JP5130701B2 - High tensile steel plate with excellent chemical conversion - Google Patents

Description

  The present invention relates to a high-strength steel plate, and particularly to a high-strength steel plate having high strength and good press formability and corrosion resistance.

  With the recent increase in awareness of environmental issues such as prevention of global warming, the automobile industry is actively reducing the weight of the vehicle body by reducing the thickness of the steel sheet in order to improve fuel efficiency. On the other hand, from the viewpoint of ensuring safety in the event of a car collision, there is a need to maintain high vehicle body strength while reducing the weight of the vehicle body. In order to achieve both the weight reduction of the vehicle body and the improvement of the safety of the vehicle body, there are an increasing number of cases where a high-strength steel plate with high strength is applied.

  Moreover, since such a steel plate is used by being press-molded into a complicated shape, it is required that the press-formability is good as well as strength and collision safety.

  In recent years, as high strength steel plates with a tensile strength of 500 MPa or more, steel materials using a composite structure of ferrite with excellent workability and hard phases such as bainite and martensite with the addition of reinforcing elements such as C, Si, and Mn. Steel sheets made of ferrite, bainite, and retained austenite that utilize the improvement in elongation due to strain-induced transformation of retained austenite during processing have been proposed and applied.

On the other hand, in steel sheets for automobiles, high-strength steel sheets are applied before painting in order to improve adhesion to the coated film from the point of improving anticorrosion properties and prevent rust from spreading even if the coating film is wrinkled. Chemical conversion treatment has been widely adopted as a surface ground treatment. In general, a zinc phosphate treatment is performed on the steel sheet by a full dip method, a spray method, etc. to form a thin film of about ² to 3 g / m ^{2 on the} surface of the steel plate as a coating underlayer, and then cation electrodeposition is applied to this underlayer. Often painted. Since the chemical conversion treatment layer becomes strongly alkaline during the cationic electrodeposition coating, it is necessary for the chemical conversion treatment layer to have sufficient alkali resistance in order to improve adhesion with the coated film after coating and to improve corrosion resistance. there were.

Corrosion of the steel sheet after painting forms a local battery in which the metal exposed part of the paint missing part elutes Fe ²⁺ ions as the anode and the lower part of the coating film around the metal exposed part generates OH ⁻ ions as the cathode. To proceed. At this time, since the pH of the lower part of the coating film is increased by OH ⁻ ions, the chemical conversion film existing at the interface between the coating film and the iron is dissolved, and the chemical conversion film is sufficient so that the corrosion due to the decrease in adhesion with the coating film does not proceed. It must have alkali resistance.

  Conventionally, a parameter called a P value has been used as an index of chemical conversion property of a steel sheet and alkali resistance of a chemical conversion layer.

  The chemical conversion property of the steel sheet can be evaluated by immersing the steel sheet in a zinc phosphate treatment solution, washing with water and drying, and measuring the adhesion amount and the P value defined below. Here, the P value refers to zinc phosphate crystals (Zn-PO: Hopeite (hereinafter referred to as H)) and iron phosphate crystals (Zn-Fe-PO) formed on the surface of the steel sheet by the zinc phosphate treatment. : It is defined by the composition ratio (P / (P + H)) of the iron phosphate crystal (P) to the phosphate film (P + H) made of phosphophyllite (hereinafter referred to as P). Usually, phosphophyllite (P) is superior in alkali resistance to hopite (H). Therefore, in order to improve the alkali resistance of the chemical conversion treatment layer, the P value is 0.8 or more, further 0.9. It is considered preferable to make it higher than the above. The P value is determined by measuring the intensities P and H of diffraction lines from the (110) plane of phosphophyllite and the (020) plane of phosphite by X-ray diffraction, and the peak intensity ratio of P / (P + H). Can be represented by

Conventionally, as a method for producing a cold-rolled steel sheet having excellent chemical conversion treatment properties, phosphoric acid that has been subjected to Fe-based electroplating on a steel sheet plated with a Zn-based intermetallic compound and then subjected to phosphate treatment thereon. There has been proposed a Zn-based plated steel sheet having excellent impact resistance adhesion, in which the phosphophyllite ratio (P value) of the salt film is 0.9 or more (see, for example, Patent Document 1). However, this method requires a plating treatment composed of a Zn-based intermetallic compound and a treatment that further performs Fe-based electroplating, and in order to perform these treatments, there is a problem that an increase in cost is inevitable. is there.

On the other hand, C: 0.16 to 0.19%, Si: 1.10 to 1.30%, Mn: 1.50 to 1.60%, and a TRIP steel sheet having a tensile strength of 780 MPa class, hot rolling Depending on the heating temperature, descaling conditions, grinding of the steel sheet surface, pickling method, etc., the average value of Si concentration on the steel sheet surface is 20 times or less of the Si concentration in steel (usually 40 to 50 of the Si concentration in steel). In addition, a steel sheet is disclosed in which the area ratio of the portion where the concentration ratio to the Si concentration in the steel in the surface Si concentration distribution is 10 or more is 95% or less (see, for example, Patent Document 2). This cold-rolled steel sheet reduces the amount of Si oxide on the steel sheet surface and reduces the variation in Si oxide distribution on the steel sheet surface, thereby mitigating local concentration of corrosion current on low-concentration sites of Si oxide, Dissolution of the chemical conversion treatment layer due to this local increase in pH (OH ^- ion generation) and adhesion degradation with the coating film can be suppressed, and as a result, the corrosion resistance of the coated steel sheet can be improved. However, according to the examination results of the inventors, the P value of the chemical conversion treatment layer of this steel plate is about 0.7 to 0.8, and the chemical conversion treatment property is sufficient to sufficiently improve the corrosion resistance after painting of the steel plate. Was not obtained.

In addition, C: more than 0.1%, Si: 0.4% or more, Si content / Mn content of 0.4 or more, high tensile strength steel sheets with a tensile strength of 700 Mpa or more, and annealing in the cold rolling process By the pickling treatment after the treatment, the coverage of the Si-based oxide (SiO ₂ ) on the steel sheet surface is 20% or less (usually about 80%), and the size of the coated region (the diameter of the largest circle inscribed) ) Has been proposed (see, for example, Patent Document 3). This cold-rolled steel sheet is subjected to a pickling treatment in which it is immersed in hydrochloric acid or sulfuric acid having a temperature of 50 ° C. or higher and a concentration of 10 mass% or higher for 7 seconds or more after the annealing treatment, and the annealing treatment is performed at a temperature of 200 to 400 ° C. The dew point is carried out under the condition of a relatively high oxygen potential from -20 ° C. to room temperature to produce a relatively coarse oxide that is easily removed by pickling.

  However, according to the examination results of the inventors, the P value of the chemical conversion treatment layer of the steel plate obtained by the pickling treatment and the annealing treatment is 0.9 or less, and the corrosion resistance after painting of the steel plate is low. A sufficient chemical conversion treatment enough to improve sufficiently could not be obtained.

  Also, C: 0.005% or less, Si: 0.1% or less, Mn: 0.5% or less, and ferrite with a tensile strength of about 300 MPa and ferrite within a thickness of 10% from the steel sheet surface The surface roughness resistance and press-molding are set such that the average crystal grain size is 90% or less of the average crystal grain size of ferrite in the center part of the plate thickness and the average crystal grain size of ferrite in the center part of the plate thickness is 10 to 30 μm. An ultra-low carbon steel sheet having excellent properties and chemical conversion treatment has been proposed (see, for example, Patent Document 4).

  This steel sheet, along with the rough surface resistance and press formability, by making the crystal grain of the steel sheet surface fine by the heating temperature in hot rough rolling, the rolling temperature and rolling rate, and the recrystallization annealing conditions in cold rolling. It promotes the elution reaction of Fe on the surface of the steel sheet and realizes a steel sheet excellent in chemical conversion treatment with a P value of 0.9 or more in the chemical conversion treatment of the steel sheet.

  However, this steel sheet is a very low carbon steel sheet having a C of 0.005% or less and a tensile strength as low as about 300 MPa, a tensile strength of 500 MPa or more, and a relatively high Si and Mn content. When symmetrized, a similar effect of improving chemical conversion cannot be expected by the above method. In addition, according to the examination results of the present inventors, the P value in the chemical conversion treatment of this steel sheet is about 0.75, which is a normal P value level, and the chemical conversion treatment property for sufficient corrosion resistance improvement after painting. Was not obtained.

Therefore, without using a special process such as plating, it is possible to achieve a high chemical conversion treatment with a P value of 0.9 or more in chemical conversion treatment according to manufacturing conditions in a normal cold rolling process. A technique capable of stably and inexpensively manufacturing a high-strength steel sheet of 500 MPa or more is desired.
JP-A-7-62563 JP 2004-204350 A JP 2004-323969 A Japanese Patent Laid-Open No. 10-158783

  Therefore, the present invention has been devised in view of the above-described problems, and its object is to perform chemical conversion treatment according to manufacturing conditions in a normal cold rolling process without using a special process such as plating. An object of the present invention is to provide a high-strength steel sheet having a tensile strength of 500 MPa or more excellent in corrosion resistance of coating capable of achieving a high chemical conversion treatment having a P value of 0.9 or more and a method for producing the same.

  (1) When the Fe of the steel sheet reacts with the phosphate solution during the chemical conversion treatment, Fe on the steel sheet surface elutes into the phosphate solution to form phosphophyllite.

  (2) The site where Fe eluting in the phosphate solution elutes is a crystal grain boundary exposed on the steel sheet surface. In other words, the phosphate solution preferentially dissolves the grain boundary on the steel sheet surface. And react.

  (3) In addition, the larger the grain boundary density exposed on the steel sheet surface, in other words, the smaller the crystal grain size exposed on the surface, the more phosphophyllite is formed.

  (4) Even in a steel sheet after annealing, when grain boundaries are exposed on the surface, the higher the grain boundary density exposed per unit area of the steel sheet surface, in other words, the exposed surface. The smaller the crystal grain size, the more phosphophyllite is formed.

  (5) The formation state of phosphophyllite can be identified through microscopic observation and is difficult to quantify, but microscopically, such as Si and Mn at grain boundaries. It has been found that the concentration element is expected to decrease.

  (6) In addition, it has also been found that elution of Fe is hindered when Si, Mn, and Al, which have a higher ionization tendency than Fe, are concentrated at the grain boundaries.

  In order to solve the above-described problems, the present inventor, based on the above-mentioned conventional knowledge (1) to (3) and newly devised knowledge (4) to (6), It was found that it is a more preferable condition that the density is increased (the crystal grain size is decreased) and Si, Mn, and Al, which have a higher ionization tendency than Fe, are not concentrated at the grain boundaries. And it was decided to solve the above-mentioned problems by inventing a high-tensile steel plate having the following configuration and a method for producing the same.

The high-tensile steel sheet according to claim 1 of the present application is in mass%, C: 0.01 to 0.3%, Si: 0.2 to 3.0%, Mn: 0.1 to 3.0%, Al: In a high-tensile steel plate containing 0.01 to 2.0%, the balance being Fe and inevitable impurities and having a tensile strength of 500 MPa or more, the average grain size of the crystal grains on the steel plate surface is 0.5 μm or less, and One or two types of silicon oxide and manganese silicate, in which an observation region having a width of 10 μm or more on the surface of the steel sheet is processed into a thin piece for cross-sectional TEM observation, and the thin piece sample is measured by TEM observation under conditions where an oxide of 10 nm or less can be observed. Oxide seeds containing 70% by mass or more of these seeds in a total amount of these are present in an amount of 30% or less with respect to the grain boundary region surface viewed from the cross section, and the depth from the steel sheet surface is 0.1 to 1.0 μm. The particle size of the oxide species present in the range is Wherein the .1μm or less.

The manufacturing method of the high-tensile steel sheet according to claim 2 of the present application is mass%, C: 0.01 to 0.3%, Si: 0.2 to 3.0%, Mn: 0.1 to 3.0%. In the method for producing a high-strength steel sheet containing Al: 0.01 to 2.0% and the balance being Fe and inevitable impurities and having a tensile strength of 500 MPa or more, the temperature of the cold-rolled steel sheet is raised from room temperature to the preheating temperature Tp. Performing the annealing process comprising a preheating step, a temperature raising step for raising the temperature from the preheating temperature Tp to the recrystallization temperature Tr, and a recrystallization step for keeping the recrystallization temperature Tr constant, the preheating step The preheating temperature Tp is set to 300 to 500 ° C., and the water vapor partial pressure ratio (PH ₂ O / PH ₂ ) to the hydrogen partial pressure in the annealing atmosphere satisfies the following formula (1) from the relationship with the preheating temperature Tp. And the recrystallization temperature T in the temperature raising step Was a 650 ° C. to 900 ° C., to satisfy the hydrogen partial from the relationship vapor partial pressure ratio (PH ₂ O / PH ₂₎ of the recrystallization temperature Tr for pressure following equation (2) conditions during annealing atmosphere and temperature The temperature rate is controlled to be 1 to 20 ° C./second, and the water vapor partial pressure ratio (PH ₂ O / PH ₂ ) to the hydrogen partial pressure in the annealing atmosphere in the recrystallization step is related to the recrystallization temperature Tr. A method for producing a high-tensile steel sheet, characterized by satisfying the condition of the following formula (3) and controlling the holding time to be 40 to 600 seconds.
log (PH ₂ O / PH ₂ ) ≦ −2.8 × 10 ⁻⁶ (Tp + 273) ² + 6.8 × 10 ⁻³ (Tp + 273) −4.8 (1)
5.3 × 10 ⁻⁸ Tr ² + 1.4 × 10 ⁻⁵ Tr−0.01 ≦ PH ₂ O / PH ₂ ≦ 6.4 × 10 ⁻⁷ Tr ² + 1.7 × 10 ⁻⁴ Tr−0.1 ... (2)
PH ₂ O / PH ₂ <5.3 × 10 ⁻⁸ Tr ² + 1.4 × 10 ⁻⁵ Tr−0.01 (3)

  In the high-tensile steel plate according to the present invention manufactured based on the above-described steps, the grain boundary density exposed per unit area of the steel plate surface can be increased and the grain boundary exposed on the steel plate surface can be increased. It becomes possible to make the thickness of the oxide formed above 10 nm or less.

  That is, in this high-tensile steel plate, an internal oxide is generated in a region having a depth of 0.1 to 0.2 μm from the steel plate surface in the preheating step. In the subsequent recrystallization process, when the steel sheet structure is recrystallized, the internal oxide exhibits a grain boundary pinning action, and the crystal of the recrystallized structure in a depth region from the steel sheet surface to 1 μm. Refine the grain. Therefore, the grain boundary density on the steel sheet surface is increased. In other words, the crystal grain size exposed on the surface is reduced. Furthermore, by maintaining the temperature for a certain time above the recrystallization temperature, Si and Mn dissolved in the steel can be absorbed in the internal oxide formed in the preheating step, so that the solid-soluble Si in the steel, The Mn concentration can be reduced. As a result, it is possible to obtain a steel sheet surface with exposed crystal grain boundaries in which almost no oxide is formed.

  That is, the high-tensile steel plate to which the present invention is applied can reduce the crystal grain size exposed per unit area of the steel plate surface and suppress the formation of an oxide layer on the grain boundary exposed on the steel plate surface. When the Fe of the steel sheet reacts with the phosphate solution during the chemical conversion treatment, Fe elutes into the phosphate solution through this grain boundary, thereby increasing the amount of phosphophyllite formed. As a result, the corrosion resistance can be improved.

  Hereinafter, the manufacturing method of the high-tensile steel plate to which the present invention is applied will be described in detail with reference to the drawings.

  The high-tensile steel plate targeted by the present invention is a high-tensile steel plate having a tensile strength of 500 MPa or more, and can maintain good workability such as press working. The steel sheet structure need not be particularly limited, but as a steel sheet structure capable of imparting excellent workability and strength due to processing-induced transformation at room temperature, it should be a multiphase structure containing a ferrite phase, a bainite phase, and an austenite phase. Is this.

  Moreover, the high-tensile steel plate which this invention makes object limits content of the basic component of C, Si, Mn, and Al in a steel plate from the point which satisfies the said tensile strength and a formability as follows.

  In the following description, “%” means “mass%” unless otherwise specified.

C: 0.01 to 0.3%
C is the most basic element for controlling the hardenability and strength of steel, and is an essential element for securing retained austenite. Specifically, it is an important element for dissolving sufficient C in the austenite phase and leaving the desired austenite phase at room temperature, and is useful for increasing the balance between strength and stretch flangeability. If the C is less than 0.01%, it is difficult to secure a retained austenite structure that is necessary as a structure-reinforced steel sheet. On the other hand, when C exceeds 0.3%, not only the effect is saturated, but also the weldability is lowered. For this reason, the C content is desirably 0.01 to 0.3%.

Si: 0.2-3.0%
Si is an element effective not only for deoxidation or strength improvement but also for the generation of stable retained austenite. If this Si is less than 0.2%, it will be difficult to ensure the required tensile strength. Further, if this Si exceeds 3.0%, the effect of increasing the strength is saturated, and the ferrite ductility in the pearlite is deteriorated, which becomes a factor of deteriorating workability. Therefore, the Si content is set to 0.2 to 3.0%.

Mn: 0.1 to 3.0%
Mn is an element utilized next to C because it has a role of increasing the strength of the base material and is inexpensive. If this Mn is less than 0.1%, the effect of increasing the strength cannot be obtained. On the other hand, if Mn exceeds 3.0%, the slab is likely to be cracked and the spot weldability is also deteriorated. Therefore, the Mn content is set to 0.1 to 3.0%.

Al: 0.01 to 2.0%
Al is an effective element as a deoxidizing element and is also an effective element for improving the toughness of steel. If the Al content is less than 0.01%, these sufficient effects cannot be obtained. Conversely, if the Al content exceeds 2.0%, the weldability is deteriorated or the increase in alumina inclusions causes the steel to increase. Degradation of toughness. Therefore, the Al content is desirably in the range of 0.01 to 2.0%.

  As long as the present invention satisfies the strength of a high-tensile steel plate having a tensile strength of 500 MPa or more and can maintain good workability such as press working, other components having the effect are added to improve various properties of the steel plate. You may contain suitably in addition to the said basic component.

  For example, in addition to the above-described main component elements, one or more of B, Ti, V, Cr, and Nb that have an effect of improving quenching, B is 0.0005% to less than 0.01%, and Ti is 0.01% or more and less than 0.1%, V is added 0.01% or more and less than 0.3%, Cr is added 0.01% or more and less than 1%, and Nb is added 0.01% or more and less than 0.1%. Also good. In the case of adding these elements, in order to sufficiently obtain the effect of improving the hardenability of the steel sheet, it is preferable to add more than the lower limit value of the above-mentioned addition amount of each element, and the amount exceeding the upper limit value of the above-mentioned addition amount. Even if it is added, the effect is saturated, and the effect of improving the hardenability to meet the cost cannot be expected.

  Further, for example, one or more of Ni, Cu, Co, and Mo, which have an effect of improving the strength, may be added within a range of 0.01% or more and less than 2.0%. When these elements are added, in order to obtain a sufficient effect of improving the strength, it is preferable to add more than the lower limit of the above-mentioned addition amount of each element, and even if an amount exceeding the upper limit of the above-mentioned addition amount is added This is not preferable because it leads to excessive strength and increased alloy costs.

  Moreover, you may contain common inevitable elements, such as P, S, and N which have the strength improvement effect.

  The gist of the present invention is as follows. The high-tensile steel plate to which the present invention is applied is one or more oxide particles selected from silicon oxide, manganese oxide, manganese silicate, manganese aluminum oxide, and manganese aluminum silicate in the region from the steel plate surface to a depth of 0.2 μm. Contains.

  These internal oxides, when recrystallizing the steel sheet structure in the recrystallization step, are crystal grains of a recrystallized structure in a depth region from the steel sheet surface to 1 μm due to the grain boundary pinning effect by the internal oxide. Is refined so that the size becomes 0.5 μm or less. By refinement of the crystal grains, the crystal grain boundary density on the steel sheet surface is increased, and the crystal grain boundary density exposed per unit area on the steel sheet surface is increased. Further, in the recrystallization process, the solid solution Si and Mn absorption in the steel by the internal oxides reduces the concentration of the solid solution Si and Mn in the steel, and the oxidation formed on the grain boundaries exposed on the steel plate surface. A steel plate surface having an object of 10 nm or less is obtained.

  Next, the surface of the steel sheet and the phosphate solution are reacted in the chemical conversion treatment, and the amount of phosphophyllite formed is increased by eluting Fe into the phosphate solution through the above-mentioned crystal grain boundaries. The P value in the coating is increased, thereby improving the corrosion resistance.

  Si: 2 to <15%, Mn: 10 to <40%, and O: 20 to <35% constitute the internal oxide particles within a depth range of 0 to 0.01 μm from the steel sheet surface. If any of Si, Mn, and O exceeds the above range, the grain boundary exposed on the surface of the steel sheet is covered with an oxide having a thickness of more than 10 nm, and the chemical conversion treatment performance deteriorates. It is desirable to suppress the lower limit of the concentration of Si, Mn, and O in the depth region as low as possible.

  Moreover, within the range of 0.01 to 0.1 μm in depth from the steel sheet surface, Si, Mn, and O are Si: 0.5 to 3.0%, Mn: 1.0 to 3.0%, O: 0.1 to 1.0% constitutes the internal oxide.

  In the range of 0.1 to 0.2 μm deep from the steel sheet surface, Si, Mn, and O are Si: 2 to 8%, Mn: 2 to 10%, and O: 0.5 to 3%. The internal oxide particles are configured. When the concentration of Si, Mn and O is less than the above lower limit, since the solid solution concentration of Si and Mn in the steel is high, Si and Mn form oxides on the steel sheet surface in the recrystallization step described later. And the grain boundary exposed on the steel plate surface is coat | covered with the oxide more than 10 nm in thickness, and chemical conversion treatment property deteriorates. If the concentration of Si, Mn and O exceeds the above upper limit, the internal oxide particles become coarse, and the pinning effect of the grain boundaries becomes weak in the recrystallization process, so the crystal grains do not become finer and A high-density grain boundary cannot be obtained.

  Moreover, in this invention, the average particle diameter of the crystal grain exposed on the steel plate surface shall be 0.5 micrometer or less. The smaller the crystal grain size, the larger the grain boundary density on the steel sheet surface, and in the chemical conversion treatment, the density of sites where Fe elutes from the steel sheet surface into the phosphate solution increases, and the amount of phosphophyllite formed increases. Therefore, if the crystal grain size is 0.5 μm or less, phosphophyllite is sufficiently formed by the chemical conversion treatment, and the P value of the chemical conversion treatment film is 0.9 or more. On the other hand, when the average particle diameter exceeds 0.5 μm, the grain boundary density on the steel sheet surface decreases, the amount of phosphophyllite formed by the chemical conversion treatment is small, and the P value of the chemical conversion film is less than 0.9. Therefore, the average grain size of the crystal grains exposed on the steel sheet surface is 0.5 μm or less.

  Next, the manufacturing method of the high-tensile steel plate to which the present invention is applied will be described. After forming the steel plate composed of the components described above, annealing is performed to remove residual stress and improve machinability. In this annealing, after forming the steel sheet, it is heated to a recrystallization temperature and held at a constant temperature, and then slowly cooled by ordinary furnace cooling.

  FIG. 1 shows an example of an annealing temperature history in a method for producing a high-tensile steel plate to which the present invention is applied. This annealing includes a preheating step S11, a temperature raising step S12, a recrystallization step S13, and a temperature lowering step S14.

First, in preheating process S11, it heats up a steel plate from room temperature to preheating temperature Tp ( degreeC ) . Tp is set to 300 to 500 ° C. When the preheating temperature Tp is less than 300 ° C., the residual stress in the steel sheet is not sufficiently removed, which is not preferable. Further, when the preheating temperature Tp exceeds 500 ° C., it is not preferable in terms of cost.

Further, the PH ₂ O / PH _{2 of the} annealing atmosphere made of the mixed gas of N ₂ and H ₂ in the preheating step S11 is log (PH ₂ O / PH ₂ ) ≦ −2.8 × 10 ⁻⁶ ( Tp + 273) ² Control is performed so that + 6.8 × 10 ⁻³ ( Tp +273) −4.8 is satisfied. When log (PH ₂ O / PH ₂ ) exceeds −2.8 × 10 ⁻⁶ ( Tp +273) ² + 6.8 × 10 ⁻³ ( Tp +273) −4.8, Fe oxide is generated on the steel plate surface. This is undesirable because it causes wrinkles on the surface of the steel sheet.

FIG. 2 shows a schematic cross-sectional view of the steel plate 5 to be annealed. In this preheating step S11, no internal oxide is generated in the steel plate 5 as shown in FIG.
In this preheating process S11, after raising the annealing temperature to the preheating temperature Tp, the process proceeds to the next temperature raising process S12.

In the temperature raising step S12, the steel plate 5 is heated from the preheating temperature Tp to the recrystallization temperature Tr (650 ° C. to 900 ° C.). In this temperature raising step S12, PH ₂ O / PH _{2 in the} atmosphere is 5.3 × 10 ⁻⁸ Tr ² + 1.4 × 10 ⁻⁵ Tr−0.01 ≦ (PH ₂ O / PH ₂ ) ≦ 6. 4 × 10 ⁻⁷ Tr ² + 1.7 × 10 ⁻⁴ Tr−0.1, and as shown in FIG. 2B, a fine internal oxide 11 is generated in the region from the surface of the steel plate 5 to a depth of 1 μm. Is done. The internal oxide is one or more oxide particles selected from the above-described silicon oxide, manganese oxide, manganese silicate, manganese aluminum oxide, and manganese aluminum silicate.

In order to generate the internal oxide 11 in the temperature raising step S12, PH ₂ O / PH ₂ is changed to 5.3 × 10 ⁻⁸ Tr ² + 1.4 × 10 ⁻⁵ Tr−0.01 ≦ PH ₂ O. / PH ₂ ) ≦ 6.4 × 10 ⁻⁷ Tr ² + 1.7 × 10 ⁻⁴ Tr−0.1. The reason is that when PH ₂ O / PH ₂ exceeds 6.4 × 10 ⁻⁷ Tr ² + 1.7 × 10 ⁻⁴ Tr−0.1, the internal oxide 11 becomes coarse, and a recrystallization step described later, The grain boundary pinning effect by the internal oxide 11 for refining crystal grains on the surface of the steel sheet is reduced, and further raising PH ₂ O / PH ₂ is not preferable because Fe oxide is generated on the surface. Further, when PH ₂ O / PH ₂ is less than 5.3 × 10 ⁻⁸ Tr ² + 1.4 × 10 ⁻⁵ Tr-0.01, the supply of oxygen from the steel sheet surface layer becomes insufficient in this temperature range, This is because the fine internal oxide 11 cannot be sufficiently generated.

  In the temperature raising step S12, the rate of temperature rise is set to 1 to 20 ° C./second. The heating rate here is an average of heating rates from the preheating temperature Tp to the recrystallization temperature, and does not prescribe heating conditions during heating. For this reason, as long as the average heating rate is within the above-described range, the heating rate may deviate from the above-described range in a certain period.

  The reason for setting the heating rate to 1 to 20 ° C./second is that when the heating rate is 1 ° C./second or less, the oxide particles are coarsened and cannot be finely dispersed in the steel sheet. This is because when the temperature rising rate is 20 ° C./second or more, the formation density of the oxide particles becomes insufficient. Incidentally, it is desirable that the rate of temperature increase be in the range of 1 ° C./second to 2 ° C./second from the viewpoint of obtaining a sufficient amount of oxide particles. After raising the annealing temperature to the recrystallization temperature in the temperature raising step S12, the process proceeds to the next recrystallization step S13.

  In the recrystallization step S13, the temperature is kept constant at the recrystallization temperature Tr (650 to 900 ° C.). When the recrystallization temperature Tr is less than 650 ° C., the recrystallization is insufficient and the steel sheet cannot have the press workability necessary for the steel sheet. In addition, annealing at a temperature at which the recrystallization temperature Tr exceeds 900 ° C. is not preferable because the cost increases.

In this recrystallization step, PH ₂ O / PH ₂ in the annealing atmosphere satisfies (PH ₂ O / PH ₂ ) ≦ 5.3 × 10 ⁻⁸ Tr ² + 1.4 × 10 ⁻⁵ Tr−0.01. Control. As a result, the internal oxide 11 generated in the temperature raising step S12 can be grown at the crystal grain boundary 12 in this recrystallization step S13 as shown in FIG. It becomes. Since the internal oxide 15 grown in the recrystallization step S13 absorbs a large amount of elements from the vicinity of the steel sheet surface or the like, its component ratio is different from that of the internal oxide 11. Further, the size of the internal oxide 15 is larger than that of the internal oxide 11.

When the hydrogen partial pressure ratio PH ₂ O / PH ₂ exceeds 5.3 × 10 ⁻⁸ Tr ² + 1.4 × 10 ⁻⁵ Tr−0.01, the internal oxide particles become coarse and Since the boundary pinning effect is reduced and the crystal grains of the steel sheet structure are coarsened, (PH ₂ O / PH ₂ ) is set within the above-described range. The holding time at the recrystallization temperature is 40 to 600 seconds. When the holding time at the recrystallization temperature is 40 seconds or less, the steel sheet structure is not sufficiently recrystallized and desired characteristics cannot be obtained. When the holding time is 600 seconds or more, the internal oxide particles are coarsened, the grain boundary pinning effect is reduced, and the crystal grains of the steel sheet surface structure are coarsened.

  The temperature increase rate in the temperature increase step S12 is set within the above-described range, the number of fine internal oxides per unit volume is increased, and recrystallization is performed in a state where the internal oxides are sufficiently formed. The internal oxide is grown by setting the holding time at the crystallization temperature within the range of 40 seconds to 600 seconds.

  Moreover, in this recrystallization process S13, the oxide 13 is formed on the steel plate surface. However, the oxide 13 may cover the crystal grain boundaries 12 exposed on the steel plate surface. Even if an oxide adheres to the crystal grain boundary 12, this is a so-called natural oxide film having a thickness of less than 10 nm. In the following, even if the oxide 13 as such a natural oxide film is formed at the grain boundary on the surface of the steel sheet, it is assumed to be equivalent to the case where the grain boundary is exposed on the surface of the steel sheet.

  In this recrystallization step S13, after the holding at the recrystallization temperature is finished, the process proceeds to the temperature lowering step S14. In the temperature lowering step S14, the steel sheet is cooled by ordinary furnace cooling.

  In each of these steps S11 to S14, in order to investigate the formation behavior of the internal oxides 11 and 15, Si, Mn, and the like constituting the oxide particles based on the glow discharge emission spectroscopic analysis method (GDS analysis method) The O content was measured.

  FIG. 3 shows the result of measuring the distribution of the internal oxide in the steel plate depth direction in the preheating step S11 by the GDS analysis method. In FIG. 3, the horizontal axis represents the depth (μm) from the steel sheet surface, and the vertical axis represents the concentration (mass (%)) of the element to be analyzed. It is shown that Si and Mn constituting the internal oxide 11 are intensively detected in the vicinity of the steel plate surface. It can also be seen that Fe slightly decreases in the vicinity of the steel plate surface.

  On the other hand, as shown in FIG. 4, the GDS distribution in the temperature raising step S12 is such that Si, Mn, and O constituting the internal oxide 11 are concentrated within a range of 0 to 0.01 μm in depth from the steel sheet surface. It can be seen that, in addition to appearing, peaks of Si, Mn, and O appear within a depth range of 0.1 to 0.2 μm from the steel sheet surface. Furthermore, a so-called deficient layer with a small amount of Si, Mn, and O is formed within a depth range of 0.01 to 0.1 μm from the steel sheet surface.

  This tendency confirms that the internal oxide 11 is generated in the temperature raising step S12. In response to the increase in the concentration of the elements constituting these internal oxides 11, the Fe concentration is within a range of 0 to 0.01 μm deep from the steel sheet surface and 0.1 to 0.2 μm deep from the steel sheet surface. is decreasing.

  Further, in the recrystallization step S13, as shown in FIG. 5, Si constituting the internal oxide 15 appearing at a depth of 0 to 0.01 μm from the steel plate surface and at a depth of 0.1 to 0.2 μm from the steel plate surface. , Mn and O peaks are shown to be high. Moreover, it is shown that the valley of the peak corresponding to what is called a deficient layer which exists in the range of 0.01-0.1 micrometer in depth from the steel plate surface becomes deep. This tendency confirms that the internal oxide has grown in the recrystallization step S13. In response to the growth of the internal oxide 15, the Fe concentration is further reduced as compared with the concentration in the temperature raising step S12.

  Moreover, in the high-tensile steel plate according to the present invention obtained through the above-described steps S11 to S14, the thickness of the oxide formed at the crystal grain boundary exposed on the steel plate surface is 10 nm (= 0.01 μm) or less. It becomes possible. That is, in this high-tensile steel plate, an internal oxide is generated at a depth of 0.1 to 0.2 μm from the steel plate surface and grown. For this reason, by maintaining the temperature for a certain time above the recrystallization temperature, Si and Mn dissolved in the steel can be absorbed into the internal oxide as the internal oxide grows. As a result, almost no oxide is formed on the surface of the steel sheet, or even if it is formed, 70% or more of the grain boundary exposed on the surface is exposed when viewed from the cross section of the steel sheet. Is possible. For this reason, it is possible to clean the grain boundary of the high-tensile steel plate obtained by going through the manufacturing method according to the present invention.

  When the grain boundary exposed on the surface is less than 70% when viewed from the cross section of the steel sheet, the P value remains at the 0.8 level and may not exceed 0.9.

  There may be a so-called natural oxide film having a thickness of less than 10 nm on the grain boundary exposed on the surface of the steel sheet obtained after annealing. This natural oxide film is formed after annealing. Thus, the composition can be distinguished from the above oxides that contain a large amount of Fe oxide and are formed during annealing. These natural oxide films have no effect on the chemical conversion treatment. Further, when the steel sheet is recrystallized in step S13, the internal oxide exhibits a pinning effect and suppresses the growth of crystal grains in the steel sheet surface structure, and the region of 1 μm from the steel sheet surface has a crystal grain size. Becomes a fine crystal of 0.5 μm or less.

  In the present invention, the steel sheet produced through the above-described annealing process is subjected to chemical conversion treatment and further subjected to electrodeposition coating or the like. By applying this chemical conversion treatment, in addition to being able to improve the corrosion resistance, it prevents peeling of the coating film by functioning as a coating base, and prevents rust from spreading even if the coating film is wrinkled. It becomes possible. As the chemical conversion treatment solution used for this chemical conversion treatment, for example, a phosphate solution or the like is used.

  As described above, the high-strength steel sheet to which the present invention is applied can clean the grain boundary and refine the crystal grain on the steel sheet surface, so that the grain boundary exposed per unit area of the steel sheet surface can be reduced. The density can be increased. When Fe of the steel plate reacts with the phosphate solution during the chemical conversion treatment, Fe elutes into the phosphate solution through this grain boundary, and the amount of phosphophyllite formed can be increased. It becomes.

  If Si, Mn, or the like, which has a higher ionization tendency than Fe, is concentrated at the grain boundary, the elution of Fe is hindered, and the phosphophyllite formation state deteriorates. By growing in the recrystallization step S13, these Si, Mn, etc. can be absorbed. Microscopically, elements such as Si, Mn, etc. at the grain boundaries are reduced, and the elution of Fe is promoted. It becomes possible to promote the formation of phyllite.

  If the amount of phosphophyllite formed can be increased, the P value (= P / (P + H)) as the ratio of the amount of phosphophyllite (hereinafter referred to as P) and the amount of phosphorite (hereinafter referred to as H). The corrosion resistance itself can be improved.

  Note that the maximum grain size is about 0.5 μm, the size is about 0.5 μm, and the grain boundary closest to the internal oxide existing at a position of about 0.5 μm from the steel sheet surface. The concentrated components (Si, Mn, etc.) are preferentially absorbed by the internal oxide.

  Steel having the composition shown in Table 1 was hot-rolled, pickled, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 0.8 mm.

  Incidentally, this Table 1 illustrates the steels of the present invention of steel types A to E within the range of the components defined in the present invention.

Next, the steel of the present invention and the comparative steel composed of the above-described components were annealed using a continuous annealing facility. In this continuous annealing equipment, the ratio PH ₂ O / PH ₂ of the steam partial pressure and the hydrogen partial pressure in the furnace can be controlled. That is, water vapor was introduced into the hydrogen gas in the furnace, and the PH ₂ O / PH ₂ in the furnace was adjusted to an atmosphere 1) as shown in Table 2.

  In addition, the hydrogen concentration and water vapor concentration in the furnace were controlled while monitoring the dew point meter at room temperature installed in the furnace and the hydrogen concentration meter.

The value of atmosphere 1) in Table 2 indicates log (PH ₂ O / PH ₂ ) in the preheating step S11 until the steel plate temperature reaches from the room temperature to the preheating temperature.

Further, the rate of temperature increase (° C./second) shown in Table 2 indicates the rate of temperature increase in the temperature increasing step S12, and the atmosphere 2) is the PH ₂ O / in the furnace in the temperature increasing step S12. PH ₂ is shown.

Furthermore, the recrystallization temperature (° C.) shown in Table 2 indicates the recrystallization temperature in the recrystallization step S13, and the atmosphere 3) is the PH ₂ O in the furnace in the recrystallization step S13. / PH ₂ is indicated.

  By the way, the heat treatment conditions were performed for eight types of HP1 to HP8, and HP1, 4, and 5 were within the range of the manufacturing conditions defined in the present invention. Also, HP2 deviated from the manufacturing conditions defined in the present invention for atmosphere 1), and HP3 deviated from the manufacturing conditions defined in the present invention for atmosphere 2). HP6 was deviated from the production conditions defined in the present invention for atmosphere 3), and HP7 was deviated from the production conditions defined in the present invention by setting the heating rate to 0.5 ° C./second. Moreover, HP8 was made to deviate from the manufacturing conditions prescribed | regulated by this invention by setting the temperature increase rate to 30 degrees C / sec.

  In addition, regarding all the heat treatment conditions HP1 to HP8, the preheating temperature was 500 ° C., and the recrystallization temperature was 800 ° C. The holding time at the recrystallization temperature in the recrystallization step S13 was 60 seconds.

  The steel of the present invention after annealing was evaluated for two items: the crystal grain size just below the surface layer, the oxide grain size just below the surface layer, and the location. Table 3 shows the evaluation results.

  The contents of Si, Mn, and O within a depth range of 0 to 0.01 μm from the steel sheet surface were measured using a GDS apparatus (JY5000RF-PSS manufactured by Jobin Yvon).

  Thus, the analysis data does not fluctuate depending on analysis conditions (for example, measurement area in GDS, measurement current, etc.).

As a result of measurement by GDS as described above, the contents of Si, Mn, and O were Si: 2 to <15%, Mn: 10 to <40%, and O: 20 to <35%. It was confirmed that. In the method of preparing the measurement sample , the test piece was further processed by a focused ion beam (FIB) apparatus, and the cross section including the steel plate surface was processed into a thin piece having a width of about 10 μm or more and a thickness of about 0.1 μm. Next, this thin piece sample was confirmed with a transmission electron microscope (TEM). At this time, the thin piece sample was confirmed under the condition that an oxide of 10 nm or less can be observed.

  As a specific method of TEM observation, in order to prevent damage during FIB processing, a carbon film of 0.2 μm and a tungsten film of 2 μm are vapor-deposited on the sample surface, and a Ga ion beam of 30 kV, 0.05 to 1 nA is used. A thin piece having a thickness of 0.1 μm and a width of 10 μm may be cut out and used as a TEM observation sample. In TEM observation, observation may be performed with an acceleration voltage of 200 kV and an observation magnification of 100,000 to 200,000 times.

  Since the crystal size of the steel plate surface layer portion is about 0.5 μm, 40 or more crystal grain boundaries can be observed in one field of view (observation region having a width of 10 μm or more). For this reason, although it is possible to measure even with one visual field in a test piece having a width of about 10 μm or more, it is preferable to observe five or more visual fields.

  Then, the positional relationship between the surface layer oxide and the grain boundary exposed on the surface layer was observed and confirmed, and a case where 70% or more of the grain boundary exposed on the surface was not less than the surface oxide film was accepted. The criterion for such acceptance is that 30% or less of the oxide species containing one or two of silicon oxide and manganese silicate in a total amount of 70% by mass or more with respect to the surface of the grain boundary region as seen from the cross section is present. It means that

This is because the number A of crystal grain boundaries whose surfaces are covered with the above oxide species and crystal grain boundaries whose surfaces are not covered with the above oxide species (exposed to the steel sheet surface) are observed from the cross section. This means that the ratio (%) calculated by the following formula is 30 or less. The grain boundaries covered with the Fe-based natural oxide are included in the number A.
Number A / (Number A + Number B) (1)

  As a result, a case where the above condition was satisfied was shown as a pass in the table as a pass, and a case where the above condition was not satisfied was a failure (a cross in the table).

  Furthermore, as a result of measuring with said GDS apparatus within the depth of 0.1-0.2 micrometer from the steel plate surface, content of Si, Mn, and O is Si: 2-8%, Mn: 2-10 %, O: 0.5 to 3%. In addition, the oxide particle size immediately below the surface layer and the presence position of the oxide immediately below the surface layer are obtained by further processing the test piece with a focused ion beam apparatus and including the steel plate surface. The cross section is processed into a thin piece with a thickness of about 0.1 μm and checked by a transmission electron microscope. When the diameter is 0.01 μm to 1.0 μm in the region from the steel sheet surface to a depth of 1 μm, the oxide just below the surface layer Was accepted (circles in the table). On the other hand, when the above conditions were not satisfied, the test was rejected (x mark in the table).

  The crystal grain size immediately below the surface was measured by observing a scanning ion image with a focused ion beam device after exposing the cross section of the steel plate cross section by fine processing with a focused ion beam device. The case where the size of the crystal grain of the steel sheet in the region from the steel sheet surface to a depth of 1 μm is 0.5 μm or less is accepted (circle mark in the table), and the case where it exceeds 0.5 μm is rejected (× mark in the table) ).

In addition, about steel of this invention after annealing, after performing degreasing | defatting, surface adjustment, and a chemical conversion treatment, it evaluated about P value. The treatment solutions used for each of these treatments were all made by Nippon Paint, degreased using Surf Cleaner SD250, surface adjustment using Surf Fine 5N-10, and chemical conversion treatment using Surf Dyne SD2500. In addition, the adhesion amount of the chemical conversion treatment film was adjusted to be about 3 g / m ² .

  The P value is represented by the peak intensity ratio of P / (P + H) by X-ray diffraction, where P and H are the diffraction line intensities from the (110) plane of the phosphorophyllite and the (020) plane of the white, respectively. It was. A P value of 0.9 or higher was accepted and less than that was rejected.

  In addition, the corrosion resistance was evaluated after electrodeposition coating was performed on the steel sheet subjected to chemical conversion treatment. For electrodeposition coating, V-50 manufactured by Nippon Paint was used, the film thickness was 25 μm, and the baking temperature was 170 ° C. After electrodeposition, a cutter is attached on the electrodeposited surface with a cutter, immersed in an aqueous solution of NaCl at 55 ° C and 5% for 240 hours, a tape peeling test is performed on the cut flange, and the maximum coating film around the cut flange is obtained. The peel width was measured. A maximum peel width of less than 2 mm was regarded as acceptable (◯ mark in the table), and 2 mm or more was regarded as unacceptable (x mark in the table).

  From Table 3, as for the heat treatment conditions HP1, 4 to 5, it was possible to obtain a pass result for each evaluation item for the steel types A to E as the steel of the present invention. On the other hand, regarding the heat treatment conditions HP2, 3, and 6 to 8, all of the steel types A to E were unacceptable for each evaluation item.

  From these results, the steel slab having the components defined in the high-strength steel sheet according to the present invention is annealed based on the heat treatment conditions defined in the production method of the present invention, and from the steel sheet surface to a depth of 1 μm. The size of the crystal grains of the steel sheet in the region can be 0.5 μm or less, and the oxide grain size immediately below the surface layer can be optimized. Similarly, it is possible to improve the corrosion resistance by stably setting the P value to a high level. That is, the results of Table 3 suggest that the intended effects of the present invention can be obtained.

It is a figure which shows an example of the annealing temperature log | history in the manufacturing method of the high strength steel plate to which this invention is applied. It is a figure which shows the cross-sectional schematic diagram of the steel plate which anneals. It is a figure which shows the result of having measured the distribution of the steel plate depth direction of the internal oxide in preheating process S11 by the GDS analysis method. It is a figure which shows the result of having measured the distribution of the steel plate depth direction of the internal oxide in temperature rising process S12 by the GDS analysis method. It is a figure which shows the result of having measured the distribution of the steel plate depth direction of the internal oxide in recrystallization process S13 by the GDS analysis method.

Explanation of symbols

5 Steel plates 11, 15 Internal oxide 12 Grain boundary 13 Oxide

Claims (2)

In mass%, C: 0.01-0.3%, Si: 0.2-3.0%, Mn: 0.1-3.0%, Al: 0.01-2.0% In the high-tensile steel plate having a tensile strength of 500 MPa or more, the balance being Fe and inevitable impurities ,
An observation region having an average grain size of 0.5 μm or less on the surface of the steel plate and a width of 10 μm or more on the surface of the steel plate is processed into a thin piece for cross-sectional TEM observation, and an oxide of 10 nm or less is observed on the thin sample. Oxide species containing one or two of silicon oxide and manganese silicate in a total amount of 70% by mass or more as measured by TEM observation under conditions that can be produced is 30% or less with respect to the grain boundary region surface as seen from the cross section. Exists,
A high-tensile steel plate excellent in chemical conversion treatment, wherein the particle size of the oxide species present in the range of 0.1 to 1.0 µm in depth from the steel plate surface is 0.1 µm or less. In mass%, C: 0.01-0.3%, Si: 0.2-3.0%, Mn: 0.1-3.0%, Al: 0.01-2.0% In the method for producing a high-strength steel sheet having a balance of Fe and inevitable impurities and having a tensile strength of 500 MPa or more, a preheating step for raising the temperature of the cold-rolled steel sheet from room temperature to a preheating temperature Tp, and a recrystallization temperature from the preheating temperature Tp Performing an annealing treatment comprising a temperature raising step for raising the temperature to Tr and a recrystallization step for keeping the recrystallization temperature Tr constant;
The preheating temperature Tp in the preheating step is set to 300 to 500 ° C., and the water vapor partial pressure ratio (PH) to the hydrogen partial pressure in the annealing atmosphere ₂ O / PH ₂ ) Is controlled to satisfy the condition of the following formula (1) from the relationship with the preheating temperature Tp,
The recrystallization temperature Tr in the temperature raising step is set to 650 ° C. to 900 ° C., and the water vapor partial pressure ratio (PH) to the hydrogen partial pressure in the annealing atmosphere ₂ O / PH ₂ ) Satisfies the condition of the following formula (2) from the relationship with the recrystallization temperature Tr, and the temperature rising rate is controlled to be 1 to 20 ° C./second,
Steam partial pressure ratio (PH to hydrogen partial pressure in the annealing atmosphere in the recrystallization step) ₂ O / PH ₂ ) Satisfies the condition of the following formula (3) from the relationship with the recrystallization temperature Tr, and is controlled so that the holding time is 40 to 600 seconds.
log (PH ₂ O / PH ₂ ) ≦ −2.8 × 10 ^-6 (Tp + 273) ² + 6.8 × 10 ^-3 (Tp + 273) -4.8 (1)
5.3 × 10 ^-8 Tr ² + 1.4 × 10 ^-Five Tr-0.01 ≦ PH ₂ O / PH ₂ ≦ 6.4 × 10 ^-7 Tr ² + 1.7 × 10 ^-Four Tr-0.1 (2)
PH ₂ O / PH ₂ <5.3 × 10 ^-8 Tr ² + 1.4 × 10 ^-Five Tr-0.01 (3)

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