JPH079035B2 - Method of manufacturing hot rolled high strength steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a hot-rolled high-tensile steel sheet having a tensile strength of 60 kgf / mm ² or more, which is mainly useful for use in rim materials and disk materials for automobile wheels. It is a thing.

(Prior Art) Hot-rolled high-tensile steel sheets used for automobile wheels are largely
It is divided into types. One is utilizing precipitation strengthening, for example, as disclosed in JP-A-61-170541, C, Si,
The one in which Ti and Nb are added to Mn and the other one is disclosed in
As disclosed in Japanese Patent Publication No. 1-79730, the basic components of C, Si and Mn are used to strengthen the structure. The former is mainly used for wheel rims, and the latter is mainly used for wheel discs. Here, the characteristics required for the wheel rim material are flash butt weldability / DC butt weldability and cold workability after welding, and the characteristics required for the wheel disc material are formability. Precipitation-strengthened steel used for wheel rim materials has a high yield point and a large yield elongation, so it is not used for wheel discs except for very simple shapes. On the other hand, since the structure-reinforced steel used for wheel discs has poor flash butt weldability / cold workability after DC bat welding, it should be used for rims except those with low tensile strength of 45 kgf / mm ² or less. There is no. Currently, hot-rolled high-strength steel sheets for wheels with a tensile strength of 60 kgf / mm ² or more are manufactured separately for rims and discs from the time of manufacture.

A common drawback of both steel types is the separation that occurs on the sheared surface and punched surface, which causes deterioration of the rim formability and wheel durability strength.

(Problems to be solved by the invention) An automobile wheel is an important safety component, and is required to have a high durability strength in the first place. However, in addition to this, fashionability is emphasized, and the shape is complicated. Anything that impairs the beauty you see is extremely disliked. The above-mentioned separation is generated in the center hole of the wheel disc and impairs the beauty, and also occurs in the decorative window portion to deteriorate the durability strength. Further, since the wheel rim material and the disc material have different required properties, they are manufactured under different components and manufacturing conditions, which complicates the work at the manufacturing site.

This invention is basically aimed at achieving the production of steel plates for both rims and discs. Therefore, it has excellent flash butt / DC butt weldability, cold workability after welding, low yield elongation, and separation resistance. It is an issue to be solved to satisfy all the characteristics of excellent durability and high durability.

(Means for Solving the Problem) The present invention provides C: 0.03 to 0.10 wt% (hereinafter simply indicated as%),
Si: 0.10% or less, Mn: 0.5% or less, P: 0.03 to 0.10% and N
b: 0.03 to 0.5%, B: 0.0001 to 0.005%, M
o: A steel slab containing 0.01 to 1.0% of 1 or 2 and the balance of Fe and unavoidable impurities is heated and soaked at 1200 to 1400 ° C, and hot rolling is finished in the temperature range of 730 to 880 ° C. Then, the method for producing a hot-rolled high-tensile steel sheet is characterized in that it is continuously cooled within 0.5 seconds to a temperature range of 700 to 300 ° C at a cooling rate of 50 ° C / s or more and wound up. This invention is C: 0.03 to 0.10%
Si: 0.10% or less, Mn: 0.5% or less, P: 0.03 to 0.10% and N
b: 0.03 to 0.5%, B: 0.0001 to 0.005%, M
o: 0.01 to 1.0% of 1 or 2 kinds, and Ca: 0.0001 to 0.001
%, REM: 0.001 to 0.01%, 1 or 2 types, balance F
Steel slab consisting of e and unavoidable impurities is 1200-1400
After soaking by heating to ℃, finish the hot rolling in the temperature range of 730 to 880 ℃, and continue within 0.5 seconds at a cooling rate of 50 ℃ / s or more to 700 ~
A method for producing a hot-rolled high-strength steel sheet, which comprises cooling to a temperature range of 300 ° C and winding.

One of the features of this invention is that the amount of Mn is low. This is because the separation that occurs in the conventional hot-rolled high-strength steel sheet is Mn.
It is based on the finding that it is due to the layered second phase generated in the segregation zone of. The decrease in TS due to the decrease in the amount of Mn is compensated by the solid solution strengthening by adding P and the precipitation strengthening by adding Nb. In addition, the large yield elongation, which is the drawback of precipitation strengthening, is
The fact that it can be suppressed by adding Mo is a new finding of the inventors and is the second feature. Furthermore, it is a great feature that, with this component system and manufacturing conditions, it was possible to maintain a tensile strength of 60 kgf / mm ² or more even if the C content was reduced,
This results in a low C equivalent and can advantageously improve weldability.

As described above, if the amount of Mn is reduced, the separation can be considerably reduced, but that alone cannot completely eliminate the separation. The inventors carefully investigated the experimental material that caused separation, and obtained the following results. That is, film-like MnS is precipitated in the layered second phase of the sample that has undergone separation. The absolute amount is M
Although it decreases when the amount of n added is reduced, it is not limited to the reduction of the amount of Mn. The massive MnS does not become the starting point of separation. When Ca and REM are added, this film-shaped MnS disappears and only massive MnS remains. Based on these findings, the present invention has found that the addition of Ca and REM can eliminate the separation. This is the third feature.

(Function) The reasons for limiting the range of each component will be described below.

C: For the wheel rim, put together the end faces of strip-shaped steel plates,
It is manufactured by welding and forming a hoop. If the hardness difference between the base material and the welded portion is large, the processing characteristics of the welded portion are deteriorated and cracking occurs during forming. The hardness of the welded portion depends on the C content of the material, and 0.10% is the upper limit that does not deteriorate the workability after welding. On the other hand, C is an element that is advantageous for increasing the strength of the steel sheet,
With the component system with reduced Mn as in the present invention, TS is 60 kgf /
If it is more than mm ^2, 0.03% or more must be added.

Si: An element with a large solid solution strengthening ability, which was effectively used, but if it exceeds 0.1%, a red scale occurs during hot rolling, and scale marks remain on the wheel surface, impairing the beauty.
Therefore, the upper limit is 0.10%.

Mn: Since it is the main cause of separation, it is desired to make it as low as possible, but in order to effectively use the strengthening ability, the upper limit is 0.5%, which does not form a layered structure that causes separation.

Without producing a red scale like P: Si,
Since it does not form a layered second phase as described above, it is an element for which it is desired to effectively utilize the solid solution strengthening ability, but if it exceeds 0.1%, embrittlement due to grain boundary segregation occurs, so this is the upper limit. If it is 0.03% or less, the contribution to the strength is small, so this is made the lower limit.

Nb: This is an important additional element in the present invention, and by the precipitation strengthening mechanism, it is possible to supplement the strength decrease due to the reduction of Mn and impart high fatigue strength. By the way, the amount of Nb added was 0.03
%, The contribution to strengthening becomes small, so this is made the lower limit. Further, the addition of more than 0.5% deteriorates the cold workability of the welded portion, so the upper limit is 0.5%.

B: In addition to the above, when B is further added, the ferrite grain boundaries have an irregular shape, the yield elongation decreases and the total elongation increases. This is an advantageous property for molding complex disks. To produce this property, it is necessary to add 0.0001% or more of B, but if it is added excessively, cold workability deteriorates, so 0.005% is made the upper limit.

Mo: Mo has the same effect as B, and 0.01% is the lower limit for the same reason. Regarding the upper limit, Mo is an expensive element, and the upper limit is 1.0% at which the required yield elongation reduction can be obtained.

In the present invention, in addition to this, one or two kinds of Ca and REM can be added. The reason for limiting the component range is as follows.

To suppress the precipitation of Ca: film-like MnS, 0.0001% or more must be added, so this is the lower limit. But,
If it is added excessively, coarse sulfide will be precipitated and the formability will be deteriorated, so 0.001% is made the upper limit.

REM: REM has the same effect as Ca. For the same reason, the lower limit is 0.001%, and 0.01% due to restrictions in the steelmaking process.
% Is the upper limit.

Note that S and N are not particularly limited, but lower ones are desirable.

Next, processing and heat treatment conditions of the steel slab satisfying the above-mentioned composition will be described.

Slab heating temperature: The slab heating temperature is set to 1200 to 1400 ° C in order to dissolve the coarse Nb carbonitride generated during solidification of the slab.

End temperature of hot rolling: Strength is improved by using precipitation strengthening in addition to compensating for strength reduction due to decrease in Mn amount, and grain refinement strengthening. Therefore, the hot rolling finish temperature is set to a range of 730 to 880 ° C.

Holding time after completion of hot rolling: It is more effective to start cooling immediately after completion of hot rolling, but the structural air cooling time of the hot rolling mill occurs. The maximum air-cooling time to prevent the strength from decreasing is 0.5 sec. Therefore, the holding time should be within 0.5 seconds.

Cooling rate: If the cooling rate is slow, α grains grow and become coarse after α transformation, and it is not possible to expect strength improvement due to the effect of refining. Therefore, the cooling rate is 50 ° C./sce or more. In addition, the faster the cooling rate is, the better the upper limit is not set.
It is about 200 ℃ / sce.

Winding temperature: A higher winding temperature is preferable because precipitation of Nb tends to occur after winding, but if the winding temperature is too high, coarse precipitates do not contribute to strength improvement, so 700 ° C is the upper limit. On the other hand, if the winding temperature is low, the steel sheet is likely to have a defective shape, so the lower limit is 300 ° C.

Hot rolling was performed under the conditions shown in Table 2 using the slabs having the components shown in Table 1. The results of the investigation of the mechanical properties of the obtained steel sheet are shown in the table.
3, shown in Tables 1-3.

Steel A has a low hot rolling end temperature, and steel E is too high, so TS is insufficient. Steels I and J have a large yield elongation because the cooling start time is too long. Steel K also has a large yield elongation because of its slow cooling rate. Other than those are application examples according to the present invention, and as is clear from FIGS. 1, 2, and 3, TS
> 60 kgf / mm ² , YE ′ <1.0%.

A steel slab having the components shown in Table 4 was hot-rolled under the conditions shown in Table 5 to finish a hot-rolled steel sheet having a plate thickness of 3.2 mm, and the following items were investigated. The coil shape during production (ear extension, belly extension, wrinkles, etc.) and the presence or absence of a red scale were visually determined. The cross-sectional structure along the rolling direction was examined by a microscope to determine the degree of the layered second phase in the central part. The tensile strength along the rolling direction was examined. Further, an in-plane bending test called side vent elongation was performed using a test piece cut out from the direction perpendicular to the rolling direction. In addition, a side bend test was also conducted on the welded portion of the test piece that had been subjected to normal flash butt welding to examine the change in side vent elongation after welding. The fracture surface after the tensile test was observed to examine the degree of separation. A wheel was prototyped using the manufactured steel sheet, and the durability strength of the wheel was examined. There are two types of wheel fatigue tests,
On the other hand, a tire was mounted and pressed against a rotating drum, and the durability was defined as the number of rotations until a crack was generated and air leakage occurred. Here, a test was conducted with a load of 1000 kgf. On the other hand, the wheel is fixed, and a bending moment is applied to the wheel so that the number of rotations until a crack is generated is the durability strength. Here, a rotating bending moment of 150 kgf · m was applied. Table 6 shows the above survey results.

Steel 1 has a low C content and therefore has a tensile strength of 60 kgf / mm ² or less. Steel 4 has too high Mn content, poor side bend elongation, and insufficient moment durability strength. Steel 7 has a too high C content, and thus has a low side bend elongation in the welded portion. Steel 8 had a high Si content and a red scale was generated. Steel 9 has a high Mn content,
The layered second phase in the middle portion is remarkable, and therefore the side bend elongation is low, separation occurs, and the moment durability strength is insufficient. Steel 12 has a low P content, so
Tensile strength does not reach 60kgf / mm ² . Steel 16 has a high P content, so the side bend elongation of the weld is low. The tensile strength of Steel 19 has not reached 60 kgf / mm ² due to the low Nb content. Steel 22 has a high amount of Nb, so the side bend elongation is poor.
Steels 5-1,5-4,5-6,5-8,5-9,5-12 have the same composition as steel 5 and are within the scope of the present invention, but the hot rolling conditions are out of range. Therefore, the tensile strength is 60kgf / mm ² or less (5-1,6,8,9), or the side bend elongation of the weld is low (5-4), the yield elongation is large (5-8,9), Or the coil shape is bad (5-1
Deterioration of various properties such as 2) was observed.

Steel 24 has a large yield elongation because neither Me nor B is added. steel
Since 30 has a large amount of B, the side bend elongation of the weld is poor.

What has been described above is a comparative example, but all other properties, that is, those manufactured according to the present invention, have obtained good characteristics. Steel 35 to 39 is Ca or REM
It is an added steel and is characterized by no separation, and therefore the moment durability of the disc is high. Other properties are similar to those of the Ca- and REM-free steels, and it is clear that they have excellent features such as low yield elongation and high TS.

(Effect of the Invention) According to this invention, the weldability as a rim material and the cold workability after welding are excellent, and the yield elongation as a disc material is small,
Excellent separation resistance for both rim and disc,
It is possible to manufacture a high-strength hot-rolled steel sheet that satisfies all of the characteristics required for a material for automobile wheels that has high durability strength. In addition, there is no need to make separate rim materials and disc materials, and cost reduction can be achieved by reducing the amount of Mn, which has a great effect on industrial development.

[Brief description of drawings]

Fig. 1 is a graph of the relationship between the hot rolling end temperature and tensile strength, Fig. 2 is a graph of the relationship between the holding time until the start of cooling after hot rolling and the yield elongation, and Fig. 3 is the cooling rate after the end of hot rolling and the yield. It is a relation graph of growth.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 61-79730 (JP, A) JP 60-39119 (JP, A) Japan Society for the Promotion of Science, Steel and Alloying Elements (19th Committee) (Below) "(Sho 41.3.25 Seibundo Shinkosha Co., Ltd.) 17 ~ 29

Claims (2)

[Claims] 1. C: 0.03 to 0.10 wt% Si: 0.10 wt% or less, Mn: 0.5 wt% or less, P: 0.03 to 0.10 wt% and Nb: 0.03 to 0.5 wt%, and B: 0.0001 to A steel slab containing 0.005 wt% and Mo: 0.01 to 1.0 wt% of 1 or 2 and the balance of Fe and unavoidable impurities is heated to 1200 to 1400 ° C and heated in the temperature range of 730 to 880 ° C after soaking. A method for producing a hot-rolled high-strength steel sheet, which comprises terminating the hot rolling and subsequently cooling it to a temperature range of 700 to 300 ° C at a cooling rate of 50 ° C / s or more within 0.5 seconds and winding. 2. C: 0.03-0.10 wt% Si: 0.10 wt% or less, Mn: 0.5 wt% or less, P: 0.03-0.10 wt% and Nb: 0.03-0.5 wt%, and B: 0.0001- 0.005wt%, Mo: 0.01-1.0wt% 1 or 2 and Ca: 0.0001-0.001wt%, REM: 0.001-0.01wt% 1 or 2 with the balance Fe and unavoidable impurities. After soaking the steel slab to 1200 to 1400 ℃, finish the hot rolling in the temperature range of 730 to 880 ℃, and continue to the temperature range of 700 to 300 ℃ within 0.5 seconds at a cooling rate of 50 ℃ / s or more. A method for producing a hot-rolled high-strength steel sheet, which comprises cooling to a temperature and winding.