EP3272892A1

EP3272892A1 - High-strength cold-rolled steel sheet and method for manufacturing same

Info

Publication number: EP3272892A1
Application number: EP16764383.2A
Authority: EP
Inventors: Shimpei Yoshioka; Katsutoshi Takashima; Kohei Hasegawa; Yoshimasa Funakawa
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2015-03-18
Filing date: 2016-02-16
Publication date: 2018-01-24
Anticipated expiration: 2036-02-16
Also published as: WO2016147549A1; KR101998652B1; CN107429344A; US20180037969A1; EP3272892B1; JP6210175B2; EP3272892A4; JPWO2016147549A1; MX2017011825A; KR20170116112A

Abstract

Provided are a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1,300 MPa or higher, a good chemical conversion property, and good formability, and a method for producing the high-strength cold-rolled steel sheet.

The high-strength cold-rolled steel sheet contains C: 0.15% or more and 0.22% or less, Si: 1.0% or more and 2.0% or less, Mn: 1.7% or more and 2.5% or less, P: 0.05% or less, S: 0.02% or less, Al: 0.01% or more and 0.05% or less, N: 0.005% or less, O: 0.01% or less, and the balance being iron and unavoidable impurities. The composition satisfies [Si]/[Mn] ≥ 0.5 ([Si] represents the Si content, and [Mn] represents the Mn content (% by mass)). The steel sheet has a structure including, in terms of area fraction, 60% or more and less than 100% of tempered martensite, 5% or less (inclusive of 0%) of untransformed austenite, and the balance being ferrite. The ferrite has an average crystal grain size of less than 3.5 µm. Less than 10 particles/100 µm² of Si-Mn compound oxide particles having a circle equivalent diameter of 5 µm or less are present on the surface of the steel sheet. The surface of the steel sheet is covered with Si-based oxide at a coverage of 1% or less.

Description

Technical Field

The present invention relates to a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1,300 MPa or higher, a good chemical conversion property, and good formability and being useful in applications pertaining to automotive components, and to a method for producing the high-strength cold-rolled steel sheet.

Background Art

In recent years, automotive bodies have been reduced in weight and increased in strength because of the need for CO₂ emission reduction and collision safety. Automotive bodies can be most effectively reduced in weight by thinning parts used for automotive bodies. Specifically, the weight reduction of automotive bodies with high strength can be most effectively achieved by thinning strengthened steel sheets used as materials for automotive parts. Currently, materials of automotive parts, namely, steel sheets for automobiles, have a tensile strength of about 980 to 1,180 MPa. However, there is an increasing demand for steel sheets having higher strength, and thus there is a need to develop high-strength steel sheets having a tensile strength of higher than 1,300 MPa while having elongation and stretch flangeability (hereinafter, elongation and stretch flangeability are collectively referred to as formability, and elongation may be referred to as ductility) similar to those in the related art.
Steel sheets for automobiles are used after the steel sheets have been painted. Before painting, such steel sheets are treated with chemical conversion such as phosphate conversion. Since chemical conversion of steel sheets is an important treatment to ensure corrosion resistance after painting, steel sheets for automobiles need to have a good chemical conversion property.
It is thus necessary to develop high-strength steel sheets having a good chemical conversion property and good formability, and various attempts have been made to form steel sheets having both high strength and high formability.
In Patent Literature 1, the balance between strength and ductility has been improved by adding a large amount of C. However, addition of a large amount of C leads to a deterioration in stretch flangeability due to a difference in hardness between two phases.
In Patent Literature 2, Si is used. In the production method described in Patent Literature 2, however, addition of a large amount of Si may cause Si oxide to form on the surface of a steel sheet in the continuous annealing line, degrading the chemical conversion property. This degradation is not desirable when using such a steel sheet for automobiles.
In Patent Literature 3, addition of a large amount of Mn causes Si-Mn compound oxides to be finely dispersed on the surface of a steel sheet and allows the Si-Mn compound oxides to serve as nucleation sites for zinc phosphate crystals, which reduces the amount of SiO₂ on the surface of the steel sheet to as low an amount as possible to ensure the chemical conversion property. It is, however, difficult to obtain a tensile strength of 1,300 MPa and an elongation of 10% or higher when the C content and the Si content are as described in Patent Literature 3.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2010-90432
PTL 2: Japanese Unexamined Patent Application Publication No. 2012-12642
PTL 3: Japanese Patent No. 3939509

Summary of Invention

Technical Problem

In light of the above-mentioned circumstances, an object of the present invention is to provide a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1,300 MPa or higher, a good chemical conversion property, and good formability and to a method for producing the high-strength cold-rolled steel sheet.

Solution to Problem

In general, to obtain a high strength of 1,300 MPa or higher at low alloying costs, the microstructure needs to be converted into a martensite single-phase structure or a ferrite-martensite composite structure. However, since the elongation of steel sheets decreases with increasing strength, it is important to optimize, for example, composition design and structural control in order to obtain both high strength and high formability.
Addition of Si and optimal structural control enables an increase in strength without a significant decrease in ductility. As described above, however, addition of Si results in a poor chemical conversion property because of the formation of Si oxide. Therefore, using Si in development of high-strength steel sheets for automobiles needs a production process for removing Si oxide.
Manganese (Mn) is effective in increasing the strength of steel sheets. However, addition of excess Mn causes segregation during casting and thus results in the formation of a steel structure in which ferrite and martensite are distributed in the form of stripes. This causes anisotropy of mechanical properties and degrades formability.
As a result of studies in light of the foregoing, it has been found that a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1,300 MPa or higher, a good chemical conversion property, and good formability can be produced by pickling a steel sheet that does not contain more Mn than necessary but contains Si and Mn in the range satisfying formula (1) below and that has been continuously annealed after cold rolling, and further re-pickling the steel sheet to remove Si-based oxide on the surface of the steel sheet: $[Si] / [Mn] \geq 0.5$
in the formula, [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 such a finding and will be summarized below.
In a first embodiment, a high-strength cold-rolled steel sheet has a composition including, in terms of % by mass, C: 0.15% or more and 0.22% or less, Si: 1.0% or more and 2.0% or less, Mn: 1.7% or more and 2.5% or less, P: 0.05% or less, S: 0.02% or less, Al: 0.01% or more and 0.05% or less, N: 0.005% or less, and the balance being iron and unavoidable impurities. The composition satisfies formula (1) below. The steel sheet has a structure including, in terms of area fraction, 60% or more and less than 100% of tempered martensite, 5% or less (inclusive of 0%) of untransformed austenite, and the balance being ferrite. The ferrite has an average crystal grain size of less than 3.5 µm. Less than 10 particles/100 µm² of Si-Mn compound oxide particles having a circle equivalent diameter of 5 µm or less are present on the surface of the steel sheet. The surface of the steel sheet is covered with Si-based oxide at a coverage of 1% or less. The steel sheet has a tensile strength of 1,300 MPa or higher: $[Si] / [Mn] \geq 0.5$
in the formula, [Si] represents the Si content (% by mass), and [Mn] represents the Mn content (% by mass).
In a second embodiment, the high-strength cold-rolled steel sheet according to the first embodiment has the composition further including, in terms of % by mass, Ti: 0.010% or more and 0.020% or less.
In a third embodiment, the high-strength cold-rolled steel sheet according to the first or second embodiment has the composition further including, in terms of % by mass, Nb: 0.02% or more and 0.10% or less.
In a fourth embodiment, the high-strength cold-rolled steel sheet according to any one of the first to third embodiments has the composition further including, in terms of % by mass, B: 0.0002% or more and 0.0020% or less.
In a fifth embodiment, the high-strength cold-rolled steel sheet according to any one of the first to fourth embodiments has the composition further including, in terms of % by mass, at least one selected from 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.
In a sixth embodiment, the high-strength cold-rolled steel sheet according to any one of the first to fifth embodiments has the composition further including, in terms of % by mass, at least one selected from Cu: 0.01% or more and 0.30% or less and Ni: 0.01% or more and 0.30% or less.
In a seventh embodiment, the high-strength cold-rolled steel sheet according to any one of the first to sixth embodiments has the composition further including, in terms of % by mass, at least one selected from Sn: 0.001% or more and 0.100% or less, Sb: 0.001% or more and 0.100% or less, Ca: 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, and REM: 0.0002% or more and 0.0050% or less.
In an eighth embodiment, a method for producing a high-strength cold-rolled steel sheet includes: heating a steel having the composition according to any one of the first to seventh embodiments to a temperature of 1,200 °C or higher, then performing hot rolling at a finish rolling delivery temperature equal to or higher than 800 °C, performing coiling at a temperature of 450 °C or higher and 700 °C or lower, and performing cold rolling; then performing an annealing treatment that involves performing heating to an annealing temperature of Ac₁ point or higher and Ac₃ point or lower where a holding time in a temperature range from Ac₁ point to Ac₃ point is 30 seconds or longer and 1,200 seconds or shorter, performing primary cooling from the annealing temperature to a primary cooling finishing temperature equal to or higher than 600 °C at an average cooling rate below 100 °C/s, and performing secondary cooling to a secondary cooling finishing temperature equal to or lower than 100 °C at an average cooling rate of 100 °C/s or higher and 1,000 °C/s or lower; then performing a tempering treatment that involves performing heating to a temperature of 100 °C or higher and 300 °C or lower where a holding time in a temperature range from 100 °C to 300 °C is 120 seconds or longer and 1,800 seconds or shorter; and performing pickling and re-pickling.
In the ninth embodiment, in the method for producing a high-strength cold-rolled steel sheet according to the eighth embodiment, the re-pickling uses, as a pickling solution, a non-oxidizing acid, which is different from a pickling solution used in the pickling.
As used herein, the term "high-strength cold-rolled steel sheet" refers to a cold-rolled steel sheet having a tensile strength (TS) of 1,300 MPa or higher.

Advantageous Effects of Invention

The present invention provides a high-strength cold-rolled steel sheet having a tensile strength of 1,300 MPa or higher, a good chemical conversion property, and good formability. Since the high-strength cold-rolled steel sheet of the present invention has a tensile strength of 1,300 MPa or higher and has a good chemical conversion property and good formability, the steel sheet of the present invention can be preferably used in applications pertaining to automotive parts and the like and offers significant advantageous effects of, for example, reducing the weight of automotive parts and improving the reliability thereof.

Description of Embodiments

The present invention will be described below in detail. The unit "%" as used hereinafter denotes % by mass unless otherwise specified.
First, the reason for limiting the composition of the steel sheet according to the present invention will be described.

C: 0.15% or more and 0.22% or less

Carbon (C) is an element effective in improving the balance between the strength and ductility of the steel sheet. At a C content below 0.15%, it is difficult to ensure a tensile strength of 1,300 MPa or higher. At a C content exceeding 0.22%, coarse cementite is precipitated, which degrades formability such as stretch flangeability. The C content is therefore in the range of 0.15% or more and 0.22% or less. The C content is preferably 0.16% or more. The C content is preferably 0.20% or less.

Si: 1.0% or more and 2.0% or less

Silicon (Si) is an element effective in ensuring strength without significantly reducing the ductility of the steel sheet. At a Si content below 1.0%, a steel sheet having high strength and high formability cannot be produced. At a Si content exceeding 2.0%, pickling and subsequent re-pickling still fail to completely remove Si oxide on the surface of the steel sheet, which results in a poor chemical conversion property. The Si content is therefore in the range of 1.0% or more and 2.0% or less. The Si content is preferably 1.0% or more. The Si content is preferably 1.5% or less.

Mn: 1.7% or more and 2.5% or less

Manganese (Mn) is an element for increasing the strength of the steel sheet. At a Mn content below 1.7%, it is difficult to obtain a tensile strength of 1,300 MPa or higher. At a Mn content exceeding 2.5%, a steel structure in which ferrite and martensite are distributed in the form of stripes is formed because of segregation during casting. As a result, anisotropy is found in mechanical properties, so that formability is degraded. The Mn content is therefore in the range of 1.7% or more and 2.5% or less. $[Si] / [Mn] \geq 0.5$
In the formula, [Si] represents the Si content (% by mass), and [Mn] represents the Mn content (% by mass).
The amounts of Si-based oxide and Si-Mn compound oxides produced depend on the balance between Si and Mn. If one oxide is produced in a much larger amount than the other oxide, pickling and subsequent re-pickling still fail to completely remove the oxides on the surface of the steel sheet, which results in a poor chemical conversion property. It is thus necessary to specify the quantitative ratio of Si to Mn. If the Mn content is much larger than the Si content, that is, [Si]/[Mn] < 0.5, excessive amounts of Si-Mn-based oxides (Si-Mn compound oxides) are produced and as a result, the chemical conversion property intended in the present invention is not obtained. Therefore, [Si]/[Mn] ≥ 0.5.

P: 0.05% or less

Phosphorus (P) is an impurity element and needs to be reduced in amount because it degrades ductility. At a P content exceeding 0.05%, grain boundary embrittlement associated with segregation of P to austenite grain boundaries during casting degrades local ductility. As a result, the balance between strength and ductility is impaired. The P content is therefore 0.05% or less. The P content is preferably 0.02% or less.

S: 0.02% or less

Sulfur (S) is present as MnS in the steel sheet. Since MnS leads to deterioration of impact resistance, strength, and stretch flangeability, the S content is preferably reduced to as low an amount as possible. The upper limit of the S content is therefore 0.02%. The S content is preferably 0.002% or less.

Al: 0.01% or more and 0.05% or less

Aluminum (Al) has an effect of improving ductility by forming Al oxide to reduce the amount of oxides such as Si oxide. However, a significant effect is not obtained at an Al content below 0.01%. If Al is added in an excess amount exceeding 0.05%, Al combines with N to form a nitride. This nitride is precipitated at austenite grain boundaries during casting to cause grain boundary embrittlement, which degrades stretch flangeability. The Al content is therefore in the range of 0.01% or more and 0.05% or less.

N: 0.005% or less

Nitrogen (N) forms nitrides with Al and Ti. These nitrides degrade stretch flangeability as described above. At an N content exceeding 0.005%, Ti nitride and Al nitride significantly degrade stretch flangeability, and an increased amount of a solid solution of N leads to a considerable reduction in elongation. The N content is therefore 0.005% or less. The N content is preferably 0.002% or less.

Ti: 0.010% or more and 0.020% or less

Titanium (Ti) has an effect of refining the structure and thus may be added as desired. At a Ti content below 0.010%, the effect of refining the structure is small. At a Ti content exceeding 0.020%, not only the effect of refining the structure may be saturated, but also coarse Ti and Nb compound carbides may be formed to impair the balance between strength and ductility and to degrade stretch flangeability. In addition, the production costs increase. Therefore, the Ti content, if added, is 0.010% or more and 0.020% or less. The Ti content is preferably 0.012% or more. The Ti content is preferably 0.018% or less.

Nb: 0.02% or more and 0.10% or less

Niobium (Nb) has an effect of refining the structure similarly to Ti and thus may be added as desired. At a Nb content below 0.02%, the effect of refining the structure is small. At a Nb content exceeding 0.10%, not only the effect of refining the structure may be saturated, but also coarse Ti and Nb compound carbides may be formed to impair the balance between strength and ductility and to degrade stretch flangeability. In addition, the production costs increase. Therefore, the Nb content, if added, is 0.02% or more and 0.10% or less. The Nb content is preferably 0.04% or more. The Nb content is preferably 0.08% or less.

B: 0.0002% or more and 0.0020% or less

Boron (B) is segregated to austenite grain boundaries during heating in continuous annealing and suppresses transformation to ferrite and transformation to bainite from austenite during cooling, which facilitates formation of tempered martensite. As a result, the steel sheet is strengthened. Therefore, B may be added as desired. At a B content below 0.0002%, this effect is small. At a B content exceeding 0.0020%, boron carbide Fe₂₃(C,B)₆ may be formed to degrade formability and reduce strength. Therefore, the B content, if added, is 0.0002% or more and 0.0020% or less.
In the present invention, to further improve the properties, the steel sheet preferably further includes at least one selected from 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.

V: 0.01% or more and 0.30% or less

Vanadium (V) combines with C to form a fine carbide, which is effective for precipitation strengthening of the steel sheet. Thus, V may be added as desired. At a V content below 0.01%, this effect is small. At a V content exceeding 0.30%, an excessive amount of the carbide may be precipitated to impair the balance between strength and ductility. Therefore, the V content, if added, is 0.01% or more and 0.30% or less.

Mo: 0.01% or more and 0.30% or less

Molybdenum (Mo) is effective for quenching strengthening of the steel sheet and also has an effect of refining the steel structure. Thus, Mo may be added as desired. At a Mo content below 0.01%, this effect is small. At a Mo content exceeding 0.30%, not only the effect may be saturated, but also formation of Mo oxide on the surface of the steel sheet may be accelerated during continuous annealing to significantly degrade the chemical conversion property of the steel sheet. Therefore, the Mo content, if added, is 0.01% or more and 0.30% or less.

Cr: 0.01% or more and 0.30% or less

Chromium (Cr) is effective for quenching strengthening of the steel sheet and thus may be added as desired. At a Cr content below 0.01%, strengthening performance is poor. At a Cr content exceeding 0.30%, formation of Cr oxide on the surface of the steel sheet may be accelerated during continuous annealing to significantly degrade the chemical conversion property of the steel sheet. Therefore, the Cr content, if added, is 0.01% or more and 0.30% or less.
In the present invention, to further improve the properties, the steel sheet preferably further includes at least one selected from Cu: 0.01% or more and 0.30% or less and Ni: 0.01% or more and 0.30% or less.

Cu: 0.01% or more and 0.30% or less

Copper (Cu) suppresses transformation to ferrite and transformation to bainite from austenite during cooling in continuous annealing, which facilitates formation of tempered martensite to strengthen the steel sheet. Thus, Cu may be added as desired. At a Cu content below 0.01%, this effect is small. At a Cu content exceeding 0.30%, the transformation to ferrite may be suppressed to an excessive degree, which may degrade ductility. Therefore, the Cu content, if added, is 0.01% or more and 0.30% or less.

Ni: 0.01% or more and 0.30% or less

Nickel (Ni) suppresses transformation to ferrite and transformation to bainite from austenite during cooling in continuous annealing, which facilitates formation of tempered martensite to strengthen the steel sheet. Thus, Ni may be added as desired. At a Ni content below 0.01%, this effect is small. At a Ni content exceeding 0.30%, the transformation to ferrite may be suppressed to an excessive degree, which may degrade ductility. Therefore, the Ni content, if added, is 0.01% or more and 0.30% or less.
In the present invention, to further improve the properties without adversely affecting the properties, the steel sheet preferably further includes at least one selected from Sn: 0.001% or more and 0.100% or less, Sb: 0.001% or more and 0.100% or less, Ca: 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, and REM: 0.0002% or more and 0.0050% or less.

Sn: 0.001% or more and 0.100% or less, Sb: 0.001% or more and 0.100% or less

Both Sn and Sb have an effect of suppressing surface oxidation, decarburization, and nitridization, and thus Sn and Sb may be added as desired. However, this effect is small at a Sn content below 0.001% and a Sb content below 0.001%. This effect is saturated at a Sn content exceeding 0.100% and a Sb content exceeding 0.100%. Therefore, the Sn content, if added, is 0.001% or more and 0.100% or less, and the Sb content, if added, is 0.001% or more and 0.100% or less. The Sn content is preferably 0.005% or more, and the Sb content is preferably 0.005% or more. The Sn content is preferably 0.010% or less, and the Sb content is preferably 0.010% or less.

Ca: 0.0002% or more and 0.0100% or less

Calcium (Ca) has an effect of improving ductility through morphology control of sulfide, grain-boundary strengthening, and solid solution strengthening. Thus, Ca can be added as desired. However, this effect is small at a Ca content below 0.0002%. In contrast, addition of an excessive amount of Ca causes grain boundary segregation or the like to degrade ductility. Therefore, the Ca content, if added, 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

Both W and Co have an effect of improving ductility through morphology control of sulfide, grain-boundary strengthening, and solid solution strengthening. Thus, both W and Co can be added as desired. However, this effect is small at a W content below 0.01% and a Co content below 0.01%. In contrast, addition of an excessive amount of W and/or Co causes grain boundary segregation or the like to degrade ductility. Therefore, the W content, if added, is 0.01% or more and 0.10% or less, and the Co content, if added, is 0.01% or more and 0.10% or less.

REM: 0.0002% or more and 0.0050% or less

REM has an effect of improving ductility through morphology control of sulfide, grain-boundary strengthening, and solid solution strengthening. Thus, REM can be added as desired. However, this effect is small at a REM content below 0.0002%. In contrast, addition of an excessive amount of REM causes grain boundary segregation or the like to degrade ductility. Therefore, the REM content, if added, is 0.0002% or more and 0.0050% or less.
In the present invention, the balance is Fe and unavoidable impurities. Examples of unavoidable impurities include 0 (oxygen). An 0 content of 0.01% or less is acceptable.
Next, the structure, which is an important matter for the steel sheet of the present invention, will be described.
The steel sheet contains, in terms of area fraction, 60% or more and less than 100% of tempered martensite, 5% or less (inclusive of 0%) of untransformed austenite, and the balance being ferrite. The ferrite has an average crystal grain size of less than 3.5 µm.
The tensile strength of steel having a structure including tempered martensite and ferrite increases with increasing area fraction of tempered martensite. This is because tempered martensite has higher hardness than ferrite and contributes to deformation resistance during tensile deformation because of its hard phase, so that a larger area fraction of tempered martensite results in a tensile strength closer to the tensile strength of tempered martensite single-phase structure. In the steel composition range of the present invention, the tensile strength is below 1,300 MPa when the area fraction of tempered martensite is below 40%. When the area of the boundaries between tempered martensite and ferrite is large, that is, the area fraction of tempered martensite is 40% or more and less than 60%, voids attributed to a difference in hardness between two phases are often generated and easily connected to each other, which accelerates development of cracks and thus degrades stretch flangeability. Therefore, the area fraction of tempered martensite needs to be 60% or more to ensure tensile strength and improve formability. However, good formability is not obtained if the area fraction of tempered martensite is 100%. In some cases, 5% or less of untransformed austenite may be unavoidably mixed. However, 5% or less of untransformed austenite is acceptable because no problem arises to obtain advantageous effects of the present invention. Therefore, the steel sheet contains, in terms of area fraction, less than 100% of tempered martensite, 5% or less (inclusive of 0%) of untransformed austenite, and the balance being ferrite. A preferred lower limit of the area fraction of tempered martensite is 70%. A preferred upper limit is 90%.
If the ferrite has an average crystal grain size of 3.5 µm or more, crystal grain refining strengthening is not enough to obtain a predetermined strength. During deformation, variations in deformation between crystal grains tend to be generated, which degrades formability. Threrefore, the average crystal grain size of ferrite is less than 3.5 µm.
The area fraction of tempered martensite, the area fraction of ferrite, and the average crystal grain size of ferrite can be determined by the methods of Examples described below.
The number of Si-Mn compound oxide particles having a circle equivalent diameter of 5 µm or less: less than 10 particles/100 µm²
The presence of Si-Mn compound oxides on the surface of the steel sheet significantly degrades the chemical conversion property. Needless to say, the presence of coarse Si-Mn compound oxide particles on the surface of the steel sheet degrades the chemical conversion property. Even for Si-Mn compound oxide particles having a circle equivalent diameter of 5 µm or less, degradation in chemical conversion property becomes obvious if the Si-Mn compound oxide particles are distributed at a number density higher than a certain number density. Therefore, the number of Si-Mn compound oxide particles having a circle equivalent diameter of 5 µm or less is set to less than 10 particles/100 µm². At 10 particles/100 µm or more, the region where zinc phosphate crystals are not formed becomes obvious, which results in a poor chemical conversion property. Therefore, the number of Si-Mn compound oxide particles having a circle equivalent diameter of 5 µm or less is preferably 0 particle/100 µm².
The number of Si-Mn compound oxide particles having a circle equivalent diameter of 5 µm or less can be determined by the method of Examples described below. The surface refers to a region extending in the thickness direction from the surface layer to a depth corresponding to 3% of the thickness.
The coverage of Si-based oxide on the surface of the steel sheet is 1% or less.
The presence of Si-based oxide on the surface of the steel sheet significantly degrades the chemical conversion property. Therefore, the coverage of Si-based oxide on the surface of the steel sheet is 1% or less. The coverage is preferably 0%. The Si-based oxide is, for example, SiO₂. The Si-based oxide can be measured by the method of Examples described below.
The structure, the number of Si-Mn compound oxide particles, and the coverage of Si-based oxide on the surface of the steel sheet can be obtained by controlling pickling after annealing, particularly re-pickling in the production method described below.
Next, a method for producing the high-strength cold-rolled steel sheet of the present invention will be described.
To produce the high-strength cold-rolled steel sheet of the present invention, a steel (steel slab) having the above-described composition is heated to a temperature of 1,200 °C or higher. The heated steel is then hot-rolled at a finish rolling delivery temperature equal to or higher than 800 °C. The resulting hot-rolled steel sheet is coiled at a temperature of 450 °C or higher and 700 °C or lower, followed by cold rolling. Next, the annealing treatment is performed as follows: performing heating to an annealing temperature of point or higher and Ac₃ point or lower where a holding time in the temperature range from point to Ac₃ point is 30 seconds or longer and 1,200 seconds or shorter, performing primary cooling from the annealing temperature to a primary cooling finishing temperature equal to or higher than 600 °C at an average cooling rate below 100 °C/s, and performing secondary cooling to a secondary cooling finishing temperature equal to or lower than 100 °C at an average cooling rate of 100 °C/s or higher and 1,000 °C/s or lower. Next, the tempering treatment is performed by performing heating to a temperature of 100 °C or higher and 300 °C or lower where the holding time in the temperature range from 100 °C to 300 °C is 120 seconds or longer and 1,800 seconds or shorter. Furthermore, pickling and re-pickling are performed and as a result, the high-strength cold-rolled steel sheet of the present invention is produced. The re-pickling preferably uses, as a pickling solution, a non-oxidizing acid, which is different from a pickling solution used in the pickling.
Ac₁ point and Ac₃ point are values (°C) calculated from the transformation expansion curve obtained by using a thermodilatometer at an average heating rate of 3 °C/s.
In the present invention, the method for smelting steel is not limited, and a known smelting method using, for example, a converter or an electric furnace can be employed. Secondary refining may be performed in a vacuum degassing furnace. Subsequently, continuous casting is preferably performed to produce a slab (steel) from the viewpoint of productivity and quality, but a known casting method, such as ingot casting-blooming rolling or thin-slab continuous casting, may be performed to produce a slab.

Heating temperature for steel: 1,200 °C or higher

Carbides are not redissolved at a heating temperature below 1,200 °C, leading to poor formability. Therefore, the heating temperature for steel is 1,200 °C or higher. Since an excessively high heating temperature leads to an increase in scale loss associated with an increase in oxidation mass, the heating temperature for steel is preferably 1,300 °C or lower. If the steel after casting is in a temperature range of 1,200 °C or higher or carbides in the steel are dissolved before hot rolling of the steel, the steel may be directly rolled without heating the steel. The conditions for rough rolling are not limited.

Finish rolling delivery temperature: 800 °C or higher

When the finish rolling delivery temperature is 800 °C or higher, a hot-rolled uniform matrix phase structure can be obtained. If the finish rolling delivery temperature is below 800 °C, the steel sheet has an uneven structure, and there is an increased risk of low ductility and various defects formed during forming. Therefore, the finish rolling delivery temperature is 800 °C or higher. The upper limit of the finish rolling delivery temperature is not limited, but is preferably 1,000 °C or lower because rolling the steel at an excessively high temperature produces scale defects and the like.

Coiling temperature: 450 °C or higher and 700 °c or lower

When the coiling temperature after hot rolling is lower than 450 °C, the deformation structure formed by hot rolling remains and imposes a large rolling load on subsequent cold rolling. When the coiling temperature is higher than 700 °C, coarse grains are produced so that the steel sheet has an uneven structure and low ductility. Therefore, the coiling temperature is 450 °C or higher and 700 °C or lower. A preferred lower limit of the coiling temperature is 500 °C. A preferred upper limit is 650 °C.
After hot rolling and coiling, optional pickling and subsequent cold rolling are performed. The pickling conditions are not limited. Cold rolling is needed to obtain a desired thickness. The cold-rolling reduction ratio is not limited but preferably 30% or higher and 80% or lower because of restrictions imposed by the manufacturing line.
Heating to an annealing temperature of point or higher and Ac₃ point or lower where the holding time in the temperature range from point to Ac₃ point is 30 seconds or longer and 1,200 seconds or shorter
If the annealing temperature is below point, austenite (transformed into martensite after quench hardening) needed to ensure a predetermined strength is not formed during annealing, and a predetermined strength is not obtained even by quench hardening after annealing. Even if the annealing temperature is over Ac₃ point, 60% or more (area fraction) of martensite can be obtained by controlling the area fraction of ferrite precipitated during cooling from the annealing temperature. However, if annealing is performed at a temperature over Ac₃ point, it is difficult to obtain a desired metallographic structure. The annealing temperature is thus Ac₁ point or higher and Ac₃ point or lower. In order to stably ensure that the area fraction of austenite in equilibrium is 60% or more in this temperature range, the annealing temperature is preferably 780 °C or higher. If the holding time at the annealing temperature is too short, the microstructure is not annealed well, which provides an uneven structure including the deformation structure formed by cold rolling and thus results in low ductility. However, if the holding time is too long, this holding time is not desirable because of long production time and high production costs. Therefore, the holding time is 30 to 1,200 seconds. A preferred lower limit of the holding time is 150 seconds. A preferred upper limit is 600 seconds.
Primary cooling from the annealing temperature to a primary cooling finishing temperature equal to or higher than 600 °c at an average cooling rate below 100 °C/s
Cooling (slow cooling) from the annealing temperature to a primary cooling finishing temperature (slow cooling finishing temperature) equal to or higher than 600 °C is performed at an average cooling rate below 100 °C/s. The balance between strength and ductility can be controlled by precipitation of ferrite during slow cooling from the annealing temperature. If the slow cooling finishing temperature (primary cooling finishing temperature) is lower than 600 °C, a tensile strength of 1,300 MPa or higher cannot be obtained because a large amount of perlite is generated in the microstructure to cause a drastic decrease in strength. To stably obtain a predetermined strength, the slow cooling finishing temperature (primary cooling finishing temperature) is preferably 680°C or higher.
If the average cooling rate is 100 °C/s or higher, good ductility cannot be obtained because an adequate amount of ferrite is not precipitated during cooling. The ductility of the metallographic structure having tempered martensite and ferrite intended in the present invention results from high work hardenability expressed by mixing hard tempered martensite and soft ferrite. However, if the average cooling rate is 100 °C/s or higher, concentration of carbon in austenite during cooling is inadequate, so that hard martensite is not obtained during rapid cooling. As a result, the work hardenability of the final structure is too low to obtain adequate ductility. Therefore, the average cooling rate is lower than 100 °C/s. The average cooling rate is preferably 5 °C/s or lower in order to cause adequate concentration of carbon in austenite.
Secondary cooling to a secondary cooling finishing temperature equal to or lower than 100 °c at an average cooling rate of 100 °c/s or higher and 1,000 °c/s or lower
After slow cooling as described above, cooling (rapid cooling) to a secondary cooling finishing temperature of 100 °C or lower is performed at an average cooling rate of 100 °C/s or higher and 1,000 °C/s or lower. Rapid cooling after slow cooling is intended to transform austenite into martensite. If the average cooling rate is below 100 °C/s, austenite is transformed into ferrite, bainite, or perlite during cooling, and thus a predetermined strength cannot be obtained. However, if the average cooling rate is higher than 1,000 °C/s, shrinkage cracks of the steel sheet due to cooling may be generated. Therefore, the average cooling rate during rapid cooling is 100 °C/s or higher and 1,000 °C/s or lower. Rapid cooling is preferably performed by water quenching.
The secondary cooling finishing temperature is 100 °C or lower. A secondary cooling finishing temperature higher than 100 °C is not desirable because such a temperature induces a decrease in area fraction of martensite due to inadequate quench hardening of austenite during rapid cooling and a decrease in material strength due to self-tempering of martensite formed by rapid cooling.
Tempering treatment involving performing heating to a temperature of 100 °c or higher and 300 °c or lower where the holding time in the temperature range from 100 °c to 300 °c is 120 seconds or longer and 1,800 seconds or shorter
To temper martensite after rapid cooling as described above, the tempering treatment is performed by performing re-heating to a temperature of 100 °C or higher and 300 °C or lower and holding in the temperature range from 100 °C to 300 °C for 120 to 1,800 seconds. This tempering softens martensite and improves formability. If tempering is performed at a temperature lower than 100 °C, martensite is softened insufficiently so that the effect of improving formability cannot be expected and there is a large difference in hardness between martensite and ferrite, which degrades stretch flangeability. If tempering is performed at a temperature higher than 300 °C, not only the production cost for re-heating increases, but also the strength decreases significantly, so that advantageous effects cannot be obtained. Tempering is preferably performed in the range of 150 °C to 250 °C. If the holding time is shorter than 120 seconds, martensite is softened insufficiently in the temperature range from 100 °C to 300 °C, so that the effect of improving formability cannot be expected. If the holding time is longer than 1,800 seconds, this holding time is not desirable because martensite is softened to an excessive degree so as to significantly reduce the strength and a long re-heating time increases the production costs.

Pickling, Re-pickling

Pickling and re-pickling remove Si oxide and Si-Mn oxides on the surface of the steel sheet and improve the chemical conversion property. The re-pickling preferably uses, as a pickling solution, a non-oxidizing acid, which is different from a pickling solution used in the pickling.
Pickling can be performed by an ordinary method, and the conditions are not limited. For example, any acid selected from nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, an acid mixture thereof can be used.
Si-based oxide and Si-Mn compound oxides on the surface of the steel sheet, which degrade the chemical conversion property, can be removed by pickling the steel sheet after the tempering treatment in, for example, larger than 50 g/L and 200 g/L or lower of a strong acid, such as nitric acid. However, the pickling in a strong acid causes Fe dissolved out of the surface of the steel sheet to form an iron-based oxide. The iron-based oxide is precipitated on the surface of the steel sheet to cover the surface of the steel sheet and thus to degrade the chemical conversion property. Therefore, in order to improve the chemical conversion property, it is necessary to dissolve and remove the iron-based oxide precipitated on the surface of the steel sheet by further performing re-pickling under the appropriate conditions after the pickling in a strong acid. On the basis of the above-mentioned reason, the re-pickling preferably uses, as a pickling solution, a non-oxidizing acid, which is different from a pickling solution used in the pickling. Examples of the non-oxidizing acid include hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid, and an acid mixture thereof. For example, an acid mixture of 0.1 to 50 g/L of hydrochloric acid, 0.1 to 150 g/L of sulfuric acid, 0.1 to 20 g/L of hydrochloric acid, and 0.1 to 60 g/L of sulfuric acid can be used preferably.
According to the foregoing, the high-strength cold-rolled steel sheet of the present invention having a tensile strength (TS) of 1,300 MPa or higher and having a good chemical conversion property and good formability is produced. Since the high-strength cold-rolled steel sheet of the present invention after annealing has good sheet shape (flatness), the process for correcting the shape of the steel sheet by, for example, rolling and leveler processing is not always needed, but the steel sheet after annealing may be rolled at an elongation rate of about several percent without causing any problem in order to adjust material properties and surface roughness. Since the properties of the high-strength cold-rolled steel sheet of the present invention are not affected by a coating process or the composition of a coating bath, any of hot-dip galvanizing, galvannealing, and electro galvanizing can be performed as the coating process.

Example 1

Sample steels A to R each having the composition described in Table 1 were each smelted under vacuum to produce a slab, which was then hot-rolled under the conditions described in Table 2 to produce a hot-rolled steel sheet. This hot-rolled steel sheet was pickled to remove surface scale and then cold-rolled (rolling reduction ratio: 60%). The steel sheet was then subjected to continuous annealing and the tempering treatment under the conditions described in Table 2 and then subjected to pickling and re-pickling.
Ac₁ point and Ac₃ point were calculated from the transformation expansion curve obtained by using a thermodilatometer at an average heating rate of 3 °C/s.
A sample was taken from the steel sheet obtained above and subjected to observation (measurement) of the metallographic structure, a tensile test, and a hole expansion test. The number of Si-Mn oxide particles having a circle equivalent diameter of 5 µm or less and the coverage of Si-based oxide on the surface of the steel sheet were obtained. The chemical conversion property was determined. The measurement methods and the calculation methods are described below.
The metallographic structure was observed as follows: cutting the sample such that the thickness cross section parallel to the rolling direction was targeted for observation, etching the thickness middle area with 1% Nital, and then observing a typical microstructure under a scanning electron microscope (SEM). The volume fraction of two phases was obtained by a point counting method on the basis of the SEM image taken at a magnification of x1,000, and the grain size of each phase was obtained by linear analysis. The obtained volume fraction was defined as the area fraction.
The tensile test was performed at a strain rate of 3.3 × 10^-3s^-1 on a JIS No. 5 sample (original gauge length: 50 mm, width of parallel part: 25 mm) cut out from the steel sheet in parallel to the rolling direction. The total elongation was determined by abutting the samples after fracture.
The hole expansion test was performed as follows: punching a circular hole with ∅10 mm (do) in a sample having a size of 100 mm × 100 mm; then forcing a conical punch having a vertical angle of 60° into the hole from below while the sample was held by a die with an inner diameter of 75 mm at a blank holding force of 9 tons; and measuring the hole diameter (d) at the time when a thickness-penetrating crack was generated in the hole edge. The hole expansion ratio λ(%) defined in the formula described below was obtained. In this test, hole punching and hole expansion were carried out in the same direction while the surface on which burrs were formed by hole punching faced upward (according to JIS 2256). $λ (%) = \{(d - d 0) / d 0\}$
In the formula, d0 represents the initial hole diameter, and d represents the hole diameter at the time when the crack penetrates through the thickness.
The number of Si-Mn oxide particles having a circle equivalent diameter of 5 µm or less was determined as follows: producing an extraction replica film for the steel surface; and counting the average number of the Si-Mn oxide particles (per 100 µm²) in freely selected 20 fields of view through TEM observation at x15,000. When the number of Si-Mn oxide particles having a circle equivalent diameter of 5 µm or less was 10 particles/100 µm² or more, the sample was defined as "positive" for the Si-Mn oxide particles. When the number of the Si-Mn oxide particles was less than 10 particles/100 µm², the sample was defined as "negative" for the Si-Mn oxide particles. The Si-Mn oxides were identified as follows: performing diffraction pattern analysis on oxides of which Si, Mn, and/or 0 had been detected by EDX analysis; and determining whether the detected spots matched with the spots from Mn₂SiO₄ or MnSiO₃.
The coverage of Si-based oxide on the surface of the steel sheet was obtained as follows: identifying Si-based oxide in the same method as described above by observing the surface of the steel sheet under a SEM in five fields of view at x1,000 and performing EDX analysis in the five fields of view; and calculating the coverage by a point counting method (a method involving drawing 15 straight lines vertically and 15 straight lines horizontally on the SEM image and calculating the probability of presence of Si-based oxide at intersections (225 points).
The chemical conversion property was evaluated as follows: performing chemical conversion using a commercial chemical conversion agent (PALBOND PB-L3065 (registered trademark) available from Nihon Parkerizing Co., Ltd.) under the conditions of a bath temperature of 35 °C and a treatment time of 120 seconds; observing the surface of the steel sheet after chemical conversion under a SEM in five fields of view at a magnification of x500; and rating the chemical conversion property as "A" which means good when 95% or more (area fraction) of a chemical-conversion crystal was evenly formed in all five fields of view, and rating the chemical conversion property as "B" which means poor when more than 5% (area fraction) of defects were found at least in one field of view.

The results obtained above are shown in Table 3.

[Table 3]

No.	Steel Type	Ferrite Grain Size (µm)	Ferrite Fraction (%)	Tempered Martensite Fraction (%)	TS (MPa)	EL (%)	λ(%)	Oxides on Surface		Chemical Conversion Property	Note
No.	Steel Type	Ferrite Grain Size (µm)	Ferrite Fraction (%)	Tempered Martensite Fraction (%)	TS (MPa)	EL (%)	λ(%)	Si-Mn Oxide (number of particles)	Si-based Oxide (%)	Chemical Conversion Property	Note
1	A	3.0	29	71	1358	12	31	negative	0	A	Invention Example
2	B	2.8	27	73	1410	11	37	negative	0	A	Invention Example
3	C	3.4	38	62	1326	14	33	negative	0	A	Invention Example
4	D	3.2	33	67	1400	13	32	negative	0	A	Invention Example
5	E	3.3	36	64	1381	14	31	negative	0	A	Invention Example
6	F	3.4	39	61	1351	15	30	negative	0	A	Invention Example
7	G	3.4	37	63	1370	14	31	negative	0	A	Invention Example
8	H	3.2	33	67	1402	13	32	negative	0	A	Invention Example
9	I	3.0	29	71	1395	12	36	negative	0	A	Invention Example
10	J	3.1	33	67	1368	13	35	negative	0	A	Invention Example
11	K	3.2	34	66	1360	13	35	negative	0	A	Invention Example
12	L	2.7	25	75	1490	15	16	negative	0	A	Comparative Example
13	M	3.6	45	55	1344	14	25	positive	20	B	Comparative Example
14	N	1.2	9	91	1608	7	32	negative	0	A	Comparative Example
15	O	1.3	11	89	1593	8	31	negative	0	A	Comparative Example
16	P	3.9	60	40	1196	20	18	negative	0	A	Comparative Example
18	R	0.3	2	98	1325	8	16	positive	0	B	Comparative Example
19	A	3.1	32	68	1264	9	30	negative	0	A	Comparative Example
20	A	5.8	74	26	1094	31	9	negative	0	A	Comparative Example
21	A	5.3	68	32	1138	25	10	negative	0	A	Comparative Example
22	A	3.7	44	56	1122	16	38	negative	0	A	Comparative Example
23	A	3.6	43	57	1385	10	9	negative	0	A	Comparative Example
24	A	3.4	38	62	1395	12	32	positive	29	B	Comparative Example

The underlines indicate the conditions out of the scope of the present invention.

According to Tables 1 to 3, Examples that meet the conditions of the present invention have a tensile strength (TS) of 1,300 MPa or higher, an elongation (EL) of 10% or higher, and a hole expansion ratio (λ) of 30% or higher and accordingly have high strength and good formability. Examples that meet the conditions of the present invention also have a good chemical conversion property.
Sample No. 12 is Comparative Example in which the C content is higher than that in the scope of the present invention. Since the C content is high, the strength of martensite is high, and the strength and the ductility are well balanced, but the stretch flangeability is significantly low because of a difference in hardness between ferrite and martensite.
Samples No. 13 and No. 14 are Comparative Examples in which the Si content is out of the scope of the present invention. Sample No. 13 fails to have a good chemical conversion property because Si oxide is present on the surface of the steel sheet even after the two-step pickling treatment. Sample No. 14 fails to have a predetermined elongation.
Samples No. 15 and No. 16 are Comparative Examples in which the Mn content is out of the scope of the present invention. Since Mn is an element that largely changes the martensite fraction, Sample No. 15 having a high Mn content fails to have a predetermined elongation. Since Sample No. 16 having a low Mn content has a low martensite fraction, Sample No. 16 fails to have a predetermined strength.
Samples No. 18 to No. 23 are Comparative Examples for which the production conditions are out of the scope of the present invention. Sample No. 18 is Comparative Example for which the composition and the production conditions are out of the scope of the present invention. Sample No. 18 not only fails to have a predetermined elongation but also has low stretch flangeability and a poor chemical conversion property.
Sample No. 19 fails to have a predetermined strength and a predetermined elongation because the annealing temperature is high.
Samples No. 20 to No. 22 have an insufficient martensite fraction and thus fail to have a predetermined strength.
Sample No. 23 has an insufficient martensite fraction and has poor stretch flangeability.
Sample No. 24 is an example produced without the pickling treatment after annealing. Since Si oxide is present on the surface of the steel sheet, Sample No. 24 has a poor chemical conversion property.

Claims

A high-strength cold-rolled steel sheet having a composition comprising, in terms of % by mass, C: 0.15% or more and 0.22% or less, Si: 1.0% or more and 2.0% or less, Mn: 1.7% or more and 2.5% or less, P: 0.05% or less, S: 0.02% or less, Al: 0.01% or more and 0.05% or less, N: 0.005% or less, and the balance being iron and unavoidable impurities, the composition satisfying formula (1) below, wherein the steel sheet has a structure including, in terms of area fraction, 60% or more and less than 100% of tempered martensite, 5% or less (inclusive of 0%) of untransformed austenite, and the balance being ferrite, and the ferrite has an average crystal grain size of less than 3.5 µm, wherein less than 10 particles/100 µm² of Si-Mn compound oxide particles having a circle equivalent diameter of 5 µm or less are present on a surface of the steel sheet, and the surface of the steel sheet is covered with Si-based oxide at a coverage of 1% or less,
wherein the steel sheet has a tensile strength of 1,300 MPa or higher: $[Si] / [Mn] \geq 0.5$
in the formula, [Si] represents a Si content (% by mass), and [Mn] represents a Mn content (% by mass).
The high-strength cold-rolled steel sheet according to Claim 1 having the composition further comprising, in terms of % by mass, Ti: 0.010% or more and 0.020% or less.
The high-strength cold-rolled steel sheet according to Claim 1 or 2 having the composition further comprising, in terms of % by mass, Nb: 0.02% or more and 0.10% or less.
The high-strength cold-rolled steel sheet according to any one of Claims 1 to 3 having the composition further comprising, in terms of % by mass, B: 0.0002% or more and 0.0020% or less.
The high-strength cold-rolled steel sheet according to any one of Claims 1 to 4 having the composition further comprising, in terms of % by mass, at least one selected from 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.
The high-strength cold-rolled steel sheet according to any one of Claims 1 to 5 having the composition further comprising, in terms of % by mass, at least one selected from Cu: 0.01% or more and 0.30% or less and Ni: 0.01% or more and 0.30% or less.
The high-strength cold-rolled steel sheet according to any one of Claims 1 to 6 having the composition further comprising, in terms of % by mass, at least one selected from Sn: 0.001% or more and 0.100% or less, Sb: 0.001% or more and 0.100% or less, Ca: 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, and REM: 0.0002% or more and 0.0050% or less.
A method for producing a high-strength cold-rolled steel sheet, the method comprising:
heating a steel having the composition according to any one of Claims 1 to 7 to a temperature of 1,200 °C or higher, then performing hot rolling at a finish rolling delivery temperature equal to or higher than 800 °C., performing coiling at a temperature of 450 °C or higher and 700 °C or lower, and performing cold rolling;

then performing an annealing treatment that involves performing heating to an annealing temperature of point or higher and Ac₃ point or lower where a holding time in a temperature range from Ac₁ point to Ac₃ point is 30 seconds or longer and 1,200 seconds or shorter, performing primary cooling from the annealing temperature to a primary cooling finishing temperature equal to or higher than 600 °C at an average cooling rate below 100 °C/s, and performing secondary cooling to a secondary cooling finishing temperature equal to or lower than 100 °C at an average cooling rate of 100 °C/s or higher and 1,000 °C/s or lower;

then performing a tempering treatment that involves performing heating to a temperature of 100 °C or higher and 300 °C or lower where a holding time in a temperature range from 100 °C to 300 °C is 120 seconds or longer and 1,800 seconds or shorter; and

performing pickling and re-pickling.
The method for producing a high-strength cold-rolled steel sheet according to Claim 8, wherein the re-pickling uses, as a pickling solution, a non-oxidizing acid, which is different from a pickling solution used in the pickling.