KR101998652B1 - High-strength cold-rolled steel sheet and method for producing the same - Google Patents

Abstract

A high strength cold rolled steel sheet having a tensile strength of 1300 MPa or more and excellent in chemical conversion treatment and workability and a method for producing the same. C: not less than 0.05%, S: not more than 0.02%, Al: not less than 0.01% and not more than 0.05%, N: not less than 0.15% (Si) of [Si] / [Mn] ≥0.5 ([Si] satisfies the Si content and [Mn] satisfies the Mn content (mass%)) Iron and inevitable impurities. The steel sheet has an area ratio of 60% or more to less than 100% of tempering martensite, 5% or less (including 0%) of untransformed austenite, the remaining ferrite and an average crystal grain size of ferrite of less than 3.5 占 퐉, On the surface, the number of Si-Mn composite oxides having a circle equivalent diameter of 5 占 퐉 or less is 10/100 占 퐉 ^{2 or} less, and the surface coverage of the steel sheet mainly composed of Si is 1% or less.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength cold-rolled steel sheet and a method of manufacturing the same. BACKGROUND ART [0002]

The present invention relates to a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1300 MPa or more and excellent in chemical conversion property and processability, which is useful for use in automobile members, and a method for producing the same.

BACKGROUND ART [0002] In recent years, light weight and high strength of an automobile body are progressing in the background of a need for reduction of CO ₂ emissions and safety of collision. In order to reduce the weight of an automobile body, the use of parts is most effective. That is, in order to reduce the weight of the automobile body while maintaining the strength of the automobile body, it is effective to reduce the thickness of the steel sheet by increasing the strength of the steel sheet as a material for automobile parts. At present, the material for automobile parts, that is, the steel sheet for automobiles, has a tensile strength of 980 to 1180 MPa. However, the demand for high-strength steel sheet has been increasing more and more, and the steel sheet has the same elongation and elongation flangeability (hereinafter, referred to as workability, elongation and elongation flangeability are also referred to as ductility in some cases) , It is required to develop a high strength steel sheet having a tensile strength exceeding 1300 MPa.

The steel sheet for automobiles is painted and used, and a chemical treatment such as a phosphate treatment is carried out as a pretreatment before the painting. Since the chemical treatment of this steel sheet is one of the important treatments for securing corrosion resistance after coating, it is also required that the automotive steel sheet has excellent chemical conversion treatment.

From the above, it is necessary to develop a steel sheet excellent in chemical conversion treatability and workability with high strength, and various experiments have been conducted so far to achieve both high strength and high porosity of a steel sheet.

In Patent Document 1, by adding a large amount of C, a balance between strength and ductility is improved. However, when C is added in a large amount, deterioration of stretch flangeability due to the difference in hardness between two phases occurs.

In Patent Document 2, Si is utilized. However, when a large amount of Si is added, in the case of the production method described in Patent Document 2, Si oxide is formed on the surface of the steel sheet in the continuous annealing line and deterioration of chemical conversion treatment is predicted, which is not preferable as a steel sheet for automobiles .

In Patent Document 3, by the addition of Mn in a large amount, the Si-Mn composite oxide to fine dispersion and, chemical conversion treatability, by as much as possible reducing the steel sheet surface SiO ₂ with a box used as a nucleation site for zinc phosphate crystals on the surface of the steel sheet . However, it is difficult to achieve a tensile strength of 1300 MPa and an elongation of 10% or more in the amounts of C and Si described in Patent Document 3.

Japanese Laid-Open Patent Publication No. 2010-90432 Japanese Laid-Open Patent Publication No. 2012-12642 Japanese Patent Publication No. 3934604

In view of such circumstances, the present invention aims to provide a high-strength cold-rolled steel sheet having a tensile strength of 1300 MPa or more and excellent in chemical conversion treatment and workability and a method for producing the same.

Generally, in order to achieve a high strength of 1300 MPa or more at a low alloy cost, it is necessary to make the microstructure into a martensite single phase structure or a ferrite-martensite composite structure. However, since the elongation decreases with the increase in the strength of the steel sheet, optimization of component design, tissue control, and the like is important in order to achieve both high strength and processability.

It is possible to achieve high strength without significantly deteriorating the ductility by addition of Si and optimal structure control. However, as described above, the deterioration of the chemical conversion treatment due to the Si oxide is caused by the addition of Si. Therefore, when Si is used for the development of a high strength steel sheet for automobiles, a manufacturing process capable of removing Si oxide is essential.

Mn is effective for increasing the strength of the steel sheet. However, when Mn is added more than necessary, the steel structure is segregated at the time of casting, and ferrite and martensite are distributed in a band shape. As a result, anisotropy is generated in the mechanical properties, and the workability is deteriorated.

In view of the above, it has been found that, as a result of the studies, it has been found that the steel sheet containing the Mn and the Mn in a range satisfying the following formula (1) without addition of Mn more than necessary is pickled after cold- , And further by re-pickling to remove the Si-based oxide on the surface of the steel sheet, a high strength cold rolled steel sheet having a tensile strength of 1300 MPa or more and excellent in chemical conversion treatment and workability can be produced.

[Si] / [Mn]? 0.5? (One)

Here, [Si] represents the Si content (% by mass) and [Mn] represents the Mn content (% by mass).

The present invention has been made on the basis of the above-mentioned recognition, and it is based on the following points.

[1] A ferritic stainless steel having a composition of C: 0.15 to 0.22%, Si: 1.0 to 2.0%, Mn: 1.7 to 2.5%, P: 0.05%, S: 0.02% Al: not less than 0.01% to not more than 0.05%, N: not more than 0.005%, satisfies the following formula (1), and the balance is iron and inevitable impurities, The average crystal grain size of the ferrite is less than 3.5 占 퐉, and the surface of the steel sheet has a circle-equivalent diameter (diameter) of not less than 60% and not more than 100%, an untransformed austenite of not more than 5% Wherein the number of Si-Mn composite oxides of 5 占 퐉 or less is 10/100 占 퐉 ^{2 or} less, the coverage of a steel sheet surface mainly coated with Si is 1% or less, and the tensile strength is 1,300 MPa or more.

[Si] / [Mn]? 0.5? (One)

Here, [Si] represents the Si content (% by mass) and [Mn] represents the Mn content (% by mass).

[2] The high strength cold rolled steel sheet according to the above [1], which contains, in mass%, 0.010% or more and 0.020% or less of Ti in addition to the above composition.

[3] The high strength cold rolled steel sheet according to the above [1] or [2], further comprising 0.02% or more and 0.10% or less of Nb in terms of mass% in addition to the above composition.

[4] The high strength cold rolled steel sheet according to any one of [1] to [3], which further contains, in mass%, B: 0.0002% or more and 0.0020% or less.

[5] The method for producing a ferritic stainless steel according to [1] above, which comprises, in mass%, at least one of V: 0.01 to 0.30%, Mo: 0.01 to 0.30% To (4) above.

[6] The steel sheet according to any one of [1] to [5] above, which further comprises at least one of Cu: at least 0.01% and not more than 0.30%, Ni: at least 0.01% High strength cold rolled steel sheet.

0.001% or more and 0.100% or less of Sb, 0.0002% or more and 0.0100% or less of Ca, 0.01% or more and 0.10% or less of W, : 0.01 to 0.10%, and REM: 0.0002 to 0.0050%, based on the total weight of the high strength cold rolled steel sheet and the high strength cold rolled steel sheet.

[8] A steel material having the composition described in any one of [1] to [7] above is heated to a temperature of 1200 ° C or higher, followed by hot rolling at a finish rolling temperature of 800 ° C or higher, Rolled at a temperature of 700 ° C or less, cold-rolled, and then heated to an annealing temperature of Ac ₁ point or more and Ac ₃ point or less, and the residence time in the temperature range from Ac ₁ point to Ac ₃ point is from 30 seconds to 1200 seconds Or less at an annealing temperature, a primary cooling at an average cooling rate of less than 100 ° C / s to a primary cooling stop temperature of 600 ° C or more, an average cooling rate of 100 ° C / s or more Annealing treatment for secondary cooling to 1000 占 폚 / s or less is performed, and then the substrate is heated to a temperature of 100 占 폚 or higher and 300 占 폚 or lower, and tempering is performed at a temperature range of 100 占 폚 to 300 占 폚 in a temperature range of 120 seconds to 1800 seconds To be treated Method for manufacturing a high strength cold rolled steel sheet, that in addition, subjected to acid washing, acid washing material.

[9] The method for producing a high strength cold rolled steel sheet according to the above [8], wherein the acid cleaning is different from the acid cleaning solution used in acid cleaning, and a non-oxidizing acid is used as an acid cleaning solution.

In the present invention, the high strength cold rolled steel sheet is a cold rolled steel sheet having a tensile strength (TS) of 1300 MPa or more.

According to the present invention, a high strength cold rolled steel sheet having a tensile strength of 1300 MPa or more and excellent in chemical conversion treatment and workability can be obtained. Since the high strength cold rolled steel sheet of the present invention has a tensile strength of 1300 MPa or more and is excellent in chemical conversion treatment and workability, it can be suitably used for applications such as structural members of automobiles, And the effect is remarkable.

(Mode for carrying out the invention)

Hereinafter, the present invention will be described in detail. The following percentages are meant by mass% unless otherwise specified.

First, the reason for limiting the composition of the inventive steel sheet will be described.

C: not less than 0.15% and not more than 0.22%

C is an effective element for improving the balance between strength and ductility of a steel sheet. When the C content is less than 0.15%, it is difficult to secure a tensile strength of 1300 MPa or more. On the other hand, when the amount of C exceeds 0.22%, coarse cementite precipitates and workability such as stretch flangeability deteriorates. Therefore, the C content is set in the range of 0.15% or more and 0.22% or less. It is preferably at least 0.16%. And preferably not more than 0.20%.

Si: 1.0% or more and 2.0% or less

Si is an effective element for ensuring strength without significantly lowering the ductility of the steel sheet. When the amount of Si is less than 1.0%, a steel sheet with high strength and high porosity can not be produced. On the other hand, if the amount of Si exceeds 2.0%, the Si oxide on the surface of the steel sheet is not completely removed even though the step of acid cleaning and acid washing is performed, resulting in deterioration of chemical conversion treatment. Therefore, the amount of Si is set in the range of 1.0% or more and 2.0% or less. It is preferably at least 1.0%. And preferably 1.5% or less.

Mn: not less than 1.7% and not more than 2.5%

Mn is an element for increasing the strength of the steel sheet. When the Mn content is less than 1.7%, it is difficult to secure a tensile strength of 1300 MPa or more. On the other hand, when the Mn content exceeds 2.5%, a steel structure in which ferrite and martensite are distributed in a band shape is formed by segregation during casting. As a result, anisotropy is generated in the mechanical properties, and the workability is deteriorated. Therefore, the amount of Mn is set in the range of 1.7% or more and 2.5% or less.

[Si] / [Mn]? 0.5

Here, [Si] represents the Si content (% by mass) and [Mn] represents the Mn content (% by mass).

The amount of each of the Si-based oxide and the Si-Mn composite oxide is determined by the balance of Si and Mn. When either one of the oxides is extremely formed, the oxides are not completely removed from the surface of the steel sheet even though the acid washing and acid washing are performed, and the chemical conversion treatment is deteriorated. Therefore, it is necessary to define a quantitative ratio of Si and Mn. (Si-Mn composite oxide) mainly composed of Si-Mn is excessively produced when the amount of Mn is excessively larger than that of Si, i.e., when [Si] / [Mn] < 0.5. Is not obtained. Therefore, [Si] / [Mn]? 0.5 is set.

P: not more than 0.05%

Since P is an impurity element and deteriorates ductility, it must be reduced. If it exceeds 0.05%, the local ductility deteriorates due to grain boundary embrittlement accompanied by P segregation into an austenite grain boundary at the time of casting. As a result, the balance between strength and ductility deteriorates. Therefore, the P content should be 0.05% or less. It is preferably 0.02% or less.

S: not more than 0.02%

S is present as MnS in the steel sheet, and the impact resistance, the strength and the stretch flangeability are lowered. Therefore, it is preferable that S is reduced as much as possible. Therefore, the upper limit is set to 0.02%. It is preferably 0.002% or less.

Al: not less than 0.01% and not more than 0.05%

Al has an effect of reducing the oxides such as Si by forming oxides thereof and improving the ductility. However, if it is less than 0.01%, no significant effect is obtained. On the other hand, when Al is excessively added in excess of 0.05%, Al and N bond to form a nitride. This nitride precipitates on the austenite grain boundary during casting to cause grain boundary brittleness, which deteriorates stretch flangeability. Therefore, the amount of Al is set in a range of 0.01% or more and 0.05% or less.

N: 0.005% or less

N forms a nitride with Al and Ti to deteriorate elongation flangeability as described above. When the amount of N exceeds 0.005%, the stretch flangeability remarkably deteriorates due to the Ti and Al nitrides, and the decrease of the elongation due to the increase of solute N is remarkable. Therefore, the N content should be 0.005% or less. It is preferably 0.002% or less.

Ti: not less than 0.010% and not more than 0.020%

Since Ti has an effect of making the texture finer, it may be added as needed. When the amount of Ti is less than 0.010%, the effect of making the structure finer is small. On the other hand, addition of more than 0.020% not only saturates the effect of texture refinement but also forms coarse Ti and Nb complex carbides, which may result in a balance of strength and ductility and deterioration of stretch flangeability. In addition, the manufacturing cost is increased. For this reason, when Ti is added, the content is made 0.010% or more and 0.020% or less. It is preferably 0.012% or more. Preferably 0.018% or less.

Nb: 0.02% or more and 0.10% or less

Since Nb has the effect of making the structure finer like Ti, it may be added as needed. If the amount of Nb is less than 0.02%, the effect of making the structure finer is small. On the other hand, if the content exceeds 0.10%, the effect of texture refinement is not only saturated but also the coarse Ti and Nb complex carbides are formed to deteriorate balance of strength and ductility and stretch flangeability. In addition, manufacturing costs increase. For this reason, when Nb is added, it is set to 0.02% or more and 0.10% or less. It is preferably at least 0.04%. And preferably 0.08% or less.

B: not less than 0.0002% and not more than 0.0020%

B is segregated in the austenite grain boundary at the time of heating in continuous annealing, suppressing ferrite transformation and bainite transformation from austenite at the time of cooling, and facilitating the formation of tempered martensite. As a result, the steel sheet is strengthened. Therefore, it may be added as needed. When the B content is less than 0.0002%, the above effect is small. On the other hand, when the amount of B exceeds 0.0020%, boron carbide Fe ₂₃ (C, B) ₆ is generated, which may cause deterioration of workability and lowering of strength. Therefore, the amount of B is 0.0002% or more and 0.0020% or less when B is added.

In the present invention, it is preferable that at least one of V: 0.01% or more and 0.30% or less, Mo: 0.01% or more and 0.30% or less, and Cr: 0.01% or more and 0.30% or less is preferable.

V: not less than 0.01% and not more than 0.30%

The fine carbide formed by the combination of V and C is effective for precipitation strengthening of the steel sheet, and V may be added as needed. When the amount of V is less than 0.01%, the above effect is small. On the other hand, if the amount of V exceeds 0.30%, the carbide is excessively precipitated and the balance between strength and ductility may deteriorate. For this reason, the amount of V is preferably 0.01% or more and 0.30% or less when added.

Mo: not less than 0.01% and not more than 0.30%

Mo is effective for quenching strengthening of a steel sheet and may be added as necessary since it has an effect of refining steel structure. When the amount of Mo is less than 0.01%, the above effect is small. On the other hand, when the amount of Mo exceeds 0.30%, not only the effect is saturated but also the formation of Mo oxide on the surface of the steel sheet during the continuous annealing is promoted, and the chemical conversion treatment of the steel sheet is remarkably lowered in some cases. Therefore, when Mo is added, the amount of Mo is set to 0.01% or more and 0.30% or less.

Cr: not less than 0.01% and not more than 0.30%

Cr is effective for strengthening the quenching of the steel sheet, and may be added as needed. When the amount of Cr is less than 0.01%, the strengthening ability is small. On the other hand, if the amount of Cr exceeds 0.30%, the formation of Cr oxide on the surface of the steel sheet at the time of continuous annealing is promoted, so that the chemical conversion treatment of the steel sheet may be remarkably lowered. Therefore, when adding Cr, the Cr content is set to 0.01% or more and 0.30% or less.

In the present invention, it is preferable that at least 0.01% to 0.30% of Cu and 0.01% or more and 0.30% or less of Ni are contained in order to further improve the characteristics.

Cu: not less than 0.01% and not more than 0.30%

Cu suppresses ferrite transformation and bainite transformation from austenite during cooling in continuous annealing to facilitate the formation of tempered martensite to strengthen the steel sheet. Therefore, it may be added as needed. When the amount of Cu is less than 0.01%, the above effect is small. On the other hand, when the amount of Cu exceeds 0.30%, the ferrite transformation is excessively suppressed and the ductility may be lowered. Therefore, the amount of Cu is set to 0.01% or more and 0.30% or less when added.

Ni: not less than 0.01% and not more than 0.30%

Ni suppresses ferrite transformation and bainite transformation from austenite during cooling in continuous annealing to facilitate the formation of tempered martensite to strengthen the steel sheet. Therefore, it may be added as needed. When the amount of Ni is less than 0.01%, the above effect is small. When the amount of Ni exceeds 0.30%, the ferrite transformation is excessively suppressed and the ductility may be lowered. Therefore, when adding Ni, the amount of Ni is set to 0.01% or more and 0.30% or less.

In the present invention, in order to further improve the characteristics in the range of not adversely affecting the characteristics, 0.001 to 0.100% of Sn, 0.001 to 0.100% of Sb, 0.0002 to 0.0100% of Ca, : 0.01% or more and 0.10% or less, Co: 0.01% or more and 0.10% or less, and REM: 0.0002% or more and 0.0050% or less.

Sn: not less than 0.001% and not more than 0.100%, Sb: not less than 0.001% and not more than 0.100%

Sn, and Sb all have an effect of inhibiting surface oxidation, decarburization, and nitriding, so that they can be contained as needed. However, when the amounts of Sn and Sb are less than 0.001% each, the above effect is small. On the other hand, if the addition amount exceeds 0.100% each, the effect is saturated. For this reason, when Sn and Sb are added, the content is 0.001% or more and 0.100% or less, respectively. It is preferably 0.005% or more. It is preferably 0.010% or less.

Ca: 0.0002% or more and 0.0100% or less

Ca has an effect of improving the ductility through controlling the shape of the sulfide, strengthening the grain boundary, and strengthening the solid solution, so that Ca can be contained as needed. However, when the amount of Ca is less than 0.0002%, the above effect is small. If too much is added, ductility deteriorates due to grain boundary segregation or the like. For this reason, when Ca is added, the content is 0.0002% or more and 0.0100% or less.

W: 0.01% or more and 0.10% or less, Co: 0.01% or more and 0.10% or less

W and Co all have an effect of improving the ductility through the shape control of the sulfide, strengthening of the grain boundary, and solid solution strengthening, so that it can be contained if necessary. However, when the amounts of W and Co are less than 0.01%, the above effect is small. On the other hand, excessive addition causes deterioration of ductility due to grain boundary segregation or the like. For this reason, when W and Co are added, they are set to 0.01% or more and 0.10% or less, respectively.

REM: not less than 0.0002% and not more than 0.0050%

The REM has an effect of improving the ductility through the shape control of the sulfide, the strengthening of the grain boundary, and the strengthening of the solid solution, so that REM can be contained as needed. However, when the REM amount is less than 0.0002%, the above effect is small. On the other hand, excessive addition causes deterioration of ductility due to grain boundary segregation or the like. Therefore, when REM is added, it is 0.0002% or more and 0.0050% or less.

In the present invention, the balance other than the above is Fe and inevitable impurities. The inevitable impurities include O (oxygen), and the content of O is 0.01% or less.

Next, the organization which is an important requirement of the steel sheet of the present invention will be described.

The steel sheet according to any one of claims 1 to 5, wherein the tempering martensite contains 60% or more and less than 100%, untransformed austenite is 5% or less (including 0%), the remainder is ferrite,

The tensile strength of the steel of the structure having tempering martensite and ferrite rises with an increase in the area ratio of the tempering martensite. In tempering martensite and ferrite, the tempering martensite has a higher hardness and the deformation resistance at the time of tensile deformation is the tempering martensite. The larger the area ratio of the tempering martensite is, the more the tempering martensite single phase structure As shown in Fig. In the steel component range of the present invention, when the area ratio of the tempering martensite is less than 40%, the tensile strength is not more than 1300 MPa. Further, when the area of the interface between the tempering martensite and the ferrite is large, that is, when the area ratio of the tempering martensite is 40% or more and less than 60%, the generation frequency of the voids due to the difference in hardness between the two phases increases, Is likely to be connected to advance the progress of the crack, so that the stretch flangeability deteriorates. From the above, in order to improve the workability while securing the tensile strength, the area ratio of the tempering martensite is required to be 60% or more. On the other hand, when the area ratio of the tempered martensite is 100%, excellent workability can not be obtained. Further, untransformed austenite of 5% or less may be inevitably mixed. However, if it is 5% or less, there is no problem in obtaining the effect of the present invention, which is acceptable. From the above, the area ratio of tempered martensite is less than 100%, the untransformed austenite is not more than 5% (including 0%), and the remainder is ferrite. The lower limit of the area ratio of suitable tempering martensite is 70%. A suitable upper limit is 90%.

When the average crystal grain size of the ferrite is not less than 3.5 탆, the crystal grain refinement strengthening is insufficient, so that the predetermined strength can not be obtained. In addition, when deformed, deformation of deformation easily occurs between crystal grains, which deteriorates workability. Therefore, the average crystal grain size of the ferrite is less than 3.5 占 퐉.

The area ratio of the tempered martensite, the area ratio of the ferrite, and the average crystal grain size of the ferrite can be measured by the methods of Examples described later.

The number of Si-Mn composite oxides having a circle equivalent diameter of 5 占 퐉 or less is 10/100 占 퐉 ^{2 or} less

If the Si-Mn composite oxide is present on the surface of the steel sheet, the chemical conversion treatment property is remarkably deteriorated. Needless to say, when the coarse Si-Mn composite oxide is present on the surface of the steel sheet, the chemical conversion property is deteriorated. Even when the Si-Mn composite oxide having a circle equivalent diameter of 5 占 퐉 or less becomes a distribution shape exceeding a certain constant density, deterioration of chemical conversion property becomes present. Therefore, the number of Si-Mn composite oxides having a circle-equivalent diameter of 5 占 퐉 or less is defined as 10/100 占 퐉 ^{2 or} less. At a ratio of 10/100 mu m < ^{2 >} or more, a region in which zinc phosphate crystals are not formed is present, deteriorating chemical conversion treatability. Preferably 0 parts / 100 mu m < ^{2 &} gt ;.

The number of Si-Mn composite oxides having a circle-equivalent diameter of 5 탆 or less can be measured by the method of Examples described later. Further, the surface is a range from the surface layer to the position of 3% with respect to the plate thickness in the plate thickness direction.

The surface coverage of the steel sheet mainly composed of Si is 1% or less

When an oxide mainly composed of Si is present on the surface of the steel sheet, the chemical conversion treatment property remarkably decreases. Therefore, the surface coverage of the steel sheet mainly composed of Si is set to 1% or less. Preferably 0%. The oxide mainly containing Si is, for example, SiO ₂ . Further, the oxide mainly composed of Si can be measured by the method of Examples described later.

The steel sheet surface coverage of the above-described structure, the number of Si-Mn composite oxides, and the oxide mainly composed of Si can be obtained by controlling the acid cleaning, particularly the acid cleaning, after annealing in the production method described later.

Next, a method of manufacturing the high-strength cold-rolled steel sheet of the present invention will be described.

In the high-strength cold-rolled steel sheet of the present invention, steel material (steel slab) having the above-mentioned composition is heated to a temperature of 1200 캜 or higher, followed by hot rolling at a finish rolling temperature of 800 캜 or higher, Lt; / RTI > and cold rolled. Then, the residence time in the temperature range from Ac ₁ point to Ac ₃ point is 30 seconds or more and 1200 seconds or less by heating to an annealing temperature of Ac ₁ point or more and Ac ₃ point or less, and at the annealing temperature, Or more to an average cooling rate of less than 100 占 폚 / s and an annealing process for secondary cooling to an average cooling rate of 100 占 폚 / s or more and 1000 占 폚 / sec or less to a second cooling stop temperature of 100 占 폚 or less. Subsequently, the substrate is heated to a temperature of 100 ° C or higher and 300 ° C or lower, and subjected to a tempering treatment in which the retention time in the temperature range from 100 ° C to 300 ° C is 120 seconds or longer and 1800 seconds or shorter. Further, acid cleaning and acid cleaning are performed The high-strength cold-rolled steel sheet of the present invention can be produced. In the acid cleaning, it is preferable to use a non-oxidizing acid as the acid cleaning liquid, which is different from the acid cleaning liquid used in the acid cleaning.

The Ac ₁ point and Ac ₃ point are values (° C) obtained from a transformation expansion curve obtained at an average heating rate of 3 ° C / s using a thermal expansion measuring apparatus.

In the present invention, the method of the solvent of the steel is not particularly limited, and a known solvent method such as a converter, an electric furnace or the like can be adopted. Further, secondary refining may be performed in a vacuum degassing furnace. Subsequently, it is preferable to make the slab (steel material) by the continuous casting method in view of the problem of productivity and quality. However, it is preferable to use a casting method such as ingot casting-blooming rolling, thin-slab continuous casting, The slab may be formed by a known casting method.

Heating temperature of steel material: 1200 ℃ or more

If the heating temperature is less than 1200 ° C, the carbide is not re-dissolved and the workability is deteriorated. Therefore, the heating temperature of the steel material should be 1200 ° C or higher. If the heating temperature is excessively high, it is connected to an increase in scale loss accompanied by an increase in the oxidation mass. Therefore, the heating temperature of the steel material is preferably 1300 DEG C or less. However, in the case of hot rolling the steel material, when the steel material after casting is in the temperature range of 1200 ° C or higher, or when the carbonaceous material of the steel material is dissolved, the steel material is directly rolled ). The condition of rough rolling is not particularly limited.

Finishing Rolling Out temperature: 800 ℃ or more

A uniform hot-rolled matrix phase structure can be obtained by setting the finishing rolling output temperature at 800 占 폚 or higher. If the finish rolling temperature is lower than 800 占 폚, the steel sheet becomes uneven and the ductility of the steel sheet deteriorates and the risk of various problems during molding increases. Therefore, the finishing rolling output temperature is set to 800 ° C or higher. The upper limit of the finishing rolling output temperature is not particularly restricted, but rolling at an excessively high temperature causes scale defects and the like, and therefore, it is preferably 1000 占 폚 or less.

Coiling temperature: 450 캜 to 700 캜

If the coiling temperature after hot rolling is below 450 캜, the processed structure generated by hot rolling remains, and the rolling load of cold rolling, which is the next step, becomes large. If the coiling temperature exceeds 700 ° C, coarse grains are generated, the steel sheet structure becomes uneven, and ductility is lowered. Therefore, the coiling temperature is set to 450 ° C or higher and 700 ° C or lower. The lower limit of the suitable coiling temperature is 500 캜. A suitable upper limit is 650 占 폚.

Hot rolled, and wound, and then subjected to pickling, if necessary, followed by cold rolling. The condition of pickling is not particularly limited. Cold rolling needs to be carried out to obtain a desired sheet thickness. There is no restriction on the cold rolling rate, but it is preferably 30% or more and 80% or less in the constraint of the production line.

Ac is heated to an annealing temperature of not less than ₁ point and not more than ₃ points and the residence time in a temperature range from Ac ₁ point to Ac ₃ point is from 30 seconds to 1200 seconds

When the annealing temperature is lower than Ac ₁ point, austenite (transformation into martensite after quenching) necessary for securing a predetermined strength is not generated during annealing, and even when quenching is performed after annealing, a predetermined strength is not obtained. Even an annealing temperature of Ac than _three, by controlling the ferrite area ratio of the precipitated during cooling from the annealing temperature, can be in the area ratio to obtain more than 60% martensite, but if annealing at Ac exceeds _three, the desired It is difficult to obtain a metal structure. Therefore, the annealing temperature is set to Ac ₁ point or more and Ac ₃ point or less. From the viewpoint of stably ensuring an equilibrium area ratio of the austenite of 60% or more in this temperature range, the annealing temperature is preferably 780 DEG C or higher. In addition, if the residence time at the annealing temperature is too short, the microstructure is not sufficiently annealed, resulting in a nonuniform structure in which the processed structure due to cold rolling exists, and the ductility is deteriorated. On the other hand, if the retention time is too long, the production time is increased, which is not preferable in terms of production cost. Therefore, the residence time is 30 to 1200 seconds. The lower limit of the suitable residence time is 150 seconds. A suitable upper limit is 600 seconds.

At the annealing temperature, the primary cooling stop temperature is 600 ° C or higher. The average cooling rate is less than 100 ° C / s,

(Slow cooling) to a first cooling stop temperature (slow cooling stop temperature) of 600 ° C or more at an average cooling rate of less than 100 ° C / s at the annealing temperature. It is possible to control the balance between strength and ductility by precipitating ferrite during the slow cooling from the annealing temperature. When the slow cooling stopping temperature (primary cooling stopping temperature) is less than 600 占 폚, a large amount of pearlite is generated in the microstructure and the strength is rapidly lowered, so that a tensile strength of 1300 MPa or more can not be obtained. In order to obtain a predetermined strength more stably, the temperature is preferably 680 DEG C or more.

Further, when the average cooling rate is 100 ° C / s or more, a sufficient amount of ferrite is not precipitated during cooling, so that excellent ductility can not be obtained. The ductility of the tempering martensite and the ferrite metal structure contemplated in the present invention is due to the high work hardening ability expressed by the mixture of hard tempering martensite and soft ferrite. However, when the average cooling rate is 100 ° C / s or more, carbon concentration into austenite during cooling becomes insufficient, and hard martensite is not obtained during quenching. As a result, the work hardening ability of the final structure deteriorates and sufficient ductility can not be obtained. In this respect, the average cooling rate is set to be less than 100 ° C / s. In order to sufficiently generate carbon enrichment into the austenite, an average cooling rate of 5 DEG C / s or less is preferable.

Secondary Cooling Stopping Temperature Below 100 ℃ Average Cooling Rate 100 ℃ / s to 1000 ℃ / s Secondary Cooling

Following the slow cooling, cooling (quenching) is carried out at an average cooling rate of 100 ° C / s or more and 1000 ° C / s or less to a second cooling stop temperature of 100 ° C or less. When the average cooling rate is less than 100 ° C / s, the austenite is transformed into ferrite, bainite or pearlite during cooling, so that a predetermined strength can be obtained none. On the other hand, if the average cooling rate exceeds 1000 캜 / s, shrinkage cracking of the steel sheet due to cooling may occur. For this reason, the average cooling rate during quench is set to 100 ° C / s or more and 1000 ° C / s or less. The quenching is preferably quenched by water quenching.

The secondary cooling stop temperature should be 100 ℃ or less. When the secondary cooling quench temperature is higher than 100 ° C, the quenching of the austenite does not sufficiently occur during the quenching, and the reduction of the area ratio of the martensite and the reduction of the material due to the self-tempering of the martensite produced by quenching It is undesirable from the viewpoint of production because it entails a decrease in strength.

Heating to a temperature of 100 ° C or more and 300 ° C or less and tempering treatment in which the residence time in the temperature range from 100 ° C to 300 ° C is 120 seconds or more and 1800 seconds or less

Following the quenching, a tempering treatment is performed to reheat the martensite to a temperature of 100 ° C or more and 300 ° C or less and to stand at a temperature range of 100 to 300 ° C for 120 to 1800 seconds. This tempering softens martensite and improves workability. When the tempering is performed at a temperature lower than 100 캜, the softening of the martensite is insufficient and the improvement in the workability can not be expected, and the difference in hardness between the ferrite and the ferrite is increased. Further, if the tempering is performed at a temperature higher than 300 DEG C, not only the manufacturing cost for reheating is increased but also the strength is remarkably lowered, and a useful effect can not be obtained. Preferably in the range of 150 to 250 캜. When the residence time is less than 120 seconds, the softening of the martensite does not sufficiently occur in the temperature range from 100 占 폚 to 300 占 폚, so that an effect of improving workability can not be expected. If the retention time exceeds 1800 seconds, the softening of the martensite proceeds excessively, whereby the strength is remarkably lowered, and the production cost is increased due to an increase in reheating time, which is not preferable.

Acid cleaning, acid cleaning

Acid cleaning and acid cleaning are performed to remove Si oxide and Si-Mn oxide on the surface of the steel sheet to improve the chemical conversion treatment. In the acid cleaning, it is preferable to use a non-oxidizing acid as the acid cleaning liquid, which is different from the acid cleaning liquid used in the acid cleaning.

Acid cleaning can be carried out by a general method, and the conditions are not particularly limited. For example, any of nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, and an acid mixture of two or more thereof can be used.

The steel sheet subjected to the tempering treatment is subjected to acid cleaning using, for example, a strong acid such as nitric acid at a concentration of more than 50 g / L and not more than 200 g / L, whereby an oxide or Si-Mn composite It is possible to remove the oxide. However, by the strong acid cleaning, Fe dissolved from the surface of the steel sheet produces an iron-based oxide, precipitating and precipitating on the surface of the steel sheet and covering the surface of the steel sheet deteriorates the chemical conversion treatment. Therefore, in order to improve the chemical conversion treatment, it is necessary to dissolve and remove the iron-based oxide precipitated on the surface of the steel sheet by subjecting the steel sheet to further acid washing after the strong acid cleaning. For the reasons described above, in the acid cleaning, it is preferable to use a non-oxidizing acid as the acid cleaning liquid, which is different from the acid cleaning liquid used in acid cleaning. The non-oxidizing acid includes, for example, hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid and an acid mixture of two or more thereof. For example, hydrochloric acid having a concentration of 0.1 to 50 g / L, sulfuric acid of 0.1 to 150 g / L, acid of 0.1 to 20 g / L of hydrochloric acid and 0.1 to 60 g / L of sulfuric acid may be suitably used.

Thus, a high strength cold rolled steel sheet having a tensile strength (TS) of 1300 MPa or more and excellent in chemical conversion treatment and workability is produced. Since the high-strength cold-rolled steel sheet of the present invention is excellent in plate formability after annealing, a step for calibrating the shape of a steel sheet such as rolling or leveling is not necessarily required, but from the viewpoint of adjusting the material and surface roughness , There is no problem even if the steel sheet after annealing is rolled at an elongation percentage of about several%. Since the high strength cold rolled steel sheet of the present invention does not affect the quality of the material even by the plating treatment or the composition of the plating bath, any of the hot dip galvanizing treatment, the galvannealed hot dip galvanizing treatment and the electro galvanizing treatment is carried out can do.

Example 1

The specimens A to R having the composition shown in Table 1 were vacuum-melted to obtain slabs, and hot-rolled under the conditions described in Table 2 to obtain hot-rolled steel sheets. The hot-rolled steel sheet was acid washed to remove the surface scale, and then cold-rolled (rolled rate: 60%) was performed. Subsequently, the continuous annealing and the tempering treatment were performed under the conditions shown in Table 2, and the acid cleaning and the acid cleaning were carried out.

Ac ₁ point and Ac ₃ point were obtained from the expansion curve obtained at an average heating rate of 3 ° C / s using a thermal expansion measuring apparatus.

A test piece was taken from the steel sheet obtained by the above, and observation (measurement) of a metal structure, tensile test and hole expansion test were carried out. In addition, the number of Si-Mn oxides with a circle-equivalent diameter of 5 占 퐉 or less and the coverage rate of the oxide mainly composed of Si on the steel sheet surface were determined. Also, the chemical conversion properties were examined. Each measurement method and calculation method is shown below.

The observation of the metal structure was performed by cutting the plate thickness section parallel to the rolling direction to be the observation surface, etching the center of the plate thickness to 1% or nital, and then observing a typical microstructure with a scanning electron microscope (SEM). Based on the SEM image at a magnification of 1000 times, the two-phase volume ratio was determined by a point counting method, and the particle size of each phase was determined by a line segment method. The obtained volume ratio was defined as the area ratio.

In the tensile test, a JIS No. 5 test piece (distance between target points: 50 mm, parallel portion width: 25 mm) was cut parallel to the rolling direction and the deformation rate was 3.3 × 10 ^-3 s ^-1 . Total elongation was measured by the butt of the specimen after fracture.

The hole expansion test was carried out by using a test piece of 100 mm x 100 mm in size. After punching a circular hole having a diameter of 10 mm (d ₀ ), it was pressed with a blank holding force of 9 tons using an inner diameter 75 mm dies , Vertical angle: 60 ° The cone punch was pushed up from below with respect to the hole, and the pore diameter (d) at the time when a plate thickness penetration crack occurred was measured at the edge of the hole. Then, the hole expanding ratio:? (%) Defined by the following formula was obtained. Further, the test was carried out so that the punching of holes and the hole expansion were made in the same direction with the surface on which burrs due to punching occurred on the upper side (in accordance with JIS 2256).

? (%) = {(d-d0) / d0}

Here, d0 is the initial pore diameter, and d is the pore diameter at the time when the crack penetrates through the plate thickness.

The number of Si-Mn oxide of the circle-equivalent diameter 5㎛ below, by making the extraction replica of a steel surface layer (replica film), and 15000-fold TEM observation this, the average number of arbitrary field of view of 20 (100㎛ ² per) I investigated. The number of Si-Mn oxide of the circle-equivalent diameter of less than 10 5㎛ / 100㎛ ² "that" is not less than, and the "none" to 10 / 100㎛ ² below. The Si-Mn oxide is subjected to EDX analysis and diffraction pattern analysis is performed on the oxide in which Si, Mn and O are detected to obtain spots of Mn ₂ SiO ₄ or MnSiO ₃ , .

The surface coverage of the steel sheet mainly composed of Si was determined by observing the five fields of view at 1000 times using the SEM using the SEM and analyzing the same field of view with EDX to identify oxides mainly composed of Si in the same manner as above And the coating rate was obtained by a point counting method (a method of drawing 15 straight lines in each of the longitudinal and lateral directions of the SEM image and finding the probability that the Si main body oxide exists at the intersection (225 points)).

The chemical conversion treatment was carried out under the conditions of a bath temperature of 35 ° C and a treatment time of 120 seconds using a commercially available chemical conversion agent (Pearlbond PB-L3065 (registered trademark) manufactured by Nippon Pharmaceutical Co., Ltd.) , And the surface of the steel sheet after the chemical conversion treatment was observed with SEM at a magnification of 500 times for 5 days so as to obtain a uniform chemical conversion with an area ratio of 95% , And when defects exceeding 5% of the area ratio were confirmed even in one field of view, the chemical conversion property was rated as " x ".

Table 3 shows the results obtained by the above.

According to Tables 1 to 3, the examples suitable for the conditions of the present invention are those having a high tensile strength (TS) of 1300 MPa or more, an elongation (EL) of 10% or more and a hole expanding rate (?) Of 30% . Also, it is excellent in chemical conversion treatment.

No. 12 is a comparative example in which the C content is higher than the range of the present invention. As the C content is high, the strength of the martensite rises, and the balance between strength and ductility is excellent, but the stretch flangeability is remarkably low due to the difference in hardness between ferrite and martensite.

Nos. 13 and 14 are comparative examples in which the Si content is out of the range of the present invention. In No. 13, Si oxide was present on the surface of the steel sheet even after the acid washing treatment in the two steps, so that the chemical treatment property was not satisfied. On the other hand, in No. 14, a predetermined elongation is not obtained.

Nos. 15 and 16 are comparative examples in which the Mn content is out of the range of the present invention. Since Mn is an element that largely changes the martensite fraction, a predetermined elongation is not obtained in No. 15 having a high content. In the case of No. 16 having a low content, the martensite fraction is small, so that the predetermined strength is not obtained.

Nos. 18 to 23 are comparative examples in which the production conditions are outside the scope of the present invention. No. 18 is a comparative example in which the composition of the components and the production conditions are outside the scope of the present invention. In addition to the fact that a predetermined elongation is not obtained, elongation flangeability and chemical conversion treatment are inferior.

Since the annealing temperature of No. 19 is high, a predetermined strength and elongation are not obtained.

In Nos. 20 to 22, a sufficient martensite fraction was not obtained, and a predetermined strength was not obtained.

In No. 23, a sufficient martensite fraction was not obtained, and the stretch flangeability was inferior.

No. 24 is an example in which acid pickling treatment after annealing is omitted. Si oxide is present on the surface of the steel sheet, and the chemical conversion property is not satisfied.

Claims (9)

Wherein the composition comprises, by mass%, C: 0.15 to 0.22%, Si: 1.0 to 2.0%, Mn: 1.7 to 2.5%, P: 0.05%, S: 0.02% At least one selected from the group consisting of Cu: not less than 0.01% and not more than 0.05%, N: not more than 0.005%, Cu: not less than 0.01% and not more than 0.30% , The balance consisting of iron and inevitable impurities,
The microstructure of the structure is such that the tempering martensite is 60% or more and less than 100%, the untreated austenite is not more than 5% (including 0%), and the remainder is ferrite, and the average crystal grain size of the ferrite is less than 3.5 占 퐉,
Wherein the number of Si-Mn composite oxides having a circle equivalent diameter of 5 占 퐉 or less is 10/100 占 퐉 ^{2 or} less on the surface of the steel sheet and the surface coverage of the steel sheet mainly composed of Si is 1%
A high strength cold rolled steel sheet having a tensile strength of 1300 MPa or more.
[Si] / [Mn]? 0.5? (One)
Here, [Si] represents the Si content (% by mass) and [Mn] represents the Mn content (% by mass). The method according to claim 1,
A high strength cold rolled steel sheet comprising at least one of the following groups (A) to (E)
(A) in mass%, Ti: not less than 0.010% and not more than 0.020%
(B) In terms of% by mass, Nb: 0.02% or more and 0.10% or less
(C)% by mass, B: not less than 0.0002% and not more than 0.0020%
(D) 0.01 to 0.30% of V, 0.01 to 0.30% of Mo, 0.01 to 0.30% of Cr,
0.001% or more and 0.100% or less of Sb, 0.0002% or more and 0.0100% or less of Ca, 0.01% or more and 0.10% or less of Co, 0.01% or more and 0.10% or less of Co, At least one of 0.0002% or more and 0.0050% or less of REM A steel material having the composition described in claim 1 or 2 is heated to a temperature of 1200 占 폚 or higher and then subjected to hot rolling at a finish rolling temperature of 800 占 폚 or higher and then heated at a temperature of 450 占 폚 to 700 占 폚 Rolled, cold rolled,
Then, the residence time in the temperature range from Ac ₁ point to Ac ₃ point is 30 seconds or more and 1200 seconds or less by heating to an annealing temperature of Ac ₁ point or more and Ac ₃ point or less, and at the annealing temperature, Annealing treatment for secondary cooling at an average cooling rate of 100 占 폚 / s or more and 1000 占 폚 / sec or less to a temperature of a second cooling stop temperature of 100 占 폚 or less and,
Subsequently, the substrate is heated to a temperature of 100 ° C or more and 300 ° C or less, and subjected to a tempering treatment in which the residence time in the temperature range from 100 ° C to 300 ° C is 120 seconds or more and 1800 seconds or less,
Further, acid washing and acid washing are carried out. The method of claim 3,
Wherein the acid cleaning is different from the acid cleaning solution used in the acid cleaning, and the non-oxidizing acid is used as an acid cleaning solution. delete delete delete delete delete