JPS6142766B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a composite structure type high ductility high strength cold rolled steel sheet, and in particular, in a manufacturing method using a box sintering method, the strength-ductility balance of the obtained steel sheet is improved by controlling rolling in the hot rolling process. This is what I did. Conventional high-strength cold-rolled steel sheets are strengthened by
Precipitation strengthening, fine grain strengthening, solid solution strengthening with substitutional elements, etc. are generally applied, but these high strength steel plates are
In addition to being prone to cold processing cracks and poor shape fixability, tool wear is large and mold galling is likely to occur.
There is a drawback in terms of cold workability. In order to deal with this problem, composite structure type high ductility high strength cold rolled steel sheets are being put into practical use in recent years. This steel plate has improved shape fixability and mold galling by lowering the yield strength, and improved stretch formability by increasing uniform pressing. However, on the other hand, compared to the conventional steel sheets, local elongation, reduction in uneven elongation due to reduction, etc. are more likely to occur, and the effect of improving total elongation cannot be obtained, making it difficult to apply to applications requiring stretch flange formability. There are often restrictions. Further, in the manufacturing process of this composite structure steel sheet, annealing treatment is performed at a temperature in a two-phase region consisting of a ferrite phase (α) and an austenite phase (γ) in a continuous annealing furnace or a box-type annealing furnace. In order to obtain a composite structure by continuous annealing, rapid cooling must be performed from a temperature in the two-phase region of (α+γ).
The required cooling rate may be relatively slow if the steel has a large amount of alloying elements added, but if the amount added is to be reduced for cost reduction purposes, a high cooling rate must be provided. However, if the cooling rate is increased too much, the non-uniform elongation of the obtained steel sheet will decrease, and the strength-ductility balance will deteriorate significantly.
On the other hand, in order to obtain a composite structure by slow cooling from the two-phase region temperature of (α + γ) by box annealing, Mn, Cr, Mo
It is necessary to add a large amount of alloying elements such as. However, adding a large amount of such alloying elements not only increases manufacturing costs, but also causes problems in material properties such as weldability, surface texture, and ductility of the steel sheet. In order to solve the problem of the above-mentioned material properties of composite structure type steel sheets and reduce manufacturing costs, the present inventors conducted detailed research on various conditions in the manufacturing process, and as a result, in the hot rolling process. By performing rolling control that adjusts the cumulative reduction rate in the unrecrystallized austenite region (temperature region where austenite recrystallization is significantly delayed) according to the alloy composition of the steel, with a relatively small amount of alloying elements, We have completed the present invention by discovering that strength and ductility can be increased and the strength-ductility balance can be improved. That is, the present invention provides a method for producing a cold-rolled steel sheet with a composite structure through hot rolling, cold rolling and annealing treatment, in which C, Mn and Si are the basic components, and Nb, V, Ti and Zr are optionally added. (hereinafter referred to as "Group A elements") and other elements, and in the hot rolling process, (i) Group A elements are added to the steel. For steel not containing, the temperature approx.
Rolling with a cumulative reduction rate of approximately 50% or more at temperatures below 950℃,
(ii) In the case of steel containing group A elements, the same reduction rate is approximately 30%.
The present invention provides a new manufacturing method in which hot rolling including the above reduction is performed, the obtained hot rolled steel sheet is cold worked, and then box-type annealing is performed at a temperature in the two-phase region of (α + γ). By this treatment, (i) 2 coexisting bainite, martensite, retained austenite, etc. as a second phase is formed in the ferrite area.
(ii) Non-uniform elongation is achieved by ultra-fine ferrite grains, promotion of ferrite purification, and adjustment of the strength ratio of ferrite and the second phase. It has high ductility, high strength, and a good strength-ductility balance. The method of the present invention will be explained in detail below. According to the present invention, in the hot rolling process in the austenite region, strong working is applied to the austenite before transformation by increasing the cumulative reduction rate in the unrecrystallized austenite region. The purpose of this strong working of unrecrystallized austenite is to promote forced precipitation of high-purity fine ferrite in the subsequent transformation process. To obtain this effect, the cumulative rolling reduction in the unrecrystallized austenite region should be about 30% or more, preferably about 50% or more. In addition, Nb, which is the group A element,
In the case of steel that does not contain any of the elements V, Ti or Zr, the cumulative reduction rate at temperatures below about 950°C is about
On the other hand, in steels containing one or more of the group A elements, austenite recrystallization is suppressed by the elements, and the unrecrystallized austenite temperature is Since the range expands to the high temperature side, the above effect can be sufficiently obtained by performing hot rolling so that the cumulative reduction ratio at a temperature of about 950° C. or less is about 30% or more, preferably about 50% or more. The thermal temperature of the steel billet during hot rolling may be adjusted appropriately according to the pass schedule so that a predetermined reduction rate can be obtained in the above temperature range, but for steels that do not contain group A elements, it should be approximately 1000 ° C or more, For steels containing group A elements, the temperature may be approximately 1050°C or higher. The heating and rolling in this hot rolling may be carried out by any appropriate method, such as using a normal slab heating furnace or by performing direct rolling after blooming. The finishing temperature of hot rolling must be higher than the Ar ₃ transformation point temperature, and it is necessary to finish the rolling with a single austenite phase. As the degree of working of unrecrystallized austenite increases, the α transformation becomes more active, and the temperature and time at which the transformation starts shifts to the high temperature and short time side (α +
γ) Rolling tends to occur in the two-phase region. Rolling in the two-phase region corresponds to α-phase warm rolling, and the final steel plate has {001}
This results in a remarkable development of collective tissue. Such steel sheets have poor cold workability, particularly poor deep drawing workability. Furthermore, rolling in the (α+γ) region after rolling in the unrecrystallized austenite region promotes the formation of a band structure (ferrite phase and second phase) in the hot-rolled sheet, worsening the strength-ductility balance of the final steel sheet. The hot-rolled steel sheet after hot rolling may be coiled at a normal temperature range (generally about 560 to 710°C). By controlling the coiling temperature to 550℃ or less, pearlite transformation is suppressed, and ferrite and bainite structures are formed.
Alternatively, the purpose is to create a ferrite and martensitic structure or a mixed structure thereof, and these structures before cold rolling effectively refine the structure of the final steel sheet. In addition, final annealing can be done more efficiently in a shorter time. As a secondary phenomenon, the amount of retained austenite increases, resulting in a low yield ratio and excellent strength-ductility balance. Thereafter, the obtained hot rolled steel sheet is subjected to pretreatment such as pickling in a conventional manner, followed by ordinary cold rolling, and then subjected to box annealing in the (α+γ) two-phase region. When ordinary hot-rolled steel that does not contain large amounts of alloying elements is cold-rolled and box-annealed, the cooling rate during annealing is extremely slow (0.01°C/sec), so the structure of the resulting cold-rolled sheet product is becomes ferrite and pearlite (cementite), and does not form the above-mentioned composite structure. Therefore, the yield ratio is also 0.7 or more, making it impossible to obtain a low yield ratio steel plate. By performing the above-mentioned low-temperature rolling (controlled rolling), 50% or more processing at 950°C or less, or 30% or more processing at 950°C or less for steel containing Nb, V, Ti, Zr, etc. It becomes possible to form a composite structure even by heat treatment with a slow cooling rate such as annealing, and a cold-rolled sheet product with a low yield ratio can be obtained. On the other hand, when the above-mentioned low-temperature rolling ends after strong working in the (α + γ) two-phase region, the {200} texture of the hot-rolled sheet develops due to the working of the α phase in the (α + γ) two-phase coexistence state. . When this hot-rolled sheet is cold-rolled and annealed, a composite structure can be obtained, but deep drawability may be impaired because the {200} texture remains strongly. Therefore, for products where deep drawability is important, it is desirable to set the rolling end temperature to Ar ₃ or higher. Normally, winding after hot rolling is carried out at a temperature range of 560 to 710°C, including low-temperature winding, normal winding, and high-temperature winding. cementite). 550
When coiled at a temperature lower than the conventional temperature of ℃ or below, the above structure changes to a composite structure of ferrite and low-temperature transformation phases such as bainite and martensite (including some residual austenite) or a structure consisting mainly of low-temperature transformation phases. do. By cold rolling and annealing a hot rolled sheet having such a structure, it is possible to obtain a cold rolled sheet having an extremely uniform and fine composite structure, and the balance between strength and ductility can be further improved. Next, the composition of the steel used in the present invention will be explained. The steel to which the present invention is applied has a basic composition system of C, Mn, and Si, and if desired, (a) is added to the basic composition system.
One or more elements of group A elements Nb, V, Ti and Zr, or (b) Cr, Ni, Mo, Cu,
Contains one or more elements selected from the group of P, Ca, and Ce. The elements (a) and (b) may be added singly or in combination. C has the effect of increasing strength. The amount added is approx.
If it is less than 0.01%, the strength will be insufficient. On the other hand, about 0.2
%, weldability deteriorates. Therefore, about 0.01
~0.2%. Mn is effective in obtaining a composite tissue. For this purpose, about 1.0% or more is added. However, addition of a large amount causes problems in melting the steel and also impairs weldability, so the upper limit is set at about 2.5%. Si has the effect of promoting ferrite transformation during the hot rolling process and purifying ferrite. Also,
The synergistic effect with the strong deformation of unrecrystallized austenite provides a good strength-ductility balance. However, since these effects are almost saturated at an addition amount of about 1.0%, there is no need to add any more. Group A elements Nb, V, Ti, and Zr have the effect of suppressing austenite recrystallization during the hot rolling process and widening the temperature range of unrecrystallized austenite to the high temperature side. The effect of each of these elements increases with the amount added, but if too much is added, the effect will saturate or even decrease, so Nb is approximately 0.1
%, V is approximately 0.15%, Ti is approximately 0.2%, and Zr is approximately 0.3%. These elements may be added singly or in combination of two or more within the respective ranges. Cr, Mo, and Ni are all used as hardenability-improving elements and reinforcing elements, either singly or in combination as necessary. However, if added in large amounts, weldability will deteriorate, so Cr should be about 1.5% or less and Mo should be about 1.5% or less.
Ni is added within a range of 0.6% or less, and Ni is added within a range of about 1.5% or less. Cu is effective in improving weather resistance. However, approximately
If it exceeds 0.6%, hot cracking is likely to occur.
The upper limit is set at approximately 0.6%. P has the effect of strengthening the solid solution and improving ductility. However, if added in a large amount, it will cause embrittlement and have a negative effect on weldability, so it should be added within a range of about 0.2% or less. Both Ca and Ce have the function of controlling the shape of nonmetallic inclusions (especially sulfide inclusions) that are harmful to material properties and rendering them harmless. However, if large amounts are added, the ductility will be impaired due to the increase in nonmetallic compounds of these elements, so each element should be added in approx.
The upper limit is 0.02%, and it can be added singly or in combination. There are no particular restrictions on the melting method for steel having the above composition, and any method such as Si killed, Al killed, or Si-Al killed may be used. Further, if necessary, degassing treatment such as RH method or DH method may be performed during the melting process. Further, the steel billets may be manufactured by either a conventional ingot-blending process or a continuous casting method. Next, the present invention will be specifically explained with reference to Examples. Example Using a test material having a chemical composition shown in Table 1 (all compositions satisfying the above-mentioned provisions of the present invention),
After hot rolling and then cold working at a rolling reduction rate of 70% to form a cold rolled steel plate with a thickness of 0.8 mm, box annealing was performed at a temperature of 750°C, and then slowly cooled at a cooling rate of 26°C/Hr. A product steel plate was obtained. Table 2 shows hot rolling conditions, hot rolled steel sheet winding conditions, and measurement results of mechanical properties. In addition, in Table 2, "rolling conditions"
"CR" in the column indicates that controlled rolling was performed according to the present invention to adjust the cumulative reduction rate at a temperature of approximately 950°C or less;
"HR" is a comparison material that was subjected to normal hot rolling, and test material No. 1 according to the present invention was heated at a heating temperature of 1100.
℃, Nos. 2 to 7 are 1150℃, Nos. 9 to 15 are 1100℃, and the cumulative reduction rate at 950℃ or less is 60 to 60℃.
Adjusted to 78%. The cumulative reduction rate of comparative material No. 8 at temperatures below 950°C is 20% or less. Also,
``Normal'' in the ``Winding Conditions'' column means a temperature of 570 to 710℃.
Here, "low temperature" indicates that the hot-rolled sheet was wound at a temperature of 180 to 550°C.

【table】

【table】

【table】

[Table] As shown in Table 2, sample materials Nos. 1 to 7 and 9 to 15 obtained by the method of the present invention have constant yield strength and tensile strength, and comparative material No. 8 It can be seen that the yield ratio is lower and the total elongation is higher compared to the above. FIG. 1 is a graph showing the relationship between yield strength (YP) and tensile strength (TS) of each sample material based on the above results. In the diagram, the "●" mark indicates normal winding.
The “〇” mark indicates the material of the present invention (No. 1) that was rolled at low temperature.
~7), the "x" mark is the comparative material (No. 8), and the straight lines indicate the yield ratio (YR) of 0.5, 0.6, and 0.7. From the same figure, it can be seen that the yield ratio of comparative material No. 8 produced by the conventional method exceeds 0.7, whereas that of materials Nos. 1 to 7 and 9 to 15 produced by the method of the present invention is as low as approximately 0.6 or less. FIG. 2 is a graph showing the relationship between the tensile strength and total elongation of each sample material. The marks in the figure are the same as in Figure 1 above, curve A is the strength-ductility balance when low-temperature winding is carried out using the method of the present invention, B is the strength-ductility balance when normal winding is carried out, and curve C is inclusive of both. The strength-ductility balance (TS x El = 1800) and D represent the strength-ductility balance of comparative material No. 8 obtained by the conventional method. From the figure, it can be seen that the strength-ductility balance according to the present invention is significantly superior to that of the conventional method, and is particularly improved by low-temperature winding. As described above, according to the present invention, a cold-rolled steel sheet with a composite structure that has high strength, ductility, and a good strength-ductility balance can be obtained, and is suitable for applications where severe forming processing is performed. It can be used as a material. Furthermore, in order to satisfy such material characteristics, it is not necessary to newly add or increase the amount of special alloying elements, which is very economically advantageous.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between tensile strength and yield strength, and FIG. 2 is a graph showing the relationship between tensile strength and total elongation.

Claims (1)

[Claims] 1 Cumulative reduction at a temperature of 950°C or less in the hot rolling process of steel consisting of 0.01 to 0.2% C, 1.0 to 2.5% Mn, 1.0% or less Si, the balance iron and unavoidable impurities. Hot rolling is carried out at ₃ points or more at finishing temperature Ar, including rolling reduction such that the rolling ratio is 50% or more, and the resulting hot rolled sheet is cold rolled to a temperature in the two-phase region of ferrite phase and austenite phase. A method for producing a complex structure type high ductility high strength cold rolled steel sheet characterized by performing box annealing treatment. 2. The manufacturing method according to item 1 above, characterized in that the winding temperature of the hot rolled sheet during hot rolling is adjusted to 550°C or less. 3 C0.01~0.2%, Mn1.0~2.5%, Si1.0% or less, and Cr1.5% or less, Ni1.5% or less, Mo0.6%
Steel containing one or more elements selected from the following groups: Cu 0.6% or less, P 0.2% or less, Ca 0.02% or less, Ce 0.02% or less, with the balance consisting of iron and unavoidable impurities. In the hot rolling process, the temperature is 950℃.
Hot rolling is carried out at ₃ or more points of rolling finish temperature Ar, including rolling with a cumulative reduction ratio of 50% or more, and the resulting hot rolled steel sheet is cold rolled, and then the ferrite phase and austenite phase are removed. A method for producing a complex structure type high ductility high strength cold rolled steel sheet characterized by performing box annealing treatment at a temperature in a two-phase region. 4. The manufacturing method according to item 3 above, wherein the hot-rolled sheet winding temperature during hot rolling is adjusted to 550°C or lower. 5 C0.01~0.2%, Mn1.0~2.5%, Si1.0% or less, and Nb0.1% or less, V0.15% or less, Ti0.2%
In the hot rolling process of steel containing one or more elements selected from the group below or Zr0.3% or less, with the remainder iron and unavoidable impurities, the cumulative reduction rate at a temperature of 950°C or less is 30%. After performing hot rolling at a finishing temperature Ar of ₃ points or more including the above rolling reduction, and cold rolling the obtained hot rolled sheet, box annealing is performed at a temperature in the two-phase region of ferrite phase and austenite phase. A method for manufacturing a composite structure type high ductility high strength cold rolled steel sheet characterized by applying 6. The manufacturing method according to item 5 above, characterized in that the hot rolled sheet winding temperature during hot rolling is adjusted to 550°C or less. 7 C0.01~0.2%, Mn1.0~2.5%, Si1.0% or less, and (i) Nb0.1% or less, V0.15% or less, Ti0.2
% or less or Zr0.3% or less 1
species or two or more elements, and (ii) Cr 1.5% or less, Ni 1.5% or less, Mo 0.6% or less, Cu 0.6% or less,
Contains one or more elements selected from the group of P0.2% or less, Ca0.02% or less, Ce0.02% or less,
In the hot rolling process of steel consisting of residual iron and unavoidable impurities, the cumulative reduction rate is 30 at temperatures below 950°C.
% or more, rolling finishing temperature
A composite structure type characterized by performing hot rolling at ₃ or more Ar points, cold rolling the obtained hot rolled steel sheet, and then performing box annealing treatment at a temperature in the two-phase region of ferrite phase and austenite phase. A method for producing high-ductility, high-strength cold-rolled steel sheets. 8. The manufacturing method according to item 7 above, characterized in that the hot-rolled sheet winding temperature during hot rolling is adjusted to 550°C or less.