What Factors Affect the Strength of Carbon Steel?

Carbon steel is an iron-carbon alloy with a carbon content of 0.0218% to 2.11%. Also called carbon steel. Generally also contains a small amount of silicon, manganese, sulfur, phosphorus. Generally, the higher the carbon content in carbon steel, the greater the hardness and the higher the strength, but the lower the plasticity.

Carbon steel

(1) Carbon steel can be divided into carbon structural steel and carbon

Carbon structural steel

Grade: Example

I. Experiment Purpose

1. Understand the basic heat treatment process of carbon steel

2. Study the relationship between cooling conditions and steel properties

3. Analysis of the effect of quenching and tempering temperature on the properties of steel

Experimental equipment and test pieces

1. Experimental equipment: SX-10M-2.5 box test resistance furnace

2. Test piece: 45 steel, 30 steel and T8 steel

3. Three specimens of 45 steel after quenching

Third, the experimental principle

Heat treatment is an important metal processing technology method, the purpose is to improve the properties of steel (use performance and process performance). The heat treatment process characteristic of steel is that the steel is heated to a certain temperature, kept for a certain time, and then cooled at a certain cooling rate. The process changes the properties of the steel.

Fourth, experiment content and steps

(I) Quenching and heat treatment of steel

The quenching heat treatment is to heat the carbon steel to above AC3 or AC1 at 30-50 ° C, and then place it in a different cooling medium for rapid cooling (the cooling rate is greater than the critical cooling rate) to obtain a martensite structure (M). The microstructure after quenching is martensite and retained austenite.

Determination of quenching temperature

Depending on the material, check its critical temperature AC3 or AC1 in Table 1, and then add 40 ° C to get its heating temperature.

Hypoeutectoid steel (45 steel, 30 steel):

Heating temperature = AC3 + 40 ° C

Hypereutectoid steel (T10 steel):

Heating temperature = AC1 + 40 ° C

So the final heating temperature of 30 steel = ° C + 40 ° C =

45 steel heating temperature = ° C + 40 ° C =

45 steel heating temperature = ° C + 40 ° C =

2. Determination of holding time

After the parts are heated with the furnace to reach the required heating temperature, a period of heat insulation is required to ensure that the entire part reaches the required temperature evenly and fully. Obviously the holding time is related to the size and shape of the workpiece.

Calculate the holding time of the test piece by measuring the size of the part and then referring to Table 2.

The size of the part is a cylindrical part with a diameter of twenty millimeters, so the heat preservation times of 30 steel, 45 steel, and T10 steel are:

3. Selection of cooling medium

Cooling is the key process of quenching. It directly affects the properties of the steel after quenching. The quenching cooling rate should be greater than the critical cooling rate to obtain a supercooled martensite structure. At the same time, during the cooling process, the internal stress in the crystallization process must be controlled to prevent deformation and cracking.

In order to ensure the quenching effect, a suitable cooling medium and cooling method should be selected. In this experiment we choose water at room temperature as the cooling medium.

4. Place the workpiece in the furnace, set the heating control temperature of the electric furnace temperature controller, and start heating.

5. After the electric furnace reaches the set temperature, start the heat preservation timing.

6. The workpiece comes out of the oven and is quickly cooled in water.

(Ii) Tempering heat treatment of steel

The martensite structure obtained after quenching of the steel is hard and brittle, and there is a large internal stress in the workpiece. The purpose of tempering is to eliminate internal stress, appropriately reduce hardness, and improve processability. According to different process requirements, tempering is divided into three high-temperature tempering, medium-temperature tempering and low-temperature tempering. The temperature selection and the changes in microstructure and performance are shown in Table 3.

The tempering cooling method is air cooling, that is, the workpiece is slowly cooled at room temperature after it is out of the oven.

1. Place the workpiece in the furnace, set the heating control temperature of the electric furnace temperature controller, and start heating;

2. After the electric furnace reaches the set temperature, the heat preservation timing starts, and the heat preservation time is 30 minutes;

3. The workpiece comes out of the oven, and it is cooled slowly at room temperature.

(1) Output hit a record high. From January to June 2013, the country's cumulative production of crude steel was 390 million tons, a year-on-year increase of 7.4%, an increase of 5.6 percentage points over the same period a year ago. In the first 6 months, the average daily output of crude steel was 2.154 million tons, equivalent to an annual output of 786 million tons of crude steel. Among them, in February it reached a record high of 2.208 million tons. Although it declined in March-June, it remained at a high level of more than 2.1 million tons. In terms of provinces and regions, the crude steel output of Hebei and Jiangsu increased by 6.8% and 13.2% year-on-year respectively from January to June. The combined new production of the two provinces accounted for 42.4% of the national increase of 26.94 million tons. Provinces such as Henan and Yunnan also increased output by more than 1 million tons. In terms of different types of enterprises, from January to June, the crude steel output of key large and medium-sized steel companies increased 5.5% year-on-year, which was 2 percentage points lower than the national average increase, but 60% of the increase in output was still from key large and medium-sized steel companies.

(2) Low steel prices. From January to June 2013, the overall performance of the domestic steel market was sluggish. With the substantial release of crude steel production capacity, the market supply and demand has fallen into an imbalance, and steel prices have entered a downward channel and have fallen weakly for more than 4 months. As of July 26, 2013, the steel price index had dropped to 100.48 points, which was lower than 6.6 points at the beginning of the year. The prices of the eight steel varieties that the Iron and Steel Industry Association focused on have all fallen to varying degrees from the beginning of the year, with an average decrease of 5.7%. In terms of different types, the prices of construction wires and rebars, which accounted for a large proportion of China's steel production, fell by 4.9% and 6.7%, and the prices of plate and hot rolled coil fell by 5.7% and 9.7%, respectively.

(3) The export of steel products has grown rapidly. The imbalance between supply and demand in the domestic steel market has stimulated exports. From January to June, China exported 30.69 million tons of steel products, an increase of 12.6% year-on-year; imported steel products were 6.83 million tons, a decrease of 1.8%, and imported steel billets and ingots were 320,000 tons, an increase of 50%. Converting billets into crude steel, the cumulative net export was 25.06 million tons, a year-on-year increase of 17.3%, accounting for 6.4% of China's crude steel output. From the perspective of export prices, the average export price of rods and wires for the first six months of the year was US $ 624.3 / ton, a year-on-year decrease of 18%; sheet materials were US $ 835.2 / ton, a year-on-year decrease of 2.8%.

(4) Steel mills and social stocks are operating at high levels. The contradiction between market supply and demand has spread to the circulation field, and domestic steel stocks have continued the growth trend at the end of the previous year. On March 15, it reached a record high of 22.52 million tons, an increase of 3.51 million tons from the previous year's highest point, of which 14.32 million tons of construction steel stocks, accounting for 63.6% of the total inventory. After that, as seasonal consumption increased, the inventory gradually dropped, and it fell to 15.4 million tons on July 26. Oversupply in the market has also pushed up steel plant inventory. In mid-March, the steel stocks of key enterprises reached a historical record of 14.51 million tons, a year-on-year increase of 29.7%. In late June, it fell to 12.68 million tons, which is still a 29.9% increase from the beginning of the year, compared with 2012 The year-on-year growth was 11.4%.

(E) The profit level of steel mills has been decreasing month by month. In the first half of 2013, the metallurgical industry realized a profit of 73.69 billion yuan, an increase of 13.7% year-on-year, of which the ferrous metal smelting and rolling processing industry realized a profit of 45.44 billion yuan, an increase of 22.7% year-on-year. From January to May, the profitability of key large and medium-sized iron and steel enterprises is far lower than the overall level of the industry, and it has shown a downward trend from month to month. Although the profit has increased by 34%, it is only 2.8 billion yuan, and the sales profit margin is 0.19%. In May of that month, 86 key large and medium-sized steel companies only realized a profit of 150 million yuan, which has been a sequential decline for 5 consecutive months, of which 34 companies lost money, with a loss of up to 40%.

(6) The growth rate of investment in fixed assets in the steel industry dropped significantly. From January to June 2013, the investment in fixed assets in the steel industry was 303.5 billion yuan, an increase of 4.3% year-on-year, of which ferrous metal smelting and rolling investment was 235.6 billion yuan, an increase of 3.3% year-on-year, down 6.1 percentage points from the same period in 2012; The selected investment was 67.9 billion yuan, a year-on-year increase of 7.8%, and the growth rate fell sharply by 15 percentage points.