JPH02149646A - High strength hot rolled steel sheet having excellent workability and weldability - Google Patents

Abstract

PURPOSE:To manufacture the title steel sheet by subjecting a low carbon steel to hot rolling under specific conditions, satisfactorily progressing ferritic transformation, cooling the steel at a cooling speed which prevents pearite transformation and coiling it. CONSTITUTION:A slab of a low carbon steel contg., by weight, 0.10 to 0.15% C, 0.5 to 3.0% Si and 0.5 to 2.5% Mn is subjected to hot rolling, and finish rolling is ended at the temp. of (Ar3-50 deg.C) to 950 deg.C. Then, the steel is cooled to the nose temp. area of ferritic transformation of 600 to 800 deg.C at 1 to 200 deg.C/sec cooling speed, is thereafter cooled until immediately before the starting of pearlite transformation at <=30 deg.C/sec cooling speed to satisfactorily progress the ferrite transformation and is furthermore cooled and coiled to 300 to 500 deg.C coiling temp. at a cooling speed of preventing the pearlite transformation. The hot rolled steel sheet having excellent workability and weldability which has the structure of which >=5% retained austenite is uniformly dispersed into a two-phase matrix of ferrite and bainite having 75mum grain size, has >=2000 strength-ductility balance (TSXEl) and >=60kgf/mm<2> tensile strength can be obtd.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a manufacturing technology for high-strength steel sheets, particularly tensile strength 6
This article relates to a hot-rolled steel sheet with a high strength of 0 kgf/+u+" or more and excellent workability and weldability, and a method for manufacturing the same. (Prior technology) High-strength steel sheets have traditionally been used in automobiles, industrial machinery, etc. In particular, steel sheets for automobiles are used to reduce the weight of automobiles,
There is an increasing demand for higher strength steel plates in order to ensure safety in the event of a collision. However, it is not only necessary to simply increase the strength of steel sheets, but also workability and weldability are required. To meet this type of demand, there has conventionally been a dual-phase steel made of hot-rolled steel sheet made of ferrite and martensite. However, this duplex steel has a better strength-ductility balance (TSXEn
), but the horn, TSxEQ'': 2゜00, had the disadvantage of not being able to withstand stricter workability requirements. (Problem to be solved by the invention) Therefore, in order to solve this disadvantage, ,TSXEI2
In order to obtain a hardness of >2000, a hot-rolled steel sheet with a structure containing retained austenite was developed. As an example, one example is performing large reduction rolling with a finishing temperature of 850°C or higher, a total reduction rate of 80% or more, a total reduction rate of 60% or more in the final three passes, and a final pass reduction rate of 20% or more, and then There is a method (Japanese Unexamined Patent Publication No. 165320/1983) of producing a hot rolled steel sheet containing retained austenite by cooling to 300°C or less at a cooling rate of 50°C/S or more. However, manufacturing such hot-rolled steel sheets requires large reduction rolling, so in order to solve this problem,
A steel containing C: 0.15 to 0.40%, Si: 0.5 to 2.0%, and Mn: 0.5 to 2.0%, with the balance consisting of iron and unavoidable impurities, is finished rolled. End temperature (Ar3-
50°C) to (Ar3+50°C), hot rolling at a total reduction rate of 80% or more, followed by cooling from 350 to 500°C at a cooling rate of 40°C/S or more and winding; In order to improve the strength-ductility balance, the steel plate is heated at a cooling rate of 30°C/hr or more for 20 minutes after winding.
A method of manufacturing a hot-rolled steel sheet containing retained austenite by cooling to 0° C. or lower (Japanese Unexamined Patent Publication No. 63-4017) has been proposed. However, among these methods, method
xE1! Although a strength-ductility balance of >2400 can be obtained, in order to obtain high ductility, it is necessary to perform mist cooling from the side of the coil or cooling the entire coil by immersing it in water, etc., to obtain high ductility. There are problems such as making the material material in the width direction of the plate significantly non-uniform. Also, ■,
In any of (2), there is a problem that adding 0.15% or more of C deteriorates weldability, which is an essential condition for automotive steel sheets, and the workability is also poor. In the present invention, the C content is reduced to 0.00 to prevent deterioration of weldability.
Even if it is kept lower than 15%, it has a strength-ductility balance of TSXEQ>2000 and a tensile strength of 60 kgf/mm.”
The purpose of the above is to provide a high-strength hot-rolled steel sheet with excellent workability and weldability, and also to provide a method for stably and reliably manufacturing the high-strength hot-rolled steel sheet. . (Means for Solving the Problem) In order to achieve the above object, the present inventor reduced the C content to 0.
In order to ensure weldability by lowering it to less than 15% and also to improve workability, we conducted extensive research on composition 1 structure and manufacturing conditions. As a result, steel with a composition containing 0.10 or more but less than 0.15% C was hot-rolled under specific conditions at a cooling rate of 350 to 500°C that could sufficiently advance ferrite transformation and prevent pearlite transformation. By cooling to a temperature of 50% and winding it up, it is possible to obtain a structure in which the amount of retained austenite is uniformly dispersed in a two-phase matrix of ferrite and bainite in a volume fraction of 5% or more, resulting in high strength, reduced C content, and It has been found that workability can be improved in relation to manufacturing conditions, and weldability can be ensured at the same time. In addition, during cooling after hot rolling the steel under specific conditions,
By cooling to 350 to 500°C and winding at a cooling rate that allows ferrite transformation to proceed sufficiently and prevents pearlite transformation, the amount of residual austenite in the two-phase matrix of ferrite and bainite is reduced without making the ferrite finer. It has been discovered that it is possible to obtain a structure in which a volume fraction of 5% or more is uniformly dispersed, and that not only high strength and weldability but also low yield strength and workability can be ensured, and the present invention has been made based on this discovery. That is, the high-strength hot-rolled steel sheet (ml of the present invention) with excellent workability and weldability according to the present invention has a C: 0.10 or more and a C: 0.1
Less than 5%, Si: 0.5-3.0% and Mn: 0.5-
2.5%, with the remainder consisting of iron and unavoidable impurities, and characterized by having 5% or more of retained austenite uniformly dispersed in a two-phase matrix of ferrite and bainite. It is. The method for producing the high-strength hot-rolled steel sheet (method 1 of the present invention) is to process steel having the above-mentioned composition at a finish rolling end temperature of (Ar,
-50°C) to 850°C with a finish rolling reduction of 80% or more, and then cooling to a coiling temperature of 300°C to 500°C at a cooling rate that prevents pearlite transformation, and one roll is taken. 6 Furthermore, the high-strength hot-rolled steel sheet (invention 112) with excellent workability and weldability according to the present invention has a C: 0.10 or more and 0.
．． less than 15%, Si: 0.5-3.0% and Mn: 0.
5 to 2.5%, with the balance consisting of iron and unavoidable impurities, and 5% or more of retained austenite is uniform in a two-phase matrix of ferrite (with grain size of 5μ or more) and bainite. It is characterized by being dispersed in The method for manufacturing the high-strength hot-rolled steel sheet (method 2 of the present invention) is to process steel having the above composition at a finish rolling end temperature of (Ar,
-50℃) to 950℃, and after hot rolling,
It is cooled at a rate of 1-b to the nose temperature range of ferrite transformation of 600 to 800°C, and then cooled at a cooling rate of 30°C/S or less until just before pearlite transformation starts to advance ferrite transformation, and then the coiling temperature is It is characterized in that it is cooled to 300 to 500°C at a cooling rate that can prevent pearlite transformation, and then wound up. The present invention will be explained in more detail below. (Function) First, the reason for limiting the chemical composition of steel in the present invention will be explained. C: Since C is an essential element for increasing the strength of steel, a minimum amount is required, but the smaller the amount, the more effective it is in improving weldability as well as the workability. However, if it is less than 0.10%, a sufficient amount of retained austenite that improves ductility cannot be obtained. On the other hand, an increase in the amount of C increases the difference in hardness between the martensite phase, which is a strain-induced transformation of the second phase of bainite or retained austenite, and the ferrite phase. When the difference in hardness between the matrix and the second phase is large, deformation is difficult to transmit at the interface between the matrix and the second phase, and this interface becomes the starting point for cracks, resulting in cracking during processing. This effect becomes noticeable in the second phase generated when the amount of C is 15% or more, and it is not preferable that the amount of carbon is more than 0.15% because it deteriorates weldability.
The amount should be in the range of 0°10 to less than 0.15%, and 0.10 to less than 0.15%.
0°13% is preferred. Si: Increasing the content of SL is advantageous for the generation and purification of ferrite that contributes to improving ductility, and is also advantageous for concentrating C in untransformed austenite to obtain retained austenite. Furthermore, Si suppresses carbide formation during bainite transformation after winding, and makes C more concentrated in untransformed austenite, which is more advantageous for obtaining retained austenite. Furthermore, increasing the amount of Si causes solid solution hardening of the ferrite phase, which has the effect of reducing the difference in hardness between the ferrite phase and the second phase, suppressing the occurrence of cracks to a high degree of workability, and as a result, improving the workability of the ferrite phase. Improves sex. Such an effect is not fully exhibited when Si is less than 0.5%, and when Si is less than 0.5%,
If it exceeds 0%, the effects of ferrite generation, purification, and retention of retained austenite are saturated, and scale properties instead occur. This is not preferable because it deteriorates weldability and impairs workability due to the formation of an ordered phase (B2). Therefore, S
The amount of i should be in the range of 0.5 to 3.0%, and 1°5 to 2.0%.
is preferred. Mn: Mn has the effect of contributing to the retention of austenite as an austenite stabilizing element. However, this effect is 0
．． If it is less than 5%, sufficient results cannot be obtained, and if it exceeds 2.5%, the effect is saturated and weldability is deteriorated, which is not preferable. therefore. The amount of Mn is in the range of 0.5 to 3.0%. In addition, there is no need to add other alloying elements to the above steel,
The remainder is iron and unavoidable impurities. Next, conditions for the method of the present invention will be explained. In order to improve the ductility of hot rolled steel sheets with the above composition, it is first necessary to uniformly contain a predetermined amount of retained austenite in the two-phase matrix of ferrite and bainite. Stabilize by concentrating elements, etc. For this reason, in the method of the present invention, two types of manufacturing conditions are adopted on the premise that the composition is adjusted as described above. That is, as the first means (method 1 of the present invention), the finish rolling temperature of the above steel is (Ar, -50°C) to 850°C.
C, finish rolling is performed at a finish reduction rate of 80% or more to cause deformation-induced ferrite transformation. In addition, as a second means (method 2 of the present invention), the above steel (A
Finish rolling is completed at ~950°C (r3-50°C), then rapidly cooled to 600~800°C near the nose of ferrite transformation, and then slowly cooled around the nose of ferrite transformation to promote ferrite transformation. This produces ferrite that is advantageous for improving ductility, promotes the concentration of elements such as C in austenite, and contributes to the retention of austenite. This condition is to stabilize the austenite and not to refine the ferrite in order to improve workability, that is, the ferrite grain size is 5 μm.
It has also been considered not to make the ferrite less than fine. In the case of the first means and the second means, the finish rolling end temperature is (A
r, −50° C.), a ferrite texture is formed and the ductility is impaired, which is not preferable. therefore,
In the case of either method, the finish rolling end temperature is set to (Ar,
-50°C) or higher. However, when utilizing deformation-induced ferrite transformation as in the first method, a sufficient amount of ferrite must be obtained during hot rolling. However, if the finish rolling end temperature is too high, a very large rolling reduction is required to sufficiently advance ferrite transformation, so the upper limit of the finish rolling temperature is 850°C in order to sufficiently advance ferrite transformation within the practicable range. It is necessary to prevent the formation of processing transformation of ferrite that impairs ductility (Ar, -50℃).
In order to sufficiently advance ferrite transformation at the above temperature, the rolling reduction ratio in finish rolling must be 80% or more. In addition, as in the second method, when the process-induced ferrite transformation is not utilized and the ferrite transformation is rapidly cooled to the vicinity of the ferrite transformation nose and the ferrite transformation nose is gradually cooled to promote the ferrite transformation, the ferrite transformation may be accelerated during finish rolling. Since there is no need to allow transformation to proceed, it is not necessary to specify the rolling reduction ratio, but when the finish rolling end temperature is extremely high, the austenite grain size is too large and it takes a long time for ferrite transformation to proceed near the nose of ferrite transformation. The upper limit of the finish rolling temperature is set at the first
The temperature must be kept at 950°C, although it is higher than in the case of conventional methods. Further, after hot rolling, a cooling rate of 1° C./S or more is required to cool the steel sheet to the ferrite transformation nose temperature range of 600 to 800° C. without pearlitic transformation. However, a cooling rate of 200° C./S or more is not desirable because it is difficult to control the cooling rate and there is a risk of overcooling. After rapidly cooling to the vicinity of the nose of ferrite transformation in this manner, the amount of ferrite can be ensured by cooling at a cooling rate of 30° C./S or less (including isothermal maintenance). In this case, if the cooling rate exceeds 30° C./S, ferrite transformation will not proceed sufficiently, and therefore a sufficient amount of ferrite, which is essential for improving ductility, cannot be secured. Therefore, after being rapidly cooled to the vicinity of the nose of ferrite transformation after hot rolling, the
It is necessary to cool at a cooling rate of /S or less. In any of the above means, when ferrite transformation ends and pearlite transformation starts during cooling, C that is effective for remaining austenite is consumed, and the remaining austenite decreases. Therefore, in any of the above methods, after the above stage, it is necessary to cool down to the coiling temperature at a cooling rate that can prevent pearlite transformation. If the winding temperature exceeds 500°C, pearlite will be generated after winding or bainite transformation will progress excessively.
Not enough residual austenite can be obtained. Also, 300
At a winding temperature below ℃, the workability (hole expandability) deteriorates. Therefore, the winding temperature is in the range of 300 to 500°C. The resulting hot rolled steel sheet has a structure in which 5% or more of retained austenite is uniformly dispersed in a two-phase matrix of ferrite and bainite. However, in the case of the first means, the ferrite grain size is fine grains of less than 5 μI, but since the amount of C is small, weldability is excellent and workability is also improved. on the other hand. In the case of the second means, the weldability is excellent due to the small amount of C, and since the ferrite grain size is 5 μm or more, the yield strength is particularly low and the workability is excellent. Of course, in either case, it has high strength and a good strength-ductility balance. Next, examples of the present invention will be shown. (Example) Steels A to D having the chemical compositions shown in Table 1 were hot rolled under the conditions shown in Table 2, coiled, and air cooled. In addition, hot rolling is 30+m → 16mm → 8III→
A 3mm pass schedule was used. In addition, NQl(
Examples of the present invention) are produced by method 1 of the present invention, and Nα4 to Nα5 (examples of the present invention) are produced by method 2 of the present invention. The mechanical properties and structure of the obtained steel plate were investigated, as well as weldability (cross tensile strength after welding) and workability (hole expansion ratio). The results are also listed in Table 2. Inventive examples Nα1, Na4 and Nα5 are all TS
It exhibits an extremely high strength-ductility balance of XEQ>2400 (see Figure 1), and has excellent weldability and workability. On the other hand, since the comparative example Ha 2 has a low Si content, it also has Na
Since No. 3 has a low C content, almost no retained austenite is formed, the ductility is low, the strength-ductility balance is low, and the workability is also poor. Comparative Example Surface 6 has low ductility because the rolling reduction is too low and the steel is rapidly cooled to the coiling temperature, so ferrite transformation does not progress sufficiently, C concentration is insufficient, and the amount of retained austenite is insufficient. Processability is also poor. In Comparative Example Nα7, the finish rolling end temperature was too low and a processed ferrite structure was formed, so the ductility was low. Processability is also poor. Comparative Example Nα8 is cooled at a slow cooling rate even below the pearlite transformation start temperature (650° C. in this case), so no retained austenite is generated and the ductility is low. Processability is also poor. In Comparative Example &9, the coiling temperature was high, residual austenite was not generated, and ductility was insufficient. Processability is also poor. Comparative example Nα10 has a high C content of 0.25%, has insufficient welding strength, has poor weldability, and has poor workability. In Comparative Example 11, the finish rolling temperature was as high as 970° C., so the ferrite transformation did not proceed sufficiently, and the ductility and workability were low. Comparative Example NQ12 has poor hole expandability (workability) because the winding temperature is too low. It also has poor ductility. FIG. 2 shows the structure of the hot-rolled steel sheet, and it can be seen that the structure (a) of Invention Example &4 has coarser ferrite grains than the structure (b) of Comparative Example NqlO.

[Left below]

(Effects of the Invention) As is clear from the above detailed description, the present invention provides a high-strength hot rolled sheet having a tensile strength of 60 kgf/mm2 or more, a good strength-ductility balance, and excellent workability and weldability. You can get steel plates. In addition, it is economical because it can be manufactured without adding any special alloying elements, and there is no need for particularly high rolling. Furthermore, it does not require special cooling after winding, so stable quality can be obtained. The above effect is very large.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between TS and EQ (strength-ductility balance). Figure 2 is a micrograph (X400) showing the metallographic structure of the hot rolled steel sheet obtained in the example. The case of the invention example is shown, and (b) shows the case of the comparative example. Patent Applicant Kobe Steel Corporation Patent Attorney Hisashi Nakamura (/,) (α) No.

Claims (4)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.10 or more
．． less than 15%, Si: 0.5-3.0% and Mn: 0.
5 to 2.5%, with the remainder consisting of iron and unavoidable impurities, and is characterized by having 5% or more of retained austenite uniformly dispersed in a two-phase matrix of ferrite and bainite. High-strength hot-rolled steel sheet with excellent workability and weldability. (2) C: 0.10 or more and less than 0.15%, Si: 0.
5 to 3.0% and Mn: 0.5 to 2.5%, with the balance consisting of iron and inevitable impurities,
Finish rolling end temperature is (Ar_3-50℃) ~ 850℃,
Workability and weldability characterized by finishing rolling at a finish rolling reduction of 80% or more, then cooling to a coiling temperature of 300 to 500°C at a cooling rate that can prevent pearlite transformation, and coiling. A method for producing superior high-strength hot-rolled steel sheets. (3) C: 0.10 or more and less than 0.15%, Si: 0.
5 to 3.0% and Mn: 0.5 to 2.5%, with the remainder consisting of iron and unavoidable impurities, and a two-phase matrix of ferrite (with a grain size of 5 μm or more) and bainite. A high-strength hot-rolled steel sheet with excellent workability and weldability, characterized by having 5% or more of retained austenite uniformly dispersed therein. (4) C: 0.10 or more and less than 0.15%, Si: 0.
5 to 3.0%, Mn: 0.5 to 2.5%, and the balance is iron and unavoidable impurities.The finish rolling temperature is (Ar_3-50℃) to 950℃. After hot rolling, cooling is performed at a cooling rate of 1 to 200°C/s to the nose temperature range of ferrite transformation of 600 to 800°C, and then pearlite transformation is carried out at a cooling rate of 30°C/s or less. It is characterized by the fact that it is cooled until just before the start of ferrite transformation to advance ferrite transformation, and further cooled to a coiling temperature of 300 to 500°C at a cooling rate that can prevent pearlite transformation, and then coiled. A method for producing high-strength hot-rolled steel sheets.