CN109182923B

CN109182923B - Heat treatment method of low-carbon microalloyed cold-rolled TRIP980 steel with high product of strength and elongation

Info

Publication number: CN109182923B
Application number: CN201811351275.4A
Authority: CN
Inventors: 许云波; 彭飞; 顾兴利; 王源
Original assignee: Northeastern University China
Current assignee: Northeastern University China
Priority date: 2018-11-14
Filing date: 2018-11-14
Publication date: 2021-09-10
Anticipated expiration: 2038-11-14
Also published as: CN109182923A

Abstract

The invention belongs to the technical field of material heat treatment, and particularly relates to a heat treatment method of low-carbon microalloyed cold-rolled TRIP980 steel with high strength-ductility product. The 980 MPa-grade cold-rolled TRIP steel with high strength-elongation product comprises the following chemical components in percentage by mass: 0.18-0.23% of C, 1.5-2.0% of Mn, 1.6-1.8% of Si, 0.025-0045% of Nb, 0.08-0.15% of Ti, less than or equal to 0.015% of P, less than or equal to 0.005% of S, and the balance of Fe and inevitable impurities. The preparation method comprises smelting, forging, hot rolling, acid washing, cold rolling, pre-quenching, dephosphorization and annealing. The steel produced by the invention realizes good strength-plasticity proportion on the basis of low alloy cost, and realizes 980 MPa-grade cold-rolled TRIP steel with high strength-plasticity product, easy molding and easy welding, wherein the yield strength is 730-.

Description

Heat treatment method of low-carbon microalloyed cold-rolled TRIP980 steel with high product of strength and elongation

Technical Field

The invention belongs to the technical field of material heat treatment, and particularly relates to a heat treatment method of low-carbon microalloyed cold-rolled TRIP980 steel with high strength-ductility product.

Background

With the increasing global energy crisis and environmental deterioration, safety, energy conservation and environmental protection have become the development trend of the automobile manufacturing industry. The novel advanced high-strength steel (AHSS) for the automobile with high strength and good plasticity and toughness can effectively realize the thickness reduction of parts, thereby realizing the weight reduction, energy conservation and consumption reduction of the automobile and being widely favored by the steel and automobile industries. The TRIP steel is transformed into martensite by the transformation induced plasticity (TRIP effect) of the residual austenite in the deformation process by a complex phase structure of ferrite, bainite and the residual austenite, so that the strength and the shape of the steel are improved, and the TRIP steel becomes a preferred strategy for producing the high-strength steel at present.

From the practical use situation, due to the requirement of stamping and forming of automobile parts, numerous automobile enterprises put forward the practical requirement of 24% elongation for 980MPa grade automobile steel. However, from the perspective of the development of the existing steel grades, the existing commercial steel grades can not meet the above-mentioned shaping indexes, so that under the background that the existing cold-rolled TRIP products can not completely meet the use requirements, the development of a new steel grade which can realize high strength (equal to or more than 980MPa) and high elongation (equal to or more than 24%) under a low alloy component system has very important application value and wide market prospect.

Disclosure of Invention

In order to solve the technical problems, the invention provides a heat treatment method of low-carbon microalloyed cold-rolled TRIP980 steel with high strength-ductility product.

The specific technical scheme is as follows:

a heat treatment method of low-carbon microalloyed cold-rolled TRIP980 steel with high product of strength and elongation comprises the following steps:

(1) forging a casting blank with a certain chemical composition into a forging blank, reheating, hot rolling, cooling by water, and curling to obtain hot rolled strip steel;

(2) pickling the hot-rolled strip steel, and then cold-rolling the pickled hot-rolled strip steel into cold-rolled strip steel;

(3) keeping the temperature for a period of time after the cold-rolled strip steel is completely austenitized, and then cooling the cold-rolled strip steel to room temperature by water to generate pre-quenched strip steel with a full martensite structure;

(4) and descaling the surface of the pre-quenched strip steel to remove an iron oxide skin layer and a decarburized layer, then reheating, annealing and preserving heat for a period of time, cooling to a certain temperature by using a salt bath, preserving heat for a certain period of time, and then cooling to room temperature by water to prepare the final finished product strip steel.

The cold-rolled TRIP980 steel comprises the following chemical components in percentage by mass: 0.18-0.23% of C, 1.5-2.0% of Mn1.6-1.8% of Si, 0.025-0.045% of Nb0.08-0.15% of Ti, less than or equal to 0.015% of P, less than or equal to 0.005% of S, and the balance of Fe and inevitable impurities.

The reheating temperature range of the forging stock in the step (1) is 1100-1200 ℃, the heat preservation time is 3-5h, the initial rolling temperature of hot rolling is 1050-1150 ℃, and the final rolling temperature is 850-900 ℃; the hot rolling is carried out for 7 times of reciprocating rolling by adopting a 4-roller reversible rolling mill, the reduction rate of the first two times is 30-50%, the reduction rate of the last five times is 20-30%, then the rolled steel is cooled to 650-750 ℃ by water and then placed into asbestos for heat preservation for 8-10h, so that the curling process is simulated, and the thickness of the hot rolled strip steel is 4-5.5 mm.

And (3) carrying out unidirectional rolling by adopting a four-roller rolling mill in the cold rolling step (2), wherein the rolling passes are 10-15 passes, including 3-5 passes of smoothing rolling, and the thickness of the final cold-rolled strip steel is 1.0-1.5 mm.

The austenitizing temperature of the cold-rolled strip steel in the step (3) is 870-920 ℃, and the austenitizing heat preservation time is 5-15 min.

And (4) removing the iron scale and the decarburized layer to 50-100 μm of the upper bottom surface and the lower bottom surface respectively.

The reheating annealing temperature of the pre-quenched strip steel in the step (4) is 780-830 ℃, and the annealing heat preservation time is 3-8 min.

The cooling speed of the salt bath in the step (4) is 100-.

Compared with the prior art, the invention has the following beneficial technical effects:

(1) the content of alloy components is not increased, and the alloy cost is low by adopting the traditional low-carbon Si-Mn series microalloying TRIP component system; (2) according to the invention, a large amount of lath-shaped high-carbon residual austenite is mainly formed by taking the martensite nanoscale lath boundary as an austenite core point, and a small amount of blocky low-carbon residual austenite is supplemented, so that the TRIP effect can be effectively generated in a large stress range, and the strong plasticity of the steel is obviously improved. (3) According to the invention, by optimizing the heat treatment process, 980 MPa-grade cold-rolled TRIP steel with high strength-elongation product, easy forming and easy welding is realized, and the potential of steel materials is further developed.

The heat treatment process can realize the following mechanical properties of the strip steel: the yield strength is 730-.

Drawings

FIG. 1 is an SEM photograph of a microstructure obtained by the present invention;

FIG. 2 is a TEM photograph of lath austenite obtained by the present invention.

Detailed Description

The invention is described in detail below with reference to the drawings and specific embodiments, but the scope of the invention is not limited by the drawings and the embodiments.

Example 1.

Smelting and casting are carried out according to the chemical component range, and the chemical components of the obtained casting blank are shown in the table 1:

table 1 example 1 corresponds to the composition of the cast slab (wt.%) and

C	Si	Mn	Nb	Ti	S	P
							0.22	1.80	1.65	0.025	0.10	0.001	0.006

then forging the casting blank into a forging blank with the section of 60 x 60mm, then cutting a steel block with the length of 150mm by a sawing machine, putting the steel block into a heating furnace (the temperature is 1200 ℃), preserving heat for 3 hours, and then carrying out hot rolling. The initial rolling temperature corresponding to hot rolling is 1150 ℃, then the steel plate is subjected to 7-pass reciprocating rolling by a 4-roller reversible rolling mill, the final rolling temperature is controlled to be 860 ℃, the reduction ratios of the first two passes are 45%, the reduction ratios of the last five passes are respectively 25%, 30%, 25%, 25% and 20%, and the final thickness is 4.5 mm. Then, an ultra-fast cooling device is adopted to cool to 650 ℃ at a cooling speed of 100 ℃/s, asbestos is put into the ultra-fast cooling device to perform a heat preservation simulation curling process, and the heat preservation time is 8 hours. The final structure is ferrite and pearlite.

The hot-rolled strip steel is soaked in 20% hydrochloric acid solution to clearly remove surface iron scale, and then washed with water to remove acid liquor, and then washed with alcohol and dried. And then, carrying out unidirectional rolling by adopting a four-roller cold rolling mill, wherein the rolling pass is 15 passes, including 5 passes of flat rolling, and the thickness of the final cold-rolled strip steel is 1.0 mm. A tensile sample with the total length of 120mm and the width of a parallel section of 12.5mm is cut along the rolling direction of the strip steel, heated to 900 ℃ in an electric resistance furnace at the heating speed of 20 ℃/s, kept warm for 300s, and then water-quenched to room temperature. The tensile specimen was then removed 100 μm from both the top and bottom surfaces to remove the surface iron oxide scale and the decarburized layer. Then the stretched sample is reheated to 810 ℃ at a speed of 20 ℃/s, and is kept warm for 360s, and then is cooled to 360 ℃, 380 ℃ and 400 ℃ respectively by a salt bath at a cooling speed of 100 ℃/s, and is kept warm for 500s, and then is cooled to room temperature by water. The mechanical properties are shown in Table 2.

Table 2 mechanical properties corresponding to example 1

The above table shows that all the mechanical properties reach the required performance indexes, and the ferrite content, the bainite and martensite content and the retained austenite content of the steel plate prepared by the process are about 32%, about 56% and about 12%. The fine structure shows that the effective grain size of the prior austenite grains is greatly reduced after the prior austenite grains are divided by ferrite and bainite, and meanwhile, the form of the retained austenite is mainly lath, which provides favorable conditions for the overall improvement of the strong plasticity.

Example 2.

Smelting and casting are carried out according to the chemical composition range, and the chemical compositions of the obtained casting blank are shown in the following table 3:

table 3 example 2 corresponding ingot composition (wt.%)

C	Si	Mn	Nb	Ti	S	P
							0.21	1.70	1.70	0.045	0.08	0.001	0.006

Then forging the casting blank into a forging blank with the section of 60 x 60mm, then cutting a steel block with the length of 150mm by a sawing machine, putting the steel block into a heating furnace (the temperature is 1100 ℃), preserving heat for 5 hours, and then carrying out hot rolling. The initial rolling temperature corresponding to hot rolling is 1050 ℃, then the steel plate is subjected to 7-pass reciprocating rolling by a 4-roller reversible rolling mill, the final rolling temperature is controlled to be 880 ℃, the reduction rates of the first two passes are both 50%, the reduction rates of the last five passes are respectively 22%, 30%, 27%, 25% and 20%, and the final thickness is 4 mm. Then, an ultra-fast cooling device is adopted to cool to 750 ℃ at a cooling speed of 100 ℃/s, asbestos is put into the ultra-fast cooling device to perform a heat preservation simulation curling process, and the heat preservation time is 10 hours. The final structure is ferrite and pearlite.

The hot-rolled strip steel is soaked in 20% hydrochloric acid solution to clearly remove surface iron scale, and then washed with water to remove acid liquor, and then washed with alcohol and dried. And then, carrying out unidirectional rolling by adopting a four-roller cold rolling mill, wherein the rolling pass is 10 passes, including 3 passes of flat rolling, and the thickness of the final cold-rolled strip steel is 1.5 mm. A tensile sample with the total length of 120mm and the width of a parallel section of 12.5mm is cut along the rolling direction of the strip steel, heated to 880 ℃ at a heating speed of 20 ℃/s in an electric resistance furnace, insulated for 900s, and then water-quenched to room temperature. The tensile sample was then removed 50 μm both from the top and bottom surfaces to remove the surface iron oxide scale and the decarburized layer. Then the stretched sample is reheated to 800 ℃ at the speed of 20 ℃/s, the temperature is preserved for 180s, then the sample is cooled to 380 ℃ by salt bath at the cooling speed of 130 ℃/s, the temperature is preserved for 300s, 500s and 600s, and then the sample is cooled to room temperature by water. The mechanical properties are shown in Table 4.

Table 4 mechanical properties corresponding to example 2

From the above table, it can be seen that all the mechanical properties reach the required performance indexes, the ferrite content of the process is about 35%, the bainite and martensite content is about 52%, and the residual austenite content is about 13%. At the moment, the tensile strength is 1035MPa, the yield strength is 756MPa, the elongation is 26.5%, the product of strength and elongation is 27.4GPa, lath-shaped carbide-free bainite exists in the structure as a matrix phase, crack propagation can be effectively hindered, the toughness of steel is improved, meanwhile, lath-shaped high-carbon austenite can effectively play the TRIP effect, and therefore the overall strength and plasticity are also effectively improved.

Claims

1. The heat treatment method of the cold-rolled TRIP980 steel with low carbon microalloying and high product of strength and elongation is characterized in that the cold-rolled TRIP980 steel comprises the following chemical components in percentage by mass: c = 0.18-0.23%, Mn = 1.5-2.0%, Si = 1.6-1.8%, Nb = 0.025-0.045%, Ti = 0.08-0.15%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, and the balance is Fe and inevitable impurities; the method comprises the following steps:

(1) forging the casting blank with the chemical components into a forging blank, heating and preserving heat for a period of time, then carrying out hot rolling, and coiling after water cooling to obtain hot rolled strip steel;

(2) carrying out acid pickling on the hot-rolled strip steel, and then carrying out cold rolling on the hot-rolled strip steel to obtain cold-rolled strip steel;

(3) completely austenitizing the cold-rolled strip steel, preserving the heat for a period of time, and cooling the cold-rolled strip steel to room temperature by water to obtain pre-quenched strip steel with a full martensite structure; the austenitizing temperature of the cold-rolled strip steel is 870-920 ℃, and the austenitizing heat preservation time is 5-15 min;

(4) descaling the surface of the pre-quenched strip steel to remove an iron oxide skin layer and a decarburized layer, then reheating, annealing and preserving heat for a period of time, cooling to a certain temperature by using a salt bath, preserving heat for a certain period of time, and then cooling to room temperature by water to prepare the final finished product strip steel; the reheating annealing temperature of the pre-quenched strip steel is 780-830 ℃, and the annealing heat preservation time is 6-8 min; the cooling speed of the salt bath is 100-;

after the cold-rolled TRIP980 steel is subjected to pre-quenching and heat treatment, the yield strength is 730-.

2. The heat treatment method of the low-carbon microalloyed high-strength plastic-product cold-rolled TRIP980 steel as claimed in claim 1, wherein the temperature range for reheating the forged blank in the step (1) is 1100-; the hot rolling is carried out for 7 times of reciprocating rolling by adopting a 4-roller reversible rolling mill, the reduction rate of the first two times is 30-50%, the reduction rate of the last five times is 20-30%, then the asbestos is put into the hot rolling mill after the water cooling is carried out to 650-750 ℃, the heat preservation is carried out for 8-10h, thereby simulating the coiling process, and the thickness of the hot rolled strip steel is 4-5.5 mm.

3. The heat treatment method of the low-carbon microalloyed cold-rolled TRIP980 steel with high product of strength and elongation as claimed in claim 1, wherein the cold rolling in the step (2) is carried out by unidirectional rolling with a four-high mill, the rolling passes are 10-15 passes, the rolling passes comprise 3-5 passes of temper rolling, and the thickness of the final cold-rolled strip steel is 1.0-1.5 mm.

4. The method for heat treating a low-carbon microalloyed high-strength high-ductility cold-rolled TRIP980 steel as claimed in claim 1, wherein the removal thickness of the scale and the decarburized layer in step (4) is 50-100mm for each of the upper and lower bottom surfaces.