JPH0555572B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for producing high-strength hot-rolled steel sheets with excellent workability and bake hardenability suitable for strong automobile parts and the like at extremely low cost. [Technical background of the invention and its problems] As a method for producing high-tensile hot-rolled steel sheets, Mn, Si
A solid solution strengthening method by adding Nb, Ti, V, etc., and a precipitation strengthening method in which carbides or nitrides are precipitated by adding Nb, Ti, V, etc. have been put into practical use for a long time. However, with the former, in principle, no effect beyond a certain level can be expected, and with the latter, there is a problem that the effect cannot be obtained depending on the degree of change in the solid solubility limit of the alloying element due to temperature. Later, Special Publication No. 58-27328 and Special Publication No. 58-48616
No. 2, etc., a hot-rolled dual-phase steel in which a martensitic phase is dispersed in a ferrite matrix by cryogenic coiling after hot rolling has been evaluated for strength-ductility balance [tensile strength (TS) and elongation]. It attracted attention as a high-strength hot-rolled steel sheet with a good yield ratio (yield stress ÷ tensile strength) and a small yield ratio (yield stress ÷ tensile strength) and excellent workability. However, in the production of such hot-rolled dual-phase steel, the coiling temperature is kept at an ultra-low temperature (350°C).
℃ or less) and at a very high cooling rate, there was a problem that there was large variation in the winding temperature, and as a result, there was a large variation in the material properties of the product. or,
In order to manufacture such hot-rolled dual-phase steel, a relatively large number of alloying elements are required to generate the martensitic phase, and therefore there are problems of resource consumption and cost increase due to the increase in the number of alloying elements. In recent years, efforts have been made to improve strength by rapidly cooling steels that are basically solid solution strengthened and converting the hard second phase into a low-temperature transformation phase such as bainite or pseudo-pearlite, which is neither pearlite nor martensite. For example, a method for producing high-tensile hot-rolled steel sheets is disclosed in Japanese Patent Application Laid-Open No.
Proposed in No. 126719. By using bainite as the hard second phase, this manufacturing method has the effect of saving resources by saving some alloying elements compared to the above-mentioned manufacturing method of hot-rolled dual-phase steel based on martensitic phase. It has advantages such as being advantageous in terms of cost, and because the winding temperature is 350°C or higher, cooling control is easy. However, this conventional technique has the problem that only insufficient workability and baking hardenability (the property of increasing hardness due to baking during painting, etc.) can be obtained. [Means for Solving the Problems] The present invention aims to solve the problems of the above-mentioned prior art and provide a method for producing high-strength hot-rolled steel sheets with excellent workability and bake hardenability at a low cost. It was completed as a result of intensive research into this purpose. That is, in the present invention, C: 0.06 to 0.16%, Si:
Ar ₃ Hot rolling is finished above the transformation point,
Then cooled at a cooling rate of 20℃/second or higher to 350℃
The present invention relates to a method for producing a high-strength hot-rolled steel sheet with excellent workability and bake hardenability, which is characterized by winding at a temperature exceeding 600°C and below 600°C. [Detailed explanation of structure and operation] In the method of the present invention, a continuously cast slab or a blooming slab of steel having a specific chemical composition, which will be explained later, is used. As this steel, Al-killed steel is preferable. When this slab is hot rolled in a hot strip mill, normal hot rolling is carried out in which the hot rolling is finished at a temperature equal to or higher than the Ar ₃ transformation point. After hot rolling, the product is cooled on a run-out table using a laminar and spray water at an average cooling rate of 20°C/second or more, followed by winding at a temperature exceeding 350°C and below 600°C. . The method of the present invention aims to produce a bainite phase, which is a low-temperature transformation phase, as a hard second phase through the above steps, and to obtain a steel plate with a good balance of strength and ductility due to the ferrite-bainite structure. Steel with a specific chemical composition used in the method of the present invention refers to C: 0.06 to 0.16% (% in the present invention).
all refer to weight%), Si: 0.03 to 1.00%,
Mn: 0.50-2.00%, S: 0.005% or less, Al: 0.001
~0.1%, N: 0.005~0.015%, and the balance is Fe and unavoidable impurity elements. The reason for limiting to such a chemical component composition is shown below. C is an element that promotes bainite transformation, and is an important element in the present invention, which aims to utilize the bainite phase as the hard second phase. However, if it exceeds 0.16%, the bainite phase becomes too large, impairing workability, and if it is less than 0.06%, the formation of bainite phase is small, making it impossible to obtain the strength of a high-tensile hot-rolled steel sheet. Si is also a solid solution strengthening element and is an element for reducing C in ferrite to obtain a high tensile strength steel sheet with good ductility. However, if Si exceeds 1.00%, not only will the surface quality of the hot rolled steel sheet deteriorate, but it will also become difficult to continuously cast slabs, making it impossible to use economical manufacturing methods. Furthermore, if Si is less than 0.30%, it is difficult to obtain a ferrite-bainite structure with a good strength-ductility balance. Mn is a solid solution strengthening element, and like C, it is an element that promotes bainite transformation, and is a basic constituent element for obtaining high-tensile hot-rolled steel sheets by utilizing the low-temperature transformation phase. However, when it exceeds 2.00%, not only does the susceptibility to weld cracking of the hot rolled steel sheet significantly increase, but also a striped structure is formed, resulting in a large decrease in ductility, while when it is less than 0.05%, it becomes difficult to form a bainite phase. S is 0.005% in consideration of deterioration of cold workability due to sulfide inclusions, especially deterioration of punching hole expandability.
The following was made. Al is an element used as a deoxidizer,
If it is less than 0.001%, there will be insufficient deoxidation in the steelmaking process, and 0.1%
If it exceeds %, the surface quality of the steel sheet will deteriorate. Note that Al is an element that easily bonds with N, and in order to leave free N in order to have the effects of N described below, it is better to have a low content, so 0.05% or less is particularly preferable. N is an element that imparts bake hardenability, and if it exceeds 0.015%, cold workability will be impaired.
If it is less than 0.005%, sufficient bake hardenability cannot be obtained. Increasing the cooling rate of steel with the above chemical structure to 20°C/sec or more after hot rolling suppresses the growth of austenite grains and promotes the formation of fine ferrite, thereby increasing the strength. This is to improve the ductility balance. Also, the winding temperature is 600
The reason why the temperature is lower than ℃ is to prevent coarsening of ferrite crystals, ensure a hard second phase,
To prevent AlN from precipitating in ferrite and reducing bake hardenability, and to obtain high strength by increasing the strength and volume fraction of the hard second phase in combination with a fast cooling rate. It's for a reason. The reason why the winding temperature is set to exceed 350°C is that if the winding temperature is too low, there will be no self-annealing effect and the material quality of the product will vary greatly. The hot-rolled steel sheet obtained in this way cannot be stretched during forming process, unlike cold-rolled steel sheet.
It has the advantage of being less likely to cause problems with strain and has excellent bake hardenability. In the method of the present invention, it is easy to maintain the above range as free N without fixing the N component by simply controlling the winding temperature. Free N imparts baking hardenability, and therefore, the hardness increases significantly by baking, but at the same time, the impact transition temperature shifts to a slightly higher temperature side. However, the extent of this is small and does not pose an industrial problem. [Examples and Comparative Examples] The present invention will be explained in more detail below using Examples and Comparative Examples. Examples 1 to 12, Comparative Examples 1 to 19 No. 1 to No. 16 having the chemical composition shown in Table 1
of steel was melted in a 90-ton converter, made into a 250 mm thick slab by soul rolling or continuous casting, heated at 1230°C and roughly rolled, then hot finished in a dandem mill under the manufacturing conditions shown in Table 2. Hot rolled steel plates having various thicknesses shown in Table 2 were manufactured by rolling and winding after rolling. The mechanical properties of each of the obtained steel plates were measured. That is, the yield point, tensile strength (TS) and elongation (El) were measured by a tensile test, and as an indicator of bake hardenability, the test piece was pre-strained at 3% and then heated at 170°C for 30 minutes. The amount of increase in tensile strength (this is called BH) was measured.
In addition, TS-El, which indicates the strength-rolling balance of the steel plate,
Values are also listed. The results are shown in Table 2.

【table】

[Table] (Note) *: Chemical composition of steel within the range specified by the present invention
**mark: Chemical composition of steel outside the range specified by the present invention

【table】

[Table] From Examples 1 to 12 in Table 2, steels No. 1 to No. 5 having the chemical composition specified in the present invention were
High tensile strength hot rolled steel sheets obtained by controlling the cooling rate and coiling temperature within the range specified by the present invention have an excellent strength-ductility balance and are also endowed with excellent bake hardenability. I understand. On the other hand, from Comparative Examples 1 to 4, even if the chemical composition of the steel is as specified by the present invention, if one or more of the manufacturing conditions is outside the specified range of the present invention, the excellent It can be seen that high tensile strength hot rolled steel sheets cannot be obtained. For example, regarding the winding temperature,
In Comparative Example 1, which is heated to over 600℃, the amount of increase BH in tensile strength, which indicates bake hardenability, is extremely small, and in Comparative Example 2, which is heated to 350℃ or lower, bake hardenability is excellent, but the strength-ductility balance is poor. Not only that, but it is not shown in the table, fluctuations in mechanical properties increase and the product value decreases. In addition, in Comparative Example 3 where the cooling rate was less than 20°C/sec and Comparative Example 4 where the finish rolling temperature was less than the Ar ₃ transformation point, the ductility was relatively good, but the corresponding strength was not obtained. Because of this, the strength-rolling balance is poor. In addition, even if all of these manufacturing conditions comply with the provisions of the present invention, if the chemical composition of the steel to be hot rolled is outside the range specified by the present invention, each implementation It is clear from other comparative examples that a high tensile strength hot rolled steel sheet with such excellent strength-rolling balance and high bake hardenability as in the example cannot be obtained. Example 13, Comparative Example 20 Steel No. 17 with the chemical composition shown in Table 3 (Example 13
) and Steel No. 18 (for Comparative Example 20) were made into a 250 mm thick slab by continuous casting, heated to 1230°C and rough rolled, then hot finish rolled at 880°C,
Cooling at a cooling rate of 30℃/sec, then winding temperature
Each hot-rolled steel plate with a thickness of 4.5 mm was manufactured by winding at 560°C.

〔effect〕

According to the method of the present invention, a high tensile strength hot rolled steel sheet with excellent workability and bake hardenability can be manufactured easily and at low cost. For high-strength hot-rolled steel sheets, which are relatively thick and require a large amount of energy to form, the properties of being soft during forming and exhibiting high strength during use after baking are extremely important properties. The present invention greatly contributes to resource and energy conservation in a wide range of industrial fields that require high-strength hot-rolled steel sheets.

[Brief explanation of drawings]

Figures 1 and 2 are Example 13 and Comparative Example, respectively.
20 is a graph showing changes in impact value and brittle fracture ratio, which are indicators of impact toughness, with temperature.

Claims (1)

[Claims] 1 C: 0.06-0.16%, Si: 0.30-1.00%, Mn:
0.50~2.00%, S: 0.005% or less, Al: 0.001~0.1
%, N: 0.005 to 0.015%, with the balance consisting of Fe and unavoidable impurity elements. Hot rolling is completed above the _Ar3 transformation point, and then the steel is cooled at a cooling rate of 20°C/second or above. , A method for producing a high-tensile hot-rolled steel sheet with excellent workability and bake hardenability, characterized by rolling at a temperature of more than 350°C and less than 600°C.