BR102013010172A2 - High strength cold rolled steel plate suitable for chemical conversion treatment and method of production - Google Patents

Abstract

High strength cold rolled steel plate suitable for chemical conversion treatment and method of production thereof. The present invention relates to the high strength cold-rolled steel plate with improved chemical conversion property, which can be suitably used in auto structure parts, has a composition including by weight% c: 0.05 % to 0.1%; si: 0.05% to 0.45%; mn: 2.5% to 3.5%; ai: 0.01% to 0.08%; p: 0.05% or less; s: 0.0050% or less; n: 0.01% or less; nb: 0.02% to 0.1%; ti: 0.001% to 0.05%; the balance being fe and the inevitable impurities. The ratio si / min is 0.02 to 0.15. The steel plate has a microstructure including 50% to 80% ferrite phase and 20% to 50% martensite phase in an area ratio to the entire microstructure. The ferrite phase and the martensite phase each have an average grain size of 0.5 <109> m to 3.0 <109> m, and the ratio of the average grain size of the ferrite phase / average grain size of the martensite phase) is 0.5 to 5.0

Description

Report of the Invention Patent for "HIGH-RESISTANCE COLD LAMINATED STEEL SHEET SUITABLE FOR CHEMICAL CONVERSION TREATMENT AND SAME PRODUCTION METHOD".

TECHNICAL FIELD The present invention relates to a high strength cold rolled steel sheet that can be suitably used in auto structural parts, and a method for producing it. The present invention aims in particular at improving the chemical conversion treatment property of such a steel sheet.

BACKGROUND ART

In recent years, car producers are striving positively to reduce CO2 emissions by improving fuel efficiency. In particular, since the reduction in car body weight is effective for improving fuel efficiency, positive measures are taken to improve the strength of steel sheets to be applied to automobile bodies and to reduce the thickness of the steel plates. steel. In addition to improving fuel efficiency, improving the strength of steel plates to be applied to the car body is often required to improve crash safety performance, such as crash space cabin protection. Therefore, high strength steel sheets having a tensile strength (TS) level of about 980 MPa are positively used in particular on a number of parts of automobile structures pressed into complex shapes.

In order to achieve high strength of the steel plate, it is a general practice to add to Fe various bonding elements such as C, Si, Mn, Ti, Nb, Cu, Ni, Cr, Mo, and V. In automobiles, there are many problems such as: (a) if parts can be formed without fractures or wrinkles, (b) if parts can be welded, (c) if a chemical conversion coating is formed thickly on the steel plate surface , and (d) if corrosion resistance after electroplating coating is sufficient, it is very important to ensure a sufficient chemical conversion treatment property, particularly in the case of high tensile strength steel sheets (TS). ) of about 980 MPa or more, including many bonding components to ensure strength compared to steel sheets with a tensile strength (TS) level of about 590 MPa. Several technologies associated with high strength cold rolled steel sheets are known. For example, high strength cold rolled steel sheets having an improved draw flanging capacity may be obtained by achieving a structure in which a predetermined amount of product material other than the ferrite phase is dispersed at regular intervals, as described in JP 2004-035905A (Patent Document 1), or by the use of a bainite thin phase as the main phase, as described in JP 2001 -226741A (Patent Document 2). In addition, JP 2000-273576A (Patent Document 3) describes a high strength cold rolled steel plate in which the generation of wrinkles at the roll forming edge is prevented by controlling the ratio of yield strength to strength. tensile strength. In addition, JP 2000-008136A (Patent Document 4) discloses a high strength cold rolled steel plate having stretch flanging capability and retarded fracture properties which are improved by controlling oxide inclusion.

As a technology for improving the chemical conversion treatment property, for example, JP 59-159987A (Patent Document 5) and JP 06-093472A (Patent Document 6) each describe a steel sheet suitable for treatment. chemical conversion and having a steel plate surface formed with a Ni oxide or Ni hydroxide coating, or a metallic Ni layer composed of precipitated NI particles. In addition, JP 2004-204350A (Patent Document 7) describes a steel plate suitable for chemical conversion treatment having a suppressed Si concentration surface.

LIST OF QUOTE PATENT LITERATURE

Patent Document 1: JP 2004-035905A

Patent Document 2: JP 2001-226741A

Patent Document 3: JP 2000-273576A

Patent Document 4: JP 2000-008136A

Patent Document 5: JP 59-159987A

Patent Document 6: JP 06-093472A

Patent Document 7: JP 2004-204350A

While describing improvements in drawing flanging capacity, cylinder forming capability and delayed fracture properties, Patent Documents 1 to 4 fail to describe the chemical conversion treatment property of steel plates that include the ferrite phase and the phase. Martensite. Thus, the technology described in Patent Documents 1 to 4 would be insufficient to consistently achieve a satisfactory chemical conversion treatment property.

Patent Documents 5 and 6 each describe a technology for improving the chemical conversion treatment property by adding Ni to a steel plate surface. However, this technology requires a special coating using Ni which is an expensive metal, which results in high cost and low productivity. There is yet another problem that expensive connection elements such as Cu, Ni, Cr, Mo, V, etc. must be contained to ensure the desired strength of the steel sheets.

Ultimately, the technology described in Patent Document 7 requires a higher than normal process cost due to two stripping, grinding, descending, and the like. In addition, since the suppression of Si concentration by such technology is insufficient, it is difficult to stably ensure a satisfactory chemical conversion treatment property.

DESCRIPTION OF THE INVENTION (Technical Problem) The present invention advantageously solves the above problems. It is an object of the present invention to provide a high strength cold rolled steel plate without expensive additional coupling elements such as Cu, Ni, Cr, Mo, and V having a tensile strength (TS) of 980 MPa or more, which gives favorable chemical conversion treatment property without requiring special surface treatment to be applied to the steel sheet. Another object of the present invention is to provide a method of producing such high strength cold rolled steel sheet. (Solution to the Problem) To solve the problems mentioned earlier, the inventors conducted intensive studies and discovered what follows. (1) Even in a ferrite martensite double phase steel made of constituents where carbon content (C) is reduced in terms of weldability and conformability without adding expensive bonding elements such as Cu, Ni, Cr By Mo and V, by properly controlling the area ratios of the ferrite phase and the martensite phase, it is possible to obtain a steel plate having a tensile strength (TS) of 980 MPa or more while maintaining the required working capacity and increased resistance. (2) In a steel having the above composition, if the Si / Μη ratio of the steel is controlled to low to suppress the Si concentration on the steel plate surface as Si02, it is possible to minimize the average grain size of the steel. ferrite phase and martensite phase to the same level and perform a fine and homogeneous dispersion of the martensite phase into the ferrite phase, thereby improving the chemical conversion treatment property without any special surface treatment for the steel sheet and controlling the conditions of heating and annealing conditions after hot rolling. The present invention is based on the aforementioned findings, and characterized by the characteristics described below.

That is, a first aspect of the present invention resides in a high strength cold rolled steel sheet suitable for chemical conversion treatment wherein the steel sheet has a composition of components including by weight%: C: 0.05 % to 0.1%;

Si: 0.05% to 0.45%;

Mn: 2.5% to 3.5%;

Al: 0.01% to 0.08%; P: 0.05% or less; S: 0.0050% or less; N: 0.01% or less;

Nb: 0.02% to 0.1%;

Ti: 0.001% to 0.05%, and the balance being Fe and the inevitable impurities, where the Si / Μη ratio is 0.02 to 0.15; and the steel sheet has a microstructure including 50% to 80% ferrite phase and 20% to 50% martensite phase in an area to total microstructure ratio, the balance being bainite phase and / or retained austenite phase , the ferrite phase and the martensite phase each having an average grain size of 0,5 μιη at 3,0 pm, where the ratio of the average grain size of the ferrite phase to the average grain size of the martensite phase (average ferrite phase grain size / average martensite phase grain size) is 0.5 to 5.0.

A second aspect of the present invention resides in a method for producing a high strength cold rolled steel plate suitable for chemical conversion treatment, the method comprising hot rolling a steel plate having the composition of components according to the first aspect. followed by stripping, heat treatment at 400 ° C to 700 ° C for 0,5 hour to 10 hours, cold rolling and annealing, wherein: annealing is performed such that during the heating step the temperature Annealing endpoint maximum is 760 ° C to 860 ° C, and the steel sheet is kept in a temperature range from a temperature below 50 ° C to the maximum endpoint temperature for a duration of 50 seconds at 100 ° C. seconds, and a subsequent cooling step is performed at an average cooling rate of 5 ° C / s to 50 ° C / s.

A third aspect of the present invention is the method for producing a high strength cold rolled steel plate suitable for chemical conversion treatment, according to the second aspect, also comprising over-aging at 150 ° C to 350 ° C for 400 seconds or less subsequent to or immediately after the annealing cooling step.

ADVANTABLE EFFECTS OF THE INVENTION The present invention makes it possible to stably provide a high strength cold-rolled steel plate having a tensile strength (TS) of 980 MPa or greater, which is highly suitable for chemical conversion treatment. The high strength cold rolled steel plate obtained by the present invention may have suitable use as automotive structural parts, as damping member materials, etc.

DESCRIPTION OF EMBODIMENTS The present invention will be described in detail below.

Initially, the reasons for restricting the composition of steel sheet components to the above mentioned ranges will be described. It should be noted that the unit of content of each element is% by mass unless otherwise specified. <C: 0.05% to 0.1%> Carbon (C) is an auste-nite stabilizing element, which has an influence on the area ratio of the martensite phase generated from the austenite phase and on the hardness of steel. When the C content in steel is less than 0.05%, an excessive ferrite phase is generated, which makes it difficult to guarantee the required strength. On the other hand, when the C content exceeds 0.1%, an excessive martensite phase is generated, which makes it difficult for the martensite phase to be evenly and finely distributed. Thus, the chemical conversion treatment property is deteriorated. In addition, the spot welding capability is significantly deteriorated. Consequently, the C content in steel should be in the range 0.05% to 0.1%. <Si: 0.05% to 0.45%> Silicon (Si) is an element that contributes to increase the strength of steel by reinforcing the solid solution of the ferrite phase. Silicon, however, has the effect of promoting ferrite phase generation during continuous cooling after cold rolling, annealing and rinsing. Therefore, when the Si content added to steel exceeds 0.45%, an excessive ferrite phase is generated, which makes it difficult to guarantee the required strength of the steel. In addition, the amount of silicon concentrated on the steel plate surface is increased, which results in a deteriorated chemical conversion treatment property. On the other hand, a Si content of less than 0.05% results in reduced generation of ferrite phase, so that an excessive martensite phase is generated. Thus, it becomes difficult for the martensite phase to be finely and evenly distributed, leading to a deteriorated chemical conversion treatment property. Consequently, the Si content in steel should be in the range 0.05% to 0.45%. <Mn: 2.5% to 3.5%> Manganese (Mn) is an austenite stabilizing element which contributes to increase steel strength by suppressing carbide precipitation during cooling after annealing and by generating a adequate amount of martensite phase from the austenite phase. The Mn content added to steel is necessarily 2.5% or more to achieve the above effect. On the other hand, when the Mn content added to the steel exceeds 3.5%, the quench hardening capacity is excessively increased, which results in an increased area ratio of the martensite phase. Thus, it becomes difficult for the martensite phase to be finely and evenly distributed.

Consequently, the content of Μη in steel should be in the range 2.5% to 3.5%. <Al: 0.01% to 0.08%> Aluminum (Al) is a useful element as a deoxidizing agent for steel, and the Al content added to steel is necessarily 0.01% or more. On the other hand, when the Al content exceeds 0.08%, inclusions such as alumina are increased in a surface portion of a steel sheet, which results in reduced bending ability of the steel sheet. In addition, an excessive Al content on the steel plate surface deteriorates the chemical conversion treatment property, corrosion resistance, and weldability of the steel plate. Consequently, the Al content in steel should be in the range of 0.01% to 0.08%. <P: 0.05% or less>

Although a large amount of phosphorus (P) affects spot welding capability, phosphorus in steel is acceptable up to 0.05%. Accordingly, the P content in steel must be 0.05% or less. Note that an excessively low P content reduces production efficiency in a steelmaking process, which results in high cost. The lower limit of P content is preferably about 0.01%. <S: 0.0050% or less> Sulfur (S) forms a sulfide inclusion such as MnS. MnS is deformed by extension due to cold rolling to be a starting point for fractures, so that steel working capacity is reduced. Therefore, the MnS content is preferably reduced as much as possible, although it is acceptable up to 0.0050%. Consequently, the S content in steel must be 0.0050% or less. Note that an excessive reduction of S is industrially difficult, which involves increased desulphurization cost in a steelmaking process and a number of productivity reductions. Accordingly, the lower limit of S content is preferably 0.0001%. <N: 0.01% or less> Nitrogen (N) is an element that affects the aging properties of steel, so the N content is preferably low. In particular, when the N content in steel exceeds 0.01%, a significant aging stress occurs. Accordingly, the N content must be 0.01% or less. Note that an excessive reduction in N content involves an increased cost of denitrification and a decrease in steel production productivity. Accordingly, the lower limit of N content is preferably 0.0001%, <Nb: 0.02% to 0.1%> Niobium (Nb) precipitates as carbide such as NbC to suppress crystal grain fouling during annealing and contribute to the refining of the crystal grains and homogenization of the microstructure of the ferrite phase and the sea-tensite phase. The Nb content added to steel is 0.02% or more to achieve the effect mentioned above. On the other hand, an Nb content exceeding 0.1% is likely to cause effect saturation, which is particularly disadvantageous in terms of alloy cost. In addition, the hardness of a hot rolled steel sheet is increased and the rolling load is increased, consequently productivity decreases. Consequently, the Nb content in steel should be in the range 0.02% to 0.1%. <Ti: 0.001% to 0.05%>

As with Nb, Titanium (Ti) precipitates as a carbide such a TiC stem, suppresses the crystal graft during annealing and contributes to the refining and homogenization of the crystal grains in the ferrite and martensite phases. In particular, titanium suppresses grain growth in the heat-slap of the hot-rolled plate, thus contributing to the refining / homogenization of the final microstructure. The Ti content added to steel is 0.001% or more to obtain the aforementioned e-made. On the other hand, an excessive Ti content exceeding 0.05% is likely to cause effect saturation. Consequently, the Ti content in the steel should be in the range of 0.001% to 0.05%.

Thus, a component composition according to the present invention has been described above. It should be noted that it is important in the present invention that not only does each component satisfy the aforementioned range, but also the Si / Mn ratio is adjusted within a suitable range. <Si / Mn ratio: 0.02 to 0.15> Silicon is contained in steel to ensure a predetermined amount of the smooth ferrite phase, which contributes to good ductility. However, in steel with Si being added, Si which is an oxidizable element is concentrated on the steel plate surface as Si02 during annealing. Silicon oxide (Si02) on the steel plate surface inhibits the co-loidal Ti absorption during surface conditioning in a pretreatment step for painting and caustic action on the steel plate in the formation of a chemical conversion coating . Similarly, Mn is also concentrated on the surface of the steel plate; however, it has less influence on the chemical conversion treatment property than Si. For a steel containing excess Si, it is difficult to suppress the generation of Si02 on the steel plate surface. However, with a Si content in the range above 0.05% to 0.45%, a Si / Mn ratio of 0.15 or less results in a dominant Mn concentration greater than the Si concentration on the steel plate surface. . Consequently, the effect of Si02 generated on the steel plate surface decreases. Thus, the property of the chemical conversion treatment can be improved. The Si / Mn ratio is preferably wanted. However, to achieve a Si / razãoη ratio of less than 0.02, an excessive addition of Mn or an excessive reduction of Si is required, leading to an increased cost. Accordingly, the Si / Μη ratio is determined to be in the range 0.02 to 0.15, preferably in the range 0.05 to 0.10.

In a steel plate of the present invention, components other than those mentioned above are iron (Fe) and the inevitable impurities. Note that components other than the above mentioned components may be contained as long as they do not adversely affect the effects of the present invention. In the following, the reasons for restricting the microstructure of a steel plate in the present invention to the aforementioned bands will be described. <area ratio of ferrite phase to the entire microstructure: 50% to 80%> The ferrite phase is soft and contributes to good ductility. When the area ratio of the ferrite phase for the entire microstructure is less than 50%, the area ratio of the hard martenite phase is relatively high, so that the strength of the steel is excessively increased. Thus, it is difficult to guarantee sufficient elongation of the steel. On the other hand, an area ratio of over 80% makes it difficult to guarantee the required strength. Consequently, the area ratio of the ferrite phase in steel to the entire steel microstructure is determined to be in the range of 50% to 80%. <area ratio of the martensite phase to the entire microstructure: 20% to 50%> The martensite phase contributes to the high strength of steel. When the area ratio of the martensite phase to the entire microstructure is less than 20%, the area ratio of the soft ferrite phase is relatively high, which makes it difficult to guarantee the required strength. On the other hand, when the area ratio exceeds 50%, steel strength is excessively increased, leading to reduced working capacity. Consequently, the area ratio of the martensite phase in steel to the entire steel microstructure is determined to be in the range of 20% to 50%. The remaining microstructure other than the ferrite phase and the martensite phase mentioned above includes the bainite phase and the retained austenite phase. The bainite phase and the retained austenite phase are preferably small to obtain a thin and homogeneous microstructure composed of ferrite and martensite phase.

In particular, in super aging after stopping cooling, the transformation of bainite generated from the austenite phase is accompanied by the progression of carbon thickening in austenite. Thus, the retained austenite phase is finally generated. The retained austenite phase has the effect of improving ductility by the introduced voltage transformation. In order to obtain a thin and homogeneous microstructure with small variations in Mn, and even C concentrations, the microstructure is preferably composed mainly of ferrite phase and martensite phase. The bainite phase and the retained austenite phase accounting for more than 5% of the area ratio for the entire microstructure indicate the presence of hard phase having a high carbon concentration. Thus, it is difficult to obtain a thin and homogeneous microstructure. Accordingly, it is preferable that the bainite phase and / or the retained austenite phase account for 5% or less, or alternatively 0% of the area ratio for the entire microstructure. <average ferrite phase grain size: 0.5 μιτι to 3.0 pm>

To obtain a thin and homogeneous microstructure which is useful for improving the chemical conversion treatment property, the average grain size of the ferrite phase is preferably as small as possible. However, excessive grain refining involves cost and technical difficulties. Accordingly, the average grain size is determined to be 0.5 pm or more. On the other hand, when ferrite grains are stiffened to have an average grain size of more than 3.0 pm, the martensite phase is located in the ferrite phase including the raw crystal grains. In addition, when the two phases of the austenite phase and the ferrite phase are separated during annealing and cooling, the volume of silicon distribution is greater in the ferrite phase than in the austenite phase. Therefore, when the ferrite phase, including raw crystal grains, resides in the final microstructure, the Si concentration varies, and the chemical conversion treatment property is deteriorated. Accordingly, the average grain size of the ferrite phase is determined to be in the range from 0.5 pm to 3.0 pm. <average martensite phase grain size: 0.5 pm to 3.0 pm>

As with the ferrite phase, the average grain size of the martensite phase is preferably as small as possible. However, excessive grain refining involves cost and technical difficulties. Consequently, the average grain size should be 0.5 pm or more. On the other hand, when the martensite grains are stiffened to have an average grain size of more than 3.0 pm, the martensite phase including the raw crystal grains is localized. When the martensite phase including raw crystal grains resides in the final microstructure, the Si concentration also varies, and the chemical conversion treatment property is deteriorated. Consequently, the average grain size of the martensite phase is determined to be in the range from 0.5 pm to 3.0 pm. <Average ferrite phase to martensite phase grain size ratio (average ferrite phase grain size / average martensite phase grain size): 0.5 to 5.0>

Thin and homogeneous microstructure with small variations in Si, Mn, and even C concentrations is effective for improving the chemical conversion treatment property. As discussed above, although the ferrite and martensite phase crystal grains are thin, the microstructure is not always homogeneous when the average grain size of the ferrite phase is vastly different from that of the martensite phase. When the ratio of the average grain size of the ferrite phase to that of the martensite phase is less than 0.5, the crystal grains of the ferrite phase are fine while the crystal grains of the martensite phase are crude. When the ratio is greater than 5.0, the ferrite phase crystal grains are crude while the martensite phase crystal grains are thin. In any case, the presence of phases having different concentration distributions of Si, Mn and even C results in microstructure that is disadvantageous to the chemical conversion treatment property. Accordingly, the ratio of the average grain size of the ferrite phase to that of the martensite phase is determined to be in the range 0.5 to 5.0, preferably in the range 0.8 to 2.0. In the following, a method for producing high strength cold rolled steel plate of the present invention will be described.

Initially, a plate is produced to have a component composition as described above. The plate may be produced by casting thin plates or by conventional casting, however it is preferably produced by continuous casting to reduce segregation.

Then, the produced plate is heated. The heating temperature of the plate is preferably 1100 ° C or higher. In particular, the upper limit of the plate heating temperature is preferably 1300 ° C in terms of reduction in scale generation and reduction in specific energy consumption. The heated plate as described above is subjected to hot rolling including roughing rolling and finishing finishing. The roughing lamination conditions are not necessarily specified, and the roughing lamination can be performed according to conventional techniques. The delivery temperature of the finisher in the finishing lamination is preferably 850 ° C or higher to prevent formation of lamellar structure consisting of ferrite, perlite and others. In particular, the upper limit of the finisher delivery temperature is preferably 950 ° C in terms of reducing scale generation, and obtaining a finer and more homogeneous microstructure by suppressing crystal grain dullness. The winding temperature after hot rolling should preferably be 450 ° C to 650 ° C in terms of cold rolling capacity and surface quality. After maintaining the winding temperature as required, stripping is performed thus removing the oxide from its surface. Pickling can be performed according to conventional techniques. <Heating conditions: 400 ° C to 700 ° C, 0.5 hour to 10 hours>

Subsequently, the obtained hot-rolled plate is heat treated. Heat treatment after hot rolling is an important process for achieving excellent chemical conversion treatment property of a cold rolled steel sheet obtained by subsequent cold rolling and annealing. The heat treatment includes the following steps: (a) eliminating an inhomogeneous strip structure derived from the segregation of P and Mn generated depending on the hot rolling end temperature, cooling rate, cooling temperature, etc., and (b ) also eliminate the location of the element by making the hot-rolled plate having ferrite, bainite, sea-tensite, and perlite phases with different distribution volumes of C, Si, and Mn, in which the elements are regularly distributed, having composite microstructure. ferrite and cementite, thus achieving a uniform distribution of C, Si, and Mn.

Here, when the heating temperature of the above heat treatment is less than 400 ° C, or when the retention time is less than 0.5 hour, the microstructure of the hot rolled plate does not change much. Consequently, the phase constitution is maintained and variations in element concentration cannot be eliminated so that elements such as Si remain unevenly distributed. Thus, recovery and recrystallization proceed unevenly in the heat treatment process after cold rolling. For this reason, the final microstructure obtained has inhomogeneous mixed grain structure including raw grains and fine grains. Accordingly, the chemical conversion treatment property cannot be improved.

On the other hand, a heating temperature exceeding 700 ° C results in perlite and martensite phases which are derived from the austenite phase, and ferrite phase in the microstructure obtained after heat treatment. Therefore, elements such as Si are irregularly distributed and homogenization is not performed. For this reason, the final microstructure has an inhomogeneous mixed grain structure including raw grains and fine grains. Accordingly, the chemical conversion treatment property cannot be improved. A retention time exceeding 10 hours is acceptable but results in reduced productivity. Consequently, the heating temperature in the heat treatment after hot rolling is determined to be in the range of 400 ° C to 700 ° C, while the retention time should be in the range of 0.5 hour to 10 hours. The hot rolled sheet obtained by heat treatment as described above is cold rolled. Cold rolling conditions are not necessarily specified, and cold rolling may be performed according to conventional techniques. The thickness of a steel plate of the present invention is preferably about 0.8 mm to 1.6 mm. The cold rolled sheet thus obtained is then annealed under the following conditions. <Maximum endpoint temperature: 760 ° C to 860 ° C>

When the maximum end point temperature in the annealing is less than 760 ° C, the area of the rinse and annealing ferrite phase is excessively high, which makes it difficult to guarantee the required strength. Under this condition, the elements C, Si, Mn, and P added to the steel do not diffuse sufficiently. In addition, a microstructure having irregular concentrations of C, Si, Mn, and P is formed after annealing under the influence of the perlite, bainite, and martensite phases generated after hot rolling. Therefore, a number of martensite phases of different hardness and size are dispersed, which results in a deteriorated chemical conversion treatment property.

On the other hand, a maximum endpoint temperature exceeding 860 ° C increases the austenitic phase area ratio in rinsing and annealing, resulting in a reduced ferrite phase area ratio after cooling and over-aging, and an increased area ratio of the martensite phase. The steel thus obtained has excessive strength which makes it difficult to guarantee sufficient elongation. When the steel plate is heated to a high temperature range of the single-phase ausiteite exceeding 860 ° C, the concentration of C, Si, Mn, and P is uniform; however, austenite grains are excessively brutalized. This leads to an increased martensite phase by including raw crystal grains in the final annealing material, and the chemical conversion treatment property of the final annealing material is consequently deteriorated. Thus, to diffuse sufficiently the component elements contained in the steel to obtain a thin and homogeneous microstructure, the maximum annealing endpoint temperature should be in the range of 760 ° C to 860 ° C, more preferably in the range of 780 ° C to 840 ° C. ° C. <Steel sheet residence time (time during which the sheet is kept) within a temperature range from a temperature of 50 ° C below the maximum endpoint temperature to the maximum endpoint temperature in the heating step; 50 seconds to 100 seconds>

When the residence time of the steel plate at temperatures ranging from a temperature of 50 ° C below the maximum endpoint temperature to the maximum endpoint temperature in a heating step is greater than 100 seconds, the crystal grains are brutalized. This makes it difficult to obtain fine crystal grains. In cases where residence time is less than 50 seconds, insufficient recrystallization after cold rolling results in a mixed grain structure including elongated ferrite grains and fine ferrite grains generated by recrystallization, so that the treatment property chemical conversion is deteriorated. Consequently, the residence time of the steel sheet for a temperature range, from a temperature lower than 50 ° C to the maximum endpoint temperature to the maximum endpoint temperature in a heating step, is determined to be within a range. from 50 seconds to 100 seconds. The above controlled heating temperature range is limited to the temperature range, from a temperature below 50 ° C to the maximum endpoint temperature to the maximum temperature of the endpoint, since the temperature range from a temperature below 50 ° C ° C at maximum endpoint temperature to maximum endpoint temperature has a major influence on incomplete recrystallization or mixed grain structure generation. <Average cooling rate: 5 ° C / s to 50 ° C / s> The average cooling rate in the cooling step is the average cooling rate from the start of cooling from the maximum endpoint temperature to 350 ° C or less. When the average cooling rate is less than 5 ° C / s, excessive ferrite phase is generated on cooling, so it is difficult to guarantee the required resistance. In cases where the average cooling rate exceeds 50 ° C / s, an excessively high hardening capacity in the tempera results in excessive martensite phase generation and suppressed ferrite phase generation, thus making it difficult to obtain a thin microstructure. homogeneous. Accordingly, the average cooling rate in the cooling step should be in the range of 5 ° C / s to 50 ° C / s, preferably in the range of 10 ° C / s to 40 ° C / s. Cooling is preferably performed by gas cooling. Alternatively, mist cooling, cylinder cooling, water cooling, or any combination thereof may be employed.

In the present invention, subsequent to or immediately after cooling mentioned above, overaging may be further performed. <Over-aging conditions: 150 ° C to 350 ° C, 400 seconds or less When the over-aging temperature is greater than 350 ° C, a small martensite phase is generated, whereby the bainite or austenite phase is excessively generated, thereby making it difficult to achieve the required strength. The over-aging temperature may be below 150 ° C, but in this case, excessive cooling equipment capacity is required, thereby increasing cost and reducing productivity. Accordingly, it is preferable that the super aging temperature be in the range of 150 ° C to 350 ° C. When the over aging time exceeds 400 seconds, the bainite phase or the retained austenite phase are generated excessively so that the martensite phase is reduced. Accordingly, it is preferable that the super aging time is 400 seconds or less. The steel plate thus obtained can also be hardened as needed. (Examples) Steel samples having the respective component compositions shown in Table 1 were each fused to obtain a plate. The plates were heated to 1200 ° C, hot rolled to a finish delivery temperature of 900 ° C, cooled immediately after rolling at a rate of 50 ° C / s, coiled to 550 ° C, and then stripped. with hydrochloric acid. Thereafter, the hot-rolled sheets thus obtained were heat treated under the conditions shown in Table 2. After cold rolling, the sheets were annealed under the conditions shown in Table 2, followed by over-aging if necessary. Thus samples of cold rolled steel sheets were produced. The steel microstructure of the cold-rolled steel sheets thus obtained was analyzed and the results are also shown in Table 2. In addition, the mechanical property and chemical conversion treatment property of each cold-rolled steel sheet were analyzed and The results are shown in Table 3.

Here, the steel microstructure, mechanical property and chemical conversion treatment property of each cold-rolled steel sheet were measured as follows. (1) Steel microstructure The steel microstructures were identified and the area ratios of the respective phases for the entire microstructure were measured as described below.

Initially, a section of each steel plate cut at a position of plate thickness x 1/4 along the rolling direction of the steel plate was observed with an optical microscope. Observation was performed with N = 5 (ie with five observation fields). The area occupied by each phase within each square area of 100 pm χ 100 pm arbitrarily chosen by image analysis was determined using each 1000 x microstructure of the microstructure. Specifically, the samples were etched with a mixed solution containing 3 wt% picral and 3 wt% sodium pyrosulfite. Black colored regions were determined as ferrite phase and the other region was determined as corresponding to martensite phase, bainite phase, and retained austenite phase using microstructure microphotography to find out the area ratio of the ferrite phase. Next, a nital causticity was performed. A region where the carbide is observed is determined as a bainite phase, and a smooth region is determined as corresponding to the martensite phase and the austenite phase retained together using a microphotograph of a SEM 5000 x cross-section microstructure to find the area ratio of the bainite phase.

In addition, to distinguish the martensite phase from the retained austenite phase, the volume fraction of the retained austenite phase was determined by x-ray diffraction using Mo K-alpha x-ray. Specifically, the volume fraction of the retained austenite phase was calculated based on the peak intensities of face 211 and face 220 of the austenite phase and of face 200 and face 220 of the ferrite phase using a steel sheet specimen. and analyzing, as a measuring surface, its surface in the vicinity of the position at 1/4 depth in the direction of plate thickness. The volume fraction of the retained austenite phase was defined as the area ratio of the retained austenite phase. Meanwhile, the area ratio of the martensite phase was discovered by subtracting the aforementioned area ratio from the retained austenite phase from the total area ratio of the martensite phase of the retained austenite phase.

The average grain sizes of the ferrite phase and the martensite phase were determined according to the nominal grain size measurement using quadrature. Observation was performed with N = 5 (ie five observation fields). After being caused with nital, the area (V) occupied by each phase within each 20 μιτι χ 20 pm square area, arbitrarily chosen by image analysis, was determined and the number (n) of each phase within the areas was counted. using a SEM microphotograph of the 5000 χ SEM microstructure cross section. Thus the average grain area (a = V / n) was calculated to finally determine the grain size (d = Va). (2) Tensile Properties A tensile test was performed in accordance with JIS Z 2241 to assess the tensile properties of a No. 5 test sample prepared in accordance with JIS Z 2201 with its longitudinal direction (tensile) being orthogonal to the direction of lamination. Tensile properties were evaluated using TS x El, and a TS x El value of 16000 MPa.% Or higher was rated as satisfactory. (3) Hole Expansion Properties The hole expansion properties were evaluated based on the Japan Iron and Steel Federation Standard JFS T 1001. A hole having an initial diameter of d0 = 10 mm was drilled in each sample. A 60 ° tapered bore was constructed to expand the hole until the fracture penetrates the thickness of the steel plate. The diameter of hole d after fracture penetration was measured to calculate the hole expansion ratio λ (%) = {(d-do) / d0} χ 100. Steel sheets referenced with the same steel sample number were tested three times to calculate the average value of hole expansion ratios, and the evaluation was performed using the average value. The hole expansion properties were evaluated by TS χ λ, and a value obtained by TS χ λ of 29000 MPa-% or more was evaluated as satisfactory. (4) Chemical Conversion Treatment Property A surface conditioner (5N-10) and a chemical conversion treatment agent (SD2800), both produced by Nippon Paint Co., Ltd., were used to treat 75% specimens. mm χ 150 mm by chemical conversion with zinc phosphate. After this, an electroplating coating was performed to a thickness of 25 pm (ink: V-50 black). Each sample was cut with a stylus to have two slits having a length of 100 mm, and was immersed in a 5 wt% NaCl liquid solution at 50 ° C for 240 hours. Then an adhesive tape was placed in the slits and peeled, thus measuring the width of the peel of the chemical conversion coating.

In assessing the chemical conversion treatment property as indicated in the following tables, the symbol "+" indicates that the following conditions have been sufficiently satisfied, and the symbol indicates that any of the following conditions have not been sufficiently satisfied. (a) Crystal grain size of the chemical conversion coating: 2 μηι to 10 μηη (b) Coating weight: 1.8 g / m2 to 2.6 g / m2 (c) Maximum stripping width: 2.5 mm or less (d) The surface of the steel plate is completely covered with a chemical conversion coating without part of it being covered with the coating, ie coated with an excessively thin coating. Microstructure observation was performed by an SEM 1000 x to measure the crystal grain sizes of each chemical conversion coating by an intercession method. The weight of the coating was determined by dissolving the chemical conversion coating after the chemical conversion treatment and comparing the weights before and after dissolution. In addition, whether the chemical conversion coating has any excessively thin aperture or part can be determined by observing the microstructure by a 1000 x SEM. co Ό co = 3 CT

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CO * Table 3 shows that all cold rolled steel sheets of the present invention are not only suitable for chemical conversion treatment, but also have a tensile strength of 980 MPa or more, which even meets TS χ El of 16000 MPa-% or more and TS χ λ of 29000 MPa% or more. Thus, the balance between resistance and working capacity is satisfactory.

On the other hand, comparative steel samples are shown to be inferior in chemical conversion treatment property. Comparative steel samples denoted by paragraphs 6, 10, 14, 16 and 17 have low martensite phase area ratio, which results in low level of strength. Meanwhile, comparative steel samples denoted by paragraphs 11 and 15 have a high martensite phase area ratio, however, the TS χ λ of 29000 MPa-% or more was not satisfied in any example.

INDUSTRIAL APPLICABILITY

In accordance with the present invention, a high strength cold rolled steel plate having a tensile strength (TS) of 980 MPa or more suitable for chemical conversion treatment can be obtained at low cost without expensive connecting elements such as such as Cu, Ni, Cr, Mo, and V being contained in the steel, and without performing any special treatment on the steel plate surface. This is made possible by controlling the Si / Μη ratio and controlling the area ratio and the average ferrite phase and martensite phase grain size. The high-strength cold-rolled steel plate of the present invention is particularly useful for car body parts, however, it can also be suitably used for other applications such as building materials or household appliances.

Claims (3)

1. High-strength cold-rolled steel plate suitable for chemical conversion treatment, characterized in that: the steel plate has a composition of components including, by weight%: C: from 0.05% to 0, 1%; Si: from 0.05% to 0.45%; Mn: from 2.5% to 3.5%; Al: from 0.01% to 0.08%; P: 0.05% or less; S: 0.0050% or less; N: 0.01% or less; Nb: from 0.02% to 0.1%; Ti: from 0.001% to 0.05%, and the balance being Fe and the inevitable impurities, where the Si / Μη ratio is from 0.02 to 0.15; and the steel plate has a microstructure including 50% to 80% ferrite phase and 20% to 50% martensite phase in an area to total microstructure ratio, the balance being bainite phase and / or aus phase. retained -tenite, the ferrite phase and the martensite phase each having a grain size of 0.5 pm to 3.0 pm, where a ratio of the average grain size of the ferrite phase to the average grain size of the martensite phase (average ferrite phase grain size / average martensite phase grain size) is 0.5 to 5.0. Method for producing a high-strength cold-rolled steel plate suitable for chemical conversion treatment, characterized in that it comprises hot-rolling a steel plate having a component composition as defined in claim 1, followed successively by pickling, heat treatment from 400 ° C to 700 ° C for from 0.5 hour to 10 hours, cold rolling, and annealing where: annealing is performed so that during the heating step the maximum temperature of the Annealing end point is 760 ° C to 860 ° C, and the steel plate is kept in a temperature range from a temperature of 50 ° C below the maximum endpoint temperature to the maximum endpoint temperature, for example. a duration of 50 seconds to 100 seconds, and a subsequent cooling step is performed at an average cooling rate of 5 ° C / s to 50 ° C / s. Method for producing a high-strength cold-rolled steel plate suitable for a-accord chemical conversion treatment according to claim 2, characterized in that it also comprises over-aging from 150 ° C to 350 ° C for 400 seconds. or less subsequent to or immediately after the cooling step in the re-cooking.