JPS5826408B2 - Manufacturing method for low yield ratio, high tensile strength hot rolled steel sheet with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a low yield ratio, high tensile strength hot rolled steel sheet with excellent workability.

The high tensile strength hot rolled steel sheet having a two-phase mixed structure of ferrite and martensite is conventional NbpTi.

Compared to precipitation-strengthened high-strength hot-rolled steel sheets containing elements such as V, it has a lower yield ratio, good ductility, and excellent workability. Various methods of manufacturing steel sheets have been developed.

These methods can be roughly divided into two types as described below.

The first method is called the as-roll method.

This is a method of forming a two-phase structure in step 11 of hot rolling.

In this method, Si-Mn-Cr or Si-M
A two-phase structure is obtained by hot rolling steel to which n-Cr-Mo is added under an appropriate combination of finishing temperature, cooling on a runout table, and coiling temperature. Although it has the advantage that it becomes a product in 11, it is difficult to operate because it is necessary to strictly control the finishing temperature, cooling rate on the runout table, and winding temperature.

In addition, it has the disadvantage that it is slightly inferior to the heat treatment type method described below in terms of material quality.

The second method is a heat treatment type, in which heat treatment is performed after hot rolling to form a two-phase structure, and depending on the heat treatment method, it is further divided into a continuous heat treatment method and a batch annealing method.

In the continuous heat treatment method, Si-Mn-V-1'L
S i -Mn -Cr or S i -Mn -Cr
After hot-rolling Mo-added steel, A is applied on a continuous heat treatment line.
The process involves heating above the c1 transformation point and cooling at an appropriate cooling rate. Although there is no problem with the material level, material uniformity, or stability of the product, in order to perform special heat treatment, the heat treatment line This requires new construction or major modification of existing heat treatment lines (for example, continuous annealing furnaces, continuous galvanizing furnaces, etc.), which poses problems in terms of equipment.

The batch annealing method involves batch annealing steel to which two or more strips of Mn have been added to form a two-phase structure, and heat treatment is performed using an ordinary batch annealing furnace or an open annealing furnace, so there are no equipment limitations. There are no problems and manufacturing is relatively easy, but with conventional high-Mn steel, the annealing heating temperature and cooling rate have a significant effect on product strength, and the outer periphery of the coil where the cooling rate is high is compared to the coil where the cooling rate is low. The strength is significantly higher than that at the center, and the strength varies greatly in the length direction of the coil, resulting in poor material uniformity and stability.

In addition, the quality of the product material is also slightly inferior to that of continuous heat treatment methods.

The present invention eliminates the deficiencies of each of the above-mentioned methods, does not require a special heat treatment line, is easy to manufacture, has excellent workability, and has a low yield ratio and high tensile strength with good material uniformity and stability. The present invention provides a method for manufacturing a hot-rolled steel sheet with a low yield ratio and high tensile strength.

C: 0.01-0.12%, Si: 1.0% or less, M
n: 1.0-3.0%, Cr: 0.2-2.0%
In particular, 2.5 to 4.0% as (Mn+Cr), SoA,
After hot rolling a steel consisting of A6: 0.020 to 0.100%, P: 0.015% or less, S: o, oio% or less, and the balance Fe and unavoidable elements at a finishing temperature of Ar3 transformation point or higher. , coiled at 700℃ or below, pickled after cooling, and then placed in a batch annealing furnace to adjust the heating temperature from AC1 transformation point to Ac
3 transformation point, and the cooling rate at 400℃ after heating is lo
It is characterized by batch annealing under conditions such as ~2oooc/hr, thereby making it possible to produce a low yield ratio, high tensile strength hot rolled steel sheet with a tensile strength of 40 kg/m or more and excellent workability. The limitations of each component in the method of the present invention are due to the following reasons.C is an important element that controls the strength of the product, but if it is less than 0.01%, martensite will not be generated. Or, even if it is formed, its volume fraction is too small to achieve a low yield ratio, and if it exceeds 0.12%, the strength-ductility balance and weldability of the product will deteriorate, so 0.01 to 0.12%. Si has the effect of improving the strength-ductility balance of products, but if it is added in excess of 1.0%, red scale will occur in hot rolling, which will have a negative effect on pickling properties and the surface of the product. It was set to 1.0% or less.

Mn and Cr are the elements that play the most important role in the method of the present invention, and by adding both elements in combination, the conventional Mn
Compared to the batch annealing method in which n is added alone, it is possible to reduce the variation in product strength due to annealing conditions and to improve the balance between strength and ductility of the product.

Mn is 1. If Mn is less than 30% or Cr is less than 0.20%, the effect of the above-mentioned composite addition is not recognized, and if Mn is less than 30% or C
If r exceeds 2.0%, steel manufacturing problems will occur, so M
n was in the range of 1.0 to 3.0%, and Cr was in the range of 0.2 to 2.0%.

In addition, (M n +Cr) needs to be 2.5 or more in order to generate martensite by batch annealing and achieve a low yield ratio, and if it exceeds 4.0%, problems in steelmaking may occur. Sob and Al in the range of 2.5 to 4.0% need to be made into killed steel to ensure the workability of the product, so P in the range of 0.020 to 0.100% is used during batch annealing. S causes grain boundary segregation and impairs the toughness of the product, so it needs to be kept low, so S is set at 0.015% or less, but for car weight it is set at 0.010% or less to ensure the workability of the product. Above, the lower one is heavier, so 0.010φ
For Toshita hot rolled materials, the quality of the material deteriorates when finished at low temperatures, so the finishing temperature is set to be above the Ar3 transformation point, and the coiling temperature is 700°C.
If the temperature exceeds 700°C, the pickling property deteriorates, so the temperature was set to 700°C or lower.Next, regarding annealing, in order to make the structure into a two-phase structure of ferrite and martensite, the temperature at which the γ phase appears, that is, Ac1
It is necessary to heat above the point transformation, but the heating temperature is Ac
If the strength-ductility balance exceeds 3 transformation points, the strength-ductility balance deteriorates, so the heating temperature was set in the range of Ac1 to Ac3 transformation points.

In addition, in order to generate martensite and form a two-phase structure, the cooling rate needs to be 75 steps from the heating temperature to an average cooling rate of 10°C/hr or more at 400'C4.

Furthermore, if the cooling rate exceeds 200°C, the ductility deteriorates, so the upper limit is set at 200°C/hr. Examples of the present invention will now be described, in which a low yield ratio, high tensile strength hot rolled steel sheet with a tensile strength T S ) of 40 kg/m or more and excellent workability can be produced by appropriately combining the following.

Example 1 As shown in Table 1, steels A, B, C, D, and E having compositions within the range according to the method of the present invention and comparative steels F, G, H, and I having other compositions were heated to 2.5 Compare the mechanical test values of products obtained by hot rolling, winding at 560°C, pickling after cooling, and patch annealing at a heating temperature of 750°C and a cooling rate of 50°C/hr. did.

The results are shown in Table 2.

According to the heat treatment conditions within the scope of the present invention, steels A to E, which are steel materials with the composition of the present invention, all have a TS of 40 kg or more, 77 or more, a ferrite-martensitic dual phase structure, a low yield ratio, and good ductility. You can get a good product.

On the other hand, even under the same heat treatment conditions, Comparative Steel F has too low a C content and does not form two phases, resulting in a high yield ratio.
In the case of Comparative Steel G, the ductility was poor compared to the steel of the present invention due to the single addition of Mn, and in the case of Comparative Steel H, the amount of Mn was low; Steel■
In the case of , the yield ratio is high because the amount of (Mn-)-Cr) is small.

This example clarifies the effectiveness of the composition range according to the method of the present invention, and shows that the combination of composition and heat treatment conditions is an essential requirement for obtaining a good product.

Example 2 Steel within the composition range according to the method of the present invention shown in Example 1 and comparative steel G of other compositions (Mn addition, no Cr addition) were subjected to batch annealing at various heating temperatures or cooling rates. The mechanical test values when annealed were compared.

The results are shown in Table 3 and FIGS. 1 and 2.

Figure 1 is a graph showing changes in tensile strength (TS) depending on annealing temperature for Mn-Cr composite addition steel and Mn single addition steel G, and Figure 2 is a graph showing changes in TS depending on cooling rate. In a certain half of FIG. 2, the range inside the lines Ld and Lu is the range of the cooling rate according to the method of the present invention.

According to the above results, the steel with the composition within the range of the present invention has a significantly smaller change in strength due to changes in annealing conditions, that is, heating temperature and cooling rate, than Comparative Steel G (single addition of Mn). It can be seen that according to the method of the invention, a product steel sheet with good material uniformity and stability can be obtained.For example, even if the composition is within the scope of the present invention (see especially Figure 2), the cooling rate is slow. If the cooling rate is too fast, the structure becomes a heptagonal light-to-barrite structure and the yield ratio is not low, and if the cooling rate is too fast, the strength increases and the ductility deteriorates, which is not preferable.

Only when the composition and annealing conditions are combined within the scope of the present invention can a good product be obtained.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the change in TS depending on the annealing temperature for steel according to the method of the present invention and comparative steel, and FIG. 2 is a graph showing the change in TS depending on the cooling rate.

Claims (1)

[Claims] IC: 0.01-0.12%, Si: 1.0% or less, Mn: 1.0-3.0%, Cr: 0.2'-
2.0%, especially (Mn+Cr): 2.5-4.0
%, Sol, A7: 0.020-0.100φ, P:
A steel consisting of O, 0.015% or less, S: 0.010% or less, and the balance Fe and unavoidable elements is hot-rolled at a finishing temperature of Ar3 transformation point or higher, then coiled at a temperature of 700°C or lower, and after cooling is acid-treated. The hot-rolled steel sheet obtained by washing is annealed to obtain Ac. A low yield ratio, high tensile strength hot-rolled steel sheet with excellent workability, characterized in that the average cooling rate at 400°C is 10-b after heating and soaking within the temperature range from the transformation point to the Ac3 transformation point. Production method.