JPS6142767B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing high-strength cold-rolled steel sheets, and in particular, to high-strength cold-rolled steel sheets with a composite structure having a low yield ratio and high ductility by rolling control in the hot rolling process and continuous annealing after cold rolling. The present invention relates to a method of manufacturing a steel plate. Conventionally, precipitation strengthening, fine-grain strengthening, solid solution strengthening with substitutional elements, etc. have been applied to strengthen high-strength cold-rolled steel sheets, but these high-strength steel sheets are prone to cold-work cracking and are difficult to shape. In addition to poor freezing performance, there were problems in terms of cold workability, such as large tool wear and mold galling. To address this problem, composite structure type 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 elongation. This composite structure type low yield ratio steel sheet is obtained by hot rolling, cold rolling, and continuous annealing at a temperature in the two-phase region of ferrite (α) phase and austenite (γ) phase. That is, after cold rolling,
By rapidly heating to the (α+γ) region and holding in the same temperature range for a short time to cause crystal grain recovery, recrystallization, and partial austenitization, and then rapid cooling, the bainite phase and marten It has a composite structure in which low-temperature transformation product phases such as site phases coexist. Therefore, the formation of a composite structure becomes easier as the cooling rate after annealing is increased. However, when the cooling rate is increased, the ductility of the obtained steel sheet is significantly impaired, and in particular, compared to the conventional reinforced steel sheet, uneven elongation such as local elongation and reduction of area is lower, and overall elongation, strength-ductility balance, etc. Because of their inferiority, their application is often restricted to applications with strict requirements for these properties. on the other hand,
If we attempt to obtain a composite structure at a relatively low cooling rate in order to avoid such adverse effects, it will be necessary to add extremely large amounts of alloying elements, which not only increases the economic burden but also reduces the weldability of the steel plate. , many problems arise in terms of material properties such as surface properties and ductility. The present inventors, in order to solve the above-mentioned problems in terms of material properties and manufacturing cost of a composite structure type high strength steel sheet,
As a result of detailed examination of each condition in the manufacturing process, we found that the cumulative rolling reduction rate in the temperature range where recrystallization of the γ phase is significantly delayed (hereinafter referred to as the "late recrystallized austenite range") in the hot rolling process. By performing rolling control to adjust the value to a constant value according to the alloy composition of the steel, and performing continuous annealing in the (α+γ) region after cold rolling, ductility can be increased with a relatively small amount of alloying elements. The present inventors have discovered that the strength-ductility balance can be improved and a low yield ratio can be provided, and the present invention has been completed. That is, the present invention is a method for producing a composite structure type high-strength cold-rolled steel sheet through each step of hot rolling, cold rolling, and continuous annealing treatment following the cold rolling, in which C, Mn, and Si are The group consisting of Nb, V, Ti and Zr (hereinafter referred to as "A
1 or 2 selected from the group elements)
Elements more than species, or Cr, Ni, Mo, Cu, P,
Using steel with a composition containing one or more elements selected from the group consisting of Ca and rare earth elements (hereinafter referred to as "group B elements"), in the hot rolling process, (a) In the case of steel that does not contain elements, the cumulative reduction rate at temperatures below about 950°C is about
After hot rolling with a reduction of 50% or more, or (b) a reduction of about 30% or more in the case of steel containing group A elements, and cold working the obtained hot rolled steel sheet, (α
The present invention provides a new manufacturing method in which continuous annealing is performed at a temperature in the two-phase region of By creating a two-phase structure in which retained austenite phase etc. coexist, and by lowering the yield ratio, it increases uniform elongation, and (ii) promotes ultra-fine refinement and purification of the ferrite matrix.
And by adjusting the strength ratio of the ferrite base and the second phase, the non-uniform elongation can be increased.
It has a tensile strength of mm ² , a low yield ratio of about 0.7 or less, and excellent ductility as shown in the examples below. According to the present invention, in the hot rolling step in the austenite region, strong working is applied to the austenite before transformation by increasing the cumulative reduction rate in the unrecrystallized austenite region. This strong working of unrecrystallized austenite is essential for promoting forced precipitation of high-purity fine ferrite in the subsequent transformation process. In addition, such purification and refinement of the ferrite base in the hot rolled steel sheet have the effect of facilitating ultra-fine refinement of the ferrite base and the formation of a composite structure after cold rolling and continuous annealing. The above-mentioned strong working for this purpose is introduced by hot rolling including a reduction in the unrecrystallized austenite region with a cumulative reduction of about 30% or more. In other words, steel containing one or more of the Group A elements is hot rolled so that the cumulative reduction at a temperature of about 950°C or less is about 30% or more, preferably about 50% or more. For steels that do not contain the group A elements, by hot rolling so that the cumulative reduction at a temperature of about 950°C or less is about 50% or more, preferably about 60% or more. , respectively, can achieve the above effects. The reason why the required rolling reduction rate when a group A element is included is lower than when the element is not included is that the element suppresses austenite recrystallization and expands the temperature range of unrecrystallized austenite to a high temperature range. by. The heating temperature of the steel billet during the above-mentioned hot rolling may be appropriately adjusted according to the pass schedule so as to obtain a predetermined rolling reduction in the above-mentioned temperature range, but in the case of steel that does not contain group A elements, the heating temperature is approximately 1000°C or higher. For steels containing group A elements, the temperature may be approximately 1050°C or higher. Also,
The steel billet used for hot rolling is produced by the usual ingot making/blanking process or by continuous casting, and the billet is heated before rolling by using a normal billet heating furnace or Either direct rolling may be performed after blooming. The finishing temperature of hot rolling must be higher than the Ar ₃ transformation point temperature, and it is necessary to finish 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. In addition, 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. Winding of hot rolled steel sheet obtained by hot rolling is
It may be carried out at a normal temperature range (generally about 560-710°C). By keeping the winding temperature below 550℃,
The purpose is to suppress pearlite transformation and create a ferrite and bainite structure, 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. The hot-rolled steel sheet obtained by the above-mentioned hot rolling is then subjected to conventional cold rolling, including pretreatment such as pickling, and then continuous annealing in the (α+γ) two-phase region. will be attached. The above-mentioned low temperature rolling (controlled rolling) at 950℃ or less
The structure of a hot rolled steel sheet can be refined by processing it by 50% or more, or by processing it by 30% or more at a temperature below 950℃ for steels containing Nb, V, Ti, Zr, etc. Since the structure of the cold-rolled sheet product after cold rolling and annealing can be made significantly finer, it is also possible to obtain a cold-rolled sheet product with an excellent balance of strength and ductility. On the other hand, when the low-temperature rolling ends after intense 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 coexistence of α+γ two phases. When this hot rolled sheet is cold rolled and annealed,
Although a composite structure can be obtained, deep drawing workability 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 in the temperature range of 560°C to 710°C, including low-temperature winding, normal winding, and high-temperature winding, so the structure of hot rolled sheets is ferrite, pearlite, etc. (cementite).
When coiled at a lower temperature than conventional methods, such as 550℃ or less, the above structure changes to a composite structure of ferrite, bainite, and martensite (including some residual austenite), or a structure consisting mainly of low-temperature transformation phases. Change. By cold rolling and annealing a hot rolled sheet having such a structure, a cold rolled sheet having an extremely uniform and fine composite structure can be obtained.
Strength and ductility balance 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 C, Mn, and Si as basic components, and if desired, in the basic components,
(a) Group A elements, i.e., one or more elements selected from the group of Nb, V, Ti, and Zr, or (b) Group B elements, i.e., Cr, Ni, Mo,
Contains one or 22 or more elements selected from the group of Cu, P, Ca, and rare earth elements. Group A elements and group B elements may be added 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
If it exceeds 0.18%, weldability will deteriorate. Therefore, about
Set to 0.01-0.18%. Mn contributes to the formation of complex tissues. For this purpose, addition of about 0.8% or more is required. However, if added in a large amount, the economic burden will increase despite the effect, and it will also cause deterioration of weldability, so the upper limit is set at about 2.2%. Si has the function of promoting ferrite transformation during the hot rolling process and purifying the inside of the ferrite grains. In addition, together with the strong working of unrecrystallized austenite, this contributes to improving the strength-ductility balance. However, if added in a large amount, ductility will be inhibited, so the upper limit is set at about 1.0%. Each of the group A elements Nb, V, Ti, and Zr has the effect of suppressing austenite recovery and recrystallization during the hot rolling process and extending the temperature range of unrecrystallized austenite to the high temperature side. This facilitates strong working by hot rolling in the unrecrystallized austenite region. The effects of each of these elements increase as the amount added increases, but
Since saturation occurs when a certain amount is reached, this amount is set as the upper limit. That is, Nb is about 0.1% or less, V is about 0.15%
Below, Ti is added in a range of about 0.2% or less, and Zr is added in a range of about 0.3% or less. These elements may be added singly or in combination of two or more within the respective ranges. Among group B elements, Cr, Mo, and Ni all work as hardenability improving elements and reinforcing elements. The amount of each element to be added is set at the upper limit of approximately 1.0% for Cr, approximately 1.0% for Ni, and approximately 0.5% for Mo, taking into account its effect and economic efficiency, and may be added singly or in combination within each range. It's fine. Cu has the effect of improving weather resistance. However, if too much is added, hot cracking is likely to occur, so add approximately
The upper limit is 0.5%. P is useful as a reinforcing element. However, since adding a large amount causes embrittlement, it should be added within a range of about 0.2% or less. Both Ca and rare earth elements have the function of controlling the shape of nonmetallic inclusions (particularly sulfide inclusions) that are harmful to material properties and rendering them harmless. However, the effect is saturated when each element exceeds about 0.02%, so the upper limit for each element is about 0.02%.
One or more types may be added within each range. There are no special restrictions on the melting of steel with the above composition, and Si
It may be killed, Al killed, or Si-Al killed. Further, if necessary, degassing treatment such as RH method or DH method may be performed during the melting process. Next, the present invention will be specifically explained with reference to Examples. Example A test material having the component composition shown in Table 1 (all component compositions satisfying the above-mentioned provisions of the present invention) was hot rolled, pickled, and then cold worked at a rolling reduction of 70% to form a plate. thickness
A 0.8 mm cold rolled steel plate was obtained. Next, annealing was carried out at a temperature of 800°C for 2 minutes, followed by cooling at a cooling rate of about 10°C/second to obtain a product steel plate. Table 2 shows the hot rolling conditions, hot rolled steel sheet coiling temperature, and measurement results of various mechanical properties. In addition, sample materials No. 1 to 9 and 11
~17 is a hot rolling process in which controlled rolling (indicated by "CR" in the "Hot rolling conditions" column in the table) including rolling with a cumulative reduction rate of 60 to 78% at a temperature of 950 ° C or less was performed. For comparison, No. 10 was subjected to normal hot rolling (indicated by "HR" in the table) at a temperature of 950° C. or less and a cumulative reduction of 25%. In addition, winding after hot rolling is normal winding at a temperature of 570 to 710°C (in the table,
"Normal" is displayed in the "Coiling temperature" column), or the temperature
The winding was carried out at a low temperature of 550°C or less (indicated as "low temperature" in the table).

【table】

【table】

【table】

[Table] As shown in Table 2, sample materials Nos. 1 to 9 and 11 to 17 produced by the method of the present invention have a certain yield strength and tensile strength, and are higher than material No. 10 produced by the conventional method. In comparison, it can be seen that the yield ratio (YR) is low and the total elongation is excellent. 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 figure, "〇" and "●" indicate No. 1 to 9 materials subjected to controlled rolling according to the present invention (however,
"〇" is low-temperature winding material, "●" is normal winding material),
"△" and "▲" are No. 10 materials made by the conventional method (however, "△" is a low-temperature rolled material, and "▲" is a normal rolled material). The number attached to each mark is the sample material number. In addition, the straight line (i) has a yield ratio (YP/TS) of 0.4,
(ii) shows 0.5, (iii) shows 0.6, (iv) shows 0.7, and (v) shows 0.8. From the figure, the yield ratio of No. 10 material using the conventional method is
While the yield ratio exceeds 0.7, the yield ratios of materials No. 1 to 9 according to the present invention are as low as about 0.6 or less, and it can be seen that the yield ratio can be further lowered especially by low-temperature winding. FIG. 2 is a graph showing the relationship between tensile strength (TS) and total elongation (El) of each sample material. Each mark in the figure is the same as that in FIG. 1 above. curve
(a) shows the strength-ductility balance (TS El＝
1550) respectively. From the figure, it can be seen that the strength-ductility balance according to the present invention is significantly better than that of the conventional method, and can be further improved by low-temperature winding. As described above, according to the present invention, a cold-rolled steel sheet with a composite structure having high strength, excellent ductility and strength-ductility balance, and a low yield ratio can be obtained, and is suitable for applications where severe forming processing is performed. It can be used as a suitable 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 advantageous economically.

[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 Accumulation at a temperature of 950°C or less in the hot rolling process of steel consisting of 0.01 to 0.18% C, 0.8 to 2.2% Mn, 1.0% or less Si, the balance iron and unavoidable impurities. After performing hot rolling at a rolling finish temperature of ₃ points or more including rolling reduction of 50% or more and cold rolling the obtained hot-rolled sheet, it is continuously annealed in the two-phase region of ferrite phase and austenite phase. 1. A method for producing a low yield ratio, high ductility, high strength cold rolled steel sheet, characterized by subjecting it to a treatment. 2. The coiling temperature of hot rolled steel sheet during hot rolling is 550℃.
The manufacturing method according to item 1 above, which is characterized by adjusting the following: 3 C0.01~0.18%, Mn0.8~2.2%, Si1.0% or less, and Cr1.0% or less, Ni1.0% or less, Mo0.5%
Contains one or more elements selected from the group of Cu 0.5% or less, P 0.2% or less, Ca 0.02% or less, or rare earth elements 0.02% or less, with the balance consisting of iron and unavoidable impurities. In the hot rolling process of steel, hot rolling is carried out at a temperature of 950°C or less with a cumulative reduction rate of 50% or more, and the finishing temperature of Ar _{is 3} or more points, and the obtained hot rolled steel sheet is cold rolled. A method for producing a low yield ratio, high ductility, and high strength cold rolled steel sheet, the method comprising subsequently performing continuous annealing treatment in a two-phase region of a ferrite phase and an austenite phase. 4. The manufacturing method according to item 3 above, characterized in that the coiling temperature of the hot rolled steel sheet during hot rolling is adjusted to 55°C or less. 5 C0.01~0.18%, Mn0.8~2.2%, Si1.0% or less, and Nb0.1% or less, V0.15% or less, Ti0.2%
1 selected from the group below or below Zr0.3%
In the hot rolling process of steel containing one or more elements, the balance being iron and unavoidable impurities, the rolling process involves rolling with a cumulative reduction rate of 30% or more at a temperature of 95°C or less, and a finishing temperature of Ar of ₃ points or more. A low yield ratio, high ductility, and high strength cold rolled sheet characterized by hot rolling, cold rolling the obtained hot rolled sheet, and then subjecting it to continuous annealing treatment in a two-phase region of ferrite phase and austenite phase. Method of manufacturing steel plates. 6. The manufacturing method according to item 5 above, characterized in that the coiling temperature of the hot rolled steel sheet during hot rolling is adjusted to 55°C or less. 7 C0.01~0.18%, Mn0.8~2.2%, Si1.0% or less, and (i) Cr1.0% or less, Ni1.0% or less, Mo0.5
% or less, Cu0.5% or less, P0.2% or less, Ca0.02% or less, or one or more elements selected from the group of rare earth elements 0.02% or less, and (ii) Nb0.1
% or less, V0.15% or less, Ti0.2% or less, or
In the hot rolling process of steel containing one or more elements selected from the group Zr0.3% or less, and the balance consisting of iron and unavoidable impurities, the cumulative rolling reduction rate is 30% or more at a temperature of 950℃ or less. It is characterized by performing hot rolling including reduction at a finishing temperature of Ar _{at 3} points or more, cold rolling the obtained hot rolled sheet, and then subjecting it to continuous annealing treatment in the two-phase region of ferrite phase and austenite phase. A method for producing a low yield ratio, high ductility, and high strength cold rolled steel sheet. 8. The manufacturing method according to item 7 above, characterized in that the winding temperature of the hot rolled steel sheet during hot rolling is adjusted to 55°C or less.