JPS638164B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a high-strength steel sheet with excellent workability, in which fine ferrite grains are formed in a hot rolling process, and then a composite structure is formed through appropriate annealing. Recently, due to the deterioration of the global oil situation, there has been a strong demand for fuel savings in automobiles, and in order to meet this demand, efforts are being made to reduce the weight of automobiles, and the use of high-tensile steel sheets is being actively considered. However, in general, as the strength of steel increases, its ductility decreases, so high-strength steel plates are difficult to form when high-level processing is required, such as in the case of steel plates for automobiles. Therefore, it is desired to develop a steel plate that has high strength and excellent ductility. To meet these demands, composite steel sheets having a soft ferrite phase and a hard second phase have been developed. This steel plate is characterized by having better ductility and lower yield strength than conventional steel plates when compared with the same strength, and can be said to be a high-strength steel plate with excellent formability. A composite steel sheet has a soft ferrite phase and a small amount of a hard second phase consisting mainly of a martensite phase and a retained austenite phase.A typical method for forming this structure is to combine ferrite and austenite. There are two methods: rapid cooling from the phase region at 10 ² to ^{10 3} °C/s, and cooling steel containing appropriate amounts of Mn, Si, Cr, etc. from the two-phase region at 1 to 10 ² °C/s. Methods include using a large amount of alloying elements, winding at a high temperature during hot rolling to save some alloying elements, hot rolling in a two-phase region,
A method has been proposed in which the material is annealed for a long time in a two-phase region before final annealing. The former method of quenching from the two-phase region has the advantage of being able to save a considerable amount of alloying elements, but it has the disadvantage of poor workability, and many improvements are needed for high-strength steel sheets for automobiles where workability is important. The latter has relatively good workability and has the advantage of being able to use conventional production equipment, but it is expensive because it uses a large amount of alloying elements, and the workability decreases in proportion to the amount of alloying elements. In the case of high-temperature coiling, there is a problem with the uniformity of the material, resulting in a decrease in yield and pickling properties.In the case of two-phase hot rolling, there is a decrease in productivity and the load during cold rolling. In the case of long-term annealing, there is an increase in cost due to the increase in the number of processes.
It has drawbacks such as: As a result of various studies in order to solve the above-mentioned problems in the production of composite structure steel sheets, the present inventors have determined that by setting the conditions of the hot rolling process to form fine ferrite grains, We have invented a method that makes it extremely easy to manufacture composite steel sheets. The gist of the present invention is that C: 0.15% or less, Mn: 1 to
2%, Al: 0.1% or less as a basic component, or further Si: 2% or less, Cr: 1% or less, Mo:
1% or less, Cu: 0.5% or less, V: 0.2% or less,
A steel containing one or more of Nb: 0.15% or less, Ti: 0.1% or less, B: 0.01% or less, and the remainder being substantially Fe, is rolled into ferrite in a hot rolling process. After the grain size has been reduced to 10μ or less, it is hot rolled or cold rolled after hot rolling, and the annealing process is performed at 1000℃ or more at A _c1 point.
Production of high-strength steel sheet with excellent workability, characterized by heating to a temperature below A _r1 point and cooling at an average cooling rate of 5°C/S or more and less than 200°C/S in a temperature range of 300°C or more below the A r1 point It's in the method. Below, the technical idea of the present invention and the reasons for limiting the constituent elements will be described. The essential element added to create a composite structure is Mn, and the higher the amount of Mn, the easier the composite structure is formed, but it is desirable that the fine ferrite grains are clean to ensure good ductility, and from this point of view From
The smaller the amount of Mn, the better. Therefore, in order to obtain a steel sheet with a composite structure and good ductility, the upper limit of the amount of Mn is set to 2%. On the other hand, when the Mn content is less than 1%, it becomes difficult to form a composite structure under the hot rolling conditions and annealing conditions according to the present invention. Like Mn, C is an austenite stabilizing element, and the higher its content, the more advantageous it is to forming a composite structure. However, if it is included in a large amount, it suppresses the formation of clean ferrite grains and reduces ductility. In addition to this, it also causes a decrease in weldability, so it should be kept at 0.15% or less. Since the composite structure is basically formed by Mn, the lower limit of the C content is not particularly limited. However, since the composite structure is suppressed by decreasing the C content, if the C content is extremely low, Mn, Cr It is desirable to see many things such as Al acts as a deoxidizing agent, and Mn, Si, Cr
It is useful for making alloying elements such as
If the content exceeds 0.1%, the cost increases and problems due to surface oxidation increase, which is undesirable, so the upper limit was set at 0.1%. Si is an effective element for improving ductility,
Ductility improves with increasing Si content, but
If the content exceeds 2%, the appropriate hot rolling temperature becomes high, making it difficult to obtain fine ferrite grains, as well as increasing problems due to surface oxidation and increasing costs. This is not preferable because it causes the occurrence of oxidation, and the upper limit thereof is set to 2% or less. The addition of Cr and Mo is an element that is effective in suppressing the transformation from austenite to pearlite, and is preferable for forming a composite structure, and is desirable for suppressing room temperature aging due to solid solution C and N. Even if the content exceeds 10%, the effect will be small, leading to a decrease in ductility and increasing cost, which is not preferable. Therefore, the upper limit is set at 1%. Cu is an element that suppresses the transformation from austenite to pearlite and is advantageous for forming a composite structure, as well as improving corrosion resistance. However, if it is contained in an amount exceeding 0.5%, it may cause hot embrittlement. I'm worried. Therefore, the upper limit of Cu is set at 0.5%. V is an element that forms fine carbonitrides, and is useful for refining ferrite grains, as well as suppressing room-temperature aging caused by solid solution C and N, and is an element that is effective in suppressing increases in yield point. If the content exceeds 0.2%, fine precipitates not only impede the ductility of the ferrite matrix but also lead to an increase in cost. Therefore, the upper limit of V is set to 0.2%. Nb and Ti are carbonitride-forming elements and have the effect of refining ferrite grains, but when added in large amounts, fine precipitates increase in the ferrite matrix and inhibit the ductility of ferrite grains. It becomes like this. Therefore, the upper limit of Nb is set to 0.15
%, the upper limit of Ti is 0.1%. In addition, a trace amount of B increases the hardenability of austenite and is advantageous for forming a composite structure, but if it is contained in an amount exceeding 0.01%, ductility becomes poor, so it should be suppressed to 0.01% or less. As mentioned above, each of these elements contributes to improving the strength of steel sheets, and the differences in the contribution of each element to the strengthening mechanism of steel sheets include solid solution strengthening, precipitation strengthening, quenching strengthening, etc. However, it has the same effect in terms of improving the strength of the steel plate,
By adding one kind or two or more kinds as necessary, a high-strength, high-tensile steel plate can be provided. In the hot rolling process, the ferrite crystal grain size of the steel sheet after hot rolling and winding must be 10μ or less. Figure 1 shows the relationship between the ferrite grain size of a hot rolled steel sheet and the strength/ductility balance (tensile strength x elongation), which is a measure of ductility. A high value of strength/ductility balance indicates that ductility is excellent at the same strength, and it can be said that the higher this value is, the more preferable it is. From this figure, when the grain size is less than 10μ, the balance between strength and ductility is good.
If it exceeds 10μ, it shows a tendency for deterioration. Although the reason why the strength/ductility balance is good when the grain size is less than 10μ is not yet clear, it can be speculated as follows. In other words, pearlite phase, bainite phase, martensite phase, retained austenite phase, etc. exist at the grain boundaries of fine ferrite grains depending on the cooling rate and coiling temperature after hot rolling, but the spacing between these phases is approximately equal to the ferrite grain size. This interval remains unchanged even if it is left as it is or is annealed under the conditions of the present invention after cold rolling. For this reason, solid solution C in ferrite grains during annealing
can diffuse into the remaining austenite phase during cooling, so the interior of the ferrite grains is cleaned, and the austenite phase is stabilized by the diffused carbon, and this part becomes marten, leaving some austenite behind. Transform the site. For this reason, a composite structure of clean ferrite grains with little solid solution C and a martensite phase (partially retained austenite) is formed, so it is assumed that a composite structure steel sheet with excellent ductility can be obtained. In addition, as a method to reduce the ferrite crystal grain size of hot rolled steel sheets to 10μ or less, the finishing temperature during hot rolling is
The winding method is done by setting the A _r3 point just above the A r3 point and cooling relatively slowly between the A _r3 and A _r1 points, and winding it relatively quickly by setting the finishing temperature just below the A _r3 point and reducing the rolling amount in the two-phase region by less than 50%. A method of cooling and winding, a method of adding elements that lower the A _r3 transformation point such as Mn and Cr and rolling at a low temperature to increase the nucleation of fine ferrite grains, a method of increasing the nucleation of fine ferrite grains, Nb,
Possible methods include adding Ti, Mo, etc. to suppress austenite recrystallization and increase the number of nuclei of fine ferrite grains. The hot-rolled steel sheet whose ferrite crystal grain size has been adjusted to 10 μm or less is subjected to an annealing process, either as it is or after cold rolling. In the annealing process, heating is performed above A _c1 point and below 1000℃.
It is necessary to maintain Figure 2 shows the relationship between annealing temperature, yield ratio (yield strength/tensile strength), and strength/ductility balance. The lower the yield ratio, the better the workability, and for composite structure steel, the value is generally 0.6 or less. From this figure, A _c1 point or more,
When heated and held at a temperature below 1000°C, the yield ratio is low and the strength/ductility balance is excellent. That is, annealing below the A _c1 point does not produce an austenite phase, and therefore no transformation from an austenite phase to a martensite phase occurs, so a composite structure cannot be formed. In addition, annealing at 1000℃ or higher causes austenite crystal grains to become coarser.
For this reason, the ferrite grains generated during cooling also become coarse, making it impossible to obtain sufficient ductility. On the other hand, when the annealing temperature is above A _c1 point and below A _c3 point, there are fine ferrite grains and austenite grains, so during cooling, clean ferrite grains and austenite →
Martensitic transformation (some residual austenite)
occurs, resulting in a highly ductile composite steel. In addition, when the annealing temperature is above A _c3 point and below 1000℃, it transforms into austenite grains, but since the temperature is not so high that the austenite grains become coarse, they are fine austenite grains, and therefore the subsequent cooling process It becomes a composite structure of fine ferrite grains and martensite phase (some retained austenite),
It shows good ductility. The cooling rate during annealing must be an average cooling rate of 5°C/S or more and less than 200°C/S in the temperature range of 300°C or more below the A _r1 point. FIG. 3 is a diagram showing the relationship between cooling rate, yield ratio, and strength/ductility balance. If it is less than 5° C./S, the transformation from austenite to pearlite or bainite progresses, and a composite structure is not formed, resulting in a high yield ratio, which is not preferable. If it is cooled to a temperature exceeding 200° C./S, a large amount of solid solution C remains in the ferrite, and a clean ferrite phase cannot be obtained, resulting in a poor balance of strength and ductility, which is not preferable. The reason why we set the upper limit of the temperature range of the cooling rate to be controlled below A _r1 point is because A _r1
This is because there is no need to particularly regulate the cooling rate in this temperature range above this temperature range, and the reason why the lower limit of the temperature range was set at 300°C was to suppress the transformation from austenite to pearlite, etc., and to prevent age hardening due to solid solution C. This is to prevent Although there is no particular limitation on the cooling rate above the A _r1 point, a slower rate is preferable for forming a composite structure, since cleaning of the ferrite grains and concentration of C, Mn, etc. in the austenite grains proceed. The steel sheet of the present invention may be plated with metals or alloys such as Zn, Sn, Cr, Al, Pb, etc., and may be tempered at 150°C or higher, such as baking paint, or lightly plated. It has the characteristic that its yield strength increases significantly if it is processed. Furthermore, rare earth elements (REM), Zr, Ca, etc. can be added for the purpose of improving stretch flangeability. Next, examples of the present invention will be described. Example 1 A steel having the composition shown in Table 1, which was formed into a slab using a converter and continuous casting method, was hot rolled to adjust the ferrite crystal grain size as shown in Table 1. A part of this was cold rolled to 0.8 mm under the conditions shown in Table 1 and annealed. Table 2 shows the mechanical property values of the steel sheets manufactured under the conditions shown in Table 1.

【table】

【table】

[Table] Samples 1 to 13 were produced by the method of the present invention, and are found to be steel sheets with good elongation properties, low yield ratios, and excellent workability. On the other hand, samples 14 to 19 were manufactured under conditions outside the scope of the present invention, that is, when the grain size of the hot rolled sheet exceeded 10μ as in sample 14, the annealing temperature was A as in sample 15. If the temperature is below _c1 , the annealing temperature exceeds 1000℃ as in sample 16, the cooling rate during annealing is slower than 5℃/s as in sample 17, or the cooling rate is 200℃/s as in sample 18. When cooled to a temperature exceeding S, when the Mn content is less than 1%, as in sample 19, the elongation properties are poor and the yield ratio is high, compared to samples 1 to 13 produced by the method of the present invention. It is a steel plate with poor processing properties. Example 2 C: 0.07%, Mn: 1.5 melted by converter
%, Al: 0.03%, Cr: 0.3% steel was made into a slab by a continuous casting method, and then hot rolled at a finishing temperature of 880℃ and a coiling temperature of 600℃ according to the usual method. Ferrite grain size in hot rolled steel sheet
15μ steel plate, finishing temperature 780℃, winding temperature 500℃,
A hot-rolled steel sheet with a ferrite crystal grain size of 7 μm was produced by slowly cooling the first stage before winding and quenching the second stage. This steel plate was cold rolled from 2.7 mm to 0.8 mm, and then annealed at 750°C for 30 seconds and cooled to 300°C at a rate of 20°C/S. The mechanical properties of the cooled steel sheet produced in this way are shown in Table 3. When the ferrite crystal grain size of the hot rolled steel sheet is within the range of the present invention, a composite structure is formed and a good balance of strength and ductility is achieved. Indicates the yield ratio. On the other hand, when a hot rolled steel sheet having a ferrite crystal grain size of 15μ, which is outside the range of the present invention, is used, a composite structure is not formed, the strength/ductility balance is poor, and the yield ratio is high.

[Table] As described above, the present invention makes it possible to inexpensively produce a composite structure steel sheet with excellent workability by reducing the crystal grain size of the steel sheet to 10μ or less in the hot rolling process. Its industrial value is extremely high.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the ferrite grain size of hot rolled steel sheets and the tensile strength x elongation of the product. Figure 2 is a diagram showing the relationship between the annealing temperature and the yield ratio and tensile strength x elongation of the product. Figure 3 is a diagram showing the relationship between cooling rate during annealing, yield ratio, and tensile strength x elongation.

Claims (1)

[Claims] 1 C: 0.15% or less, Mn: 1 to 2%, Al: 0.1%
A steel having the following basic components and the remainder substantially Fe is rolled in a hot rolling process to reduce the ferrite crystal grain size to 10 μm or less after being rolled up, and then either as hot rolled or cooled after hot rolling. After rolling, the annealing process heats the material to a temperature above ₁ point A and below 1000°C.
A Temperature range of 300°C or more below _r1 point at 5°C/s or more
A method for producing a high-strength steel sheet with excellent workability, characterized by cooling at an average cooling rate of less than 200°C/s. 2 C: 0.15% or less, Mn: 1-2%, Al: 0.1%
The following are the basic ingredients: Si: 2% or less, Cr: 1%
Below, Mo: 1% or less, Cu: 0.5% or less, V: 0.2
% or less, Nb: 0.15% or less, Ti: 0.1% or less, B:
Steel containing one or more of the following 0.01% or less, with the remainder being substantially Fe, is rolled in a hot rolling process to reduce the ferrite crystal grain size to 10 μm.
After the following, cold rolling is performed as it is hot rolled or after hot rolling, and heated to a temperature of A _c1 point or more and less than 1000°C in an annealing process, and A _r1 point or less 300°C
A method for manufacturing a high-strength steel sheet with excellent workability, characterized by cooling in the above temperature range at an average cooling rate of 5°C/s or more and less than 200°C/s.