JP2005248281A - High strength cold rolled steel sheet having excellent adhesion of coating film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet having excellent adhesion of a coating film, and having a tensile strength of ≥550 MPa. <P>SOLUTION: The high strength cold rolled steel sheet is a ferrite-martensite based DP (Dual Phase) steel sheet satisfying prescribed components and having a tensile strength of ≥550 MPa, where, in the cross-section in a direction orthogonal to the surface of the steel sheet, at the time when the region in which depth from the surface of the steel sheet is 2 μm and the length in the surface of the steel sheet is 10 μm is observed at the magnification of 5,000 times using an electron microscope, Mn-Si multiple oxides with a major axis of 0.01 to 5 μm in which the atomic ratio between Mn and Si (Mn-Si) is ≥0.5 are present by ≥10 pieces, and also, the occupancy ratio of oxides consisting essentially of Si and having a cross-sectional thickness of ≥0.01 μm in the length of 10 μm in the surface of the steel sheet is ≤10% (inclusive of 0%) on the average of 5 places optionally selected in the surface of the steel sheet. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in coating film adhesion, and in particular, has a tensile strength of 550 MPa or more and excellent coating film adhesion, and is optimal as a steel sheet for automobile parts. The present invention relates to a cold rolled steel sheet.

  Higher strength of steel materials is demanded against the background of improving fuel economy and weight reduction of automobiles, and high tempering (hardening) is also progressing in the field of cold-rolled steel sheets. On the other hand, since cold-rolled steel sheets are press-formed at the time of component production, it is assumed that sufficient ductility such as elongation is secured. Addition of alloying elements is effective for increasing the strength, but generally the ductility tends to decrease as the amount of alloying elements increases.

  However, among alloy elements, Si is an element with a relatively small reduction in ductility, and is an element effective for increasing the strength while ensuring ductility. However, when the Si content is increased, the chemical conversion treatment property is deteriorated, and the coating film adhesion after coating is lowered. For this reason, when the chemical conversion property is important, the Si content must be reduced. Further, when the Si content is increased, cracks due to Si-containing grain boundary oxides generated on the steel sheet surface are likely to occur, which has been a factor of deteriorating coating film adhesion.

  Until now, as a technology to achieve both mechanical properties and chemical conversion treatment, the steel sheet surface is coated with a clad material, and a low Si concentration layer is provided on the steel sheet surface to improve chemical conversion treatment. There is a technique for ensuring the target characteristics (for example, Patent Document 1). However, since the clad structure is required, the manufacturing process becomes complicated and the cost increases.

  In addition, there is a conventional technique in which a special alloy element is added so that Si that inhibits chemical conversion treatment does not concentrate on the surface (for example, Patent Document 2 and Patent Document 3). In this method, by adding Ni or Cu, concentration of Si on the steel sheet surface layer is suppressed, and chemical conversion processability is ensured. However, this method has a problem that the cost is increased because expensive Ni or Cu is used.

  Further, these techniques relate to a so-called IF steel sheet in which the C content is as low as 0.005% or less, and the deep drawing property is improved by regulating the recrystallization temperature and controlling the texture. However, it is difficult to achieve the high strength as intended by the present invention with such an IF steel sheet with a very small amount of C.

In Patent Document 4, NbC is precipitated, and this is used as a nucleation site for zinc phosphate crystals to ensure chemical conversion treatment. However, this technique is also a technique that secures deep drawability by controlling the texture in a low C concentration range of 0.02% or less. Although the C concentration is slightly higher than the IF steel, the strength is insufficient. Absent. According to the technique, in the first invention 55kgf ^/ mm 2 (539MPa), in the second invention is the upper limit of 60kgf ^/ mm 2 (588MPa) strength, in the second invention is greater than 550 MPa. However, this strength is realized by increasing the P and Mo contents, and if these elements are contained, sufficient weldability cannot be secured.

Patent Document 5 proposes a retained austenite-containing steel sheet that ensures chemical conversion processability by defining the SiO ₂ / Mn ₂ SiO ₄ ratio of the surface layer. In this technique, in order to control the surface layer oxide or to control the element ratio of Si / Fe, the surface after continuous annealing is pickled or brushed to remove the Si oxide, or below the Ac ₁ transformation point. It is necessary to adjust the dew point to −30 ° C. or higher with temperature to suppress the amount of Si oxide produced.

However, when the pickling or brush treatment is performed, the manufacturing cost increases due to an increase in the number of steps. Although the dew point control is performed in a continuous annealing furnace, as far as the examples shown in the literature are concerned, even if the dew point is controlled, the SiO ₂ / Mn ₂ SiO ₄ ratio in the outermost layer is about 1.0, Since SiO ₂ that inhibits the formation of the chemical conversion coating crystal is generated to the same extent as Mn ₂ SiO ₄ , it is difficult to say that the chemical conversion treatment performance is sufficiently improved.

  In addition, this technique is a steel sheet using retained austenite, and contains a large amount of alloying elements such as C, Mn, Si, Al, etc. in order to secure retained austenite, and therefore has a problem of poor weldability. In the examples shown in the above publications, not only steel sheets having a tensile strength of 750 MPa or more, but also steel sheets having a strength of 650 MPa or more contain a large amount of alloying elements that inhibit the weldability, and excellent weldability cannot be expected. I think that the.

  Patent Document 6 proposes a technique for observing the surface of a steel sheet by XPS and suppressing the ratio of Si and Mn (Si / Mn) constituting the oxide to 1 or less to improve chemical conversion property.

  As steel having a Si / Mn ratio of 1 or less, for example, it is generally known that mild steel having almost no Si content is excellent in chemical conversion treatment. However, as described above, in order to increase both the high strength and the ductility, it is necessary to contain Si to some extent, and there is a limit in reducing the Si amount and making the Si / Mn ratio 1 or less. Further, it was found that a steel sheet exhibiting good chemical conversion treatment properties could not be obtained stably even when the Si / Mn ratio was controlled to 1 or less by controlling the Mn amount while securing an appropriate amount of Si.

In the above technique, the surface of the steel sheet is observed by XPS, but the measurement depth by XPS is generally several tens of angstroms, and when a thicker Si-containing oxide exists, the Si-containing oxide is accurately controlled. I think I can't. Therefore, in order to reliably improve the chemical conversion property, it is necessary to expand the control region in consideration of the size of the Si-containing oxide to be formed.
Japanese Patent Laid-Open No. 5-78752 Japanese Patent No. 2951480 Japanese Patent No. 3266328 Japanese Patent No. 3049147 JP 2003-201538 A JP-A-4-276060

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a cold-rolled steel sheet having excellent coating film adhesion at a tensile strength of 550 MPa or more and further excellent in weldability. It is in.

The high-strength cold-rolled steel sheet according to the present invention is expressed by mass% (the same applies to chemical components below), C: 1% or less (not including 0%), Si: 0.3-2%, Mn: 1-5 %, A ferrite-martensitic DP (Dual Phase) steel sheet that satisfies the following formula (1) and has a tensile strength of 550 MPa or more,
In the cross section in the direction perpendicular to the steel plate surface, when the depth of 2 μm from the steel plate surface and the steel plate surface length of 10 μm was observed with an electron microscope at a magnification of 5000 times or more,
There are 10 or more Mn-Si complex oxides having a major axis of 0.01 μm or more and 5 μm or less with an atomic ratio of Mn to Si (Mn / Si) of 0.5 or more, and a cross-sectional thickness of 0.01 μm or more. The ratio of the main oxide to the steel sheet surface length of 10 μm is characterized in that it is 10% or less (including 0%) on the average of five arbitrarily selected steel sheet surfaces (hereinafter referred to as “present invention steel sheet 1”). Sometimes).

[Si] / [Mn] ≦ 0.4 (1)
{In the formula, [Si] indicates the Si content (% by mass) and [Mn] indicates the Mn content (% by mass)}
The “Si-based oxide” refers to an element other than oxygen constituting the oxide in which Si accounts for more than 67% by atomic ratio (atomic%). It is considered amorphous. Further, the “steel plate surface” refers to a steel plate substrate surface.

  In addition to the above-mentioned requirements, the present invention further satisfies Si: 0.3 to 1.5% and uses a SEM to observe a cross section near the steel sheet surface at a magnification of 2000 times. In the following, a high-strength cold-rolled steel sheet having no cracks with a depth of 5 μm or more is also defined (in this manner, in addition to the requirements defined as the steel plate 1 of the present invention, a steel plate that satisfies the above requirements is referred to as “the steel plate 2 of the present invention”. There is).

  The width and depth of the cracks refer to the part shown in FIG. 1 (schematic cross-sectional view of steel plate) when the vicinity of the surface of the steel plate cross-section is observed at 2000 times using SEM (S-4500, manufactured by Hitachi, Ltd.). And

  In these steel plates, as a further additional requirement, it is preferable to adjust the components so as to satisfy the following formulas (2) and (3), since excellent weldability can be secured.

[P] +3 [S] +1.54 [C] <0.25 (2)
[C] + [Si] / 30 + [Mn] / 20 + 2 [P] +4 [S] <0.34 (3)
{Wherein [C], [Si], [Mn], [P], [S] represents the content (% by mass) of each element}

  The steel sheet of the present invention effectively utilizes Si and Mn, so that it can easily secure a strength of 550 MPa or more, exhibits excellent coating adhesion, and is optimal for automobiles with excellent weldability. Steel plates can be provided efficiently and inexpensively without the need for constituting a clad or using expensive elements.

  As a result of studying to obtain a steel sheet having excellent coating film adhesion, the inventors have found that the following requirement (I), and further, the following requirement (II) may be satisfied. Furthermore, while satisfying these requirements, the component composition and production conditions were also examined in order to ensure a tensile strength and ductility of 550 MPa or more.

(I) In a cross section in a direction perpendicular to the steel plate surface, a region having a depth of 2 μm from the steel plate surface and a steel plate surface length of 10 μm (hereinafter sometimes referred to as “steel plate surface region”) is magnified using an electron microscope. When observed at 5000 times or more,
(i) 10 or more Mn—Si composite oxides having a major axis of 0.01 to 5 μm with an atomic ratio of Mn to Si (Mn / Si) of 0.5 or more, and
(ii) The ratio of the length of the Si-based oxide having a cross-sectional thickness of 0.01 μm or more in the steel sheet surface length of 10 μm is 10% or less (including 0%) on average at five locations on the steel sheet surface selected arbitrarily. To be.

  (II) When observing a cross section in the vicinity of the steel sheet surface at a magnification of 2000 using an SEM, no cracks having a width of 3 μm or less and a depth of 5 μm or more are present in any 10 visual fields.

  The reason why the requirements (I) and (II) are specified will be described in detail below.

<Atomic ratio of Mn to Si (Mn / Si) in the steel sheet surface layer region is 0.5 or more Mn-Si composite oxide having a major axis of 0.01 to 5 μm: 10 or more>
The present inventors have been researching for a long time to obtain a high-strength steel sheet excellent in coating film adhesion, and have already proposed a chemical conversion treatment improving technique for a steel sheet containing a relatively large amount of Si (Japanese Patent Application). 2003-106152). This technique aims to improve chemical conversion treatment by finely dispersing amorphous Si oxide that adversely affects chemical conversion treatment by controlling the annealing atmosphere. However, in the region where the Si concentration is relatively low, not the amorphous Si oxide but the Mn—Si composite oxide is generated as the main oxide. Although this composite oxide also reduces the adhesion of the coating film in the same manner as the amorphous Si oxide, it is considered that the influence is smaller than that of the Si oxide. Therefore, the Mn—Si composite oxide is considered to be actively used for improving chemical conversion treatment, and research has been advanced along that line.

  As a result, by making Si constituting the oxide exist as a Mn-Si composite oxide, the Si activity is reduced and the Si oxide that remarkably inhibits the chemical conversion treatment reaction is suppressed and the Mn-Si composite oxidation is suppressed. It was found that the chemical conversion can be improved by finely dispersing the material so that an "electrochemical heterogeneous field" that acts as a nucleation site for zinc phosphate crystals is easily formed. The reason why the Mn—Si composite oxide defined in the present invention effectively acts on the nuclei of zinc phosphate crystals is not clear, but is considered as follows.

  In the chemical conversion treatment process, it is known that zinc phosphate crystals are likely to be generated in the “electrochemical inhomogeneous field” formed around the grain boundaries and around the Ti colloid previously deposited on the steel plate surface during the surface conditioning treatment. It has been. Also in the present invention, an electrochemical heterogeneous field is formed around the Mn-Si composite oxide, so that zinc phosphate crystals are easily attached during chemical conversion treatment, and good chemical conversion treatment performance is exhibited. It is thought that.

  It is said that the zinc phosphate crystal after the chemical conversion treatment is preferably several μm or less from the viewpoint of coating film adhesion. Therefore, it is considered that the above-mentioned electrochemical non-uniform field is desirably on the order of several μm or less. Accordingly, 10 or more Mn—Si composite oxides having a major axis of 0.01 to 5 μm are present in the steel sheet surface layer region (region where the depth from the steel plate surface is 2 μm and the steel plate surface length is 10 μm), and the composite oxide particles The average particle interval was set to several μm or less so that an electrochemical heterogeneous field having the above size was easily formed.

  As the size of the Mn—Si composite oxide to be present, the target having a major axis of 0.01 μm or more is the minimum size that can be confirmed by observation with TEM or the like in addition to the size that easily exhibits the above effect. Because it is the limit level. Preferably, 0.05 μm or more is present. On the other hand, when the size of the Mn—Si composite oxide exceeds 5 μm, the adverse effect such as inhibition of the refining of phosphate crystals by the oxide is worse than the improvement of the chemical conversion treatment property due to the formation of an electrochemical heterogeneous field. It is because it becomes large and it becomes impossible to ensure the excellent chemical conversion property. Preferably, a Mn—Si composite oxide of 1 μm or less is present.

In all the Mn-Si complex oxides present, the electrochemical heterogeneous field is not always effectively formed, so preferably 50 or more, more preferably 100 or more per steel sheet surface layer region, More preferably, 150 or more of the above Mn—Si composite oxide should be present. The above Mn-Si composite oxide is an Mn-based Mn-Si composite oxide having an Mn / Si atomic ratio (Mn / Si) of 0.5 or more, which has an adverse effect on the chemical conversion treatment than the Si oxide. For example, Mn ₂ SiO ₄ can be mentioned.

  The Mn—Si composite oxide is included in a region where the depth from the steel plate surface is 2 μm and the steel plate surface length is 10 μm and is not visible when the steel plate surface is viewed in plan. It is. Even in the presence of Mn-Si composite oxide in such a state, it is considered that it contributes to the formation of an electrochemical inhomogeneous field. This is because the above effect is exhibited.

<Proportion of Si-based oxide with a cross-sectional thickness of 0.01 μm or more in the steel sheet surface length of 10 μm: 10% or less (including 0%) on average of five steel sheet surfaces selected arbitrarily>
Even if an appropriate amount of a Mn-Si composite oxide effective as a zinc phosphate crystal nucleus is present, if there is another substance that inhibits the chemical conversion treatment, the excellent chemical conversion treatment performance will not be exhibited. It becomes inferior to adhesiveness.

  As described above, when an oxide mainly composed of Si is present on the surface of the steel sheet, zinc phosphate crystals are not generated at the portion, and the chemical conversion treatment property is deteriorated. As described above, the present inventors have proposed a technique for finely dispersing an oxide mainly composed of Si to improve the chemical conversion treatment property. In the present invention that utilizes the above-described action of the Mn—Si composite oxide, It has been found that it is preferable that the Si-based oxide is not present as much as possible. In particular, since a Si-based oxide having a large cross-sectional thickness significantly hinders the formation of zinc phosphate crystals, in the present invention, a Si-based oxide having a cross-sectional thickness of 0.01 μm or more (other than oxygen constituting the oxide). It was decided to suppress an oxide having a Si ratio of more than 67% by atomic ratio among elements. Specifically, the proportion of the Si-based oxide having a cross-sectional thickness of 0.01 μm or more in the steel plate surface length of 10 μm is 10% or less (including 0%) on average at five steel plate surfaces selected arbitrarily. I found out that I should do it. Preferably, it is 5% or less on the average of five steel plate surfaces selected arbitrarily, and most preferably 0%.

<When observing a cross section near the steel sheet surface at 2000 times using SEM, there should be no cracks with a width of 3 μm or less and a depth of 5 μm or more in any 10 fields of view>
If sharp cracks are present on the surface of the steel plate, it is considered that zinc phosphate crystals do not adhere to the site during the chemical conversion treatment, and as a result, corrosion of the site is likely to proceed, resulting in a decrease in coating film adhesion. That is, in order to improve the adhesion of the coating film, it is important to suppress as much as possible sharp cracks to which zinc phosphate crystals do not adhere.

  The present inventors have already proposed a technique for improving the adhesion of a coating film by setting the existing depth of a linear compound (width: 300 nm or less) containing Si and oxygen to 10 μm or less. In this technique, it is assumed that pickling is not performed after continuous annealing, but the steel sheet is more often subjected to pickling after continuous annealing, in which case, the linear oxide is removed and cracks occur. Occurs.

  Although the quantitative relationship between the crack depth and the linear oxide is not clear, it is considered that the linear oxide is dissolved in the acid as described above or mechanically dropped to cause cracks. Even after the oxide is removed, the crack portion is dissolved by an acid or the like, so the crack is considered to be deeper than the size of the linear oxide.

  Therefore, in the present invention, the coating film adhesion is more reliably controlled by controlling cracks regardless of the presence or absence of pickling after annealing, rather than defining the existence depth of the linear oxide as in the proposed technique. We thought that it might be possible to increase the thickness of the crack, and investigated the form of cracks to be controlled. As a result, if the width of the crack is the same as or smaller than the crystal grain size of zinc phosphate, the zinc phosphate crystal is difficult to adhere to the crack, and in particular, the zinc phosphate crystal is in the crack having a depth of 5 μm or more. Since it is difficult to adhere, cracks having a width of 3 μm or less and a depth of 5 μm or more were targeted for suppression. And when the cross section near the steel plate surface was observed at 2000 times using SEM, it was made a requirement that the crack does not exist in any 10 visual fields.

  In the present invention, the Mn—Si composite oxide is efficiently precipitated, the prescribed Si-based oxides and cracks are suppressed, and the chemical components are specified as follows in order to ensure the characteristics as a high-strength steel plate.

Since the Si-based oxide adversely affects the chemical conversion processability as described above, it is preferable to suppress it as much as possible rather than finely dispersing the oxide. Therefore, the inventors suppress the oxide mainly composed of Si by setting the ratio of [Si] / [Mn] to 0.4 or less as shown in the following formula (1). [Si] / [Mn] is preferably 0.3 or less.
[Si] / [Mn] ≦ 0.4 (1)
{In the formula, [Si] indicates the Si content (% by mass) and [Mn] indicates the Mn content (% by mass)}

<C: 1% or less (excluding 0%)>
C is an element necessary for ensuring the strength and should be contained in an amount of 0.05% or more. However, if it is present in excess, weldability is lowered. Therefore, C content is suppressed to 1% or less. Preferably it is 0.23% or less, More preferably, it is 0.15% or less.

<Si: 0.3 to 2.0% (in the case of the steel sheet 1 of the present invention)>
<Si: 0.3 to 1.5% (in the case of the steel plate 2 of the present invention)>
Si is necessary for increasing the strength, and it is necessary to contain at least 0.3%, preferably 0.5% or more, more preferably 0.7% or more. On the other hand, when the Si content is excessive, generation of the surface Si oxide layer cannot be avoided, and sufficient chemical conversion treatment property cannot be obtained. Therefore, the amount of Si is suppressed to 2.0% or less. In particular, the amount of Si needs to be suppressed to 1.5% or less in order to suppress the occurrence of cracks defined as the above requirement (II) that causes deterioration of coating film adhesion.

<Mn: 1.0 to 5.0%>
Mn is also an element necessary for securing the strength, and is also necessary for the generation of the Mn—Si composite oxide. In order to exhibit such an effect, Mn is contained in an amount of 1.0% or more, preferably 2.0% or more. However, if it becomes excessive, the ductility deteriorates, so it is suppressed to 5.0% or less, preferably 3.5% or less.

  The contained elements specified in the present invention are as described above, and the remaining component is substantially Fe. However, 1% or less of Al, 0% as an element brought into the steel depending on the status of raw materials, materials, manufacturing equipment, etc. Of course, inevitable impurities such as N (nitrogen) of 0.01% or less and O (oxygen) of 0.01% or less are allowed to be included, as long as the effects of the present invention are not adversely affected. Further, Cr, Mo, Ni, Ti, Nb, V, P or B can be positively contained as other elements.

  That is, Cr, Mo, Ni, Ti, Nb, V, P, and B may be added from the viewpoint of increasing the strength of the steel sheet. Cr: 0.1% or more, Mo: 0.1% or more, Ni: 0.1% or more, Ti: 0.005% or more, Nb: 0.005% or more, V: 0.0005% or more, P: 0.005% or more, B: 0.0003% or more However, if added excessively, ductility and weldability are deteriorated. Therefore, Cr, Mo and Ni are each 1% or less, Ti, Nb and P are each 0.1% or less, and V and B are each 0.01%. It is preferable to keep it below.

  By the way, the left side of the following formulas (2) and (3) is known as a parameter for evaluating spot weldability {Tanaka et al .: Nippon Steel Pipe Technical Report, No. 105 (1984), Heuschkel, J .: Weld J26 (10 ), P560S (1947)}, the higher the parameter value, the lower the weldability. In the present invention, ([P] +3 [S] +1.54 [C]) is 0.25 or more in the following formula (2), or ([C] + [Si] / 30 + [Mn] in the following formula (3). ] / 20 + 2 [P] +4 [S]) is 0.34 or more, it has been confirmed that spot weldability deteriorates.

[P] +3 [S] +1.54 [C] <0.25 (2)
[C] + [Si] / 30 + [Mn] / 20 + 2 [P] +4 [S] <0.34 (3)
{Wherein [C], [Si], [Mn], [P], [S] represents the content (% by mass) of each element}

  In the present invention, a steel sheet having a tensile strength of 550 MPa or more (preferably 750 MPa or more, more preferably 900 MPa or more) is used. When the tensile strength is less than 550 MPa, it is not necessary to add a large amount of Si for high strength and high ductility, and there is no problem of deterioration of chemical conversion treatment due to the generation of Si oxide as described above. Because.

  In order to secure weldability when adjusting the contents of C, Mn, and Si or securing P in order to ensure the strength, it is desirable to satisfy the following range according to the strength level.

TS: 550 to 650 MPa
[P] +3 [S] +1.54 [C] <0.14
[C] + [Si] / 30 + [Mn] / 20 + 2 [P] +4 [S] <0.21
TS: More than 650 to 750 MPa
[P] +3 [S] +1.54 [C] <0.18
[C] + [Si] / 30 + [Mn] / 20 + 2 [P] +4 [S] <0.27
TS: More than 750 to 1050 MPa
[P] +3 [S] +1.54 [C] <0.22
[C] + [Si] / 30 + [Mn] / 20 + 2 [P] +4 [S] <0.30
TS: More than 1050 MPa
[P] +3 [S] +1.54 [C] <0.25
[C] + [Si] / 30 + [Mn] / 20 + 2 [P] +4 [S] <0.34

  The present invention is directed to a ferrite-martensitic DP (Dual Phase) steel sheet. In addition to the above structure only (ie, ferrite and martensite), pearlite, bainite, and retained austenite that may inevitably remain in the production process of the present invention may be included within a range not impairing the function of the present invention. . However, the smaller these, the better.

  Although this invention does not prescribe | regulate to the manufacturing method of the said steel plate, in the area | region of 2 micrometers in depth from the steel plate base surface, an appropriate amount of prescription | regulation Mn-Si complex oxide exists, and Si main oxide is suppressed. It is very effective to perform batch annealing instead of CAL (continuous annealing). If the batch method is used, heating for a long time is possible, so the soaking temperature (heating temperature in FIG. 2 described later) can be lowered. If the soaking temperature is low, the alloying elements are compared with the diffusion of oxygen into the interior. This is because since diffusion to the surface is slow, alloy elements such as Si do not concentrate on the surface and oxides tend to be generated inside, and concentration of Si on the surface can be suppressed. Further, unlike CAL exposed to an oxidizing atmosphere (air, water-cooled equipment) in a high temperature state, the atmosphere can be controlled to a low temperature (about 200 ° C.) during cooling, and surface oxidation and cracks can be suppressed.

As a manufacturing condition, a hot rolled material wound up at 430 to 500 ° C. is used, and after immersion for 40 to 90 seconds in a 3 to 18 mass% hydrochloric acid solution at a liquid temperature of 50 to 85 ° C., the atmosphere is mixed with a nitrogen-hydrogen mixed gas. In a controlled batch type annealing furnace, the dew point was -55 to -40 ° C, the heating temperature shown in Fig. 2 described later was annealed at 740 to 820 ° C, and after soaking, 3 x 10 ^{-3 to} 3 x It is recommended to cool in a nitrogen-hydrogen mixed gas to 200 ° C. or lower at a cooling rate of 10 ⁻¹ ° ^C./second .

  In particular, in order to prevent the generation of cracks, it is very effective to set the dew point to −55 to −40 ° C. in the batch-type annealing furnace in the production process.

  The present invention is not limited to other production conditions, and may be cast after melting and hot-rolled as usual. Moreover, in the Example mentioned later, although pickling is performed after annealing, the presence or absence of this pickling is not ask | required.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  A steel material having the chemical composition shown in Table 1 was melted and hot-rolled using a slab obtained by casting, and a 300 mm × 220 mm test piece was collected from the hot-rolled material. Then, the test piece was pickled under the following conditions, ground, and then cold-rolled, and then annealed in an infrared image furnace (batch furnace) or CAL.

Pickling (hydrochloric acid treatment) conditions ・ Hydrochloric acid concentration: 15% by mass
・ Liquid temperature: 80 ℃
Immersion time: 60 seconds The annealing treatment was performed with the heat pattern schematically shown in FIG. 2 in the case of an infrared image furnace and in FIG. 3 in the case of CAL. The dew point and the heating and tempering temperatures shown in FIGS. 2 and 3 are as shown in Table 2. The dew point is an infrared image furnace or CAL atmosphere dew point. In the heat pattern diagrams of FIGS. 2 and 3, in the infrared image furnace, after heating, the furnace was cooled to 200 ° C. (slow cooling), and in CAL, water cooling (water quenching) was performed from the end point of the slow cooling.

  The mechanical properties, coating film adhesion and weldability of the obtained steel sheet were evaluated as follows. The obtained steel sheets all had a structure mainly composed of two phases of ferrite and martensite.

  Mechanical properties were measured by collecting JIS No. 5 test pieces and determining tensile strength (TS), El (total elongation), and yield ratio (YP). The stretch flangeability was evaluated using a disk-shaped test piece having a diameter of 100 mm and a plate thickness of 1.4 mm. Specifically, after punching out a hole with a diameter of 10 mm in a test piece with a punch, the hole expansion rate (λ) at the time of crack penetration was measured by performing hole expansion processing with a burr facing up with a 60 ° conical punch ( Steel Federation Standard JFST 1001).

  As the coating film adhesion, chemical conversion property and presence of cracks were examined. For the chemical conversion treatment, the precipitation state of the Mn-Si composite oxide and the Si-based oxide in the surface layer region of the steel sheet is examined as follows, and the chemical conversion treatment is performed under the following conditions, and the steel sheet surface after the chemical conversion treatment is multiplied by 1000 times. SEM observation was conducted to examine the state of adhesion of zinc phosphate crystals in 10 fields of view. And in all 10 fields of view, there was no unprecipitated portion of the zinc phosphate crystal and the maximum crystal size was 10 μm or less, and “○” was evaluated when the unprecipitated portion of the zinc phosphate crystal or the coarse crystal was present. .

・ Chemical conversion treatment solution: Zinc phosphate coating treatment solution manufactured by Nihon Parkerizing Co., Ltd.
Product Name: Palbond L 3020
・ Chemical conversion treatment process: degreasing → washing → surface adjustment → chemical conversion treatment

  The number of Mn—Si composite oxides having a major axis of 0.01 μm or more and 5 μm or less is a region where the depth from the steel sheet surface is 2 μm and the steel sheet surface length is 10 μm in the cross section perpendicular to the steel sheet surface. A dark-field scanning transmission electron image (D-STEM) is observed at a magnification of 15000 times or more using an electron microscope (HF-2000 manufactured by Hitachi, Ltd.), and the atomic ratio (Mn / Si) of Mn and Si existing in the region is 0. The number of Mn—Si composite oxides having a major axis of 0.01 μm or more and 5 μm or less that is 0.5 or more was measured. The atomic ratio between Mn and Si was determined by performing an oxide composition analysis using EDX (SIGMA manufactured by KEVEX, an energy dispersive X-ray detector) attached to the TEM. Such measurement was performed at five arbitrarily selected sites (5 fields of view), and the average value of the number of Mn—Si composite oxides at the five sites was obtained.

  Moreover, the ratio of the Si-based oxide having a cross-sectional thickness of 0.01 μm or more occupying the steel sheet surface length of 10 μm (the ratio of Si exceeding 67% by atomic ratio among elements other than oxygen constituting the oxide) It investigated in the same visual field as the measurement of the said Mn-Si oxide. That is, the total distance occupied by the Si-based oxide was determined at an arbitrary steel plate surface length of 10 μm in the same field of view as the measurement of the Mn—Si oxide, and the ratio was calculated. In this case as well, measurement was performed at five locations on the surface of the steel sheet arbitrarily selected, and the average value at the five locations was determined. These results are also shown in Table 2.

  The presence or absence of cracks was examined using an SEM (S-4500, manufactured by Hitachi, Ltd.) at a magnification of 2000 and by observing any 10 visual fields (1 visual field: 65 μm × 55 μm) in the vicinity of the surface of the steel sheet cross section.

  Weldability was examined as follows. Two test steel plates were stacked and spot-welded by the direct method (detailed conditions are as follows) to prepare a specimen for a tensile shear test, and a tensile shear test was performed to evaluate the fracture state after the test. . Tensile shear tests were performed using 10 test pieces for each test steel plate. When all the 10 ruptures were button ruptures in a sound rupture state, “◯”, and 9 or less ruptures of button ruptures. In some cases, it was evaluated as “x”. These results are also shown in Table 2.

Electrode: Dome radius type, φ6mm (tip diameter)
Applied pressure: 4.32 kN
Energizing time: 17 cycles Welding current value: (minimum current value at which scattering occurs) +0.5 kA

From Tables 1 and 2, it can be considered as follows (note that the following No. indicates the experiment No.).
That is, no. Since No. 6 and No. 11 satisfy | fill the prescription | regulation requirement as this invention steel plate 1, the outstanding chemical conversion property is exhibited and it can exhibit the outstanding coating-film adhesiveness. This No. 6 and 11, cracks should be suppressed to further improve the coating film adhesion, and in order to provide excellent weldability, the above formulas (2) and (3) should be satisfied. Is good.

  No. No. 9 satisfies the prescribed requirements as the steel sheet 1 of the present invention and also satisfies the above formulas (2) and (3), and therefore exhibits excellent chemical conversion property and excellent weldability. This No. In order to further improve the adhesion of the coating film 9, it is preferable to suppress cracks.

  No. Nos. 8 and 10 satisfy the requirements specified by the steel plate 2 of the present invention, that is, the requirements specified as the steel plate 1 of the present invention, as well as the requirements related to cracks. In order to exhibit excellent weldability, it is preferable to satisfy the above formulas (2) and (3) in the component composition.

  No. 1 to 5 and 7 satisfy the above formulas (2) and (3) together with the requirements defined as the steel sheet 2 of the present invention, so that excellent chemical conversion properties can be secured and the occurrence of cracks is suppressed. It can exhibit excellent coating film adhesion and can also exhibit excellent weldability.

  No. Since Nos. 12 to 18 do not satisfy even the requirements defined as the steel sheet 1 of the present invention, excellent coating film adhesion cannot be expected, or the tensile strength does not reach the specified level. That is, no. Nos. 12 and 18 were manufactured by the method recommended in the present invention. No. 13 has a relatively large amount of Si, and the Si / Mn ratio is also outside the specified range. No. 15 had an excessive amount of Si, and the Si / Mn ratio was also outside the specified range, so that both of them were inferior in chemical conversion treatment and many cracks were generated.

  No. No. 14 has an insufficient amount of Si. No 16 had a sufficient amount of Mn, so neither could satisfy the specified strength.

  No. No. 17 is excellent in coating film adhesion, but cannot be formed satisfactorily because the Mn content is too high and the ductility to be provided as a steel sheet is poor.

  For reference, a D-STEM observation photograph of the steel sheet obtained in this example and a composition analysis result of oxides and the like in the steel sheet surface layer region by EDX are shown. 4 and 5 (photos of observing region A in FIG. 4 at high magnification) are No. 1 as a comparative example. FIG. 4 and FIG. 5 show that the steel plate surface layer region is covered with a layer (black portion). FIG. 6 shows the result of composition analysis of the layer (region B) in FIG. 5 by EDX, and it can be seen from FIG. 6 that the layer is made of an oxide mainly composed of Si. That is, from FIGS. It can be confirmed that the steel plate surface layer region 15 is covered with an oxide mainly composed of Si.

  On the other hand, FIGS. 7 and 8 (photographs of region C of FIG. 1 is a D-STEM observation photograph in a cross section of the steel plate of No. 1, in the surface layer region of the steel plate. There is almost no Si-based oxide that lowers the chemical conversion processability as in FIG. 15, and particulate matter (black part) is dispersed instead. 9 and FIG. 10 are the results of the composition analysis of the analysis position 1 (granular material) and the analysis position 2 (granular material) of FIG. 8 by EDX, respectively, and FIG. 11 is the analysis position 3 (granular material of FIG. 8). This is the result of the composition analysis of the nonexistent portion) by EDX. From the comparison of FIG. 9, FIG. 10, and FIG. 11, the granular material observed in FIG. 8 is an Mn—Si composite effective in improving the chemical conversion treatment property. It turns out that it is an oxide.

It is the figure which showed typically the crack in a steel plate cross section. It is a figure which shows the manufacturing process (part) in an Example. It is a figure which shows another manufacturing process (part) in an Example. No. in the examples. It is a D-STEM observation photograph (magnification: 15000 times) of a steel plate section of No. 15 (comparative example). It is a D-STEM observation photograph (magnification: 100000 times) of the area | region A of the said FIG. 6 is an X-ray spectrum showing the result of composition analysis by EDX in region B of FIG. No. in the examples. 1 is a D-STEM observation photograph (magnification: 15000 times) of a cross section of a steel sheet of 1 (Example of the present invention). It is a D-STEM observation photograph (magnification: 100000 times) of the area | region C of the said FIG. 9 is an X-ray spectrum showing a composition analysis result by EDX at analysis position 1 in FIG. 8. 9 is an X-ray spectrum showing a composition analysis result by EDX at analysis position 2 in FIG. FIG. 9 is an X-ray spectrum showing a composition analysis result by EDX at analysis position 3 in FIG. 8. FIG.

Claims (3)

% By mass (the same applies to chemical components)
C: 1% or less (excluding 0%),
Si: 0.3-2%,
Mn: containing 1-5%,
A ferrite-martensitic DP (Dual Phase) steel plate that satisfies the following formula (1) and has a tensile strength of 550 MPa or more,
In the cross section in the direction perpendicular to the steel plate surface, when the depth of 2 μm from the steel plate surface and the steel plate surface length of 10 μm was observed with an electron microscope at a magnification of 5000 times or more,
There are 10 or more Mn-Si complex oxides having a major axis of 0.01 μm or more and 5 μm or less with an atomic ratio of Mn to Si (Mn / Si) of 0.5 or more, and a cross-sectional thickness of 0.01 μm or more. The ratio of the main oxide to the steel sheet surface length of 10 μm is 10% or less (including 0%) on the average of five steel sheet surfaces selected arbitrarily, and high strength with excellent coating film adhesion Cold rolled steel sheet.
[Si] / [Mn] ≦ 0.4 (1)
{In the formula, [Si] indicates the Si content (% by mass) and [Mn] indicates the Mn content (% by mass)} Si: satisfying 0.3 to 1.5%,
The high-strength cold-rolled steel sheet according to claim 1, wherein a crack having a width of 3 µm or less and a depth of 5 µm or more does not exist in an arbitrary 10 field of view when a cross section near the steel sheet surface is observed at 2000 times using SEM. The high-strength cold-rolled steel sheet according to claim 1 or 2, wherein the following formulas (2) and (3) are satisfied.
[P] +3 [S] +1.54 [C] <0.25 (2)
[C] + [Si] / 30 + [Mn] / 20 + 2 [P] +4 [S] <0.34 (3)
{Wherein [C], [Si], [Mn], [P], [S] represents the content (% by mass) of each element}