CN101643880B - High tensile strength hot-rolled ferrite-bainite dual-phase steel and manufacturing method thereof - Google Patents

Abstract

The invention relates to a hot-rolled ferritic bainite dual-phase steel with a tensile strength ≥ 490 MPa and a preparation process thereof. The dual-phase steel is composed of the following components in weight percentage: 0.05% to 0.15% carbon, 0.5% silicon ～1.0%, manganese 1.0～1.5%, phosphorus≤0.02%, sulfur≤0.010%, aluminum 0.03～0.05%, nitrogen≤0.0060%, and the balance of Fe and impurities. The preparation process of the dual-phase steel is as follows: after the steel ingot is subjected to hot rolling treatment, it is then treated according to the subsection cooling process, and finally the steel material is coiled. The dual-phase steel of the invention has high tensile strength, excellent hole expansion performance and formability, and low cost. At the same time, the preparation process is simple, no expensive alloy elements need to be added, and different strength levels can be obtained through process control. The invention can be widely and mainly applied to the forming of parts with complex shapes such as automobile wheels and suspension parts.

Description

High tensile strength hot-rolled ferritic bainite dual-phase steel and manufacturing method thereof

technical field

The invention relates to a high tensile strength steel and its preparation process, in particular to a hot-rolled ferritic bainite dual-phase steel with a tensile strength of 490 MPa and the preparation of the hot-rolled iron by segmental cooling after hot rolling. Method for bainitic dual-phase steels.

Background technique

In recent years, people have paid more and more attention to environmental protection issues, and put forward higher requirements for energy saving and emission reduction for vehicles and other vehicles. To achieve this goal, one of the commonly used methods is to reduce the weight of automobiles as much as possible while maintaining their safety performance, which promotes the application of various high-strength steels in automobiles. For example, the car chassis, wheels, and their surrounding parts account for more than 25% of the total weight of the car body, and they are mainly made of hot-rolled steel plates (such as ferritic martensitic hot-rolled steel plates with high ductility and good fatigue properties). duplex steel) production. However, due to the complex shape of the running parts of the automobile, the steel plate is required to have good stamping performance and high hole expansion performance during forming, and the ferritic martensitic dual-phase steel has a large microhardness difference between the two phases, and small voids are easy to form. Production and dissemination will make the hole expandability worse, which is not conducive to the forming of parts such as automobiles that require high hole expandability. However, the ferrite and pearlite hot-rolled steels that were commonly used in the preparation of wheels in the past limited the further improvement of their overall strength because the second phase was pearlite. Considering that the bainite transformation temperature is between pearlite and martensite, it has both the ductility and toughness of high-temperature transformation products and the strength of low-temperature transformation products, and it is between ferrite and pearlite, ferrite and bainite. Among the three two-phase structures of ferrite and martensite, the microhardness difference between ferrite and bainite is the smallest, and its hole expansion performance is also the best. For example, the tensile strength of hot-rolled high hole expansion steel developed by Baosteel is Above 590MPa, the elongation rate is between 18% and 32%, and the hole expansion rate is greater than 75%. Therefore, due to its excellent performance, hot-rolled high hole expansion rate ferritic and bainitic dual-phase steel is expected to be widely used in some automotive parts such as wheels that require strict flange processing.

The invention patent with the patent publication number CN101033522A discloses a fine-grained ferritic bainite dual-phase steel based on supercooled austenite deformation, which has good mechanical properties and low yield ratio, but silicon and aluminum The content is high, which increases the difficulty of the steelmaking process, and the process requirements are also high, especially the cooling rate after rolling is required to be controlled within a certain range. At the same time, a certain amount of Ma Tensile, which has an adverse effect on the hole expansion properties of steel. The invention patent with the patent publication number CN1190997A discloses a dual-phase steel with a second phase of mainly polygonal ferrite and carbon-rich pearlite, which achieves high strength, good surface quality and matching of cold deformation properties of steel , but a certain amount of microalloying elements need to be added to the steel, which increases the production cost, and the high content of aluminum elements also brings certain difficulties to the steelmaking process. In addition, the invention patents with patent publication numbers US20050133124A1 and US20080202645A1 also proposed multi-phase steels with good strength, elongation and hole expandability. The microstructure of these steels contains 3-30% martensite and/or residual Austenite, the other is one or a combination of ferrite and bainite. Due to the existence of martensite, the hole expansion performance of steel is adversely affected to a certain extent. Also, the invention patent with the patent publication number CN101353757A discloses a hot-rolled high hole expansion rate steel plate with a tensile strength of 440 MPa. The hole expansion rate is very high, but its steel structure is a full bainite structure, and wherein More niobium, so the yield ratio is high and the stamping performance is poor.

In summary, it can be seen that there are many defects in the existing dual-phase steel materials. Therefore, how to develop a kind of steel with simple preparation process and good mechanical properties, such as high tensile strength, good plasticity, and hole expansion rate High steel has been a long-standing concern in the industry.

Contents of the invention

One of the purposes of the present invention is to propose a hot-rolled ferritic bainite dual-phase steel with a tensile strength above 490 MPa, excellent hole expansion performance and formability, low cost, and simple preparation process to overcome the existing Weaknesses in technology.

In order to realize the above-mentioned purpose of the invention, the present invention has adopted following technical scheme:

A hot-rolled ferritic bainite dual-phase steel with high tensile strength, which is composed of the following components in weight percentage: carbon 0.05-0.15%, silicon 0.5-1.0%, manganese 1.0-1.5%, phosphorus ≤ 0.02%, sulfur≤0.010%, aluminum 0.03-0.05%, nitrogen≤0.0060%, and the balance of Fe and impurities.

Further speaking, the tensile strength of the hot-rolled ferritic-bainite dual-phase steel is ≥490MPa.

The hot-rolled ferritic bainite dual-phase steel is composed of the following components in weight percentage: 0.07-0.10% carbon, 0.60-0.90% silicon, 1.40-1.50% manganese, phosphorus≤0.011%, sulfur≤0.0050% , aluminum 0.035-0.040%, nitrogen ≤ 0.0050%, and the balance of Fe and impurities.

Another object of the present invention is to propose a process for preparing hot-rolled ferritic bainite dual-phase steel with high tensile strength as described above, which has a simple flow process, is easy to operate, and has low cost. The method is as follows:

Heat the steel ingot to be processed to 1200-1250°C, and perform soaking treatment, and then carry out hot rolling treatment on the steel ingot, the rolling temperature is controlled at 820-1150°C, and then the rolled steel is treated according to the staged cooling process, The temperature of the steel is cooled to 450-550° C., and finally the steel is coiled. The tensile strength of the steel is ≥490 MPa.

Specifically, the soaking process lasts for 1-2 hours.

The technological conditions of the hot rolling treatment are as follows: the rolling start temperature is 1100-1150°C, the finish rolling start temperature is 950-1000°C, and the finish rolling temperature is 820-860°C.

Described subsection cooling process is specifically:

(1) Rapid cooling to between 680°C and 720°C at a cooling rate greater than 20°C/s;

(2) Air cooling treatment for 3-5s;

(3) Rapid cooling to between 450 and 550°C at a cooling rate greater than 20°C/s.

The method specifically includes the following steps:

(1) Heating the steel ingot to be processed to 1200-1250°C, and soaking for 1-2 hours;

(2) Roll the steel ingot after heat treatment, and control the starting rolling temperature to be 1100-1150°C, the finish rolling start temperature to be 950-1000°C, and the finishing rolling temperature to be 820-860°C;

(3) Cool the rolled steel, cool it quickly to 680°C-720°C at a cooling rate greater than 20°C/s, then air-cool it for 3-5s, and then cool it at a cooling rate greater than 20°C/s Fast cooling to between 450 and 550°C;

4) The cooled steel is coiled at a coiling temperature of 450-550°C.

Aiming at the technical problems that have not been solved in the prior art for a long time, the inventors of this case have found through long-term research that polygonal ferrite and shellfish can be obtained based on conventional C-Si-Mn steel by adopting appropriate controlled rolling and segmental cooling processes. The hot-rolled ferritic bainitic dual-phase steel with high strength makes the steel have certain plasticity and excellent hole expansion rate, and has low production cost, and the application prospect is very good. More specifically, considering the role of each conventional component in conventional C-Si-Mn steel, namely:

Carbon: the most effective strengthening element in steel, if the carbon content is too low, the strength will not meet the requirements, and if the carbon content is too high, it will be unfavorable to welding performance and hole expansion performance;

Silicon: acts as solid solution strengthening in steel, accelerates the formation of polygonal ferrite, delays the formation of pearlite, and is beneficial to obtain ferrite and bainite dual-phase structure and increase the hole expansion rate, but too high will deteriorate the steel plate. surface properties;

Manganese: a solid solution strengthening element, and can delay the formation of ferrite and pearlite, too low for insufficient strength of steel, too high to reduce plasticity;

Phosphorus: impurity elements in steel, the content should be controlled at a low level;

Sulfur: Sulfur is easy to form inclusions represented by MnS, which is easy to produce intermediate segregation, which is unfavorable to hole expansion performance, and the lower the content, the better;

Through long-term practice, the inventors of this case have based on the steel of conventional C-Si-Mn composition without adding expensive micro-alloying elements such as niobium, vanadium, titanium, etc., and by adopting hot rolling method, especially based on segmental cooling process, And control the content of each component in the steel within a certain range, so as to produce hot-rolled ferritic bainite dual-phase steel. The microstructure of this dual-phase steel is polygonal ferrite + bainite, and its strength reaches Above 490MPa, the elongation reaches about 25%.

Compared with the prior art, the beneficial effect of the present invention is that: the high tensile strength hot-rolled ferritic bainite dual-phase steel has a tensile strength above 490MPa, has excellent hole expansion performance and formability, and has low material cost. It is cheap, and at the same time, the preparation process of the high tensile strength hot-rolled ferritic bainitic dual-phase steel is simple, no need to add expensive alloy elements, easy to operate, and different strength levels can be obtained through process control. The invention can be widely and mainly applied to the forming of parts with complex shapes such as automobile wheels and suspension parts.

Description of drawings

Fig. 1 is the subsection cooling process schematic diagram in embodiment 1～3;

FIG. 2 is a schematic diagram of a ferrite-bainite dual-phase structure in Example 1. FIG.

Detailed ways

The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Example 1: The steel ingot was smelted in a vacuum induction furnace, and its chemical composition is shown in Table 1. Rolling was performed on a laboratory 550mm reversing mill. The rolling process is as follows: heating the steel ingot to about 1200°C, holding it for 1 hour, the start temperature of rough rolling is about 1150°C, the start temperature of finish rolling is 954°C, and the finishing temperature is 847°C. After 2 passes of rough rolling and 5 passes of finishing rolling, the thickness of the material is rolled from 90mm to 10mm, and the reduction ratio is distributed at 30%-30%-30%-25%-25%-25%-20%. As shown in Figure 1, the cooling process is C1: about 30°C/s; air cooling start temperature T1: 718°C; air cooling time t1: 5s, C2: 20°C/s; coiling temperature T2: 487°C (heat treatment furnace Simulated coiling), the mechanical properties of the obtained steel are shown in Table 2, and its structure is shown in Figure 2.

Example 2: Steel ingots were smelted in a vacuum induction furnace, and the chemical composition is shown in Table 1. Rolling was performed on a laboratory 550mm reversing mill. The rolling process is as follows: heating the steel ingot to about 1220°C and holding it for 1 hour, the start temperature of rough rolling is about 1136°C, the start temperature of finish rolling is 981°C, and the finishing temperature is controlled at 859°C. After 2 passes of rough rolling and 5 passes of finishing rolling, the thickness of the material is rolled from 90mm to 10mm, and the reduction ratio is distributed at 30%-30%-30%-25%-25%-25%-20%. The cooling process is C1: about 30°C/s, air cooling start temperature T1: 693°C, air cooling time t1: 3s, C2: 33°C/s; coiling temperature T2: 457°C (simulated coiling with heat treatment furnace), the obtained steel The mechanical properties are shown in Table 2, and the structure is similar to Example 1.

Example 3: Steel ingots were smelted in a vacuum induction furnace, and the chemical composition is shown in Table 1. Rolling was performed on a laboratory 550mm reversing mill. The rolling process is as follows: heat the steel ingot to about 1230°C, hold it for 1 hour, start the rough rolling at about 1143°C, start the finish rolling at 961°C, and control the finishing temperature at 835°C. After 2 passes of rough rolling and 5 passes of finishing rolling, the thickness of the material is rolled from 90mm to 10mm, and the reduction ratio is distributed at 30%-30%-30%-25%-25%-25%-20%. As shown in Figure 1, the cooling process is C1: about 30°C/s, air cooling start temperature T1: 706°C, air cooling time t1: 5s, C2: 40°C/s; coiling temperature T2: 451°C (heat treatment furnace Simulated coiling), the mechanical properties of the steel obtained are as shown in Table 2, and the structure is similar to that of Example 1.

Table 1. Example chemical composition

The mechanical performance test is carried out according to GB6397-86, and the gauge length of the tensile specimen meets: $L_0=5.65\sqrt{S_0};$

Table 2. Example mechanical properties

Claims (3)

1. high tensile strength hot rolled ferrite bainite dual phase steel, it is characterized in that, tensile strength 〉=the 490MPa of this hot-rolled ferrite-bainite dual-phase steel, it is made up of following component by weight percentage: the Fe and the impurity of carbon 0.05～0.15%, silicon 0.5～1.0%, manganese 1.0～1.5%, phosphorus≤0.02%, sulphur≤0.010%, aluminium 0.03～0.05%, nitrogen≤0.0060% and surplus;

This hot-rolled ferrite-bainite dual-phase steel is prepared as follows:

(1) steel ingot to be processed is heated to 1200～1250 ℃, and all thermal treatment 1～2 hour;

(2) steel ingot after the heat treated is rolled, and control start rolling temperature be 1100～1150 ℃, finish rolling begins 950～1000 ℃ of temperature, 820～860 ℃ of finishing temperatures;

(3) to carrying out cooling process through Rolled Steel, be chilled to soon between 680 ℃～720 ℃ with the speed of cooling greater than 20 ℃/s, air cooling is handled 3～5s thereafter, then is chilled to soon between 450～550 ℃ with the speed of cooling greater than 20 ℃/s;

4) cooled steel are batched 450～550 ℃ of coiling temperatures.

2. high tensile strength hot rolled ferrite bainite dual phase steel according to claim 1, it is characterized in that this hot-rolled ferrite-bainite dual-phase steel is made up of following component by weight percentage: the Fe and the impurity of carbon 0.07～0.10%, silicon 0.60～0.90%, manganese 1.40～1.50%, phosphorus≤0.011%, sulphur≤0.0050%, aluminium 0.035～0.040%, nitrogen≤0.0050% and surplus.

3. the preparation method of high tensile strength hot rolled ferrite bainite dual phase steel as claimed in claim 1, this hot-rolled ferrite-bainite dual-phase steel is made up of following component by weight percentage: the Fe and the impurity of carbon 0.05～0.15%, silicon 0.5～1.0%, manganese 1.0～1.5%, phosphorus≤0.02%, sulphur≤0.010%, aluminium 0.03～0.05%, nitrogen≤0.0060% and surplus, it is characterized in that this method is:

(1) steel ingot to be processed is heated to 1200～1250 ℃, and all thermal treatment 1～2 hour;

(2) steel ingot after the heat treated is rolled, and control start rolling temperature be 1100～1150 ℃, finish rolling begins 950～1000 ℃ of temperature, 820～860 ℃ of finishing temperatures;

(3) to carrying out cooling process through Rolled Steel, be chilled to soon between 680 ℃～720 ℃ with the speed of cooling greater than 20 ℃/s, air cooling is handled 3～5s thereafter, then is chilled to soon between 450～550 ℃ with the speed of cooling greater than 20 ℃/s;

4) cooled steel are batched 450～550 ℃ of coiling temperatures.

Images