after annealing. For this reason, it is preferable to make finish rolling temperature more than Ar3 point. In addition, Ar3 point can be calculated | required by measuring the temperature at which a ferrite transformation starts when it cools continuously at 1 degree-C / s from an austenite single-phase temperature range using a transformation point measuring apparatus (for example, a master tester).

Retention time in the temperature range of finish rolling temperature-650 degreeC: 10 second or less

In the cooling after hot rolling, the coarsening of the average grain size of the ferrite can be suppressed by appropriately controlling the residence time in the temperature range of the finish rolling temperature to 650 ° C. For this reason, the said cooling conditions are important in this invention. In the cooling after finishing rolling, when the residence time in the temperature range of finishing rolling temperature-650 degreeC exceeds 10 second, coarse Nb carbide, Ti carbide, and V carbide will precipitate excessively after winding of hot rolling, and at the time of annealing, Ferrite particles tend to coarsen and the yield ratio decreases because the average grain size of ferrite exceeds 15.0 µm. Therefore, the residence time in the temperature range of finishing rolling temperature-650 degreeC in the said cooling shall be 10 second or less. The lower limit of the residence time is not particularly limited, but it is preferable to stay at least 1 second from the viewpoint of uniformly depositing Nb carbide, Ti carbide or V carbide at the time of annealing to make the ferrite crystal grain size uniform. The lower limit of the temperature range in which the residence time is controlled is 650 ° C for the reason that the average particle diameter of Nb carbide or the like is outside the scope of the present invention or the total amount of precipitates such as Nb carbide is outside the scope of the present invention. Shall be.

Winding temperature: 500 to 700 ℃

The coiling temperature is important for controlling the ferrite average crystal grain size after annealing to 15.0 µm or less by adjusting the precipitation amounts of Nb carbide, Ti carbide and V carbide and their average particle diameter. In the center of the width direction of the steel sheet, when the coiling temperature is less than 500 ° C., the carbides are not sufficiently precipitated during the cooling after the coiling, coarse carbides are precipitated during heating and cracking of the annealing, and the ferrite grain size is coarsened. The yield ratio is not obtained and the tensile strength is also small. If the coiling temperature exceeds 700 ° C, coarse Nb carbide, Ti carbide or V carbide precipitates during cooling after coiling, and the ferrite grain size coarsens during annealing, so that a high yield ratio is not obtained and the tensile strength is also reduced. . Therefore, coiling temperature shall be 500-700 degreeC. Preferable winding temperature is 550 degreeC or more with respect to a minimum. A preferable winding temperature is 650 degrees C or less with respect to an upper limit.

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 shall be 75% or less. Preferably it is 30 to 75%. When the rolling rate exceeds 75%, the average particle diameter of the carbide is coarse, and high YR for the purpose of the present application is not obtained, so 75% or less is required. If the rolling ratio is 30% or more, the ferrite is completely recrystallized at the time of annealing, so that isotropic tensile properties are obtained.

Annealing consists of a process of cooling after after heating up to a cracking temperature using a continuous annealing furnace. The annealing step in the present invention means that the cold rolled steel sheet obtained by the cold rolling step is held in a continuous annealing furnace at a temperature range of 650 ° C to 750 ° C at the time of elevated temperature: 60 seconds or less, and cracks after the retention. Temperature: 760-880 degreeC, a crack time; It cracks on the conditions of 120 second or less, and it is a process of cooling on the conditions which the residence time of the 400-500 degreeC temperature range after the crack is 100 seconds or less.

Retention time in the temperature range of 650-750 degreeC at the time of temperature rising: 60 second or less

The residence time at 650-750 degreeC at the time of temperature rising is important manufacturing conditions, in order to control the average aspect ratio of the ferrite after annealing to 5.0 or less. When the residence time at 650-750 degreeC at the time of temperature rising exceeds 60 second, since ferrite becomes easy to grain grow in a rolling direction, the average aspect ratio of ferrite exceeds 5.0. Therefore, the residence time in 650-750 degreeC at the time of temperature rising shall be 60 second or less. Preferably, the residence time in 650-750 degreeC at the time of temperature rising shall be 50 second or less. The lower limit of the residence time is not particularly limited, but if the residence time is too short, the recrystallization of ferrite does not proceed sufficiently, so the residence time is preferably 5 seconds or more.

Crack temperature: 760-880 ℃, crack time: 120 seconds or less

Crack temperature and crack time are important conditions for controlling the ferrite average grain size. If the cracking temperature is less than 760 DEG C, the recrystallization of ferrite becomes insufficient and the anisotropy of the tensile properties increases. If the cracking temperature exceeds 880 ° C, the ferrite average crystal grain size becomes coarse, the yield ratio intended for the present invention is not obtained, and the tensile strength is also reduced. For this reason, a crack temperature shall be 760-880 degreeC. If the crack time exceeds 120 seconds, the ferrite average crystal grain size is coarsened, so that the tensile strength and the high yield ratio of the present invention are not obtained. For this reason, a crack time shall be 120 second or less. Preferably it is 60 seconds or less. The lower limit of the cracking time is not particularly limited. However, the cracking time is preferably 30 seconds or more because it is preferable to completely recrystallize the ferrite from the viewpoint of reducing the anisotropy of the tensile properties.

The heating method at the time of temperature rising and a crack is not specifically limited, It can carry out by a radiant tube system, direct heating method, etc.

As for cooling conditions in cooling after a crack, the residence time of the temperature range of 400-500 degreeC is 100 second or less. The residence time of 100 seconds or less is necessary in order to make the average particle diameter of carbide into 50 nm or less. The lower limit of the residence time is not particularly limited, but an extremely short time is preferably 5 seconds or more because the solid solution C in the ferrite increases and the aging resistance deteriorates or excessive investment is required for the cooling equipment. More preferably, it is 10 second or more. Here, "the residence time in the temperature range of 400-500 degreeC" means the sum total of time when the steel plate in cooling becomes the temperature of 400-500 degreeC, and if cooling stop temperature is 400 degreeC or more, it is 500 from a cooling stop temperature. It means the sum total of time which is set to ° C. In addition, the retention in this temperature range corresponds to the overaging treatment. Moreover, although other cooling conditions are not specifically limited, The conditions of a cooling stop temperature of 400-500 degreeC and an average cooling rate of 30 degrees C / s or less are mentioned.

The surface of the high strength steel plate obtained as mentioned above can be plated. Zinc plating is preferable, and a zinc plating layer is formed on a high strength steel plate by performing zinc plating on the high strength steel plate of this invention. Among the galvanizing (electro galvanizing, hot dip galvanizing, etc.), hot dip galvanizing which is immersed in the hot dip galvanizing bath is preferable.

An alloying hot dip galvanizing layer is formed by performing an alloying process on the hot dip galvanizing layer formed by performing hot dip galvanizing on a high strength steel sheet. When performing alloying process, when holding temperature is less than 450 degreeC, alloying may not fully advance and plating adhesiveness and corrosion resistance may deteriorate. Moreover, when holding temperature exceeds 560 degreeC, alloying may advance excessively and a problem, such as powdering, may arise at the time of press. For this reason, it is preferable to set retention temperature as 450-560 degreeC. If the holding time is less than 5 seconds, alloying does not proceed sufficiently and plating adhesion and corrosion resistance may deteriorate. Therefore, the holding time is preferably 5 seconds or more.

Then, you may perform temper rolling of 0.1 to 5.0% of elongation rate as needed.

By the above, the high strength steel plate made into the objective of this invention is obtained. Even if the high strength steel plate of this invention is subjected to surface treatment, such as chemical conversion treatment and organic coating treatment, and coating, the characteristic made into the objective of this invention is not impaired.

Example

Hereinafter, an Example demonstrates this invention in detail.

After cooling the steel slab of the steels A to O having the component compositions shown in Table 1 at 1250 ° C. for 1 hour after rolling under conditions of a finish plate thickness of 3.2 mm and a finish rolling temperature of 900 ° C. of at least Ar3 point, the steel slab was cooled under the conditions shown in Table 2 It wound up at the winding temperature shown in Table 2. After pickling the manufactured hot rolled sheet steel, it cold-rolled to the thickness of 1.4 mm of finish plates, it was made into a cold rolled sheet steel, it annealed on the conditions shown in Table 2, and No. High strength steel sheets of 1 to 31 were produced. Moreover, as for the cooling conditions of the cooling in annealing, the residence time in the cooling stop temperature is 480 degreeC, the average cooling rate 20 degrees C / s or less, and 400-500 degreeC temperature range (temperature range of 500 degreeC-cooling stop temperature). It was 30 seconds. Annealing was performed using CAL, when plating was not performed. In addition, when performing plating, hot dip galvanization or alloying hot dip galvanization was performed using CGL. When making a plating layer into the alloying hot dip galvanizing layer, the alloying process hold | maintained at 510 degreeC for 10 second was implemented.

With respect to the obtained high strength steel sheet, the stress test and the tensile test were performed.

The area ratio of the steel structure is SEM in the range of sheet thicknesses 1/8 to 3/8 centered on the 1/4 position in the plate thickness direction from the steel plate surface side of the cross section perpendicular to the rolling width direction. Was observed and found by the point count method described in ASTM E 562-05. The average grain size of the ferrite was determined by observing the range of sheet thickness 1/8 to 3/8 centered on the sheet thickness quarter position by SEM, and calculating the circle equivalent diameter from the observation area and the number of crystal grains. . In the cross section perpendicular | vertical to a rolling width direction, the average aspect ratio of ferrite observes 1/4 position from the steel plate surface in the plate | board thickness direction with an optical microscope, and averages the line segment length per crystal grain of Table 1 of JIS G 0551. The average crystal grain length of the rolling direction and the plate | board thickness direction was computed by the method of calculating | required, and it calculated | required by (average crystal grain length of a rolling direction) / (average crystal grain length of a plate | board thickness direction). The average particle diameter of carbides (Nb carbide, Ti carbide, V carbide) performed TEM observation, and calculated | required the circle equivalent diameter by image processing. The total volume fraction of Nb carbide, Ti carbide and V carbide was determined by the extraction residue method. All observation was performed in each 10 visual field, and the average was computed. In addition, the results are shown in Table 2 (Table 2-1 and Table 2-2 are referred to as Table 2), and α in Table 2 means ferrite, P is pearlite, M is martensite, and θ is cementite. , α particle size means the ferrite average crystal grain size, M (C, N) particle size means the average particle diameter of carbide, M (C, N) volume ratio means the sum of the precipitation amount of Nb carbide, Ti carbide and V carbide . In addition, M in said M (C, N) means Nb, Ti, or V. FIG.

Tensile strength (TS) and yield ratio (YR) were calculated | required by the tensile test based on JISZ22241 using the JIS No. 5 tensile test piece extract | collected so that the tension direction might become perpendicular to a rolling direction. The anisotropy of tensile characteristics is evaluated by the difference between the yield strength in the tensile test performed in the direction parallel to the rolling direction, and the yield strength in the tensile test performed in the direction perpendicular to the rolling direction. The thing exceeding "(circle)" and 30 Mpa was made into "x" what was 30 Mpa or less. Moreover, the tensile strength of 330 Mpa-less than 500 Mpa and the yield ratio of 0.70 or more were evaluated as favorable.

TABLE 2-1

Table 2-2

Table 2 shows the results of the observation of the steel structure and the tensile test results. No. Since 1 to 3, 6, 8, 9, 12 to 16, 18, 19, 22, 24, 25, and 28 satisfy all of the requirements of the present invention, the high yield ratio and the anisotropy of the tensile property which are the object of the present invention. This small high strength steel sheet is obtained. On the other hand, No. 4, 5, 7, 10, 11, 17, 20, 21, 23, 26, 27, 29, 30, 31 are not shown because the component composition or manufacturing conditions are outside the scope of the present invention, and the desired emphasis is not obtained. The high strength steel sheet for which the invention was intended was not obtained.

Industrial availability

The high strength steel sheet of this invention is suitable for the field | area where the high yield ratio and the isotropy of tensile property are calculated | required centering on the board | plate part etc. in automobiles.

Claims (11)

The component composition is, in mass%, C: 0.02% to less than 0.10%, Si: less than 0.10%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01 to 0.10%, N: 0.010% or less, Nb: 0.005 to 0.070%, Ti: 0.100% or less (including 0%), V: 0.100% or less (including 0%) and Nb, Ti and V in total, 0.005 to 0.100% Containing, the balance consists of Fe and inevitable impurities,
The steel structure consists of an area ratio of ferrite: 90% or more, a total of pearlite and cementite: 0 to 10%, a total of martensite and residual austenite: 0 to 3%,
The average crystal grain size of the ferrite is 15.0 µm or less, the average aspect ratio is 5.0 or less,
Nb carbide and Ti carbide and / or V carbide, the average particle diameter of the Nb carbide and Ti carbide and / or V carbide is 5 to 50 nm,
The sum total of precipitation amount of Nb carbide, Ti carbide, and V carbide is 0.005 to 0.050% by volume ratio,
Tensile test performed in the tensile strength of 330 MPa or more and less than 550 MPa, yield ratio of 0.70 or more, and tensile strength in the direction parallel to the rolling direction, and tensile direction perpendicular to the rolling direction. The high strength steel plate whose difference in yield strength in is 30 MPa or less. The method of claim 1,
The component composition is, in mass%, any one of Cr: 0.3% or less, Mo: 0.3% or less, B: 0.005% or less, Cu: 0.3% or less, Ni: 0.3% or less, and Sb: 0.3% or less. High strength steel sheet containing two or more species. The method according to claim 1 or 2,
High strength steel sheet having a galvanized layer on the surface. The method of claim 3, wherein
A high strength steel sheet wherein the galvanized layer is a hot dip galvanized layer. The method of claim 4, wherein
A high strength steel sheet wherein the hot dip galvanized layer is an alloyed hot dip galvanized layer. The method of claim 3, wherein
A high strength steel sheet wherein the galvanized layer is an electrogalvanized layer. As a manufacturing method of the high strength steel plate of Claim 1 or 2,
A hot rolling process of hot rolling the steel, cooling the steel sheet under conditions of 10 seconds or less in a residence time in the temperature range of the finish rolling temperature to 650 ° C, and winding it at 500 to 700 ° C 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,
In the continuous annealing furnace, the cold-rolled steel sheet obtained by the cold rolling step is held at a temperature range of 650 ° C to 750 ° C at the time of temperature increase at a residence time of 60 seconds or less, and after the retention, a crack temperature of 760 to 880 ° C and a crack. Time: The manufacturing method of the high strength steel plate which has the annealing process of making it crack on the conditions of 120 second or less, and cooling on the conditions which the residence time of the temperature range of 400-500 degreeC is 100 seconds or less. The method of claim 7, wherein
The manufacturing method of the high strength steel plate which has a plating process of plating the cold rolled steel plate after the said annealing process. The method of claim 8,
The said plating process is a manufacturing method of the high strength steel plate which is hot dip galvanizing process. The method of claim 9,
The manufacturing method of the high strength steel plate which has the alloying process of alloying the cold rolled steel plate after the said plating process. The method of claim 8,
The said plating process is a manufacturing method of the high strength steel plate which is an electro zinc plating process.